Upload
others
View
4
Download
0
Embed Size (px)
Citation preview
Thinking Matters: The Profile of Executive Functioning Associated with Cannabis
Use in Schizophrenia and its Functional Outcome Correlates
by
Khristin Highet, B.Psych (Hons); M.Psych (Clin)
This thesis is submitted in partial fulfilment of the requirements for the degree of
Doctor of Psychology (Clinical)
at Murdoch University
2014
I declare that this thesis is my own account of my research and contains as its main
content work which has not previously been submitted for a degree at any tertiary
institution
.........................................................
Khristin Highet
Abstract
Not only is there a high prevalence of cannabis use in schizophrenia, but long-term or
heavy cannabis users are also considered to demonstrate deficits similar to the cognitive
endophenotypes of the illness itself. The specific neurocognitive domain of executive
function has particular clinical relevance as executive dysfunction has been found to be
significantly associated with the functional outcomes of schizophrenia patients. The
current study sought to examine the pattern of executive functioning in schizophrenia
patients with cannabis use and its functional outcome correlates. Eleven measures
guided by a hierarchical model of executive function were administered to 28
schizophrenic outpatients with cannabis use and 28 matched-controls with a similar
cannabis use history. Premorbid IQ and ‘real-world’ functional outcome were also
assessed. A series of ANCOVA’s revealed that patients with cannabis use demonstrated
poorer performances on a number of executive function measures relative to controls.
However, further analysis indicated that a significantly larger proportion of this sample
showed performances that did not fall in the range of clinical impairment. Using
multiple regression analyses, a retrospective memory and an interference control task
were found to significantly contribute to ‘real-world’ functional outcome. A lack of
clinical impairment in the executive functioning of a representative sample of
schizophrenia patients with cannabis use may be suggestive of differential and subtle
deficits in this subgroup. Retrospective memory and interference control abilities may
also present as potential targets helping to better inform rehabilitation planning efforts
for such patients. Evaluating different subgroups of schizophrenia from a
neurocognitive viewpoint should be an important consideration for treating clinicians
particularly when attempting to achieve more successful functional outcomes.
ii
Acknowledgements
There are a number of individuals and organisations that have contributed to the
development and completion of this project. Firstly, I would like to express my sincere
and utmost gratitude to my chief research supervisor at Murdoch University, Dr
Bethanie Gouldthorp, for her continual mentorship, dedication and confidence in my
study. I also wish to thank my co-supervisors, Dr Allen Gregg Harbaugh and Dr
Corinne Reid, for their collaboration on the project and offering their invaluable
guidance. In addition, I would like to thank Dr Marjorie Collins, a previous faculty
member and supervisor at Murdoch University, and Dr Janet Bryan at the University of
South Australia, for their assistance and support with the original design, methodology
and initial supervision of the project. Without all of their support, direction and
expertise, this study would not have been possible.
I would also like to show my appreciation to the many public health and mental
health organizations, as well as their staff, that have helped contribute to and support the
various activities that were crucial elements in this research project. I am extremely
grateful to the Queen Elizabeth Hospital and Lyell McEwin Hospital Service
(specifically Port Adelaide Mental Health Service and Beaufort Clinic), Flinders
Medical Centre (specifically Southern Mental Health Service and Adaire Clinic), The
Royal Adelaide Hospital, as well as the Mental Illness Fellowship of South Australia
(MIFSA), for their participation in the study through providing not only access to
prospective clinical participant populations, but for also allowing me access to their
facilities and liaison with their mental health treatment teams. In addition, I wish to
thank all of the local mental health community centres and mental health supported
accommodation/hostels which also took part by posting advertisements for the study
within their facilities. An enormous thank you also goes out to the individuals who took
iii
part in study itself by taking the time to undertake comprehensive interviews and
assessments. Without the extensive multi-site involvement and support of these
organisations and individuals, the present research would not have been achievable.
Another important acknowledgement goes to the Department of Psychiatry at
Monash University for providing distance training and support in the administration,
scoring, and interpretation of the Positive and Negative Symptom Scale (PANSS) and to
the Brain Injury Rehabilitation Community Hospital (BIRCH) team in South Australia
for affording me the opportunity to utilise difficult to access neuropsychological test
resources. I would also like to thank Andrew Rothwell, for his time, professional
networks, neuropsychological knowledge and invaluable feedback.
A special expression of gratitude is dedicated to my dear friend, Rachel, for her
tireless and enduring unconditional support and enthusiasm throughout this journey. A
huge thank you also goes out to another dear friend and fellow student, Carlo. Your out-
of-this-world statistical mind astounds me and I cannot thank you enough for not only
the guidance you have provided me in this area, but also the continual motivation, moral
support and understanding since we have met. We finally made it! To my partner Mike,
for all of the amazing home-cooked meals, cups of tea, massages and even practical
assistance in the final editing and cross-checking of my extensive reference list, I
sincerely thank you. Without your patience and encouragement during moments of
doubt, I could not have brought this project to finality.
To my family and friends, I wish to thank you all for your unwavering
accommodation, flexibility, love and support. Your belief in me has truly played a vital
role in the completion of this thesis.
iv
TABLE OF CONTENTS
ABSTRACT.......................................................................................................................
i
ACKNOWLEDGEMENTS...............................................................................................
ii
LIST OF ABBREVIATIONS............................................................................................
vii
LIST OF TABLES.............................................................................................................
x
LIST OF FIGURES...........................................................................................................
xi
CHAPTER
INTRODUCTION............................................................................................................
Part A
The Issue of Comorbid Substance Use and Schizophrenia
The Relationship Between Cannabis use, Schizophrenia and Cognition
1
Cognitive impairments associated with cannabis use
The role of executive function
Executive function deficits associated with cannabis use
Attention
Working memory
Inhibition and impulsivity
Verbal fluency
Decision-making
Cognitive impairments associated with schizophrenia
The importance of executive function to individuals with schizophrenia
Cognitive impairments associated with cannabis use and schizophrenia
Limitations to Previous Research
Part B
The Assessment of Executive Function
Models of executive function
West’s model of executive function
Damasio’s somatic marker hypothesis
The Validity of Tests of Executive Function
‘Traditional’ tests of executive function
Assessing Functional Outcome
The Current Study
Hypotheses
METHOD......................................................................................................................... 70
Participants
Recruitment
Measures
Demographics
Clinical symptoms
Neuropsychometric battery
v
Premorbid intelligence
Executive Function
The temporal organisation of behaviour
Retrospective memory
Prospective memory
Interference control
Inhibition of prepotent responses
Verbal fluency
Emotion-based decision-making
Functional outcome
Procedure
RESULTS......................................................................................................................... 89
Data Screening
Part A
Group Differences in Executive Function
Level of executive function impairment between groups
The Effect of Cannabis-Use Status on the Recovery of Executive Function
The Effect of Length of Abstinence on the Recovery of Executive Function
The Validity of West’s Theoretical Model of Executive Function
Principal Components Analysis
Part B
The Relationship Between Executive Function and Functional Outcome
The predictive ability of executive function measures in predicting functional
outcome ratings
Personal Management (Independent Living Skills Inventory)
Hygiene and Grooming (Independent Living Skills Inventory)
Clothing (Independent Living Skills Inventory)
Basic Skills (Independent Living Skills Inventory)
Interpersonal skills (Independent Living Skills Inventory)
Home Maintenance (Independent Living Skills Inventory)
Money Management (Independent Living Skills Inventory)
Cooking (Independent Living Skills Inventory)
Resource Utilization (Independent Living Skills Inventory)
General Occupational Skills (Independent Living Skills Inventory)
Medication Management (Independent Living Skills Inventory)
Total ILSI Score (Independent Living Skills Inventory)
Moderation effects on executive function measures for functional outcome
Hygiene and Grooming (Independent Living Skills Inventory)
Clothing (Independent Living Skills Inventory)
Interpersonal Skills (Independent Living Skills Inventory)
Home Maintenance (Independent Living Skills Inventory)
Money Management (Independent Living Skills Inventory)
Cooking (Independent Living Skills Inventory)
Resource Utilization (Independent Living Skills Inventory)
Medication Management (Independent Living Skills Inventory)
Total ILSI Score (Independent Living Skills Inventory)
vi
GENERAL DISCUSSION.............................................................................................. 125
Part A
Group Differences in Executive Function
Level of executive function impairment between groups
The Effect of Cannabis-Use Status on the Recovery of Executive Function
The Effect of Length of Abstinence on the Recovery of Executive Function
The Validity of West’s Theoretical Model of Executive Function
Part B
The Relationship Between Executive Function and Functional Outcome
Practical Applications of the Findings
Limitations and Future Directions
Conclusions
REFERENCES................................................................................................................. 166
APPENDICES.................................................................................................................. 217
Appendix A: Structured Interview
Appendix B(i): Advertisement for Clinical Participants – Cannabis Users
Appendix B(ii): Advertisement for Clinical Participants – Non-drug Users
Appendix C: Clinical Participant Information Form
Appendix D: Advertisement for Control Participants
Appendix E: Control Participant Information Form
Appendix F: Consent Form for Clinical Participants
Appendix G: Consent Form for Control Participants
Appendix H: Debrief Information
vii
LIST OF ABBREVIATIONS
Analysis of Covariance (ANCOVA)
Behavioral Assessment of the Dysexecutive Syndrome (BADS)
Brief Assessment of Cognition in Schizophrenia (BACS)
Brief Visual Memory Test – Revised (BVMT-R)
Cambridge Neuropsychological Test Automated Battery (CANTAB)
Cannabis Use Disorder (CUD)
Continuous Performance Test (CPT)
Continuous Performance Test – Second Edition (CPT-II)
Controlled Oral Word Association Test (COWAT)
Delis-Kaplan Executive Function System (D-KEFS)
Delta-9-tetrahydrocannabinol (∆9-THC)
Depression Anxiety Stress Scale (DASS)
Diagnostic and Statistical Manual of Mental Disorders – Fourth Edition (DSM-IV)
Digit Symbol Substitution Test (DSST)
Dorsolateral Prefrontal Cortex (DLPFC)
Dysexecutive Questionnaire (DEX)
Event-Related Potentials (ERP’s)
Full Scale Intelligence Quotient (FSIQ)
Functional Magnetic Resonance Imaging (fMRI)
Global Assessment of Functioning (GAF)
Goal Management Training (GMT)
Independent Living Skills Inventory (ILSI)
Instrumental Activities of Daily Living (IADLs)
Intelligence Quotient (IQ)
viii
Iowa Gambling Task (IGT)
Letter-Number Sequencing (LNS)
Mayer-Salovey-Caruso Emotional Intelligence Test (MSCEIT)
Modified Six Elements (MSE)
National Adult Reading Test – Second Edition (NART-II)
Neurocognitive Enhancement Therapy (NET)
Orbitofrontal Cortex (OFC)
Positive and Negative Syndrome Scale (PANSS)
Positron Emission Tomography (PET)
Prefrontal Cortex (PFC)
Premorbid Adjustment Scale (PAS)
Principal Components Analysis (PCA)
Standard Deviation (SD)
Statistical Package for the Social Sciences (SPSS)
Structured Clinical Interview for the Positive and Negative Syndrome Scale (SCI-
PANSS)
Test de Aprendizaje Verbal Espan ˜a-Complutense (TAVEC)
Test of Grocery Shopping Skills (TGSS)
Trail Making Test (TMT)
UCSD Performance-based Skills Assessment (UPSA)
Ventromedial Prefrontal Cortex (VPFC)
Verbal Intelligence Quotient (VIQ)
Wechsler Adult Intelligence Scale – Fourth Edition (WAIS-IV)
Wechsler Adult Intelligence Scale – Revised (WAIS-R)
Wechsler Adult Intelligence Scale – Third Edition (WAIS-III)
ix
Wechsler Memory Scale – Revised (WMS-R)
Wechsler Memory Scale – Third Edition (WMS-III)
Wisconsin Card Sorting Test (WCST)
x
LIST OF TABLES
Table 1 Sociodemographic characteristics of the sample………………………..
74
Table 2 Results of ANCOVAs of executive function performance for Group
with covariate of premorbid IQ................................................................
92
Table 3 The proportion (n) of each sample group and effect size of proportional
differences in executive function performance across each of the
impairment level categories......................................................................
94
Table 4 Results of ANCOVAs of executive function performance for Group
and cannabis-use status with covariate of duration of use.......................
97
Table 5 Results of ANCOVAs of executive function performance for Group
and time since cessation categories with covariate of duration of use.....
101
Table 6 Factor loadings based on a Principal Components Analysis utilising
orthogonal (varimax) rotation of 10 executive function measures...........
104
Table 7 Correlations between executive function measures and ILSI sub-scales
and total ILSI score for the schizophrenia group.....................................
108
Table 8 Correlations between executive function measures and ILSI sub-scales
and total ILSI score for the matched-control group.................................
109
Table 9 Correlations between executive function measures and ILSI sub-scales
and total ILSI score for the overall sample..............................................
110
Table 10 Regression models for predicting ILSI ratings.........................................
116
xi
LIST OF FIGURES
Figure 1 Moderation effect between groups on Digit Span for Hygiene and
Grooming ratings........................................................................................
117
Figure 2 Moderation effect between groups on Symbol Search for Interpersonal
Skills ratings...............................................................................................
119
Figure 3 Moderation effect between groups on Symbol Search for Cooking
ratings..........................................................................................................
121
Figure 4 Moderation effect between groups on Symbol Search for Resource
Utilization ratings.......................................................................................
122
Figure 5 Moderation effect between groups on Symbol Search for Medication
Management ratings....................................................................................
123
Figure 6 Moderation effect between groups on Symbol Search for Total ILSI
Score...........................................................................................................
125
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 1
Thinking Matters: The Profile of Executive Functioning Associated with Cannabis
Use in Schizophrenia and its Functional Outcome Correlates
Substance use and its misuse have been associated with detrimental effects on
cognition (Beatty, Katzung, Moreland, & Nixon, 1995; O’Malley, Adamse, Heaton, &
Gawin, 1993). Cognitive impairment is also considered to be a core feature of the illness
of schizophrenia (Bleuler, 1911, 1950; Green, 1996; Herman, 2004; Hoff, Riordan,
O'Donnell, Morris, & DeLisi, 1992; Kraeplin, 1919, 1971; Saykin, Gur, Gur, Mozley,
Mozley, Resnick, & Stafniak, 1991; Sharma & Antonova, 2003), and executive
dysfunction is found to be particularly implicated (Pantelis, Barnes, Nelson, Tanner,
Weatherley, Owen, & Robbins, 1997; Reichenberg & Harvey, 2007; Shallice, Burgess,
& Frith, 1999). Executive function is considered an important cognitive domain as it has
been strongly linked to the functional outcomes of patients (Greenwood, Landau, &
Wykes, 2005; McClure, Bowie, Patterson, Heaton, Weaver, Anderson, & Harvey, 2007;
Penadés, Boget, Catalán, Bernardo, Gastó, & Salamero 2003; Simon, Giacomini,
Ferrero, & Mohr, 2012; Williams, Whitford, Flynn, Wong, Liddell, Silverstein et al.,
2008). Not only are there high rates of substance abuse in patients with schizophrenia
(Drake, Osher, & Wallach, 1991; Mueser, Yarnold, Levinson, Singh, Bellack, Kee et
al., 1990; Regier, Farmer, Rae, Locke, Keith, Judd, & Goldwin, 1990), but cannabis is
the most commonly used illicit drug among patients with the illness (Barnes, Mutsatsa,
Hutton, Watt, & Joyce, 2006). Impaired cognitive function has also been found in
cannabis abusers alone (Pope, Gruber, & Yurgelun-Todd, 1995). In addition, the
widespread use of cannabis amongst individuals with schizophrenia has been found to
be strongly related to the severity of symptoms, the course of their illness, and poor
outcome (Caspari, 1999; Grech, Van Os, Jones, Lewis, & Murray, 2005; Linszen,
Dingemans, & Lenior, 1994). Therefore, examining the relative differences in the
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 2
executive functioning of schizophrenia patients with cannabis use has important clinical
significance when considering treatment targets for comorbid patients and ultimately
more successful functional outcomes. Considering the high rates of cannabis use in this
particular clinical population, both the neuropsychological and functional outcome
correlates of cannabis use and schizophrenia will be specifically examined.
Part A
The Issue of Comorbid Substance Use and Schizophrenia
There is a high prevalence of substance use and misuse in individuals with first-
episode psychosis (Cantwell, Brewin, Glazebrook, Dalkin, Fox, Medley, & Harrison,
1999). In addition, amongst those with psychotic disorders approximately 52% of
patients are not only found to have comorbid diagnoses, but the most common of these
diagnoses is substance use disorder (Stratowski, Maurico, Stoll, Faedda, Mayer,
Kolbrener et al., 1993). Substance abuse is also the most prevalent comorbid condition
associated with the illness of schizophrenia (Cuffel, Heithoff, & Lawson, 1993).
Evidence from community studies suggests that schizophrenia patients in particular, are
more prone to comorbid substance use disorders than the general population and that the
rate of substance abuse has been found to be 4.6 times higher than those without the
illness (Regier et al., 1990). The estimated lifetime prevalence rates of substance abuse
in this clinical population are reported to approximate 50% and the prevalence rates
relating to recent or current substance use range from 20% to 40% (Drake et al., 1991;
Mueser et al., 1990). Not only are substance abuse prevalence rates high in this
population, but substance use in schizophrenia is associated with the greater utilisation
of services as well as higher service costs (Bartels, Teague, Drake, Clark, Bush, &
Noordsy, 1993).
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 3
In comparison to other types of psychiatric patients or healthy individuals, it has
been found that schizophrenia patients in particular are more likely to use cannabis than
other drugs of abuse (Degenhardt, Hall, & Lynskey, 2003; Warner, Taylor, Wright,
Sloat, Springett, Arnold, & Weinberg, 1994). With lifetime use reported to be as high as
64.4%, cannabis is the most commonly used illicit drug reported among patients with
schizophrenia (Barnes et al., 2006). More specific to the Australian population,
approximately 25% of a nationally represented sample of individuals with psychotic
disorders in contact with services, were found to have reported a lifetime history of
cannabis abuse (Hall, Teesson, Lynskey, & Degenhardt, 1999). Twenty-four percent of
these patients were reported to be daily or near-daily users. Data from an Australian
survey also showed that cannabis dependence was associated with an 11-fold higher
odds of screening positively for schizophrenia or schizoaffective disorders (Degenhardt
et al., 2003). Green and colleagues (2005) have considered that the variation in
prevalence findings across studies is contributed to by a number of factors. These
include age, the percentage of male representation in studies, the methods used as well
as the clinical experience of researchers in applying diagnostic criteria to individuals
with psychoses, the variation in specificity of diagnostic inclusion criteria, and the
variance in the nature of the interviews used to collect information and the method of
data collection (e.g., self-report versus sample analysis). Such variability in factors
across studies makes prevalence rates more difficult to precisely define. However, the
general consensus suggests that cannabis use and its misuse is a commonality amongst
this particular clinical population.
In a meta-analysis of studies examining cannabis use in schizophrenia occurring
between 1996 and 2008, a median current prevalence rate of 16.0% was reported and a
median lifetime prevalence rate was estimated to be 27.1% (Koskinen, Löhönen,
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 4
Koponen, Isohanni, & Miettunen, 2010). It was found that the median rate of cannabis
use disorders (CUD’s) was higher in first-episode patients compared to long-term
patients, with rates for current CUD’s being 28.6% and 22.0% respectively, and lifetime
prevalence rates being 44.4% and 12.2% respectively. The overall findings of the meta-
analysis indicated that approximately 1 in 4 schizophrenia patients in the included
studies had a diagnosis of CUD. The reviewed studies also suggest that countries such
as Australia are an example of a country that evidences a high consumption of cannabis
not only in schizophrenia patients, but also in the general population. Data from the
Low Prevalence arm of the Australian National Survey of Mental Health and Wellbeing
which targeted a sample of 970 individuals diagnosed with psychosis, showed that
26.5% of schizoprenia patients had a comorbid lifetime diagnosis of cannabis abuse or
dependence (Kavanagh, Waghorn, Jenner, Chant, Carr, Evans, et al., 2004). Cannabis
was also found to be the most frequently used drug with 40.9% of the overall sample
using this particular illicit substance. In a review of population-based studies, it has
been reported that cannabis use appears to overall be associated with a two-fold increase
in the relative risk for later schizophrenia or schizophreniform disorders when compared
to the general population (Arseneault, Cannon, Witton, & Murray, 2004).
It is considered that many young people in Australia use cannabis and that this
use usually occurs during late adolescence and early adulthood, which is a period of
development considered to be where the risk of psychoses is at its peak (Hall,
Degenhardt, & Teesson, 2004). Australian survey data suggests that the age of cannabis
use initiation now occurs at an earlier age in young people, compared to the age of
cannabis use onset reported in the 1980’s (Degenhardt, Lynskey, & Hall, 2000). It is
reasonable to consider that if the onset of psychosis symptoms occurs around this time
in development; consequently, so does a diagnosis of the illness. However, research
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 5
findings have suggested a more direct link between earlier age of cannabis use onset and
increased risk of the development of psychosis. Arsenault and colleagues (2002)
examined the relationship between cannabis use in adolescence and psychosis in young
adulthood. Results of their study not only demonstrated a significant relationship
between cannabis use by the age of 15 and an increased risk of reported psychotic
symptoms by the age of 26, but also the specificity of effects of this particular substance
on such symptoms. In other words, no significant relationship was found between
psychotic disorders and other drugs of abuse. In addition to this, no relationship was
demonstrated between depression and cannabis use. An interaction between age of
cannabis use onset and psychosis risk was also found, showing earlier cannabis use
onset being significantly related to psychosis. In line with these findings, results from a
meta-analysis indicated that the age of psychoses onset in cannabis users is 2.7 years
earlier in comparison to non-cannabis using individuals (Large, Sharma, Compton,
Slade, & Nielssen, 2011). It has also been shown that earlier initiation of cannabis abuse
is associated with the onset of high-risk symptoms for psychosis at a younger age
(Dragt, Nieman, Becker, van de Fliert, Dingemans, de Haan et al., 2010). In light of the
research evidence, it appears that earlier age of cannabis use onset has a significant
association with earlier age of illness onset. In addition, not only is cannabis use in
schizophrenia highly prevalent around the world, as well as within Australia, but that
the age of cannabis use onset has become younger over the years which places such
individuals at greater risk considering the abovementioned relationships.
Studies investigating the subjective effects of substances based on the self-report
of users, suggest that patients with schizophrenia use drugs such as cannabis to help
self-medicate. Cannabis has been reported to be used in order to relieve negative affect,
‘to increase pleasure’ , ‘to get high’, ‘to relax,’, ‘to satisfy curiosity’, ‘to give more
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 6
interests’, as well as to ‘get along with the group’ (Addington & Duchak, 1997;
Goswami, Mattoo, Basu, & Singh, 2004). However, despite the many reported reasons
for using cannabis by individuals with schizophrenia, extensive research has
demonstrated that there are also many detrimental consequences that are associated with
cannabis use in this particular patient population. While it has been proposed that
cannabis may help to relieve negative symptoms (Dixon, Haas, Weiden, Sweeney, &
Frances, 1991; Peralta & Cuesta, 1992), it has been also reported to have a negative
impact on the symptom dimensions of the illness and its course (Linszen et al., 1994).
More specifically, the research examining substance abuse and outcome in
schizophrenia has suggested that cannabis use is associated with an increase in positive
symptoms and significantly more and earlier psychotic relapses (Linszen et al., 1994).
This association is also found to be stronger among heavy cannabis users. Those with a
history of cannabis use have significantly more rehospitalisations, poorer psychosocial
functioning, as well as increased ratings of thought disorder and hostility (Caspari,
1999). Moreover, not only is there is a significant association between increased
cannabis use and increased severity of positive symptoms, but also a more continuous
course of the illness itself (Grech et al., 2005). While substance use in schizophrenia has
been found to be associated with generally fewer negative symptoms, more social
contacts and better functioning in social-leisure areas, issues in relation to interpersonal
relationships and family dynamics are still reported to be associated with cannabis use
in schizophrenia (Salyers & Mueser, 2001). Therefore, given the high rates of
comorbidity between schizophrenia and substance abuse, as well as considering the vast
detrimental clinical effects that can be associated with the misuse of substances in
psychotic disorders, further insight into the implications of substance abuse in
schizophrenia are of particular relevance when considering the management of such
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 7
individuals. The following section provides an overview of the literature examining the
individual relationships between cannabis use and cognition, and schizophrenia and
cognition. More specifically, the relationships between cannabis use, schizophrenia and
the neurocognitive domain of executive function. The role of this neurocognitive
domain and its clinical relevance to patients with schizophrenia will then be outlined,
particularly in relation to the functional outcome of such individuals. Lastly, the
challenges associated with the assessment of executive function and the ecological
validity of traditional psychometric measures will be discussed.
The Relationship Between Cannabis Use, Schizophrenia and Cognition
In addition to the many clinical implications associated with cannabis use in
schizophrenia, cognitive functioning has also been found to be significantly affected.
Cognition involves mental processes that enable day-to-day functioning in humans
spanning over a range of personal, social and/or occupational domains (Sharma &
Antonova, 2003). Research has found deleterious cognitive impairments associated with
cannabis abuse (for review, see Pope, Gruber, & Yurgelun-Todd, 1995), as well as
substantial cognitive impairments have also been demonstrated in schizophrenia
patients without a substance abuse history (Hoff et al., 1992; Saykin, Shtasel, Gur,
Kester, Mozley, Stafiniak, & Gur, 1994). Therefore, given the high rates of cannabis use
in schizophrenia and its association with poor outcome, a better understanding of the
relationship between cannabis use, schizophrenia and cognition is warranted. In order to
be able to fully evaluate the clinical significance of the relationship between dual-
diagnosis and cognition, it is important to firstly review the individual relationships
between cannabis use, schizophrenia and cognition separately.
Cognitive impairments associated with cannabis use. Cannabis consumption
in otherwise healthy subjects has been demonstrated to be accompanied by cognitive
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 8
deficits similar to the cognitive endophenotypes found in schizophrenia patients (for
review, see Solowij & Michie, 2007). It has been found that early onset, higher dose,
higher frequency of use and duration of use are considered as risk factors when
examining the relationship between patterns of cannabis use and the extent of cognitive
impairment (Bolla, Brown, Eldreth, Tate, & Cadet, 2002; Harvey, Sellman, Porter, &
Frampton, 2007; Pope, Gruber, Hudson, Cohane, Huestis, & Yurgelun-Todd, 2003).
Evidence has also suggested that the cognitive impairments associated with cannabis
use not only persist beyond the acute intoxication state, but that worse cognitive
performance has been found to be specifically associated with cumulative years of use
(Solowij, Stephens, Roffman, Babor, Kadden, Miller et al., 2002). While there has been
an increase in the empirical evidence suggesting that the deficits demonstrated in
cannabis users have been shown to persist beyond the acute effects of intoxication, the
findings related to the recovery of such functions following cannabis use abstinence is
inconsistent (Solowij & Battisti, 2008; Solowij & Michie, 2007).
Frontal lobe function, particularly prefrontal, superiorfrontal and central areas,
have been found to be negatively affected by long-term cannabis use (Lundqvist,
Jönsson, & Warkentin, 2001). The frontal lobe of the brain is a structure thought to be
associated with attentional abilities, the facilitation of memory functions and retrieval
strategies, and the support of higher-level cognitive functions (Lezak, Howieson, &
Loring, 2004). Such higher-level cognitive abilities, otherwise known as the executive
functions, are considered to be crucial in decision-making processes where one is faced
with novel or unfamiliar situations (Crean, Crane, & Mason, 2011). Patients with
prefrontal cortex (PFC) lesions in the ventromedial sector and individuals with
substance dependence have been found to show similar behaviours where they not only
have reduced awareness of their difficulties, but also demonstrate a tendency to choose
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 9
a course of action that presents themselves with rewards of an immediate nature and
ignore the risk of future negative consequences (Bechara, 2001; Bechara, Dolan,
Denburg, Hindes, Anderson, & Nathan, 2001; DiClemente, 1993). In a study conducted
by Bechara and Martin (2004), it was found that individuals with substance dependence
who demonstrated decision-making impairments also showed severe deficits in working
memory performance. However, the pattern of results found were suggestive of deficits
in the ‘executive’ process of working memory (i.e., switching and response inhibition
components) in comparison to the memory storage aspect of the task. While it was
found that overall as a group, individuals with substance dependence showed poor
performance on measures of decision-making and working memory, not every subject
demonstrated impaired performances. The authors concluded that the different pattern
of deficits across individuals with substance dependence may be a result of such
individuals demonstrating any one, or a combination, of a number of different
mechanisms which are thought to be involved in decision-making processes and
inhibitory control supported by the PFC region. As executive dysfunction can result in
self-monitoring difficulties, reduced ability to modify or control inappropriate
behaviours, as well as impact rational decision-making processes, it is also considered
that executive dysfunction may result in the perpetuation of addictive behaviour (Bolla,
Cadet, & London, 1998). It is postulated that such decision-making deficits could be
potentially attributable to the cumulative effects of heavy substance use, but may also
result in the continuation of substance use and unsuccessful abstinence attempts (Bolla,
Eldreth, London, Kiehl, Mouratidis, Contoreggi et al., 2003; Paulus, Tapert, &
Schuckit, 2005). In addition, reduced decision-making abilities may make an individual
more susceptible to the commencement of substance use and further, becoming a
substance abuser. Therefore, it appears that executive functions may play an important
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 10
role in the engagement as well as maintenance of substance abuse; consequently, the
following discussion will specifically focus on the effects of cannabis use in relation to
this particular neurocognitive domain.
The role of executive function. Due to previous neuroanatomical research
findings suggesting associations between impairments in executive function and frontal
lobe pathology, the use of the term ‘executive function’ was traditionally equated with
the term ‘frontal lobe function’ (Benton & Hamsher, 1976; Fuster, 1989; Katz, Tadmor,
Felzen, & Hartman-Maeir, 2007; Luria, 1973; Milner, 1963; Roberts, Robbins, &
Weiskrantz, 1998; Stuss & Benson,1986). However, the use of the functional definition
of ‘executive function(s)’ over the more anatomical-based definition of ‘frontal
functions’ became increasingly more emphasised over the years. This was largely due to
the number of regions in the brain that were postulated to be implicated in such
cognitive processes which included those falling outside of the frontal lobes alone
(Baddeley, 1996). Executive functions are conceptualised as higher-order cognitive
processes which are considered pertinent to forming intentions, planning and
organisation, the translation of a plan into purposive action, and self-monitoring and
self-regulation (Lezak et al., 2004). The term ‘executive function’ is an umbrella term
that has been used to describe, ‘capacities that enable a person to engage successfully in
independent, purposive and self-serving behaviour’ (Lezak et al., 2004, p. 35).
While it is agreed that executive functions are considered to be complex, as well
as highly important to adaptive human behaviour, there are an array of definitions for
this particular neurocognitive construct (Jurado & Rosselli, 2007; Lyon & Krasnegor,
2001). Executive function is considered to be a multidimensional construct referring to
a number of ‘higher-order cognitive processes including initiation, planning, hypothesis
generation, cognitive flexibility, decision making, regulation, judgement, feedback
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 11
utilization, and self-perception that are necessary for effective and contextually
appropriate behavior’ (Spreen & Strauss, 1998, p. 171). Such functions have been
considered to be supported by the dorsolateral prefrontal cortex (DLPFC) and referred
to as the ‘cold’ components of executive functioning due to their corresponding
processes involving relatively ‘mechanistic’ or logically-driven abilities (Grafman &
Litvan, 1999). Those executive functions thought to be supported by the ventromedial
prefrontal cortex (VPFC) are considered to reflect those cognitive processes that are
more emotionally-driven. These include the experience of reward and punishment,
decision-making processes related to interpersonal and social behaviour, and the
interpretation of complex emotions, and are often referred to as ‘hot’ components of
executive functioning (Bechara, Damasio, Damasio, & Lee, 1999; Bechara, Tranel,
Damasio, & Damasio, 1996; Damasio, 1995; Grafman & Litvan, 1999).
Executive functions are also thought to act as those cognitive processes which
are considered vital for success in the ‘Instrumental Activities of Daily Living’ (IADLs)
(Chevignard, Taillefer, Picq, Poncet, Noulhiane, & Pradat-Diel, 2008; Shallice &
Burgess, 1991). Such activities may involve a range of domestic and community
activities, occupational activities, interpersonal and social behaviours, and the temporal
organisation of daily activities as a whole (Chevignard et al., 2008). Research
investigating the effects of executive function impairments have found that such deficits
may have a negative impact on an individual’s activities of everyday life such as ‘real-
world’ planning abilities (e.g., financial management), the ability to attend to
educational and work pursuits, independent living, or even the development and/or
maintenance of social relationships and understanding of social rules (Goel, Grafman,
Tajik, Gana, & Danto, 1997; Grafman, Schwab, Warden, Pridgen, Brown, & Salazar,
1996; Green, 1996; Green, Kern, Braff, & Mintz, 2000).
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 12
Executive function deficits associated with cannabis use. When considering the
more long-term effects of cannabis use in the literature, tasks that employ executive
control abilities have at least in some way been shown to be affected (Grant, Gonzalez,
Carey, Natarajan, & Wolfson, 2003; Pope, Gruber, Hudson, Huestis, & Yurgelun-Todd,
2001). The impact of cannabis use on executive functioning can be evaluated by its
acute, residual and long-term effects. However, it is acknowledged that any
neurocognitive deficits observed following a relatively minimal period of abstinence
may be associated with drug residues in the brain or even the effects of drug
withdrawal, therefore it is considered that the interpretation of such results would be
difficult. With this in mind, the residual (7 hours to 20 days after last use) and longer-
term (over 21 days after last use) effects are of greater interest and will be the focus of
the following section. In addition, studies examining adolescent subjects were not
included in the following review due to the consideration that the effects of cumulative
cannabis use/longer lifetime-use potentially not being fully achieved in such samples. In
relation to the specific effects of cannabis use on executive processes, the areas of
attentional processing, working memory, inhibition and impulsivity, verbal fluency and
decision-making will be discussed.
Attention. In relation to attentional abilities, no significant differences were
found between current heavy cannabis users (i.e., those that used at least 5000 times in
their lifetime and were daily users), individuals with a history of past heavy use (i.e.,
those that used at least 5000 times in their lifetime, but less than 12 times in the last 3
months), and control subjects with limited cannabis use experience (Pope et al., 2001,
2002). Subjects were assessed following 0, 1, 7 and 28 days of abstinence. Jager and
colleagues (2006) not only assessed the cognitive performance of moderate cannabis
users (i.e., median lifetime use of 1300 joints and median last year use of 350 joints) in
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 13
the area of attention, but also measured alterations of brain activity with functional
Magnetic Resonance Imaging (fMRI) after one week of abstinence. Findings of the
study were similar to Pope and colleagues’ results showing no significant differences in
task performance compared to controls and, in addition to this, no significant
differences in associated brain activity. Grant and colleagues (2012) examined the
cognitive performances between cannabis users (with a 3.1 ± 2.2 times per week mean
frequency of use within the past 12 months; n= 16) and controls (n=214). However, the
authors did not impose a specific abstinence period as they wished to evaluate cognition
under normal circumstances in day-to-day life. Findings from their study also indicated
that there were no significant differences between groups in performance on sustained
attention (as measured by the Rapid Visual Information Processing task from the
Cambridge Neuropsychological Test Automated Battery (CANTAB; Sahakian & Owen,
1992). Conversely, Solowij and colleagues (1991) found that long-term cannabis users
(with a mean of 11.2 years of use, mean level of use of 4.77 days a week, and a mean
consumption of 766 mg per week) demonstrated significantly poorer performance than
controls on a selective attention task as well as a reduced ability to filter out irrelevant
information, following a minimum 12-hour period of abstinence. In a later study, it was
found that heavy cannabis users (i.e., those that used more than or equal to 3 days per
week) had slower reaction times during cognitive tasks compared to not only controls,
but to light cannabis users (i.e., those that used less than or equal to 2 days per week)
following a minimum of 24 hours abstinence (Solowij, Michie, & Fox, 1995). Findings
from the study also indicated that the ability for cannabis users to filter out irrelevant
information was progressively impaired as a function of duration of use. In addition,
both long-term (with a mean of 23.9 years of regular use) and short-term cannabis (with
a mean of 10.2 years of regular use) users were found to make more errors on a speed of
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 14
comprehension task following a minimum 12-hour period of abstinence in comparison
to controls (Solowij et al., 2002). Slower processing rates were also demonstrated in
long-term cannabis users compared to short-term cannabis users (Solowij et al., 2002).
Verdejo-García et al. (2013) examined whether genetic polymorphisms moderated the
effects of cannabis use on executive function. The study included 86 cannabis users
(with daily use of cannabis of >7 joints per week and for a minimum duration of 3
years) and 58 non-drug using controls. Both groups were genotyped and matched for
genetic makeup, sex, age, education and Intelligence Quotient (IQ). The authors found
no significant differences between users and controls in sustained attention performance
(as measured by the Rapid Visual Information Processing test; CANTAB) following a
72-hour abstinence period. Further findings from the study suggested that certain
genotypes of the COMT and SLC6A4 genes moderated the impact of cannabis use on
executive functions. More specifically, results showed that cannabis users carrying the
COMT val/val genotype exhibited lower accuracy of sustained attention than non-users
of the same genotype. Research into the long-term effects of cannabis use on
attentional abilities has also produced mixed findings. It was found that heavy cannabis
users (with a mean consumption of 93.9 joints per week, a mean level of use of 7 days a
week, and a mean of 5.3 years duration of use) who were abstinent for a period of
approximately 28 days were found to demonstrate deficits in reaction time (Bolla et al.,
2002). In addition, results from Solowij’s (1995) study found significant deficits in the
ability to process irrelevant information in ex-cannabis users who were abstinent from a
minimum of 6 weeks to an average of 2 years. However, in a study examining
monozygotic twins, no impairments in attention or concentration were reported in
cannabis-using twins who were abstinent for a minimum period of 1 year, in
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 15
comparison to non-using co-twins (Lyons, Bar, Panizzon, Toomey, Eisen, Xian, &
Tsuang, 2004).
Working memory. Findings from Scholes & Martin-Iverson’s (2010) study
showed that cannabis users (n=36; with a median total duration of cannabis use of 10
years and a median of 2 usage times per day) showed no significant differences from
controls (n=35) on Letter-Number Sequencing (LNS; Wechsler Memory Scales – Third
Edition (WMS-III); Wechsler, 1997), Spatial Span (WMS-III; Wechsler, 1997), and
overall working memory (i.e., the summed score of LNS and Spatial Span). Cannabis
users were instructed to not alter their pattern of cannabis use on the day of testing and
therefore a set abstinence period was not imposed. This was considered by the authors
to reduce the likelihood of abstinence syndrome. Of the cannabis users, 21 were
daily/nearly daily users; 12 were weekly users; one was a monthly user and two used
cannabis less than monthly. Sixteen of these users reported using other drugs in the last
month. A similar pattern of results were also found regarding spatial working memory
performance (measured by the Spatial Working Memory task from the CANTAB) in
Grant and colleagues’ (2012) study. However, results also showed that cannabis users
demonstrated significant impairments on the One Touch Stockings of Cambridge Task
(from the CANTAB) relative to controls. While the test developers indicate that
working memory is assessed by this task, spatial planning abilities are also tapped into
by this measure. In addition, Gruber et al. (2012) found no significant differences
between controls and marijuana users on tasks of visuospatial construction and working
memory (measured by the Rey-Osterrieth Complex Figure test: for test description, see
Lezak et al., 2004), as well as attention and working memory (measured by Digit Span).
No significant differences in working memory (measured by Digit Span) were found
between cannabis users (categorised as heavy and light) and a control group following a
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 16
minimum period of 19 hours abstinence (Pope & Yurgelun-Todd, 1996). Additional
studies have replicated similar findings when assessing working memory with
auditory/verbal tasks including computation span, omitted numbers, and a spatial task
(e.g., a modified Sternberg item recognition task) (Fisk & Montgomery, 2008; Jager et
al., 2006; Solowij et al., 2002). In a fMRI study, it was found that cannabis users
demonstrated greater and more distributed brain activation during the performance of a
spatial working memory task (measured by a perception task and a short-delay response
task) in comparison to control subjects following 6-36 hours after last cannabis use
(Kanayama, Rogowska, Pope, Gruber, & Yurgelun-Todd, 2004). Thames and
colleagues (2014) examined neurocognitive performances across recent users (n=68;
those who reported use in the last 4 weeks), past users (n=41; those who used more than
28 days prior to testing) and non-users (n=49). Results indicated that recent users
performed more poorly than past users and non-users in attention/working memory (a
composite score of performance on the Trail Making Test – Part A, Reitan, 1958;
Stroop Test [colour and word conditions], Golden, 1978; and the Letter-Number
Sequencing test from the Wechsler Adult Intelligence Scale – Fourth Edition (WAIS-
IV); Wechsler, 2008). However, it is noted that the criteria of what constituted a user
regarding frequency, amount and duration of use was not reported by the authors.
Inhibition and impulsivity. Results from Pope and Yurgelun-Todd’s (1996)
study indicated that heavy cannabis users (with a median use of 29 days in the past 30
days) demonstrated significantly greater perseverations on a card sorting measure (i.e.,
Wisconsin Card Sorting Task (WCST); Heaton, 1981), in comparison to light users
(with a median use of 2 days in the past 30 days) following a minimum period of 19
hours of abstinence. Solowij and colleagues (2002) found that performance differences
between task trials on the Stroop test suggested that cannabis users had greater
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 17
vulnerability to task complexity and increased demands following a minimum of 12
hours abstinence relative to controls. In addition, there was also an inverse relationship
between the number of task items completed and duration of cannabis use. In the same
study, there were no significant differences between groups on WCST performance.
Gruber et al. (2012) examined the relationship between age of onset of marijuana use
and executive function performance in 34 chronic, heavy marijuana smokers (with a
mean frequency of 19.24 ±19.58 smokes per week; mean amount of 10.86 ± 14.95
grams per week; and mean duration of 7.24 ± 7.30 years) and 28 non-using controls
following a minimum 12-hour abstinence period. From the results of the study,
significantly poorer performances were found on the WCST, as well as in both the
Word Reading and Interference condition of the Stroop test. Findings also suggested
that early onset marijuana users were significantly more impaired than late onset users
on both measures. However, Battisti et al. (2010) conducted a neurophysiological
measure study using event-related potentials (ERP’s) to examine discrete cognitive
events. Event-related brain potentials to neutral, congruent, and incongruent trials were
compared between 21 cannabis users (with a mean of 16.4 years of nearly daily use) and
19 non-using controls following at least a 12 hour abstinence period. Results of the
study indicated that chronic cannabis use produced changes in accuracy and brain
activity associated with performance on the Stroop task during the unintoxicated state. It
was shown that users displayed increased errors on colour-incongruent trials (e.g.,
“RED” printed in blue ink) suggesting more susceptibility to the colour-naming
interference effect. However, no significant differences were found between users and
controls on colour-congruent (“RED” printed in red ink) or neutral trials (e.g., “*****”
printed in green ink). Further, earlier age of cannabis use onset was found to
significantly predict accuracy on incongruent trials. Results imply that users may have
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 18
difficulty suppressing more automatic responses. Attentional inhibition dysfunction
(assessed by a visuospatial negative priming task) was demonstrated in current cannabis
users in comparison to past cannabis users and controls following a minimum 48-hour
abstinence period in a study conducted by Skosnik, Spatz-Glenn, and Park (2001). The
authors proposed that as current cannabis users had significantly faster reactions times
when performing a simple detection task, the demonstrated deficit was not the result of
visual inattention, but instead reflective of increased disinhibition. Poorer performances
demonstrated by cannabis users have also been reported in other studies using the
WCST (Bolla et al., 2002; Pope, Yurgelun, & Todd, 1996; Pope et al., 2001, 2002;
2003). Conversely, findings from Lyons and colleagues’ (2004) study did not indicate
reduced performance on the WCST by users. Other studies have also reported no effects
on inhibition or impulsivity to be associated with cannabis use (Gruber & Yurgelun-
Todd, 2005; Hermann, Sartorius, Welzel, Walter, Skopp, Ende, & Mann, 2007;
Whitlow, Liguori, Brooke Livengood, Hart, Mussat-Whitlow, Lamborn et al., 2004).
Additionally, when inhibition was measured using the Stroop test, no significant
differences were reported between cannabis users and controls (Lyons et al., 2004; Pope
& Yurgelun-Todd, 1996; Pope et al., 2001, 2002, 2003). It is possible that differences in
results could possibly be due to the WCST and Stroop test measuring different
underlying executive function subdomains. While both have been used as measure of
inhibition and impulsivity, the WCST is also thought to be a measure of cognitive
flexibility and abstraction ability (Milner, 1963), whereas the Stroop test also measures
sustained attention with or without interference (MaCleod, 1991). In Verdejo-García et
al.’s (2013) study no differences were found between users and controls in
monitoring/shifting (as measured by the CANTAB Intradimensional/Extradimensional
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 19
set-shifting task). However, it was found that users carrying the COMT val allele
committed more monitoring/shifting errors than users carrying the met/met genotype.
Verbal fluency. While verbal fluency tasks do not traditionally fall into the
category of being solely a frontal lobe executive function task due to also tapping into
functions of the temporal lobe (Raskin & Rearick, 1996), verbal fluency tasks are
thought to load heavily on executive function (Rabbitt, 1997). It is also considered a
well established measure of executive functioning and has been commonly used to
assess the integrity of the pre-frontal region (Kolb & Whishaw, 1985; Parkin & Java,
1999; Warkentin & Passant, 1997). In addition, due to the tendency of verbal fluency
measures heavily relying on processes involving strategic searching, such tests are
deemed to be some of the most sensitive and best validated measures of dysfunction in
the frontal lobe (Benton, 1968; Bryan & Luszcz, 2000; Lezak, 1995; Parker &
Crawford, 1992; Stuss & Benson, 1986). Following a minimum 12-hour abstinence
period, Gruber et al. (2012) found no significant differences between marijuana users
and controls in verbal fluency performance (as measured by the Controlled Oral Word
Association Test (COWAT; Benton, Hamsher, & Sivan, 1983). In Pope and Yurgelun-
Todd’s (1996) study, no significant differences were found between light and heavy
cannabis users on verbal fluency following a minimum 19-hour abstinence period.
However, a significant interaction was found between Verbal Intelligence Quotient
(VIQ) and cannabis-use status indicating that out of those subjects with the lowest
VIQ’s, the heavy users demonstrated reduced verbal fluency in comparison to light
users. In addition, those subjects with average to high VIQ scores demonstrated no
verbal fluency deficits. Findings from Fisk and Montgomery’s (2008) study showed no
significant differences in verbal fluency (both letter and semantic) between cannabis
users and controls following a minimum of 24 hours abstinence. Both recent (i.e., had
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 20
used in the last 7 days) and abstinent cannabis users (i.e., had not used in the last 7 days)
were found to demonstrate deficits in verbal fluency (phonemic verbal fluency)
following a minimum 24-hour abstinence period compared to controls (McHale & Hunt,
2008). Pope and colleagues (2003) reported a significant difference between groups in
verbal fluency (COWAT - FAS) following a minimum of 28 days abstinence. In this
study, the cannabis users were grouped according to age of onset (early and late).
Cannabis users with early age of onset (i.e., those who commenced use before 17 years
of age) were found to demonstrate significant verbal fluency deficits compared to the
control group. However, the researchers’ earlier studies (2001, 2002) found no
significant differences on this domain. Results from the more recent study suggest that
age of onset may be an influencing factor (Pope et al., 2003).
Decision-making. In a study conducted by Whitlow and colleagues (2004) long-
term, heavy cannabis users (i.e., those that were daily users for at least 25 out of 30 days
and for at least 5 years duration) were found to demonstrate impaired performance on
the Iowa Gambling Task (IGT; Bechara, Damasio, Damasio, & Anderson, 1994), in
comparison to controls (i.e., those who had a minimal use of 1-50 lifetime uses and did
not use in the last year). Subjects in the user group were required to abstain for a
minimum of 12 hours prior to testing. Findings from the study suggest that long-term,
heavy cannabis users made decisions which led to larger immediate gains, but more
losses overall in comparison to individuals with minimum cannabis use in their history.
The authors concluded that long-term, heavy cannabis users may have a reduced ability
to effectively weigh up costs and rewards and that this deficit may contribute to the
continuation of their drug-use. However, it is also acknowledged by the researchers that
such deficits may be the consequence of cannabis use, or may alternatively reflect the
presence of pre-existing differences regarding genetics or behaviour. Additional
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 21
findings from Grant et al.’s (2012) study also indicated significant impairments in
decision-making performance (as measured by the Cambridge Gamble Task from the
CANTAB) in cannabis users. Bolla et al. (2005) examined brain activity using Positron
Emission Tomography (PET) during the administration of the IGT. Not only did the
authors find similar results where decision-making deficits were demonstrated in
cannabis users (i.e., those who currently smoked at least 4 days a week, for at least 2
years duration) following a 25-day abstinent period, but cannabis users also showed
different patterns of brain activation than non-users. More specifically, cannabis users
showed less activation in the right dorsolateral prefrontal cortex (DLPFC) and right
lateral orbitofrontal cortex (OFC), in addition to greater brain activation in the left
cerebellum. One study, examined the relationship between cocaine and cannabis users
(i.e., those that used at least 4 times a week, for at least 2 years duration) and IGT
learning (Verdejo-García, Benbrook, Funderburk, David, Cadet, & Bolla, 2007). Results
suggested that while both cocaine and cannabis users demonstrated worse performance
on the total IGT net score than controls, the pattern of learning indicated that the
cannabis-using group showed less learning than the cocaine-using group. Dose-related
measures of cannabis use (i.e., joints per week), were also found to predict IGT
performance where the heavier the use, the lower the performance demonstrated.
Contrastingly, Quednow and colleagues (2007) did not find IGT decision-making
deficits in chronic cannabis users who had been abstinent for a mean of 7 days. The
chronic cannabis users in this study (with a mean consumption of 3.89 times a week and
6.55 years duration of use) did report using other drugs of abuse in their history, but
were included in the study as long as they were deemed to not have substantially used
amphetamine derivatives like MDMA or have substantial previous use of cocaine. In
Verdejo-García et al.’s (2013) study, a significant difference was found between groups
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 22
on the IGT (block 3), which is considered to index the learning phase of the task. In
addition, users carrying the 5-HTTLPR s/s genotype had worse decision-making skills
than s/s non-users.
The abovementioned studies indicate that the cognitive impairments associated
with cannabis use are demonstrated across a range of important executive function
processes. However, differences in findings across studies could be attributed to
variability in sample characteristics, abstinence periods and differences in frequency,
severity/quantity, or duration of use/cumulative amount. In addition, results indicating
no significant differences suggest that while some of the measures mentioned above
may be sensitive enough to tap into the neurocognitive domain of interest, they may not
support the specificity of the measures to solely detect deficits in that specific domain.
In other words, such measures may not serve as specific markers for deficits in the
associated domain in question. In addition, differences in the actual tests used to assess
specific domains may have impacted the consistency of the findings. It would also not
be surprising that multiple brain regions and connections would be involved in a
neurocognitive process as complex as executive function and thus, isolating the domain
by a single psychometric task presents another concern. While the findings suggest that
some executive function impairments related to cannabis use have been found to
improve following the discontinuation of cannabis use (Pope et al., 2001, 2002, 2003),
deficits have been also shown to persist following cannabis use cessation (Bolla et al.,
2005). It is also proposed that the specific factors of age of cannabis use onset, duration
and amount of cannabis use may play a part in the manifestation of differing executive
functioning impairments (Grant et al., 2003; Pope et al., 2003; Solowij, 1995; Solowij et
al., 2002). Considering the differences in findings across studies, it is thought that the
more precise and adequate assessment of executive function abilities, as well as of such
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 23
cannabis use parameters, is crucial in order to ensure that the measures used are
sensitive enough to detect impairment if it indeed exists.
Cognitive impairments associated with schizophrenia. In 1896, the first
clinical account of schizophrenia was made by Kraepelin and was termed ‘dementia
praecox’ (Kraepelin, 1919, 1971, cited in Sharma & Antonova, 2003). Cognitive
impairment was emphasized to be a core part of the illness and that such impairments
comprised of deficits in attention, problem-solving, motivation, learning and memory
(Sharma & Antonova, 2003). In 1911, Bleuler used the term ‘schizophrenia’ and also
conceived cognitive impairment as being a fundamental aspect of the illness (Bleuler,
1911, 1950, cited in Sharma & Antonova, 2003). Through extensive research over the
years, it is now universally recognised that cognitive impairment is a central and
enduring feature of the illness and that such deficits have greater predictive ability of
functional outcome than clinical symptoms (Green, 1996). More contemporary
understandings of the illness indicate that cognitive impairments are more specifically
demonstrated in the domains of sustained attention, language skills, executive functions,
verbal learning and memory (Bilder, Lipschutz-Broch, Reiter, Geisler, Mayerhoff, &
Lieberman, 1992; Riley, McGovern, Mockler, Doku, O’Ceallaigh, Fannon, & Sharma,
2000).
The cognitive impairment associated with schizophrenia is not thought to be the
result of symptomatology, nor the effects of antipsychotic medication (Green, 2006).
Research has suggested that the cognitive impairments associated with schizophrenia
are present prior to the onset of clinical symptomatology and show relative stability
across the course of the illness (Finkelstein, Cannon, Gur, Gur, & Moberg, 1997; Gold,
2004). Cognitive dysfunction in schizophrenia has also been found to be related to, but
distinct from, negative symptoms (Harvey, Lombardi, Leibman, White, Parrella,
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 24
Powchik, & Davidson, 1996; Hughes, Kumari, Soni, Das, Binneman, Drozd, & Sharma,
2003). Impairments have also been observed to already be present in first-episode
patients with schizophrenia (Hutton, Puri, Duncan, Robbins, Barnes, & Joyce, 1998). In
addition, cognitive deficits have not only been found in patients with schizophrenia
prior to illness onset, but there is evidence of similar deficits in their first degree
relatives (Cornblatt, Obuchowski, Schnur, & O’Brien, 1998). The cognitive
impairments found in first-episode patients with schizophrenia are also considered to be
important predictors of long-term outcome (Green, Kern, & Heaton, 2004).
A wide range of cognitive impairments have been associated with the illness of
schizophrenia. While deficits in global performance have been described (Braff, Heaton,
Kuck, Cullum, Moranville, Grant, & Zisook, 1991), as well as broad deficits spanning
across a number of cognitive domains (Heinrichs & Zakzanis, 1998), other findings
have shown selective cognitive deficits which are deemed over-and-above general
cognitive impairment (Saykin et al., 1991, 1994). Meta-analytic, epidemiological and
twin-study findings suggest that the most severe impairments are consistently observed
in the domains of episodic memory and executive function (Reichenberg & Harvey,
2007). Therefore, considering the postulated relationship between executive function
and substance abuse maintenance, this neurocognitive domain in particular will once
again be focussed on in the following section.
Many pathophysiological accounts of schizophrenia suggest that the illness
results from neural network dysfunction where the PFC is considered an intrinsic part of
these networks (Reichenberg & Harvey, 2007). The most prevalent findings from brain-
imaging studies are impairments in frontal lobe functions and ‘hypofrontality’
associated with the illness of schizophrenia (Davidson & Heinrichs, 2003). It is also
found that medication variables are not significant contributors to these
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 25
pathophysiological differences (Davidson & Heinrichs, 2003). It has been reported that
the similar pattern of deficits demonstrated between individuals with schizophrenia and
frontal lesion patients not only suggest frontostriatal circuitry disturbance, but that there
are even greater impairments in executive functioning among schizophrenia patients
than those individuals with frontal lobe lesions (Pantelis et al., 1997).
It has been argued that executive function deficits are a commonality across the
cognitive deficits seen in the illness of schizophrenia (Shallice et al., 1999). The study
of executive functions in schizophrenia is of great importance considering deficits in
this domain have been associated with psychosis proneness (Franke, Maier, Hain, &
Klinger, 1992). Schizophrenia patients also demonstrate profound impairments in
executive function at the beginning of their illness and that such deficits are thought to
mainly involve planning and strategy use, but not attentional set-shifting (Hutton et al.,
1998). Such impairments are found to progress over the course of the illness. Findings
have also suggested that schizophrenia patients demonstrate a distinctive pattern of
decision-making impairment on specific cognitive tasks with which performance is also
dependent on executive function (Shurman, Horan, & Neuchterlein, 2005). Once again,
such evidence is suggestive of specific frontal lobe function abnormalities, supporting
the notion that the cognitive impairments associated with the illness are related to
frontostriatal dysfunction (Robbins, 1990).
Results from Katz et al.’s (2007) study suggested that while schizophrenia
patients demonstrated executive deficits, those patients in the more chronic stages of the
illness demonstrated worse executive function performance in comparison to those in a
more acute phase. Such deficits have been shown to be associated with treatment-
refractory symptoms, such as negative symptoms (Kerns & Berenbaum, 2002;
Voruganti, Heslegrave, & Awad, 1997), as well as with poor functional outcome
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 26
(Greenwood et al., 2005; McClure et al., 2007). Impairments in this particular cognitive
domain have also been suggested to be associated with performance in daily
independent living skills, occupational and community functioning (Green, 1996;
Sharma & Antonova, 2003).
Impaired performances have been specifically demonstrated on ‘traditional’
executive functions measures such as the Wisconsin Card Sorting Test (WCST), tests of
verbal fluency, and the Trail Making Test (TMT) (Chen, Chen, Chan, Lam, & Lieh-
Mak, 2000; Chen, Lam, Chen, Nguyen, & Chan, 1996; Franke, Maier, Hardt, Frieboes,
Lichterman, & Hain, 1993; Goldberg, Torrey, Berman, & Weinberger, 1994; Hutton et
al., 1998; Zalla, Joyce, Szöke, Schürhoff, Pillon, Komano et al., 2004). However, the
extent of impairment on such measures has been found to vary among individuals with
schizophrenia depending on illness presentation. For example, findings have suggested
that there is a poor correlation between positive symptoms and reduced performance on
executive function measures (Morris, Rushe, Woodruffe, & Murray, 1995). Some
authors report that negative symptoms have been found to be significantly correlated
with the severity of executive dysfunction (Voruganti et al., 1997). However, in a study
by Rodriguez-Jimenez and colleagues (2008), it was found that while those
schizophrenia patients who performed poorly on the WCST also exhibited more
negative symptoms, there was no association found between WCST performance and
negative symptoms in dually-diagnosed schizophrenia patients with substance use
disorders. No significant association between neuroleptic medication and executive
functioning impairment in schizophrenia has also been found (Verdoux, Magnin, &
Bourgeois, 1995). Therefore, it is possible that there may be certain subgroups among
the schizophrenia population that may comprise of individuals with differing illness
pathology and possibly different executive abilities. Due to the high prevalence of
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 27
substance use, more specifically cannabis use, in schizophrenia, investigation into the
neurocognitive abilities of such a subgroup is of particular interest.
The importance of executive function to individuals with schizophrenia. The
illness of schizophrenia has significant economic implications and it is considered as the
most costly of the psychiatric disorders (Capri, 1994; Knapp, 1997; Torrey, 2002).
Functional impairment in particular has been identified as one the primary factors
contributing to the illness’ high cost (Kenny & Meltzer, 1991). In an extensive review
of the measurement of outcome in schizophrenia, Green (1996) defined functional
outcome as ‘the result of competence in a large number of constituent social and
instrumental role tasks’ (p. 323). Research findings have shown that cognitive
impairments account for significant variance in functional status measures (Green,
1996). Both cross-sectionally (Green, 1996) and longitudinally (Green, Kern, & Heaton,
2004), the cognitive impairments found in schizophrenia have been shown to be
associated with impairments in everyday functioning. Past research has found that
cognition is a stronger predictor of functional outcome than symptomatology (Harvey,
Howanitz, Parrella, White, Davidson, Mohs, & Davis, 1998; Harvey, Napolitano, Mao,
& Gharabawi, 2003; Kurtz, Moberg, Ragland, Gur, & Gur, 2005; Perlick, Rosenheck,
Kaczynski, Bingham, & Collins, 2008; Velligan, Mahurin, Diamond, Hazleton, Eckert,
& Miller, 1997). Therefore, in order for rehabilitation efforts to be more precise and
thus ultimately have a higher success rate, identifying the specific cognitive domains
found to be particularly associated with successful independent living is crucial.
Current treatments targeting functional impairments typically involve
behavioural techniques aimed at social, occupational, self-care and independent living
skills training (Sharma & Antonova, 2003). However, despite the levels of residential
independence demonstrated in schizophrenia outpatients (i.e., those that reside in the
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 28
community), widespread functional impairments are still evidenced (Auslander,
Lindamer, Delapena, Harless, Polichar, Patterson et al., 2001). It has been postulated
that one of the main reasons for the lack of improvement in the functional outcome of
patients with schizophrenia is the lack of success in sufficiently treating those features
of schizophrenia which are considered strong predictors of functional outcome, such as
negative symptoms and cognitive impairment (Harvey, Green, Keefe, & Velligan,
2004). Therefore, there appears to possibly be a mediating factor in influencing the
effectiveness of such approaches and it is cognitive impairment that is considered to be
this link (Green, 1996; Green et al., 2000).
Specific cognitive domains have been found to be closely related to functional
outcome (Green & Neuchterlein 1999; Green et al., 2000). It has been found that the
utilisation of complex and higher-order cognitive abilities (i.e., executive functions)
which include flexibility with set-shifting, reasoning, decision-making, planning and
working memory are related to successful treatment outcomes (Gordon, Kennedy, &
McPeake, 1988; O’Leary, Donovan, Chaney, & Walker, 1979; Weinstein & Shaffer,
1993). Among the range of cognitive deficits which have been most commonly found in
individuals with schizophrenia, it is acknowledged that executive functions play a
crucial role when considering outcome (Simon et al., 2003). Through extensive reviews
in the area of cognition in schizophrenia, performance on measures assessing executive
functioning was consistently found to be associated with poor community outcomes in
particular (i.e., independent living) (Green, 1999; Green et al., 2000). In addition,
Penadés and colleagues (2003) examined the effects of cognitive rehabilitation
strategies (i.e., the cognitive differentiation and social perception cognitive modules
from the Integrated Psychological Therapy program, implemented in 24 group sessions
over a 12-week period), on cognition and functional outcome in patients with
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 29
schizophrenia. The authors found that changes in encoding and executive function were
specifically associated with changes in functional outcome.
Several research studies have suggested that tests measuring executive functions
are the best predictors of performance in activities of daily living, particularly when
considering patients with schizophrenia (Dickenson & Coursey, 2002; Ihara, Berrios, &
McKenna, 2003; Semkovska, Stip, Godbout, Pacquet, & Bedard, 2002). Results from a
study conducted by McClure and colleagues (2007) indicated that two domains of
functional capacity (i.e., social and living skills) had different neuropsychological
correlates in schizophrenia. More specifically, it was found that both working and
episodic memory, as well as verbal fluency abilities, were related to social competence.
It was also found that processing speed, episodic memory and executive function were
associated with everyday living skills. Katz et al.’s (2007) findings suggested that
performance on executive function measures were more strongly related to performance
in complex areas of daily living (which included social communication, IADLs, and
occupational skills), in comparison to performance on a basic cognitive skill measure. It
is also considered that executive function measures in general have better predictive
ability regarding lack of insight in comparison to premorbid intellectual ability, IQ,
memory, or language measures (Burgess, Alderman, Evans, Emslie, & Wilson, 1998).
It has been well established that cognitive impairment is a core feature of
schizophrenia. In addition, executive dysfunction is not only particularly implicated in
the illness of schizophrenia, but is also identified as an important predictor of functional
outcome status. This neurocognitive domain has been considered as an area of
importance especially when evaluating the independent living capacity of patients in the
community. Therefore, if this particular domain is considered as a potential target for
treatment, then the comprehensive assessment of this neurocognitive system has
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 30
significant clinical importance. Without obtaining knowledge with regard to which
specific deficits are linked to ‘real-world’ functioning, efforts towards cognitive
rehabilitation will continue to remain not only less clearly focused, but ultimately less
successful in their utility. In the current study, executive functions are targeted due to
their close connections to community functioning, as well as their association with
negative symptoms which are also linked to functional outcome. Considering the
relationship between executive function and illness presentation, as well as outcome,
this particular neurocognitive domain has specific relevance in the examination of
cognitive functioning in schizophrenia.
Cognitive impairments associated with cannabis use and schizophrenia.
Since previous research findings have suggested an association between cannabis use in
schizophrenia and worsened illness prognosis, in addition to the reported adverse effects
of cannabis use alone on cognitive function without the presence of psychotic illness, it
would be reasonable to suggest that poorer cognitive function would be expected in
schizophrenia patients with cannabis use. However, relatively few studies have
specifically examined the relationship between cannabis use and cognitive functioning
in the schizophrenia spectrum disorders, with mixed results being reported. The current
literature review will further outline these studies. Once again, only neurocognitive
effects associated with chronic or long-term use cannabis use will be focussed on in the
following review.
Some studies have reported poorer performance amongst schizophrenia patients
with cannabis use. Mata and colleagues (2008) examined the relationship between pre-
illness cannabis abuse and cognitive performance in 61 cannabis-abusing and 71 non-
abusing patients experiencing their first episode of schizophrenia-spectrum psychosis.
The patients were grouped according to their cannabis abuse prior to illness onset and
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 31
cannabis abuse status was based on the criteria of at least weekly use during the
previous year. In their study, a cognitive battery was administered which included tasks
thought to utilise the DLPFC, such as a measure of working memory (assessed by Digit
Span - backwards condition), a measure of verbal fluency (measured by phonemic
fluency task – FAS), and a set-shifting task (measured by the TMT), as well as an OFC
related task designed to measure decision-making capacity (e.g., IGT). Findings from
their research showed no significant differences between patients who had abused
cannabis before their psychosis onset and patients who had not abused cannabis before
their psychosis onset on any of the DLPFC related tasks. However, results also showed
that those patients who had abused cannabis before their psychosis onset demonstrated
poorer total performance and lower improvement in performance on the IGT. The
authors suggested a possible explanation for the findings is that decision-making
differences between cannabis abusers and non-abusers may reflect a pre-existing
condition that led abusers to make costly decisions in the gambling task.
In a recent study conducted by Sánchez-Torres and colleagues (2013), the
relationship between lifetime cannabis use (over 10 years of follow-up) and current
cognitive performance was examined in a sample of 42 patients with schizophrenia, 35
of their unaffected siblings, and 42 healthy control subjects. Drug abuse was assessed in
patients and siblings longitudinally using the Composite International Diagnostic
Interview (World Health Organisation, 1993). The controls were only assessed at
follow-up and reported current consumption. A lifetime estimate of consumption was
determined for all participants (using information from subject reports, family and
charts) and subjects were given a rating from 0 (no consumption) to 5 (every-day
consumption). In the patient group, longitudinal cannabis consumption was found to
have a negative relationship with performance in a social cognition task (assessed by the
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 32
Managing Emotions section of the Mayer-Salovey-Caruso Emotional Intelligence Test
(MSCEIT; Mayer, Salovey, Caruso, 2009). The investigators did not find a significant
relationship between cannabis use in the patient group for the three time points and the
composite scores for processing speed, attention, declarative memory, working memory
and executive function. In the unaffected sibling group, a significant negative
correlation between cannabis use at baseline 10 years prior to assessment, current use
and the declarative memory composite score (assessed by the Spanish version of the
California Verbal Learning Test, the Test de Aprendizaje Verbal Espan ˜a-Complutense
(TAVEC; Benedet & Alejandre, 1998) and the Brief Visual Memory Test-Revised
(BVMT-R; Benedict, 1997)) was found. However, while cannabis use was not found to
predict cognitive performance in the healthy control group, a negative association
between lifetime cannabis and tobacco use together and processing speed and social
cognition performance was found. Results indicated that longitudinal cannabis use was
associated with poorer social cognition in patients and that current cannabis use was
associated with poorer performance in working memory, particularly when low
premorbid IQ and earlier age of illness onset was accounted for in the explanatory
model.
Other research findings have suggested no differences in cognitive performance
between cannabis-using and non-cannabis using patients with schizophrenia. Sevy and
colleagues (2007) compared the performance of 14 schizophrenia patients with
concurrent cannabis use disorders and 13 patients without concurrent substance use
disorders on a number of measures of cognition as well as on a decision-making task
(i.e., IGT), with that of 20 healthy normal subjects. Results of the study showed that
both schizophrenia groups demonstrated more cognitive impairments and worse
performance on the IGT, compared to healthy controls. However, no significant
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 33
differences on most of the cognitive tests (including the IGT) were found between
schizophrenia patients with cannabis use disorders and schizophrenia patients without.
While it is noted that the study conducted by Sevy and colleagues (2007) included
subjects who also used substances other than cannabis, due to the inclusion of the IGT
measure in their assessment of cognition (and its particular relevance to the
neurocognitive construct of interest), the results of the study were deemed worthy of
review.
Scholes and Martin-Iverson (2010) investigated the cognitive performances of
22 cannabis-using schizophrenia patients, 49 non-using schizophrenia patients, 36
otherwise healthy cannabis-using controls and 35 non-using healthy controls.
Performances on tasks measuring attentional control (Stroop test), auditory working
memory (LNS), spatial working memory (Spatial Span) and card-sorting/set-shifting
and peserveration (WCST), were assessed. Results from the study showed that while
both schizophrenia groups demonstrated significantly poorer performance across all
tasks relative to controls, no significant differences in cognitive performance between
cannabis-abusing schizophrenia and non-using schizophrenia patients were found. In
other words, cannabis use in schizophrenia was not associated with further
neurocognitive decrements. Scholes and Martin-Iverson’s findings also suggested that
cannabis use in healthy individuals was not associated with deficits in the measures
administered that resemble those deficits associated with schizophrenia. However, one
exception to this finding was noted and this was increased perseveration demonstrated
on the WCST. From these results, the authors concluded that cannabis use has subtle
effects on the cognitive performance of both healthy individuals and patients with
schizophrenia across a range of tasks.
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 34
Surprisingly and contrary to expectations, there has also been a number of
studies showing superior cognitive performance amongst schizophrenia patients with
cannabis use. Stirling and colleagues (2003, 2005) administered a neuropsychological
battery to 69 patients at a 10-12 year follow-up. The patients in this study were recruited
out of 112 individuals in 1987 who had experienced their first episode of psychosis
within 2 years prior to this time. Those who had a history of cannabis use (categorised
as ‘no use’ and ‘some use’) at the time of the onset of their illness were noted. Thirty-
seven patients were assessed with a neurocognitive battery at onset, 24 were re-assessed
at follow-up and an additional 25 from the parent sample were also assessed at follow-
up. Findings from the study showed that patients with cannabis use demonstrated better
performance on a verbal fluency task (COWAT – FAS), as well as on other
neurocognitive measures such as design memory (Memory for Design; Graham &
Kendall, 1960), visuospatial organisation (object assembly, block design, picture
completion, picture arrangement), and recognition memory (face recognition) at follow-
up. More specifically, after controlling for the effects of age of cannabis use onset
significantly better performance on measures of memory, verbal fluency, sequencing,
visuospatial organisation and recognition memory was found to be associated with a
history of cannabis use. In addition to these findings, the 26 patients who reported a
level of continued cannabis use at follow-up demonstrated significantly better
performance in several of these domains in comparison to those who had not sustained
their cannabis use at this time. It was suggested by the authors that while impairment in
executive function may be present in first episode patients, they do not appear to
deteriorate over 10-12 years. As both the patients with cannabis use and non-using
patients were not able to be distinguished based on their performance on any of the
premorbid adjustment measures or in relation to social/behavioural functions (as
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 35
assessed by the Premorbid Adjustment Scale (PAS); Cannon-Spoor, Potkin, & Wyatt,
1982), the authors concluded that their findings were not indicative of the presence of
differences between groups related to premorbid social functioning. Results suggested
that the cannabis-using group may represent a subgroup with a subtly different profile,
characterised by relatively preserved neurocognition and fewer negative symptoms.
While better performances demonstrated by schizophrenia patients with
cannabis use could be considered counterintuitive, other studies have also reported a
similar pattern of results. Jockers-Scherübl and colleagues (2007) compared the
cognitive functioning of 39 schizophrenia patients (19 cannabis abusers and 20 non-
abusers) with 39 healthy control subjects (18 cannabis abusers and 21 non-abusers)
following a minimum abstinence period of 28 days. The researchers excluded those
patients with any drug abuse or dependence history other than that involving cannabis.
The cannabis-abusing group in the study included only those individuals whom had
consumed an average of at least 0.5g of cannabis per day for a minimum of 2 years prior
to illness onset. Main effects of diagnostic group (i.e., schizophrenia vs. controls)
indicated cannabis-using patients with schizophrenia demonstrated worse performance
in most neurocognitive tests in comparison to cannabis-using controls. However, when
only the main effect of age of cannabis use onset was considered, irrespective of
diagnostic group, there was no significant difference found. A significant interaction
between diagnostic group and age of onset was found for performance on a processing
speed and working memory task (measured by the Digit Symbol Substitution Test
(DSST): Wechsler, 1981) and performance on another measure of working memory
(assessed by ‘other errors’ on the WCST). More specifically, control subjects with
cannabis abuse who had commenced regular use at 16 years or younger demonstrated
worse cognitive performance than those individuals whose age of cannabis use onset
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 36
was 17 years or older. Contrastingly, schizophrenia patients with cannabis abuse who
commenced their abuse at the age of 16 years or younger performed better on the DSST,
but demonstrated worse performance relative to controls when their cannabis abuse had
started at 17 years or older. No significant difference in performance between control
subjects and schizophrenia patients with early onset of regular cannabis abuse were also
observed on this test. A similar pattern of results was observed on ‘other errors’ of the
WCST. These results suggested that regular cannabis abuse commencing before the age
of 17 (and prior to illness onset) was associated with better neurocognitive performance
in patients with schizophrenia. However in the control group, early age of onset of
regular cannabis abuse was found to be associated with worse performance. It was noted
that once the authors made corrections to the alpha level in order to account for multiple
statistical tests, the difference in performance remained significant for ‘other errors’ on
the WCST (when age of onset was applied), but only approached significance for
performance on the DSST. The authors considered that a possible explanation for the
findings showing better performance by patients with cannabis abuse on such measures
who commenced their cannabis use prior to the age of 17 (and also illness onset) may be
potentially due to the latter measure not only assessing processing speed abilities. It is
thought that the DSST in particular may also tap into a number of neurocognitive
abilities including attention, sustained attention, psychomotor speed, working memory
and other executive functions (Lezak et al., 2004). Therefore, it was considered by the
authors that such a measure may be possibly more sensitive in detecting subtle
differences that exist between groups. It is postulated that when there are slight deficits
on a collection of neurocognitive abilities, the need to simultaneously integrate these
abilities may consequently lead to overall impairments in performance becoming more
evident. Therefore, the authors proposed that a possible explanation for the findings of
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 37
better performance on this test may suggest that those patients with cannabis abuse with
an age of cannabis abuse onset prior to illness onset could possibly possess a better
capacity to integrate several cognitive functions despite demonstrating poor
performance in individual tests.
Coulston, Perdices and Tennant (2007) examined the relationship between
neuropsychological performance and three different indices of cannabis use in 60 males
with schizophrenia/schizoaffective disorder and 17 male healthy control subjects. Forty-
four schizophrenia patients met the Diagnostic and Statistical Manual of Mental
Disorders - Fourth Edition (DSM-IV; American Psychiatric Association, 2004) criteria
for lifetime cannabis abuse/dependence. The indices used by the researchers included
lifetime abuse/dependence (assessed according to the DSM-IV criteria of substance
abuse/dependence), frequency of use, and recency of use. In line with the pattern of
results of previous studies, findings indicated that cannabis use was associated with
enhanced neurocognitive functioning in schizophrenia. More specifically, results
showed that a larger proportion of those schizophrenia patients with lifetime cannabis
abuse/dependence demonstrated performance classified in the normal range on one of
the executive function measures administered (i.e., the TMT), in comparison to those
without lifetime abuse/dependence. However, the actual component of the measure
which was found to be significantly associated with better performance was that aspect
of the test which assessed psychomotor speed. In this study, cognitive performance was
classified as ‘impaired’, ‘normal’, and ‘above average’ based on scores derived from the
control group. Subjects were deemed to demonstrate cognitive ‘impairment’ if
performance fell 1.5 – 2.0 standard deviations or more below the mean of the control
group (Spreen & Strauss, 1998; Ruff & Allen, 1996, both cited in Coulston et al., 2007).
In addition, above average performance was determined if scores were 0.5 standard
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 38
deviations or more above the mean of the control group. Intriguingly, high frequency
cannabis use was found to be associated with better performance on Principal
Component Analysis derived cognitive components of speed of information processing,
divided attention, visual conceptual switching, and planning efficiency. However in
contrast, a larger proportion of subjects categorised as medium frequency cannabis users
fell in the ‘impaired’ range on the cognitive components of planning efficiency and
complex information processing. It was noted that while premorbid IQ was not found to
be significantly different between frequency groups, the medium frequency group had a
mean premorbid IQ at almost half a standard deviation lower than the high and low
frequency cannabis user groups. The cannabis use indices measuring recency (i.e.,
cannabis abuse/dependence in the past week; cannabis use at a non-dependent level in
the past month, but prior to the past week; cannabis use at a non-dependent level prior
to the past month), were also found to be associated with better performance on the
cognitive components reflective of speed of information processing, complex
information processing, psychomotor speed, inhibition accuracy, planning efficiency,
planning and organisation, and complex perceptual organisation. Worse performance on
a non-executive cognitive component of immediate memory was however, associated
with the recency category of non-dependent cannabis use in the past week. In addition,
cannabis use at a non-dependent level in the past week was associated with worse
performance on the cognitive components of planning and organisation and complex
information processing. As the cognitive components which were found to be associated
with better performance, as well as with the cannabis use parameters of frequency and
recency of use, were attention/processing speed and executive function domains, the
authors postulated that cannabis use may possibly stimulate prefrontal
neurotransmission.
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 39
Further evidencing the surprising observation of better neurocognitive
performance, Schnell and colleagues (2009) investigated the residual impact of cannabis
use and specific parameters of cannabis consumption on cognition in a sample of
schizophrenia patients with a lifetime diagnosis of comorbid cannabis use disorder. The
indices of cannabis use assessed included age of onset of use, the duration of regular
use, time since last dose, average frequency of use (joints per month), and maximum
frequency of use (joints per month). A cognitive test battery was administered to 34
schizophrenia patients and 35 currently abstinent schizophrenia patients with cannabis
use disorder. The minimum period of abstinence from cannabis was 3 weeks. The
schizophrenia patients with cannabis use disorder had poorer academic achievement and
lower vocabulary scores. However, after potential confounds were accounted for
schizophrenia patients with cannabis use disorder were found to demonstrate better
performance in tests of verbal memory (measured by the Auditory Verbal Learning and
Memory Test; Heubrock, 1992), working memory (LNS; Gold, Carpenter, Randolph,
Goldberg, & Weinberger, 1997), and visuomotor speed and executive function (DSST;
Tewes, 1991; TMT, Part B; Reitan 1958; Reitan & Wolfson, 1993), in comparison to
schizophrenia patients without comorbid cannabis use disorder. Results of the study
were in support of previous findings (Coulston et al., 2007; Jockers-Scherübl et al.,
2007; Stirling et al., 2005). More frequent cannabis use was also associated with better
performance in tasks of attention (Continuous Performance Test (CPT); Cornblatt,
Risch, Faris, Friedman, & Erlenmeyer-Kimling, 1988) and working memory (LNS). In
addition, DeRosse and colleagues (2010) compared lifetime measures of psychotic
symptoms, as well as cognitive performance in 280 schizophrenia patients without a
history of cannabis use disorder and 175 schizophrenia patients with a history of
cannabis use (51 patients with comorbid cannabis abuse; 124 patients with cannabis
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 40
dependence). Similar results were found where schizophrenia patients with a history of
cannabis use disorder demonstrated significantly better performances on measures of
processing speed (TMT, Part A & B), verbal fluency (animal naming) and verbal
learning and memory (California Verbal Learning Test). This group also had better
Global Assessment of Functioning (GAF) scores than schizophrenia patients without a
history of cannabis use disorder.
Rodríguez-Sánchez et al (2010) conducted a study amongst 107 first-episode
patients with schizophrenia (47 cannabis users and 57 non-users) and 37 healthy
controls. Patients were considered to be cannabis users if there had been at least weekly
use during the year prior to program entry. The aim was to compare subjects on clinical
features, cognitive performance and premorbid adjustment, both cross-sectionally and
longitudinally. Results showed that cannabis users demonstrated better performance in
attention (as measured by the CPT) and executive function (as measured by the TMT-B)
at baseline and following 1 year of treatment. In addition, cannabis users were found to
have better social premorbid adjustment, particularly during childhood and adolescence.
They were also found to have earlier age of illness onset. Findings also indicated that
the amount of cannabis consumed and duration of use was not associated with cognitive
performance. While it was noted by the investigators that some patients regularly used
other drugs of abuse in addition to their cannabis use, it was found that results relating
to executive function performance remained significant after the omission of these
subjects from analyses. The authors argued that the findings of the higher presence of
positive symptoms at baseline, better premorbid adjustment and higher performance on
frontal related cognitive functions suggest that cannabis users may represent a specific
subgroup of patients with better overall premorbid abilities.
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 41
Leeson and colleagues (2011) aimed to explore the role of age of onset and
cognition on outcomes in cannabis users with first-episode schizophrenia. In addition,
the investigators aimed to evaluate the effect of cannabis use parameters such as dose
and cessation. Ninety-nine patients were divided into 65 lifetime cannabis users (as
defined by those who reported having used the drug at all during their lifetime) and 34
never-users. The cannabis-using group were further parsed into a high frequency group
(i.e., those who reported either daily or almost daily use; n = 30) and a low frequency
group (i.e., those who reported use of either twice a week or less; n = 35). Results of the
study showed that cannabis users had better premorbid IQ and social function at
psychosis onset. Cannabis users also demonstrated better performance in verbal learning
(assessed by the Rey Auditory Verbal Learning Task), working memory span (Spatial
Span) and planning (assessed by a task analogous to the Tower of London), but not on a
working memory manipulation task (self-ordered search task from the CANTAB), in
comparison to non-users. However, when premorbid IQ was entered into analyses as a
covariate, these between group differences were no longer significant except for
planning. It was found that premorbid IQ did not explain the between group difference
in social functioning. Low frequency users were found to have significantly higher
current IQ than high frequency users and cannabis users had an earlier age of illness
onset than non-users. There was also a strong relationship found between age at first
cannabis use and age of onset of both prodromal and psychotic symptoms. The authors
concluded from their overall findings that cannabis use may trigger the onset of
psychosis in individuals who otherwise have good prognostic features (i.e., higher
intellectual and social functioning prior to psychosis onset). It was also considered that
the cannabis-using patients in the study had higher ‘cognitive reserve’ not only as
evidence by better premorbid abilities, but by also better social function demonstrated
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 42
over the first 15 months of illness. It was considered that the earlier age of illness onset
in cannabis users may therefore be due to the toxic action of cannabis instead of it
reflecting of a form of illness that is more severe in nature.
Yücel and colleagues (2010) conducted a meta-analysis of the research in the
area examining the cognitive functioning of patients with established schizophrenia with
and without comorbid cannabis use. Findings suggested that patients with history of
cannabis use were found to have superior cognitive functioning. These particular results
were predominantly influenced by those studies which included schizophrenia patients
who had a lifetime history of cannabis use (defined elsewhere e.g., Schnell et al., 2009
as those that were assessed to have a lifetime diagnosis of comorbid CUD according to
DSM-IV-TR), rather than current or recent use factors. In a second study conducted by
the authors, the cognitive performance of 59 first-episode patients with psychosis with a
cannabis use history was examined in comparison to 26 patients with first-episode
psychosis patients without a history of cannabis use and 43 healthy non-using control
patients. Findings showed that first-episode patients with a cannabis use history
demonstrated better performance in the neurocognitive domains of visual memory (as
measured by Visual Reproduction (Part I) from the Wechsler Memory Scale - Revised
(WMS-R); Wechsler, 1987), working memory (as measured by ‘SWM-errors’ on
Spatial Working Memory from the CANTAB), and planning and reasoning (as
measured by the Tower of London; Shallice, 1982). In addition to these results, it was
also found that earlier onset of cannabis use was associated with less cognitive
impairment in comparison to a later age of cannabis use onset which is in support of
previous findings (Jockers-Scherübl et al., 2007).
Rabin and colleagues (2011) also conducted their own meta-analysis, but instead
only included studies that examined the direct effects of cannabis on cognition in
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 43
schizophrenia, with no other substance use included. Similar findings were reported,
denoting overall superior cognitive performance in schizophrenia patients associated
with lifetime cannabis use in comparison to non-using patients. However, the authors
noted that the magnitude of the effect sizes found were considered to be in the small to
medium range (mean d = .06 - .48) which were also similar to the effect sizes reported
in Yücel and colleagues’ (2010) meta-analysis. As it is considered that effect sizes of
0.50 or greater are deemed to be clinically significant (Lezak et al., 2004), it is possible
that results of the meta-analysis may be reflective of variability in methodological
design instead of between-group differences. Therefore, effect sizes of these magnitudes
may serve to question the clinical significance of the found relationship between
cannabis use and neurocognition in schizophrenia amongst these studies. In a recent
study by the same authors (2013), the cognitive performance in schizophrenia as a
function of cannabis use indices was examined. The study included 18 schizophrenia
outpatients with current cannabis dependence and 29 patients with no current cannabis
dependence. The latter group was then parsed into subgroups which included 21
patients with lifetime cannabis dependence and 8 patients with no lifetime dependence.
Results indicated no significant differences in cognitive performance between patients
with current cannabis dependence and no current cannabis dependence. However,
never-dependent patients were found to demonstrate poorer reaction time on a measure
of sustained attention, concentration, psychomotor speed, response inhibition and
impulsivity (Continuous Performance Test II (CPT-II); Conners, 2000), than current and
former dependent patients. These results remained significant after controlling for age.
In current dependent patients, negative correlations were found between years of
cannabis exposure and performance on the CPT-II, a measure used to assess working
memory (Digit Span; Wechsler, 1997), another measure of executive function (WCST),
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 44
a measure of verbal memory (California Verbal Learning Test—Second Edition; Delis,
Kramer, Kaplan, & Ober, 2000), and long delayed cued recall. It is important to note
that the reported associations with years of cannabis use exposure were found on the
Digit Span forwards condition only. There was no association between performance on
tests of impulsivity and risky decision making (assessed by the Kirby Delayed
Discounting Task; Kirby, Petry, & Bickel, 1999 and the IGT - Computerized version;
Bechara et al., 1994) and cumulative cannabis use. Results of the study suggest that the
cessation of cannabis use was not associated with worse or better performance. Also,
while poorer reaction times were demonstrated by never-dependent patients, reduced
performance on a range of cognitive tasks was found to be associated with cumulative
use in current dependent patients suggesting that years of cannabis use exposure may be
a more important contributing factor than cannabis use status.
The increasing picture of better cognitive performance associated with cannabis
use in schizophrenia patients is not only counterintuitive, but rather unexpected
considering the impact of cannabis use on cognition in otherwise healthy users. One
possible explanation for these findings is that better cognitive functioning may serve to
be a risk factor in patients with schizophrenia for the development of cannabis use
disorders. In other words, this cohort of individuals may have higher social competence
(i.e., as demonstrated by the ability to seek, obtain and maintain drug use) and therefore
be reflective of more intact cognitive functions (DeRosse et al., 2010; Jockers-Scherübl
et al. 2007; Leeson et al., 2011; Rodríguez-Sánchez et al., 2010). Arndt and colleagues
(1992) found better premorbid adjustment in those individuals with schizophrenia who
abused alcohol or cannabis. A two-stage model was proposed by the authors to attempt
to explain substance abuse in these individuals. The authors postulated that those with
better premorbid adjustment were more sociable in nature and thus would possibly be
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 45
exposed to more opportunities for substance use experimentation. Other authors have
also suggested better premorbid abilities in substance-using schizophrenia patients,
where such cohorts have been considered to reflect a relatively distinct group who differ
from non-using patients in terms of premorbid social adjustment which is thought to be
required in order to initiate and maintain drug-seeking behaviour (Wobrock, Sittinger,
Behrendt, D’Amelio, Falkai, & Caspari, 2007). Similarly, in a study conducted by
Kumra and colleagues (2005) which examined performances on measures of intellectual
functioning in adolescent inpatients with schizophrenia and schizoaffective disorder,
findings suggested that a better Full Scale Intelligence Quotient (FSIQ) and Verbal
Intelligence Quotient (VIQ) was associated with a history of cannabis
abuse/dependence. However, this perspective is contrary to previous findings which
failed to identify significant differences in premorbid adjustment and social/behavioural
functioning between groups (Coulston et al., 2007; Stirling et al., 2005). It is also noted
that the sample in Kumra et al.’s (2005) study were adolescents.
Limitations to Previous Research
The results of the abovementioned sudies cannot be adequately evaluated
without reviewing the limitations in methodology across each study. In some studies, it
was more difficult to establish the direct impact of cannabis on cognition because
additional substances had also been used by patients and these had not been controlled
for in analyses (Sevy et al., 2007; Yücel et al., 2010). Yücel and colleagues (2010)
included studies in their meta-analysis as long as cannabis was the most preferred
substance of the sample. Therefore, some cannabis-using groups also included patients
who did not only use cannabis. However, it is noted that in studies which included other
substance use one would assume that this would only serve to increase the likelihood of
detecting poorer performance in the drug-using patient group, but surprisingly most
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 46
found better performances. It is considered that the inclusion of other substances may
potentially make it more difficult to establish the precise contributing factors to this
level of performance (i.e., whether superior performances are associated with cannabis
use related brain functioning per se, or are just simply associated with the
neurocorrelates of substance use in schizophrenia in general). It is acknowledged that
this issue is considered to be practically unavoidable, or at least in part a contributing
factor, making the recruitment of larger, more specific patient sample groups more
difficult to achieve. Some studies also only examined the effects of cannabis use prior to
the first psychotic episode and failed to investigate the effects of continuing cannabis
use after illness onset (Jockers-Scherübl et al., 2007; Mata et al., 2008).
Such inconsistent findings in the existing literature may also be explained by the
level of variability in population characteristics between the studies. One source of this
variability in sampling includes the types of patients used. Reported better performances
amongst cannabis-using patients have involved a relatively young sample (Coulston et
al., 2007). It was considered that the effects of cumulative substance use may not be
fully developed in the tested sample and therefore, the effects associated with longer
lifetime cannabis use could not be established. Another possibility is that the deficits
found may be a reflection of age-related cognitive decline, as there is increasing
evidence in the literature suggesting age-related deficits in executive functioning (Fisk
& Sharp, 2004; Fisk & Warr, 1996; Fristoe, Salthouse, & Woodard, 1997; Salthouse &
Babcock, 1991; Van der Linden, Beerten, & Pesenti, 1998). In other words, the
neurocognitive performances observed in these previous studies may not be directly
related to the effects of cannabis use, but instead a function of age differences. In Rabin
and colleagues’ (2013) study, never-dependent patients were found to be significantly
older than current-dependent and former-dependent patients. In addition, never-
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 47
dependent patients were found to have a significantly older age of diagnosis than
former-dependent patients. Similar differences in age of diagnosis between never-
dependent and current-dependent patients approached significance.
Further, the way in which cannabis users were classified or identified also varied
between studies. For example, some studies have utilised a binary system grouping
participants as being either users or non-users (Mata et al., 2008; Scholes & Martin-
Iverson, 2010; Sevy et al., 2007; Stirling et al., 2005) and cannabis use parameters such
as severity/quantity, frequency, and duration were not accounted for in the researchers’
classification of what characterised a user. In most of these studies, either no significant
differences were reported or worse cognitive performance was associated with cannabis.
However, Stirling and colleagues (2005) reported better performance. Contrastingly,
those studies which classified participants as having a cannabis use disorder diagnosis
or being defined by a minimum amount or duration of use showed only better cognitive
performance associated with cannabis (Coulston et al., 2007; DeRosse et al., 2010;
Jockers-Scherübl et al., 2007; Leeson et al., 2011; Rabin et al., 2013; Rodríguez-
Sánchez et al., 2010; Schnell et al., 2009). In addition, two studies reported a
relationship between higher frequency of cannabis use and superior cognitive
functioning (Coulston et al., 2007; Schnell et al., 2009). Therefore, considering the
differences in findings it seems that assessing the different parameters of cannabis use is
of particular importance when examining the effect of cannabis on cognition in
schizophrenia.
Studies also differed in the range of tests used to assess cognitive functioning or
the types of measures used to tap into specific cognitive functions. Further, it is
acknowledged that the selection of measures used in previous studies appears to be
relatively atheoretically driven and instead based upon face validity (i.e., they appear to
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 48
assess these functions) in measuring their proposed cognitive constructs. In addition, the
cognitive assessment conducted in some studies were restricted to only a few cognitive
domains (DeRosse et al., 2010; Schnell et al., 2009; Scholes & Martin-Iverson, 2009).
The means in which these domains were assessed were also limited by the diminutive
number of measures used to do so. Performance on the measures utilised in previous
research may not precisely enough reflect the range of deficits implicated in
schizophrenia, as well as in cannabis users, particularly when considering the multi-
faceted neurocognitive processes involved in the executive function system. Therefore,
it is possible that these studies could fall short of being able to adequately assess the full
range of abilities or components that are theoretically conceptualised to be involved in
the executive functioning system. Considering the relationship between executive
function and functional outcome in schizophrenia, consequently it may be more difficult
to fully evaluate the clinical relevance of previous findings regarding overall cognitive
performances. A methodological problem commonly involved in neuropsychological
research, involves the issue of how the multi-faceted and complex construct that is
termed ‘executive function’ has been mainly assessed by relatively few psychometric
measures (e.g., the WCST, the Stroop test, verbal fluency or TMT). This potential
inadequacy in assessment approach has been considered an important issue (Green,
1996; Green et al., 2000; Thoma, Wiebel, & Daum, 2007). Therefore, previous
neuropsychological evidence may not be reflective of the true nature of, or sufficiently
narrow down, the various subcomponents which could be specific to the executive
deficits deemed to be crucial to the illness of schizophrenia. It is possible that the
inconsistencies in previous findings may be related to the methodological difficulties
that are involved in precisely defining and measuring the construct of executive
function, as well as assessing the range of sub-processes which fall under the executive
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 49
function domain. Neurocognitive test batteries should aim to target and delineate the
construct(s) of interest (e.g., executive function) more precisely. In addition, a lack of
examination of whether performance on such measures is related to day-to-day
functioning abilities (i.e., functional outcome) questions the ecological validity of the
employed measures used in previous studies (i.e., are they are adequately reflective, and
predictive, of the abilities required to function in the ‘real-world’?). It is possible that
outside of the testing context, there are more complex demands and distractions that an
individual may encounter in more naturalistic settings. Therefore, any absence of
deficits demonstrated in a psychometric assessment situation may not necessarily reflect
intact neurocognitive function. It is possible that utilising more ecologically valid
measures could potentially allow for the detection of more subtle deficits by creating
conditions that could serve to place greater ‘real-world’ demands on an individual.
The inconsistency in past findings suggests that the evaluation of neurocognitive
function in schizophrenia patients with cannabis use requires further examination.
Considering the results in previous studies indicating superior cognitive performances in
cannabis-using schizophrenia patients, in comparison to patients without a cannabis-use
history, it is possible that the use of a matched-pairs control group with cannabis use
could potentially help to further tease apart or further explain these observations. An
otherwise healthy control group with a similar cannabis use history could possibly allow
for more information in addition to the previous findings, by assisting to determine
whether cannabis acts correspondingly in otherwise healthy users in comparison to
patient users, or whether there is something truly unique to the schizophrenia patient
with cannabis use population. Considering the prevalence rates of cannabis use in this
particular clinical population, cannabis use in schizophrenia is an often unavoidable
confounding variable that needs to be carefully accounted for. Indices of cannabis use
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 50
are also often highly variable across individuals and consequently cannabis use per se
can be difficult to define. Therefore, in order to address this issue it is imperative that a
detailed assessment of the different parameters of cannabis use is conducted (e.g.,
severity/quantity, frequency, duration, time since last use). The use of a control group
with a similar cannabis use history would also help to control for any confounding
influence contributed to by such variations in crucial cannabis-use indices which have
been considered to have an impact on cognitive performance. In addition, as executive
impairments have been found to be associated with age-related cognitive decline, age is
also deemed an important variable to control for.
The previously mentioned methodological issues also provide scope for the
further development of neuropsychological battery measures in not only being able to
more effectively examine complex cognitive constructs such as executive function, but
in addition sufficiently validate individual neurocognitive sub-components against
various aspects of functional outcome. In order to critically examine the validity of
various tests of executive function, further exploration into the concept of executive
function and its specific components, will be discussed in the following section.
Part B
The Assessment of Executive Function
Over the years, establishing a precise definition, as well as developing the
effective measurement, of executive function has had its challenges. This can be
explained in part by there being much debate about the validity of specific measures in
being able to capture the full range of abilities which fall under this domain (i.e., being
able to successfully ‘fractionate’ the executive system). While various studies have
aimed to assess executive functioning in patients with schizophrenia, many of these
studies have appeared to examine only a unitary concept of the construct using a limited
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 51
number of assessment measures (for review, see Green, 1996; Green et al., 2000).
Therefore, many studies have failed to directly address the fractionation of the executive
function system in schizophrenia and hence, it is possible that performance on selected
measures may not adequately reflect the range of processes that fall within the construct
that is termed ‘executive function’. It is thought that the more successful fractionation of
the executive functions is largely dependent on the ability to develop specific theoretical
models of this neurocognitive system (Baddeley, 1998; Burgess et al., 1998; Shallice &
Burgess, 1991). While there is a long history of various models conceptualising the
construct of executive function in the neurocognitive literature, for the purposes of this
study one specific model (chosen on the basis of its comprehensive features), will be
discussed in detail.
Models of executive function. Developing a good understanding of the specific
mechanisms and components of executive functioning has unfolded over various
approaches. The conceptualisations, definitions and models of executive functioning
subscribe to various research findings arising from neuroanatomical, neurophysiological
and neural circuitry evidence from animal brain-behavioural relationship observations
(for overview, see Goldman-Rakic, 1987), the examination of the relationship between
cognitive and behavioural deficits and frontal lobe damage (e.g., Fuster, 1989;
Goldman-Rakic, 1996; Luria, 1973; Royall, Lauterbach, Cummings, Reeve, Rummans,
Kaufer et al., 2002), as well as theoretical model fit from statistical approaches such as
confirmatory factor analysis methods (e.g., Daigneault, Braun, & Whitaker, 1992).
More recently, there has been various theoretical and psychometric approaches
that have been developed in an attempt to determine whether executive functioning
should be conceptualised as a unitary system or whether is it is more adequately
described by a model involving multi-processes (Baddeley, 1998; Miyeake, Friedman,
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 52
Emerson, Witzky, & Howerter, 2000; Parkin, 1998; Stuss & Alexander, 2000).
Increasingly it has become more apparent that executive functioning cannot be
considered as a unitary construct as various executive function components have been
able to be individualised and while found to be related, are shown to be independent to
each other (Chevignard et al., 2008; Duncan, Johnson, Swales, & Freer, 1997; Miyake
et al., 2000; Robbins, James, Owen, Sahakian, Lawrence, McInnes, & Rabbit, 1998).
Models of executive function which have emerged over the years view the overall
system as comprising of individual and distinct subcomponents (Andres, 2003;
Baddeley, Della Sala, Robbins, & Baddeley, 1996; Goethals, Audenaert, Van de Wiele,
& Dierckx, 2004) which are then conceptualised as being structured either in a
hierarchical manner (e.g., Fuster, 1989; West, 1996) or as a distinction between
'hot/cold’ processes (e.g., Damasio, 1995).
West’s model of executive function. West’s (1996) model of executive function
builds upon a previous hierarchical model originally proposed by Fuster (1989), which
incorporates the conceptualisation of superordinate and subordinate functions.
According to Fuster’s model, it is postulated that executive functioning involves an
overarching or ‘superordinate’ function called the ‘temporal organisation of behaviour’
which is considered to be responsible for the initiation and maintenance of behaviours
that have a common goal. This function is conceptualised to be subserved by three
subordinate functions. These include provisional memory which involves the
recollection of previous experience; prospective memory which is thought to be
responsible for the storage and cueing of information that is required to perform a future
goal; and interference control which serves to suppress interferance or conflicts
irrelevant to the task at hand. This model emphasizes the concept of ‘temporal gestalt’,
meaning that such functions support the organisation of behaviour over time in
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 53
sequence, connecting past actions and future events or goals in conjunction with the
integration of information that is task-relevant and the disregard of task-irrelevant
information (Fuster, 1989). It is acknowledged that Fuster’s model affords, at least in
part, a way of theoretically explaining the executive function system as a hierarchy of
cognitive processes. However, considering that many previous studies investigating
executive function have also examined the cognitive process of inhibition, Fuster’s
model appears to be somewhat lacking. Therefore, additional models of executive
function will also be discussed in order to incorporate other executive function domains
which appear to not be accounted for in this model alone.
In addition to Fuster’s (1989) conceptualised executive processes, West included
a fourth subordinate function to his own model which is termed the inhibition of
prepotent responses. West also used the term ‘retrospective memory’ instead of
provisional memory. According to West, retrospective memory involves the retrieval
and maintenance of information that is task-relevant from its storage in memory. This
function has also been conceptualised to be similar to the construct of working memory
(Baddeley, Della Sala, Papagno, & Spinnler, 1997). In West’s model, prospective
memory is thought to aid in the preparation of an individual for an upcoming action or
behavioural response. Interference control serves to manage the intrusion of task-
irrelevant information incoming from external sources. Such task-irrelevant information
can be incorporated into retrospective memory and, therefore, subsequently interfere
with correct task execution if it is not adequately controlled. Finally, the inhibition of
prepotent responses is thought to inhibit more dominant, habitual behavioural responses
particularly when such responses are not considered to be optimal or appropriate for the
present task. It is acknowledged that West’s (1996) model encompasses a much broader
range of cognitive functions than other models of ‘executive functions’. However, as
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 54
previously mentioned Lezak et al. (2004) describes the structure of the frontal lobe as
being associated with not only higher-order cognitive functions, but also the facilitation
of memory functions and retrieval strategies. In addition, Fuster (2000) considers that it
would be erroneous to localise various executive functions to the prefrontal cortex alone
and that only part of the networks involved in memory abilities can be localised to this
region. Both retrospective memory and prospective memory abilities are described to
complement each other at the service of frontal executive function playing the critical
role in the mediation of contingencies of action across time.
There have been a number of different measures designed and utilized to capture
the different components highlighted in this model. The following section discusses an
approach in the measurement of each of these domains guided by West’s (1996)
theoretical model of executive function, as well as a comprehensive investigation in
evaluating the validity of the theoretical constructs involved.
Num (2006) evaluated the construct validity of a hierarchical model of the
executive functions proposed by West (1996). In the study, participants were
administered a battery of neurocognitive tests individually selected based on the
theoretical concepts of the individual processes outlined in West’s model. The study
utilised the Zoo Map task from the Behavioural Assessment of the Dysexecutive
Syndrome test battery (BADS; Wilson, Alderman, Burgess, Emslie, & Evans, 1996), a
modified version of the Self-Ordered Pointing Task (Bryan & Luszcz, 2001), and the
Modified Six Elements task also from the BADS, in order to assess the construct of the
temporal organisation of behaviour. Retrospective memory was assessed using memory
for items from the Zoo Map task, recall of items from the Self-Ordered Pointing Task,
and a measure of working memory called the Reading Span test (Daneman & Carpenter,
1980). Prospective memory was assessed using both event-based and time-based tasks
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 55
which have been used in previous research aimed to measure this particular cognitive
construct (Einstein, McDaniel, Richardson, Guynn, & Cunfer, 1995; Park, Hertzog,
Kidder, Morrell, & Mayhorn, 1997; Smith & Bayen, 2005). Interference control was
assessed using the Symbol Search subtest of the Wechsler Adult Intelligence Scale –
Third Edition (WAIS-III; Wechsler, 1997), and the Letter Cancellation test (Diller, Ben-
Yishay, Gerstman, Goodkin, Gordon, & Weinberg, 1974). The inhibition of prepotent
responses was measured using the Stroop test (Dodrill, 1978), a modified version of the
Rule Shift card test from the BADS, and the Trail Making Test from the Delis-Kaplan
Executive Function System test battery (D-KEFS; Delis, Kaplan, & Kramer, 2001).
Results of the study demonstrated that measures of the temporal organisation of
behaviour, retrospective memory, prospective memory, interference control and the
inhibition of prepotent responses significantly correlated within each of the constructs
tested. However, it was also found that only retrospective memory performance
predicted temporal organisation of behaviour performance after controlling for the
effects of fluid intelligence. Through factor analysis these measures were compared in
order to determine whether the pattern of results served to support West’s hierarchical
model of executive function. Overall, results of the study suggested that ‘executive
functions’ operate as a group of individual, though related, functions. It was also found
that performance on these functions were found to be independent of performance on a
measure of fluid intelligence, suggesting that such functions were distinct from general
intellectual ability. Findings of the study were in support of West’s (1996) theoretical
hierarchical model of executive function. These findings are particularly relevant to the
current study as previous reseach investigating the executive functioning of
schizophrenia patients with cannabis use appear to fall short of examining the range of
abilities that have been considered to be reflective of the executive function construct as
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 56
a whole. It should be noted that this particular testing protocol has also yet to be utilised
among clinical samples.
Damasio’s somatic marker hypothesis. Taking a different perspective,
Damasio’s ‘somatic marker hypothesis’ model (Damasio, 1995; Damasio, Everitt, &
Bishop, 1996; Damasio, Grabowski, Frank, Galaburda, & Damasio, 1994)
conceptualises the role of frontal lobe functions in emotion and interpersonal behaviour,
specifically in relation to reasoning and decision-making. It was considered by Damasio
that emotion is mediated by the prefrontal regions, via the complex links found between
the cortical (which involve the ventromedial cortex) and subcortical (which involve the
mediodorsal nucleus of the thalamus, the amygdala, and the hypothalamus) linkages.
This model considers and emphasizes the ‘hot’ or emotional-related component of
executive function and its associated relationship with ‘cold’ or more mechanistic
cognitive components of executive function with regard to decision-making processes
and the development and/or maintenance of social relationships. Damasio (1995)
developed the model to help explain impairments such as dramatic personality change,
emotional and interpersonal problems which are commonly seen after VPFC damage in
patients. It is thought that individuals with damage to the VPFC experience difficulties
in behaviour regulation due a reduced ability to utlise emotion-related somatic signals
which are used to ‘mark’ inappropriate behaviours. Therefore, considering the
conceptualised relationship between the prefrontal cortex and executive function,
Damasio’s model provides further insight into the construct of executive function and
its potential subcomponents.
The Iowa Gambling Task (IGT) was developed by Damasio and his colleagues
in order to test the somatic marker hypothesis (Bechara et al., 1994; Bechara, Damasio,
Tranel, & Damasio, 1997; Bechara, Tranel, & Damasio, 2000; Bechara, Tranel,
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 57
Damasio, & Damasio, 1996). Studies have shown the IGT to demonstrate sound
sensitivity in individuals with VPFC lesions. Previous research has also utilised the IGT
in the examination of cognitive function related to the frontal regions in cannabis users,
schizophrenia patients, as well as schizophrenia patients who also abuse cannabis (Mata
et al., 2008; Sevy et al., 2007; Shurman et al., 2005; Whitlow et al., 2004). However,
studies have reported that performance on ‘traditional’ measures of executive function,
such as the WCST, is poorly correlated with performance on the IGT (Ritter, Meador-
Woodruff, & Dalack, 2004; Wilder, Weinberger, & Goldberg, 1998). These findings
therefore suggest that the IGT is more sensitive in tapping into other components of
executive function which have yet to be adequately measured by more traditional tests
of executive function, and fails to be addressed through West’s (1996) model alone.
The Validity of Tests of Executive Function
One of the difficulties in the neuropsychological examination of executive
functioning (and its potential in determining corresponding rehabilitation regimes) lies
in the ability of traditional psychometric tests in providing a true and accurate
assessment of the executive functions per se (i.e., whether such tests are really
measuring what they claim to measure), as well as whether performance on such
measures relates to performance in the real world. In other words, the question is
whether commonly used psychometric measures of executive function are ‘ecologically
valid’. Although meta-analytic results have shown that performance on neurocognitive
tests have the ability to predict everyday functioning on a relatively global scale, more
often than not, the typical standardised neuropsychological tests used to conduct such
assessments are selected on the basis of their ability to detect pathology (Green et al.,
2000). Therefore, it is plausible that such measures may not truly capture the essence of
the demands and routines involved in everyday life. Wilson and colleagues (1997)
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 58
emphasised that some clinical patients who exhibit deficits in day-to-day functioning
can also demonstrate normal performance on executive functioning tests. Accordingly,
it seems reasonable that the evaluation of the ecological validity of executive
functioning tests as an empirical research target has specific clinical relevance to the
schizophrenia population.
Many common neuropsychological test approaches involve presenting the
subject with a single explicit task at any one time, relatively short task trials, prompts or
instructions by the examiner regarding when to initiate behaviour, and some type of
indication of the intended/expected successful outcome (Shallice & Burgess, 1991).
However, through the process of administering a set of standard instructions and scoring
systems in such approaches, it is reasonable to consider that these approaches may not
adequately correspond to the naturalistic demands involved in everyday life. The use of
quantitative approaches in research is mainly thought to capture an individual’s
performance at either the pathological or impairment level, but fails to tap into more
clinical concerns such as functional everyday life deficits (Whyte, Polanski, Cavallucci,
Fleming, Lhulier, & Coslet, 1996). It is considered that much of the discrepancy
between cognitive performances demonstrated in formal testing and real-world
situations could possibly be explained by the notion that such testing contexts provide a
structured and often well-controlled environmental setting for the examinee (Chaytor &
Schmitter-Edgecombe, 2003; Lezak et al., 2004; Manchester, Priestley, & Jackson,
2004, Shallice & Burgess, 1991; Whyte et al., 1996). Due to these characteristics, many
traditional tests of executive function are said to lack ecological validity (Acker, 1990;
Cripe, 1996), and consequently may fail to sufficiently assess the complex interplay of
behaviours that are involved in ‘real-world’ living.
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 59
A general limitation involved in standardised test procedures is that such
experimental contexts strongly prompt the participant toward exhibiting certain
behaviours. It is therefore plausible that situations that require behaviours such initiative
or volition may not be able to adequately be assessed and represented by such measures.
This notion is particularly relevant to the assessment of executive functioning, which
can be conceptualised as reflecting not solely what an individual does, but how they do
it as well as whether or not the individual exhibits the behaviour in the first place (Lezak
et al., 2004). It is thought that measures sensitive to executive dysfunction should
involve those contexts which are ill-structured and capture strategic planning, and
retrospective memory and prospective memory abilities, particularly when considering
the relationship between performance on such measures and performance in daily living
(Burgess, Veitch, de Lacy Costello, & Shallice, 2000; Fortin, Godbout, & Braun, 2003;
Godbout, Grenier, Braun, & Gagnon, 2005; Goel et al., 1997). Due to the structured and
standardised nature of many traditional executive function tests, it is argued that such
measures may not be adequate in their sensitivity to detect true executive deficits, such
as the ability to deal with multi-task demands that are required for day-to-day
functioning (Burgess, 2000; Shallice & Burgess, 1991).
‘Traditional’ tests of executive function. Typical measures which have been
deemed to assess executive abilities are usually those which have been found to be
related and sensitive to dysfunction of the frontal lobe. Examples of such measures
include the WCST (Milner, 1963). Deficits have also been shown on other widely used
measures of executive functioning such as the TMT (Reitan, 1958), verbal fluency
(Allen, Liddle & Frith, 1993; Benton, 1968), and the Stroop test (MacLeod, 1991;
Stroop, 1935). However, it is considered that the construct validity of many traditional
tests of executive function, such as the WCST, verbal fluency, the Tower of Hanoi, and
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 60
the TMT, is not well established despite their extensive use in neuropsychological
assessment (Kafer & Hunter, 1997; Phillips, 1997; Rabbit, 1997). In the schizophrenia
literature, the WCST is the single most commonly applied measure in the investigation
of executive functioning (Reicheberg & Harvey, 2007). However, despite its
widespread use not all schizophrenia patients have been found to demonstrate deficits in
WCST performance (Braff et al., 1991; Goldstein, 1990; Goldstein, Beers &
Shemansky, 1996). Reports of normal performances in schizophrenia patients on the
WCST further suggests that the construct of executive function is not only complex, but
comprises a range of different components. Therefore, the ability of psychometric
measures to fractionate the executive functions seems crucial in order to be more
sensitive to individual variation in executive functioning performance. Considering the
importance of executive functions in schizophrenia, previous research has failed to
employ a range of neurocognitive measures aimed at tapping into the various
subcomponents theorized to be involved in the construct of executive function. It also
appears that the construction of many commonly used measures of executive function
has been fundamentally atheoretical in nature. The selection of such tests which are
hypothesised to assess this neurocognitive domain more often than not, rely on their
apparent face validity which is largely subjectively based (Salthouse, Atkinson, &
Berish, 2003). New research in this area should endeavour to take into account this issue
as an important methodological consideration when assessing a neurocognitive domain
that is as complex and multi-faceted as executive function.
However, there are some more theoretically-driven psychometric measures that
exist which have been specifically developed in order to attempt to tap into the
processes conceptualised be involved in executive function and validated to assess their
reliability in the detection of executive dysfunction. Such tests include the Tower of
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 61
London test (Shallice, 1982), the Cognitive Estimates Test (Shallice & Evans, 1978),
and the Self-Ordered Pointing Task (Petrides & Milner, 1982; Bryan & Luszcz, 2001).
However, establishing the construct validity of such tests has not been without
difficulty. For example, Kafer and Hunter (1997) utilised a structural equation
modelling approach in order to evaluate the relationship among four tests
conceptualised to measure planning/problem-solving abilities (i.e., the Tower of London
test, the Six Element Test, the Twenty Questions Test, and the Rey Complex Figure
Test) and the latent construct of planning/problem-solving. Findings indicated that an
adequate statistical model was not able to be estimated in their analyses. Such results
therefore served to question the construct of planning/problem-solving, as well as
problems were specifically identified with the psychometric structure of the Tower of
London test. O’Carroll, Egan and MacKenzie (1994) conducted a study in order to
provide normative data for the Cognitive Estimates Test from a representative sample of
the general population. A Principal Component Analysis resulted in a five-factor
solution, suggesting that the test does not measure a single factor. In addition, while the
test demonstrated adequate inter-rater reliability, it was also found to have poor internal
reliability. From the results obtained, the investigators concluded that the Cognitive
Estimates Test is psychometrically unsatisfactory. Issues in establishing the validity of
measures has also been the case where various classic tests of executive function (e.g.,
WCST, COWAT/phonemic verbal fluency, and the Stroop test), have shown sensitivity
to frontal lobe lesions, but fail to demonstrate specificity to frontal lobe pathology alone
(Alvarez & Emory, 2006).
Despite the importance of the various theoretical models of executive function, it
was not until approximately the last two decades that specific constructs were
incorporated into the development of clinical and experimental assessments of executive
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 62
functions. For example, the Behavioural Assessment of the Dysexecutive Syndrome
(BADS) is an executive function test battery that is specifically designed to assess the
everyday deficits in executive functioning among clinical groups (Wilson, Alderman,
Burgess, Emslie, & Evans, 1996) and was developed as a modification of the Six
Elements Test and Multiple Errands Test (Shallice & Burgess, 1991). It consists of six
subtests aimed at measuring different facets of executive functioning such initiation and
task implementation, as well as planning and cognitive flexibility. However, this test
battery as a whole has been limited in its use when assessing schizophrenia patients
(Evans, Chua, McKenna, & Wilson, 1997; Katz et al., 2007; Krabbendam, de Vugt,
Derix, & Jolles, 1999). Findings from Katz and colleagues’ (2007) study confirmed the
ecological validity of the BADS test battery in its sensitivity and predictive validity in
determining ‘real-world’ functioning in individuals with schizophrenia.
Another major difficulty related to the assessment of the executive functions lies
in the ability for performance on one particular test of executive function in not only
having predictive value in performance on another test of executive function, but also in
coping and performance in the functional domains of ‘real world’ contexts (Burgess,
1997; Burgess et al., 1998). Due to this issue, reduced performance on executive
function tests may be due to many different reasons and could be independent of actual
executive dysfunction. It is also possible that individuals may demonstrate adequate
performance on such measures and still experience significant difficulties in their ability
to navigate the complex demands of day-to-day life activities (Shallice & Burgess,
1991). Therefore, incorporating more ‘real-world’ characteristics into specific
assessment measures has been considered imperative when attempting to make progress
in this area (Schwartz, Reed, Montgomery, Palmer, & Mayer, 1991; Shallice & Burgess,
1991). Particular assessment methods have been developed to attempt to achieve this
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 63
such as in the Modified Six Elements test from the BADS test battery (MSE; Evans et
al., 1997; Wilson, Alderman, Burgess, Emslie, & Evans, 1996). The MSE aims to assess
planning abilities by allowing subjects to complete several tasks and navigate their work
according to their own self-driven plan, as well as a set of prescribed rules. This test has
been shown to demonstrate superior ecological validity, as well as sensitivity, in
comparison to other subtests in schizophrenia patients (Evans et al., 1997). There
appears to be many factors to take into account when considering the more precise and
effective measurement of specific cognitive domains, particularly in the case of those
conceptualised to be multi-faceted and fractionable in nature, such as executive
function.
Assessing Functional Outcome
A number of methods have also been employed to assess functional outcome
status. In order to achieve a more ecological approximation in the measurement of
executive functions, several approaches have been taken such as the use of
questionnaires (Bennett, Ong, & Ponsford, 2005; Broadbent, Cooper, Fitzgerald, &
Parkes, 1982; Burgess, Alderman, Wilson, Evans, & Emslie, 1996), or through
laboratory tasks approximating real-life situations, such as those tasks used in the
BADS (Evans et al., 1997; Katz et al., 2007; Krabbendam et al., 1999; Wilson et al.,
1996; 1998). Other means involve observing and assessing the behaviours demonstrated
by patients in daily activity simulations (Knight, Titov, & Crawford, 2006; Zalla,
Plassiart, Pillon, Grafman, & Sirigu, 2001; Zhang, Abreu, Seale, Masel, Christiansen, &
Ottenbacher, 2003). In a study by Rempfer, Hamera, Brown and Cromwell (2003), the
relationship between performance on various cognitive tests and performance on a
functional outcome measure, namely the Test of Grocery Shopping Skills (TGSS), was
examined. The TGSS evaluates the performance of shopping skills within the natural
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 64
context of a grocery store, with the measure yielding three outcome scores (1) accuracy;
(2) redundancy (a measure of shopping efficiency); and (3) time. Associations were
found between shopping accuracy and measures from the first two trials of the Stroop
test, namely word reading and colour naming. There were also correlations between
accuracy and verbal recall on the Rey Auditory Verbal Learning Test (Rey, 1964) and
perseverative errors on the WCST. In addition, ‘redundancy’ while shopping was found
to be significantly related with poorer performance on three measures of executive
functioning (WCST total correct and perseverative errors, verbal fluency/FAS, and
performance on the colour-naming and colour-word trials of the Stroop test). Total time
on the Letter Cancellation test was also correlated with redundancy. Verbal memory
was shown to be associated with both shopping accuracy and redundancy. However,
while this investigative approach provides a good demonstration of the ecological
validity of certain standardised psychometric tasks, the results of this study were limited
specifically to the particular skill of independent living that was simulated (e.g., grocery
shopping). Therefore, addressing real-world functioning in a more precise manner by
making distinctions amongst the various aspects of social and community living
activities are warranted.
Keefe and colleagues (2006) examined the relationship between cognitive
performance in schizophrenia with functional capacity and functional outcome. In their
study, functional capacity was assessed using the UCSD Performance-based Skills
Assessment (UPSA; Patterson, Goldman, McKibbin, Hughs, & Jeste, 2001) and ‘real-
world’ functional outcome was measured using the Independent Living Skills Inventory
(ILSI; Menditto, Wallace, Liberman, Wal, Jones, & Stuve, 1999). The UPSA requires
patients to perform a number of tasks reflective of a range of skills considered important
for functioning in the community. Patients are then rated on their performance on these
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 65
tasks. The ILSI is a functional assessment tool which also aims to measure an
individual’s competence in a broad range of skills considered pertinent for successful
autonomous community living. Ratings are made on the basis of an interview with the
patient or the reports/observations of care givers. Findings from the study indicated that
cognitive performance (as assessed by the Brief Assessment of Cognition in
Schizophrenia (BACS); Keefe, Goldberg, Harvey, Gold, Poe, & Coughenour, 2004),
was found to be significantly correlated with both functional capacity and real-world
functional outcome. Results also suggested that measures of cognition and functional
capacity significantly predicted variance in real-world functional outcome. However,
only the measures of cognition independently accounted for significant variance in real-
world functional outcome, when compared to the contribution of performance-based
functional skills. The authors concluded that the latter measure appeared to fail to
uniquely contribute to real-world outcomes above and beyond that which was accounted
for by the cognitive performance measures. Results of the study are suggestive that
performance-based skills assessment may not necessarily have better predictive capacity
when considering ‘real-world’ functional outcome in schizophrenia in comparison to the
cognitive measures used.
While there are a range of methods in assessing functional outcome, it is
acknowledged that such an assessment can be difficult to achieve through observation
of real-world performance on activities in a research setting. In addition, the observation
of performance on specific daily activities is not only time consuming, but is limited to
the assessment of only the activity or task in question. While skill demonstrations and
task simulations have their important merits, the number and wide range of daily skills
thought to be required in successful community functioning would render such an
approach difficult when attempting to make a thorough assessment. Thus, a critical
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 66
advantage of the questionnaire method (e.g., the ILSI) is that it permits a more
comprehensive assessment of functional outcome across a variety of independent living
skill domains while at the same time allowing for a more resource efficient approach as
opposed to task simulation methods.
The Current Study
It has been widely established that cognitive impairment is a fundamental
characteristic of the illness of schizophrenia. In addition, the cognitive domain of
executive function is not only considered to be particularly implicated, but is also shown
to be significantly related to functional outcome. As cannabis is a commonly used
substance of abuse in schizophrenia, but in addition has also been found to contribute to
detrimental effects on the illness’ presentation, the examination of the relationship
between cannabis use, schizophrenia and neurocognitive performance has clinical
significance. There is already substantial evidence comparing schizophrenia patients
with non-schizophrenia controls, demonstrating deficits across different executive tasks.
Similarly, there is also extensive research demonstrating executive function impairment
associated with cannabis use. However, there have been some mixed and surprising
findings in relation to the associations between schizophrenia, cannabis use and
neurocognition. Contrary to expectation (which has been largely based on the premise
of well established cognitive deficits associated with the illness of schizophrenia, as
well as cannabis use alone, thus leading to a hypothesis of additive effects), some of the
findings that have emerged over the years have suggested that cannabis use in
schizophrenia is associated with better neurocognitive performances. Factors related to
variability in sample characteristics, neurocognitive assessment protocols, research
design, and failure to adequately control for potential confounds have been considered
as possible contributors to the inconsistent results.
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 67
Many previous studies have employed measures of executive function which are
adopted from frontal lobe neuropsychological paradigms based on lesion studies. On the
whole, it appears that the majority of schizophrenia research regarding cognition has
failed to adopt comprehensive enough theoretical approaches and assessment
methodologies in order to adequately tap into the range of executive abilities, and
therefore fall short of achieving the successful fractionation of the executive function
system. The current study employed the use several theory-driven selected tests of
executive function guided by West’s (1996) hierarchical model of executive function.
The current study proposes to apply a theoretical model of executive function, in
particular West’s model, to examine the executive functioning of schizophrenia patients
with a cannabis use history. A verbal fluency and an emotion-based decision-making
measure were also included in the present study. Verbal fluency (particulary letter
verbal fluency) has been shown to be sensitive to frontal dysfunction (for review, see
Parker & Crawford, 1992) and deficits on this measure is found to be consistently
observed in patients with schizophrenia (Chen et al., 2000, Crawford, Obonsawin,
Bremner, 1993, Gruzelier, Seymour, Wilson, Jolley, & Hirsch, 1988). In addition,
emotions are known to play a key role in decision-making processes (Bechara et al.,
1997). These additional components do not appear to be accounted for in West's model.
Therefore, this assessment approach will permit us to examine executive functioning
with a range of measures sampling both the conceptualised mechanistic and emotion-
related components of executive function.
Considering the inconsistency in the literature regarding the assessment of
executive function, it is important to evaluate and further develop the theoretical
conceptualisation of this neurocognitive domain that will in turn lead to the better
refinement, clarification and successful measurement of the construct. As it has been
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 68
shown in previous research that there are executive functioning deficits in individuals
with schizophrenia, as well as in individuals who use cannabis, it is important to
ascertain whether cannabis use in schizophrenia is associated with poorer executive
functioning and hence greater impairment and poorer adjustment in everyday life. While
previous studies have examined the relationship between executive function and various
aspects of functional outcome in schizophrenia, there appears to be a paucity of a
comprehensive examination of the the link between performance on tests of executive
function and functional outcome measures amongst the schizophrenia with cannabis use
population.
In the first part of the present study, the aim was to identify the relative
differences in executive functioning associated with cannabis use in schizophrenia
compared to those which are associated with cannabis use alone. Exploring the pattern
of executive functioning in schizophrenia patients with cannabis use and non-
schizophrenia controls with a similar cannabis use history will help to tease out such
differences. Since performance on executive function tests is rarely evaluated for its
'real-world' significance, in the second part of the study another aim was to examine the
relationship between performance on the tests selected in the current research and
ratings on a measure of ‘real-world’ functional outcome. This will enable not only the
comprehensive evaluation of executive functioning in schizophrenia patients with a
cannabis-use history, but also allow for the examination of whether performance on a
range of tests of executive function serve as effective predictors of performance in
specific ‘real-world’ functional outcome domains. This in turn, will permit an
evaluation of whether such executive function measures are 'ecologically valid' (i.e.,
reflective of 'real world' abilities and adjustment). The assessment of this clinical
population with ecologically valid measures may therefore help to better inform and
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 69
guide future treatment planning. This may be achieved by being able to more precisely
identify the various executive functioning components thought to more closely relate to
the specific functional skills that are required for successful independent living. From
the results of the study, it is hoped that through the neurocognitive sub-typing of
individuals with schizophrenia who use cannabis, specific cognitive rehabilitation
and/or behavioural strategies can be better directed and consequently help to assist in
the prediction of those who are more likely to successfully respond to such intervention.
Hypotheses. The aims of the current study will thus be achieved by dividing the
study into two parts. Part A of the study specifically aimed to examine the relative
differences in executive functioning associated with cannabis use in schizophrenia
patients, in comparison to individuals with no psychiatric or mental illness diagnoses
with a similar pattern of cannabis use. It was predicted that schizophrenia patients with
cannabis use will demonstrate poorer performance in constructs of executive
functioning, compared to healthy controls with a similar cannabis use history. Further,
performances falling greater than 1.0 standard deviation below normative mean criteria
will be described in schizophrenia patients with cannabis use, compared to controls. In
addition, it was hypothesised that the discontinuation or abstinence from cannabis use
will be associated with the recovery of executive function (i.e., ‘non-current users’ will
demonstrate significantly better performance on executive function measures than
‘current’ users). It was also predicted that former users who have ceased their cannabis
use for a longer period of time will have significantly better executive function
performance, in comparison to those individuals who have ceased cannabis use for a
shorter period of time.
Part B of the study aimed to further our understanding of the functional
outcome correlates associated with the executive functioning of schizophrenia patients
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 70
with cannabis use, relative to matched-control subjects with a similar cannabis use
history. The investigation into the relationship between the executive function
components belonging to West’s (1996) model and functional outcome is exploratory in
nature. However, due to reports of a significant association between executive function
and functional outcome in the literature, it is predicted that executive function scores
will have a significant relationship with functional outcome ratings. More specifically, it
was hypothesised that executive function scores will have significant predictive ability
in relation to functional outcome ratings.
Method
Participants
Participants included in the current study were aged between 18 and 55 years.
Schizophrenia patients were recruited as outpatients from public mental health services
in South Australia. Clinical participants were eligible for the study if they had a current
diagnosis of schizophrenia according to the DSM-IV (American Psychiatric
Association, 2004), which was verified by the patient’s clinical file information upon
their consent. Control subjects were eligible for the study if they did not have any
current psychotic, anxiety, or mood disorder diagnoses. While this was based on the
self-report of the healthy control group, subjects were also screened for elevated and
clinically significant levels of depression, anxiety and stress symptoms (see below for
further detail regarding the determination of the presence of group differences in
depression, anxiety and stress symptoms). An overall sample of 76 was recruited for the
study. However, data from 4 participants in the schizophrenia group were excluded
from the study due to these individuals not having a reported cannabis use history. Due
to the total sample size of this subgroup being too low, the maintenance of these cases
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 71
in the final data set was considered insufficient for meaningful between-subject
statistical analyses to be conducted. Data from an additional participant from the
schizophrenia patient group who did however have a reported history of cannabis use,
was also excluded due to the participant not being able to complete the full
neurocognitive battery. It was observed by the researcher that the participant
demonstrated significant difficulty in being able to understand the instructions of the
psychometric measures and was therefore assessed to be ineligible for the continuation
of testing. Data from two schizophrenia patients with cannabis use were also excluded
from the study, due to these participants not being able to be adequately matched to a
corresponding control subject on the matched variables. In addition, data from 13
otherwise healthy control subjects with cannabis use were also excluded from the study,
due to these participants not being able to be adequately matched to a corresponding
schizophrenia patient on the matched variables. The final sample included 28
individuals with schizophrenia (both former and current users) who had a regular
cannabis use history and 28 matched-controls (both former and current users) with a
similar cannabis use history. The process of recruiting matched controls involved
finding a suitable matched pair for an already recruited patient subject. Premorbid IQ
was matched with a suitable pair within +/- .05 standard deviation (SD) of the
schizophrenia patient’s score. The age, years of education, severity/quantity, duration,
and time since cessation variables were matched with a suitable pair in the control
groups within +/- 1.0 SD of the schizophrenia patient’s score. The frequency variable
was matched with a suitable pair according to frequency group category (i.e., light users
≤ 2 times per week; heavy users ≥ 3 times per week). Gender was equally matched
between groups. While the +/- 1.0 SD matching criteria was acknowledged to be much
less conservative than a +/- 0.5 SD matching criteria, it was considering necessary due
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 72
to the large variability found in such variables and consequently attempting to match
within +/- 0.5 SD would have made the process increasingly more difficult considering
the timeframes and resources available. However, independent sample t-tests indicated
that no significant differences were found between groups on the cannabis use
parameters (all p’s > .05), suggesting that the two groups were well matched on all of
these indicators of cannabis use. Participants were considered to be regular cannabis
users if they used cannabis for more than 2 years duration, with a frequency of use of
one or more days per week. Further, 34 participants (17 schizophrenia patients and 17
matched-controls) who reported regular cannabis use in their history, were currently
abstinent at the time of testing. In addition, 22 participants (11 schizophrenia patients
and 11 matched-controls) who reported to be regular users of cannabis were current
users at the time of testing. All participants were required to provide written informed
consent, as well as have the physical capacity to participate in cognitive assessment.
Participants with ages falling outside the range of 18 to 55 years, who had any
other mental illness diagnoses (other than schizophrenia for the patient group only), a
medical history of head or brain injury, a history of neurological disorder, or an
intellectual or developmental disability, were excluded from the study. Subjects who
regularly used other drugs of dependence or had a history of alcohol abuse/dependence
were also excluded from the study. This criteria was assessed in a structured interview
(see Appendix A) via self-report. Participants were required to abstain from cannabis
use for at least 24 hours prior to testing (if still a current user), as well as abstain from
the consumption of caffeine and nicotine use within 1 hour of testing. Adherence to the
24-hour and 1-hour abstinence period was assessed using self-report. Participants were
also required to be proficient in understanding and speaking the English language. All
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 73
participants were given a $25 Coles-Myer voucher as a token of appreciation for their
participation in the study.
The sociodemographic characteristics of the two groups are detailed in Table 1.
Independent sample t-tests indicated that no significant differences were found between
groups on the sociodemographic variables (all p’s > .05), suggesting that the groups
were well matched on all of these characteristics. No significant difference was found
between groups in nicotine use (p > .05). The groups also did not significantly differ on
self-reported depression, anxiety or stress symptoms (as measured by the Depression
Anxiety Stress Scales (DASS); S. H. Lovibond & P. H. Lovibond, 1995) (all p’s >.05).
It was noted that the between group differences in reported anxiety symptoms
approached significance (p = .052). However, the mean ratings for the schizophrenia
group fell in the moderate range for depressive and anxious symptomatology and in the
mild range for stress symptomatology. Mean ratings for the matched-controls group fell
in the mild range for depressive, anxious and stress symptomatology. A chi-square test
for goodness of fit examining symptom categories (as assessed by the the Structured
Clinical Interview for the Positive and Negative Syndrome Scale (SCI-PANSS); Kay,
Fiszbein & Opler, 1987) indicated that there was no significant difference in the
proportion of schizophrenia patients with cannabis use, falling in either the positive
symptom subtype or negative symptom subtype category, χ2(1, N = 27) = .333, p= .564,
w = .111. All but one schizophrenia patient were taking antipsychotic medications. Four
patients were taking typical antipsychotics, 16 were taking atypical antipsychotics, 7
were on combined therapy (i.e., taking both typical and atypical antipsychotic
medications), and 1 patient was currently not taking any medication.
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 74
Table 1. Sociodemographic characteristics of the sample
Total
(N=56)
Schizophrenia Patients
(n=28)
Matched Controls
(n=28)
Mean (SD) Mean (SD) Range Mean (SD) Range
Age 34.50 (7.20) 34.57 (7.71) 23-51 34.43 (6.78) 25-49
Male:Female 38:18 19:9 19:9
Years of
Education
11.83 (1.77) 11.77 (2.05) 8.0-16.0 11.89 (1.47) 9.0-14.0
Estimated
Premorbid IQ
99.09 (6.17) 98.75 (7.02) 82-108 99.43 (5.31) 88-107
Nicotine Use 11.21 (10.76) 13.50 (13.16) 0-40 8.93 (7.19) 0-25
Frequency
(days per week)
5.43 (2.08) 5.61 (2.15) 1.0-7.0 5.25 (2.03) 1.0-7.0
Severity
(grams)
1.42 (1.03) 1.44 (1.21) .25-5.0 1.39 (.82) .25-4.0
Duration
(years)
13.14 (8.85) 13.57 (10.04) 2.0-38.0 12.71 (7.64) 3.0-33.0
Time Since
Cessation
(years)
3.63 (5.40) 3.71 (5.74) .00-28.0 3.55 (5.14) .00-25.0
DASS
Depression 13.18 (11.23) 16.00 (13.84) 0-42 10.36 (6.99) 0-20
Anxiety 9.98 (7.80) 12.00 (9.36) 1-30 7.96 (5.27) 0-14
Stress 12.88 (8.57) 14.21 (10.02) 0-34 11.54 (6.74) 0-24
PANSS
Positive 18.25 (6.97) 7-36
Negative 18.79 (5.71) 10-33
General 39.18 (12.07) 16-69
Composite -.54 (4.84) -9-12
Recruitment
Advertisements (see Appendix B: (i) and (ii)) were distributed to the Royal
Adelaide Hospital, the Queen Elizabeth Hospital, Lyell McEwin Hospital Service,
Flinders Medical Centre, various local public mental health service locations (Port
Adelaide Mental Health Service, Beaufort Clinic, Southern Mental Health Service,
Adaire Clinic), a community-based mental health organisation (The Mental Illness
Fellowship of South Australia), mental health community centres and supported
residential facilities, requesting for patient volunteers willing to participate in the
current research study. The contact details of the researcher were provided for any
prospective patient participants to contact in order to express their interest. The
advertisement also informed potential participants that a form could alternatively be
completed and posted at the reception desk (found in each location) in order for the
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 75
participant to leave their contact details should they wish to be contacted by the
researcher instead. An information sheet about the study (see Appendix C) was then
provided through reception, upon request. In addition, the researcher attended the local
clinics on a arranged basis in order to be available to talk to individuals who were
interested in participating in the study.
Advertisements (see Appendix D) were also posted in various locations in the
local community (e.g., shopping/public community centre notice boards) requesting for
otherwise healthy control volunteers willing to participate in the research study.
Potential control subjects were initially screened using a telephone interview gathering
demographic details and information pertaining to patterns of cannabis use. Once
individuals were deemed eligible for inclusion in the study, an appointment time was
scheduled to attend a testing location in order to be assessed and an information sheet
(see Appendix E) was then sent out to the participant via post.
The current study employed a matched-control design to ensure that any
individual differences occurring between groups (outside of the variables of interest),
were controlled for. Due to the variation in indices of cannabis use (i.e., frequency,
severity/quantity, duration and time since last use), the interpretation of results based on
comparisons between groups is made more difficult. Therefore, the current study aimed
to take these indices into account when matching control subjects. Control subjects were
matched with schizophrenia subjects based on age, gender, years of education,
estimated premorbid IQ, frequency of cannabis use, severity of cannabis use, duration
of cannabis use and time since cannabis use cessation.
Measures
Demographics. The demographic information gathered included age, gender,
years of education, medical history (e.g., head/brain injury, neurological conditions,
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 76
stroke, cardiac problems etc.), medication, frequency, severity/quantity, duration of
cannabis use, time since cessation (if relevant), as well as other types of drugs used and
their patterns of use. Average days of use per week, average grams consumed per using-
day (where 1 joint was estimated to be .5 grams (Zeisser et al., 2011); and 1 cone was
estimated to be .25 grams), years of use, and years since last use were determined as
measures of frequency, severity/quantity, duration and time since cessation,
respectively. This was assessed through the structured interview conducted prior to
neurocognitive testing. This information was also gathered to assess the exclusion
criteria.
Clinical symptoms. To assess the positive and negative symptom profile of the
schizophrenia group, the Structured Clinical Interview for the Positive and Negative
Syndrome Scale (SCI-PANSS) was administered. The SCI-PANSS was used to
evaluate the presence and severity of positive, negative and general symptoms of
schizophrenia. Thirty items were rated on a 7-point scale from 1 = absent to 7 = extreme
and total scores were then calculated for the positive, negative and general symptom
areas. High internal reliability (r = .73 - .83) (Kay et al., 1987; Kay, Opler, &
Lindenmayer, 1988) is reported across each scale of the instrument.
To assess whether there were any significant differences between groups in
affective state the Depression Anxiety Stress Scales (DASS; S. H. Lovibond & P. F.
Lovibond, 1995) was administered. The DASS is divided into 3 scales (constituting 14-
items each) which correspond to the syndromes of depression, anxiety and stress. Each
syndrome has been found to be correlated, but distinct from each other (P. F. Lovibond
& S. H. Lovibond, 1995; S. H. Lovibond & P. F. Lovibond, 1995). Subjects were asked
to indicate the presence of symptoms occurring over the past week only, with responses
to the items based on a 4-point severity/frequency scale, ranging from ‘did not apply to
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 77
me at all’ (0) to ‘applied to me very much, or most of the time’ (3). The DASS has been
reported to have internal reliability coefficients of r = .87 - .95.
Neuropsychometric battery. Standardised measures of estimated premorbid
intelligence and executive function were chosen from the Behavioural Assessment of
the Dysexecutive Syndrome (BADS; Wilson et al., 1996), the Delis-Kaplan Executive
Function System (D-KEFS; Delis, Kaplan, & Kramer, 2001), the Wechsler Adult
Intelligence Scale – Third Edition (WAIS-III; Wechsler, 1997), and also included tests
modified or developed by previous researchers (e.g., Num, 2006). Considering a
number of tests used in the current study originate from well validated test batteries, the
validity of the batteries themselves will be discussed first. Individual test information
will then be further discussed in the sections following.
The main purpose of the BADS battery was to predict the presence and severity
of everyday executive problems amongst brain injured groups. The technical manual
reports correlations (using Pearson’s method), between the BADS profile score, each of
the six individual subtests, age, the National Adult Reading Test - Second Edition
(NART-II; Nelson & Willison, 1991), and WAIS FSIQ, with others’ and self-ratings on
the Dysexecutive Questionnaire (DEX) for a brain injured patient group. The degree of
insight, which is measured by the discrepancy between others’ and self-ratings is also
reported. Significant moderate negative correlations between each of the six individual
subtests and others’ ratings of executive problems were found. The negative correlations
indicate that satisfactory performance on any subtest in the BADS is indicative of low
ratings by significant others who know the patient well regarding the presence or
severity of executive problems, demonstrating the construct and predictive validity of
the BADS tests in detecting every day executive problems. Katz and colleagues (2007)
examined the construct validity and sensitivity of the BADS in differentiating between
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 78
different types of schizophrenia patients. The predictive validity of the BADS with
regard to functional outcomes within the outpatient group was also examined.
Performances were compared between adult schizophrenic inpatients who were in an
acute episode of the illness, adult schizophrenic outpatients who were in the more
chronic stages of the illness, and healthy control subjects. Results of the study showed
not only significant differences between schizophrenia patients and control subjects, but
also between both of the schizophrenic patient groups such that patients in the
outpatient (chronic) group demonstrated greater executive function deficits. It was also
found that within the outpatient group that performance on the BADS significantly
predicted Independent Activities of Daily Living (IADLs) and communication outcome
areas. These results were demonstrated to be beyond that which was accounted for by a
measure of basic cognitive skills. Not only do the results of the study show the BADS to
be a valid measure in the assessment of executive function of individuals with
schizophrenia by distinguishing between patients and healthy subjects, but in addition
distinguishing between patients with differing illness phases.
The technical manual for the D-KEFS (Delis et al., 2001) reports evidence for
the validity of the D-KEFS measures. The range of tests included in the current study
(see below for further discussion of specific tests used) are those which were either
developed by the test authors or modified versions of well-established, standardised
executive function measures. The modified D-KEFS instruments (i.e., versions of the
Stroop test, Trail Making Test, verbal and design fluency tests, Tower Task, the
Twenty-Questions test, and proverb interpretation) have demonstrated validity
evidenced across a number of neuropsychological studies spanning over 50 years.
Reviews of these studies are reported in Lezak (1995, cited in Delis et al., 2001, p. 47)
and Spreen and Strauss (1998, cited in Delis et al., 2001, p. 47). Validity studies have
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 79
also indicated that various D-KEFS measures have reasonable sensitivity in the
assessment of executive functioning of different clinical populations which include
patients with focal frontal-lobe lesions (Baldo, Shimamura, Delis, Kramer, & Kaplan,
2001; Delis, Squire, Bihrle, & Massman, 1992; Dimitrov, Phipps, Zahn, & Grafman,
1999), as well as schizophrenia (Beatty, Jocic, Monson, & Katzung, 1994; Savla,
Twamley, Thompson, Delis, Jeste, & Palmer, 2011). The current study only selected
those tests from the D-KEFS that were modifications of pre-existing, long-standing
clinical tests with proven validity.
The WAIS-III is a popular and widely used scale of intellectual ability. In a
study conducted by Dickinson, Iannone and Gold (2002), exploratory and confirmatory
analyses revealed that a four-factor model of WAIS-III performance (which was similar
to that reported by the developers of the scale), adequately fit data from a sample of
schizophrenia patients. In fact, the factor structure suggested by the developers (i.e.,
with correlated factors for verbal comprehension, perceptual organisation, working
memory, and processing speed) fit the clinical sample just as well as it did for the
comparison sample of non-clinical subjects. These results suggest the construct validity
of the scale and its subtests in the assessment of individuals with schizophrenia.
The test selections in the current study were guided by West’s (1996) model of
executive functioning and were also similar measures to those used in Num’s (2006)
study. A total of 12 tests were administered, including a measure of estimated
premorbid intelligence and 11 measures of executive functioning. All of the tests were
in paper-and-pencil format, with some verbal responses recorded on a tape recorder, and
time recorded using a stopwatch. All scores from the standardised tests used were
converted to standard scores based on norms which were reported in each test manual.
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 80
Premorbid intelligence. The National Adult Reading Test - Second Edition,
(NART-II; Nelson & Willison, 1991), was administered to assess estimated premorbid
intelligence and to determine if any differences in premorbid general cognitive ability
existed between groups. The NART-II consists of a list of 50 irregularly spelt words in
the English language. In this test, the reader is required to ‘know’ the word, rather than
rely on phonology in order to be able to read it correctly (e.g., ‘topiary’). Scores
represent the number of correct responses, with higher scores representing higher
estimated premorbid intelligence. The NART test manual reports a test-retest reliability
of r = .98.
Crawford and colleagues (1992) aimed to determine whether the National Adult
Reading Test would provide a valid estimate of premorbid intelligence in a sample of
schizophrenia patients. The overall sample comprised of a group of patients who resided
in long-stay wards and a group of patients residing in the community. Schizophrenia
subjects were individually matched with healthy control subjects on the variables of
age, sex, and education. Findings from the study indicated that a significant discrepancy
was found between the two IQ measures administered, where current WAIS IQ scores
were significantly lower than NART-derived estimated premorbid IQ scores. Such
results indicated a decline in cognitive functioning was present in the sample. These
results are indicative of the NART measure providing a valid measure of estimated
premorbid IQ in a schizophrenia sample. O’Carrol et al. (1992) also found NART
scores estimated significantly higher IQ scores than a current measure of IQ (i.e., the
Wechsler Adult Intelligence Scale - Revised (WAIS-R); Wechsler, 1981) in a
schizophrenia patient sample. Results of the study were consistent with Crawford et
al.’s (1992) findings.
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 81
Executive Function
The temporal organisation of behaviour. Two measures of the temporal
organisation of behaviour were used: the Zoo Map task from the BADS (Wilson et al.,
1996), and the Modified Six Elements test also from the BADS. In the Zoo Map task,
participants were required to plan and execute a trip on a map of a zoo visiting various
locations which were specified prior to commencing the test. They were also required to
follow rules involving using designated paths with which some can be travelled along
multiple times and others only once. Other rules in the task required the participant to
begin the trip at the entrance and finish at the picnic area. Two trials of the task were
administered. The first trial is of high demand (i.e., participants are required to negotiate
the trip independently) and the second trial is of low demand (i.e., participants are given
directions as to the route required). Scores on this measure reflected overall planning
time (i.e., the time that has elapsed between the participant finishing reading the
instructions and when they began the trial), the total time to complete the trial itself, the
number of places correctly visited and number of rules correctly adhered to. A profile
score was then calculated reflecting the extent to which participants followed the correct
path sequence minus error deductions, with a low profile score reflecting poor planning
and temporal organisation of behaviour. The BADS manual reports the Zoo Map test to
have inter-rater reliabilities of r = .88 – 1.0 and a test-retest reliability correlation of r =
.39.
The Modified Six Elements test comprised of six sub-tasks which require the
solving of two sets of arithmetic problems, the identification of pictures in two sets of
picture booklets, and two verbal tasks involving dictation (e.g., one task describing the
best holiday the participant has ever experienced and the second task describing any
memorable event in their life). Participants were required to attempt each task within an
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 82
allocated 10-minute period and attempt to spend equal amounts of time on each task.
However, participants were not allowed to attempt two tasks from the same superset
consecutively (e.g., not attempting two arithmetic tasks in a row). If the participant
failed to attempt every task within the allocated 10 minutes, if they spent more than 271
seconds on a single task, or if the participant attempted two tasks from the same
superset consecutively, then errors were incurred. A profile score was calculated based
on the number of tasks completed minus error deductions. A high profile score on this
measure reflected higher planning and temporal organisation of behaviour ability. For
this test, inter-rater reliabilities of r = .90 – .97 and a test-retest reliability of r = .33 is
reported.
Retrospective memory. Retrospective memory was assessed using two
measures: memory for items from the Zoo Map task from the BADS (which assessed
task-specific retrospective memory), and the Digit Span subtest from the WAIS-III.
Measures of task-specific retrospective memory were derived from Num’s (2006) study.
For memory for items from the Zoo Map task, participants were required to remember
the places they were asked to visit in the original Zoo Map task. Scores ranged from 0 to
7, with higher scores representing higher retrospective memory performance.
In the Digit Span subtest of the WAIS-III, participants were read a series of
numbers and were required to repeat them back to the examiner in sequence. The length
of the series of numbers was increased until the participant was no longer able to repeat
them back to the examiner in correct sequence across two trials. Two forms of digit-
spans were presented, one in which the participant was required to repeat the series of
numbers forwards and the other in which the participant was required to repeat the
series of numbers in reverse order to how they were verbally presented. Each length of
number series has two trials, with participants obtaining 1 point if they passed only one
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 83
trial and 2 points if they passed both trials. The maximum score on digits forwards is 16
points and maximum score on digits backwards is 14 points. The sum of digits forwards
and backwards gave a total Digit Span score. Higher scores on this measure represented
higher retrospective memory performance. The Digit Span subtest has a test-retest
reliability correlation of r = .90.
Prospective memory. For the current study, an event-based prospective
memory task was used. This measure was also derived from Num’s (2006) study. In the
event-based prospective memory task, participants were informed at the beginning of
neurocognitive testing to place a paperclip into a plastic cup at the completion of every
second task administered throughout the testing session. Paperclips and the plastic cup
were positioned in front of the participant on the table used for test administration.
Scores ranged from 0 to 6, with higher scores reflecting higher performance on
prospective memory.
Interference control. Interference control was assessed using two measures:
the Symbol Search subtest from the WAIS-III, and the Trail Making Test from the D-
KEFS. In the Symbol Search test, participants were required to indicate whether either
of two symbols which were printed on the left of the page, were present in a row of a
five symbols printed on the right of the page. They were given a time limit of 120
seconds to complete as many rows (trials) as possible. Scores represented the number of
trials completed correctly minus the number of incorrect trials, with higher scores
indicating an increased ability to control for interference. The Symbol Search test has a
test-retest reliability correlation of r = .77.
Trial 4 of the Trail Making Test from the D-KEFS was also used to assess
interference control. In this trial of the Trail Making Test, the participant was presented
with an array of 16 circles with consecutive numbers in them and 16 circles with
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 84
consecutive letters in them. Participants were required to join the numbers and letters
with a continuous line, by alternating between them (e.g., 1-A-2-B-3-C and so forth) in
the quickest time possible, without sacrificing accuracy. Scores represented the number
of errors and the time taken to complete the task. If the participant took longer than 240
seconds then they were required to stop completion of the task. Higher scores reflected
a lower ability to control interference. A Trial 4 Contrast score was also calculated to
provide a measure of the time taken to complete Trial 4 divided by the time taken to
complete both Trial 2 and Trial 3 (which provide an index of visuo-motor speed). Trial
2 of the TMT involved joining an array of circles with consecutive numbers in them
with a continuous line, in sequence and in the quickest time possible. Trial 3 of the
TMT involved joining an array of circles with consecutive letters in them with a
continuous line, in sequence and in the quickest time possible. Both Trial 2 and Trial 3
were administered prior to the participant commencing Trial 4 as part of an overall
TMT subtest administration procedure. The D-KEFS manual reports a test-retest
reliability correlation of r = .36 for this trial.
Inhibition of prepotent responses. The inhibition of prepotent responses was
assessed using two measures: the Color-Word Interference subtest (D-KEFS) and a
modified version of the Rule Shift card test from the BADS. Trial 3 of the Color-Word
Interference subtest was used in the current study, which is a variant of the Stroop test.
In this trial, participants were presented with a page of names of colours which were
printed in incongruent coloured ink. The participant is asked to name the colour of the
ink that each word is printed in, as quickly as possible but not naming the actual word
printed. The time taken to complete the trial, as well as the number of errors, was
recorded. A higher score reflected a lower ability to inhibit a prepotent response. In
addition, a Color-Word Trial 3 Contrast score was calculated to provide a measure of
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 85
the time taken to complete Trial 3 divided by the time taken to complete Trial 1 (which
is an index of processing speed). Trial 1 of the Color-Word subtest required the
participant to name colours (presented as colour patches on the page) in the quickest
time possible. Trial 1 was administered prior to the participant commencing Trial 3 as
part of the overall Color-Word Interference subtest administration procedure. Test-retest
reliabilities are reported to be in the moderate to high range (r = .60 - .90).
A modified version of the Rule Shift card test from the BADS was derived from
Num’s (2006) study. In this task, participants were provided with a booklet of 20
playing cards which were presented in random order throughout the booklet. In the first
trial, the participant was required to say ‘yes’ if the colour of the card suit was red and
‘no’ if the colour of the card suit was black, until the participant went through all of the
cards in the booklet. Using the same 20-card booklet, on the second trial participants
were required to say ‘yes’ if the colour of the card suit was black and ‘no’ if the colour
of the card suit was red. In the third trial (which was a modified addition to the original
test), participants were required to once again go through the booklet and say ‘yes’ if the
card colour was the same colour as the previous card and ‘no’ if the card colour was
different to the colour of the card previously presented. In the fourth trial (also a
modified addition to the original test), participants were required to say ‘no’ if the card
colour was the same colour to the previous card and ‘yes’ if the card colour was
different to the colour of the card previously presented. The participant was required to
go through each trial as quickly as they could, without sacrificing accuracy. The overall
time taken to complete each trial, as well as the number of errors, was recorded. Profile
scores were calculated for performance on trial 2 and trial 4 of the test by the summing
the number of errors incurred minus an error deduction if the completion time on either
of these two trials was over 67 seconds. A Rule Shift Trial 2 and 4 combined score was
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 86
calculated to represent the average score across both trials. A lower profile score
reflected a lower ability to inhibit a response. A test-retest reliability coefficient of r =
.76 is reported for the original version of this test.
Verbal fluency. The Letter Fluency subtest of the D-KEFS was used to assess
verbal fluency. Participants were asked to generate as many words as they could,
beginning with a specified letter (e.g., F, A, S), within a timeframe of 60 seconds for
each letter. Participants were asked to exclude proper nouns, numbers, and multiple
forms of the same word. The total score was the number of correct responses summed
across the three letters. Sound internal consistency is reported for this subtest ranging
from .60 to .90, as well as adequate test-retest reliability (r = .60 - .90).
Emotion-based decision-making. The Iowa Gambling Task (IGT; Bechara et al.,
1994) was used to measure emotion-related decision-making. A computerized version
of the IGT was administered where participants were required to choose between decks
of cards which yielded high immediate gains but large future loss, and decks which
yielded lower immediate gains but a smaller future loss. The goal of the task was to
optimise profit on a loan of play money. Participants were presented with four decks of
cards (1, 2, 3 & 4), and were given a $2000 loan of play money. Participants were
informed that the game requires a series of card selections, one card at a time, from any
of the four decks, until they were told to stop (which was after 100 trials). Decks 1 and 2
were ‘disadvantageous’ because they resulted in larger overall loss, whereas decks 3
and 4 were ‘advantageous’ because they resulted in larger overall gain. A total score
was calculated by subtracting the total number of cards selected from the two
disadvantageous decks from the total number of cards selected from the two
advantageous decks. Lower total scores reflected poorer decision-making abilities.
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 87
The ability of the IGT to detect decision-making impairments has been demonstrated
across a variety of clinical samples which have included populations with VPFC lesions
(Bechara et al., 1994; Bechara et al., 1997; Bechara et al., 2001; Ernst, Grant, London,
Contoreggi, Kimes, & Spurgeon, 2003). These findings are suggestive of the construct
validity of this test. More specifically to the current study, individuals with
schizophrenia have been found to demonstrate a distinct pattern of impaired decision-
making (as assessed by the Iowa Gambling Task), that is somewhat different to that
found in patients with OFC lesions, as well as healthy participants (Ritter et al., 2004;
Shurman et al., 2005). Published reliability information on the computerised version of
the test is not available and internal consistency on this measure is difficult to calculate
due to the IGT not consisting of test items that usually make up conventional
neuropsychological tests (Dunn, Dalgleish, & Lawrence, 2006). There are also
difficulties examining the reliability of the measure as the ability to assess the temporal
stability of the test could be potentially impacted by practice effects on retesting
(Beulow & Shur, 2009).
Functional outcome. The Independent Living Skills Inventory (ILSI; Menditto
et. al., 1999), was used to measure functional outcome. This inventory measures the
extent to which individuals are able to perform competently on a broad range of skills
which have been found to be important for successful community living. The inventory
was completed on the basis of obtaining ratings from the patient’s key worker or
community support worker. The original version of the ILSI included 89 items divided
into 11 subscales. Items assessed a range of skills including personal management,
hygiene and grooming, clothing, basic skills, interpersonal skills, home maintenance,
money management, cooking, resource utilisation, general occupation skills and
medication management. Each item was rated in relation to the extent in which the
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 88
individual was considered to be able to perform particular skills in conjunction to the
extent in which assistance or support is required. Ratings ranged from ‘No Competence’
(0) to ‘Independent Competence’ (3). A total ILSI score was calculated as the average
score across all of the ILSI subscales. The present study used the current version of the
inventory which contained 55 items. The shorter version was based on the psychometric
properties of items from the original scale, as well as on the feedback from raters about
their usefulness. The internal consistency among the items of the orginal version of this
scale is 0.82 (Cronbach’s coefficient alpha). This measure has been used in previous
research investigating the relationship between cognitive impairments in individuals
with schizophrenia and 'real-world' functional outcome (Keefe et al., 2006). As the
matched-control participants were not clinical patients, the measure was completed as a
self-report questionnaire by these subjects.
Procedure
After patient participant recruitment, an information sheet about the study was
provided and a mutually convenient time for testing was scheduled. Testing took place
at various local public mental health service suburban clinics. At the beginning of each
testing session, participants were informed that participation was voluntary and they had
the right to withdraw at any given time. Informed consent was then obtained (see
Appendix F). The structured interview gathering demographic information was
conducted after informed consent was obtained and this took approximately 10 minutes
to complete. The DASS and the SCI-PANSS were administered afterwards, taking
approximately 30 minutes to complete. Neurocognitive testing followed taking
approximately 60 to 90 minutes to complete. Tests were administered in a standardised
order so as to minimize measurement error due to test order interaction. A break was
also provided to participants depending on the level of participant motivation and
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 89
fatigue demonstrated/reported. All testing was completed by the researcher who is a
registered psychologist and has professional training in the administration and scoring
of neuropsychometric assessments, as well as additional professional training in the
administration and scoring of the SCI-PANSS. The Independent Living Skills Inventory
was given to the rater which was nominated by the patient participant as being able to
give the most accurate evaluation of their independent living skills. This questionnaire
was then returned to the researcher via post by a self-addressed stamped envelope which
was either provided to the participant at the time of testing or posted directly to the
nominated rater.
The matched-control subjects underwent a similar procedure. Testing took place
at one of the available testing locations selected by the participant. Informed consent
was obtained prior to the testing session (see Appendix G) and overall testing time for
this group of participants was approximately 60 to 90 minutes. The SCI-PANSS was
not administered to this group. For matched-control subjects, the Independent Living
Skills Inventory was self-completed in their own time and then sent back to the
researcher via post.
Debriefing information (see Appendix H) was given at the conclusion of testing
detailing the use of the obtained data from the research.
Results
Data Screening
Prior to testing hypotheses, data was entered into a statistical analysis program,
SPSS Version 22.0 (2013), and checked for missing values, outliers and departures from
normality. There were no missing values in the final data set. Outliers were detected on
two variables, Color-Word Trial 3 and the Iowa Gambling Task. The distributions of
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 90
these variables were Winsorised whereby the ranks of extreme scores (i.e., values
falling outside of 3.29 standard deviations from the mean) were maintained by assigning
them a value of the closest score that was not greater than 3.29 standard deviations from
the mean (for discussion, see Erceg-Hurn & Mirosevich, 2008; Field, 2009). Two scores
on the Color-Word Trial 3 variable and one score on the Iowa Gambling Task variable
were dealt with using this protocol in order to maintain the integrity of the data and
protect against any departures from normality. Following winsorisation of these
variables, normality was achieved. Further data screening demonstrated that there were
no departures from normality on any of the other variables.
Part A
Group Differences in Executive Function
As the first aim of the current study was to determine if schizophrenia patients with
cannabis use demonstrate poorer performance in constructs of executive functioning
compared to a matched-control group with a similar cannabis use history, the focus of
the initial statistical analyses was to compare the executive function performance of
these two groups. For further exploration, the proportion of schizophrenia patients with
cannabis use which demonstrated executive function performance falling in the
‘impaired’ range (compared to normative criteria), was described. Prior to analyses,
standardised scores (z) were calculated using normative means and standard deviations.
However, as Zoo Map recall, Rule Shift Trial 4, the Rule Shift Trial 2 and 4 combined
score, and the event-based prospective memory task did not have normative mean
criteria available, scores relating to these measures were left in their original form. In
order to examine the executive function performances of schizophrenia patients with
cannabis use, relative to matched-controls with a similar cannabis use history, a series
of Analysis of Covariance (ANCOVA) analyses were conducted to determine the effect
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 91
of Group on executive function performance when accounting for the effects of
premorbid IQ. Examination of Shapiro-Wilk statistics and histograms for each group
indicated that the ANCOVA assumption of normality was supported. Scatterplots
indicated that the relationship between the covariate (premorbid IQ) and the dependent
variables (performance on the individual executive function measures) was linear.
Finally the assumptions of homogeneity of regression slopes and homogeneity of
variances were supported by the absence of a significant interaction between the
independent variable and the covariate. The ANCOVA results will be categorised
according to the executive function components defined by West’s (1996) hierarchical
model of executive function specifically: (i) the Temporal Organisation of Behaviour,
(ii) Retrospective Memory, (iii) Interference Control, (iv) the Inhibition of Prepotent
Responses and (v) Prospective Memory. Performance in the additional executive
function domains included in the current study that were not outlined in West’s model,
specifically verbal fluency and emotion-based decision making, will also be discussed.
Table 2 displays the results of the covariate analyses of executive function performance
for the schizophrenia group with cannabis use and matched-control group with a similar
cannabis use history, with premorbid IQ as a covariate. Premorbid IQ was found to have
an impact on Modified Six Elements, Digit Span and Letter Fluency task performance.
However, the main effects of Group remained significant for Digit Span after
accounting for the effects of premorbid IQ. An examination of the adjusted means
indicated that schizophrenia patients with cannabis use demonstrated significantly lower
scores on the Temporal Organisation of Behaviour (except for on the Modified Six
Elements measure), Retrospective Memory, Interference Control (except for the Trail
Making Test Trial 4 Contrast score), and the Inhibition of Prepotent Responses (except
for the Color-Word Trial 3 Contrast score), relative to controls. Adjusted means
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 92
Table 2. Results of ANCOVAs of executive function performance for Group with covariate of premorbid
IQ Executive Function
Performance
Adjusted Mean
Effects
Group Premorbid IQ
SCH /C
n=28
MC
n=28
F p ηp2 F p ηp
2
TOB
Zoo Map .055 .936 11.46 .001** .178 .095 .759 .002
Modified Six Elements -.008 .315 3.03 .088 .054 10.97 .002** .172
RM
Zoo Map Recall 4.79 6.39 21.42 <.001** .288 .008 .930 .000
Digit Span -.511 .392 26.93 <.001** .337 7.47 .009** .123
IC
Symbol Search -.662 .377 20.05 <.001** .275 .456 .502 .009
TMT Trial 4 -.729 .420 34.05 <.001** .391 2.48 .121 .045
TMT Trial 4 Contrast .043 -.019 .081 .777 .002 1.32 .255 .024
IPR
Rule Shift Trial 2 -.260 .518 7.60 .008**† .125 .206 .652 .004
Rule Shift Trial 4 2.01 3.31 25.32 <.001** .323 3.32 .074 .059
RS Trial 2 and 4 5.37 7.27 27.84 <.001** .344 2.50 .120 .045
Color-Word Trial 3 -.817 .531 26.23 <.001** .331 .985 .325 .018
Color-Word Trial 3
Contrast
.484 .778 1.85 .179 .034 .555 .460 .010
PM
Event-based task 3.05 3.52 1.52 .224 .028 1.94 .170 .035
Verbal Fluency
Letter Fluency -.552 -.210 2.28 .137 .041 9.84 .003** .157
Emotion-based
decision making
Iowa Gambling Task 15.23 13.57 .062 .804 .001 .319 .574 .006
df = 2
SCH/C schizophrenia patients with cannabis use, MC matched-controls with a similar cannabis-use
history, TOB Temporal Organisation of Behaviour, RM Retrospective Memory, IC Interference Control,
IPP Inhibition of Prepotent Responses, RS Rule Shift, PM Prospective Memory, TMT Trail Making Test
Note.*p< .05, **p< .01, †non sig. following post hoc analyses
comparisons also revealed no significant differences between schizophrenia patients
with cannabis use and matched-control subjects with a similar cannabis use history on
the event-based prospective memory, verbal fluency and emotion-based decision
making tasks. Post hoc analyses using a Holm-Bonferroni correction method revealed
that no significant differences were detected between groups on Rule Shift Trial 2
performance while controlling for family-wise Type I error rate at level α = .05.
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 93
Level of executive function impairment between groups. To gain further
insight into the extent and level of executive function impairment in the sample groups,
a number of calculations using normative data and impairment criteria or impairment
cut-off scores were conducted prior to analysis. The z-scores calculated prior to
analyses were used for this examination. Used as a guide in Coulston et al.’s (2007)
study, ‘impairment’ status was defined if neurocognitive performance fell 1.5 – 2.0
SD’s or more below the mean of the control group. Lezak et al. (2004) also suggests
that a neurocognitive test performance change of 1.0, 1.5 and 2.0 SD’s may be used for
determining impairment. Lezak considers that while a test performance reflective of a
1.0 SD change may not be sufficient enough to indicate a difference of statistical
significance, some examiners may still classify this performance as ‘probably impaired’.
However, as approximately 15% of individuals who are considered ‘intact’ are expected
to obtain scores greater than 1.0 SD below normative means and therefore, potentially
resulting in too many false positives, a change in performance of 2.0 SD’s is considered
to be a clear indication of impairment (Lezak, 2004). Therefore, the current study aimed
to examine performances between groups across the different impairment criteria of 1.0,
1.5 and 2.0 SD’s below the normative mean in order to better our understanding of the
nature of any impairment that may be shown. Table 3 displays the chi-square results for
proportional differences in each group that demonstrated neurocognitive performance
across the different impairment ranges (i.e., greater than 1.0, 1.5 and 2.0 standard
deviations below normative mean criteria) for each of the six executive function
measures where significant differences were demonstrated in the previous analyses. It
should be noted that as Zoo Map recall, Rule Shift Trial 4, the Rule Shift Trial 2 and 4
combined score, and the event-based prospective memory task, did not have normative
mean criteria available they were therefore not included in the table. The Iowa
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 94
Table 3. The proportion (n) of each sample group and effect size of proportional differences in executive function performance across each of the impairment level categories Executive
Function
Performance
Mean
(SD) Proportion of sample >
-1.0 SD’s below
normative mean
n
Proportion of sample
between
-1.0 to -1.49 SD’s below
normative mean
n
Proportion of sample
between
-1.5 to -1.99 SD’s below
normative mean
n
Proportion of sample <
-2.0 SD’s below
normative mean
n
Effect Size of difference
between frequencies
within each group
w
SCH/C
(n= 28)
MC
(n= 28)
SCH/C
(n= 28)
MC
(n= 28)
SCH/C
(n= 28)
MC
(n= 28)
SCH/C
(n= 28)
MC
(n= 28)
SCH/C
(n= 28)
MC
(n= 28)
SCH/C
(n= 28)
MC
(n= 28)
TOB
Zoo Map .05 (1.14) .94 (.74) 23 27 2 1 0 0 3 0 1.04** .929**
RM
Digit Span -.52 (.64) .40 (.73) 19 27 7 1 1 0 1 0 1.05** .929**
IC
Symbol
Search
-.67 (-.87) .38 (.85) 16 26 8 2 3 0 1 0 .827** .857**
TMT Trial 4 -.74 (.91) .43 (.53) 14 27 8 1 3 0 3 0 .647** .929**
IPP
Rule Shift
Trial 2
-.26 (.56) .52 (.24) 21 28 0 0 0 0 7 0 .500**
CW Trial 3 -.81 (1.29) .52 (.52) 16 28 6 0 0 0 6 0 .505**
Note.**p< .01
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 95
Gambling Task will then be discussed due to impairment criteria for this measure
involving a cut-off score instead of normative means and standard deviations.
Symbol Search and Trail Making Test Trial 4 had the highest proportion of
participants that performed at greater than 1.0 below the normative mean. A chi-square
test for goodness of fit (with α = .01) was used to assess whether certain levels of
performance (according to the impairment level criteria of 1.0, 1.5 and 2.0 SD’s below
the normative mean) were more common than others in each sample group. The chi-
square test was statistically significant on all measures for the schizophrenia group with
cannabis use (p < .001), indicating that some impairment criteria levels were achieved
with significantly greater frequencies than others. Results indicate that a significantly
larger proportion of schizophrenia patients with cannabis use demonstrated performance
on all of the 6 measures above the impairment level criteria of greater than 1.0, 1.5 and
2.0 standard deviations below normative mean than patients with cannabis use falling
below these criteria. In other words, a larger proportion of schizophrenia patients with
cannabise use demonstrated performances which were classified in the normal range on
these executive function measures than those which were deemed to be at the probably
impaired or clinically impaired range. These findings suggest that a statistically
significant larger proportion of patients with schizophrenia with cannabis use did not
perform at a level that was considered to be even ‘probably impaired’, let alone fall into
the clinically impaired category and this was significantly different than chance (p <
.001). Similar results were found for matched-controls (p < .001) indicating a
significantly larger proportion of the group demonstrating performances in the normal
range. An examination of the index of effect size Cohen’s w ranged from 0.50 to 1.05,
which can be considered large. These results suggest that the previously reported
significant results showing significant differences between groups is more reflective of
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 96
‘poorer’ performance by the schizophrenia group, but does not reflect ‘impaired’
performance relative to normative criteria.
Adjusted mean comparisons also reveal that neither group performed below the
cut-off score for impaired performance on the Iowa Gambling Task (i.e., net score <10)
(Bechara et al., 2001). This result also suggests that neither the schizophrenia group
with cannabis use (M = 15.23), nor the matched-controls group (M = 13.57), in the
current study demonstrated impairment on this measure according to this criterion.
The Effect of Cannabis-Use Status on the Recovery of Executive Function
As another aim was to investigate whether continued/current cannabis use is
associated with the non-recovery of executive function, the groups were partitioned into
current cannabis users and abstinent cannabis users based on self-reported current use at
the time of testing. In order to examine this relationship, a series of two-way
ANCOVA’s were conducted to determine the effect of Group and cannabis-use status
on executive function, when controlling for the effects of duration of use. Once again,
the results will be grouped according to the executive function domains outlined in
West’s (1996) model and the additionally included measures of executive function will
then be reported.
The ANCOVA assumption of normality was supported via the examination of
Shapiro-Wilk statistics and histograms for each group and cannabis-use status
categories. Scatterplots indicated that the relationship between the covariate (duration of
use) and the dependent variables (executive function performance) was linear. Finally
the assumptions of homogeneity of regression slopes and homogeneity of variances
were supported by the independence of the covariate and both of the independent
variables. Table 4 outlines the ANCOVA results of executive function performance for
each group, with duration of use as a covariate. No statistically significant interactions
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 97
Table 4. Results of ANCOVAs of executive function performance for Group and cannabis-use status with covariate of duration of use
Cognitive
performance
Adjusted Mean
Effects
Group Cannabis-Use Status Group Cannabis-Use Status Duration of Use
SCH/C
n=28
MC
n=28
Current
Users
n=22
Non-Current
Users
n=34
F p ηp2 F p ηp
2 F p ηp
2
TOB
Zoo Map .021 .958 .461 .518 112.23 .001** .193 .043 .836 .001 .826 .368 .016
Modified Six Elements -.038 .339 .135 .165 3.15 .082 .058 .019 .892 .000 .108 .743 .002
RM
Zoo Map Recall 4.72 6.40 5.44 5.68 21.98 <.001** .301 .421 .519 .008 .039 .845 .001
Digit Span -.508 .429 .053 -.133 24.33 <.001** .323 .909 .345 .018 .687 .411 .013
IC
Symbol Search -.674 .378 -.172 -.124 18.75 <.001** .269 .038 .846 .001 .028 .869 .001
TMT Trial 4 -.747 .449 -.122 -.176 32.71 .001** .391 .065 .800 .001 .000 .989 .000
TMT Trial 4 Contrast .024 -.024 -.055 .055 .047 .829 .001 .227 .636 .004 1.30 .260 .025
IPP
RS Trial 2 -.298 .527 .044 .184 8.09 .006** .137 .220 .641 .004 .013 .909 .000
RS Trial 4 2.06 3.36 2.96 2.47 23.80 <.001** .318 3.18 .080 .059 1.60 .212 .030
RS Trial 2 and 4 5.39 7.33 6.55 6.17 26.50 <.001** .342 .96 .332 .018 .92 .341 .018
CW Trial 3 Contrast .430 .778 .478 .730 2.65 .110 .049 1.32 .256 .025 1.56 .218 .030
PM
Event-based task 3.03 3.57 3.37 3.24 1.77 .189 .034 .100 .754 .002 .594 .445 .012
Verbal Fluency
Letter Fluency -.495 -.146 -.039 -.602 2.05 .159 .039 5.05 .029* .090 .036 .850 .001
Emotion-based decision
making
Iowa Gambling Task 15.54 13.46 14.96 14.03 .091 .756 .002 .017 .896 .000 .292 .591 .006
Note.*p< .05, **p< .01
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 98
between the independent variables and executive function performance were found after
adjustment for duration of use. Duration of use was not found to have an impact on
performance for the majority of the executive function measures. However, a significant
interaction between the covariate of duration of use and the independent variables of
Group and cannabis-use status on Color-Word Trial 3 was found, F(6, 49) = 3.01, p
=.039, ηp2 = .156. Therefore, the assumption of the homogeneity of regression slopes
was violated in this instance and an ANCOVA was not conducted. Inspection of the
regression slopes revealed a poor association between duration of use and Color-Word
Trial 3 scores across the test groups (i.e., schizophrenia patients with cannabis use and
matched-control subjects with a similar cannabis use history). However, for the
cannabis-use status groups, an association between duration of use and Color-Word
Trial 3 scores was found. As a general pattern, the regression slopes indicated that in the
current-user group, a longer duration of use was associated with higher Color-Word
Trial 3 scores indicating better performance on this measure due to the conversion to z-
scores using normative criteria. Contrastingly, in the non-current user group, longer
duration of use was found to be associated with lower Color-Word Trial 3 scores
indicating poorer performance.
A two-way ANOVA revealed a significant main effect of Group on Color-Word
Trial 3, F(3, 52) = 26.86, p <.001, ηp2 = .341. Main effects of cannabis-use status, F(3,
52) = 3.41, p = .071, ηp2= .062 and the interaction, F(3, 52) = .446, p = .512, ηp
2= .008
were not significant. Comparison of means indicated that the schizophrenia group with
cannabis use (M = -.81) had significantly lower scores on Color-Word Trial 3, than
matched-controls (M = .53). Although, current users (M = -.44) and non-current users
(M = .05) showed similar performance on this measure.
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 99
An examination of the adjusted means for the other executive function domains
indicated that schizophrenia patients with cannabis use had significantly lower scores on
the Temporal Organisation of Behaviour (except on the Modified Six Elements
measure), Retrospective Memory, Interference Control (except for the Trail Making
Test Trial 4 Contrast score), and the Inhibition of Prepotent Responses (except for the
Color-Word Trial 3 Contrast score), relative to controls. Adjusted means comparisons
also revealed no significant differences between schizophrenia patients with cannabis
use and matched-control subjects with a similar cannabis use history on the event-based
Prospective Memory, Verbal Fluency and Emotion-based Decision-making tasks.
Further, results showed no significant differences in performance on all of the executive
function domains between current users and non-current users, except for in the domain
of verbal fluency. Findings revealed that current users (M = -.04) had higher scores on
Letter Fluency than former users (M = -.60), suggesting that continued cannabis use is
associated with better performance on Letter Fluency. The effect size of this result is
considered moderate. Holm-Bonferroni correction was applied to adjust for multiple
comparisons, with all results remaining statistically significant following correction.
The Effect of Length of Abstinence on the Recovery of Executive Function
An additional aim of the study was to investigate if a longer period of abstinence
is associated with the recovery of executive function. In order to examine this
relationship, a series of two-way ANCOVA’s were conducted to determine the effect of
Group and time since cessation (i.e., abstinence for less than 5 years and abstinence for
equal to, or greater than, 5 years) on executive function, when controlling for duration
of use. For this analysis, non-current users were divided into two groups using a median
split. Following this, 16 subjects were in the less than 5 years abstinence group and 18
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 100
subjects were in the equal to, or greater than, 5 years abstinence group. The following
analyses were only conducted on the sample of non-current cannabis users (n = 34).
Examination of Shapiro-Wilk statistics and histograms for each group and time since
cessation categories indicated the ANCOVA assumption of normality was supported.
Scatterplots indicated that the relationship between the covariate (duration of use) and
the dependent variables (performance on the individual executive function measures)
was linear. Finally the assumptions of homogeneity of regression slopes and
homogeneity of variances were supported by the absence of a significant IV-by-
covariate interaction. Table 5 outlines the ANCOVA results of executive function
performance for Group and time since cessation categories amongst the non-current
user sample, with duration of use as a covariate. There were no statistically significant
interactions found between either group, or time since cessation categories, and
executive function performance after adjustment for duration of use. Duration of use
was not found to have an impact on performance for the majority of the executive
function measures, except for Color-Word Trial 3. However, the main effect of group
on Color-Word Trial 3 remained significant after accounting for the effects of duration
of use. Comparison of adjusted means show that schizophrenia patients with cannabis
use had significantly lower scores on Retrospective Memory, Interference Control
(except for the Trail Making Test Trial 4 Contrast score), and the Inhibition of Prepotent
Responses domains, relative to controls (except for Rule Shift Trial 2 and the Color-
Word Trial 3 Contrast score). Adjusted means comparisons also revealed no significant
differences between schizophrenia patients with cannabis use and matched-control
subjects with a similar cannabis use history on the Temporal Organisation of Behaviour
(on the Modified Six Elements measure only), Prospective Memory, Verbal Fluency
and Emotion-based Decision-making domains. Post hoc analyses using the Holm-
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 101
Table 5. Results of ANCOVAs of executive function performance for Group and time since cessation categories with covariate of duration of use
Cognitive
performance
Adjusted Mean
Effects
Group Time Since Cessation Group Time Since Cessation Duration of Use
SCH+C
n=28
C CTRL
n=28
< 5 years
n=16
≥5 years
n=18
F p ηp2 F p ηp
2 F p ηp
2
TOB
Zoo Map .210 .830 .920 .110 3.17 .086 .098 4.57 .041* .136 .116 .736 .004
Modified Six Elements .014 .311 .290 .034 1.25 .273 .041 .767 .388 .026 .619 .438 .021
RM
Zoo Map Recall 5.08 6.30 5.98 5.40 6.86 .014*† .191 1.29 .266 .043 1.10 .302 .037
Digit Span -.664 .345 -.244 -.075 14.35 .001* .331 .337 .566 .011 2.72 .110 .086
IC
Symbol Search -.584 .373 -.126 .085 9.11 .005** .239 .014 .906 .000 2.14 .154 .069
TMT Trial 4 -.677 .338 -.143 -.196 11.43 .002** .283 .027 .872 .001 .644 .429 .022
TMT Trial 4 Contrast .111 .024 .056 .079 .078 .782 .003 .004 .947 .000 .266 .610 .009
IPP
RS Trial 2 -.085 .480 .308 .086 3.44 .074 .106 .441 .512 .015 .043 .837 .001
RS Trial 4 1.76 3.17 2.24 2.70 12.95 .001** .309 1.16 .290 .039 .098 .756 .003
RS Trial 2 and 4 5.26 7.11 6.04 6.33 14.37 .001** .331 .295 .591 .010 .124 .727 .004
CW Trial 3 -.484 .606 .050 .067 17.16 <.001** .372 .003 .956 .000 6.55 .016* .184
CW Trial 3 Contrast .722 .728 .671 .779 .001 .976 .000 .185 .670 .006 2.79 .105 .088
PM
Event-based task 3.12 3.43 3.34 3.20 .377 .544 .013 .065 .801 .002 .132 .719 .005
Verbal Fluency
Letter Fluency -.821 -.367 -.427 -.762 1.72 .200 .056 .776 .386 .026 .002 .964 .000
Emotion-based decision
making
Iowa Gambling Task 14.00 14.96 11.07 17.90 .009 .923 .000 .403 .531 .014 .006 .938 .000
Note.*p< .05, **p< .01, †non sig. following post hoc analyses
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 102
Bonferroni correction method revealed that significant differences no longer remained
between groups on Zoo Map recall performance after adjusting the alpha level for
multiple comparisons. Further, results showed no significant differences in performance
on all of the executive function domains between non-current users who were abstinent
for less than 5 years and non-current users who were abstinent for equal to, or greater
than, 5 years, except for in the Temporal Organization of Behaviour domain (on the Zoo
Map measure only). Findings revealed that non-current users who were abstinent for
less than 5 years (M = .92) had higher scores on Zoo Map than non-current users who
were abstinent for equal to, or greater than, 5 years (M = .11), suggesting that shorter
abstinence periods (i.e., less than 5 years) was associated with better performance on
this measure. The effect size of this result is considered moderate.
The Validity of West’s Theoretical Model of Executive Function
Principal Components Analysis. Principal Components Analysis (PCA) was
conducted to determine what, if any, factor structures underlie the following executive
function measures: Zoo Map, Zoo Map recall, Digit Span, Symbol Search, Trail Making
Test Trial 4, Rule Shift Trial 2, Color-Word Trial 3, the event-based prospective
memory task, Letter Fluency, and the Iowa Gambling Task. Rule Shift Trial 4 and the
Rule Shift Trial 2 and 4 combined score were not included in the analyses as they
involved modified versions of the original measures. The Trail Making Test Trial 4
Contrast score and Color-Word Trial 3 Contrast score were also not included in PCA as
the Trail Making Test Trial 4 and Color-Word Trial 3 appeared to be more successful in
detecting differences between groups. This was considered an important concern when
attempting to determine any underlying factors amongst the executive function
measures administered which may serve to be more sensitive and thus, reflective of the
fundamental construct in question. The purpose of the PCA was exploratory in nature.
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 103
This type of analysis was conducted in order to determine which executive function
measures were related to each other in a theoretically consistent way.
Criteria to determine the best factor model included: (a) exclusion of measures
or items exhibiting Kaiser’s measurement of sampling adequacy (MSA), Kaiser-Meyer-
Olkin (KMO) or communality values < .5; (b) Bartlett’s test of sphericity exhibiting
significance (p < .0001); (c) retention of factors with eigenvalues > 1 that were
supported by scree plot analysis and clinical/theoretical plausibility of factors; (d) factor
loadings ≥ .4 for interpretation of factors (explaining ≥ 16% of total variance); and (e)
stability of factor solutions across analyses employing oblique and orthogonal rotations
(Field, 2009; Tabachnick & Fidell, 2007).
PCA was conducted utilising both orthogonal (varimax) and oblique (oblimin)
rotations. During initial analysis, one item (the Iowa Gambling Task) failed to meet the
minimum criterion of .5 for Kaiser’s MSA and was excluded in further analyses.
Subsequently, PCA of the remaining 10 executive function measures yielded near-
identical factor solutions across orthogonal (varimax) and oblique (oblimin) rotations.
However, results of the former are reported only as varimax rotation provided the best
defined factor structure (see Table 6). A three-factor solution explaining 61.57% of the
total variance in executive function was extracted. Of this variance, 40.26% was
accounted for by factor 1 (‘Mental Control and Self-Regulation’; 8 variables), 11.22%
by factor 2 (‘Voluntary Response Generation and Disinhibition’; 2 variables), and
10.09% by factor 3 (‘Maintenance of Intention for Future Action’; 2 variables).
The outcomes of the PCA indicate that the executive function measures which
loaded onto factor 1 include mostly those tests which were guided by West’s (1996)
theoretical model of executive function. However, verbal fluency (which was included
as an additional executive function component for exploration and did not appear to be
104
Table 6. Factor loadings based on a Principal Components Analysis utilising orthogonal (varimax)
rotation of 10 executive function measures (N=56)
Loading
Theoretical
Construct
Item Measure Factor 1 Factor 2 Factor 3
Temporal
Organisation of
Behaviour
Zoo Map .576
Modified Six
Elements .513
Retrospective
Memory
Zoo Map Recall .646
Digit Span .692
Interference
Control
Symbol Search .703
Trail Making Test
Trial 4 .798
Inhibition of
Prepotent
Responses
Rule Shift Trial 2 .707 -.473
Color-Word Trial
3 .637 .405
Prospective
Memory
Event-based task .903
Verbal Fluency Letter Fluency .847
Note. Factor loadings less than the absolute value of ± .4 are omitted
included in West’s model), appeared to load onto a separate factor relative to the
remaining measures (i.e., factor 2). Taken together with the negative loading of Rule
Shift Trial 2 onto factor 2, this may indicate that the underlying structure of this
component involves response generation/fluency abilities or cognitive responses that
require disinhibition. Moreover, the positive loadings of both Color-Word Trial 3 and
the event-based prospective memory onto factor 3 potentially reflect an underlying
structure involving cognitive responses that require the ability to remember an intention
for future action. Finally, the Iowa Gambling Task (which did not meet criterion for
PCA and was excluded in the subsequent analysis) may not have been sensitive enough
to sufficiently tap into the executive function constructs of interest, albeit in this
particular sample.
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 105
While the sample size in the current study is notably low, it has been suggested
by Stevens (1986; 2002) that as low as 5 subjects per variable is the minimum needed in
Principle Components Analysis based on Gorsuch’s (1983) guidelines. The current
study’s sample size is just over 50 with the combined sample and is therefore just above
the minimum requirements in the recommended guidelines. In addition,
recommendations arising from a study by Guadagnoli and Velicer (1988) outline that
‘components with four or more loadings above .60 in absolute value are reliable,
regardless of sample size’ (cited in Stevens, 2002, p. 395). As the results indicate 6
loadings above .60, it can be considered that at least factor 1 is a reliable component.
Another recommendation arising from Guadagnoli and Velicer’s (1988) study is that
‘components with only a few low loadings should not be interpreted unless sample size
is at a least 300’ (Stevens, 2002, p. 395). It is outlined that a low loading would include
those .40 and below. However, Stevens (2002) suggested that a factor which was
defined by only a few loadings was not considered as much of a factor and instead, the
factor could be considered as variable specific. Therefore, as factor 2 included loadings
of only two variables (Rule Shift Trial 2 at -.473 and Letter Fluency at .847), and factor
3 including loadings of only two variables (Color-Word Trial 3 at .405 and Prospective
Memory at .903) this could indeed represent some of those cases.
Overall, these results are in partial support of the construct validity of the
selected executive function measures (which were those guided by West’s model). That
is, the measures which loaded on factor 1 seemed to adequately tap into a single
theoretical neurocognitive construct, namely executive function. However, the results
also suggest that the while the measure of Letter Fluency may assess executive function
abilities, it may also rely upon other cognitive abilities that are separate, or independent,
to executive function alone. In addition, the event-based prospective memory measure
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 106
utilised may not have been sensitive enough to detect the true construct of prospective
memory abilities. In alternative, the prospective memory measure used may have tapped
into other cognitive processes that fall outside of the executive function domain.
Part B
The Relationship Between Executive Function and Functional Outcome
The predictive ability of executive function measures in predicting
functional outcome ratings. The investigation into the relationship between the
executive function domains belonging to West’s (1996) model and functional outcome
is exploratory in nature. However, due to reports of an association between executive
function and functional outcome in the literature, it was hypothesized that executive
function performance would be significantly related to functional outcome. More
specifically, scores on executive function measures would have significant predictive
ability in relation to functional outcome ratings (as measured by the Independent Living
Skills Inventory (ILSI)). The purpose of the following analyses is to assist in identifying
a measure (or measures) of executive functioning that significantly predicts a specific
domain of functional outcome in schizophrenia patients (e.g., if measures of prospective
memory are shown to predict a given functional outcome domain then cognitive
remediation specific to enhancing prospective memory abilities could be utilised to
ultimately target improvements in that functional outcome domain). From this
perspective, the goal of the analyses (and in turn the consequent results) is to assist in
identifying whether there is potential for improvements to specific cognitive abilities
which may in turn affect associated functional outcomes.
In order to identify the predictors that would be considered important to enter
into regression analyses, Pearson correlation analyses between each of the individual
executive function measures and each of the ILSI sub-scale ratings, as well as the Total
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 107
ILSI score, were first computed. Correlations were calculated for the schizophrenia
group with cannabis use, the matched-control group with a similar cannabis use history,
and the overall combined sample. Table 7, 8 and 9 display the correlation matrices.
A series of multiple regression and hierarchical multiple regression analyses
(divided into three stages) were conducted for each functional outcome sub-domain, as
well as for overall functional outcome. Only the schizophrenia group with cannabis use
were used the analyses (n = 28). In the first stage of regression analysis, the measure(s)
showing significant correlations in the schizophrenia group with cannabis use were
entered as predictors into a multiple regression analysis. For stage 2, at step 1 these
same predictors are entered simultaneously. At step 2, only the predictor from Stage 1
showing a significant slope was then entered. At step 3, this same predictor as well as
those executive function measures showing significant correlations in the matched
control group (not including those already entered), were then entered as predictors. At
stage 3, all of the previous steps were used again. However, in step 4 only the
predictor(s) from previous steps showing a significant slope were entered. In the final
step, these same significant predictors in addition to those measures showing significant
correlations in the combined sample (not including those already entered previously),
were entered as predictors. Before interpreting the results of the regression analyses, a
number of assumptions were tested and checks were performed. Firstly, an inspection of
the normal probability plot of standardised residuals and the scatterplot of standardised
residuals against standardised predicted values indicated that the assumptions of
normality, linearity and homoscedasticity of residuals were met. In addition,
Mahalanobis distance did not exceed the critical χ2
for degrees of freedom for any cases
in the data, indicating that multivariate outliers were not of concern. Finally, relatively
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 108
Table 7. Correlations between executive function measures and ILSI sub-scales and total ILSI score for
the schizophrenia group
Executive Function Measures
Independent
Living Skills
Zoo
Map
Zoo
Map
Recall
Digit
Span
Symbol
Search
Trail
Making
Test
Trial 4
Rule
Shift
Trial 2
Rule
Shift
Trial 4
Rule Shift
Trial 2 and
4
Combined
Color-
Word
Trial 3
Personal
Management
.223 .220 -.097 .316 .038 -.067 -.209 -.063 .310
Hygiene &
Grooming
.185
.123
.429*
.335
.089
.146
.015
-.084
.353
Clothing
-.051
.108
.278
.118
-.108
-.207
-.120
-.215
.260
Basic Skills
-.083
-.057
.216
.170
.043
-.266
.154
-.276
.145
Interpersonal
skills
.178
.257
.181
.457*
.241
-.135
.138
.007
.361
Home
Maintenance
.160
.274
.083
.251
-.038
-.171
-.137
-.203
.300
Money
Management
-.087
.307
.194
.209
-.107
-.202
-.063
-.173
.162
Cooking
.145
.331
.169
.456*
.119
-.178
.138
-.021
.371
Resource
Utilization
.097
.304
.113
.390*
.199
-.009
-.042
-.035
.288
General
.225
.053
.012
.289
.136
-.172
.105
-.040
.352
Occupational
Skills
Medication
Management
.197
.368
.074
.431*
.252
-.016
.074
.040
.309
Total
.
141
.271
.183
.389*
.096
-.152
.036
-.074
.328
Note. *p<.05, **p<.01
high tolerances indicated that multicollinearity would not interfere with the ability to
interpret the outcome of the regression analyses.
Personal Management (Independent Living Skills Inventory). There were no
significant correlations found in each of the separate groups (i.e., schizophrenia group
with cannabis use and matched-controls group with a similar cannabis use history),
therefore stage 1 and 2 of the regression analyses were not conducted. For stage 3,
results indicate that the overall model which included Zoo Map, Zoo Map recall, Digit
Span, Symbol Search, Trail Making Test Trial 4, Rule Shift Trial 4, the Rule Shift Trial
2 and 4 combined score, and Color-Word Trial 3 as predictors, did not significantly
predict ratings in Personal Management on the Independent Living Skills Inventory in
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 109
Table 8. Correlations between executive function measures and ILSI sub-scales and total ILSI score for
the matched-control group
Executive Function Measures
Independent
Living Skills
Zoo
Map
Zoo
Map
Recall
Digit
Span
Symbol
Search
Trail
Making
Test
Trial 4
Rule
Shift
Trial 2
Rule
Shift
Trial 4
Rule Shift
Trial 2 and
4
Combined
Color-
Word
Trial 3
Personal
Management
-.055 -.115 .287 -.195 -.280 .173 -.027 .016 -.367
Hygiene &
Grooming
.070
.226
.088
-.211
-.233
.556**
.157
.270
-.098
Clothing
.337
.098
.416*
-.095
.140
.694**
.321
.448*
-.260
Basic Skills
-
-
-
-
-
-
-
-
-
Interpersonal
skills
.290
.199
.158
-.176
-.287
.125
.011
.039
-.558**
Home
Maintenance
.213
.110
.049
.168
.226
.662**
.222
.353
-.092
Money
Management
.051
-.148
.387*
-.224
-.238
.306
-.130
-.045
-.322
Cooking
.284
.150
-.062
-.307
-.150
.540**
.188
.294
-.263
Resource
Utilization
.314
.041
.419*
.032
.074
.970**
.306
.499**
-.128
General
Occupational
Skills
-.117
-.105
.199
-.220
-.336
-.037
-.184
-.173
-.307
Medication
Management
.282
-.087
.245
-.231
-.317
.243
-.104
-.036
-.430*
Total
.240
.032
.331
-.233
-.218
.647**
.106
.246
-.414*
Note. *p<.05, **p<.01
schizophrenia patients with cannabis use, R2 = .247, R
2adj = -.070, F(8, 19) = .780, p =
.625.
Hygiene and Grooming (Independent Living Skills Inventory). At stage 1,
Digit Span significantly accounted for 15.3% of the variance in Hygiene and Grooming
ratings on the Independent Living Skills Inventory in schizophrenia patients with
cannabis use, R2 = .184, R
2adj = 153, F(1, 26) = 5.87, p = .023. By Cohen’s (1988)
conventions, an effect of this magnitude can be considered ‘medium’ (f 2
= .23). At
stage 2, Rule Shift Trial 2 was added as a predictor to the regression equation and
accounted for an additional 4.2% of the variance in Hygiene and Grooming ratings.
However, this change was not statistically significant, ∆R2 = .042, ∆F(1, 25) = 1.345, p
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 110
Table 9. Correlations between executive function measures and ILSI sub-scales and total ILSI score for
the overall sample
Executive Function Measures
Independent
Living Skills
Zoo
Map
Zoo
Map
Recall
Digit
Span
Symbol
Search
Trail
Making
Test
Trial 4
Rule
Shift
Trial
2
Rule
Shift
Trial 4
Rule Shift
Trial 2 and
4
Combined
Color-
Word
Trial 3
Personal
Management
.392** .441** .358** .459** .399** .188 .345** .342** .520**
Hygiene &
Grooming
.334*
.350**
.488**
.418**
.347**
.071
.297*
.247
.508**
Clothing
.205
.342**
.451**
.324*
.264*
.034
.235
.184
.451**
Basic Skills
.108
.176
.331*
.301*
.271*
-.086
.122
.042
.325*
Interpersonal
skills
.393**
.489**
.455**
.544**
.519**
.142
.444**
.388**
.562**
Home
Maintenance
.355**
.464**
.370**
.439**
.350**
.090
.260
.231
.514**
Money
Management
.193
.446**
.453**
.369**
.271*
.061
.269*
.222
.396**
Cooking.
.354**
.490**
.380**
.476**
.398**
.084
.409**
.344*
.526**
Resource
Utilization
.355**
.497**
.458**
.517**
.504**
.250
.364**
.388**
.519**
General
Occupational
Skills
.405**
.379**
.391**
.471**
.473**
.119
.425**
.363**
.571**
Medication
Management
.385**
.506**
.384**
.505**
.490**
.203
.370**
.367**
.509**
Total
.369**
.487**
.
.467**
.508**
.448**
.138
.394**
.350**
.547**
Note. *p<.05, **p<.01
= .257. At stage 3, results indicate that the overall model which included Zoo Map, Zoo
Map recall, Symbol Search, Trail Making Test Trial 4, Rule Shift Trial 4, and Color-
Word Trial 3 as additional predictors, accounted for an additional 15.1% of the variance
in Hygiene and Grooming ratings. However, again this change was not statistically
significant, ∆R2 = .151, ∆F(6, 19) = .766, p = .605.
Clothing (Independent Living Skills Inventory). There were no significant
correlations found in the schizophrenia group with cannabis use for Clothing ratings.
Therefore, stage 1 of the regression analyses was not conducted. At stage 2, Digit Span,
Rule Shift Trial 2 and the Rule Shift Trial 2 and 4 combined score were entered as
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 111
predictors to the regression equation. This model was not found to be statistically
significant, R2 = .198, R
2adj = .098, F(3, 24) = 1.975, p = .145. Due to the non-significant
results in the previous stage, at stage 3 only Zoo Map recall, Symbol Search, Trail
Making Test Trial 4, and Color-Word Trial 3 were entered as predictors in the model.
Results indicate that the overall model did not significantly predict the variance in
Clothing ratings, R2 = .125, R
2adj = -.027, F(4, 23) = .819, p = .526.
Basic Skills (Independent Living Skills Inventory). As there were no significant
correlations found in each of the separate groups for Basic Skills ratings, stage 1 and 2
of the regression analyses were not conducted. At stage 3, Digit Span, Symbol Search,
Trail Making Test Trial 4, and Color-Word Trial 3 were entered as predictors. Results
indicate that the overall model did not significantly predict ratings in Basic Skills, R2 =
.068, R2
adj = -.094, F(4, 23) = .418, p = .794.
Interpersonal skills (Independent Living Skills Inventory). At stage 1, Symbol
Search significantly accounted for 17.9% of the variance in Interpersonal Skills ratings
in schizophrenia patients with cannabis use, R2 = .209, R
2adj = .179, F(1, 26) = 6.881, p
= .014. The effect size is considered to be ‘medium’ (f 2
= .26) (Cohen, 1988). At stage
2, Color-Word Trial 3 was added as a predictor to the regression equation and
accounted for an additional 0.9% of the variance in Interpersonal Skills ratings, with
this change being non-significant, ∆R2 = .009, ∆F(1, 25) = 2.83, p = .599. At stage 3,
the overall model which included Zoo Map, Zoo Map recall, Digit Span, Trail Making
Test Trial 4, Rule Shift Trial 4, and the Rule Shift Trial 2 and 4 combined score as
additional predictors, accounted for an additional 0.7% of the variance in Interpersonal
Skills ratings. Once again, this change was not statistically significant, ∆R2 = .074,
∆F(6, 19) = .328, p = .914.
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 112
Home Maintenance (Independent Living Skills Inventory). There were no
significant correlations found in the schizophrenia group with cannabis use, hence stage
1 of the regression analyses was not conducted. At stage 2, Rule Shift Trial 2 was
entered as a predictor into the regression equation. This model was not found to be
statistically significant, R2 = .029, R
2adj = -.008, F(1, 26) = .783, p = .384. Due to this
non-significant result, at stage 3 only Zoo Map, Zoo Map recall, Digit Span, Symbol
Search, Trail Making Test Trial 4, and Color-Word Trial 3 were entered as predictors,
with again no significant results, R2 = .193, R
2adj = -.038, F(6, 21) = .835, p = .557.
Money Management (Independent Living Skills Inventory). No significant
correlations were found in the schizophrenia group with cannabis use. Therefore, stage
1 of the regression analyses was not conducted. At stage 2, Digit Span was entered as a
predictor into the regression equation with the model not being statistically significant,
R2 = .037, R
2adj = .000, F(1, 26) = 1.013, p = .324. Due to this result, at stage 3 only Zoo
Map recall, Symbol Search, Trail Making Test Trial 4, Rule Shift Trial 4, and Color-
Word Trial 3 were entered as predictors. No significant results were found, R2 = .211,
R2
adj = .032, F(5, 22) = 1.179, p = .351.
Cooking (Independent Living Skills Inventory). At stage 1, Symbol Search
significantly accounted for 17.7% of the variance in Cooking ratings in schizophrenia
patients with cannabis use, R2 = .207, R
2adj = .177, F(1, 26) = 6.807, p = .015. The effect
size is ‘medium’ (f 2
= .26) (Cohen, 1988). At stage 2, Rule Shift Trial 2 was added as a
predictor to the regression equation and accounted for an additional 7.3% of the
variance in Cooking ratings with this change not being statistically significant, ∆R2 =
.073, ∆F(1, 25) = 2.502, p = .126. For stage 3, the overall model which included Zoo
Map, Zoo Map recall, Digit Span, Trail Making Test Trial 4, Rule Shift Trial 4, Rule
Shift Trial 2 and 4 combined score, and Color-Word Trial 3 as additional predictors,
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 113
accounted for an additional 13.5% of the variance in Cooking ratings, with no further
significant results found, ∆R2 = .135, ∆F(7, 19) = .557, p = .781.
Resource Utilization (Independent Living Skills Inventory). At stage 1, Symbol
Search significantly accounted for 11.9% of the variance in Resource Utilization ratings
in schizophrenia patients with cannabis use, R2 = .152, R
2adj = .119, F(1, 26) = 4.663, p
= .040, with a ‘medium’ effect size (f 2
= .18) (Cohen, 1988). At stage 2, Digit Span,
Rule Shift Trial 2 and the Rule Shift Trial 2 and 4 combined score were added as
additional predictors to the model and accounted for an additional 10.9% of the variance
in Resource Utilization ratings. However, this change was not significant, ∆R2 = .109,
∆F(3, 23) = 1.133, p = .357. At stage 3, the overall model which included Zoo Map,
Zoo Map recall, Trail Making Test Trial 4, Rule Shift Trial 4, and Color-Word Trial 3
as additional predictors, accounted for an additional 13.2% of the variance in Resource
Utilization ratings with this change being non-significant, ∆R2 = .132, ∆F(5, 21) = .775,
p = .579.
General Occupational Skills (Independent Living Skills Inventory). No
significant correlations were found in each of the separate groups, therefore stage 1 and
2 of the regression analyses were not conducted. At stage 3, Zoo Map, Zoo Map recall,
Digit Span, Symbol Search, Trail Making Test Trial 4, Rule Shift Trial 4, the Rule Shift
Trial 2 and 4 combined score, and Color-Word Trial 3 were entered as predictors.
Results indicate that the overall model did not significantly predict ratings in General
Occupational Skills in schizophrenia patients with cannabis use, R2 = .200, R
2adj = -.136,
F(8, 19) = .595, p = .771.
Medication Management (Independent Living Skills Inventory). At stage 1,
Symbol Search significantly accounted for 15.5% of the variance in Medication
Management ratings in schizophrenia patients with cannabis use, R2 = .186, R
2adj = .155,
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 114
F(1, 26) = 5.945, p = .022, with a ‘medium’ effect size (f 2
= .23) (Cohen, 1988). At
stage 2, Color-Word Trial 3 was added to the equation and accounted for an additional
0.2% of the variance in Medication Management ratings with results not being
statistically significant, ∆R2 = .002, ∆F(1, 25) = .069, p = .795. At stage 3, the overall
model included Zoo Map, Zoo Map recall, Digit Span, Trail Making Test Trial 4, Rule
Shift Trial 4, the Rule Shift Trial 2 and 4 combined score, and Color-Word Trial 3 as
additional predictors and accounted for an additional 9.6% of the variance in
Medication Management ratings. Again, no further significant results were found, ∆R2 =
.096, ∆F(6, 20) = .448, p = .838.
Total ILSI Score (Independent Living Skills Inventory). At stage 1, Symbol
Search significantly accounted for 11.9% of the variance in the Total ILSI Score in
schizophrenia patients with cannabis use, R2 = .151, R
2adj = .119, F(1, 26) = 4.630, p =
.041. The effect size is considered ‘medium’ (f 2
= .18) (Cohen, 1988). At stage 2, Rule
Shift Trial 2 and Color-Word Trial 3 were included as additional predictors to the
regression equation and accounted for an additional 6.2% of the variance in the Total
ILSI Score with the results however, being non-significant, ∆R2 = .062, ∆F(2, 24) =
.938, p = .405. At stage 3, the overall model included Zoo Map, Zoo Map recall, Digit
Span, Trail Making Test Trial 4, Rule Shift Trial 4, and the Rule Shift Trial 2 and 4
combined score as additional predictors and accounted for an additional 12.3% of the
variance in the Total ILSI Score. However, again this change was not statistically
significant, ∆R2 = .123, ∆F(6, 20) = .566, p = .753.
Following post hoc Holm-Bonferroni correction, the abovementioned significant
results were found to no longer remain. However, considering the small sample size and
extent to the nature of the analyses performed, it is acknowledged that the current study
was likely underpowered in order to detect more significant results. Therefore, the
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 115
significant results prior to correction will still be reported and further considered in the
following chapter.
Table 10 displays a summary of significant regression statistics for the
individual ILSI sub-scale models and total ILSI score model.
Moderation effects on executive function measures for functional outcome.
The results outlined in Table 7 & Table 8 indicate that the relationships between
executive function test performance and functional outcome differ across groups and
hence suggestive of possible moderation effects on 8 of the functional outcome sub-
domains, namely: Hygiene & Grooming, Clothing, Interpersonal Skills, Home
Maintenance, Money Management, Cooking, Resource Utilization, Medication
Management, as well as on the Total ILSI Score. Therefore, additional regression
analyses were conducted in order to further explore these potential moderation
relationships. In this phase of regression analyses, 3 variables were entered into the
regression equation in order to examine possible moderation effects. Firstly, a new
variable was computed which calculated the interaction between Group (schizophrenia
patients with cannabis use and matched-controls with a similar cannabis use history)
and the executive function measure of interest. This variable was then entered, in
addition to the executive function measure of interest, as well as the Group variable.
Once again, only those executive function measures which showed a significant
correlation with the functional outcome sub-scale ratings (from the Pearson correlation
analyses), or the Total ILSI score, in each of the separate groups (i.e., schizophrenia
group with cannabis use and matched-controls group with a similar cannabis use
history) were entered into the regression analyses. Regression analyses in this phase
were conducted using the overall combined sample (N = 56). Once again, before
interpreting the results of the regression analyses, assumption tests and checks were
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 116
Table 10. Regression models for predicting ILSI ratings
Outcome
Variable
Predictor
Variable
B Standard
error
Beta
(β)
Effect size
(f 2) p-value
Hygiene &
Grooming
Digit Span .150 .062 .429 .23 .023*
Interpersonal
Skills
Symbol Search .123 .047 .457 .26 .014*
Cooking Symbol Search .141 .054 .456 .26 .015*
Resource
Utilization
Symbol Search .110 .051 .390 .18 .040*
Medication
Management
Symbol Search .156 .064 .431 .23 .022*
Total ILSI
Score
Symbol Search .098 .045 .389 .18 .041*
Note. *p<.05, **p<.01
performed with results indicating assumptions of normality, linearity and
homoscedasticity of residuals were met. Further, multivariate outliers and
multicollinearity were not found to be an issue.
Hygiene and Grooming (Independent Living Skills Inventory). Results indicate
that the overall model which included Digit Span significantly accounted for 34.3% of
the variance in Hygiene and Grooming ratings on the Independent Living Skills
Inventory in the combined sample, R2 = .379, R
2adj = .343, F(3, 52) = 10.591, p < .001.
Digit Span (B = .297, β = 1.33, p =.004) and Group (B = 1.76, β = 1.61, p = .004) were
found to have a significant impact on Hygiene and Grooming ratings at an individual
level. The Group*Digit Span variable was also found to have a significant impact on the
outcome variable (B = -.146, β = -2.07, p = .017), suggesting that a moderation effect
between groups on Digit Span was present for Hygiene and Grooming ratings. Figure 1
displays the moderation effect between groups on Digit Span for Hygiene and
Grooming ratings. The difference in slopes between groups shown in Figure 1 indicate
that Digit Span has more predictive power for the schizophrenia group, than it does for
the matched-control group in relation to predicting Hygiene and Grooming ratings.
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 117
Figure 1. Moderation effect between groups on Digit Span for Hygiene and Grooming ratings
The overall model which included Rule Shift Trial 2 as the executive function
measure of interest, significantly accounted for 22.2% of the variance in Hygiene and
Grooming ratings in the combined sample, R2 = .264, R
2adj = .222, F(3, 52) = 6.218, p =
.001. However at an individual level, Rule Shift Trial 2 (B = -.481, β = -.748, p = .362),
Group (B = -.967, β = -.884, p = .629), and the Group*Rule Shift Trial 2 variable (B =
.393, β = 1.748, p = .438) were not found to have a significant impact on Hygiene and
Grooming ratings.
Clothing (Independent Living Skills Inventory). The overall model which
included Digit Span significantly accounted for 28.2% of the variance in Clothing
ratings in the combined sample, R2 = .321, R
2adj = .282, F(3, 52) = 8.201, p < .001.
However, results additionally indicate that Digit Span (B = .155, β = .781, p = .094),
Group (B = 1.04, β = .553, p = .066), and the Group*Digit Span variable (B = -.069, β =
-1.101, p = .217) were not found to have any individual significant impact on Clothing
ratings.
Results also indicate that the model which included Rule Shift Trial 2,
significantly accounted for 26.1% of the variance in Clothing ratings in the combined
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 118
sample, R2 = .301, R
2adj = .261, F(3, 52) = 7.467, p < .001. No further significant results
were found at an individual level for Rule Shift Trial 2 (B = -.537, β = .455, p = .244),
Group (B = -1.127, β = -1.153, p = .518), and the Group*Rule Shift Trial 2 variable (B =
.427, β = 2.126, p = .333).
The model including the Rule Shift Trial 2 and 4 combined score significantly
accounted for 25.9% of the variance in Clothing ratings in the combined sample, R2 =
.299, R2
adj = .259, F(3, 52) = 7.403, p < .001. At the individual level, no additional
significant results were found for Rule Shift Trial 2 and 4 combined score (B = -.195, β
= -.658, p = .162), Group (B = -.242, β = -.248, p = .759), and the Group*Rule Shift
Trial 2 and 4 combined score variable (B = .121, β = 1.271, p = .280).
Interpersonal Skills (Independent Living Skills Inventory). By including
Symbol Search as a predictor, the model significantly accounted for 52.1% of the
variance in Interpersonal Skills ratings in the combined sample, R2 = .547, R
2adj = .521,
F(3, 52) = 20.939, p < .001. In addition, Symbol Search (B = .252, β = 1.152, p = .002),
Group (B = 1.913, β = 1.461, p < .001) and the Group*Symbol Search variable (B = -
.129, β = -1.617, p = .010) were found to each have a significant impact on the outcome
variable. The significant results of the interaction term suggest a moderation effect
between groups on Symbol Search for Interpersonal Skills ratings (see Figure 2). Figure
2 displays the difference in the regression slopes between groups which suggest that
Symbol Search had better predictive ability for Interpersonal Skills ratings for the
schizophrenia group, than for matched-controls.
The model which included Color-Word Trial 3, significantly accounted for
47.7% of the variance in Interpersonal Skills ratings in the combined sample, R2 = .506,
R2
adj = .477, F(3, 52) = 17.731, p < .001. Significant individual results for Color-Word
Trial 3 (B = .165, β = .887, p = .034) and Group (B = 1.741, β = 1.330, p = .019), were
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 119
Figure 2. Moderation effect between groups on Symbol Search for Interpersonal Skills ratings
also found. However, there were no further significant results for the Group*Color-
Word Trial 3 variable (B = -.099, β = -1.289, p = .127).
Home Maintenance (Independent Living Skills Inventory). Including Rule
Shift Trial 2 as a predictor resulted in a model which significantly accounted for 32.5%
of the variance in Home Maintenance ratings in the combined sample, R2 = .362, R
2adj =
.325, F(3, 52) = 9.843, p < .001. At the individual level, no further significant results
were found for Rule Shift Trial 2 (B = -.639, β = -.693, p = .364), Group (B = -.988, β =
-.630, p = .711), and the Group*Rule Shift Trial 2 variable (B = .501, β = -1.552, p =
.459).
Money Management (Independent Living Skills Inventory). Results indicate
that the model including Digit Span, significantly accounted for 30.7% of the variance
in Money Management ratings in the combined sample, R2 = .344, R
2adj = .307, F(3, 52)
= 9.103, p < .001. However, no significant individual results were found for Digit Span
(B = .126, β = .435, p = .338), Group (B = 1.060, β = .744, p = .187), and the
Group*Digit Span variable (B = -.044, β = -.476, p = .585).
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 120
Cooking (Independent Living Skills Inventory). The model including Symbol
Search as a predictor, significantly accounted for 41.4% of the variance in Cooking
ratings in the combined sample, R2 = .446, R
2adj = .414, F(3, 52) = 13.972, p < .001.
Results additionally indicated that Symbol Search (B = .308, β = 1.311, p = .001),
Group (B = 2.198, β = 1.567, p < .001), and the Group*Symbol Search variable (B = -
.167, β = -1.948, p = .005) were found to have a statistically significant impact on the
outcome variable at an individual level. The significant result of the interaction term
suggests that a moderation effect between groups on Symbol Search is present for
Cooking ratings (see Figure 3). Figure 3 highlights the differences in regression slope
between groups showing that Symbol Search had better predictive ability for Cooking
ratings in the schizophrenia group, than for the matched-control group.
The model including Rule Shift Trial 2, significantly accounted for 30.4% of the
variance in Cooking ratings in the combined sample, R2 = .342, R
2adj = .304, F(3, 52) =
9.013, p < .001. However, no further significant results were found for Rule Shift Trial
2 (B = -.879, β = -1.066, p = .172), Group (B = -2.119, β = -1.510, p = .384), and the
Group*Rule Shift Trial 2 variable (B = .750, β = 2.600, p = .224), at an individual level.
Resource Utilization (Independent Living Skills Inventory). The overall model
including Symbol Search as a predictor, significantly accounted for 46.3% of the
variance in Resource Utilization ratings in the combined sample, R2 = .492, R
2adj = .463,
F(3, 52) = 16.818, p < .001. In addition, Symbol Search (B = .218, β = .941, p = .013)
and Group (B = 1.738, β = 1.256, p = .002) were each found to have a statistically
significant impact on Interpersonal Skills ratings. However, following Holm-Bonferroni
adjustment the result related to the individual impact of Symbol Search no longer
remained significant. The impact of the Group*Symbol Search variable on the outcome
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 121
Figure 3. Moderation effect between groups on Symbol Search for Cooking Ratings
variable was found to approach significance (B = -.108, β = -1.274, p = .051),
suggesting a trend towards a moderation effect between groups on Symbol Search for
Resource Utilization ratings (see Figure 4). The regression line differences seen in
Figure 4 illustrate the higher predictive ability of Symbol Search for Resource
Utilization ratings in the schizophrenia group, than in the matched-control group.
By including Digit Span into the regression equation, the model significantly
accounted for 38.8% of the variance in Resource Utilization ratings in the combined
sample, R2 = .422, R
2adj = .388, F(3, 52) = 12.647, p < .001. No individual significant
results were found for Digit Span (B = .050, β = .178, p = .675), Group (B = .836, β =
.604, p = .253), and the Group*Digit Span variable (B = -.007, β = -.073, p = .929).
The model including Rule Shift Trial 2, significantly accounted for 41.1% of the
variance in Resource Utilization ratings in the combined sample, R2 = .443, R
2adj = .411,
F(3, 52) = 13.813, p < .001. However, additional results were not significant for Rule
Shift Trial 2 (B = -1.003, β = -1.234, p = .087), Group (B = -3.065, β = -2.215, p =
.167), and the Group*Rule Shift Trial 2 variable (B = .997, β = 3.503, p = .077), at the
individual level.
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 122
Figure 4. Moderation effect between groups on Symbol Search for Resource Utilization ratings
Including the Rule Shift Trial 2 and 4 combined score in the regression equation,
the model significantly accounted for 38.5% of the variance in Resource Utilization
ratings in the combined sample, R2 = .418, R
2adj = .385, F(3, 52) = 12.456, p < .001. No
additional significant results were found for the Rule Shift Trial 2 and 4 combined score
(B = -.148, β = -.354, p = .407), Group (B = -.062, β = -.045, p = .952), and the
Group*Rule Shift Trial 2 and 4 combined score variable (B = .134, β = .946, p = .356),
when examining individual effects.
Medication Management (Independent Living Skills Inventory). With Symbol
Search included as a predictor, the overall model was found to significantly account for
43.7% of the variance in Medication Management ratings in the combined sample, R2 =
.468, R2
adj = .437, F(3, 52) = 15.241, p < .001. Additional results found Symbol Search
(B = .324, β = 1.175, p= .003), Group (B = 2.359, β = 1.431, p = .001), and the
Group*Symbol Search variable (B = -.168, β = -1.670, p = .013) to each have a
significant impact on the outcome variable. Results suggest a moderation effect between
groups on Symbol Search for Medication Management ratings (see Figure 5). Once
again, differences in regression slopes are displayed in Figure 5 indicating that Symbol
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 123
Figure 5. Moderation effect between groups on Symbol Search for Medication Management ratings
Search has better predictive ability for Medication Management ratings in the
schizophrenia group, than for matched-controls.
By adding Color-Word Trial 3 into the regression equation, the model
significantly accounted for 37.8% of the variance in Medication Management ratings in
the combined sample, R2 = .412, R
2adj = .378, F(3, 52) = 12.127, p < .001. No further
individual significant results were found for Color-Word Trial 3 (B = .189, β = .808, p =
.076), and the Group*Color-Word Trial 3 variable (B = -.114, β = -1.173, p = .201). The
individual impact of Group on the outcome variable was found to approach significance
(B = 1.985, β = 1.204, p = .050).
Total ILSI Score (Independent Living Skills Inventory). The overall model
which included Symbol Search, significantly accounted for 48.6% of the variance in
Total ILSI scores in the combined sample, R2 = .514, R
2adj = .486, F(3, 52) = 18.320, p <
.001. Examining individual effects, it was found that Symbol Search (B = .203, β =
.995, p = .007), Group (B = 1.671, β = 1.368, p = .001), and the Group*Symbol Search
variable (B = -.106, β = -1.416, p = .027) each had a statistically significant impact on
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 124
the outcome variable. The significant result of the interaction term suggests a
moderation effect between groups on Symbol Search for the Total ILSI score (see
Figure 6). The regression line differences shown in Figure 6 demonstrate that Symbol
Search has better predictive power for the schizophrenia group regarding the Total ILSI
score, than for the matched-control group.
By including Rule Shift Trial 2 as a predictor into the regression equation, the
model significantly accounted for 41.4% of the variance in Total ILSI scores in the
combined sample, R2 = .446, R
2adj = .414, F(3, 52) = 13.941, p < .001. No additional
individual significant results were found for Rule Shift Trial 2 (B = -.483, β = -.673, p =
.345), Group (B = -.707, β = -.579, p = .715), and the Group*Rule Shift Trial 2 variable
(B = .394, β = 1.567, p = .423).
The model including Color-Word Trial 3, significantly accounted for 46.1% of
the variance in Total ILSI scores in the combined sample, R2 = .490, R
2adj = .461, F(3,
52) = 16.685, p < .001. Color- Word Trial 3 (B = .135, β = .780, p = .066), and the
Group*Color-Word Trial 3 variable (B = -.080, β = -1.109, p = .194) were not found to
have any individual significant impact on Total ILSI scores. However, Group was found
to have a significant impact on the outcome variable (B = 1.497, β = 1.226, p = .033).
Once again, it is important to note that considering the small sample size and the
extent to the nature of the abovementioned analyses performed, it is acknowledged that
the current study was likely underpowered in order to detect more significant results
following post hoc Holm-Bonferroni correction. Therefore the results that no longer
remained significant following adjustment will still be reported and considered.
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 125
Figure 6. Moderation effect between groups on Symbol Search for Total ILSI Score
The overall reported regression analyses results indicate, as well as confirm, that
Digit Span is a significant predictor of Hygiene and Grooming ratings, and Symbol
Search significantly predicts Interpersonal Skills, Cooking, Resource Utilization,
Medication Management ratings, as well as the Total ILSI score, for schizophrenia
patients with cannabis use. However, results also suggest that these measures have
greater predictive power for the schizophrenia group, than they have for the matched-
control group, in predicting ratings in the abovementioned ILSI areas, and Total ILSI
score.
General Discussion
The current study sought to further our understanding of the neurocognitive
profiles associated with schizophrenia patients with cannabis use and more specifically,
their distinct profiles in relation to executive function. Previous research has explored
this by examining group differences in neurocognitive performance between cannabis-
using schizophrenia patients and non-cannabis using schizophrenia patients. However,
the literature evaluating these differences has produced contradictory findings. Such
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 126
mixed results are likely due to variations in methodology, the failure to account for
various indicators of cannabis use (e.g., a range of cannabis use parameters such as
frequency, severity/quantity, duration and length of abstinence), a lacking in the
examination of clinical ‘impairment’ levels, and varying approaches in classifying
cannabis users. The present research aimed to address these issues not by directly
investigating the possible additive relationship between cannabis use and schizophrenia
on neurocognitive performance, but instead by examining the differential effects of
cannabis use on cognitive performance in individuals with schizophrenia in comparison
to otherwise healthy subjects with a similar cannabis use history, in relation to one
specific neurocognitive system namely, executive function.
While the neurocognitive profiles of individuals with schizophrenia have been
already well established in the literature, little is known about specific patterns that may
exist in the schizophrenia patient with cannabis use population particularly in relation to
the different aspects of executive function. The neurocognitive system of executive
function is considered to have particular clinical relevance as executive function deficits
have been found to be specifically associated with the illness of schizophrenia (Pantelis
et al., 1997; Reichenberg & Harvey, 2007, Shallice et al., 1999), and have also been
associated with functional outcome (Greenwood et al., 2005; McClure et al., 2007;
Williams et al., 2008).
The current study was divided into two parts. Part A of the research aimed to
determine whether differences exist in executive function performance between
schizophrenia patients with cannabis use and otherwise healthy controls with a similar
cannabis use history. This was achieved by assessing the neurocognitive performance of
both groups on several measures of executive functioning which were guided by West’s
(1996) theoretical model of executive function. Additional executive function constructs
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 127
which appeared to not be described in West’s model, were also included in the study for
exploration purposes. In line with this further exploration, the construct validity of
West’s theoretical model of executive function in the current sample was also
examined. The present research also permits some comment on whether cannabis use
status and length of abstinence have an impact on differences in executive function
performance. Part B to the study aimed to explore the relationship between executive
function performance and functional outcome. Of particular interest was determining
whether performance on specific executive function measures predicted performance in
different functional outcome sub-domains, as well as overall functional outcome.
Overall, there are four principal findings from Part A of the present study: a)
schizophrenia patients with cannabis use demonstrated poorer performances on a range
of executive function domains when compared to participants from the matched-control
group with a similar cannabis use history; however, a significantly larger proportion of
the schizophrenia group with cannabis use demonstrated performances that were not at
an impaired level compared to those falling in the impaired range; b) an underlying
structure was found to exist on a number of selected tests of executive function,
providing partial support for the construct validity of West’s (1996) theoretical model of
executive function in the current sample; c) the discontinuation of, or abstinence from,
cannabis use was mainly not found to be associated with the recovery of executive
functioning; d) longer abstinence periods were mainly not found to be associated with
the recovery of executive function.
The main findings from Part B of the study were that: a) performance on two
executive function measures were shown to significantly predict specific functional
outcome domains, namely Digit Span and Symbol Search, and that: b) moderation
effects on the executive function measures were found for 8 of the individual functional
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 128
outcome sub-domains, as well as for overall functional outcome. Each of the main
findings will now be discussed.
Part A
Group Differences in Executive Function
With the interpretation of results being conceptually guided by West’s (1996)
model, schizophrenia patients with cannabis use were found to have poorer performance
in the Temporal Organisation of Behaviour (on the Zoo Map measure only),
Retrospective Memory (on both Zoo Map recall and Digit Span measures), Interference
Control (on both Symbol Search and Trail Making Test Trial 4 measures), and the
Inhibition of Prepotent Responses (on both the Rule Shift Trial’s 2 and 4 and Color-
Word Trial 3 measures), when compared to matched-controls with a similar cannabis
use history. These differences were found after accounting for the possible effects of
estimated premorbid IQ. However, while the Modified Six Elements test was also used
to assess the Temporal Organisation of Behaviour domain, no significant differences
between the two groups was demonstrated on this particular measure. In addition, no
significant differences were found between the two groups in the executive function
domains of Prospective Memory (assessed by an event-based prospective memory task),
Verbal Fluency (assessed by Letter Fluency), and Emotion-based Decision-making
abilities (assessed by the Iowa Gambling Task). Overall these findings could be
interpreted as cannabis possibly playing a different role for patients with schizophrenia,
in comparison to healthy individuals. The pattern of results found in the current study is
somewhat similar to the findings of Jockers-Schrübel and colleagues (2007) showing
that overall, cannabis-using schizophrenic patients performed worse than cannabis-
using healthy controls in most of the neurocognitive tests administered in their study.
Scholes and Martin-Iverson (2010) also found poorer performances across cognitive
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 129
tasks in cannabis-using patients relative to healthy cannabis-using controls. In addition
to these findings, contrastingly Jockers-Schrübel and colleagues found that cannabis use
was however associated with better test performance in schizophrenia patients, and
conversely worse performance in healthy control subjects when the regular consumption
of cannabis commenced prior to the age of 17.
It is difficult to compare the current study’s findings when evaluating group
differences in neurocognitive performance as we did not aim to specifically examine
additive effects. However, despite schizophrenia patients with cannabis use
demonstrating poorer performance on a number of executive function measures relative
to controls, a significantly larger proportion of the schizophrenia group with cannabis
use demonstrated performances in the normal range and were not reflective of even
probable neurocognitive impairment. This was determined following examining their
performances against normative mean criteria. As the current study compared the
neurocognitive performances of individuals with schizophrenia to individuals without a
mental illness (with cannabis use being controlled for), it would be reasonable to
interpret the current results as potentially reflecting the effects of schizophrenia alone.
However, as schizophrenia patients have been found to demonstrate neurocognitive
performance on a variety of tests at 1.0 - 2.0 standard deviations below the norm of
healthy populations (Heinrichs & Zakzanis, 1998; Wilk, Gold, Humber, Dickerkson,
Fenton, & Buchanan, 2004), the significantly larger proportion of the schizophrenia
group with cannabis use in the current study demonstrating performances falling in the
normal range is contrary to such observations. Therefore, the current findings are
unlikely to be reflective of just the simple effects of the illness of schizophrenia on
neurocognitive performance alone. If the current findings specifically related to the
schizophrenia group with cannabis use are considered to be a mere reflection of the
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 130
effects of the illness of schizophrenia alone, then one would expect performances falling
even in the probable impairment range. However, this was not the case in the present
study.
Despite there being mainly poorer neurocognitive performances demonstrated
by schizophrenia patients with cannabis use on a number of executive function domains
in comparison to those in the matched-control group, there were no significant
differences shown in performance in the domains of Prospective Memory, Verbal
Fluency and Emotion-based Decision-making. These results potentially suggest that
schizophrenia patients with cannabis use show nominal disruptions in performance in
these particular executive function domains. In the current literature review, no research
to date has been located where measures assessing the domain of Prospective Memory
have been used to directly examine the neurocognitive performance of schizophrenia
patients with cannabis use in particular. Therefore, a comparison of the results on this
executive function domain to past findings is difficult. However, in a meta-analytic
review of 11 studies examining prospective memory performance in schizophrenia
patients, impairments were found on time-, event-, and activity-based prospective
memory tasks compared to controls (Wang, Cui, Chan, Deng, Shi, Hong et al., 2009). In
addition, performances on time-based prospective memory tasks were demonstrated to
be more impaired than performances on event-based tasks. It was considered by the
authors that time-based prospective memory measures may require a greater level of
self-initiation than tests of event-based prospective memory. Therefore, while the
present findings may suggest that schizophrenia patients with cannabis use show
nominal disruptions in this domain, in alternative it could also indicate that the event-
based prospective memory measure used may not have sufficiently loaded onto
prospective memory abilities. Previous research has also found no difference in
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 131
performance amongst cannabis-abusing patients in verbal fluency in comparison to non-
abusers (Mata et al., 2008; Schnell et al., 2009). The current results involving emotion-
based decision making abilities were somewhat inconsistent with Sevy and colleagues’
study (2007), where it was reported that both schizophrenia groups (i.e., both cannabis
users and non-users) were found to be more cognitively impaired, and demonstrated
worse performance on the Iowa Gambling Task, when compared to healthy controls.
However, the healthy participants in this study were not matched for cannabis use and
included six individuals who reported smoking marijuana less than 10 times in their
lifetime, as well as individuals who did not report any cannabis use history. Therefore, it
is possible that the measures selected in order to assess these domains may actually be
sensitive enough measures in detecting deficits in executive function, but do not
demonstrate specificity in identifying differences in this particular schizophrenia sub-
population.
While the Temporal Organisation of Behaviour was also measured by the
Modified Six Elements test in the current study, a lack of significant difference in
performance between groups on this measure may suggest that this particular test was
also perhaps not sensitive enough, nor did not sufficiently load onto this particular
executive function domain, in order to detect differences in these samples. It is also
possible that the samples recruited in the current study may have high heterogeneity,
thus creating more variability within groups and making differences between groups
harder to locate. However, as group differences were demonstrated on another measure
of the Temporal Organisation of Behaviour (namely the Zoo Map task), as well as many
other measures tapping into additional executive function domains, it is unlikely that
this was the case. In addition, estimated premorbid IQ was found to have an impact on
Modified Six Elements task performance and this could suggest that while this test may
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 132
assess an aspect of executive function, it may also rely upon other cognitive abilities
specifically related to premorbid IQ. There has also not been any research located to
date directly examining the performance of cannabis-using schizophrenia patients in
particular on the Modified Six Elements test which limits the ability to compare and
contrast the current results with previous findings.
Level of executive function impairment between groups. The different
performances of the two groups, requires further comment. Due to the lack of clinically
impaired performances observed in a significant proportion of the schizophrenia group
with cannabis use and the expected performance of 1.0 - 2.0 standard deviations below
the norm for cannabis-naive schizophrenia patients, it appears that cannabis use in
schizophrenia may have relatively nominal clinical effects on executive function
performance when compared to normative criteria. A possible explanation for this result
may be due to cannabis use in schizophrenia being potentially associated with outcomes
of a more preserving nature. Cannabinoids have been shown to possess characteristics
serving to protect individuals from brain insult and are associated with improvements in
a range of illnesses which are considered neurodegenerative in nature (Sarne &
Mechoulam, 2005). Thus, the findings of the present study may be reflective of
differential effects of cannabis on the brain of individuals with schizophrenia, as
opposed to a healthy brain, potentially as a consequence of neuroprotective effects.
As the examination of impairment level was based on normative criteria, it is
also reasonable to consider that schizophrenia patients with cannabis use may represent
a distinct schizophrenia sub-population that has specific executive function abilities
remaining at a non-impaired level when compared to individuals with schizophrenia
who are cannabis naive. It has been postulated that the onset of psychosis in this
subgroup may be aetiologically different to non-using schizophrenic patients, such that
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 133
cannabis use may serve as a catalyst to the onset of the illness in these individuals who
may already possess better premorbid adjustment, social skills and hence a better
prognosis, than their non-using counterparts (Dixon et al., 1991, Leeson et al., 2011;
Rodríguez-Sánchez et al., 2010). It has also been suggested that this sub-group may
have a lower vulnerability to the illness compared to those patients whose illness onset
occur independent of the context of cannabis use (Schnell et al., 2009). Therefore, it is
possible that such individuals already have better cognitive abilities due to not
possessing as many neurodevelopmental risk factors, as well as having more enhanced
prognostic characteristics (Leeson et al., 2011). Findings from Kumra and colleagues’
study (2005) showed that a history of cannabis use/dependence was associated with
better FSIQ, as well as better VIQ, scores which also suggests the presence of better
cognitive adjustment in cannabis-using schizophrenics. The lack of clinical impairment
demonstrated by schizophrenia patients with cannabis use in the current research is
consistent with the theme that has emerged from previous studies suggesting better
neurocognitive performances in this group when compared to non-using schizophrenia
patients.
The Effect of Cannabis-Use Status on the Recovery of Executive Function
Cannabis-use status (i.e., current users and non-current users) was mainly not
found to have an impact on executive function performance, when controlling for the
effects of duration of use (although there were a few notable exceptions to this, as
discussed below). In other words, in general, cannabis use cessation or abstinence was
not associated with the recovery of executive function. These results are also somewhat
consistent with previous findings indicating no significant differences between patients
with current cannabis dependence and patients without a current cannabis dependence
diagnosis (which included both former users and those with minimal or no use) (Rabin
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 134
et al., 2013). However, in the present study this was not the case for performance on
Letter Fluency, such that current cannabis users demonstrated better performance on
this task compared to non-current users. A possible explanation for this contrasting
result may be related to premorbid IQ being shown to significantly influence
performance on the Letter Fluency measure, in the earlier analyses of the current study.
Therefore, this finding could provide possible further evidence for current cannabis
users representing a group of individuals with better premorbid IQ, independent of
whether or not they have the illness of schizophrenia. Further, it is plausible that
individuals with better premorbid IQ may be more likely to continue cannabis use due
to possessing the abilities required to continually seek avenues in order to maintain their
drug use. In a recent study which specifically compared the relationship of cannabis use
to premorbid IQ in a group of first episode psychosis patients, results showed that those
patients who had ever smoked cannabis in their lifetime had higher premorbid IQ than
those who had never used cannabis in their lifetime (Ferraro, Russo, O'Connor, Wiffen,
Falcone, Sideli et al., 2013). However, as the current study controlled for premorbid IQ
through the matching process, the findings may not solely be related to premorbid IQ
(as measured by an estimated verbal premorbid IQ measure). Another possible
explanation for these results are in line with previous suggestions that this subgroup of
schizophrenia appears to demonstrate characteristics of having a less severe form of the
illness in relation to neurocognitive abilities and therefore remain relatively cognitively
preserved once psychosis develops potentially in the presence of early cannabis use.
Findings from Barnett, Salmond, Jones, and Sahakian’s (2006) study indicated that
superior cognitive function (i.e., cognitive reserve) may act as a protective function
from cognitive decline. Cognitive reserve is defined as the ‘individual differences in
how people process tasks which allow some to cope better than others with brain
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 135
pathology’ (Stern, 2009, p. 2016). In other words, individuals with higher cognitive
reserve are less affected by the same extent of neurological damage caused by either the
effects of the normal aging process, or from the effects of unhealthy aging (i.e., the
result of illnesses which may serve to compromise cognitive function). Barnett and
colleagues (2006) concluded from their results that this higher level of cognitive
function may impact on neuropsychiatric disorders by either influencing the risk for
developing the disorder, the expression of illness symptomatology, or the functional
outcome of the individual. Therefore, the current results suggest that it may not be that
cannabis is neuroprotective in nature, but instead findings implicate the possibility of a
higher overall cognitive reserve in individuals who may belong to this specific
schizophrenia subgroup.
The Effect of Length of Abstinence on the Recovery of Executive Function
The findings of the current study also showed that the length of abstinence was
mainly not associated with the recovery of executive function, independent of duration
of use. More specifically, shorter abstinence periods were not related to poorer
performance even after duration of use was taken into account. Surprisingly, it was
additionally found that former cannabis users with an abstinence period of less than 5
years demonstrated better skills in the Temporal Organisation of Behaviour compared to
former cannabis users with an abstinence period of equal to, or greater than, 5 years,
although this result was shown for performance on the Zoo Map measure only. This is
somewhat consistent with previous findings which showed that in addition to frequency,
recency of cannabis use was also associated with better neurocognitive performance
related to attention/processing speed and executive function in schizophrenia patients
(Coulston et al., 2007). Stirling and colleagues (2005) also found that schizophrenia
patients who had sustained their cannabis use at follow-up demonstrated better
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 136
neurocognitive performance on a variety of measures (i.e., design memory, verbal
fluency, object assembly, block design, picture completion, picture arrangement, face
recognition memory), than those individuals who did not continue their use until this
time. An explanation for this finding may involve the possibility that those individuals
with more recent use may still possess intact temporal organisational skills even after
cannabis use cessation. It could also be considered that perhaps these abilities begin to
reduce over time from the lack of exposure to drug-obtaining and drug use maintenance
activities, as well as the lack of rehearsal of the behaviours necessary to achieve these
objectives. This is a plausible explanation considering the temporal organisation of
behaviour involves temporally organizing steps of a process which serve as a guide in
selecting appropriate behaviors required to complete tasks (Fuster, 1989). It could be
also speculated that such processes are considered to be involved in the perpetuation of
drug use. Results of the current study suggest that these abilities also appear to be intact,
irrespective of the presence or absence of the illness of schizophrenia.
The Validity of West’s Theoretical Model of Executive Function
While there was no specific hypothesis outlined regarding the construct validity
of West’s (1996) theoretical model of executive functioning, findings indicated the
presence of an underlying structure on a number of selected tests of executive function
in the current study. The executive function measures which significantly loaded onto
factor 1 (namely Mental Control and Self-Regulation) were those which were guided by
West’s theoretical model. In addition, the executive function measures which were
added for exploration due to not appearing to be accounted for in West’s model (i.e.,
Verbal Fluency and an Emotion-based Decision-making task) seemed to either load
onto a separate component or failed to meet criterion for PCA. The overall results
appear to not only in part, confirm the construct validity of West’s theoretical model of
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 137
executive function, but also provide support for the convergent validity, as well as
divergent validity, of the model. Such results are also in part, supported by Num’s
(2006) findings. It is possible that the additional executive function domains that
weren’t accounted for in West’s model are not in fact, reflective of ‘pure’ executive
function domains due to having loaded onto a different component or not being able to
be included in PCA. This may be due to such measures tapping into other
neurocognitive domains in addition to those falling under the executive function
umbrella. While tests of verbal fluency (namely phonemic fluency) are one of the most
frequently utilised measures in the assessment of executive function (Baddley, 1996;
Baldo et al., 2001; Stuss & Levina, 2002), they also are known to activate non-frontal
brain areas (Frith, Friston, Herold, Silbersweig, Fletcher, Cahill et al., 1995; Paulesu,
Goldacre, Scifo, Cappa, Gilardi, Castiglioni et al., 1997). Interestingly, an additional
factor was produced as an outcome of the analysis, namely ‘Maintenance of Intention
for Future Action’. A possible explanation for why the event-based prospective memory
task loaded onto a different factor to factor 1 in the PCA, may have been due to the
specific measure that was selected in the current study not being sensitive enough to tap
into the true prospective memory construct. Similar to Wang et al.’s (2009) conclusions,
time-based prospective memory tasks have been thought to place a greater demand on
self-initiation for successful performance due to the lack of external cues provided in
such a measure and consequently more heavily relies on executive functioning
processes (McDaniel & Einstein, 1993). In addition, the result of Kinch and
McDonald’s (2001) study, suggest that successful performance in an event-based
prospective memory task was primarily attributed to retrospective memory abilities.
Therefore, it is possible that the abilities assessed by this particular type of prospective
memory task may not just reflect a more crucial underlying executive function process
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 138
such as retrospective memory, but in addition may tap into abilities falling outside of
the executive function domain alone. It is also reasonable to consider that Trial 3 of the
Color-Word Interference subtest of the D-KEFS may not just reflect the inhibition of
more dominant, habitual responses, but also measure the maintenance of information
required to perform a future action.
It has been argued that while various neuropsychological measures may have
good face validity, demonstrated high construct validity is imperative in order for such
measures to represent a construct accurately (Salthouse et al., 2003). Therefore, results
suggest that the majority of the executive function measures which loaded onto factor 1
demonstrate an ability to be sensitive to the executive processes conceptualised by
West’s model, but in addition they also demonstrate specificity in being able to tap into
these specific neurocognitive processes as opposed to others.
In order to conduct the PCA, the sub-samples were combined due to their small
sizes. As it was considered in previous literature (e.g., Solowij & Michie, 2007) that
cannabis users demonstrate deficits on cognitive measures similar to those shown in
schizophrenia patients, there is a chance that the sub-samples in the current study are
not all that dissimilar in cognition factor structure. Therefore, combining the samples
would be appropriate. However, it is considered that if the sub-samples are indeed
dissimilar to each other then this may help to explain why some executive function
measures did not load onto factor 1. Considering the PCA was intended to be
exploratory in nature, the results of this analysis were also not used in further analyses
and therefore, the interpretation of results should only be considered on a theoretical
basis. Prospective studies with larger samples should seek to further examine the factor
structure of cognition in schizophrenia patients with cannabis use, and cannabis users
with a similar cannabis use history, in order to clarify any similarities or dissimilarities.
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 139
It is also acknowledged that the current study did not aim to examine the actual
hierarchy of functions from West’s (1996) hierarchical model of executive function.
However, was considered that an analysis of this type would require much larger
samples. Future studies should look to examine the hierarchy of functions in West’s
model in patient populations in order to assist in further evaluating the validity of the
hierarchical nature of the model.
As the current study is the only study to date to have utilised this theoretical
model of executive function to guide test selections in order to assess the executive
functioning of individuals with schizophrenia, further studies are warranted to replicate
the present findings. Taking into account the abovementioned results, future
investigations should also consider the use of a time-based prospective memory
measure in order to better clarify the role, as well as confirm or disconfirm the
inclusion, of this particular cognitive domain in the executive function model. However,
the assessment methodology of the current study may provide a more effective approach
to future research, particularly when attempting to comprehensively assess and
fractionate the executive function system.
Part B
The Relationship Between Executive Function and Functional Outcome
Findings from the present study not only showed that different aspects of
executive functioning were predictive of ‘real world’ functional outcome in
schizophrenia patients with cannabis use, but that these aspects also moderated the level
of functional outcome in different groups. From the variety of executive function
measures administered in the current research, it was found that performance on the
specific measures of Digit Span and Symbol Search significantly predicted performance
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 140
in particular areas of functional outcome for schizophrenia patients with cannabis use,
in comparison to that for the matched-control group.
Findings suggest that performance on Digit Span (a measure used to assess
Retrospective Memory), was significantly related to those functional outcome skills
associated with hygiene and grooming. It can be speculated why this aspect of executive
functioning would be related to this particular functional outcome sub-domain. It is
reasonable to consider that Retrospective Memory, which involves the maintenance of
task-relevant information online and provides a reference by which a task sequence is
formed (Fuster, 1989), has particular importance in maintaining adequate independent
skills in hygiene and grooming. Schroots, Dijkum, and Assink (2004) define
retrospective autobiographical memory as the ability to retrieve certain memories,
experiences, or past events in the present time. Dritschel et al. (1998) argued that
participants with traumatic brain injury would be able to plan further activities more
easily if they first tried to retrieve specific autobiographical memories of where they
have executed similar activities in the past and described step-by-step what they did. In
addition, Burgess and Shallice (1997) argued that the application of retrospective
memory processes, in particular those that are implicated in the recollection of previous
experiences play a critical part in prospective memory (i.e., the set of abilities that
enable an individual to carry out a future intention or plan-following behaviour). In
support of this, Kinch and McDonald (2001) also considered that a crucial component
of remembering the intention to commit a future action or task (i.e., prospective
memory) is considered to be largely retrospective in nature. Therefore, it appears that
retrospective memory processes in some form have the capacity to assist individuals to
carry out planned future intentions and one could expect such processes to be in
involved in the daily requirements of hygiene and grooming activities.
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 141
While in the current study retrospective memory has been considered a
component of executive function in West’s (1996) theoretical model of executive
function, it is acknowledged that the retrospective memory does include some more
general cognitive abilities that are not necessarily specific to the frontal lobe system.
However, Fuster (2000) refers to retrospective memory as short-term memory or
sensory working memory and uses the term ‘active short-term memory’ to encompass
the construct (p. 67). It has also been well documented that short-term active memory
involves prefrontal neurons (e.g., Funahashi, 1989; Fuster, 1973; Niki, 1974). In
addition, West (1996) conceptualises retrospective memory in his model as involving
the retrieval and maintenance of information that is task-relevant from its storage in
memory and is conceptualised to be similar to working memory. Much consistency
between the types of memory deficits experienced by patients with lesions of the frontal
lobe and by healthy older adults has been observed due to both types of subjects
demonstrating deficits on measures of memory which are thought to heavily require
processes that are strategic in nature and hence, executive functioning (Moscovitch &
Winocur, 1992). Further, it is important to note there have been a number of measures
utilised when assessing retrospective memory abilities in the literature. Some of these
include span, working memory, recall and recognition type measures. In the current
study, two measures were used in an attempt to assess retrospective memory abilities,
namely Digit Span (forwards assessing span, and backwards assessing working
memory) and Zoo Map recall (assessing free recall). While it is acknowledged that total
Digit Span provides a combination of span, as well as working memory abilities,
previous research has also utilised forward Digit Span as one approach to assessing
retrospective memory abilities (Maylor, Smith, Della Salla, & Logie, 2002). The authors
described retrospective memory being usually assessed by tasks that involve the
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 142
presentation of information that is subsequently required to be recalled or recognized
following a cue. In addition, the results of Maylor and colleagues’ (2002) factor analytic
study showed that more traditional Retrospective abilities (as measured by free recall
and recognition) and working memory abilities (as measured by Digit Span and
Sentence Span) loaded highly on one factor in a sample patients with Alzheimer’s
Disease and healthy older adults.
It is noted that the findings of Dickinson et al.’s (2008) study indicate that the
neuropsychological impairments associated with the illness of schizophrenia relative to
healthy control subjects is largely mediated through a common ability factor and that
further analyses revealed direct diagnosis effects on verbal memory and processing
speed. Therefore, it is reasonable to consider that processing speed and simple
maintenance working memory may have a more direct impact on the everyday
outcomes of schizophrenia patients based on such results. However, it may be erroneous
to apply the same conclusions about the cognition of schizophrenia patients with
cannabis use, particularly because the current study’s results related to impairment
levels (in conjunction with previous findings) suggest that this subgroup of the
schizophrenia patients may be dissimilar with regard to cognitive abilities.
Findings also showed that performance on Symbol Search (used to assess
Interference Control) is predictive of performance in the functional outcome areas of
interpersonal skills, cooking, resource utilization, medication management, as well as
overall functional outcome. These results are suggestive that the ability to deal with
interference or conflict from recently presented information that was once relevant, but
is now irrelevant, could be necessary to maintain independence in the abovementioned
areas and that these specific areas may also be important to the overall functional
outcome of this particular subgroup of individuals with schizophrenia. This could be
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 143
due to the possibility that poorer interference control abilities may limit the capacity of
individuals with schizophrenia to filter out previously attained information (that is no
longer relevant to the situation at hand) and adapt to new situations. It has been
considered that interference control abilities help to clear inappropriate information to
the task at hand from retrospective memory storage (West, 1996). It is possible that the
abovementioned sub-domains of functional outcome (as opposed to the sub-domains of
personal management, clothing, basic skills, home maintenance, money management,
and general occupational skills) may be more related to interference control deficits in
this particular schizophrenia sample. One could assume that such independent living
skills are not entirely guided by routine, but instead may involve modified action
sequences depending on the individual’s day-to-day circumstances (i.e., presenting the
individual with new information or demands). For example, this could occur when an
individual receives a phone call in the middle of following a recipe. Whereas the other
sub-domains not predicted by interference control abilities could either possibly
subscribe to more basic cognitive skill functions or even those cognitive abilities that
were not assessed for in the current study. It is also considered that proactive
interference is a major contributing factor to memory failure and in an attempt to
resolve the effects of interference from prior learning or encoding, individuals will
typically slow down and employ some type of cognitive control strategy (Persson &
Reuter-Lorenz, 2008). For example, this may involve either efforts to make a mental
note of where the individual left off before the interference/distraction, or even using a
compensatory approach such as marking the recipe at the interrupted step. Gaining a
better understanding of the nature of the neurocognitive domains which may serve to
significantly contribute to performance in specific functional outcome areas has the
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 144
potential to more effectively guide cognitive rehabilitation efforts and ultimately
increase their effectiveness and success.
Interestingly, the predictors identified as being significantly related to outcome
in the current study are in fact the variables that are not specific to executive functions
alone. While Symbol Search involves a measurement of interference control, it is also a
measure of processing speed. In this test, the participant is required to match a symbol
to an identical target that is displayed among several distractor stimuli that share similar
physical features. Interference control is assessed through the selection of correct
matches identified from the variety of distractors and speed is assessed by the number of
correct items the participant completes within the time limit. Definitions of interference
control commonly involve reference to the selection of relevant information and the
ignoring of irrelevant information (Dempster, 1993; Harnishfeger, 1995; Nigg, 2000). In
a study examining the relationship between inhibition and interference control, the
authors assessed ‘resistance to distractor interference’ with tasks in which participants
were required to select targets that were presented with irrelevant distracters (Friedman
& Miyake, 2004). For example, the authors utilised the Eriksen flanker task (Eriksen &
Eriksen, 1974) to measure this construct in which participants were required to identify
a target letter that was presented either alone or with response-incompatible letters
beside it, as quickly as possible without sacrificing accuracy. It is considered that
‘resistance to distractor interference’ is being assessed when distracting information is
presented simultaneously with the target information and is irrelevant to the response
required (Friedman et al., 2004). It would be reasonable to assume that the approach in
which to quantify interference control abilities would inevitably involve not only
determining one’s ability to control interference/distraction (i.e., by calculation of
accuracy via a number of errors score), but also the efficiency of one’s ability to control
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 145
interference/distraction (i.e., by calculation of the speed in which one can complete the
task). Therefore, the examination of one’s performance on such measures in the context
of both accuracy and speed seems fitting to determine an individual’s success on these
tasks. As the current findings also indicate that Trail Making Test performance (which
was also used as measure of Interference Control) did not significantly predict
functional outcome, this may suggest that interference from distraction could be more
directly related to the functional outcome domains of interpersonal skills, cooking,
resource utilization, medication management, as well as overall functional outcome,
compared to abilities that involve more of an active suppression process.
In addition, it is acknowledged that while there are some functional outcome
domains assessed by the Independent Living Skills Inventory that apply more to a
patient population, the current study required an inventory that was considered a valid
instrument to utilise in the assessment of this specific clinical population. As mentioned
previously the purpose of the regression analyses was to identify whether there is
potential for improvements to specific cognitive abilities which in turn may affect
associated functional outcome. Without a comprehensive assessment of a number of
functional outcome domains, the clinical relevance of potentially targeting the specific
functional outcome domains deemed to be involved in patient independent living skills
would be lost.
These results are not only in line with previous research suggesting a significant
relationship between executive functioning and functional outcome, but also extend
previous findings by being able to identify which specific aspects of executive function
(according to a theoretical model of executive function) significantly predict particular
sub-domains of functional outcome. By using a range of measures to assess the different
theoretical components of executive function and identifying which of these have better
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 146
predictive ability in relation to ‘real world’ functional outcome, we were able to more
comprehensively evaluate this relationship as well as potentially increase the ability to
detect differences between groups on this multi-faceted and complex neurocognitive
domain.
The results demonstrating moderation effects indicate that the abovementioned
measures in particular also have greater predictive power for schizophrenia patients
with cannabis use than they have for healthy matched-controls with a similar cannabis
use history. One explanation for this finding could be related to previous research
suggesting that cannabis may have differential effects on individuals with
schizophrenia, in comparison to healthy individuals (Jockers-Schrübl et al., 2007). Due
to a potential higher cognitive reserve (as suggested by their cannabis use history), it
would be expected that any preservation in cognitive performance would translate to
relative preservations in functional outcome areas. Therefore, it is possible that those
with more preserved outcome (across similar functional outcome sub-domains) in this
subpopulation may possess more preserved retrospective memory and interference
control abilities. Such findings suggest that the measures assessing these specific
executive function domains could be considered as useful tools in predicting the
functional outcome of this subgroup of individuals with schizophrenia, particularly in
relation to identifying areas for improvement regarding the associated day-to-day
independent living skills.
Practical Applications of the Findings
Given the significant association between cognition and functional outcome in
schizophrenia, over-and-above the effects of positive, negative and disorganisation
symptoms (Green, 1996; Green et al., 2000), it is considered imperative that both
researchers and clinicians look to develop specific and effective strategies in order to
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 147
improve cognitive function in this clinical population. The current study demonstrates
that through comprehensive and more construct-targeted assessment approaches, it is
possible to identify appropriate strategies that are both practical and customised to the
individual’s specific deficits. The treatment of cognition in schizophrenia has been
largely influenced and inspired by the field of neurological impairment rehabilitation
(Medalia & Choi, 2009). The cognitive problems associated with the ‘dysexecutive
syndrome’ are considered to represent a major challenge to the functional adaptation
and recovery of individuals affected by such deficits and consequently, serve as crucial
rehabilitation targets (Hewitt, Evans, & Dritschel, 2006). Therefore, clinical
neurocognitively-based rehabilitation techniques, particularly those focused on the
training of executive skills, are ideally suited to address the relationship between
cognitive dysfunction and functional outcome.
The present findings have value in that the current neurocognitive assessment
methods utilised can help to possibly attribute performance in a number of functional
outcome areas, specifically to difficulties in retrospective memory and interference
control abilities in schizophrenia patients with cannabis use. The results also indicate
that this relationship mainly exists between retrospective memory and interference
control, and various functional outcome areas, in comparison to the other executive
function domains assessed. Such findings suggest that targeting these specific executive
function domains in rehabilitation efforts may not only facilitate improvements in
performance on measures assessing these particular executive function domains, but
may also translate to performance improvements in day-to-day real-world functional
outcome areas in this subgroup. This notion is supported in the literature which
proposes that cognitive enhancement in individuals with schizophrenia may not just
generalise to neurocognitive task performance, but may also be observed in an
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 148
individual’s performance across everyday activities particularly if the skills required in
such activities tap into similar neurocognitive domains (for discussion, see Medalia &
Choi, 2009).
There are a number of rehabilitation programs which have been developed in
order to remediate cognitive impairments more specific to executive function, in
schizophrenia patients. The intention of cognitive remediation approaches is to assist
individuals to develop cognitive skills which are deemed fundamental in helping to
improve their function in day-to-day activities (Medalia & Choi, 2009). Some cognitive
rehabilitation interventions have been designed to target a number of neurocognitive
domains which include those that involve executive function abilities (Bell, Bryson,
Greig, Corcoran, & Wexler, 2001; Bellack, Dickinson, Morris, & Tenhula, 2005). Other
intervention approaches are tailored in order to target more specific executive processes,
such as working memory, cognitive flexibility, and planning abilities (Wykes, Reeder,
Landau, Everitt, Knapp, Patel, & Romeo 2007), and problem-solving abilities (Medalia,
Revheim, & Casey, 2002). A well-validated intervention aimed at the rehabilitation of
those executive function abilities involved in the self-regulation of behaviour is Goal
Management Training (GMT) (Levine, Robertson, Clare, Carter, Hong, Wilson et al.,
2000; Levine, Stuss, Winocur, Binns, Fahy, Mandic et al., 2007; Robertson, 1996;
Robertson, Levine, & Manly, 2005). GMT is a theoretically derived cognitive training
protocol based on Duncan’s theory of ‘goal neglect’ (Duncan, 1986; Duncan, Emslie,
Williams, Johnson, & Freer 1996), which describes deficits in strategic self-regulation
associated with the dysexecutive syndrome. The approach is a standardised, interactive
manual-based cognitive rehabilitation program that aims at improving an individual’s
attentional and organisation skills in order to improve the ability to achieve goal-
directed plans. The program provides a specific focus on goal-directed behaviours found
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 149
in everyday contexts. GMT procedures encompass training 5 steps, with each
emphasizing and corresponding to an important aspect of goal-directed behaviour. The
strategy involves training individuals as a first step to ‘stop and think’ by taking pauses
during the task at hand and direct their awareness to pertinent goals. Once relevant goals
are selected at step two, the individual is then required to subdivide the goal into smaller
and more manageable subgoals at step three. The fourth step trains the individual to
encode and maintain the goal, and more simple subgoals, in memory. At step five, the
outcome is then compared with and checked against the intended goal. This approach is
repeated in the event that the action outcome is mismatched with the original goal.
GMT is considered to be a top-down approach that aims to train processes across
several neurocognitive domains and incorporate different aspects involved in the stages
of goal management such as attention and task orientation, goal definition, problem-
solving, encoding, retention and retrieval strategies, and monitoring (Levine et al., 2000,
2007; Levaux, Larøi, Malmedier, Offerlin-Meyer, Danion, & Van der Linden, 2012).
Such an approach aims to promote generalisation where different behaviours or
activities that may not specifically be targeted in the actual GMT intervention may be
also subject to improvements, or the skills trained can be applied to various other
contexts that require the formulation of goal-directed plans (Levaux et al., 2012).
Levaux and colleagues (2012), sought to apply this method to a patient with
schizophrenia suffering from executive deficits in a case study. In their research they
applied the GMT strategies across three steps: (1) psychoeducation and learning GM
steps; (2) training GM principles on pencil-and-paper tasks; and (3) training in practical,
real-life situations. Results of the study showed improvements in planning and
dominant verbal response inhibition, as well as increased care and attention to tasks
(with the patient taking more contextual information into account). Progress was found
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 150
to be maintained 2 years later and continued to evolve, and generalisation of the effect
of GMT was observed on both non-trained laboratory and non-trained everyday tasks. A
significant increase in the patient’s self-esteem score was also found. Overall, the results
of the case study suggest that GMT is a promising technique for the rehabilitation of
everyday executive difficulties in people with schizophrenia. However, it was important
to note that there was no change in impaired flexibility, response inhibition and
attentional functions following the GMT intervention. In addition, working memory and
language flexibility remain preserved, and non-verbal process speed remained slowed.
Beneficial effects on a number of executive function deficits have been
demonstrated in patients with traumatic brain injury and older adults (Levine et al.,
2000, 2007; van Hooren, Valentijn, Bosma, Ponds, Van Boxtel, Levine et al., 2007), as
well as potentially for schizophrenia patients (Levaux et al., 2012), as a result of GMT
interventions. However, the approach has rarely been utilised in the context of substance
abuse. Alfonso and colleagues (2011), developed a program which not only included
methods derived from the GMT protocol, but in addition also incorporated aspects of
mindfulness-based meditation. Mindfulness-based meditation was considered to serve
as an effective compliment to traditional GMT training methods in order to improve
attention scanning and ‘reading’ of emotional signals involved in adaptive decision-
making. Mindfulness-based mediation practices promote paying attention to the present
moment, in a purposeful, non-judgemental, and emotionally aware manner (Kabat-Zinn,
Massion, Kristeller, Peterson, Fletcher, Pbert, Lenderking, & Santorelli, 1992; Segal,
Williams, & Teasdale, 2002). The use of mindfulness techniques has been considered a
promising approach for the treatment of problematic substance-use related behaviours
and supporting relapse prevention efforts (Bowen, Chawla, Collins, Witkiewitz, Hsu,
Grow et al., 2009; Bowen & Marlatt, 2009; Witkiewitz, Bowen, Douglas, & Hsu, 2013).
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 151
Results of the study showed that the GMT and mindfulness meditation program
significantly improved performance on working memory, selective attention/response
inhibition and decision-making. The authors concluded from the findings of the study
that improving competence in executive abilities may have the capacity to improve the
clinical prognosis of substance abusers. However, it is important to note that there were
no significant improvements in the neurocognitive domains of planning and flexibility
following the GMT and mindfulness meditation program. It was considered by the
authors that a possible explanation for this may perhaps involve the presence of
impairments in other more basic, underlying skills. Shallice, Burgess and Robertson
(1996) consider that an important process that serves to assist in appropriate strategy
generation and planning is episodic memory retrieval. In other words, retrospective
episodic memory processes. It is plausible that the recollection of previous similar
experiences would be beneficial when attempting to formulate solutions to unfamiliar
situations. It has been reported that retrieving more specific memories when devising
strategies to low frequency activities (i.e., unfamiliar situations), is associated with more
effective solutions as well as solutions containing more relevant steps (Dritschel et al.,
1998). This suggests that the recollection of previous similar experiences can be
beneficial for planning and problem-solving contexts. Einstein and McDonald (1990)
propose that the aspect of an activity that involves retrospective memory relates to one’s
ability to maintain information regarding action and context. Burgess and Shallice
(1997) also consider the view that the earlier stages of intention creation are not only
facilitated by executive control systems, but that such systems mediate the development
of intentions which involve complex retrieval. These findings are once again, consistent
with the view that retrospective memory processes play an important role in strategy
generation and planning future intentions.
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 152
The current study is believed to be an important first step in evaluating the
applications of more comprehensive neuropsychological assessment which has been
guided by a theoretical model of executive function (West, 1996). Considering the
potential practical applications of the current findings, further research is required in
order to demonstrate whether rehabilitation methods aimed at training specific executive
function deficits can improve performance in more practical, everyday situations. It has
been consistently found that cognitive remediation efforts aimed at specific cognitive
abilities is an effective approach for cognitive enhancement (Green & Harvey, 2014).
However, it is considered that such improvements may not necessarily translate to
functional change. With the view to improve the functional outcomes of individuals
with the illness, it is considered that the treatment of cognitive dysfunction as a sole
approach may fall short in its effectiveness (Harvey, 2007). To enable a more effective
translation of cognitive improvements into successful functional change, it is proposed
that other forms of intensive support or assistance may be required which target those
factors outside the illness that appear to also influence outcomes (e.g., demographics
and psychosocial variables) (Harvey, 2007). Bell and colleagues (2008) conducted a
study evaluating the effects of Neurocognitive Enhancement Therapy (NET) in
conjunction with a supported employment program (VOC) on the enhancement of
functional outcomes in patients with schizophrenia and schizoaffective disorder. The
NET+VOC program consisted of computer-based cognitive training, work feedback and
a social information information-processing group. Findings from the study suggested
that the combination of cognitive retraining methods with additional skills training in
other areas has the capacity to promote more beneficial functional gains. In a recent
study conducted by Bowie, McGurk, Mausbach, Patterson, and Harvey (2012),
individuals with schizophrenia living in community settings were randomly assigned to
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 153
either a cognitive remediation, functional skills training, or combined treatment group.
It was reported that those patients who either received cognitive remediation, or the
combined treatment, showed improved neurocognitive test performance from baseline
to end of treatment. At a 12-week durability assessment, these effects were observed to
be maintained. However, improvements in neurocognitive performance were not
observed following the functional skills training alone intervention. Those individuals
who either received functional skills training, or the combined treatment, demonstrated
improved social competence (as measured by the Social Skills Performance
Assessment) from baseline to end of treatment. In addition, patients who either received
functional skills training, or the combined treatment, showed improvements in
functional competence (as measured by a computed composite score from three
measures of everyday functional skills) from baseline to end of treatment. However, the
functional competence improvements demonstrated following functional skills training
were no longer significant at the 12-week durability assessment when compared to
baseline. It was noted that greater and more durable functional competence
improvements were demonstrated in those patients who were assigned to the combined
treatment group. Improvements in real-world community activities (as measured by the
Specific Levels of Functioning Scale; Schneider & Struening, 1983) were found to be
greater in patients who received the combined treatment, compared to the functional
skills training group. Patients who were assigned to the either the functional skills
training group, or the combined treatment group, showed improvements in real-world
occupational skills (also measured by the Specific Levels of Functioning Scale) at
baseline to end of treatment and these effects were maintained at the durability
assessment. However, the treatment effects in the combined treatment group were found
to be significantly larger than the functional skills group and the cognitive remediation
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 154
group. Overall findings from the study suggest that cognitive enhancement strategies in
isolation do not result in functional improvements in real-world behaviour. The authors
concluded that the transfer of cognitive improvements to everyday functioning is more
likely to occur in the context of receiving supplementary functional skills training in
addition to cognitive remediation intervention. While previous findings have indicated
that the use of cognitive remediation strategies as a standalone treatment may be
insufficient in translating to improvements in functional behaviour, it is considered that
an ideal approach may involve treating professionals placing additional focus on the
most appropriate avenues in supporting the efforts of schizophrenia patients and for
empirical studies to aim to identify which of these support methods are most effective
(Harvey, 2007).
In order for cognitive rehabilitation efforts to be optimal in their effectiveness,
there is also a need to better profile the patient samples of interest and the precise nature
of the deficits needing to be addressed. Identifying different intervention factors that
might serve to maximise the effectiveness of cognitive remediation efforts is also an
area that has the potential to optimise the efficacy of future intervention efforts (e.g.,
different instructional techniques, learning and presentation formats, and materials
used). There is also a need for the continued development of appropriate outcome
measures to ensure that interventions translate into meaningful changes in real-world
functioning. Further, future research should also not only aim to determine whether
training in executive processes can be effectively translated into benefits in daily life,
but to assess whether these benefits are maintained over time.
Finally, as only a little more than half of the variance of a number of functional
outcome areas were explained by performance on Digits Span or Symbol Search in the
current research, this suggests that other variables which were not examined in this
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 155
study could also have a significant impact on functional outcome. Therefore, our results
should be regarded as preliminary in nature. In addition, such results suggest that if only
certain aspects of executive function are significantly related to various areas in the
functional outcomes of schizophrenia patients with cannabis use, then other
neurocognitive domains may serve to also be targeted or emphasized in a strength-based
approach to cognitive training efforts. In other words, effective cognitive rehabilitation
protocols involving this particular clinical population should not only involve cognitive
training in identified deficits areas, but should also focus on making use of their intact
neurocognitive abilities. The current findings have clinical importance in that they
support the concept of developing interventions to treat neuropsychological deficits that
may contribute to the more successful functional outcomes of such patients.
Limitations and Future Directions
The results reported in the present research need to be assessed and interpreted
within the context of its limitations. Firstly, findings of the study should be considered
to be exploratory in nature considering the small sample sizes. However, unlike some of
the previous studies, the current research sought to collect more detailed information
about potential confounding variables which have been found to have a relationship
with cognitive impairment. These factors include cannabis use parameters such as
frequency, severity/quantity, duration and time since cannabis use cessation (i.e.,
recency). Also, individuals with other neuropsychiatric or neuromedical illnesses were
excluded from the study. Since the recruitment of participants also involved recruiting
suitable matched-pairs (based on a strict matching criteria on a range of variables in
order to restrict the possible impact of these potential confounds), the end result of
smaller sample sizes was difficult to avoid but arguably, offset by the reduction in
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 156
extraneous variation in the sample. Prospective studies with larger sample sizes that
maintain this strict matching process will help to extend the current findings.
Another potential limitation in the current study involves combining both former
users and current users in the primary ANCOVA analyses (due to small sub-sample
sizes). However, as previously mentioned a carefully matched-controls design was used
in an attempt to control for any confounding influence contributed to by variations in
crucial cannabis-use indices (which have been considered to have an impact on
cognitive performance) across individuals. Future studies could look to further examine
the separate cannabis status groups across the executive function measures of interest.
However, it is acknowledged that there would be difficulty in doing so reliably given
that the time since cessation variable is largely self-report in nature. In addition, it is
important to note the impact of small sample sizes on the validity of specific statistical
approaches such as PCA and multiple regression analyses. However, the sample size was
deemed sufficient for exploratory PCA. While findings from some of the multiple
regression analyses failed to reach statistical significance, it is still considered important
to particularly consider the effect sizes of all correlational analyses as indicated by the
shared variance statistic. It is acknowledged that as p values are heavily influenced by
small sample sizes and also do not provide an indication of the size or importance of the
observed effect, future studies with larger samples may inevitably produce additional
statistically significant results. As the shared variances found in the current study are
notably similar to Green et al.’s (2000) meta-analytic findings regarding the range of
detected effect sizes for the relationship between cognition and everyday outcomes in
schizophrenia patients, the results of the present research are in support of previous findings.
Therefore, future research with larger samples is also warranted for further evaluation of
similar effect sizes. The small samples size of the current study also inevitably reduces
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 157
the statistical power of being able to detect what could possibly have been significant
relationships. However, as mentioned the results of the present research are not all that
dissimilar from previous findings.
While the Holm-Bonferroni correction method is not considered as conservative
in controlling for family-wise Type I error as the Bonferroni approach, as with all
corrections the likelihood of Type II error is also increased. Therefore, truly important
clinical differences may be deemed non-significant following correction. It is
considered that the non-significant results emerging from the current study following
Holm-Bonferroni adjustment are plausibly a consequence of the small sample size and
reduced power, instead of reflective of the fact that none of these relationships
exist. Given the similar effect sizes reported in previous research related to the amount
of variance contributed to functional outcome by cognitive variables in the
schizophrenia population (and thus, raising question against the likelihood that the null
hypothesis is actually true), it is thought that it is entirely plausible for such
relationships to not only exist, but also have statistical significance. Therefore, those
results which had reached significance prior to Holm-Bonferroni correction have still
been interpreted in the current study, with the precaution that at least some of these
results may be truly reflective of Type I error. It is also considered that due to the small
sample size, once again it is recommended that effect sizes be taken into account not
just the overall significance level. Prospective studies should look to conduct similar
comparisons with larger samples sizes in order to detect results of larger statistical
significance and robustness.
It is also acknowledged that as the current study included matched-control subjects
with cannabis use as a comparison sample, it is difficult to differentiate between exceptional
performance and cannabis use associated impairment. In other words, the minimal impairment
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 158
shown in the present study could indeed be a reflection of schizophrenia patients with cannabis
use being more likely to be cognitively intact. However, it may also be that the matched-
control sample could be in some way cognitively impaired due to their cannabis use and that
the patient sample was exceptional in their performance. In addition, it is possible that the
cognitive signal for long-term cannabis use is relatively small in comparison to the signal for
schizophrenia such that additional increments are difficult to detect. The latter would not be
implausible as the cognitive deficits associated with chronic or heavy cannabis use are similar
to those seem in schizophrenia, which could serve to narrow the gap between controls and
patients. Although the similarities referred to here are related to deficits, the current
study’s results indicate a significantly larger proportion of schizophrenia patients with
cannabis use demonstrating performances in the clinically non-impaired range. While the
current study did not aim to examine the cumulative effects of schizophrenia and cannabis use,
future studies adopting a similar matched-control design, as well as including the addition of a
cannabis-naive schizophrenia patient sample would help to further elucidate these
relationships.
While it is acknowledged above that the current study did not have a cannabis
naive schizophrenia group or a cannabis-naive healthy control group for comparison,
this is the first study to directly compare between schizophrenic and non-schizophrenic
samples with carefully matched cannabis use histories. We also categorised our subjects
according to their current cannabis use status (i.e., currently using schizophrenia
patients with cannabis use, non-currently using schizophrenia patients with cannabis
use, currently using healthy controls with a similar cannabis use history and non-
currently using healthy controls with a similar cannabis use history) in subsequent
analyses. Rabin and colleagues (2013), also parsed their participants in terms of current
cannabis use status, but in addition included a group of patients containing 8 subjects
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 159
with minimal/no lifetime use. The current study also recruited a similar group, but
instead found individuals with no reported historical use at all (i.e., not even to a
minimal extent). The general pattern of recruited schizophrenia subjects in the present
research included either those patients who reported regular current or former use, or no
lifetime use at all. However, only 4 cannabis-naive individuals with schizophrenia were
recruited over a period of well over a 24-month timeframe. The small sample size of
this similar type of subgroup in Rabin and colleagues’ study may be reflective of related
difficulties faced when recruiting from this population. It is not entirely clear why such
limited numbers in this particular subgroup sample were achieved. One possible
explanation is that similarly to Rabin and colleagues’ (2013) study, recruitment was
community-based with only outpatients being included in these studies. It is reasonable
to consider that the cohort of individuals who are cannabis-naive may have much
smaller numbers in the community due to differing illness presentation, than there are in
the inpatient settings. The current study did not attempt to recruit individuals with
schizophrenia from an inpatient setting in order to reduce the likelihood of the acute
effects of the illness influencing results. The premise suggesting that schizophrenia
patients with cannabis use have better premorbid adjustment, social skills and
prognosis, supports this possibility. Future studies including all possible comparison
groups (i.e., patients with cannabis use (both current and former users), cannabis-naive
patients, healthy matched-controls with a similar cannabis use history (both current and
former users), and cannabis-naive healthy normals), will not only allow for more
informative between-subject comparisons, but will overall increase the generalisability
of findings. In addition, prospective investigations ascertaining the population
dispersion in both inpatient and outpatient settings in relation to patients with cannabis
use versus patients with no reported historical use, may help to provide a better
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 160
understanding of the overall schizophrenia population characteristics, but may also
serve to either additionally confirm or disconfirm the hypothesis related to better
premorbid adjustment in the cohort with cannabis use. Due to the cognitive
heterogeneity associated with this particular disorder (Joyce & Roiser, 2007), future
research should also look to adopt longitudinal designs that use within-subjects
approaches to statistical analyses so that not just cannabis use status, but the cumulative
effects of cannabis use over time, can be further examined within each of these groups.
A common limitation across many of the studies, including this one, when
investigating the relationship between cannabis use and neurocognitive performance
involves the ability to adequately measure the variability regarding different
concentrations of Delta-9-tetrahydrocannabinol (∆9-THC). Over the years the ∆
9-THC
content in cannabis has been seen to progressively increase with the average level of ∆9-
THC content from the 1960’s to 1980’s reported to approximate 1.5-3.5%, with
modern efforts to enhance its potency resulting in concentrations reaching up to 20%
(for discussion, see Adams & Martin, 1996). In addition to this, through more frequent
and deeper inhalation techniques experienced cannabis users are able to modify the dose
if desired, thus further altering concentration levels (Iversen, 2000). This issue poses
challenges regarding the range of variability in ∆9-THC concentration across users and
the difficulty in controlling for this factor in empirical research. However, by
controlling for differences in frequency, severity/quantity, duration of use and length of
abstinence through the matching process in the current study, the present research aimed
to address at least those measurable cannabis-use parameters which are associated with
variability in cannabis use. The objective measurement of ∆9-THC concentration in
future studies would help to address this issue and allow for more control when
considering the variations in cannabis use as a potential confound.
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 161
In the current study, we also did not directly examine the effects of frequency of
use and severity/quantity on neurocognitive performance. However, as mentioned
above, the present research did allow for the assessment of these parameters and
attempted to control for any possible differences via the matched-pairs design. While
the importance of taking into account the possible effects of these particular parameters
in cannabis use research is acknowledged, previous research has suggested that duration
of use may place more of a significant contribution to the development of
neurocognitive impairment when compared to the cannabis use parameters of frequency
of use or quantity (Solowij et al., 2002). Future investigations should look to closely
examine the possible effects of frequency, severity/quantity, duration of use, and time
since last use across all of the abovementioned relevant comparison groups in order to
determine whether these parameters have differential effects depending on group
membership.
As the current study only required the minimum of a 24-hour period of cannabis
use abstinence, it is possible that for the individuals that were current users their results
only represent the early stages of neurocognitive recovery, or quite possibly the
remaining effects from intoxication. However, following ∆9-THC inhalation peak
plasma concentrations and the associated psychotropic effects are thought to reach their
maximum level within 15-30 minutes and that the intoxication effects of the drug begin
to diminish within 2-3 hours (Grotenhermen, 2003). Previous studies have also adopted
a 24-hour abstinence period in an attempt capture the narrow window which occurs
between intoxication and drug withdrawal (Coulston et al., 2007; DeRosse et al., 2010).
In a study conducted by Budney and colleagues (2003) the withdrawal symptoms and
patterns following cannabis use abstinence were examined. Findings showed that the
onset of withdrawal symptoms usually occurred during the first day following cannabis
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 162
use cessation and that the peak of withdrawal symptom effects occurred between 2 and
6 days. It was also found that most effects lasted between 4 and 14 days. However, the
current study did aim to investigate effects associated with different lifetime use
categories (i.e., across current users and abstinent users) and therefore a much longer
period of abstinence prior to testing would have inevitably ruled out the ability to
compare performance with ‘current’ users.
Another limitation of the current study is that the history of drug consumption
was assessed using the self-report of subjects. Given the variability in cannabis use
across individuals, it would be difficult to precisely calculate an individual’s history and
pattern of substance use across their lifetime. Adherence to the 24-hour abstinence
period was also assessed using self-report. However, it has been reported that
information given by subjects about their drug use tends to be relatively reliable
(Brown, Kranzler, & Del Boca, 1992; Harrison, Haaga, & Richards, 1993; Pope et al.,
2001). Self-reports of substance use have also been found to correspond with urine
screen results (Fowler, Carr, Carter, & Lewin, 1998; Pope et al., 2001). In addition, the
quantification of various cannabis use parameters was also based on self-report. While it
has been identified that self-reporting bias is a potential limitation of the current study,
reporting the different aspects of their use is the best proxy that was available (for
example, reporting time since cessation in comparison to actual measurement of this
variable via longitudinal design). It is also acknowledged that the determination of the
validity and reliability of self-report measures of drug use in people with schizophrenia
is currently lacking. This may be particularly problematic given their difficulties with
memory and other cognitive abilities. Future studies aiming to replicate the current
study’s findings should consider the utilisation of drug-usage screening processes in
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 163
order to supplement the information provided in user self-reports and help to provide an
overall more accurate picture of actual use.
Conclusions
The main aims of the present study were to examine the patterns of executive
functioning of schizophrenia patients with cannabis use in comparison to otherwise
healthy matched-controls with a similar cannabis use history. An examination of these
complex neurocognitive profiles has clinical significance due to the reported
relationship between executive function and the functional outcome of individuals with
the illness. An additional aim was to also investigate the relationship between level of
executive functioning and ‘real world’ functional outcome. The identification of
specific functional outcome predictors is also considered a research area with important
clinical applications due to its potential in not only being able to more effectively guide
rehabilitation plans, but ultimately increasing their efficacy by offering specific
information regarding possible practical rehabilitation modifications. For example, in
order to better assist and support individuals with retrospective memory deficits,
strategies aimed at improving hygiene and grooming skills may not only involve more
structured approaches, but that these approaches could be chunked into smaller parts in
order to reduce the number of cognitive and behavioural units held online and
ultimately facilitate the more effective recall and implementation of these learned
strategies. In addition, for those with difficulties in interference control, strategies aimed
at improving interpersonal, cooking, resource utilisation and medication management
skills could involve once again more structured approaches, but also include the
repeated practice and rehearsal of scenarios where there are changing or interfering
circumstances, or even with distractors present. Such rehearsal could help to enhance
more fluency in behavioural adaptation to differing/interfering environmental demands
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 164
and possibly reduce the tendency for responses to be based on task-inappropriate
habitual behavioural patterns.
The strengths of the current study lie in the comprehensive assessment of a
multi-faceted neurocognitive construct thought to be related to functional outcome.
While the results are encouraging, this study also highlights the difficulties and
complexities associated in adequately assessing executive function. This is the first
study to use this type of assessment methodology in a clinical population such as
schizophrenia. The present findings also reinforce the importance of considering
executive function as a complex cognitive construct that is not always adequately
tapped into via the use of traditional executive functioning measures and this may help
to explain the previous mixed findings. Despite the noted limitations in the present
study, the comprehensive assessment of executive function has afforded an extension to
the existing knowledge about the neurocognitive profiles of schizophrenia patients with
cannabis use. This study also builds upon previous research by examining the relative
contribution of different components of executive function in explaining performance in
functional outcome. Specifically, the findings of this study suggest that schizophrenia
patients with cannabis use reported to demonstrate inadequate functional outcome in the
particular areas of hygiene and grooming, interpersonal skills, cooking, resource
utilization, and medication management, as well as overall functional outcome related
to independent living skills, may be partially due to less intact or poorer retrospective
memory and interference control abilities.
The neurocognitive measures used in the current study represented a theoretical
hierarchical model of the construct of executive function, allowing us to tap into a wide
range of executive processes. However, what remains unclear is which particular facets
of executive function contribute to the most crucial areas of functional outcome. Given
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 165
this identified gap, future research is needed in order to determine which aspects of
functional outcome are the most important to successful independent living.
While further research is required to replicate the study’s findings, the current
work suggests that neuropsychological evidence can be instrumental in identifying
specific neurocognitive domains and their likely functional outcome correlates. The
results of the present study and the applications of its findings, also suggest that
tailoring rehabilitation plans utilising neurocognitive information specific to particular
clinical subgroups has the potential to improve the functional outcomes of patients.
Prospective studies should endeavour to build upon such approaches and while the
future of research in the area may present its associated challenges, it also has the
potential to offer considerable promise. ‘Schizophrenia in the past was a grim diagnosis
with a poor prognosis. At the present time, it can probably be better described as a
serious condition, with plenty of reasons to be hopeful’ (Green & Harvey, 2014, p. 7).
The methodological approach of the current study is considered a first step in the
development of neurocognitive process-specific training protocols for this particular
subgroup of the schizophrenia population.
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 166
References
Acker, M. B. (1990). A review of the ecological validity of neuropsychological tests. In
D. E. Tupper & K. D. Cicerone (Eds.), The Neuropsychology of Everyday Life:
Assessment and Basic Competencies (Vol. 2, pp. 19–55). Boston, MA: Kluwer
Academic Publishers.
Adams, I. B., & Martin, B. R. (1996). Cannabis: pharmacology and toxicology in
animals and humans. Addiction, 91(11), 1585–1614. doi: 10.1046/j.1360-
0443.1996.911115852.x
Addington, J., & Duchak, V. (1997). Reasons for substance use in schizophrenia. Acta
Psychiatrica Scandinavica, 96(5), 329–333. doi:10.1111/j.1600-
0447.1997.tb09925.x
Allen, H. A., Liddle, P. F., & Frith, C. D. (1993). Negative features, retrieval processes
and verbal fluency in schizophrenia. The British Journal of Psychiatry, 163(6),
769–775. doi:10.1192/bjp.163.6.769
Alfonso, J. P., Caracuel, A., Delgado-Pastor, L. C., & Verdejo-García, A. (2011).
Combined goal management training and mindfulness meditation improve
executive functions and decision-making performance in abstinent
polysubstance abusers. Drug and Alcohol Dependence, 117(1), 78–81.
doi:10.1016/j.drugalcdep.2010.12.025
Alvarez, J. A., & Emory, E. (2006). Executive function and the frontal lobes: A meta-
analytic review. Neuropsychology Review, 16(1), 17–42. doi:10.1007/s11065-
006-9002-x
Andres, E. (2003). Drug-induced hepatotoxicity. New England Journal of Medicine,
349(20), 1974–1976. doi:10.1056/NEJM200311133492021
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 167
American Psychiatric Association (1994). Diagnostic and Statistical Manual of Mental
Disorders (Fourth Edition). Washington, D.C.: American Psychiatric
Association.
Arndt, S., Tyrrell, G., Flaum, M., & Andreasen, N. C. (1992). Comorbidity of substance
abuse and schizophrenia: the role of pre-morbid adjustment. Psychological
Medicine, 22(2), 379–388. doi:10.1017/S0033291700030324
Arseneault, L., Cannon, M., Poulton, R., Murray, R., Caspi, A., & Moffitt, T. E. (2002).
Cannabis use in adolescence and risk for adult psychosis: longitudinal
prospective study. British Medical Journal, 325(7374), 1212–1213.
doi:10.1136/bmj.325.7374.1212
Arseneault, L., Cannon, M., Witton, J., & Murray, R. M. (2004). Causal association
between cannabis and psychosis: Examination of the evidence. The British
Journal of Psychiatry, 184(2), 110–117. doi:10.1192/bjp.184.2.110
Auslander, L. A., Lindamer, L. L., Delapena, J., Harless, K., Polichar, D., Patterson, T.
L., ... Jeste, D. V. (2001). A comparison of community‐dwelling older
schizophrenia patients by residential status. Acta Psychiatrica Scandinavica,
103(5), 380–386. doi:10.1034/j.1600-0447.2001.00262.x
Baddeley, A. (1996). Exploring the central executive. The Quarterly Journal of
Experimental Psychology: Section A, 49(1), 5–28. doi:10.1080/713755608
Baddeley, A. (1998). The central executive: A concept and some misconceptions.
Journal of the International Neuropsychological Society, 4(5), 523–526.
doi:10.1017/s135561779800513x
Baddeley, A. D., Della Sala, S., Papagno, C., & Spinnler, H. (1997). Dual-task
performance in dysexecutive and non-dysexecutive patients with a frontal lesion.
Neuropsychology, 11(2), 187–194. doi:10.1037/0894-4105.11.2.187
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 168
Baddeley, A., Della Sala, S., Robbins, T. W., & Baddeley, A. (1996). Working memory
and executive control [and discussion]. Philosophical Transactions of the Royal
Society of London, Series B: Biological Sciences, 351(1346), 1397–1404.
doi:10.1098/rstb.1996.0123
Baldo, J. V., Shimamura, A. P., Delis, D. C., Kramer, J., & Kaplan, E. (2001). Verbal
and design fluency in patients with frontal lobe lesions. Journal of the
International Neuropsychological Society, 7(5), 586–596. Retrieved from
http://www.ebire.org/aphasia/baldo/verbal_design.PDF
Barnes, T. R. E., Mutsatsa, S. H., Hutton, S. B., Watt, H. C., & Joyce, E. M. (2006).
Comorbid substance use and age at onset of schizophrenia. The British Journal
of Psychiatry, 188(3), 237–242. doi:10.1192/bjp.bp.104.007237
Barnett, J. H., Salmond, C. H., Jones, P. B., & Sahakian, B. J. (2006). Cognitive reserve
in neuropsychiatry. Psychological Medicine, 36(8), 1053–1064.
doi:10.1017/S0033291706007501
Bartels, S. J., Teague, G. B., Drake, R. E., Clark, R. E., Bush, P. W., & Noordsy, D. L.
(1993). Substance abuse in schizophrenia: Service utilization and costs. Journal
of Nervous Mental Disease, 181(4), 227–232. doi:10.1097/00005053-
199304000-00003
Battisti, R. A., Roodenrys, S., Johnstone, S. J., Respondek, C., Hermens, D. F., &
Solowij, N. (2010). Chronic use of cannabis and poor neural efficiency in verbal
memory ability. Psychopharmacology, 209(4), 319–330. doi: 10.1007/s00213-
010-1800-4
Beatty, W. W., Jocic, Z., Monson, N., & Katzung, V. M. (1994). Problem solving by
schizophrenic and schizoaffective patients on the Wisconsin and California Card
Sorting Tests. Neuropsychology, 8(1), 49–54. doi:10.1037/0894-4105.8.1.49
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 169
Beatty, W. W., Katzung, V. M., Moreland, V. J., & Nixon, S. J. (1995).
Neuropsychological performance of recently abstinent alcoholics and cocaine
abusers. Drug and Alcohol Dependence, 37(3), 247–253. doi:10.1016/0376-
8716(94)01072-S
Bechara, A. (2001). Neurobiology of decision-making: risk and reward. Seminars in
Clinical Neuropsychiatry, 6(3), 205–216. doi:10.1053/scnp.2001.22927
Bechara, A., Damasio, A. R., Damasio, H., & Anderson, S. W. (1994). Insensitivity to
future consequences following damage to human prefrontal cortex. Cognition,
50(1-3), 7–15. doi:10.1016/0010-0277(94)90018-3
Bechara, D., Damasio, H., Damasio, A. R., & Lee, G. P. (1999). Different contributions
of the human amygdala and ventromedial prefrontal cortex to decision-making.
Journal of Neuroscience, 19(13), 5473–5481. Retrieved from
http://www.jneurosci.org/content/19/13/5473.full.pdf+html
Bechara, A., Damasio, H., Tranel, D., & Damasio, A. R. (1997). Deciding
advantageously before knowing the advantageous strategy. Science, 275(5304),
1293–1295. doi:10.1126/science.275.5304.1293
Bechara, A., Dolan, S., Denburg, N., Hindes, A., Anderson, S. W., & Nathan, P. E.
(2001). Decision-making deficits, linked to a dysfunctional ventromedial
prefrontal cortex, revealed in alcohol and stimulant abusers. Neuropsychologia,
39(4), 376–389. doi:10.1016/S0028-3932(00)00136-6
Bechara, A., & Martin, E. M. (2004). Impaired decision making related to working
memory deficits in individuals with substance addictions. Neuropsychology,
18(1), 152–162. doi:10.1037/0894-4105.18.1.152
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 170
Bechara, A., Tranel, D., & Damasio, H. (2000). Characterization of the decision-making
deficit of patients with ventromedial prefrontal cortex lesions. Brain, 123(Pt 11),
2189–2202. doi:10.1093/brain/123.11.2189
Bechara, A., Tranel, D., Damasio, H., & Damasio, A. R. (1996). Failure to respond
autonomically to anticipated future outcomes following damage to prefrontal
cortex. Cerebral Cortex, 6(2), 215–225. doi:10.1093/cercor/6.2.215
Bell, M., Bryson, G., Greig, T., Corcoran, C., & Wexler, B. E. (2001). Neurocognitive
enhancement therapy with work therapy: effects on neuropsychological test
performance. Archives of General Psychiatry, 58(8), 763–768.
doi:10.1001/archpsyc.58.8.763.
Bell, M. D., Zito, W., Greig, T., & Wexler, B. E. (2008). Neurocognitive enhancement
therapy with vocational services: work outcomes at two-year follow-up.
Schizophrenia Research, 105(1), 18–29. doi:10.1016/j.schres.2008.06.026
Bellack, A. S., Dickinson, D., Morris, S. E., & Tenhula, W. N. (2005). The development
of a computer-assisted cognitive remediation program for patients with
schizophrenia. Israel Journal of Psychiatry and Related Sciences, 42(1), 5–14.
Retrieved from
http://gender.mednet.co.il/upload/infocenter/info_images/1405200621046PM@
Pages%20from%20IJP-42-1-5.pdf
Benedet, M. J., & Alejandre, M. A. (1998). Test de Aprendizaje Verbal Espan˜a-
Complutense. Madrid: TEA ediciones.
Benedict, R. H. B. (1997). Brief Visuospatial Memory Test - Revised. Odessa, Florida:
Psychological Assessment Resources.
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 171
Bennett, P. C., Ong, B., & Ponsford, J. (2005). Assessment of executive dysfunction
following traumatic brain injury: Comparison of the BADS with other clinical
neuropsychological measures. Journal of the International Neuropsychological
Society, 11(5), 606–613. doi:10.1017/s1355617705050721
Benton, A. L. (1968). Differential behavioral effects in frontal lobe disease.
Neuropsychologia, 6(1), 53–60. doi:10.1016/0028-3932(68)90038-9
Benton, A. L., & Hamsher, K. (1976). The Multilingual Aphasia Examination. Iowa
City, IA: University of Iowa Press.
Benton, A. L., Hamsher, K., & Sivan, A. B. (1983). Multilingual Aphasia Examination
(Third Edition). Iowa City, IA: AJA Associates.
Bilder, R. M., Lipschutz-Broch, L., Reiter, G., Geisler, S. H., Mayerhoff, D. I., &
Lieberman, J. A. (1992). Intellectual deficits in first-episode schizophrenia:
evidence for progressive deterioration. Schizophrenia Bulletin, 18(3), 437–448.
doi:10.1093/schbul/18.3.437
Bleuler, E. (1911). (Translated by J. Zinkin, published in 1950). Dementia Praecox of
the Group of Schizophrenias. New York: International Universities Press.
Bolla, K. I., Brown, K., Eldreth, D., Tate, K., & Cadet, J. L. (2002). Dose-related
neurocognitive effects of marijuana use. Neurology, 59(9), 1337–1343.
doi:10.1212/01.wnl.0000031422.66442.49
Bolla, K. I., Cadet, J., &London, E. D. (1998). The neuropsychiatry of chronic cocaine
use. Journal of Neuropsychiatry and Clinical Neurosciences, 10(3), 280–289.
Bolla, K. I., Eldreth, D. A., London, E. D., Kiehl, K. A., Mouratidis, M., Contoreggi, C.,
... Ernst, M. (2003). Orbitofrontal cortex dysfunction in abstinent cocaine
abusers performing a decision-making task. Neuroimage, 19(3), 1085–1094.
doi:10.1016/S1053-8119(03)00113-7
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 172
Bolla, K., Eldreth, D., Matochik, J., & Cadet, J. (2005). Neural substrates of faulty
decision-making in abstinent marijuana users. Neuroimage, 26(2), 480–492.
doi:10.1016/j.neuroimage.2005.02.012
Bowen, S., Chawla, N., Collins, S. E., Witkiewitz, K., Hsu, S., Grow, J., ... Marlatt,
A. (2009). Mindfulness-based relapse prevention for substance use disorders: A
pilot efficacy trial. Substance Abuse, 30(4), 295–305. doi:
10.1080/08897070903250084
Bowen, S., & Marlatt, A. (2009). Surfing the urge: brief mindfulness-based intervention
for college student smokers. Psychology of Addictive Behaviors, 23(4), 666–671.
doi:10.1037/a0017127
Bowie, C. R., McGurk, S. R., Mausbach, B., Patterson, T. L., & Harvey, P. D. (2012).
Combined cognitive remediation and functional skills training for schizophrenia:
effects on cognition, functional competence, and real-world behavior. American
Journal of Psychiatry, 169(7), 710–718. doi:10.1176/appi.ajp.2012.11091337
Braff, D. L. (1993). Information processing and attention dysfunctions in schizophrenia.
Schizophrenia Bulletin, 19(2), 233–259. doi.org/10.1093/schbul/19.2.233
Braff, D. L., Heaton, R., Kuck, J., Cullum, M., Moranville, J., Grant, I., & Zisook, S.
(1991). The generalized pattern of neuropsychological deficits in outpatients
with chronic schizophrenia with heterogeneous Wisconsin Card Sorting Test
results. Archives of General Psychiatry, 48(10), 891–898.
doi:10.1001/archpsyc.1991.01810340023003
Broadbent, D. E., Cooper, P. F., FitzGerald, P., & Parkes, K. R. (1982). The cognitive
failures questionnaire (CFQ) and its correlates. British Journal of Clinical
Psychology, 21(Pt 1), 1–16. doi:10.1111/j.2044-8260.1982.tb01421.x
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 173
Brown, J., Kranzler, H. R., & Del Boca, F. K. (1992). Self‐reports by alcohol and drug
abuse inpatients: factors affecting reliability and validity. British Journal of
Addiction, 87(7), 1013–1024. doi:10.1111/j.1360-0443.1992.tb03118.x
Bryan, J., & Luszcz, M. A. (2000). Measurement of executive function: considerations
for detecting adult age differences. Journal of Clinical and Experimental
Neuropsychology, 22(1), 40–55. doi.org/10.1076/1380-3395(200002)22:1;1-
8;FT040
Bryan, J., & Luszcz, M. A. (2001). Adult age differences in self-ordered pointing task
performance: Contributions from working memory, executive function and
speed of information processing. Journal of Clinical and Experimental
Neuropsychology, 23(5), 608–619. doi:10.1076/jcen.23.5.608.1250
Budney, A. J., Moore, B. A., Vandrey, R. G., & Hughes, J. R. (2003). The time course
and significance of cannabis withdrawal. Journal of Abnormal Psychology,
112(3), 393–402. doi:10.1037/0021-843X.112.3.393
Buelow, M. T., & Suhr, J. A. (2009). Construct validity of the Iowa Gambling Task.
Neuropsychology Review, 19(1), 102–114. doi: 10.1007/s11065-009-9083-4
Burgess, P. W. (1997). Theory and methodology in executive function research. In P.
Rabbit (Ed.), Methodology of Frontal and Executive Function (pp. 81–116). East
Sussex, UK: Psychology Press Ltd.
Burgess, P. W. (2000). Strategy application disorder: The role of the frontal lobes in
human multitasking. Psychological Research, 63(3-4), 279–288.
doi:10.1007/s004269900006
Burgess, P. W., Alderman, N., Evans, J., Emslie, H., & Wilson, B. A. (1998). The
ecological validity of tests of executive function. Journal of the International
Neuropsychological Society, 4(6), 547–558. doi:10.1017/S1355617798466037
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 174
Burgess, P. W., Alderman, N., Wilson, B. A., Evans, J. J., & Emslie, H. (1996). The
dysexecutive questionnaire. Behavioural Assessment of the Dysexecutive
Syndrome. Bury St. Edmunds, U.K.: Thames Valley Test Company.
Burgess, P. W., Veitch, E., de Lacy Costello, A., & Shallice, T. (2000). The cognitive
and neuroanatomical correlates of multitasking. Neuropsychologia, 38(6), 848–
863. doi:10.1016/s0028-3932(99)00134-7
Cannon-Spoor, H. E., Potkin, S. G., & Wyatt, R. J. (1982). Measurement of premorbid
adjustment in chronic schizophrenia. Schizophrenia Bulletin, 8(3), 470–484.
doi:dx.doi.org/10.1093/schbul/8.3.470
Cantwell, R., Brewin, J., Glazebrook, C., Dalkin, T., Fox, R., Medley, I., & Harrison, G.
(1999). Prevalence of substance misuse in first-episode psychosis. The British
Journal of Psychiatry, 174(2), 150–153. doi:10.1192/bjp.174.2.150
Capri, S. (1994). Methods for evaluation of the direct and indirect costs of long-term
schizophrenia. Acta Psychiatrica Scandinavica, 89(S382), 80–83.
doi:10.1111/j.1600-0447.1994.tb05871.x
Caspari, D. (1999). Cannabis and schizophrenia: Results of a follow-up study.
European Archives of Psychiatry and Clinical Neuroscience, 249(1), 45–49.
doi:10.1007/s004060050064
Chaytor, N., & Schmitter-Edgecombe, M. (2003). The ecological validity of
neuropsychological tests: A review of the literature on everyday cognitive skills.
Neuropsychology Review, 13(4), 181–197.
doi:10.1023/B:NERV.0000009483.91468.fb
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 175
Chen, E. Y. H., Lam, L. C. W., Chen, R. Y. L., Nguyen, D. G. H., & Chan, C. K. Y.
(1996). Prefrontal neuropsychological impairment and illness duration in
schizophrenia: A study of 204 patients in Hong Kong. Acta Psychiatrica
Scandinavica, 93(2), 144–150. doi:10.1111/j.1600-0447.1996.tb09816.x
Chen, R. Y. L., Chen, E. Y. H., Chan, C. K. Y., Lam, L. C. W., & Lieh-Mak, F. (2000).
Verbal fluency in schizophrenia: Reduction in semantic store. Australian and
New Zealand Journal of Psychiatry, 34(1), 43–48. doi:10.1046/j.1440-
1614.2000.00647.x
Chevignard, M. P., Taillefer, C., Picq, C., Poncet, F., Noulhiane, M., & Pradat-Diehl, P.
(2008). Ecological assessment of the dysexecutive syndrome using execution of
a cooking task. Neuropsychological Rehabilitation, 18(4), 461–485.
doi:10.1080/09602010701643472
Conners, C. K. (2000). Conners’ Continuous Performance Test II: Computer Program
for Windows Technical Guide and Software Manual. North Tonawanda, NY:
Multi-Health Systems.
Cohen, J. (1988). Statistical Power Analysis for the Behavioral Sciences (Second
Edition). Hillsdale, NJ: Erlbaum.
Cornblatt, B. A., Risch, N. J., Faris, G., Friedman, D., & Erlenmeyer-Kimling, L.
(1988). The Continuous Performance Test, identical pairs version (CPT-IP): I.
New findings about sustained attention in normal families. Psychiatry Research,
26(2), 223–238. doi:10.1016/0165-1781(88)90076-5
Cornblatt, B., Obuchowski, M., Schnur, D., & O'Brien, J. D. (1998). Hillside study of
risk and early detection in schizophrenia. The British Journal of Psychiatry,
172(33), 26–32. Retrieved from http://0-bjp.rcpsych.org.
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 176
Coulston, C., Perdices, M., & Tennant, C. (2007). The neuropsychological correlates of
cannabis use in schizophrenia: Lifetime abuse/dependence, frequency of use,
and recency of use. Schizophrenia Research, 96(1-3), 169–184.
doi:10.1016/j.schres.2007.08.006
Crawford, J. R., Besson, J. A., Bremner, M., Ebmeier, K. P., Cochrane, R. H., &
Kirkwood, K. (1992). Estimation of premorbid intelligence in schizophrenia.
The British Journal of Psychiatry, 161(1), 69–74. doi: 10.1192/bjp.161.1.69
Crawford, J. R., Obonsawin, M. C., & Bremner, M. (1993). Frontal lobe impairment in
schizophrenia: relationship to intellectual functioning. Psychological Medicine,
23(3), 787–787. doi:10.1017/S0033291700025563
Crean, R. D., Crane, N. A., & Mason, B. J. (2011). An evidence-based review of acute
and long-term effects of cannabis use on executive cognitive functions. Journal
of Addiction Medicine, 5(1), 1–8. doi:10.1097/ADM.0b013e31820c23fa
Cripe, L. I. (1996). The ecological validity of executive function testing. In R. J.
Sbordone & C. J. Long (Eds.), Ecological Validity of Neuropsychological
Testing (pp. 171–202). Delray beach, FL: GR Press/St. Lucie Press.
Cuffel, B. J., Heithoff, K. A., & Lawson, W. (1993). Correlates of patterns of substance
abuse among patients with schizophrenia. Hospital and Community Psychiatry,
44(3), 247–251.
Daigneault, S., Braün, C. M. J., & Whitaker, H. A. (1992). An empirical test of two
opposing theoretical models of prefrontal function. Brain and Cognition, 19(1),
48–71. doi:10.1016/0278-2626(92)90037-m
Damasio, A. R. (1995). Review: Toward a neurobiology of emotion and feeling:
Operational concepts and hypotheses. The Neuroscientist, 1(1), 19–25.
doi:10.1177/107385849500100104
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 177
Damasio, A. R., Everitt, B. J., & Bishop, D. (1996). The somatic marker hypothesis and
the possible functions of the prefrontal cortex [and discussion]. Philosophical
Transactions of the Royal Society of London, Series B: Biological Sciences,
351(1346), 1413–1420. Retrieved from http://www.jstor.org/stable/3069187
Damasio, H., Grabowski, T., Frank, R., Galaburda, A., & Damasio, A. (1994). The
return of phineas gage: Clues about the brain from the skull of a famous patient.
Science, 264(5162), 1102–1105. doi:10.1126/science.8178168
Daneman, M., & Carpenter, P. A. (1980). Individual differences in working memory
and reading. Journal of Verbal Learning and Verbal Behaviour, 19(4), 450–466.
doi:10.1016/s0022-5371(80)90312-6
Davidson, L. L., & Heinrichs, R. W. (2003). Quantification of frontal and temporal lobe
brain-imaging findings in schizophrenia: A meta-analysis. Psychiatry Research:
Neuroimaging, 122(2), 69–87. doi:10.1016/s0925-4927(02)00118-x
Degenhardt, L., Hall, W., & Lynskey, M. (2003). Exploring the association between
cannabis use and depression. Addiction, 98(11), 1493–1504. doi:10.1046/j.1360-
0443.2003.00437.x
Degenhardt, L., Lynskey, M., & Hall, W. (2000). Cohort trends in the age of initiation
of drug use in Australia. Australian and New Zealand Journal Public Health,
24(4), 421–426. doi:10.1111/j.1467-842X.2000.tb01605.x
Delis, D. C., Kaplan, E., & Kramer, J. H. (2001). The Delis-Kaplan Executive Function
System. San Antonio, TX: The Psychological Corporation, p. 47.
Delis, D. C., Kramer, J. H., Kaplan, E., & Ober, B., (2000). California Verbal Learning
Test (Second Edition) (CVLT-II). San Antonia, TX: The Psychological
Corporation.
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 178
Delis, D. C., Squire, L. R., Bihrle, A., & Massman, P. (1992). Componential analysis of
problem-solving ability: Performance of patients with frontal lobe damage and
amnesic patients on a new sorting test. Neuropsychologia, 30(8), 683–697.
doi:10.1016/0028-3932(92)90039-O
Dempster, F. N. (1993). Resistance to interference: Developmental changes in a basic
processing mechanism. In M. L. Howe & R. Pasnak (Eds), Emerging Themes in
Cognitive Development (pp. 3-27). New York: Springer New York. doi:
10.1007/978-1-4613-9220-0_1
DeRosse, P., Kaplan, A., Burdick, K. E., Lencz, T., & Malhotra, A. K. (2010). Cannabis
use disorders in schizophrenia: Effects on cognition and symptoms.
Schizophrenia Research, 120(1-3), 95–100. doi:10.1016/j.schres.2010.04.007
Dickinson, D., & Coursey, R. D. (2002). Independence and overlap among
neurocognitive correlates of community functioning in schizophrenia.
Schizophrenia Research, 56(1-2), 161–170.
doi:10.1016/s0920-9964(01)00229-8
Dickinson, D., Iannone, V. N., & Gold, J. M. (2002). Factor structure of the Wechsler
Adult Intelligence Scale–III in schizophrenia. Assessment, 9(2), 171–180. doi:
10.1177/10791102009002008
Dickinson, D., Ragland, J. D., Gold, J. M., & Gur, R. C. (2008). General and specific
cognitive deficits in schizophrenia: Goliath defeats David?. Biological
Psychiatry, 64(9), 823–827. doi: 10.1016/j.biopsych.2003.12.010
DiClemente, C. C. (1993). Changing addictive behaviors: A process perspective.
Current Directions in Psychological Science, 2(4), 101–106. Retrived from
http://www.jstor.org/stable/20182215
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 179
Diller, L., Ben-Yishay, T., Gerstman, L. J., Goodkin, R., Gordon, W., & Weinberg, J.
(1974). Studies in cognition and rehabilitation in hemiplegia. New York, NY:
University Medical Centre.
Dimitrov, M., Phipps, M., Zahn, T. P., & Grafman, J. (1999). A thoroughly modern
Gage. Neurocase, 5(4), 345–354. doi: 10.1080/13554799908411987
Dixon, L., Haas, G., Weiden, P. J., Sweeney, J., & Frances, A. J. (1991). Drug abuse in
schizophrenic patients: Clinical correlates and reasons for use. American Journal
of Psychiatry, 148(2), 224–230.
Dodrill, C. B. (1978). A neuropsychological battery for epilepsy. Epilepsia, 19(6), 611–
623. doi:10.1111/j.1528-1157.1978.tb05041.x
Dragt, S., Nieman, D. H., Becker, H. E, van de Fliert, R., Dingemans, P. M., de Haan,
L., van Amelsvoort, T. A., & Linszen, D. H. (2010). Age of onset of cannabis
use is associated with age of onset of high-risk symptoms for psychosis.
Canadian Journal of Psychiatry, 55(3), 165–71.
Drake, R. E., Osher, F. C., & Wallach, M. A. (1991). Homelessness and dual diagnosis.
American Psychologist, 46(11), 1149–1158. doi:10.1037/0003-066x.46.11.1149
Dritschel, B., Kogan, L., Burton, A., Burton, E., & Goddard, L. (1998). Everyday
planning difficulties following traumatic brain injury: A role for
autobiographical memory. Brain Injury, 12(10), 875–886.
doi:10.1080/026990598122098
Duncan, J. (1986). Disorganisation of behaviour after frontal lobe damage. Cognitive
Neuropsychology, 3(3), 271–290. doi: 10.1080/02643298608253360
Duncan, J., Emslie, H., Williams, P., Johnson, R., & Freer, C. (1996). Intelligence and
the frontal lobe: The organization of goal-directed behavior. Cognitive
Psychology, 30(3), 257–303. doi:10.1006/cogp.1996.0008
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 180
Duncan, J., Johnson, R., Swale, M., & Freer, C. (1997). Frontal lobe deficits after head
injury: Unity and diversity of function. Cognitive Neuropsychology, 14(5), 713–
741. doi:10.1080/026432997381420
Dunn, B. D., Dalgleish, T., & Lawrence, A. D. (2006). The somatic marker hypothesis:
A critical evaluation. Neuroscience & Biobehavioral Reviews, 30(2), 239–271.
doi:10.1016/j.neubiorev.2005.07.001
Einstein, G. O., McDaniel, M. A., Richardson, S. L., Guynn, M. J., & Cunfer, A. R.
(1995). Aging and prospective memory: examining the influences of self-
initiated retrieval processes. Journal of Experimental Psychology: Learning,
Memory, and Cognition, 21(4), 996–1007. doi:10.1037/0278-7393.21.4.996
Elliott, R., McKenna, P. J., Robbins, T. W., & Sahakian, B. J. (1995).
Neuropsychological evidence for frontostriatal dysfunction in schizophrenia.
Psychological medicine, 25(3), 619–630. doi:10.1017/S0033291700033523
Erceg-Hurn, D. M., & Mirosevich, V. M. (2008). Modern robust statistical methods: an
easy way to maximize the accuracy and power of your research. American
Psychologist, 63(7), 591–601. doi:10.1037/0003-066X.63.7.591
Eriksen, B. A., & Eriksen, C. W. (1974). Effects of noise letters upon the identification
of a target letter in a nonsearch task. Perception & Psychophysics, 16(1), 143–
149. doi: 10.3758/BF03203267
Ernst, M., Grant, S. J., London, E. D., Contoreggi, C. S., Kimes, A. S., & Spurgeon, L.
(2003). Decision making in adolescents with behavior disorders and adults with
substance abuse. American Journal of Psychiatry, 160(1), 33–40.
doi:10.1176/appi.ajp.160.1.33
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 181
Evans, J. J., Chua, S. E., McKenna, P. J., & Wilson, B. A. (1997). Assessment of the
dysexecutive syndrome in schizophrenia. Psychological Medicine, 27(3), 635–
646. doi:10.1017/s0033291797004790
Ferraro, L., Russo, M., O'Connor, J., Wiffen, B. D., Falcone, M. A., Sideli, L., ... Di
Forti, M. (2013). Cannabis users have higher premorbid IQ than other patients
with first onset psychosis. Schizophrenia Research, 150(1), 129–135.
doi:10.1016/j.schres.2013.07.046
Field, A. (2009). Discovering statistics using SPSS (and sex and drugs and rock ‘n’roll)
(Third Edition). London: Sage publications.
Finkelstein, J. R. J., Cannon, T. D., Gur, R. E., Gur, R. C., & Moberg, P. (1997).
Attentional dysfunctions in neuroleptic-naive and neuroleptic-withdrawn
schizophrenic patients and their siblings. Journal of Abnormal Psychology,
106(2), 203–212. doi:10.1037//0021-843x.106.2.203
Fisk, J., & Montgomery, C. (2008). Real-world memory and executive processes in
cannabis users and non-users. Journal of Psychopharmacology, 22(7), 727–736.
doi:10.1177/0269881107084000
Fisk, J. E., & Sharp, C. A. (2004). Age-related impairment in executive functioning:
Updating, inhibition, shifting, and access. Journal of Clinical and Experimental
Neuropsychology, 26(7), 874–890. doi: 10.1080/13803390490510680
Fisk, J. E., & Warr, P. (1996). Age and working memory: the role of perceptual speed,
the central executive, and the phonological loop. Psychology and Aging, 11(2),
316–323. doi:10.1037/0882-7974.11.2.316
Fortin, S., Godbout, L., & Braun, C. (2003). Cognitive structure of executive deficits in
frontally lesioned head trauma patients performing activities of daily living.
Cortex, 39(2), 273–291. doi:10.1016/s0010-9452(08)70109-6
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 182
Fowler, I. L., Carr, V. J., Carter, N. T., & Lewin, T. J. (1998). Patterns of current and
lifetime substance use in schizophrenia. Schizophrenia bulletin, 24(3), 443–455.
Retrieved from http://schizophreniabulletin.oxfordjournals.org/content/24/3/443
Franke, P., Maier, W., Hain, C., & Klingler, T. (1992). Wisconsin Card Sorting Test:
An indicator of vulnerability to schizophrenia? Schizophrenia Research, 6(3),
243–249. doi:10.1016/0920-9964(92)90007-r
Franke, P., Maier, W., Hardt, J., Frieboes, R., Lichtermann, D., & Hain, C. (1993).
Assessment of frontal lobe functioning in schizophrenia and unipolar major
depression. Psychopathology, 26(2), 76–84. doi:10.1159/000284803
Friedman, N. P., & Miyake, A. (2004). The relations among inhibition and interference
control functions: a latent-variable analysis. Journal of Experimental
Psychology: General, 133(1), 101–135. doi: 10.1037/0096-3445.133.1.101
Fristoe, N. M., Salthouse, T. A., & Woodard, J. L. (1997). Examination of age-related
deficits on the Wisconsin Card Sorting Test. Neuropsychology, 11(3), 428–436.
doi:10.1037/0894-4105.11.3.428
Frith, C. D., Friston, K. J., Herold, S., Silbersweig, D., Fletcher, P., Cahill, C., ... Liddle,
P. F. (1995). Regional brain activity in chronic schizophrenic patients during the
performance of a verbal fluency task. The British Journal of Psychiatry, 167(3),
343–349. doi:10.1192/bjp.167.3.343
Funahashi, K. I. (1989). On the approximate realization of continuous mappings by
neural networks. Neural Networks, 2(3), 183–192. doi: 10.1016/0893-
6080(89)90003-8
Fuster, J. M. (1973). Unit activity in prefrontal cortex during delayed-response
performance: neuronal correlates of transient memory. Journal of
Neurophysiology, 36, 61–78.
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 183
Fuster, J. M. (1989). The Prefrontal Cortex (Second Edition). New York, NY: Raven
Press.
Fuster, J. M. (2000). Executive frontal functions. Experimental Brain Research, 133(1),
66–70. doi: 10.1007/s002210000401
Godbout, L., Grenier, M. C., Braun, C. M. J., & Gagnon, S. (2005). Cognitive structure
of executive deficits in patients with frontal lesions performing activities of daily
living. Brain Injury, 19(5), 337–348. doi:10.1080/02699050400005093
Goel, V., Grafman, J., Tajik, J., Gana, S., & Danto, D. (1997). A study of the
performance of patients with frontal lobe lesions in a financial planning task.
Brain, 120(10), 1805–1822. doi:10.1093/brain/120.10.1805
Goethals, I., Audenaert, K., Van de Wiele, C., & Dierckx, R. (2004). The prefrontal
cortex: Insights from functional neuroimaging using cognitive activation tasks.
European Journal of Nuclear Medicine and Molecular Imaging, 31(3), 408–
416. doi:10.1007/s00259-003-1382-z
Gold, J. M. (2004). Cognitive deficits as treatment targets in schizophrenia.
Schizophrenia Research, 72(1), 21–28. doi:10.1016/j.schres.2004.09.008
Gold, J. M., Carpenter, C., Randolph, C., Goldberg, T. E., & Weinberger, D. R. (1997).
Auditory working memory and Wisconsin Card Sorting Test performance in
schizophrenia. Archives of General Psychiatry, 54(2), 159–165.
doi:10.1001/archpsyc.1997.01830140071013
Goldberg, T. E., Gold, J. M., Greenberg, R., Griffin, S., Schulz, S. C., Pickar, D.
... Weiberger, D. R. (1993). Contrasts between patients with affective disorders
and patients with schizophrenia on a neuropsychological test battery. American
Journal of Psychiatry, 150(9), 1355–1362. Retrieved from http:// proquest.com.
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 184
Goldberg, T. E., Torrey, E. F., Berman, K. F., & Weinberger, D. R. (1994). Relations
between neuropsychological performance and brain morphological and
physiological measures in monozygotic twins discordant for schizophrenia.
Psychiatry Research: Neuroimaging, 55(1), 5–61. doi:10.1016/0925-
4927(94)90011-6
Golden, C. J. (1978). Stroop Color and Word Test: A Manual for Clinical and
Experimental Uses (pp. 1–46). Chicago: Stoelting.
Goldman-Rakic, P. S. (1987). Circuitry of the frontal association cortex and its
relevance to dementia. Archives of Gerontology and Geriatrics, 6(3), 299–309.
doi:10.1016/0167-4943(87)90029-x
Goldman-Rakic, P. S. (1996). Regional and cellular fractionation of working memory.
Proceedings of the National Academy of Sciences, 93(24), 13473-13480.
Retrieved from
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC33633/?tool=pmcentrez
Goldstein, G. (1990). Neuropsychological heterogeneity in schizophrenia: a
consideration of abstraction and problem-solving abilities. Archives of Clinical
Neuropsychology, 5(3), 251–264. doi:10.1016/0887-6177(90)90024-J
Goldstein, G., Beers, S. R., & Shemansky, W. J. (1996). Neuropsychological
differences between schizophrenic patients with heterogeneous Wisconsin Card
Sorting Test performance. Schizophrenia Research, 21(1), 13–18.
doi:10.1016/0920-9964(96)00019-9
Gordon, S. M., Kennedy, B. P., & McPeake, J. D. (1988). Neuropsychologically
impaired alcoholics: Assessment, treatment considerations, and rehabilitation.
Journal of Substance Abuse Treatment, 5(2), 99–104. doi:10.1016/0740-
5472(88)90019-0
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 185
Gorsuch, R. L. (1983). Three methods for analyzing limited time-series (N of 1) data.
Behavioral Assessment, 5, 141–154.
Goswami, S., Mattoo, S. K., Basu, D., & Singh, G. (2004). Substance-abusing
schizophrenics: Do they self-medicate? American Journal on Addictions, 13(2),
139–150. doi:10.1080/10550490490435795
Grafman, J., & Litvan, I. (1999). Importance of deficits in executive functions. The
Lancet, 354(9194),1921–1923. doi:10.1016/S0140-6736(99)90438-5
Grafman, J., Schwab, K., Warden, D., Pridgen, A., Brown, H. R., & Salazar, A. M.
(1996). Frontal lobe injuries, violence, and aggression: A report of the Vietnam
head injury study. Neurology, 46(5), 1231–1231. doi:10.1212/wnl.46.5.1231
Graham, F. K., & Kendall, B. S. (1960). Memory-for-designs test: Revised general
manual. Perceptual and Motor Skills, 11(2), 147–188. Retrieved from
http://www.amsciepub.com/doi/pdf/10.2466/pms.1960.11.2.147
Grant, I., Gonzalez, R., Carey, C. L., Natarajan, L., & Wolfson, T. (2003). Non-acute
(residual) neurocognitive effects of cannabis use: A meta-analytic study. Journal
of the International Neuropsychological Society, 9(5), 679–689.
doi:10.1017/s1355617703950016
Grant, J. E., Chamberlain, S. R., Schreiber, L., & Odlaug, B. L. (2012).
Neuropsychological deficits associated with cannabis use in young adults. Drug
and Alcohol Dependence, 121(1), 159–162.
doi: 10.1016/j.drugalcdep.2011.08.015
Grech, A., Van Os, J., Jones, P. B., Lewis, S. W., & Murray, R. M. (2005). Cannabis
use and outcome of recent onset psychosis. European Psychiatry, 20(4), 349–
353. doi:10.1016/j.eurpsy.2004.09.013
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 186
Green, M. F. (1996). What are the functional consequences of neurocognitive
deficits in schizophrenia? American Journal of Psychiatry, 153(3), 321–330.
Retrieved from http://psycnet.apa.org/psycinfo/1996-00417-002
Green, M. F. (1999). Interventions for neurocognitive deficits: Editor's introduction.
Schizophrenia Bulletin, 25(2), 197–200.
doi:10.1093/oxfordjournals.schbul.a033373
Green, M. F. (2006). Cognitive impairment and functional outcome in schizophrenia
and bipolar disorder. The Journal of Clinical Psychiatry, 67(10), e12–e12.
Retrieved from http://europepmc.org/abstract/MED/16965182
Green, M. F., & Harvey, P. D. (2014). Cognition in schizophrenia: Past, present, and
future. Schizophrenia Research: Cognition, 1, e1–e9.
doi:10.1016/j.scog.2014.02.001
Green, B., Young, R., & Kavanagh, D. (2005). Cannabis use and misuse prevalence
among people with psychosis. The British Journal of Psychiatry, 187(4), 306–
313. doi:10.1192/bjp.187.4.306
Green, M. F., Kern, R. S., Braff, D. L., & Mintz, J. (2000). Neurocognitive deficits and
functional outcome in schizophrenia: Are we measuring the "right stuff"?
Schizophrenia Bulletin, 26(1), 119–136.
doi:10.1093/oxfordjournals.schbul.a033430
Green, M. F., Kern, R. S., & Heaton, R. K. (2004). Longitudinal studies of cognition
and functional outcome in schizophrenia: implications for MATRICS.
Schizophrenia Research, 72(1), 41–51. doi:10.1016/j.schres.2004.09.009
Green, M. F., & Nuechterlein, K. H. (1999). Should schizophrenia be treated as a
neurocognitive disorder? Schizophrenia Bulletin, 25(2), 309–319.
doi:10.1093/oxfordjournals.schbul.a033380
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 187
Greenwood, K. E., Landau,S., & Wykes, T. (2005). Negative symptoms and specific
cognitive impairments as combined targets for improved functional outcome
within cognitive remediation therapy. Schizophrenia Bulletin, 31(4), 910–921.
doi: 10.1093/schbul/sbi035
Grotenhermen, F. (2003). Pharmacokinetics and pharmacodynamics of cannabinoids.
Clinical Pharmacokinetics, 42(4), 327–360. doi:10.2165/00003088-200342040-
00003
Gruber, S. A., Sagar, K. A., Dahlgren, M. K., Racine, M., & Lukas, S. E. (2012). Age of
onset of marijuana use and executive function. Psychology of Addictive
Behaviors, 26(3), 496–506. doi: 10.1037/a0026269
Gruber, S. A., & Yurgelun-Todd, D. A. (2005). Neuroimaging of marijuana smokers
during inhibitory processing: A pilot investigation. Cognitive Brain Research,
23(1), 107–118. doi:10.1016/j.cogbrainres.2005.02.016
Gruzelier, J., Seymour, K., Wilson, L., Jolley, A., & Hirsch, S. (1988). Impairments on
neuropsychologic tests of temporohippocampal and frontohippocampal
functions and word fluency in remitting schizophrenia and affective disorders.
Archives of General Psychiatry, 45(7), 623–629.
doi:10.1001/archpsyc.1988.01800310027003.
Guadagnoli, E., & Velicer, W. F. (1988). Relation of sample size to the stability of
component patterns. Psychological Bulletin, 103(2), 265–275. doi:
10.1037/0033-2909.103.2.265
Hall, W., Degenhardt, L., & Teesson, M. (2004). Cannabis use and psychotic disorders:
an update. Drug and Alcohol Review, 23(4), 433–443.
doi: 10.1080/09595230412331324554
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 188
Hall, W., Teesson, M., Lynskey, M., & Degenhardt, L. (1999). The 12‐month
prevalence of substance use and ICD‐10 substance use disorders in Australian
adults: findings from the National Survey of Mental Health and Well‐Being.
Addiction, 94(10), 1541–1550. doi: 10.1046/j.1360-0443.1999.9410154110.x
Harnishfeger, K. K. (1995). The development of cognitive inhibition: Theories,
definitions, and research evidence. In F. N. Dempster & C. J. Brainerd (Eds.),
Interference and Inhibition in Cognition (pp. 175–204). San Diego, CA:
Academic Press. doi: 10.1016/B978-012208930-5/50007-6
Harrison, E. R., Haaga, J., & Richards, T. (1993). Self-reported drug use data: what do
they reveal?. The American Journal of Drug and Alcohol Abuse, 19(4), 423–
441. doi:10.3109/00952999309001632
Harvey, P. D. (2007). Will improving cognition change functional outcomes in
schizophrenia?. Psychiatry, 4(4), 58–602. Retrieved from
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2921243/
Harvey, P. D. (2013). Cognitive aspects of schizophrenia. Wiley Interdisciplinary
Reviews: Cognitive Science, 4(6), 599–608. doi: 10.1002/wcs.1253
Harvey, M., Sellman, J., Porter, R., & Frampton, C. (2007). The relationship between
non-acute adolescent cannabis use and cognition. Drug and Alcohol Review,
26(3), 309–319. doi:10.1080/09595230701247772
Harvey, P. D., Green, M. F., Keefe, R. S. E., & Velligan, D. I. (2004). Cognitive
functioning in schizophrenia. Journal of Clinical Psychiatry, 65(3), 361–372.
doi:10.4088/JCP.v65n0312
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 189
Harvey, P. D., Howanitz, E., Parrella, M., White, L., Davidson, M., Mohs, R. C., &
Davis, K. L. (1998). Symptoms, cognitive functioning, and adaptive skills in
geriatric patients with lifelong schizophrenia: A comparison across treatment
sites. American Journal of Psychiatry, 155(8), 1080–1086.
doi.org/10.1176/ajp.155.8.1080
Harvey, P. D., Lombardi, J., Leibman, M., White, L., Parrella, M., Powchik, P., &
Davidson, M. (1996). Cognitive impairment and negative symptoms in geriatric
chronic schizophrenic patients: A follow-up study. Schizophrenia Research,
22(3), 223–231. doi:10.1016/s0920-9964(96)00075-8
Harvey, P. D., Napolitano, J. A., Mao, L., & Gharabawi, G. (2003). Comparative effects
of risperidone and olanzapine on cognition in elderly patients with schizophrenia
or schizoaffective disorder. International Journal of Geriatric Psychiatry, 18(9),
820–829. doi:10.1002/gps.929
Heaton, R. K., (1981). Wisconsin Card Sorting Test Manual. Odessa, FL: Psychological
Assessment Resources.
Heinrichs, R. W., & Zakzanis, K. K. (1998). Neurocognitive deficit in schizophrenia: A
quantitative review of the evidence. Neuropsychology, 12(3), 426–445.
doi:10.1037//0894-4105.12.3.426
Herman, M. (2004). Neurocognitive functioning and quality of life among dually
diagnosed and non‐substance abusing schizophrenia inpatients. International
Journal of Mental Health Nursing, 13(4), 282–291. doi: 10.1111/j.1440-
0979.2004.00346.x
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 190
Hermann, D., Sartorius, A., Welzel, H., Walter, S., Skopp, G., Ende, G., & Mann, K.
(2007). Dorsolateral prefrontal cortex n-acetylaspartate/total creatine (naa/tcr)
loss in male recreational cannabis users. Biological Psychiatry, 61(11), 1281–
1289. doi:10.1016/j.biopsych.2006.08.027
Hewitt, J., Evans, J. J., & Dritschel, B. (2006). Theory driven rehabilitation of executive
functioning: Improving planning skills in people with traumatic brain injury
through the use of an autobiographical episodic memory cueing procedure.
Neuropsychologia, 44(8), 1468–1474.
doi:10.1016/j.neuropsychologia.2005.11.016
Hoff, A. L., Riordan, H., O'Donnell, D. W., Morris, L., & DeLisi, L. E. (1992).
Neuropsychological functioning of first-episode schizophreniform patients.
American Journal of Psychiatry, 149(7), 898–903.
Hughes, C., Kumari, V., Soni, W., Das, M., Binneman, B., Drozd, S., & Sharma, T.
(2003). Longitudinal study of symptoms and cognitive function in chronic
schizophrenia. Schizophrenia Research, 59(2-3), 137–146. doi:10.1016/s0920-
9964(01)00393-0
Hutton, S. B., Puri, B. K., Duncan, L. J., Robbins, T. W., Barnes, T. R. E., & Joyce, E.
M. (1998). Executive function in first-episode schizophrenia. Psychological
Medicine, 28(2), 463–473. doi:10.1017/s0033291797006041
Ihara, H., Berrios, G. E., & McKenna, P. J. (2003). The association between negative
and dysexecutive syndromes in schizophrenia: A cross-cultural study.
Behavioral Neurology, 14(3-4), 63–74. doi:10.1155/2003/304095
Iversen, L. L. (2000). The Science of Marijuana [Review]. Oxford: Oxford
University Press.
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 191
Jager, G., Kahn, R. S., Van Den Brink, W., Van Ree, J. M., & Ramsey, N. F. (2006).
Long-term effects of frequent cannabis use on working memory and attention:
An fMRI study. Psychopharmacology, 185(3), 358–368. doi:10.1007/s00213-
005-0298-7
Jockers-Scherübl, M. C., Wolf, T., Radzei, N., Schlattmann, P., Rentzsch, J., Gómez-
Carrillo de Castro, A., & Kühl, K. P. (2007). Cannabis induces different
cognitive changes in schizophrenic patients and in healthy controls. Progress in
Neuro-Psychopharmacology and Biological Psychiatry, 31(5), 1054–1063.
doi:10.1016/j.pnpbp.2007.03.006
Joyce, E. M., Collinson, S. L., & Crichton, P. (1996). Verbal fluency in schizophrenia:
relationship with executive function, semantic memory and clinical alogia.
Psychological Medicine, 26(1), 39–49. doi:10.1017/S0033291700033705
Joyce, E. M., & Roiser, J. P. (2007). Cognitive heterogeneity in schizophrenia. Current
Opinion in Psychiatry, 20(3), 268–272. doi:10.1097/YCO.0b013e3280ba4975
Jurado, M. B., & Rosselli, M. (2007). The elusive nature of executive functions: a
review of our current understanding. Neuropsychology Review, 17(3), 213–233.
doi:10.1007/s11065-007-9040-z
Kabat-Zinn, J., Massion, A. O., Kristeller, J., Peterson, L. G., Fletcher, K. E., Pbert, L.,
Lenderking, W. R., & Santorelli, S. F. (1992). Effectiveness of a meditation-
based stress reduction program in the treatment of anxiety disorders. American
Journal of Psychiatry, 149(7), 936–943.
Kafer, K. L., & Hunter, M. (1997). On testing the face validity of planning/problem-
solving tasks in a normal population. Journal of the International
Neuropsychological Society, 3(2), 108–119.
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 192
Kanayama, G., Rogowska, J., Pope, H. G., Gruber, S. A., & Yurgelun-Todd, D. A.
(2004). Spatial working memory in heavy cannabis users: A functional magnetic
resonance imaging study. Psychopharmacology, 176(3-4), 239–247.
doi:10.1007/s00213-004-1885-8
Katz, N., Tadmor, I., Felzen, B., & Hartman-Maeir, A. (2007). The behavioural
assessment of the dysexecutive syndrome (BADS) in schizophrenia and its
relation to functional outcomes. Neuropsychological Rehabilitation, 17(2), 192–
205. doi:10.1080/09602010600685053
Kavanagh, D. J., Waghorn, G., Jenner, L., Chant, D. C., Carr, V., Evans, M., ...
McGrath, J. J. (2004). Demographic and clinical correlates of comorbid
substance use disorders in psychosis: Multivariate analyses from an
epidemiological sample. Schizophrenia Research, 66(2-3), 115–124.
doi:10.1016/s0920-9964(03)00130-0
Kay, S. R., Fiszbein, A., & Opler, L. A. (1987). The Positive and Negative Syndrome
Scale (PANSS) for schizophrenia. Schizophrenia Bulletin, 13(2), 261–276.
Retrieved from http://www.psycontent.com/content/p81u7t14n7303377/
Kay, S. R., Opler, L. A., & Lindenmayer, J. P. (1988). Reliability and validity of the
Positive and Negative Syndrome Scale for schizophrenics. Psychiatry Research,
23(1), 99–110. doi:10.1016/0165-1781(88)90038-8
Keefe, R. S. E., Goldberg, T. E., Harvey, P. D., Gold, J. M., Poe, M., & Coughenour, L.
(2004). The Brief Assessment of Cognition in Schizophrenia: Reliability,
sensitivity, and comparison with a standard neurocognitive battery.
Schizophrenia Research, 68(2-3), 283–297.
doi.org/10.1016/j.schres.2003.09.011
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 193
Keefe, R. S., Poe, M., Walker, T. M., Kang, J. W., & Harvey, P. D. (2006). The
Schizophrenia Cognition Rating Scale: an interview-based assessment and its
relationship to cognition, real-world functioning, and functional capacity.
American Journal of Psychiatry, 163(3), 426–432. doi:
10.1176/appi.ajp.163.3.426
Kenny, J. T., & Meltzer, H. Y. (1991). Attention and higher cortical functions in
schizophrenia. Journal of Neuropsychiatry, 3(3), 269–275.
Kerns, J. G., & Berenbaum, H. (2002). Cognitive impairments associated with formal
thought disorder in people with schizophrenia. Journal of Abnormal Psychology,
111(2), 211–224. doi:10.1037/0021-843x.111.2.211
Kinch, J., & McDonald, S. (2001). Traumatic brain injury and prospective memory: An
examination of the influences of executive functioning and retrospective
memory. Brain Impairment, 2(2), 119–130. doi:
http://dx.doi.org/10.1375/brim.2.2.119
Kirby, K. N., Petry, N. M., & Bickel, W. K. (1999). Heroin addicts have higher discount
rates for delayed rewards than non-drug-using controls. Journal of Experimental
Psychology: General, 128(1), 78–87. doi: http://dx.doi.org/10.1037/0096-
3445.128.1.78
Knapp, M. (1997). Costs of schizophrenia. The British Journal of Psychiatry, 171(6),
509–518. doi:10.1192/bjp.171.6.509
Knight, R. G., Titov, N., & Crawford, M. (2006). The effects of distraction on
prospective remembering following traumatic brain injury assessed in a
simulated naturalistic environment. Journal of the International
Neuropsychological Society, 12(1), 8–16. doi:10.1017/s1355617706060048
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 194
Kolb, B., & Whishaw, I. Q. (1985). Fundamentals of Human Neuropsychology (Second
Edition). New York: Freeman.
Koskinen, J., Löhönen, J., Koponen, H., Isohanni, M., & Miettunen, J. (2010). Rate of
cannabis use disorders in clinical samples of patients with schizophrenia: A
meta-analysis. Schizophrenia Bulletin, 36(6), 1115–1130.
doi:10.1093/schbul/sbp031
Krabbendam, L., de Vugt, M. E., Derix, M. M. A., & Jolles, J. (1999). The behavioural
assessment of the dysexecutive syndrome as a tool to assess executive functions
in schizophrenia. The Clinical Neuropsychologist (Neuropsychology,
Development and Cognition: Section D), 13(3), 370–375.
doi:10.1076/clin.13.3.370.1739
Kraepelin, E. (1919). (Translated by R. M. Barclay, published in 1971). Dementia
Praecox and Paraphrenia. New York: Krieger Publishing Company.
Kumra, S., Thaden, E., DeThomas, C., & Kranzler, H. (2005). Correlates of substance
abuse in adolescents with treatment-refractory schizophrenia and schizoaffective
disorder. Schizophrenia Research, 73(2-3), 369–371.
doi:10.1016/j.schres.2004.08.006
Kurtz, M. M., Moberg, P. J., Ragland, J. D., Gur, R. C., & Gur, R. E. (2005). Symptoms
versus neurocognitive test performance as predictors of psychosocial status in
schizophrenia: A 1- and 4-year prospective study. Schizophrenia Bulletin, 31(1),
167–174. doi:10.1093/schbul/sbi004
Large, M., Sharma, S., Compton, M. T., Slade, T., & Nielssen, O. (2011). Cannabis use
and earlier onset of psychosis: a systematic meta-analysis. Archives of General
Psychiatry, 68(6), 555–61. doi:10.1001/archgenpsychiatry.2011.5.
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 195
Leeson, V. C., Harrison, I., Ron, M. A., Barnes, T. R., & Joyce, E. M. (2011). The
effect of cannabis use and cognitive reserve on age at onset and psychosis
outcomes in first-episode schizophrenia. Schizophrenia Bulletin, 38(4), 873–
880. doi:10.1093/schbul/sbq153
Levaux, M. N., Larøi, F., Malmedier, M., Offerlin-Meyer, I., Danion, J. M., & Van der
Linden, M. (2012). Rehabilitation of executive functions in a real-life setting:
Goal Management Training applied to a person with schizophrenia. Case
Reports in Psychiatry, 2012(Article ID 503023), 1–15.
doi:10.1155/2012/503023
Levine, B., Robertson, I. H., Clare, L., Carter, G., Hong, J., Wilson, B. A., ... Stuss,
D. T. (2000). Rehabilitation of executive functioning: An experimental–clinical
validation of goal management training. Journal of the International
Neuropsychological Society, 6(3), 299–312. Retrieved from
http://www.tara.tcd.ie/bitstream/handle/2262/477/JINS2000.pdf;jsessionid=2C6
C4DB8366C1382F3B31D127FB7A9CC?sequence=1
Levine, B., Stuss, D. T., Winocur, G., Binns, M. A., Fahy, L., Mandic, M., ...
Robertson, I. H. (2007). Cognitive rehabilitation in the elderly: effects on
strategic behavior in relation to goal management. Journal of the International
Neuropsychological Society, 13(1), 143–152. doi:10.1017/S1355617707070178
Lezak, M. D., Howieson, D. B., Loring, D. W., Hannay, J. H., & Fischer, J. S. (1995).
Neuropsychological Assessment (Third Edition). New York, NY: Oxford
University Press.
Lezak, M. D., Howieson, D. B., & Loring, D. W. (2004). Neuropsychological
Assessment (Fourth Edition). New York, NY: Oxford University Press, p. 35.
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 196
Liddle, P. F., & Morris, D. L. (1991). Schizophrenic syndromes and frontal lobe
performance. The British Journal of Psychiatry, 158(3), 340–345.
doi:10.1192/bjp.158.3.340
Linszen, D. H., Dingemans, P. M., & Lenior, M. E. (1994). Cannabis abuse and the
course of recent-onset schizophrenic disorders. Archives of General Psychiatry,
51(4), 273–279. doi:10.1001/archpsyc.1994.03950040017002
Lovibond, P. F., & Lovibond, S. H. (1995). The structure of negative emotional states:
Comparison of the Depression Anxiety Stress Scales (DASS) with the Beck
Depression and Anxiety Inventories. Behaviour Research and Therapy, 33(3),
335–343. doi:10.1016/0005-7967(94)00075-U
Lovibond, S. H., & Lovibond, P. F. (1995). Manual for the Depression Anxiety Stress
Scales. Sydney: Psychology Foundation.
Lundqvist, T., Jönsson, S., & Warkentin, S. (2001). Frontal lobe dysfunction in long-
term cannabis users. Neurotoxicology and Teratology, 23(5), 437–443.
doi:10.1016/S0892-0362(01)00165-9
Luria, A. R. (1973). The Working Brain. London, UK: Penguin.
Lyons, M. J., Bar, J. L., Panizzon, M. S., Toomey, R., Eisen, S., Xian, H., & Tsuang, M.
T. (2004). Neuropsychological consequences of regular marijuana use: A twin
study. Psychological Medicine, 34(7), 1239–1250.
doi:10.1017/s0033291704002260
Lyon, G. R., & Krasnegor, N. A. (1996). Attention, Memory, and Executive
Function. Baltimore, MD: Paul H. Brookes Publishing Co.
MacLeod, C. M. (1991). Half a century of research on the Stroop effect: An integrative
review. Psychological Bulletin, 109(2), 163–203. doi:10.1037//0033-
2909.109.2.163
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 197
Manchester, D., Priestley, N., & Jackson, H. (2004). The assessment of executive
functions: Coming out of the office. Brain Injury, 18(11), 1067–1081.
doi:10.1080/02699050410001672387
Mata, I., Rodríguez-Sánchez, J. M., Pelayo-Terán, J. M., Pérez-Iglesias, R., González-
Blanch, C., Ramírez-Bonilla, M., ... Crespo-Facorro, B. (2008). Cannabis abuse
is associated with decision-making impairment among first-episode patients with
schizophrenia-spectrum psychosis. Psychological Medicine, 38(9), 1257–1266.
doi:10.1017/S0033291707002218
Mayer, J. D., Salovey, P., & Caruso, D. R. (2009). Mayer-Salovey-Caruso Emotional
Intelligence Test (Spanish version). Madrid: TEA ediciones.
Maylor, E. A., Smith, G., Della Sala, S., & Logie, R. H. (2002). Prospective and
retrospective memory in normal aging and dementia: An experimental study.
Memory & Cognition, 30(6), 871-884. doi: 10.3758/BF03195773
McClure, M. M., Bowie, C. R., Patterson, T. L., Heaton, R. K., Weaver, C., Anderson,
H., & Harvey, P. D. (2007). Correlations of functional capacity and
neuropsychological performance in older patients with schizophrenia: evidence
for specificity of relationships?. Schizophrenia Research, 89(1), 330–338.
doi:10.1016/j.schres.2006.07.024
McDaniel, M. A., & Einstein, G. O. (1993). The importance of cue familiarity and cue
distinctiveness in prospective memory. Memory, 1(1), 23–41.
doi:10.1080/09658219308258223
McHale, S., & Hunt, N. (2008). Executive function deficits in short‐term abstinent
cannabis users. Human Psychopharmacology: Clinical and Experimental, 23(5),
409–415. doi:10.1002/hup.941
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 198
Medalia, A., & Choi, J. (2009). Cognitive remediation in schizophrenia.
Neuropsychology Review, 19(3), 353–364. doi: 10.1007/s11065-009-9097-y
Medalia, A., Revheim, N., & Casey, M. (2002). Remediation of problem-solving skills
in schizophrenia: evidence of a persistent effect. Schizophrenia Research, 57(2),
165–171. doi:10.1016/S0920-9964(01)00293-6
Menditto, A. A., Wallace, C. J., Liberman, R. P., Wal, J. V., Jones, N. T., & Stuve, P.
(1999). Functional assessment of independent living skills. Psychiatric
Rehabilitation Skills, 3(2), 200–219. doi:10.1080/10973439908408384
Milner, B. (1963). Effects of different brain lesions on card sorting: The role of the
frontal lobes. Archives of Neurology, 9(1), 90–100.
doi:10.1001/archneur.1963.00460070100010.
Miyake, A., Friedman, N. P., Emerson, M. J., Witzki, A. H., Howerter, A., & Wager, T.
D. (2000). The unity and diversity of executive functions and their contributions
to complex “frontal lobe” tasks: A latent variable analysis. Cognitive
Psychology, 41(1), 49–100. doi:10.1006/cogp.1999.0734
Moscovitch, M., &Winocur, G. (1992). The neuropsychology of memory and aging. In
F. I. M. Craik & T. A. Salthouse (Eds.), The Handbook of Aging and Cognition
(pp. 315-372). Hillsdale, NJ: Erlbaum.
Morris, R., Rushe, T., Woodruffe, P. W., & Murray, R. M. (1995). Problem solving in
schizophrenia: A specific deficit in planning ability. Schizophrenia Research,
14(3), 235–246. doi:10.1016/0920-9964(94)00044-9
Mueser, K. T., Yarnold, P. R., Levinson, D. F., Singh, H., Bellack, A. S., Kee, K., ...
Yadalam, K. G. (1990). Prevalence of substance abuse in schizophrenia:
Demographic and clinical correlates. Schizophrenia Bulletin, 16(1), 31–56.
doi:10.1093/schbul/16.1.31
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 199
Nelson, H. E., & Willison, J. R. (1991). National Adult Reading Test (NART) – Second
Edition. Windsor, UK: NFER-Nelson.
Nigg, J. T. (2000). On inhibition/disinhibition in developmental psychopathology:
views from cognitive and personality psychology and a working inhibition
taxonomy. Psychological Bulletin, 126(2), 220–246. doi: 10.1037/0033-
2909.126.2.220
Niki, H. (1974). Differential activity of prefrontal units during right and left delayed
response trials. Brain Research, 70(2), 346–349. doi: 10.1016/0006-
8993(74)90324-2
Num, S. (2006). The application of a hierarchical model in investigating the construct
validity of the executive functions in young adults (Unpublished honours thesis).
University of South Australia, South Australia, Australia.
O'Carroll, R., Egan, V., & MacKenzie, D. M. (1994). Assessing cognitive estimation.
British Journal of Clinical Psychology, 33(2), 193–197. doi: 10.1111/j.2044-
8260.1994.tb01110.x
O'Leary, M. R., Donovan, D. M., Chaney, E. F., & Walker, R. D. (1979). Cognitive
impairment and treatment outcome with alcoholics: Preliminary findings.
Journal of Clinical Psychiatry, 40(9), 397–398.
O’Malley, S., Adamse, M., Heaton, R. K., & Gawin, F. H. (1992). Neuropsychological
impairment in chronic cocaine abusers. American Journal of Drug and Alcohol
Abuse, 18(2), 131–144. doi: 10.3109/00952999208992826
Pantelis, C., Barnes, T. R., Nelson, H. E., Tanner, S., Weatherley, L., Owen, A. M., &
Robbins, T. W. (1997). Frontal-striatal cognitive deficits in patients with chronic
schizophrenia. Brain, 120(10), 1823–1843. doi:10.1093/brain/120.10.1823
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 200
Park, D. C., Hertzog, C., Kidder, D. P., Morrell, R. W., & Mayhorn, C. B. (1997).
Effect of age on event-based and time-based prospective memory. Psychology
and Aging, 12(2), 314–327. doi:10.1037//0882-7974.12.2.314
Parker, D. M., & Crawford, J. R. (1992). Assessment of frontal lobe function. In J. R.
Crawford, D. M. Parker, & W. W. McKinlay (Eds.), A Handbook of
Neuropsychological Assessment (pp. 267–291). London: Erlbaum.
Parkin, A. J. (1998). The central executive does not exist. Journal of the International
Neuropsychological Society, 4(5), 518–522. doi:10.1017/S1355617798005128
Parkin, A. J., & Java, R. I. (1999). Deterioration of frontal lobe function in normal
aging: Influences of fluid intelligence versus perceptual speed.
Neuropsychology, 13(4), 539–545. doi:10.1037/0894-4105.13.4.539
Patterson, T. L., Goldman, S., McKibbin, C. L., Hughs, T., & Jeste, D. V. (2001).
UCSD performance-based skills assessment: Development of a new measure of
everyday functioning for severely mentally ill adults. Schizophrenia Bulletin,
27(2), 235–245. doi:10.1093/oxfordjournals.schbul.a006870
Paulesu, E., Goldacre, B., Scifo, P., Cappa, S. F., Gilardi, M. C., Castiglioni, I., ...
Fazio, F. (1997). Functional heterogeneity of left inferior frontal cortex as
revealed by fMRI. Neuroreport, 8(8), 2011–2016.
Paulus, M. P., Tapert, S. F., & Schuckit, M. A. (2005). Neural activation patterns of
methamphetamine-dependent subjects during decision making predict relapse.
Archives of General Psychiatry, 62(7), 761–768. doi:10.1001/archpsyc.62.7.761
Penadés, R., Boget, T., Catalán, R., Bernardo, M., Gastó, C., & Salamero, M. (2003).
Cognitive mechanisms, psychosocial functioning, and neurocognitive
rehabilitation in schizophrenia. Schizophrenia Research, 63(3), 219–227.
doi:10.1016/s0920-9964(02)00359-6
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 201
Peralta, V., & Cuesta, M. J. (1992). Influence of cannabis abuse on schizophrenic
psychopathology. Acta Psychiatrica Scandinavica, 85(2), 127–130.
doi:10.1111/j.1600-0447.1992.tb01456.x
Perlick, D. A., Rosenheck, R. A., Kaczynski, R., Bingham, S., & Collins, J. (2008).
Association of symptomatology and cognitive deficits to functional capacity in
schizophrenia. Schizophrenia Research, 99(1-3), 192–199.
doi:10.1016/j.schres.2007.08.009
Persson, J., & Reuter-Lorenz, P. A. (2008). Gaining Control: Training Executive
Function and Far Transfer of the Ability to Resolve Interference [retracted].
Psychological Science, 19(9), 881–888. doi:10.1111/j.1467-9280.2008.02172.x
Petrides, M., & Milner, B. (1982). Deficits on subject-ordered tasks after frontal- and
temporal-lobe lesions in man. Neuropsychologia, 20(3), 249–262.
doi:10.1016/0028-3932(82)90100-2
Phillips, L. H. (1997). Do “frontal tests” measure executive function? Issues of
assessment and evidence from fluency tests. In P. Rabbit (Ed.), Methodology of
Frontal and Executive Function (pp. 191–213.). East Sussex, UK: Psychology
Press.
Pope, H., Gruber, A. J., & Yurgelun-Todd, D. (1995). The residual neuropsychological
effects of cannabis: The current status of research. Drug and Alcohol
Dependence, 38(1), 25–34. doi:10.1016/0376-8716(95)01097-i
Pope, H. G., Gruber, A. J., Hudson, J. I., Cohane, G., Huestis, M. A., & Yurgelun-Todd,
D. (2003). Early-onset cannabis use and cognitive deficits: What is the nature of
the association? Drug and Alcohol Dependence, 69(3), 303–310.
doi:10.1016/s0376-8716(02)00334-4
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 202
Pope, H. G., Gruber, A. J., Hudson, J. I., Huestis, M. A., & Yurgelun-Todd, D. (2001).
Neuropsychological performance in long-term cannabis users. Archives of
General Psychiatry, 58(10), 909–915. doi:10.1001/archpsyc.58.10.909.
Pope, H. G., Gruber, A. J., Hudson, J. I., Huestis, M. A., & Yurgelun-Todd, D. (2002).
Cognitive measures in long-term cannabis users. The Journal of Clinical
Pharmacology, 42(S1), 41S–47S. doi:10.1002/j.1552-4604.2002.tb06002.x
Pope, H. G., & Yurgelun-Todd, D. (1996). The residual cognitive effects of heavy
marijuana use in college students. The Journal of the American Medical
Association, 275(7), 521–527. doi:10.1001/jama.1996.03530310027028.
Potvin, S., Joyal, C. C., Pelletier, J., & Stip, E. (2008). Contradictory cognitive
capacities among substance-abusing patients with schizophrenia: a meta-
analysis. Schizophrenia Research, 100(1), 242–251.
doi:10.1016/j.schres.2007.04.022
Quednow, B. B., Kühn, K. U., Hoppe, C., Westheide, J., Maier, W., Daum, I., &
Wagner, M. (2007). Elevated impulsivity and impaired decision-making cognition in
heavy users of MDMA (“Ecstasy”). Psychopharmacology, 189(4), 517–530.
doi: 10.1007/s00213-005-0256-4
Rabbit, P. (1997). Introduction: Methodologies and models in the study of executive
function. In P. Rabbit (Ed.), Methodology of Frontal and Executive Function
(pp. 1–38). East Sussex, UK: Psychology Press.
Rabin, R. A., Zakzanis, K. K., Daskalakis, Z. J., & George, T. P. (2013). Effects of
cannabis use status on cognitive function, in males with schizophrenia.
Psychiatry Research, 206(2-3), 158–165. doi:10.1016/j.psychres.2012.11.019
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 203
Rabin, R. A., Zakzanis, K. K., & George, T. P. (2011). The effects of cannabis use on
neurocognition in schizophrenia: A meta-analysis. Schizophrenia Research,
128(1-3), 111–116. doi:10.1016/j.schres.2011.02.017
Raskin, S. A., & Rearick, E. (1996). Verbal fluency in individuals with mild traumatic
brain injury. Neuropsychology, 10(3), 416–422. doi:10.1037/0894-
4105.10.3.416
Regier, D. A., Farmer, M. E., Rae, D. S., Locke, B. Z., Keith, S. J., Judd, L. L., &
Goodwin, F. K. (1990). Comorbidity of mental disorders with alcohol and other
drug abuse. Journal of the American Medical Association, 264(19), 2511–2518.
doi:10.1001/jama.1990.03450190043026
Reichenberg, A., & Harvey, P. D. (2007). Neuropsychological impairments in
schizophrenia: Integration of performance-based and brain imaging findings.
Psychological Bulletin, 133(5), 833–858. doi:10.1037/0033-2909.133.5.833
Reitan, R. M. (1958). Validity of the Trail Making Test as an indicator of organic brain
damage. Perceptual and Motor Skills, 8(3), 271–276.
doi:10.2466/pms.1958.8.3.271
Reitan R. M., & Wolfson, D. (1993). The Halstead-Reitan Neuropsychological Test
Battery: Theory and Clinical Interpretation (Second Edition). South Tuscon:
Neuropsychology Press.
Rempfer, M. V., Hamera, E. K., Brown, C. E., & Cromwell, R. L. (2003). The relations
between cognition and the independent living skill of shopping in people with
schizophrenia. Psychiatry Research, 117(2), 103–112. doi:10.1016/s0165-
1781(02)00318-9
Rey, A. (1964). L’Examen Clinique en Psychologie. Paris: Universitaires de France.
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 204
Riley, E. M., McGovern, D., Mockler, D., Doku, V. C. K., ÓCeallaigh, S., Fannon, D.
G., & Sharma, T. (2000). Neuropsychological functioning in first-episode
psychosis — evidence of specific deficits. Schizophrenia Research, 43(1), 47–
55. doi:10.1016/s0920-9964(99)00177-2
Ritter, L., Meador-Woodruff, J. H., & Dalack, G. W. (2004). Neurocognitive measures
of prefrontal cortical dysfunction in schizophrenia. Schizophrenia Research,
68(1), 65–73. doi:10.1016/s0920-9964(03)00086-0
Robbins, T. W. (1990). The case for frontostriatal dysfunction in schizophrenia.
Schizophrenia Bulletin, 16(3), 391–402. doi:10.1093/schbul/16.3.391
Robbins, T. W., James, M., Owen, A. M., Sahakian, B. J., Lawrence, A. D., McInnes,
L., & Rabbitt, P. M. A. (1998). A study of performance on tests from the
CANTAB battery sensitive to frontal lobe dysfunction in a large sample of
normal volunteers: Implications for theories of executive functioning and
cognitive aging. Journal of the International Neuropsychological Society, 4(5),
474–490. doi:10.1017/s1355617798455073
Roberts, A. C., Robbins, T. W., & Weiskrantz, L. (1998). The Prefrontal Cortex:
Executive and Cognitive Functions. London, UK: Oxford University Press.
Robertson, I. H. (1996). Goal Management Training: A Clinical Manual. Cambridge,
UK: PsycConsult.
Robertson, I.H., Levine, B., & Manly, T., (2005). Goal Management Training.
Baycrest: Rotman Research Institute.
Rodríguez-Sánchez, J. M., Ayesa-Arriola, R., Mata, I., Moreno-Calle, T., Pérez-
Iglesias, R., González-Blanch, C., ... Crespo-Facorro, B. (2010). Cannabis use
and cognitive functioning in first-episode schizophrenia patients. Schizophrenia
Research, 124(1), 142–151. doi:10.1016/j.schres.2010.08.017
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 205
Rodriguez-Jimenez, R., Aragües, M., Jimenez-Arriero, M. A., Ponce, G.,
Martinez, I., Hoenicka, J., Rubio, G., & Palomo, T. (2008). Psychopathology
and Wisconsin Card Sorting Test performance in male schizophrenic patients:
Influence of dual diagnosis. Psychopathology, 41(1), 58–64.
doi.10.1159/000110627
Royall, D. R., Lauterbach, E. C., Cummings, J. L., Reeve, A., Rummans, T. A., Kaufer,
D. I., ... Coffey, C. E. (2002). Executive control function: A review of its
promise and challenges for clinical research. A report from the Committee on
Research of the American Neuropsychiatric Association. The Journal of
Neuropsychiatry and Clinical Neurosciences, 14(4), 377–405.
doi:10.1176/appi.neuropsych.14.4.377
Ruff, R. M., & Allen, C. C., (1996). Ruff 2 & 7 Selective Attention Test: Professional
Manual. Odessa, Florida: Psychological Assessment Resources.
Sahakian, B. J., & Owen, A. M. (1992). Computerized assessment in neuropsychiatry
using CANTAB: discussion paper. Journal of the Royal Society of Medicine,
85(7), 399–402. doi:10.1177/014107689208500711
Salthouse, T. A., Atkinson, T. M., & Berish, D. E. (2003). Executive functioning as a
potential mediator of age-related cognitive decline in normal adults. Journal of
Experimental Psychology: General, 132(4), 566–594. doi:10.1037/0096-
3445.132.4.566
Salthouse, T. A., & Babcock, R. L. (1991). Decomposing adult age differences in
working memory. Developmental psychology, 27(5), 763–776.
doi:10.1037/0012-1649.27.5.763
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 206
Salyers, M. P., & Mueser, K. T. (2001). Social functioning, psychopathology, and
medication side effects in relation to substance use and abuse in schizophrenia.
Schizophrenia Research, 48(1), 109–123. doi:10.1016/s0920-9964(00)00063-3
Sánchez-Torres, A. M., Basterra, V., Rosa, A., Fañanás, L., Zarzuela, A., Ibáñez, B., ...
Cuesta, M. J. (2013). Lifetime cannabis use and cognition in patients with
schizophrenia spectrum disorders and their unaffected siblings. European
Archives of Psychiatry and Clinical Neuroscience, 263(8), 643–653.
doi:10.1007/s00406-013-0404-5
Savla, G. N., Twamley, E. W., Thompson, W. K., Delis, D. C., Jeste, D. V., & Palmer,
B. W. (2011). Evaluation of specific executive functioning skills and the
processes underlying executive control in schizophrenia. Journal of the
International Neuropsychological Society, 17(1), 14–23.
doi:10.1017/S1355617710001177
Sarne, Y., & Mechoulam, R. (2005). Cannabinoids: between neuroprotection and
neurotoxicity. Current Drug Targets-CNS & Neurological Disorders, 4(6), 677–
684. 10.2174/156800705774933005
Saykin, A. J., Gur, R. C., Gur, R. E., Mozley, P. D., Mozley, L. H., Resnick, S. M., &
Stafiniak, P. (1991). Neuropsychological function in schizophrenia. Archives of
General Psychiatry, 48(7), 618–624.
doi:10.1001/archpsyc.1991.01810310036007
Saykin, A. J., Shtasel, D. L., Gur, R. E., Kester, D. B., Mozley, L. H., Stafiniak, P., &
Gur, R. C. (1994). Neuropsychological deficits in neuroleptic naive patients with
first-episode schizophrenia. Archives of General Psychiatry, 51(2), 124–131.
doi:10.1001/archpsyc.1994.03950020048005
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 207
Segal, Z. V., Williams, J. M. G., & Teasdale, J. D. (2002) Mindfulness-based Cognitive
Therapy for Depression - A New Approach to Preventing Relapse. New York,
NY: Guilford Press.
Schneider, L. C., & Struening, E. L. (1983). SLOF: a behavioral rating scale for
assessing the mentally ill. Social Work Research & Abstracts, 19(3), 9–21.
Schnell, T., Koethe, D., Daumann, J., & Gouzoulis-Mayfrank, E. (2009). The role of
cannabis in cognitive functioning of patients with schizophrenia.
Psychopharmacology, 205(1), 45–52. doi: 10.1007/s00213-009-1512-9
Scholes, K. E., & Martin-Iverson, M. T. (2010). Cannabis use and neuropsychological
performance in healthy individuals and patients with schizophrenia.
Psychological Medicine, 40(10), 1635–1646. doi:10.1017/S0033291709992078
Schroots, J. J., Dijkum, C. V., & Assink, M. H. (2004). Autobiographical memory from
a life span perspective. The International Journal of Aging and Human
Development, 58(1), 69–85. doi: 10.2190/7A1A-8HCE-0FD9-7CTX
Schwartz, M. F., Reed, E. S., Montgomery, M., Palmer, C., & Mayer, N. H. (1991). The
quantitative description of action disorganisation after brain damage: A case
study. Cognitive Neuropsychology, 8(5), 381–414.
doi:10.1080/02643299108253379
Seifert, C. (1994). The role of goals in retrieving analogical cases. In K. Holyoak & J.
Barnden (Eds.), Analogy, Metaphor and Reminding (pp. 95–125). Westport, CT:
Ablex Publishing.
Semkovska, M., Stip, E., Godbout, L., Paquet, F., & Bedard, M. A. (2002). Behavioral
disorganization in schizophrenia during a daily activity: The kitchen behavioral
scoring scale. Brain Cognition, 48(2-3), 546–553. Retrieved from http://0-
www.sciencedirect.com.
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 208
Sevy, S., Burdick, K. E., Visweswaraiah, H., Abdelmessih, S., Lukin, M., Yechiam, E.,
& Bechara, A. (2007). Iowa gambling task in schizophrenia: A review and new
data in patients with schizophrenia and co-occurring cannabis use disorders.
Schizophrenia Research, 92(1-3), 74–84. doi:10.1016/j.schres.2007.01.005
Shallice, T. (1982). Specific impairments of planning. Philosophical Transactions of the
Royal Society B: Biological Sciences, 298(1089), 199–209.
doi:10.1098/rstb.1982.0082
Shallice, T., & Burgess, P. W. (1991). Deficits in strategy application following frontal
lobe damage in man. Brain, 114(2), 727–741. doi:10.1093/brain/114.2.727
Shallice, T., Burgess, P. W., & Frith, C. D. (1991). Can the neuropsychological case-
study approach be applied to schizophrenia? Psychological Medicine, 21(3),
661–673. doi:10.1017/S0033291700022303
Shallice, T., Burgess, P., & Robertson, I. (1996). The domain of supervisory processes
and temporal organization of behaviour [and discussion]. Philosophical
Transactions of the Royal Society of London, Series B: Biological Sciences,
351(1346), 1405–1412. doi: 10.1098/rstb.1996.0124
Shallice, T., & Evans, M. E. (1978). The involvement of the frontal lobes in cognitive
estimation. Cortex, 14(2), 294–303. doi:10.1016/s0010-9452(78)80055-0
Sharma, T., & Antonova, L. (2003). Cognitive function in schizophrenia: deficits,
functional consequences, and future treatment. Psychiatric Clinics of North
America, 26(1), 25–40. doi:10.1016/S0193-953X(02)00084-9
Shurman, B., Horan, W. P., & Nuechterlein, K. H. (2005). Schizophrenia patients
demonstrate a distinctive pattern of decision-making impairment on the Iowa
gambling task. Schizophrenia Research, 72(2-3), 215–224.
doi:10.1016/j.schres.2004.03.020
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 209
Simon, A. E., Giacomini, V., Ferrero, F., & Mohr, S. (2003). Dysexecutive syndrome
and social adjustment in schizophrenia. Australian and New Zealand Journal of
Psychiatry, 37(3), 340–346. doi: 10.1046/j.1440-1614.2003.01186.x
Skosnik, P. D., Spatz-Glenn, L., & Park, S. (2001). Cannabis use is associated with
schizotypy and attentional disinhibition. Schizophrenia Research, 48(1), 83–92.
doi:10.1016/s0920-9964(00)00132-8
Smith, R. E., & Bayen, U. J. (2005). The effects of working memory resource
availability on prospective memory: A formal modeling approach. Experimental
Psychology, 52(4), 243–256. doi:10.1027/1618-3169.52.4.243
Solowij, N. (1995). Do cognitive impairments recover following cessation of cannabis
use? Life Science, 56(23–24), 2119–2126. doi:10.1016/0024-3205(95)00197-E
Solowij, N., & Battisti, R. (2008). The chronic effects of cannabis on memory in
humans: A review. Current Drug Abuse Reviews, 1(1), 81–98.
doi:10.2174/1874473710801010081
Solowij, N., & Michie, P. T. (2007). Cannabis and cognitive dysfunction: parallels with
endophenotypes of schizophrenia? Journal of Psychiatry and Neuroscience,
32(1), 30–52. Retrieved from
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1764544/?tool=pmcentrez
Solowij, N., Michie, P. T., & Fox, A. M. (1991). Effects of long-term cannabis use on
selective attention: an event-related potential study. Pharmacology,
Biochemistry and Behavior, 40(3), 683–688.
doi:10.1016/0091-3057(91)90382-C
Solowij, N., Michie, P. T., & Fox, A. M. (1995). Differential impairments of selective
attention due to frequency and duration of cannabis use. Biological Psychiatry,
37(10), 731–739. doi:10.1016/0006-3223(94)00178-6
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 210
Solowij, N., Stephens, R. S., Roffman, R. A., Babor, T., Kadden, R., Miller, M., ...
Vendetti, J. (2002). Cognitive functioning of long-term heavy cannabis users
seeking treatment. Journal of the American Medical Association, 287(9), 1123–
1131. doi:10.1001/jama.287.9.1123
Spreen, O., & Strauss, E., 1998. A Compendium of Neuropsychological Tests:
Administration, Norms, and Commentary (Second Edition). Oxford: Oxford
University Press, p. 171.
Stern, Y. (2009). Cognitive reserve. Neuropsychologia, 47(10), 2015–2028.
doi:10.1016/j.neuropsychologia.2009.03.004
Stevens, J. P. (1986). Applied Multivariate Statistics for the Social Sciences. New
Jersey: Lawrence Erlbaum Associates, p. 345.
Stevens, J. P. (2002). Applied Multivariate Statistics for the Social Sciences (Fourth
Edition). New Jersey: Lawrence Erlbaum Associates, p. 395.
Stirling, J., Lewis, S., Hopkins, R., & White, C. (2005). Cannabis use prior to first onset
psychosis predicts spared neurocognition at 10-year follow-up. Schizophrenia
Research, 75(1), 135–137. doi:10.1016/j.schres.2004.10.006
Stirling, J., White, C., Lewis, S., Hopkins, R., Tantam, D., Huddy, A., & Montague, L.
(2003). Neurocognitive function and outcome in first-episode schizophrenia: A
10-year follow-up of an epidemiological cohort. Schizophrenia Research, 65(2-
3), 75–86. doi:10.1016/s0920-9964(03)00014-8
Stratowski, S. M., Maurico, I., Stoll, A. L., Faedda, G., Mayer, P. V., Kolbrener, M. I.,
& Goodwin, D. C. (1993). Comorbidity in psychosis at first hospitalization.
American Journal of Psychiatry, 150(5), 752–757.
Stroop, J. R. (1935). Studies of interference in serial verbal reactions. Journal of
Experimental Psychology, 18(6), 643–662. doi:10.1037/h0054651
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 211
Stuss, D. T., & Alexander, M. P. (2000). Executive functions and the frontal lobes: a
conceptual view. Psychological Research, 63(3-4), 289–298.
doi:10.1007/s004269900007
Stuss, D. T., & Benson, D. F. (1986). The Frontal Lobes. New York: Raven Press.
Stuss, D. T., & Levine, B. (2002). Adult clinical neuropsychology: lessons from studies
of the frontal lobes. Annual Review of Psychology, 53(1), 401–433.
doi:10.1146/annurev.psych.53.100901.135220
Tabachnick, B. G., & Fidell, L. S. (2007). Using Multivariate Statistics (Fifth Edition).
New York: Allyn and Bacon.
Thames, A. D., Arbid, N., & Sayegh, P. (2014). Cannabis use and neurocognitive
functioning in a non-clinical sample of users. Addictive Behaviors, 39(5), 994–
999. doi: 10.1016/j.addbeh.2014.01.019
Thoma, P., Wiebel, B., & Daum, I. (2007). Response inhibition and cognitive flexibility
in schizophrenia with and without comorbid substance use disorder.
Schizophrenia Research, 92(1-3), 168–180. doi:10.1016/j.schres.2007.02.004
Torrey, E. F. (2002). Surviving schizophrenia: A manual for families, consumers and
providers. New York, NY: Harper Collins.
Van der Linden, M. , Beerten, A., & Pesenti, M. (1998). Age-related differences in
random generation. Brain and Cognition, 38(1), 1–16.
doi:10.1006/brcg.1997.0969
van Hooren, S. A., Valentijn, S. A., Bosma, H., Ponds, R. W., Van Boxtel, M. P.,
Levine, B., ... Jolles, J. (2007). Effect of a structured course involving Goal
Management Training in older adults: a randomised controlled trial. Patient
Education and Counseling, 65(2), 205–213. doi:10.1016/j.pec.2006.07.010
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 212
Velligan, D. I., Mahurin, R. K., Diamond, P. L., Hazleton, B. C., Eckert, S. L., &
Miller, A. L. (1997). The functional significance of symptomatology and
cognitive function in schizophrenia. Schizophrenia Research, 25(1), 21–31.
doi:10.1016/s0920-9964(97)00010-8
Verdejo-García, A., Benbrook, A., Funderburk, F., David, P., Cadet, J. L., & Bolla, K. I.
(2007). The differential relationship between cocaine use and marijuana use on
decision-making performance over repeat testing with the Iowa Gambling Task.
Drug and Alcohol Dependence, 90(1), 2–11.
doi:10.1016/j.drugalcdep.2007.02.004
Verdejo-García, A., Fagundo, A. B., Cuenca, A., Rodriguez, J., Cuyás, E., Langohr, K.,
... de la Torre, R. (2013). COMT val158met and 5-HTTLPR genetic
polymorphisms moderate executive control in cannabis users.
Neuropsychopharmacology, 38(8), 1598–1606. doi: 10.1038/npp.2013.59
Verdoux, H., Magnin, E., & Bourgeois, M. (1995). Neuroleptic effects on
neuropsychological test performance in schizophrenia. Schizophrenia Research,
14(2), 133–139. doi:10.1016/0920-9964(94)00018-4
Voruganti, L. N., Heslegrave, R. J., & Awad, A. G. (1997). Neurocognitive correlates of
positive and negative syndromes in schizophrenia. Canadian Journal of
Psychiatry, 42(10), 1066–1071.
Wang, Y., Cui, J., Chan, R. C., Deng, Y., Shi, H., Hong, X., ... Shum, D. (2009).
Meta-analysis of prospective memory in schizophrenia: nature, extent, and
correlates. Schizophrenia Research, 114(1), 64-70.
doi: 10.1016/j.schres.2009.07.009
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 213
Warkentin, S., & Passant, U. (1997). Functional imaging of the frontal lobes in organic
dementia. Dementia and Geriatric Cognitive Disorders, 8(2), 105–109.
doi:10.1159/000106614
Warner, R., Taylor, D., Wright, J., Sloat, A., Springett, G., Arnold, S., & Weinberg, H.
(1994). Substance use among the mentally ill: Prevalence, reasons for use and
effects on illness. American Journal of Orthopsychiatry, 64(1), 30–39. doi:
10.1037/h0079489
Wechsler, D. (1981). Wechsler Adult Intelligence Scale - Revised. New York: The
Psychological Corporation.
Wechsler, D. (1987). Wechsler Memory Scale - Revised. San Antonio, TX: The
Psychological Corporation.
Wechsler, D. (1997). Wechsler Adult Intelligence Scale (Third Edition). San
Antonio, TX: The Psychological Corporation.
Wechsler, D. (1997). Wechsler Memory Scale (Third Edition). San Antonio, TX: The
Psychological Corporation.
Wechsler, D. (2008). Wechsler Adult Intelligence Scale (Fourth Edition). San Antonio,
TX: NCS Pearson.
Weinstein, C. S., & Shaffer, H. J. (1993). Neurocognitive aspects of substance abuse
treatment: A psychotherapist's primer. Psychotherapy: Theory, Research,
Practice, Training, 30(2), 317–333. doi:10.1037/0033-3204.30.2.317
West, R. L. (1996). An application of prefrontal cortex function theory to cognitive
aging. Psychological Bulletin, 120(2), 272–292. doi:10.1037/0033-
2909.120.2.272
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 214
Whitlow, C. T., Liguori, A., Brooke Livengood, L., Hart, S. L., Mussat-Whitlow, B. J.,
Lamborn, C. M., ... Porrino, L. J. (2004). Long-term heavy marijuana users
make costly decisions on a gambling task. Drug and Alcohol Dependence, 76(1),
107–111. doi:10.1016/j.drugalcdep.2004.04.009
Whyte, J., Polansky, M., Cavallucci, C., Fleming, M., Lhulier, J., & Coslett, H. B.
(1996). Inattentive behavior after traumatic brain injury. Journal of the
International Neuropsychological Society, 2(4), 274–281.
doi:10.1017/s1355617700001284
Wilder, K. E., Weinberger, D. R., & Goldberg, T. E. (1998). Operant conditioning and
the orbitofrontal cortex in schizophrenic patients: Unexpected evidence for
intact functioning. Schizophrenia Research, 30(2), 169–174. doi:10.1016/s0920-
9964(97)00135-7
Wilk, C. M., Gold, J. M., Humber, K., Dickerson, F., Fenton, W. S., & Buchanan, R. W.
(2004). Brief cognitive assessment in schizophrenia: normative data for the
Repeatable Battery for the Assessment of Neuropsychological Status.
Schizophrenia Research, 70(2), 175–186. doi:10.1016/j.schres.2003.10.009
Williams, L. M., Whitford, T. J., Flynn, G., Wong, W., Liddell, B. J., Silverstein, S., ...
& Gordon, E. (2008). General and social cognition in first episode
schizophrenia: identification of separable factors and prediction of functional
outcome using the IntegNeuro test battery. Schizophrenia Research, 99(1), 182–
191. doi:10.1016/j.schres.2007.10.019
Wilson, B. A., Alderman, N., Burgess, P., Emslie, H., & Evans, J. J. (1996).
Behavioural Assessment of the Dysexecutive Syndrome. Bury St. Edmunds,
Suffolk: Thames Valley Test Company.
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 215
Wilson, B. A., Evans, J. J., Emslie, H., Alderman, N., & Burgess, P. (1998). The
development of an ecologically valid test for assessing patients with a
dysexecutive syndrome. Neuropsychological Rehabilitation, 8(3), 213–228.
doi:10.1080/713755570
Wilson, B. A., Evans, J. J., Emslie, H., & Malinek, V. (1997). Evaluation of NeuroPage:
a new memory aid. Journal of Neurology, Neurosurgery & Psychiatry, 63(1),
113–115. doi:10.1136/jnnp.63.1.113
Wobrock, T., Sittinger, H., Behrendt, B., D’Amelio, R., Falkai, P., & Caspari, D.
(2007). Comorbid substance abuse and neurocognitive function in recent-onset
schizophrenia. European Archives of Psychiatry and Clinical Neuroscience,
257(4), 203–210. doi:10.1007/s00406-006-0707-x
World Health Organisation (1993). Composite International Diagnostic Interview,
Version 1.1. Geneva: WHO.
Wykes, T., Reeder, C., Landau, S., Everitt, B., Knapp, M., Patel, A., & Romeo, R.
(2007). Cognitive remediation therapy in schizophrenia: Randomised controlled
trial. The British Journal of Psychiatry, 190(5), 421–427. doi:
10.1192/bjp.bp.106.026575
Yücel, M., Bora, E., Lubman, D. I., Solowij, N., Brewer, W. J., Cotton, S. M., ...
Pantelis, C. (2010). The impact of cannabis use on cognitive functioning in
patients with schizophrenia: A meta-analysis of existing findings and new data
in a first-episode sample. Schizophrenia Bulletin, 38(2), 316–330.
doi:10.1093/schbul/sbq079
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 216
Zalla, T., Joyce, C., Szöke, A., Schürhoff, F., Pillon, B., Komano, O., ... & Leboyer, M.
(2004). Executive dysfunctions as potential markers of familial vulnerability to
bipolar disorder and schizophrenia. Psychiatry Research, 121(3), 207–217.
doi:10.1016/S0165-1781(03)00252-X
Zalla, T., Plassiart, C., Pillon, B., Grafman, J., & Sirigu, A. (2001). Action planning in a
virtual context after prefrontal cortex damage. Neuropsychologia, 39(8), 759–
770. doi:10.1016/s0028-3932(01)00019-7
Zhang, L., Abreu, B. C., Seale, G. S., Masel, B., Christiansen, C. H., & Ottenbacher, K.
J. (2003). A virtual reality environment for evaluation of a daily living skill in
brain injury rehabilitation: Reliability and validity. Archives of Physical
Medicine and Rehabilitation, 84(8), 1118–1124. doi:10.1016/s0003-
9993(03)00203-x
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 217
Appendices
Appendix A: Structured Interview
Demographic Information
“I’d like to ask you a few questions regarding some personal information about yourself”
(1) What is your age?_____ years _____ months (DOB:___________)
(2) Gender Male Female
(3) How many years of full-time education have you completed? ______ years
What grades (on average) did you achieve? _______
(4) Have you experienced any of the following medical conditions/disabilities:
Head/Brain injury specify ______________________
Neurological conditions (e.g. epilepsy, MS) specify ______________________
Mental Illness specify ______________________
Intellectual disability/learning difficulties specify ______________________
Stroke
Cardiac Arrest
(5) Are you currently using medication? Yes No
If yes, please specify___________________________
Approximately, how long have you been taking this medication? ____________
(6) Do you use drugs/alcohol (including cigarettes)? Yes No
If yes:
Please specify and how often? ________________________(days per week)
Approximately, how much of the drug do you use per using-day __________(e.g., cones, joints)
Approximately, how long have you been using the drug ________(months/years)
When was the last time you used the drug?______________
Has your use of drugs/alcohol ever:
Had a negative effect on your interpersonal specify ______________________
relationships
Interfered with your ability to attend/persist specify ______________________
with work/school activities
Put your physical safety (e.g. while driving a specify ______________________
vehicle or operating machinery), at risk
Resulted in legal issues (e.g. arrests for specify ______________________
substance related disorderly conduct)
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 218
Appendix B(i): Advertisement for Clinical Participants – Cannabis Users
RESEARCH STUDY
Do you want to contribute to valuable new research???
This study is about schizophrenia, cannabis use and thinking processes. Your
contribution would be most appreciated.
We’re asking for adults, both male and female aged between 18 and 55 years
with a diagnosis of schizophrenia (no other mental illness diagnoses included), and who
are also regular cannabis users (but do not use any other illicit drug), to come and be a
part of this research. Participation is voluntary and confidentiality will be strictly
maintained!!!
If you agree to volunteer, you will be asked some questions about you (e.g., age,
gender, educational level, health status, symptoms, and cannabis use), as well as asked
to fill out a questionnaire about your mood. In addition you will also be asked to
complete a range of tasks involving solving puzzles, planning trips and card
games.
As a token of appreciation for your participation you will receive a $25
voucher for Coles-Myer.
The researcher Khristin Highet (Doctor of Psychology (Clinical) student), can be
contacted on the following email address for more information:
or on:
0415 494 551
OR
Alternatively, you can leave your contact details on the form provided at the
reception desk, in order for the researcher to contact you.
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 219
Appendix B(ii): Advertisement for Clinical Participants – Non-drug Users
RESEARCH STUDY
Do you want to contribute to valuable new research???
This study is about schizophrenia, cannabis use and thinking processes. Your
contribution would be most appreciated.
We’re asking for adults, both male and female aged between 18 and 55 years
with a diagnosis of schizophrenia (no other mental illness diagnoses included), and who
do not use any illicit drug, to come and be a part of this research. Participation is
voluntary and confidentiality will be strictly maintained!!!
If you agree to volunteer, you will be asked some questions about you (e.g., age,
gender, educational level, health status, and symptoms), as well as asked to fill out a
questionnaire about your mood. In addition you will also be asked to complete a range
of tasks involving solving puzzles, planning trips and card games.
As a token of appreciation for your participation you will receive a $25
voucher for Coles-Myer.
The researcher Khristin Highet (Doctor of Psychology (Clinical) student), can be
contacted on the following email address for more information:
or on:
0415 494 551
OR
Alternatively, you can leave your contact details on the form provided at the
reception desk, in order for the researcher to contact you.
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 220
Appendix C: Clinical Participant Information Form
CENTRAL NORTHERN ADELAIDE HEALTH SERVICE
(CNAHS)
The Queen Elizabeth Hospital & Lyell McEwin Hospital
PARTICIPANT INFORMATION SHEET
Title: Relationships between schizophrenia and cannabis use on everyday adjustment
Application Number: 2009010
INVITATION TO PARTICIPATE We invite you to participate in a study which we believe is of potential importance to
people with schizophrenia. But, before you decide whether you wish to participate, we
need to be sure that you understand:
why we are doing it, and
what it would involve if you agreed.
So, please read the following carefully and be sure to ask any questions you have. I will
be happy to discuss it with you and answer your questions. You are also free to discuss
it with others if you wish (i.e., family, friends, your local Doctor and / or your clinical
treatment team).
You do not have to make an immediate decision.
PARTICIPATION IS VOLUNTARY
This is a research project and you do not have to be involved. Your medical care or
clinical treatment will not be affected in any way if you choose not to participate. Also,
if you agree to participate, you are free to change your mind and withdraw at any stage.
BACKGROUND TO THE STUDY
What is the research about?
My name is Khristin Highet. I am completing this research as part of my Doctor of
Psychology (Clinical) degree through Murdoch University in Western Australia. I’m
looking at the relationship between schizophrenia, cannabis use, thinking processes and
everyday life skills.
Who is sponsoring it, and are they paying the researcher or her department to do the
research?
I’m a post-graduate student of Murdoch University and the university is sponsoring this
study. I will not be paid for conducting this research.
Why is the research being done?
I hope that the study will help treating professionals to make better informed decisions
about treatment plans for people with schizophrenia. In addition, it may be of help when
developing treatment plans for patients who may also use cannabis.
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 221
Who can participate?
I’m inviting both male and female adults with schizophrenia, aged between 18 and 55
years, and who do NOT have a history of serious head/brain injury, mental illness
diagnoses other than schizophrenia, learning disability, chronic substance use (other than
cannabis use), stroke and/or a neurological condition, to take part in the study.
How many other people have been asked to consider participating?
Around 60 participants will be included in the study. This will include people with
schizophrenia who attend The Queen Elizabeth Hospital and the Lyell McEwin
Hospital, or any other affiliated public mental health service under CNAHS.
What is involved if I choose to participate?
If you agree to participate, a testing session will be scheduled. During this session, I will
then ask some questions about you (e.g., your age, gender, years of education, health
status, cannabis use, symptoms) and your mood. This will take about 30-45 minutes.
Afterwards, I will ask you to do some tasks looking at your reading abilities, memory,
planning, attention and everyday skills, which will take about 60-90 minutes. Most
people find these tasks enjoyable and you can have a break at any time.
With your permission, I will also ask a member of your treatment team, or your
community support worker (if applicable), to fill in a questionnaire outlining your
everyday skills. If you do not want your treatment team or support person to be
involved, this will not stop you from being involved in the study. A family member or
significant other may answer the questions related to your everyday skills.
DISCOMFORTS, RISKS AND SIDE EFFECTS
Who should I contact if I am worried about any effects that I experience?
No risks or side effects are anticipated as a result of the current research. But, if you
should experience undue emotional distress, or I observe a significant change in your
condition during the research process, you can stop the session, and I will encourage
you to contact your clinical treatment team for consultation. You can recommence the
study at a later stage if you wish to.
WHAT WILL HAPPEN TO THE INFORMATION COLLECTED?
All information you give is confidential. No names or other information that might
identify you will be used in any publication arising from the study. The information
collected from the research will be stored in locked cabinets located at Murdoch
University. Only my research supervisors and I will have access to the data.
WHAT ARE MY RIGHTS?
If you become injured during this study, and your injury is a direct result of the effects
of study procedures, The Queen Elizabeth Hospital will provide reasonable medical
treatment. Your participation in this study shall not affect any other right to
compensation you may have under common law. Should you wish to obtain more
information about your rights as a participant, you can contact the Executive Officer of
the Ethics Of Human Research committee, on (08) 8222 6841.
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 222
IS THERE ANY PAYMENT FOR PARTICIPATION?
We understand that involvement in this study requires a time commitment and we are
extremely appreciative of your participation and time given. As a token of appreciation
for your participation you will receive a $25 voucher for Coles-Myer.
BENEFITS OF THE RESEARCH
I hope that this study will assist in helping people with schizophrenia to successfully
transition out of hospital care and return to community living. However, these benefits
may not directly affect you.
WHAT IF I HAVE A QUESTION ABOUT THE STUDY?
If you have any questions or concerns about your involvement in the study, you may
contact the chief project supervisor, Dr Marjorie Collins via email:
The Central Northern Adelaide Health Service Ethics of Human Research Committee
(TQEH & LMH) has approved this study.
Should you wish to speak to a person not directly involved in the study in relation to
matters concerning policies,
information about the conduct of the study
your rights as a participant, or
should you wish to make a confidential complaint, you may contact The Executive
Officer of this committee, on (08) 8222 6841.
Thank you,
Khristin Highet
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 223
Appendix D: Advertisement for Control Participants
RESEARCH STUDY
Do you want to contribute to valuable new research???
This study is about schizophrenia, cannabis use and thinking processes. Your
contribution would be most appreciated.
We’re asking for adults, both male and female aged between 18 and 55 years
who do not have any mental illness diagnosis and are regular cannabis users (but do
not use any other illicit drug), to come and be a part of this research. Participation is
voluntary and confidentiality will be strictly maintained!!!
If you agree to volunteer, you will be asked some questions about you (e.g., age,
gender, educational level, health status, and cannabis use), as well as asked to fill out a
questionnaire about your mood. In addition you will also be asked to complete a range
of tasks involving solving puzzles, planning trips and card games.
As a token of appreciation for your participation you will receive a $25
voucher for Coles-Myer.
The researcher Khristin Highet (Doctor of Psychology (Clinical) student), can be
contacted on the following email address for more information:
or on:
0415 494 551
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 224
Appendix E: Control Participant Information Form
MURDOCH UNIVERSITY
School of Psychology
PARTICIPANT INFORMATION SHEET
Title: Relationships between schizophrenia and cannabis use on everyday adjustment
Permit Number: 2008/168
INVITATION TO PARTICIPATE We invite you to participate in a study which we believe is of potential importance to
people with schizophrenia. But, before you decide whether you wish to participate, we
need to be sure that you understand:
why we are doing it, and
what it would involve if you agreed.
So, please read the following carefully and be sure to ask any questions you have. We
will be happy to discuss it with you and answer your questions. You are also free to
discuss it with others if you wish (i.e., family, friends, and/or your local Doctor).
You do not have to make an immediate decision.
PARTICIPATION IS VOLUNTARY
This is a research project and you do not have to be involved. Participation is voluntary.
Also, if you agree to participate, you are free to change your mind and withdraw at any
stage.
BACKGROUND TO THE STUDY
What is the research about?
My name is Khristin Highet. I am completing this research as part of my Doctor of
Psychology (Clinical) degree through Murdoch University in Western Australia. I’m
looking at the relationship between schizophrenia, cannabis use, thinking processes and
everyday life skills.
Who is sponsoring it, and are they paying the researcher or her department to do the
research?
I’m a post-graduate student of Murdoch University and the university is sponsoring this
study. I will not be paid for conducting this research.
Why is the research being done?
I hope that the study will help treating professionals to make better informed decisions
about treatment plans for people with schizophrenia. In addition, it may be of help when
developing treatment plans for patients who may also use cannabis. Please note: The
involvement of participants without a mental illness, such as yourself, will help me
to better understand the effects of both cannabis use and schizophrenia, in
comparison to the effects of cannabis use alone (i.e., your results will be used in a
comparison group; comparing your results to those who have schizophrenia, but
also use cannabis in addition).
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 225
Who can participate?
I’m inviting both male and female current or past regular cannabis users, aged between
18 and 55 years, who do NOT have a history of serious head/brain injury, mental illness
diagnosis, learning disability, chronic substance use (other than cannabis use), stroke
and/or a neurological condition, to take part in the study.
How many other people have been asked to consider participating?
Around 60 participants will be included in the study. This will include individuals with
schizophrenia who use cannabis, as well as members of the community who do not have
a mental illness diagnosis who are either current or past cannabis users.
What is involved if I choose to participate?
If you agree to participate, I will initially ask you some questions via telephone about
you (e.g. your age, gender, years of education, health status, cannabis use) in order to
determine your eligibility for the study. This will take about 10-15 minutes. If you are
considered eligible for the study, you will be invited to a testing session where you will
be asked some questions about your mood, as well as I will ask you to complete some
tasks looking at your reading abilities, memory, planning, and attention, which will take
about 45-60 minutes. Most people find these tasks enjoyable and you can have a break at
any time.
DISCOMFORTS, RISKS AND SIDE EFFECTS
Who should I contact if I am worried about any effects that I experience?
No risks or side effects are anticipated as a result of the current research. But, if you
should experience undue emotional distress, you can stop the session, and I will
encourage you to contact your local doctor for consultation. You can recommence the
study at a later stage if you wish to.
WHAT WILL HAPPEN TO THE INFORMATION COLLECTED?
All information you give is confidential. No names or other information that might
identify you will be used in any publication arising from the study. The information
collected from the research will be stored in locked cabinets located at Murdoch
University. Only my research supervisors and I will have access to the data.
IS THERE ANY PAYMENT FOR PARTICIPATION?
We understand that involvement in this study requires a time commitment and we are
extremely appreciative of your participation and time given. As a token of appreciation
for your participation you will receive a $25 voucher for Coles-Myer.
BENEFITS OF THE RESEARCH
I hope that this study will assist in helping people with schizophrenia who may use
cannabis to successfully transition out of hospital care and return to community living.
However, these benefits may not directly affect you.
WHAT IF I HAVE A QUESTION ABOUT THE STUDY?
If you have any questions or concerns about your involvement in the study, you may
contact the chief project supervisor, Dr Marjorie Collins via email:
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 226
If you have any ethical concerns about the project or questions about your rights as a
participant, please contact The Murdoch University Human Research Ethics Committee
on (08) 9360 6677 or e-mail: [email protected]
Thank you,
Khristin Highet
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 227
Appendix F: Consent Form for Clinical Participants
CENTRAL NORTHERN ADELAIDE HEALTH SERVICE
(CNAHS)
The Queen Elizabeth Hospital & Lyell McEwin Hospital
CONSENT FORM
Title: Relationships between schizophrenia and cannabis use on everyday adjustment.
Researcher’s name: Khristin Highet
Supervisor’s name: Dr Marjorie Collins
Protocol Number: 2009010
I, the undersigned ..................................................................................... hereby consent
to my involvement in the research project explained above.
I have read the information sheet, and I understand the reasons for this study. The
researcher has explained the ways in which it will affect me. My questions have
been answered to my satisfaction. My consent is given voluntarily. I am also aware
that I may change my mind and stop at any time without this affecting my treatment,
either now or in the future.
I have been given the opportunity to have a family member, a friend or a member of
my clinical treating team present while the project was explained to me.
I understand that I may not directly benefit from taking part in the study.
I understand the statement on the information sheet concerning payment for taking
part in this study.
I understand that parts of the testing session will be audio-taped and that all records
and responses (including those audio-taped) will be kept in a safe and secure place
at Murdoch University where only the researcher and supervisors will have access.
I understand that all information provided is treated as confidential and will not be
released by the researcher unless required to do so by law.
I agree that research data gathered for this study may be published provided my
name or other information which might identify me is not used.
PATIENT SIGNATURE....................................................... DATE ……./……./…….
WITNESS (optional)............................................................... DATE ……./……./…….
______________________________________________________________________
I have provided information about the research to the research participant and believe
that he/she understands what is involved.
RESEARCHER........................................................................ DATE ……./……./…….
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 228
Appendix G: Consent Form for Control Participants
MURDOCH UNIVERSITY
School of Psychology
CONSENT FORM
Title: Relationships between schizophrenia and cannabis use on everyday adjustment
Researcher’s name: Khristin Highet
Supervisor’s name: Dr Marjorie Collins
Permit Number: 2008/168
I, the undersigned .................................................................................. hereby consent
to my involvement in the research study explained above.
I have read the information sheet, and I understand the reasons for this study. The
researcher has explained the ways in which it will affect me. My questions have
been answered to my satisfaction. My consent is given voluntarily. I am also aware
that I may change my mind and stop at any time without this resulting in prejudice,
either now or in the future.
I understand that I may not directly benefit from taking part in the study.
I understand the statement on the information sheet concerning payment for taking
part in this study.
I understand that parts of the testing session will be audio-taped and that all records
and responses (including those audio-taped) will be kept in a safe and secure place
at Murdoch University where only the researcher and supervisors will have access.
I understand that all information provided is treated as confidential and will not be
released by the researcher unless required to do so by law.
I agree that research data gathered for this study may be published provided my
name or other information which might identify me is not used.
PARTICIPANT’S SIGNATURE.......................................... DATE ……./……./…….
______________________________________________________________________
I have provided information about the research to the research participant and believe
that he/she understands what is involved.
RESEARCHER....................................................................... DATE ……./……./…….
CANNABIS USE, SCHIZOPHRENIA & EXECUTIVE FUNCTION 229
Appendix H: Debrief Information
ABOUT THIS STUDY
Thank you for participating in this study. Your time and efforts are much appreciated.
The aim of the research is to examine the relationship between schizophrenia, cannabis
use, and ‘executive functions’. Executive functions are a group of thinking processes
that generally involve planning, problem solving, self-monitoring, and stopping a
response that is not the best one for the task at hand. The tasks you have undertaken
today are thought to assess such processes.
Another aim of our research is to look at the relationship between these thinking
abilities and everyday skills. We hope that the results of the research project will help to
better inform treating professionals in developing more effective treatment plans for
individuals with schizophrenia who may present with cannabis abuse issues.
If any aspect of the research process has raised any concerns, we encourage you to
consult with your clinical treatment team/doctor. You may also contact the following
service for help if necessary:
Assessment & Crisis Intervention Service (ACIS) - 24 hour Emergency Mental Health
Service Ph. No: 13 14 65
Thank you once again for your contribution and participation in our project.