Neuropsychologia 43 (2005) 1721–1728

Rightward shift in spatial awareness with declining alertness

Tom Manlya,∗, Veronika B. Doblerb, Christopher M. Doddsa, Melanie A. Georgea

a UK Medical Research Council Cognition and Brain Sciences Unit, Box 58 Addenbrooke’s Hospital, Cambridge CB2 2QQ, UKb Department of Experimental Psychology, University of Cambridge, Cambridge, UK

Received 15 September 2004; received in revised form 10 February 2005; accepted 16 February 2005Available online 24 March 2005

Abstract

Although transient neglect of contralesional space occurs following damage to either hemisphere, persistent forms are overwhelminglyassociated with right hemisphere lesions. This has led to the suggestion that impairments in other right hemisphere systems—in particularthose that mediate alertness—may undermine recovery. Reductions in neglect severity with stimulation, exacerbation with sedatives and thepoor performance of chronic neglect patients on sustained attention tasks are consistent with this view. However, the question of whetherchanges in alertness exert a specific influence over spatial attention—or simply improve performance across many domains—is difficult toa ain spatialt bias wouldd of the lefta with sleepd effect. Ther tionsf©

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ddress using only patient studies. Here, we examine this question with individuals from the healthy adult population. On certasks, adults show a modest but reliable leftward attentional bias. On the basis of the neglect studies, we hypothesised that thisiminish—or even reverse—as alertness levels declined. In the first study, participants were asked to judge the relative lengthsnd right sections of a line when sleep deprived and when well rested. A significant rightward shift in attention was associatedeprivation. A rightward shift was also observed over the course of the session. The second study replicated this time-on-taskesults suggest that a diminution in alertness may besufficient to induce a rightward shift in visual attention in the healthy brain. Implicaor the persistence of neglect in patients, for spatial biases in children and for normal free viewing asymmetries are discussed.

2005 Elsevier Ltd. All rights reserved.

eywords: Spatial attention; Spatial neglect; Pseudoneglect; Alertness; Sustained attention

. Introduction

Unilateral neglect refers to a set of difficulties in detectingr acting on information appearing in contralesional space

hat cannot be fully explained by basic sensory loss. It is aemarkably common consequence of cerebrovascular acci-ent (CVA). In one study, aspects of this spatial bias wereeported in 82% of patients with right hemisphere (RH) le-ions and 65% of patients with left hemisphere (LH) lesions,hen tested within 3 days of the CVA (Stone, Halligan, &reenwood, 1993).Recovery or adaptive compensation fortunately occurs

ather rapidly for many patients. However, there is a strongsymmetry in these improvements such that almost all pa-

ients with chronic forms of the disorder have RH lesions andeglect left space. For example, the proportion of RH patients

∗ Corresponding author. Tel.: +44 1223 355 294; fax: +44 1223 516 630.E-mail address: tom.manly@mrc-cbu.cam.ac.uk (T. Manly).

who showed neglect on a cancellation test at 3 monthsstroke was reported in one study as 33%. Of the 44% opatients who had shown a spatial bias on this measurmediately after stroke, none showed this pattern by 3 mo(Stone et al., 1991).

Perhaps the most commonly argued view in accounfor the asymmetry in the incidence of neglect is that itflects RH dominance for certain spatial functions (Heilman,Valenstein, & Watson, 2000). Other authors, noting that nglect quite commonly occurs in acute LH cases, have plmore emphasis on other characteristics of the RH, whicdamaged, may impede recovery (Posner, 1993; Robertson &Manly, 1999; Samuelsson, Hjelmquist, Jensen, EkholmBlomstrand, 1998). The prime suspects in this respect acluster of abilities variously termed alerting, arousal, vlance or sustained attention.

The link between difficulties in maintaining an alstate—particularly in the absence of strong extestimulation—and the RH first emerged in neuropsycholog

028-3932/$ – see front matter © 2005 Elsevier Ltd. All rights reserved.oi:10.1016/j.neuropsychologia.2005.02.009

1722 T. Manly et al. / Neuropsychologia 43 (2005) 1721–1728

studies. RH patients were noted, for example, to have partic-ular difficulty in maintaining a readiness to respond acrosslong intervals between targets in reaction time studies (Boller,Howes, & Patten, 1970; Howes & Boller, 1975) or in main-taining a count of stimuli if these were presented at a slow,tedious rate (Wilkins, Shallice, & McCarthy, 1987). Subse-quent human functional imaging has broadly supported thisassociation. Increased activation associated with slow count-ing tasks was reported primarily within right dorsolateral pre-frontal and right parietal cortices (Lewin et al., 1996; Pardo,Fox, & Raichle, 1991). More recently, it has been suggestedthat general wakefulness is mediated by a predominantly rightlateralised network involving frontal, parietal, thalamic andbrain stem regions (Sturm et al., 2004, although seeCoull,Nobre, & Frith, 2001andThiel, Zilles, & Fink, 2004for di-verging results). Such findings are particularly intriguing interms of our current hypothesis, as we will discuss further,because they show considerable overlap with regions com-monly implicated in the aetiology of spatial neglect (Husain& Rorden, 2003).

In linking deficits in the maintenance of an alert state topersistence in neglect, the clinical observation that chronicneglect patients often appear under aroused and to have dif-ficulty in maintaining attention to even non-spatial tasks hasreceived experimental support.Robertson et al. (1997)askeda large group of RH hemisphere patients to perform the slowta , thism ntin-u asks.S tsw inr ucha al at-t tom allya atialf raryc in thee n,a uda evenr liseds eref eakep d int ucedb ousal,i alsobS tedt

spa-t wards lt ne-g er-

activity disorder (ADHD) diagnosis (Dobler, Manly, Verity,Woolrych, & Robertson, 2003; Manly, Robertson, & Verity,1997; Nigg, Swanson, & Hinshaw, 1997; Sheppard, Brad-shaw, Mattingley, & Lee, 1999; Voeller & Heilman, 1988). Anumber of studies have linked the condition to differences inright hemisphere structure and function (Casey et al., 1996;Castellanos et al., 1996; Lou, Henriksen, Bruhn, Borner, &Nielsen, 1989; Pueyo et al., 2000; Rubia et al., 1999) and tochronically reduced alertness (Antrop, Roeyers, Van Oost, &Buysee, 2000; Brown & Modestino, 2000). Where childrenwith ADHD have shown an unusual rightward bias in visualattention, a reduction in this bias has been reported—as withadult patients—with stimulant medication (Sheppard et al.,1999) and alerting tones (Dobler et al., 2003).

There are, therefore, reasonable grounds to support an ef-fect on changes in alertness levels with shifts in spatial at-tention in adult patients with right hemisphere lesions andat least some children with ADHD. A substantial questionmark remains, however, over whether this reflects a directinfluence on spatial awareness or whether spatial awarenessis simply one of the potential beneficiaries of a more gen-eral effect. This is difficult to assess purely using clinicalstudies in which theinitial level of spatial performance is it-self deficient. Here, we examine whether changes in alertnessare accompanied by a rightward shift in visual attention inhealthy adult participants.

of 9a t butv sks.T beste thiss ontall be.M oft sid-e lthyp er,s mildi thisn curea (leftt ,BR ing,t ofd tiva-t rdsc rp he acto tiallya tten-t s, &C ichh ence( ,r bias

one counting measure previously described byWilkins etl. (1987). Despite the absence of any spatial demandseasure proved a better predictor of which patients coed to show left neglect than some widely used spatial tamuelsson et al. (1998)found that chronic neglect patienere similarly compromised in bridging long intervals

eaction time tasks. A potential confound in attributing sssociations to a direct effect of poor alertness on spati

ention is the probability that large MCA lesions will leadore persistent spatial difficulties and disrupt anatomicdjacent functions. In this respect, modulation of sp

unction by manipulations designed to produce tempohanges in alertness forms a stronger test. Here agavidence is relatively strong.Robertson, Mattingley, Rordend Driver (1998)reported that the presentation of a lond somewhat unexpected tone significantly reduced oreversed left neglect on subsequent trials of a lateratimuli temporal order judgement task. As the benefits wound even when the tones were presented via a loudsplaced to the patient’s right, the effect was interprete

erms of generalised phasic increases in arousal indy the tone. Pharmacological agents that increase ar

ncluding bromocriptine and methylphenidate, haveeen shown to temporarily reduce neglect severity (Hurford,tringer, & Jann, 1998), whilst sedation has been repor

o exacerbate the rightward bias (Lazar et al., 2002).The hypothesis that deficient alertness is linked to

ial bias has also been used in accounting for the rightpatial bias—occasionally as severe as that seen in adulect patients—in children with the attention deficit hyp

r

It has been noted that children over about the agend young to middle-aged adults show a very modesery reliable leftward spatial bias in a number of tahis phenomenon, termed pseudoneglect, is perhapsxplained with reference to the line bisection task. Inimple measure, participants are asked to look at a horizine and judge/mark where they perceive its center to

any patients with left neglect will typically ignore muchhe left extent of the line and therefore bisect the line conrably to the right of its actual midpoint. In contrast, heaarticipants will often bisect slightly to the left of centuggesting an overestimation of the leftward extent/nattention to the right extent of the line. The reasons forormal leftward bias in attention remain somewhat obsnd findings relating the effect to reading scanning

o right versus right to left) have proved mixed (Chokronartolomeo, Perenin, Helft, & Imbert, 1998; Nicholls &oberts, 2002). It has been suggested, in terms of orient

hat the two cerebral hemispheres co-exist in a formynamic competition whereby relative increases in ac

ion in one hemisphere will tend to bias attention towaontralateral space (Kinsbourne, 1970). In accounting foseudoneglect, it has therefore been suggested that tf performing a spatial task, in as much as this preferenctivates the right hemisphere, is sufficient to move a

ion leftwards (McCourt, Freeman, Tahmahkera-Stevenhaussee, 2001). Within this view other factors such as whand is used to perform the task exert an additional influFailla, Sheppard, & Bradshaw, 2003). A further possibilityelated to our current hypothesis, is that the modest left

T. Manly et al. / Neuropsychologia 43 (2005) 1721–1728 1723

Fig. 1. The effects of spatial bias on line bisection judgements. A bias tothe right (seen dramatically in patients with spatial neglect) exaggerates therightward extent of the line and minimises the left—leading to the illusionthat a central bisecting mark is to the left. The reverse occurs with a leftwardbias.

may stem from an interaction between networks mediatingvisual attention and those mediating alertness.Bellgrove,Dockree, Aimola, and Robertson (2004)have recentlydemonstrated that pseudoneglect was significantly reducedin participants from the normal population who performedpoorly on a (non-spatial) sustained attention task. Such cross-sectional studies, whilst instructive, nevertheless have theinherent potential confound that other factors, such as a moregeneralised right hemisphere inefficiency, may underpin theresults. The clearer test lies in whetherwithin-subject changesin spatial bias with declining alertness can be observed.

In Study 1, we examined the performance of healthyadult participants on a computerised task that is sensitive topseudoneglect (McCourt et al., 2001). In the Landmark task(Milner, Brechmann, Roberts, & Forster, 1993), participantsare asked to view a horizontal line with a vertical divider atsome point along its length. Their task is to judge whether thesection to the left or right of the divider is the longer. In keep-ing with their performance on line bisection tasks, patientswith left spatial neglect have a strong tendency to underesti-mate the leftward extent of such lines and hence require thedivider to be considerably to the right before judging the leftsection to be longer. In contrast, healthy individuals have atendency to slightly underestimate the length of the rightwardsegment of the line and therefore generally require the dividerto be somewhat to the left of center before seeing the rights

f thist carep undt f ourt eriod

without sleep, immediately before they were due to go tobed. The other was conducted at a time when they would bewell rested. Our hypothesis was that the participants wouldshow the normal subtle leftward bias in attention when feel-ing alert but that this would be significantly reduced or evenreversed during the sleep deprived session. Long periods oftime spent performing the same task are also associated withreductions in various indices of alertness (e.g.Paus et al.,1997). In addition to examining a main effect of sleep depri-vation we also, therefore, examined whether any changes inspatial bias were apparent over the course of each session.

2. Study 1

2.1. Methods

2.1.1. ParticipantsWritten, informed consent was obtained from 10 volun-

teers (6 men and 4 women; mean age 38.4 years; S.D. 10.1).Participants with any history of head injury, other neurologi-cal or psychiatric illness were not seen. The volunteers wererecruited by advertisement within Addenbrooke’s Hospitalfor staff who, due to shift-working patterns, would be avail-able for testing after a period of relative sleep deprivation (forexample, working a double shift that included overnight ac-t akep taff,n

22 hv inb f a2b lock,t nterb ialt t andr thes ked tor o in-d ight)o d ofra pat-t f thep

2the

p axi-m lertc rtici-p t thatt es of

ection as the longer (seeFig. 1).We asked the participants to perform repeated trials o

ask at two sessions. The volunteers were largely healthrofessionals who, by dint of shift patterns, regularly fo

hemselves in a state of relative sleep deprivation. One oesting sessions was conducted at the end of such a p

ivity) as well as in a rested state (for details of hours awrior to testing see below). Volunteers included security sursing assistants and nurses.

.1.2. Materials

.1.2.1. Landmark test. Horizontal lines (200 mm) witertical dividers (10 mm× 0.2 mm) were presentedlack against a white background at the center o15 mm× 288 mm screen (seeMilner et al., 1993). Fivelocks of 42 trials were presented sequentially. In each b

he dividers deviated to the left and right of objective cey 5 mm (5% of trials), 3 mm (24%) or 1.5 mm (71%). Tr

ype was selected at random and equal numbers of lefight deviating trials were presented. Lines remained oncreen until a response was given. Participants were aseport which end of the line appeared the shorter and ticate their response by pressing one of two keys (left/rn a standard keyboard. To control for any effect of hanesponse (see, for example,Dobler et al., 2001), blocks werelternated in terms of the responding hand. A masking

ern was used between trials to disrupt visual retention orevious line for comparison.

.1.3. ProcedureSleepy condition session times were arranged with

articipants to coincide with their anticipated period of mum (relative) sleep deprivation over the coming week. A

ondition sessions were scheduled to occur when paants would feel rested and alert. Shift patterns mean

he alert and sleepy sessions occurred at different tim

1724 T. Manly et al. / Neuropsychologia 43 (2005) 1721–1728

day for different participants (alert sessions varied from 8.15a.m. to 8.00 p.m. and sleepy sessions from 6.45 a.m. to 3.30p.m.). In each session, before being administered the Land-mark test, participants completed a brief questionnaire onthe number of hours they had been awake prior to the currenttest and the number of hours sleep they had had in the last24 h. They also completed a subjective measure, the Stand-ford Sleepiness Scale (Hoddes, Zarcone, Smythe, Phillips,& Dement, 1973)—in which a rating of zero reflected feel-ing ‘very awake’ and a score of seven reflected feeling ‘verysleepy’—at the beginning and the end of the testing session.Testing took place in a quiet, slightly darkened room. Halfof the participants completed their alert session first, half thesleepy session first. Half of the participants began using theirright hands to make responses and half began using their lefthands. Re-testing was completed within a week. Session du-ration was approximately 40 min. The study was approvedby the Local Research Ethics Committee.

2.2. Results

2.2.1. Subjective sleepiness and sleep deprivationIn the alert condition, the mean number of hours awake

at time of testing was 6.25 (S.D. 3.43) and the mean num-ber of hours sleep in the previous 24 period was 7.0 (S.D.1.67). The Stanford Sleepiness score at start of session wasa meano thef aleda nessot ouslya eanh .76).M (S.D.1 ses-s rse oft ,P thep t theo pys ntp

2s

be-i eree torso on inw ori-z ed tom effecto tes( geo -

als and 27.2 errors (S.D. 10.04) on rightward deviating trials.This can be interpreted in line with existing literature on nor-mal leftward attention bias.

Although error rates in the sleepy session (mean = 40.5;S.D. 24.99) were slightly higher than in the alert session(mean = 32.9; S.D. 16.62), the effect was not statisticallysignificant (F(1,9) = 0.639,P = 0.45). Crucially, there wasa significant interaction between session and directionof bisection deviation (F(1,9) = 15.25, P = 0.004). Whencompared with the alert condition, error rates on left-bisectedtrials were significantly reduced in the sleepy condition(P = 0.01). There was a corresponding trend for increasederrors on rightward deviated trials (P = 0.05). The pattern istherefore consistent with a loss of leftward bias/a rightwardshift in attention associated with relative sleep deprivationand subjective sleepiness.

There was no significant main effect of the hand usedto make the response (F(1,9) = 2.06),P = 0.18), no signif-icant interaction between hand of response and condition(F(1,9) = 0.017,P = 0.90), no interaction between hand ofresponse and the deviation of the bisections (F(1,9) = 0.92,P = 0.36).

2.2.3. Landmark task performance—within-sessionchanges2.2.3.1. Alert session. Previous research (Dobler et al.,2 and-m t ofo in or-db ando nd 5( Errorr ses-s ime( nd-m fecto c-t ea ant(c

TM ing thes 1)

B

s

135

135

mean of 1.91 (S.D. 0.83), and at end of session af 2.55 (S.D. 0.69). A repeated measures ANOVA with

actor of time (beginning versus end of session) revestatistically significant increase in subjective sleepi

ver the course of the alert session (F(1,9) =P = 0.01). Inhe sleepy session, the mean number of hours continuwake prior to testing was 18.32 (S.D. 4.65). The mours of sleep within the previous 24 was 3.55 (S.D. 2ean sleepiness score at start of session was 4.18.25), changing to 5.91 (S.D. 0.70) by the end of theion. The increase in subjective sleepiness over the couhe session was again statistically significant (F(1,9) = 14.79= 0.003). In terms of differences between conditions,articipants reported being significantly more sleepy autset (F(1,9) = 35.11,P < 0.001) and at the end of the sleeession (F(1,9) = 190.14,P < 0.001) than in the equivaleoints within the alert session.

.2.2. Landmark task performance—sleepy versus alertessions

Errors (reporting a leftward deviating bisecting line asng to the right or vice versa) in the Landmark test wxamined in a repeated measures ANOVA with the facf session (alert versus sleep deprived) and the directihich the dividing line deviated from the center of the hontal line (left versus right) and the hand participants usake a response. As expected, there was a significantf the direction of deviation of the bisecting line on error raF(1,9) = 6.59,P = 0.03) with participants making an averaf 46.2 errors (S.D. 24.9) onleftward deviating bisection tri

001) has shown an effect of hand movements on the lark task. Although, this was effectively controlled ouur between conditions comparison described above,er to examine possible changes in the pattern of errorsacrosslocks of performance within a single session (in which hf response was alternated) data from blocks 1, 3 ain which the same hand was used) were analysed.ates in these blocks were first examined for the alertion in a repeated measures ANOVA with the factor of tblocks 1, 3 and 5) and the direction of deviation of the Laark (left versus right). There was a significant main eff time (F(2,18) = 3.99,P = 0.037) and of deviation dire

ion (F(1,9) = 9.99,P = 0.012). The interaction between timnd direction of deviation was also statistically significF(2,18) = 5.61,P = 0.013). As can be seen inTable 1, ac-uracy of judgements of lines bisected to theleft remains

able 1ean error rates and standard deviations (in parenthesis) in blocks us

ame hand of response across the alert and sleepy conditions (Study

lock Alert session

Errors on leftwarddeviating Landmark trials

Errors on rightwarddeviating Landmark trial

5.9 (4.56) 1.9 (1.66)5.5 (3.54) 4.9 (3.32)6.9 (5.38) 6.3 (4.83)

Sleepy session5.10 (6.40) 3.80 (3.19)6.10 (6.56) 5.70 (5.19)6.90 (6.16) 7.60 (4.86)

T. Manly et al. / Neuropsychologia 43 (2005) 1721–1728 1725

reasonably constant throughout the alert session. The inter-action is driven by participants initially making very few er-rors onrightward deviating trials (the pseudoneglect effect).Corrected pairwise comparisons show that this accuracy ad-vantage on right deviating trials shows a trend to reductionby block 3 (P = 0.07) and a significant reduction by block5 (P = 0.04). Blocks 3 and 5 did not significantly differ oncorrected comparison (P = 0.21).

2.2.3.2. Sleepy session. This analysis was repeated for thesleepy session. Here, there was a trend towards an increasein error rates over the course of the session (F(2,18) = 3.43,P = 0.055) but no significant main effect of the direction ofdeviation of the bisecting mark (F(1,9) = 0.088,P = 0.77)—inother words, and as expected from the between conditionanalysis, the normal pseudoneglect effect was absent. Therewas no hint of a significant interaction between time anddirection of deviation in the sleepy condition (F(2,18) = 0.67,P = 0.52).

In summary, participants showed the expected leftwardbias in the alert condition that was absent from the sleepycondition. Over the course of the alert condition, the normalleftward bias was strongest at the outset and had diminishedsignificantly by block 5. In the sleepy condition, there wasno evidence of an initial leftward bias and no significant shiftin performance in this respect—although error rates showeda

e ofS

3

oth-e ns ina dultv thel ap-p es-s mento hent r thes othe-s s butr initialp ablya halfo Thea ssionm eryt e tof easei ent isd e-o Thea task

shift in a different group of healthy participants. In addition,to increase the potential sensitivity of the Landmark measurewe now included trials (50%) in which the presented lineswere bisectedexactly. By continuing to ask participants tojudge whether the left or right extent appeared longer (i.e.not providing the response option of “equal”), very subtlebiases in attention may be assessed.

3.1. Methods

3.1.1. ParticipantsSix women and four men, recruited from the MRC Cog-

nition and Brain Sciences Volunteer Panel, gave written in-formed consent to take part (mean age 21.67 years; S.D. 4.30).Participants with any history of head injury, other neurologi-cal or psychiatric illness were not seen. One participant pro-duced responses considerably outside of the normal range(>3 S.D.) and was excluded from further analysis.

3.1.2. Materials and procedures3.1.2.1. Landmark test. Horizontal lines (200 mm) with avertical divider were presented on a computer monitor as de-scribed in Study 1. On 50% of trials, the vertical divider wasexactly in the center of the horizontal line. On the remain-ing 50% of trials, the divider was 5 mm to the left or rightof center. These clearly deviated trials were included to dis-c Trialo atedw dis-c 00 msf re-s ger?”a rialsw singt antsh . Thet re-s eingc r in-t ance,i antsm e re-s

3rmal

w . Att leteda at alls

3

3e in

s value

tendency to increase with time overall.The results from Study 1 will be discussed with thos

tudy 2 at the end of the paper.

. Study 2

The results from Study 1 are consistent with the hypsis that sleep deprivation—and by extension reductiolertness—is associated with a rightward shift in normal aisual attention/loss of the normal ‘pseudoneglect’ onandmark task. The results of the time-on-task analysisear to be consistent with this view. Initially, in the alert sion, participants showed a tendency to view the left segf the line as longer, as indexed by a low rate of errors w

his was actually the case. This tendency diminished oveession. It is possible that, although operating in the hypised direction, this does not reflect a change in alertnesather some erratic response tendency associated witherformance of a novel measure. The effect was noticebsent, however, from the sleepy condition which, forf the participants, was their first experience of the task.bsence of this time-on-task effect from the sleepy seay reflect some kind of floor effect in alertness in this v

ired group of participants (some of whom were very closalling asleep). However, as there was a significant incrn error rates generally across this session, this argumifficult to fully sustain and raises the possibility that the timn-task effects observed in Study 1 may not be reliable.im of Study 2 was therefore to replicate this time-on-

ourage guessing and were excluded from our analysis.rder was random. Line judgement trials were interpolith trials from a temporal order judgement task to beussed elsewhere. Lines remained on the screen for 10ollowed by a mask with the next trial beginning once aponse had been made. Two questions, “which end is lonnd “which end is shorter?”, were used in alternating tith responses being given via a key press (left/right) u

wo buttons on a standard computer keyboard. Participad as much time to make a response as they required

ask was stopped after 60 min, which, given variation inponse times, led to slightly different numbers of trials bompleted by different participants (see below). As ouerest was purely on within-session changes in performn order to maintain consistency across all trials particip

ade their responses (one of two keys representing thponse left or right) using their right hand.

.1.3. ProcedureSessions took place at various times within the no

orking day. No participant reported sleep deprivationhe outset and end of the session, the participants compsingle 100 mm visual analogue sleepiness scale (‘not

leepy’ (0 mm) to ‘very sleepy’ (100 mm)).

.2. Results

.2.1. Subjective alertnessSleepiness ratings confirmed a subjective increas

leepiness over the hour-long session. The initial mean

1726 T. Manly et al. / Neuropsychologia 43 (2005) 1721–1728

Fig. 2. The percentage of “left longer” responses on evenly bisected Land-mark trials over four blocks in Study 2.

on the visual analogue scale of 39.89 mm (S.D. 23.52) in-creased to 61.67 mm (S.D. 17.5; repeated measures ANOVAF(1,8) = 4.98,P = 0.028 one-tailed).

3.2.2. Landmark testDue to differences in the speed of participants’ responses,

there was some variability in the total number of trials com-pleted over the hour session (mean = 463.4, S.D. 83.9). Onlythe potentially most sensitive evenly bisected trials were con-sidered. Trials for each participant were divided into fourequal blocks and the proportion with a “left longer/rightshorter” response calculated for each. During the first block,participants judged 59.7% of evenly bisected lines as “leftlonger/right shorter” (S.D. 26.26%). This dropped to an av-erage of 54.05% (S.D. 25.80%) in block 2, 43.11% (S.D.17.03%) in block 3 and 45.39% (S.D. 28.75%) in block 4(seeFig. 2). To examine this statistically, proportionate datawere first arc sine transformed (this approximately standard-ises the variance across binomial probability data makingthem suitable for ANOVA analysis) and then submitted to arepeated measures ANOVA with the factor of time (blocks1–4). The reduction in “left longer” responses over time wasstatistically significant (F(3,24) = 3.32,P = 0.037 two-tailed(0.018 one-tailed)).

4

1 sub-f thelthye leftone-

ionalf dif-3;

2. When the same individuals were tested after a periodof relative sleep deprivation (and when their subjectiveratings of sleepiness were significantly increased)—therewas a significant change in their performance, consistentwith a rightward shift in attention.

3. A consistent effect was observed over the course of the‘alert’ session. Over the 40 or so minutes of continu-ous performance the participants displayed a significantshift in errors consistent with a reduction in leftwardbias.

4. In Study 2, using a Landmark task in which the horizontallines in key trials were exactly bisected—and therefore po-tentially sensitive to very subtle biases—the time-on-taskeffect of Study 1 was replicated. In addition, the increasedsensitivity of this task allowed us to observe that percep-tion of the left and rightward sides of the line at the end ofthe session was not even (as would be suggested by 50%left and 50% right responses) but actually appeared to bemoderately biased to the right.

We would argue that these preliminary results may havea number of important implications for the study of spa-tial neglect and for understanding the pseudoneglect phe-nomenon. As discussed, it has been suggested that recoveryfrom pathological spatial biases may be impeded in a propor-tion of RH patients by other consequences of their lesions,i per-f ntiont thaeh tiont tten-t port,p ge n tot er ith parto morea ion tot case.

higha left-w ntiont ain-t ,1 91;W inK skso biasa t thata mag-i mores erica )a ral

. General discussion

The results of these studies show:

. In a task in which participants were asked to maketle spatial judgements about the relative magnitude oleft and right extents of a horizontal line (Study 1), heaadult volunteers showed a reliable tendency to view thend as the longer. This normal pattern, termed pseudglect, has been interpreted in terms of a subtle attentbias and has been frequently observed in a number oferent spatial tasks (Chokron, 2002; Failla et al., 200McCourt et al., 2001).

n particular by deficits in alertness. The generally poorormance of chronic neglect patients on sustained atteasks (Robertson et al., 1997), the reductions in neglect wilerting tones or medication (Hurford et al., 1998) and thexacerbation of neglect with sedatives (Lazar et al., 2002)ave been consistent with this view. Similarly, the predic

hat other groups with alertness difficulties may show aional biases away from the left has received some suparticularly in the case of ADHD (Dobler et al., 2003; Nigt al., 1997). In these contexts, however, where attentio

he left is deficient, it has been difficult to assess whethas been aspecific beneficiary of increased alertness orf a more widespread improvement associated with alert state. The studies presented here, in which attent

he left was normal, suggest that the former may be theHere, the results suggest that periods of relatively

lertness were associated with overestimation of theard extent of a line and, by inference, enhanced atte

o the left side of space. Given RH dominance for the menance of alertness (Boller et al., 1970; Cohen & Semple988; DeRenzi, 1982; Lewin et al., 1996; Pardo et al., 19ilkins et al., 1987) this effect could be interpreted withinsbourne’s (1970)activation model suggesting that tar actions that differentially activate one hemisphere willttention in a contralateral direction. Our findings suggeslertness may be one such factor. Recent functional i

ng evidence suggests, however, that the effect may bepecific than that at least implied by the term ‘hemisphctivation’. In a series of studies,Sturm et al. (2004, 1999nd Sturm and Willmes (2001)have reported that gene

T. Manly et al. / Neuropsychologia 43 (2005) 1721–1728 1727

alertness was associated with increased activation in a pre-dominantly right lateralised network involving frontal, pari-etal, thalamic and brain stem regions. The authors note thatthere is considerable overlap between these regions and thoseimplicated in the aetiology of left neglect (seeHusain &Rorden, 2003; Mort et al., 2003). It is therefore tempting tospeculate that increases in general alertness in the RH havea more direct effect on other regions within the same—orclosely adjacent network—that are engaged in mediatingspatial attention. These findings may be interpreted withina general framework proposed byDehaene, Kerszberg, andChangeux (2001). They argue that fronto-parietal networksform a ‘Global Workspace’ required for effortful cognitivetasks. An important notion within this model is that rever-berative feedback in activity between the global workspaceand more basic sensory regions is necessary to support acon-scious experience of a stimulus. There is good evidence thatneglect patients without basic sensory loss can process infor-mation within neglected space to a considerable degree with-out necessarily being able to report its presence (e.g.Berti &Rizzolatti, 1992). Given the overlap between brain regionsin which lesions most commonly lead to neglect and thoseimplicated in the global workspace, it is tempting to con-ceptualise neglect as a form of unilateral global workspacedeficit affecting the conscious experience of stimuli from oneside of space. Within this framework, other factors such asa ffectg e ons e dram maget lationw ilderf

ry. Itr e de-s nsitivet k (al-t dt

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Chokron, S., Bartolomeo, P., Perenin, M. T., Helft, G., & Imbert, M.(1998). Scanning direction and line bisection: A study of normal sub-jects and unilateral neglect patients with opposite reading habits.Cog-nitive Brain Research, 7, 173–178.

Cohen, R. M., & Semple, W. E. (1988). Functional localization of sus-tained attention.Neuropsychiatry, Neuropsychology and BehaviouralNeurology, 1, 3–20.

Coull, J. T., Nobre, A. C., & Frith, C. D. (2001). The noradrenergic a2agonist clonidine modulates behavioural and neuroanatomical corre-lates of human attentional orienting and alerting.Cerebral Cortex, 11,

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lertness, reward and motivation—in as much as they alobal workspace function—could have a direct influencpatial awareness and bias. Our findings suggest that thatic effects on conscious awareness resulting from da

o these networks in patients and the subsequent moduith changes in alertness may be echoed in a much m

orm in healthy participants.The results presented here are, of course, prelimina

emains an open question as to whether the effect wcribe here is general, specific to measures that are seo pseudoneglect, or indeed specific to the Landmark tashough seeDobler et al., in press). Further work is requireo clarify these issues.

cknowledgements

We gratefully acknowledge the support of the Medesearch Council and the NHS Executive (Eastern) forupport to Veronika Dobler. Thanks to Julia Darling forareful preparation of the manuscript.

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