Neuropsychologia 50 (2012) 2573–2576

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Neuropsychologia

0028-39

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n Corr

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journal homepage: www.elsevier.com/locate/neuropsychologia

Testing the activation–orientation account of spatial attentional asymmetriesusing transcranial direct current stimulation

A.M. Loftus a,n, M.E.R. Nicholls b

a School of Psychology and Speech Pathology, Curtin University, GPO Box U1987, Perth, WA 6845, Australiab School of Psychology, Flinders University, Australia

a r t i c l e i n f o

Article history:

Received 30 April 2012

Received in revised form

27 June 2012

Accepted 6 July 2012Available online 20 July 2012

Keywords:

Pseudoneglect

Transcranial direct current stimulation

Attention

Activation

Posterior parietal cortex

32/$ - see front matter & 2012 Elsevier Ltd. A

x.doi.org/10.1016/j.neuropsychologia.2012.07

esponding author. Tel.: þ61 8 9266 3036; fax

ail address: andrea.loftus@curtin.edu.au (A.M

a b s t r a c t

The general population shows an attentional bias to the left, known as pseudoneglect. This bias is

thought to be driven by higher levels of activation in right parietal areas. Using transcranial direct

current stimulation (tDCS) to manipulate activation, this study examined whether tDCS over the left

and right posterior parietal cortices (PPC) affects pseudoneglect. Normal participants received tDCS

over the left or right PPCs (15 in each group). Pseudoneglect was measured using the greyscales task,

which requires a forced-choice discrimination of luminance between two opposing luminance

gradients. The greyscales task was administered both before and after; (a) anodal (b) cathodal and

(c) sham tDCS. Participants who received tDCS over the left PPC demonstrated pseudoneglect for the

greyscales task, which was significantly reduced by anodal tDCS, but was unaffected by sham or

cathodal tDCS. In contrast, for those participants who received right PPC tDCS, pseudoneglect for the

greyscales task was unaffected by tDCS. Anodal tDCS, which is known to elevate neural excitation, may

have overcome lower levels of activation in the left PPC, resulting in decreased pseudoneglect. These

findings provide convincing evidence in support of an activation–orientation model of pseudoneglect

and have implications for models of left neglect.

& 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Where left neglect patients demonstrate a rightward atten-tional bias (Heilman, Watson, & Valenstein, 1993), neurologicallyintact adults demonstrate a leftward attentional bias, referred toas ‘pseudoneglect’ (Nicholls, Bradshaw, & Mattingley, 1999;Nicholls & Loftus, 2007). Although less extreme than clinicalneglect, Pseudoneglect is a reliable tendency to overestimatethe features on the left side of a stimulus relative to the rightand manifests on physical line bisection tasks (Jewell & McCourt,2000), judgements of luminance, size and numerosity (Nichollset al., 1999), and the mental representation of numbers (Loftus,Nicholls, Mattingley, & Bradshaw, 2008) and letters (Nicholls &Loftus, 2007).

Given the performance similarities (McCourt & Jewell, 1999)and the mutual involvement of the right posterior parietal cortex(PPC) (Foxe, McCourt, & Javitt, 2003; Waberski et al., 2008), manyaccounts of pseudoneglect mirror models of neglect. Theactivation–orientation account suggests that spatial attention isbiased in the direction opposite to the most activated hemisphere(Kinsbourne, 1970, 1987, 1993; Reuter-Lorenz, Kinsbourne, &

ll rights reserved.

.003

: þ61 8 9266 2464.

. Loftus).

Moscovitch, 1990). Neuroimaging studies demonstrate preferen-tial activation of the right PPC during visuospatial tasks (Fink,Marshall, Weiss, Toni, & Zilles, 2002; Foxe, McCourt, & Javitt,2003; Harris & Miniussi, 2003; Gobel, Calabria, Farne, & Rossetti,2006). In accord with an activation–orientation account, prefer-ential activation of the right PPC during visuospatial tasks leads toa leftward attentional bias, which increases the salience offeatures in the left hemispace. In the case of physical linebisection, the left side of the line is subsequently perceived tobe longer than the right, resulting in a leftward shift in theperceived midpoint of the line (Bultitude & Davies, 2006).

Siman-Tov et al. (2007) proposed a revised activation–orientation model, in which pseudoneglect and neglect areexplained in terms of asymmetric interhemispheric neural activa-tion and connectivity. Neuroimaging experiments in which parti-cipants completed visual attention tasks revealed a left visualfield advantage, which Siman-Tov et al. suggested originates fromtwo distinct neural mechanisms. First, that advantageous con-nectivity within the right hemisphere facilitates its dominance forspatial attention. Second, that connectivity between the twohemispheres favours the passage of information from the rightto the left hemisphere. This account therefore suggests that aright hemisphere advantage for spatial attention, coupled with astrong right-to-left interhemispheric transfer of information,leads to a leftward bias of spatial attention.

A.M. Loftus, M.E.R. Nicholls / Neuropsychologia 50 (2012) 2573–25762574

The present study tested the effect of unilateral hemisphericactivation proposed in the models by Kinsbourne (1970, 1987,1993) and Siman-Tov et al. (2007) using transcranial directcurrent stimulation (tDCS). This relatively recent technique is anon-invasive method of brain stimulation that can be used tomodulate cortical excitability. When applied to the skull, tDCSpenetrates the underlying cortex and increases (anodal) ordecreases (cathodal) cortical excitability in that area (Lang et al.,2011; Nitsche & Paulus, 2000; Zaghi, Acar, Hultgren, Boggio, &Fregni, 2010).

Studies using tDCS have shown altered visual detection inhealthy controls and left neglect patients (Sparing et al., 2009). Inhealthy controls, anodal tDCS over the PPC improved visualdetection in the contralateral visual field and cathodal tDCS overthe PPC improved performance in the ipsilateral visual field. Forleft neglect patients, anodal tDCS over the lesioned PPC andcathodal tDCS over the unlesioned PPC reduced left neglect on aphysical line bisection task. Based on these findings, the authorssuggested that tDCS modulates visual attention by reducinginterhemispheric imbalance.

In the present study, pseudoneglect was assessed using thegreyscales task, which requires a forced choice luminance judge-ment between two mirror-reversed luminance gradients. Neuro-logically intact participants typically select the stimulus that isdarker on the left (Nicholls et al., 1999; Okubo & Nicholls, 2006).The greyscales task is highly sensitive to spatial biases of atten-tion and allows us to examine the effects of tDCS on a purelyperceptual task. The techniques associated with tDCS are still adeveloping science (Jacobson, Koslowsky, & Lavidor, 2012) andexact predictions are difficult to make. It is possible that tDCS willhave symmetrical effects on the hemispheres. That is, cathodaland anodal stimulation will excite and inhibit (respectively)neural activity—resulting in systematic increases and decreasesin pseudoneglect for left and right tDCS. However, given thatactivation techniques, such as visual spatial cueing (Nicholls &Roberts, 2002; Sosa, Clarke, & McCourt, 2011) and prismaticadaptation (Loftus et al., 2008) often only affect left hemisphereactivity, we predicted that stimulation over the left PPC was morelikely to have an effect than stimulation over the right PPC. Inaddition, tDCS itself appears to have differential effects, withanodal (excitatory) stimulation more likely to yield an effect oncognitive tasks than cathodal (inhibitory) stimulation (Jaconsonet al., 2012). Therefore, if unilateral activation affects asymme-tries in attention, we predicted that increased excitation of theleft PPC (using anodal tDCS) should equalise hemispheric activa-tion and reduce pseudoneglect for the greyscales task.

2. Methods

2.1. Participants

Thirty neurologically intact, right-handed (in accord with the Edinburgh

Handedness Inventory, Oldfield, 1971) undergraduate students took part in the

study (6 males, mean age 26 years). All participants had normal or corrected to

normal vision. The study was approved by a University human ethics committee

and was performed in accord with the 1964 Declaration of Helsinki.

Fig. 1. Examples of the greyscales stimuli used in the present study. The experimenta

darker overall by stating out loud ‘top’ or ‘bottom’.

2.2. Materials and methods

Participants were randomly assigned to one of two stimulation groups: (i) left

PPC or (ii) right PPC (15 each group). All participants completed three (anodal,

cathodal, and sham) tDCS conditions over three different testing sessions sepa-

rated by 2 weeks. Participants were naıve to the tDCS condition for a given session.

Within each session, participants completed the following tasks in this order:

(i) pre-tDCS greyscales task, (ii) either anodal, cathodal or sham tDCS, and (iii)

post-tDCS greyscales task.

2.2.1. Greyscales task

For each trial, participants viewed two greyscales stimuli simultaneously on a

350 mm computer monitor. Each mm on the screen translates into a viewing

angle of .111 when viewed at a distance of 500 mm. Each greyscales stimulus was

defined by a thin black rectangle (.6 mm) and displayed against a white back-

ground. The stimuli were either 85 mm (short) or 102 mm (long) long and 18 mm

high. The horizontal midline of each stimulus pair was aligned with the centre of

the screen. The vertical midline of the upper and lower stimuli was positioned

30 mm above and below the centre of the screen, respectively. Each stimulus

changed in luminance from one end to the other by a linear adjustment of the

ratio of white to black pixels. The stimuli were mirror-reversals of one another

(see Fig. 1) and were equiluminant at a global level (for full details on the

construction of the greyscales stimuli, please see Nicholls et al., 1999). The

stimulus combinations of length (85 mm, 102 mm) and polarity (upper stimulus

dark on left, lower stimulus dark on right and vice versa) were repeated 12 times,

resulting in 48 trials in total. The combinations changed from trial to trial in a

pseudo-randomised order.

Participants were seated so that the centre of the monitor was located along

their midline at eye height. A chinrest was used to maintain head position at

500 mm from the computer monitor. Each stimulus pair was presented for 300 ms

to minimise eye movements. There was a 1.5 s inter-trial interval, during which

participants fixated a cross in the centre of the screen, which appeared at the end

of each trial, and signalled a new trial. Participants indicated which greyscales

stimulus they thought was darker overall by stating out loud ‘top’ or ‘bottom’

when the central fixation cross was presented. The experimenter recorded

participants’ verbal responses. All participants completed four practice trials prior

to commencing the greyscales task. The entire greyscales task lasted approxi-

mately 120 s.

2.2.2. Transcranial direct current stimulation (tDCS)

Immediately following the pre-tDCS greyscales task, each participant com-

menced the tDCS phase of the study. tDCS stimulation was delivered by a battery-

driven, constant current stimulator (Empis). A constant current of 1 mA was

applied for 20 min with a pair of 35 cm2 sponge electrodes soaked in saline

solution (current density¼ .03 mA/cm2, total charge¼34 C/cm2). There was a

ramp up/ramp down period of 30 s at the start and end of tDCS. For the left PPC

stimulation group, the first electrode was placed over P3 (in accord with the

international 10–20 system) and the reference electrode at Cz (to avoid current

flow through the frontal lobe). For the right PPC stimulation group, the first

electrode was placed over P4 and the reference electrode at Cz. The configurations

of electrodes were changed between sessions to deliver anodal or cathodal

stimulation. For the sham condition, tDCS was applied in the same way as anodal,

but the stimulator was turned off after 30 s. This ensured the sham condition felt

the same as the other tDCS conditions, but without continuing stimulation.

Participants watched a short movie clip during tDCS administration. Following

administration of tDCS, participants completed the post-tDCS greyscales task,

which was the same as the pre-tDCS task.

3. Results

All participants successfully completed the experiment andthere were no missing data. For each trial, participants selectedthe stimulus that was darker on the left or the right, classified as‘left’ and ‘right’ responses, respectively. A measure of bias for the

l task required participants to select which greyscales stimulus they thought was

A.M. Loftus, M.E.R. Nicholls / Neuropsychologia 50 (2012) 2573–2576 2575

greyscales task was calculated by subtracting the number of leftresponses from the number of right responses and converting theresult into a percentage. Bias scores could therefore range from�100% to þ100% with negative and positive scores indicating apreference for selecting the stimulus that was dark on the left orright, respectively.

The data were analysed using an analysis of variance (ANOVA),with group (left, right) as a between subjects factor and tDCScondition (sham, anodal, cathodal) and time (pre, post) as withinsubjects factors. For all analyses, effect sizes are expressed aspartial Z2 squared values. There was no reliable effect of group,F (1, 28)¼ .96, p4 .05, or tDCS condition, F (2, 56)¼2.04, p4 .05.There was an effect of time, F (1, 28)¼17.37, po .05, Zp2

¼ .38,which interacted reliably with group, F (1, 28)¼5.84, po .05,Zp2¼ .17, and tDCS condition, F (1, 56)¼4.39, po .05, Zp2

¼ .14.There was a significant three-way interaction between group,tDCS condition and time, F (2, 56)¼4.89, po .05, Zp2

¼ .15.To analyse the interactions in the omnibus ANOVA more

thoroughly, the data for each group (left PPC stimulation, rightPPC stimulation) were analysed separately using repeated mea-sures ANOVA with tDCS condition (anodal, cathodal, sham) andtime (pre, post) as within subjects factors. To determine whetherthe bias was different from zero (no bias), a series of Bonferroni

-100

-80

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20

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pons

e B

ias

Pre tDCSPost tDCS

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***

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pons

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ias

Pre tDCSPost tDCS

*** ** *

ShamZero biasAnodal Cathodal

ShamZero biasAnodal Cathodal

Fig. 2. Mean bias (with SE bars) for the pre and post greyscales tasks across the

three tDCS conditions (sham, anodal, and cathodal) for the (a) left and (b) right

tCDS groups. Negative and positive bias scores indicate selection of the greyscales

that was darker on the left and right, respectively. Results of one-sample t-tests

comparing the means with zero are shown for each of the six conditions within

each graph. Significant differences are indicated by an asterisk (po .05).

adjusted paired samples t-tests (zero paired with actual bias) foreach tDCS condition was used.

For the left PPC stimulation group, while there was no effect oftDCS condition, F (2, 28)¼3.41, p4 .05, there was an effect oftime, F (1, 14)¼27.46, po .05, Zp2

¼ .66, qualified by a tDCScondition� time interaction, F (2, 28)¼8.77, po .05, Zp2

¼ .39.Post-hoc tests revealed a significant effect of time for the anodalcondition, t (14)¼�5.13, po .05, but not for the sham, t

(14)¼� .89, p4 .05, or cathodal, t (14)¼� .92, p4 .05, conditions.A significant leftward bias was found for all conditions (allpso .05) except the anodal post-tDCS condition, as can be seenin Fig. 2a.

For the right PPC stimulation group, Mauchly’s test indicated aviolation of sphericity so Huynh–Feldt corrections were applied.There was no reliable main effect of tDCS condition, F (1.02,14.26)¼ .19, p4 .05, or time, F (1, 14)¼1.27, p4 .05, and noreliable tDCS� time interaction, F (1, 14)¼ .008, p4 .05. Theresults of one-sample t-tests demonstrated a significant leftwardbias for all six (time� tDCS) conditions (all pso .05), as can beseen in Fig. 2b.

4. Discussion

Participants demonstrated pseudoneglect for the greyscalestask by selecting the greyscale that was darker on the left as beingdarker overall, regardless of group inclusion. This leftward bias isconsistent with other studies using the greyscales task (Nichollset al., 1999; Okubo & Nicholls, 2006). Pseudoneglect was sig-nificantly reduced following 20 min of anodal tDCS over the leftPPC, but remained unaffected by cathodal or sham tDCS over theleft PPC. This leftward bias was also unaffected by sham, anodaland cathodal tDCS over the right PPC.

It is proposed that exciting the left PPC with anodal stimulationrebalanced an activation asymmetry between the left and rightPPCs. In line with activation–orientation accounts (Kinsbourne,1970, 1987, 1993; Reuter-Lorenz et al., 1990; Siman-Tov et al.,2007), equalizing hemispheric activation reduces over-activationin the right hemisphere, thus alleviating pseudoneglect for thegreyscales task. Also in line with this interpretation, cathodal tDCSover the left PPC does not impact upon pseudoneglect becauseattention was already asymmetrically distributed to the left hemi-space. Although the present study did not seek to identify thespecific impact of tDCS in terms of the two distinct neuralmechanisms proposed by Siman-Tov et al., it is apparent thatsimply increasing excitability of the left PPC significantly reducedpseudoneglect in the present study.

The present finding that anodal tDCS over the left PPC reducedpseudoneglect for the greyscales task is analogous to the impactof left-shifting prisms on pseudoneglect for the greyscales (Loftus,Vijakumar & Nicholls, 2009). We previously suggested thatadaptation to left-shifting prisms shifted spatial attention fromthe left to the right hemispace, alleviating pseudoneglect for thegreyscales. In light of the present findings, the effect of left-shifting prisms may be better explained in terms of an activation–orientation account. Adaptation to left and right shifts in thevisual field is associated with increased activation of the leftintraparietal sulcus and left posterior cerebellum (Chapman et al.,2010; Clower et al., 1996; Danckert et al., 2008). It may be thatthe increased left hemispheric activation that is observed duringprism adaptation acts in the same way as anodal stimulation ofthe left PPC to rebalance asymmetric hemispheric activation andalleviate pseudoneglect.

The lack of an effect of cathodal tDCS in the present studyis surprising, since it is reasonable to suggest that decreasingexcitability of the right PPC would also rebalance asymmetric

A.M. Loftus, M.E.R. Nicholls / Neuropsychologia 50 (2012) 2573–25762576

hemispheric activation. The lack of a cathodal effect in the presentstudy is in contrast to the finding of a cathodal effect on perfor-mance of a visual detection task in healthy controls (Sparing et al.,2009), but is consistent with the finding that cathodal tDCS is lesslikely to impact upon behaviour in cognitive tasks (see Jacobsonet al., 2012, for a review). As one reviewer kindly noted, the issue isprobably much more complex than a simple lack of an effect ofcathodal tDCS. It is increasingly apparent that anodal/cathodal tDCSdoes not systematically improve or impair task performance (Vallar& Bolognini, 2011). Vallar and Bolognini (2011) suggest that amultitude of factors contribute to the effect of tDCS on taskperformance, such as the duration, localisation and intensity of thestimulation and the complexity of the task. This, coupled with thefact that increasing numbers of tDCS studies involve only anodalstimulation and not cathodal, makes it difficult to delineate anysystematic impact of anodal–cathodal tDCS on behaviour.

Another, somewhat speculatory, contributing factor is thattDCS may be dependent on initial activation level at the site ofstimulation. In terms of the present study, anodal tDCS of the leftPPC was sufficient to increase neuronal firing at an alreadyactivated area. In contrast, cathodal stimulation may not havebeen sufficient to inhibit an already highly activated right PPC.Hence, cathodal stimulation of the right PPC did not rebalance theactivation asymmetry between the hemispheres and, as a result,had no impact on pseudoneglect for the greyscales task.

The present study provides convincing evidence in support ofan activation–orientation account of pseudoneglect and hasimplications for the development of tDCS as a therapeutic toolfor left neglect. If pseudoneglect and neglect are indeed ‘‘twinmanifestations’’ of a common set of cognitive and neuronalmechanisms (McCourt & Jewell, 1999, p. 853), the present find-ings suggest that different pseudoneglect/neglect behaviours willbe differentially affected by anodal or cathodal tDCS. Tasks with agreater cognitive component may not respond to cathodal tDCSintervention alone, but may be more responsive to dual anodal–cathodal stimulation or multi-site stimulation.

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