Neuropsycholo`ia\ Vol[ 25\ No[ 00\ pp[ 0070Ð0077\ 0887Þ 0887 Published by Elsevier Science Ltd[ All rights reserved\ Pergamon Printed in Great Britain

9917Ð2821:87 ,08[99¦9[99PII] S9917Ð2821"86#99062Ð4

The role of executive function in imagerymnemonics] evidence from multiple sclerosis

MARY CANELLOPOULOU� and JOHN T[ E[ RICHARDSON

Department of Human Sciences\ Brunel University\ Uxbridge\ Middlesex UB7 2PH\ UK

"Received 15 May 0886^ accepted 5 December 0886#

Abstract*The use of imagery in learning and memory involves metacognitive skills that seem to depend upon executive function aswell as motivational mechanisms[ This implies that the e.cacy of imagery mnemonics should be impaired in neurological patientswho show either executive dysfunction or a}ective disorders[ This hypothesis was tested in patients with multiple sclerosis[ Imageryinstructions led to improved recall in three di}erent tasks\ although experimenter!generated imagery proved to be more e}ectivethan subject!generated imagery and there was little evidence that patients maintained the use of imagery mnemonics on follow!uptesting[ Three other tasks de_ned a unitary trait of executive processing e.ciency\ and this was signi_cantly correlated with thebene_t obtained from the use of the Method of Loci in a free!recall tasks\ but not with the bene_t gained from other imagerymnemonics[ The patients| scores\ on a widely used depression inventory\ were not signi_cantly correlated with the bene_t obtainedfrom the use of imagery mnemonics[ Nonetheless\ it is concluded that executive processing capacity determines the bene_t that isderived from the use of more complex forms of imagery mnemonic in verbal!learning tasks[ Þ 0887 Published by Elsevier ScienceLtd[ All rights reserved[

Key Words] depression^ executive processing^ learning^ metacognition^ memory[

Introduction

In research on human learning and memory\ imageryis commonly regarded as a set of strategic operationsthat are under conscious executive control[ Under somecircumstances\ at least\ these operations are associatedwith high levels of performance\ whether they areemployed spontaneously or in response to speci_cinstructions or training ð37Ł[ From a theoretical point ofview\ these e}ects can be ascribed either to the involve!ment of a distinct representational system\ that is spe!cialized for the storage of nonverbal information ð31Ł\ orto increased speci_city and distinctiveness of the infor!mation that is encoded within a single representationalsystem ð22Ł[ The latter point of view is supported by thefact that the e.cacy of imagery mnemonic instructionsdepends upon the relational organization of the encodedimages rather than their pictorial properties ð7\ 09Ł[

The use of imagery and other strategies presupposesan awareness of one|s cognitive processes^ this awarenesshas been called {{metacognition|| ð05Ł or {{metamemory||ð06Ł[ It re~ects an individual|s knowledge of how\ when\and where techniques such as imagery should beemployed ð35Ł[ This provides a framework for acquiring

�To whom all correspondence should be addressed[

0070

and understanding new strategies ð67Ł\ and relevant met!acognitive knowledge is activated by the administrationof imagery mnemonic instructions ð01Ł[ Some aspects ofmetacognitive ability involve the executive functions typi!cally attributed to the frontal lobes ð48Ł\ while othersappear to involve a}ective and motivational mechanismsð18\ 25\ 30Ł[ It follows that the e.cacy of imagery instruc!tions should be disrupted in neurological patients whoexhibit executive dysfunction or a}ective disturbances[

Both kinds of impairment have been reported in pat!ients with multiple sclerosis "MS#[ This is characterizedby the widespread occurrence within the nervous systemof patches of demyelination followed by gliosis[ Theseare thought to represent an immunologically mediatedin~ammatory response to a combination of genetic andenvironmental processes[ Although in some cases the dis!ease is progressive from the outset\ most patients experi!ence neurological symptoms that recover fully at _rst[Further episodes occur at a random frequency and foran unpredictable period[ Nevertheless\ many patientseventually cease to experience episodic symptoms\ andthereafter the course of the disease is slowly progressiveð03Ł[

The routine clinical examination of the mental statusof MS patients typically reveals impaired mentation inonly a small proportion of cases ð04\ 17\ 34\ 47Ł\ and until

M[ Canellopoulou and J[ T[ E[ Richardson:Imagery mnemonics0071

recently most authorities concluded that de_cits in highercortical function in MS patients were rare[ However\neuropsychological testing indicates that brief neuro!logical examinations often signi_cantly underestimate theincidence of dysfunction ð19\ 34Ł[ In formal cognitivetests\ especially those that involve recent memory\ sus!tained attention\ verbal ~uency\ conceptual reasoning\and visuospatial perception\ patients with MS have beenreliably found to produce impaired performance "see Ref[ð36Ł for review#\ even in the early stages of the disease ð07Ł[Clinical assessment can also reveal a}ective disturbances\particularly depression^ these may be in~uenced by dis!ease activity\ but they also vary with the experience ofthe illness and concomitant physical disablement ð5\ 02\40Ł[

Accordingly\ the following study was carried out toobtain measures of executive dysfunction and depressionin a sample of community!based MS patients and torelate these measures to the e.cacy of imagery mnemonicinstructions[ In this context\ {{executive function|| is usedto refer to the forms of controlled processing that areinvolved in classic {{frontal|| tests without assuming thatperformance in these tasks depends critically and solelyon neural structures in the frontal lobes ð1\ 52Ł[ A batteryof three di}erent tests was employed to evaluate executivefunction\ and statistical analysis was carried out to assesswhether scores on the three tests de_ned a single theor!etical construct\ termed {{executive processing e.ciency||[Estimated scores on this construct were compared withthose obtained on an inventory commonly used to assessdepression according to self!reports\ and both measureswere related to changes in performance on three memorytasks after the administration of imagery instructions[The _ndings were also compared with the self!reportedquality of experienced imagery and with demographicand clinical characteristics of the patients[

Method

Subjects

The subjects were recruited from MS patients on a well!documented national register ð28Ł\ all of whom had been diag!nosed as having either de_nite or probable MS ð23Ł between0866 and 0872[ The register was known to have a dis!proportionate number of female entries in comparison with theincidence of the disease within the UK[ In a previous study ð49Ł\a sample of 109 patients had been constructed\ which consistedof all the male patients on the register and a random sample ofthe female patients\ chosen to achieve a male ] female ratio equalto the UK estimate of 0 ] 0[4[ During 0882\ the patients in thissample were recontacted on an unsystematic basis and wereasked to participate in the present study until results had beencollected from a total of 49 individuals[

The _nal sample included 08 men and 20 women\ who wereaged between 24 and 65 years "with a mean of 40[9 years#\ andwho had received between 09 and 07 years of full!time education"with a mean of 02[9 years#[ They had su}ered from MS for amean of 19[5 years from the time of the onset of symptoms\and for a mean of 04[7 years from the time of its diagnosis[ The

pattern of the clinical development of the disease was classi_edas chronicÐprogressive in 12 cases and as relapsingÐremittingin the others[ In 29 cases\ the disease could be described as{{stable||\ insofar as there had been no worsening in the patients|condition on the Ambulation Index or the Expanded DisabilityStatus Scale in the previous two years ð08\ 16Ł\ while in theremaining 19 cases it was classi_ed as {{worsening||[

For scoring the Cognitive Estimation Test "see below#\ con!trol data were obtained from close relatives or carers of 27 ofthe patients\ most commonly their spouses[ This control groupconsisted of 05 women and 11 men\ who were aged between 23and 67 years "with a mean of 42[7 years#\ and who had receivedbetween 09 and 08 years of full!time education "with a mean of01[8 years#[ Statistical tests found no sign of any di}erencebetween the MS patients and the control group in terms ofeither their age or their level of education[

Procedure

A number of the patients were elderly\ physically disabled\or su}ering from psychiatric illness[ This investigation thereforefollowed at least one previous neuropsychological study of MSð11Ł in assessing patients in their own homes[ This helped toensure the inclusion of the full range of severity within a com!munity!based sample[ The _rst part of the procedure involvedthe administration of the instruments concerned withdepression\ experienced imagery\ and executive function[ Thesecond part involved the administration of tests that requiredthe recall of "a# a list of paired associates\ "b# a shopping list\and "c# a geographical route[ Patients performed each of thesetasks under instructions that did not specify any particularstrategy to obtain a baseline score[ They then performed thetasks with the support of appropriate material to measure thee.cacy of experimenter!generated imagery[ The _rst two taskswere carried out for a third time with instructions to use imagerybut without the support of appropriate material to measure thee.cacy of subject!generated imagery\ and after a delay of 09min[ they were carried out for a _nal time without explicitinstructions to measure the maintenance of imagery on follow!up[ The di}erent instruments were administered in the sameorder to all the patients\ and the time required for their assess!ment was roughly 1[4 h[

Beck depression inventory "BDI#[ The revised version of theBDI ð6Ł was administered[ In previous research on MS\ scoreson the BDI have been found to be correlated with both physicaldisability and cognitive decline ð50Ł[ They have also provedvaluable in detecting endogenous depression when combinedwith the results of psychiatric interviews ð53Ł[ The total scoreon the revised BDI was taken as a measure of depression[

Test of visual ima`ery control "TVIC#[ The revised proceduredevised by Start and Richardson ð59Ł was used[ Subjects areasked to manipulate mental images of familiar scenes\ andrespond {{yes|| "scoring 1#\ {{unsure|| "scoring 0#\ or {{no|| "scor!ing 9#[ The total score across all 01 items was taken as a measureof imagery control[

Vividness of visual ima`ery questionnaire "VVIQ#[ This instru!ment was devised by Marks ð21Ł[ Subjects rate the vividness oftheir visual imagery when thinking about familiar scenes alonga scale between 0 and 6\ where a low score indicates vividimagery[ The procedure is carried out twice\ once with regardto visual imagery experienced with the eyes open\ and againwith regard to visual imagery experienced with the eyes closed[The total scores across the 05 items in these two conditionswere highly correlated with each other "r�¦9[66#\ and so thesewere simply added together to yield a single overall measure ofimagery vividness[

Benton verbal ~uency test "BVFT#[ ð8Ł This test forms the oralcounterpart of the original Thurstone test ð51Ł[ Subjects are

M[ Canellopoulou and J[ T[ E[ Richardson:Imagery mnemonics 0072

asked to say aloud as many words beginning with a particularletter of the alphabet as possible within 0 min[ They are testedin turn on the letters F\ A\ and S[ They are instructed to excludenumerals and proper nouns and to avoid alternative forms ofthe same word[ The total number of acceptable words acrossall three trials was taken as a measure of verbal ~uency[

Co`nitive estimation test "CET#[ This test employed thematerials and procedure devised by Shallice and Evans ð45Ł[Subjects are asked to provide estimates in response to 04 factualquestions of general knowledge for which no immediately obvi!ous cognitive strategy is available] for instance\ {{What is thewidth of a double!decker bus<|| The responses were classi_edalong a 3!point scale "{{normal||\ {{quite extreme||\ {{extreme||\and {{very extreme||\ scoring 0Ð3\ respectively# by being com!pared with the distributions of responses that were producedby the control subjects[ Each patient was assigned a total scoreacross all 04 questions\ and the total time needed for the com!pletion of this test was also recorded[

Modi_ed card sortin` test "MCST#[ This employed a modi_edversion of the original Wisconsin Card Sorting Test ð26Ł thathad been devised by Nelson ð39Ł[ In this simpli_ed version\ eachof the 13 response cards shares only one attribute with thestimulus cards in order to eliminate ambiguity and anxiety[Following Nelson|s procedure\ several measures of per!formance were obtained] the number of categories achieved^the total number of errors made^ the number of correctresponses^ the number of perseverative errors^ the number ofnonperseverative errors^ and the percentage of errors that wereperseverative errors[ Finally\ the time taken to complete thistest was also recorded[

Paired associates[ Four lists of eight concrete paired associ!ates were constructed by selecting 53 words with imagery ratingsof 4[4 or more according to published norms ð32Ł[ Each list wasassigned at random to be presented in one of four experimentalconditions[ In the _rst condition "Baseline#\ the pairs were readaloud by the experimenter at a rate of one every 09 s\ and thenthe _rst word in each pair was read aloud at a rate of one every4 s[ The patients| task was to give the appropriate responsewithin the time interval allowed\ and the measure of per!formance was the total number of correctly recalled items[ Inthe second condition "Experimenter Generated#\ the patientswere given preliminary training in the construction of inter!active images\ and each pair was accompanied by a line drawingthat showed the relevant objects interacting in some way[ In thethird condition "Subject Generated#\ the patients wereinstructed to make up their own interactive images to enhancetheir recall performance[ Finally\ in the fourth condition "Fol!low!up#\ the pairs were read aloud without any explicit instruc!tions[

Shoppin` list[ Four di}erent lists of eight items were con!structed by selecting 21 generic products from a supermarket|slist[ Once again\ the four lists were randomly assigned to thefour experimental conditions[ They were read out by the exper!imenter at a rate of one item every 09 s\ and immediatelyafterwards the patients were tested by means of oral free recallwithout any time limit[ After the _rst condition "Baseline#\ theywere taught to use the Method of Loci as an imagery!basedmnemonic ð54Ł by referring to locations around their homes assuitable loci[ In the second condition "Experimenter Gener!ated#\ each of the items was accompanied by an appropriatelocation in the patients| own homes\ and the entire list oflocations was presented during the recall test[ In the third con!dition "Subject Generated#\ the subjects were instructed to usethe Method of Loci using their own choice of locations and touse these locations as retrieval cues during the recall test[Finally\ in the fourth condition "Follow!up#\ no explicit instruc!tions were given concerning the mnemonic strategy to beemployed[

Route learnin`[ In this test\ the patients| task was to remember

a journey described in the form of a simple narrative text[ Twodescriptive passages were constructed in the form of tour guidesdescribing the routes towards the centres of two _ctitious towns[Each passage described the sequence of _ve steps that had tobe taken to reach the destination[ In addition\ two maps of the_ctitious towns were drawn corresponding to the informationin the descriptive texts[ In the _rst condition\ patients werepresented with the appropriate map for 2 s to familiarize themwith the testing material[ The map was removed\ and the textwas read aloud by the experimenter[ Finally\ the map waspresented for a second time\ and the patients were instructed totrace the relevant route with a pencil[ Before the secondcondition\ the patients were instructed to use imagery in remem!bering geographical descriptions and directions[ The secondtext was then presented using the same procedure as before[

Results

Executive processing ef_ciency

The administration of the BVFT\ the CET\ and theMCST resulted in a total of 09 possible measures ofexecutive function[ The matrix of correlations betweenthese 09 measures is shown in Table 0[ The hypothesisthat these were measuring a single underlying constructwas assessed by means of two principal componentsanalyses[

The _rst was carried out on the seven measures of cardsorting from the MCST[ The plot of the eigenvaluesindicated a strong _rst component which explained69[5) of the total variance\ and a weaker second com!ponent which explained 08[7) of the total variance[ Theloadings on the latter showed that it mainly measurednonperseverative errors rather than other aspects of card!sorting behaviour\ and so it was ignored in the subsequentanalysis[ The _rst principal component showed high posi!tive loadings on the number of categories achieved andon the number of correct responses\ and moderate tohigh negative loadings on all the remaining measures[The patients| estimated scores on this principal com!ponent could therefore be interpreted as an overall mea!sure of their card!sorting ability[

These scores were used in the second principal com!ponents analysis\ along with the measure of verbal ~u!ency and the two scores from the CET[ The two latterscores "representing errors and latency\ respectively# werenot highly correlated with each other "r�¦9[16# andthus seem to re~ect di}erent aspects of cognitive esti!mation[ The plot of the eigenvalues indicated only astrong _rst component which explained 42[6) of thetotal variance[ It showed positive loadings on the mea!sures of verbal ~uency and card!sorting ability and nega!tive loadings on the number of errors and the total timeon the CET[ This outcome is consistent with the idea thatthe BVFT\ the CET\ and the MCST measure a singleunderlying construct\ and therefore the patients| esti!mated scores on this principal component were inter!preted as a measure of their executive processinge.ciency[

M[ Canellopoulou and J[ T[ E[ Richardson:Imagery mnemonics0073

Table 0[ Intercorrelations between measures of executive functioning

Variable 0 1 2 3 4 5 6 7 8 09

Benton Verbal Fluency Test0[ Verbal ~uency ¦0[99 −9[23 −9[28 ¦9[43 ¦9[42 −9[43 −9[34 −9[20 −9[35 −9[25

Cognitive Estimation Test1[ Error score −9[23 ¦0[99 ¦9[16 −9[22 −9[39 ¦9[26 ¦9[37 −9[96 ¦9[42 ¦9[052[ Completion time −9[28 ¦9[16 ¦0[99 −9[23 −9[17 ¦9[14 ¦9[23 −9[96 ¦9[23 ¦9[20

Modi_ed Card Sorting Test3[ Categories ¦9[43 −9[22 −9[23 ¦0[99 ¦9[82 −9[83 −9[66 −9[43 −9[63 −9[564[ Correct ¦9[42 −9[39 −9[17 ¦9[82 ¦0[99 −9[82 −9[74 −9[39 −9[68 −9[465[ Errors −9[43 ¦9[26 ¦9[14 −9[83 −9[82 ¦0[99 ¦9[73 ¦9[44 ¦9[65 ¦9[536[ Perseverative −9[34 ¦9[37 ¦9[23 −9[66 −9[74 −9[73 ¦0[99 ¦9[90 ¦9[81 ¦9[397[ Nonperseverative −9[20 −9[96 −9[96 −9[43 −9[39 ¦9[44 ¦9[90 ¦0[99 −9[91 ¦9[448[ ) Perseverative −9[35 ¦9[42 ¦9[23 −9[63 −9[68 ¦9[65 ¦9[81 −9[91 ¦0[99 ¦9[27

09[ Solution time −9[25 ¦9[05 ¦9[20 −9[56 −9[46 ¦9[53 ¦9[39 ¦9[44 ¦9[27 ¦0[99

Individual differences

Individual di}erences related to demographic andclinical variables were investigated in four areas]depression\ executive processing e.ciency\ imagerycontrol\ and imagery vividness[ Multiple regression tech!niques were used to try to predict these four dependentvariables based upon the independent variables of age\years of education\ disease type "chronicÐprogressive ver!sus relapsingÐremitting#\ disease activity "stable versusworsening#\ disease duration since the onset of the symp!toms\ and disease duration since the original diagnosis[

The predictive capacity of these independent variableswas assessed by means of a stepwise procedure[ In thistechnique\ an additive model is set up to predict thedependent variable from a subset of the independent vari!ables[ The analysis proceeds in an iterative fashion\ asfollows[ At each step\ the analysis considers\ for the vari!able in the model with the least predictive capacity\whether a signi_cant amount of variation would be addedto that unexplained if that variable were removed fromthe model^ if not\ then the variable is removed[ The analy!sis then considers\ for the variable not in the model withthe greatest predictive capacity\ whether a signi_cantamount of variation would be explained by adding it tothe model^ if so\ then the variable is inserted into themodel[ The analysis continues until no change in themodel is made at a given step[

The results were as follows]

"0# Depression according to the BDI was lower in pat!ients whose condition was judged to be stable"P³9[914#\ and it also tended to be inversely relatedto the number of years of education "P³9[95#[

"1# Executive processing e.ciency tended to be inverselyrelated to the duration of the disease since the originaldiagnosis "P³9[95#[

"2# Neither imagery control nor imagery vividness couldbe signi_cantly related to any of the demographic andclinical variables when the e}ects of the remainingvariables had been statistically controlled[

Finally\ depression was not signi_cantly related to execu!tive processing e.ciency "r�−9[00#[

Memory performance

Table 1 shows the mean percentage correct per!formance obtained by the MS patients on the four con!ditions of the paired!associates task and the shopping!list task and on the two conditions of the route!learningtask[ One!way analyses of variance showed that therewas signi_cant variation in recall performance across thedi}erent conditions in all three tasks "F�49[63^ df�2\036^ P³9[990^ F�12[19^ df�2\ 036^ P³9[990^ andF�3[58^ df�0\ 38^ P³9[94\ respectively#[ These e}ectswere explored by carrying out a posteriori tests on thedata from the paired!associates task and the shopping!list task comparing baseline performance with each ofthe three subsequent conditions[ On both tasks\ therewas a highly signi_cant increase in performance withexperimenter!generated imagery "F�097[04^ df�0\ 036^P³9[990^ and F�02[53^ df�0\ 036^ P³9[990\ respec!tively#[ With subject!generated imagery\ there was anincrease in performance on the paired!associates task"F�5[44^ df�0\ 036^ P³9[91# but a decrease in per!formance on the shopping!list task "F�8[18^ df�0\ 036^P³9[994#[ In the follow!up condition\ there was no sig!

Table 1[ Mean percentage correct performance in three memorytasks

Condition

0 1 2 3

Paired associates 24[64 56[14 32[49 23[14Shopping list 55[49 65[49 47[14 45[14Route learning 47[99 56[59

0 � baseline^ 1 � experimenter!generated imagery^ 2 � sub!ject!generated imagery^ 3 � follow!up[

M[ Canellopoulou and J[ T[ E[ Richardson:Imagery mnemonics 0074

ni_cant di}erence with the baseline on the paired!associ!ates task "F³0#\ but a decrease in performance on theshopping!list task "F�03[23^ df�0\ 036^ P³9[990#[

Stepwise regression techniques were used to determinethe predictive capacity of depression\ executive pro!cessing e.ciency\ imagery control\ imagery vividness\and the various demographic and clinical variables thatwere described earlier with regard to baseline per!formance in each memory task[ In the paired!associatestask\ the baseline performance tended to vary directlywith executive processing e.ciency "P³9[09#\ but notwith any of the other variables[ In the shopping!list task\the baseline performance was better in patients whosecondition was stable than in those whose condition wasworsening "P³9[91#\ it tended to vary inversely with age"P³9[09#\ and it tended to be better in patients with thechronicÐprogressive form of the disease than in thosewith the relapsingÐremitting form "P³9[96#[ In the route!learning task\ the baseline performance varied directlywith executive processing e.ciency "P³9[994#[

Similar analyses were carried out upon various changescores in the three tasks[ The _rst stet of change scoreswas obtained by subtracting the baseline performancefrom the performance with experimenter!generated ima!gery[ These scores measure the advantage gained frominstructions to use experimenter!generated imagery com!pared with standard instructions[ In the paired!associatestask\ this change score tended to vary directly with thenumber of years of education "P³9[09#\ but not withany of the other variables[ In the shopping!list task\ thischange score varied directly with executive processinge.ciency "P³9[90#\ it was higher in patients with therelapsingÐremitting form of MS than in those with thechronicÐprogressive form "P³9[914#\ and it was higherin patients whose condition was worsening than in thosewhose condition was stable "P³9[990#[ "It may be notedthat the patients whose condition was worsening beganfrom a lower baseline than the patients whose conditionwas stable and thus had more scope for improvement ina situation where the overall performance was relativelyhigh[# In the route!learning task\ this change score wasnot signi_cantly related to any of the independent vari!ables when the e}ects of the other variables were stat!istically controlled[

The second set of change scores was obtained by sub!tracting the baseline performance from the performancewith subject!generated imagery[ These scores measurethe advantage gained from instructions to use subject!generated imagery compared with standard instructions[In the paired!associates task\ this change score tended tovary inversely with the duration of the disease since theonset of symptoms "P³9[95#\ but not with any of theother variables[ In the shopping!list task\ this changescore was higher in patients whose condition was worsen!ing than in those whose condition was stable "P³9[91#[

The _nal set of change scores was obtained by sub!tracting the baseline performance from the follow!up per!formance[ These scores measure whether the bene_ts of

imagery mnemonics were maintained if the subjects wereno longer provided with explicit instructions[ In thepaired!associates task\ this change score was not sig!ni_cantly related to any of the independent variableswhen the e}ects of the other variables were statisticallycontrolled[ In the shopping!list task\ this change scorewas directly related to executive processing e.ciency"P³9[94#\ and it tended to be higher in patients whosecondition was worsening than in those whose conditionwas stable "P³9[96#[

Discussion

Previous research has demonstrated that instructionsand training in the use of imagery mnemonics often leadto improved retention in brain!damaged individuals[However\ they tend to bene_t more when images areprovided by the experimenter in the form of simple linedrawings or linking phrases than when images have to begenerated by the patients themselves[ This is in contrastto the pattern obtained in neurologically intact subjects\who tend to show at least as much bene_t from self!generated imagery as from experimenter!generatedimagery[ This suggests that brain!damaged patients haveproblems with the spontaneous use of mental imageryð38Ł[ This study has con_rmed these _ndings in a sampleof 49 patients with MS[

First\ these patients showed a clear improvementacross a range of memory tasks following the admin!istration of imagery instructions and the explicit pro!vision of appropriate mediators[ Second\ performancedeclined when the patients were asked to use imagery buthad to make up appropriate mediators by themselves[ Inthe paired!associates task\ which employed a proceduresimilar to that used in previous studies on imagerymediation in brain!damaged patients ð12\ 27Ł\ the useof experimenter!generated imagery was confounded withthe presentation of relevant line drawings\ and it is knownthat perceptual images stored within visual memory arefunctionally di}erent from images generated by themanipulation of separate components ð15Ł[ However\ thiswas not true in the free!recall task\ where mediators werenamed by the experimenter and the patients had to makeup appropriate images under both experimenter!gen!erated and subject!generated conditions[ Finally\ even thevery limited bene_t of subject!generated imagery failed tosurvive in a follow!up test when no explicit instructionswere given[

The e}ective deployment of imagery and other mne!monic techniques depends upon the individual|s met!acognitive skills\ and it was suggested earlier that theseshould depend both upon executive processing capacityand upon a}ective factors\ both of which are knownto be disturbed in MS ð36Ł[ In this investigation\ threedi}erent instruments were employed to measure execu!tive processing capacity\ based on verbal ~uency\ cog!nitive estimation\ and card sorting[ Performance

M[ Canellopoulou and J[ T[ E[ Richardson:Imagery mnemonics0075

measures that were derived from these three tasks wereshown by principal components analysis to measure asingle latent variable\ and this is consistent with thehypothesis of a unitary trait of executive processinge.ciency[

In the recall of paired associates and of geographicalroutes\ scores on executive processing e.ciency were notsigni_cantly correlated with the bene_t derived fromexperimenter!generated or self!generated imagery\ butthey did tend to be correlated with the baseline level ofperformance[ This _nding is not strictly relevant to thefocus of the present study\ but it does con_rm that execu!tive processes are involved in the encoding and retrievalof information in long!term memory ð0Ł[ Indeed\ evidencefrom brain imaging has implicated speci_c regions withinthe prefrontal cortex in the encoding and retrieval ofverbal paired associates ð46Ł[ In the free!recall task\ execu!tive processing e.ciency was signi_cantly correlated withthe bene_t derived from experimenter!generated imagery\but not with the absolute level of performance[ Althoughthis trait was not signi_cantly correlated with the bene_tderived from self!generated imagery in this task\ MS pat!ients with higher executive e.ciency showed less of adecline in performance in the follow!up test[ Never!theless\ the patients| depression scores on a widely!usedinventory were not related to the bene_t derived fromexperimenter!generated or self!generated imagery[

In short\ this study has shown that executive processingcapacity is related to the bene_t gained by MS patientsfrom imagery instructions in one of the three test pro!cedures employed[ This result was not obtained in theother two tests\ which indicates that the e}ect dependsupon the particular cognitive demands of the imagerymnemonic being employed[ In the free!recall test\ thepatients were asked to use the Method of Loci\ which isa relatively complex mnemonic device[ In normal indi!viduals\ it is known that the usefulness of this deviceis drastically reduced when the subjects are required toperform a concurrent tracking task\ implying that thetwo tasks rely on the same limited pool of processingresources\ whereas simpler devices are much less vul!nerable ð3Ł[ Similarly\ older people tend to show lessbene_t than younger people when asked to learn longlists of words using the Method of Loci or other complexdevices ð4\ 13\ 14Ł\ but they show more bene_t than youn!ger people when simply asked to construct linking imagesin remembering paired associates ð00\ 10\ 41Ł[

These results concur with the _ndings of the presentexperiment in showing that executive processing capacitydetermines the bene_t gained from the use of more com!plex forms of imagery mnemonic in verbal!learning tasks[There is\ in fact\ increasing evidence that the constructionand manipulation of complex mental images relies heav!ily upon general!purpose attentional or executiveresources ð20\ 24\ 33\ 42Ł\ and this notion has beenembodied in recent accounts of visuospatial workingmemory ð15\ 29Ł[ The evidence has come primarily fromthe use of dual!task procedures\ and this implies that the

relevant resources are involved in the coordination ofseparate activities[ This in turn seems to be important inpredicting gross disturbances of executive function indaily social activities ð2Ł[

Nevertheless\ several authors have proposed thatexecutive resources should be {{fractionated|| into a num!ber of functionally distinct components or subsystems ð0\1\ 44Ł[ Performance on classic {{frontal|| tests shows onlya modest correlation with that in dual!task procedures\and it may be unrelated to behavioural disturbances inpatients with frontal!lobe damage ð2\ 43Ł[ This suggeststhat the trait of executive processing e.ciency identi_edin the present investigation does not measure the capa!bility to construct mental images per se^ rather\ it re~ectsthe capability to make appropriate and e}ective use ofthese images in learning and remembering[ In short\ thepresent _ndings support the view that metacognitiondepends on the executive functions traditionally attri!buted to the frontal lobes[

References

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