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Neuropsychologia 49 (2011) 3351–3360

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Neuropsychologia

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Selective modulations of attentional asymmetries after sleep deprivation

Rémy Schmitz, Gaétane Deliens, Alison Mary, Charline Urbain, Philippe Peigneux ∗

UR2NF [Unité de Recherches en Neuropsychologie et Neuroimagerie Fonctionnelle], Université Libre de Bruxelles, Campus du Solbosch CP191,Avenue F.D. Roosevelt 50, B-1050 Brussels, Belgium

a r t i c l e i n f o

Article history:Received 9 May 2011Received in revised form 28 July 2011Accepted 9 August 2011Available online 17 August 2011

Keywords:AttentionCerebral dominanceGreyscales taskLandmark taskPseudoneglectSleep deprivation

a b s t r a c t

Pseudoneglect is a slight but consistent misplacement of attention toward the left visual field, commonlyobserved in young healthy subjects. This leftward attentional bias is thought to result from a right hemi-spheric dominance in visuospatial processing. Changes in endogenous levels of alertness may modulateattentional asymmetries and pseudoneglect in particular. In line with this hypothesis, it has been shownthat sleep deprived shift-workers present a reversal of their attentional bias in a landmark (LDM) task(Manly, T., Dobler, V. B., Dodds, C. M., & George, M. A. (2005). Rightward shift in spatial awareness withdeclining alertness. Neuropsychologia, 43(12), 1721–1728). However, circadian disturbances and fatigueeffects at the end of a shift work may have contributed to this reversal effect. In a first experiment, weshow that sleep deprivation (SD) under controlled conditions does not markedly change the leftwardbias, observable both at 21:00 and at 07:00 after SD. In a second experiment, we tested the hypothesisthat a drastic reduction or inversion in the attentional bias would be present only when both the circadiandrive for sleep propensity is maximal (i.e. around 05:00) and homeostatic sleep pressure is high. Thusparticipants were tested at 21:00 and under SD conditions at 05:00 and 09:00. Additionally, we used thegreyscales (GS) task well-known to evidence a leftward bias in luminance judgments. Although resultsevidenced a consistent leftward bias both in the LDM and GS, we found a suppression of the leftwardbias at the circadian nadir of alertness (05:00) after SD only for the GS, but not for the LDM. Noticeably,the leftward bias in the GS vanished at 05:00 after SD but reappeared at 09:00 despite continued SD,suggesting a predominant circadian influence on attentional asymmetries in the GS. Additionally, inter-sessions correlations evidenced a reproducible, consistent bias both in the LDM and GS, with no consistentrelationship between the two tasks, suggesting independence of the neural networks subtending perfor-mance in LDM and GS. Overall, our results suggest that SD per se does not impede the leftward bias bothin LDM and GS, whereas circadian-related variations in vigilance may impact attentional asymmetries inluminance judgments.

© 2011 Elsevier Ltd. All rights reserved.

1. Introduction

Visuospatial information is not symmetrically representedwithin the brain (Vogel, Bowers, & Vogel, 2003). For instance,unilateral neglect characterized by deficits in reacting, reportingor orienting toward stimulations presented within the con-tralesional hemispace (Heilman, Watson, & Valenstein, 2002) ismostly reported after right hemispheric (RH) cerebral damage(Bartolomeo, 2007), suggesting a RH dominance for visuospatialprocessing. Thus, when asked to divide horizontal lines into twoequal parts in a line bisection task (LB), most neglect patients errconsiderably to the right of the objective middle (Fischer, 2001).Also in healthy participants, there is a consistent trend for a later-

∗ Corresponding author. Tel.: +32 02 650 26 39; fax: +32 02 650 22 09.E-mail address: Philippe.Peigneux@ulb.ac.be (P. Peigneux).URL: http://dev.ulb.ac.be/ur2nf (P. Peigneux).

alized bias when performing the LB, although the bias is subtle,of lower amplitude and directed toward the left side (Jewell &McCourt, 2000), as if the right visuospatial hemifield was tuned out.This “healthy” leftward bias was named pseudoneglect (Bowers& Heilman, 1980) in reference to the neglect syndrome. Neglectand pseudoneglect are also evidenced using another widely usedparadigm, the landmark task (LDM) in which participants are askedto decide which side of an evenly divided line is the longest or theshortest (Milner, Brechmann, & Pagliarini, 1992; Milner, Harvey,Roberts, & Forster, 1993; Reuter-Lorenz, Kinsbourne, & Moscovitch,1990; Schmitz & Peigneux, 2011). In healthy participants, evenlybisected lines are more often judged as longer (vs. shorter) on theirleft (vs. right) side, whereas an exaggerated reversed pattern isobserved in neglect patients. Neglect and pseudoneglect may sharecommon attentional mechanisms (McCourt & Jewell, 1999).

Neglect and/or pseudoneglect have received several, not alwaysmutually exclusive theoretical accounts. Based on the observationthat neglect is more often observed after RH than left hemisphere

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3352 R. Schmitz et al. / Neuropsychologia 49 (2011) 3351–3360

(LH) damage, Heilman and Valenstein (1979) proposed that theright visuospatial hemifield is represented in both hemispheres,whereas the left visuospatial hemifield is represented in the RHonly. Alternatively, the neglect syndrome might be explained byspecific impairments in covert attentional orienting mechanisms(Bartolomeo & Chokron, 2002; Posner & Driver, 1992). In thisperspective, neglect symptoms may stem from the patients’ dif-ficulties to disengage their attention from the healthy visuospatialhemifield, as if “glued” on the currently processed stimuli. Asimilar phenomenon was observed for pseudoneglect in experi-mental conditions artificially inducing an attentional magnificationtoward one side of the visual field (Toba, Cavanagh, & Bartolomeo,2011). Finally, the activation/orientation model (Kinsbourne, 1970)proposes that behavioural asymmetries arise from a differen-tial cerebral hemispheric activation biasing the attentional focuscontralaterally to the more activated hemisphere, which in turntransiently inhibits its homologue. According to this model, thespatial nature of LB and LDM activates more the RH. Consequently,if the RH inhibits the LH and orients the attentional focus towardthe left visuospatial hemifield in healthy participants, the objectivemiddle and/or the size of the two parts of a line will be misperceivedeventually, with a bias toward the left side. Whether pseudoneglectresults from an over-representation of the left visuospatial field,and/or an under-representation of the right still remains an openissue.

Neuroimaging studies further support the hypothesis that theRH is primarily involved in LB and LDM, in disclosing a predom-inantly right-lateralized occipito-parieto-frontal network duringtask processing (e.g. Cicek, Deouell, & Knight, 2009; Fink et al., 2000;Foxe, McCourt, & Javitt, 2003; Weiss, Marshall, Zilles, & Fink, 2003).Likewise, repetitive transcranial magnetic stimulation (rTMS) stud-ies evidenced the involvement of the right posterior parietal cortex(PPC) in neglect and pseudoneglect (e.g. Bjoertomt, Cowey, &Walsh, 2002; Brighina et al., 2002; Ellison, Schindler, Pattison, &Milner, 2004; Fierro et al., 2006; Ghacibeh, Shenker, Winter, Triggs,& Heilman, 2007). In line with the activation/orientation model(Kinsbourne, 1970), rTMS on the right PPC during LDM enhancesa leftward bias, whereas rTMS on the left PPC induces a right-ward bias (Kim et al., 2005). In neglect patients, lesions in the rightoccipito-parieto-frontal network are associated with a strong right-ward attentional bias in the LDM task (Vossel, Eschenbeck, Weiss,& Fink, 2010), and the corpus callosum integrity has been shown tobe important both in LB (Goldenberg, 1986; Hausmann, Corballis,& Farbi, 2003; Heilman, Bowers, & Watson, 1984; Kashiwagi,Kashiwagi, Nishikawa, Tanabe, & Okuda, 1990; Plourde & Sperry,1984) and LDM (Corballis, 1995).

Interestingly, alertness levels might shape pseudoneglect aswell, as suggested by a study showing a reversal of the leftwardto a rightward bias in healthy subjects after a sleep deprivation(SD) night (Manly, Dobler, Dodds, & George, 2005). In addition,a rightward attentional inversion was also found after practicingthe LDM for 1 h (but see Dufour, Touzalin, & Candas, 2007 for anegative result), suggesting an interaction between visuospatialand attentional processes. According to Posner and collaborators(Fernandez-Duque & Posner, 2001; Posner & Petersen, 1990; Raz &Buhle, 2006), attention can be subdivided into three functionallyand neuroanatomically partially independent networks: orient-ing, executive and alerting. Parietal activity supports the orientingnetwork with the ability to select (overtly or covertly) relevantinformation among distracters. The executive network, subtendedby activity in anterior cingulate and dorsolateral prefrontal cortices,supports the broad construct of effortful control and coordinationprocesses involved in inhibition, task switching, conflict resolution,error detection and planning. These orienting and executive sys-tems rely upon cholinergic and dopaminergic neurotransmissionpathways, respectively (Posner & Petersen, 1990). Finally, alertness

can be defined as the capacity to maintain an internal control ofwakefulness and arousal to cope with the immediate environment.Efficient levels of arousal are provided by the alerting networkencompassing right lateralized frontal, parietal, thalamic and brain-stem regions (Sturm et al., 1999, 2004; Sturm & Willmes, 2001).Noradrenergic (NA) transmission in the locus coeruleus (LC) is cru-cial to mediate this attentional component, and animal studies havedemonstrated higher density of NA-LC projections in right frontaland parietal regions (Posner & Petersen, 1990), further supportingthe hypothesis of a role for the RH in alertness regulation. Therefore,intrinsic levels of alertness are tightly related to visuospatial andattentional processing (Coull, 1998, 2001; Mesulam, 1999; Posner& Petersen, 1990).

A predominantly right-lateralized network subtending thealerting system is in line with observations that extended RHlesions are often associated with deficits in sustained attention(Hjaltason, Tegner, Tham, Levander, & Ericson, 1996; Robertson,Manly, et al., 1997; Robertson, Ridgeway, Greenfield, & Parr, 1997).Indeed, patients with right frontal lesions seem to be particularlyimpaired in monotonous tasks requiring continuous processing ofrelatively rare stimulations (Koski & Petrides, 2001; Manly et al.,2003; Rueckert & Grafman, 1996; Ruff, Niemann, Allen, Farrow, &Wylie, 1992; Shallice, Stuss, Alexander, Picton, & Derkzen, 2008;Vendrell et al., 1995; Wilkins, Shallice, & McCarthy, 1987). Difficul-ties to maintain attention over a prolonged period of time are alsoevidenced in cases of right parietal lesions (Malhotra, Coulthard, &Husain, 2009) and callosotomy (Dimond, 1976, 1979b; Ellenberg& Sperry, 1979), especially when the RH is required (Dimond,1979a; Dimond & Beaumont, 1972, 1973). Furthermore, vigilancedecrements are associated with callosal transfer efficiency levels(Rueckert, Baboorian, Stavropoulos, & Yasutake, 1999; Rueckert &Levy, 1996; Rueckert, Sorensen, & Levy, 1994). Overall, these datasuggest that both the integrity of the RH alerting network and agood interhemispheric coordination are required to maintain opti-mal levels of vigilance.

As mentioned above, sleep deprivation (SD) that impairs alert-ness and vigilance levels may also represent an effective paradigmto investigate the interactions between visuospatial and attentionalprocesses. Indeed, neuroimaging studies have consistently showedthat SD markedly affects activation patterns in the thalamo-fronto-parietal network during various attentional tasks (e.g. Chee &Chuah, 2007; Chee & Tan, 2010; Chee, Tan, Parimal, & Zagorodnov,2010; Chee et al., 2006; Chee et al., 2008; Drummond et al., 2005;Drummond, Gillin, & Brown, 2001; Mander et al., 2008; Portas etal., 1998; Thomas et al., 2000, 2003; Tomasi et al., 2009). How-ever, although it is well established that SD markedly impactsbehaviourally both vigilance and executive functions (e.g. Killgore,2010; Lim & Dinges, 2008, 2010), only a few studies have care-fully investigated SD-related changes in the orienting system.These studies have shown that SD may impact visuospatial ori-enting (Mander et al., 2008; Martella, Casagrande, & Lupianez,2011; Trujillo, Kornguth, & Schnyer, 2009; for a negative resultsee Casagrande, Martella, Di Pace, Pirri, & Guadalupi, 2006), espe-cially when cues are displayed laterally in the visual fields (Bocca &Denise, 2006; Fimm, Willmes, & Spijkers, 2006; Sanders & Reitsma,1982; Versace, Cavallero, De Min Tona, Mozzato, & Stegagno, 2006).In line with a RH-lateralized attentional network, other studieshave shown that attentional processes in the RH can be selec-tively impaired after SD (Fimm et al., 2006; Kendall, Kautz, Russo, &Killgore, 2006). Indeed, SD participants more easily covertly shiftedtheir attention toward the right visual field (RVF) when cues werepresented on the left and target were located to the right of thefixation point (Fimm et al., 2006). Accordingly, simple visual detec-tion (i.e. alerting) is impaired after SD only for stimuli displayedwithin the leftmost part of the visual field (Kendall et al., 2006). Fur-thermore, higher sensitivity of the RH to SD has been evidenced in

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Fig. 1. Experimental design of study 1. Black and white rectangles represent nights and days, respectively. The flash represents the experimental sleep deprivation night. Allparticipants were tested at 21:00 and 07:00 according to the group in which they were assigned.

auditory dichotic discrimination (Johnsen, Laberg, Eid, & Hugdahl,2002), lateralized facial recognition (Pallesen et al., 2004) and long-term preference judgments (Peigneux, Schmitz, & Willems, 2007).

To sum up, data gathered in healthy and pathological popu-lations suggest a great overlap between the networks involvingvisuospatial attention and alertness (Coull, 1998, 2001; Husain& Rorden, 2003). In this respect, RH impairment may be associ-ated with a decrease in alerting levels (e.g. Hjaltason et al., 1996;Malhotra et al., 2009; Robertson, Manly, et al., 1997). Conversely, adecrease of vigilance may selectively impact RH-related processes(e.g. Fimm et al., 2006; Johnsen et al., 2002; Kendall et al., 2006;Pallesen et al., 2004; Peigneux et al., 2007) and modulate the left-ward attentional bias evidenced in a LDM task. In line with thishypothesis, an inversion of the leftward bias in LDM was shown ina population of health care professionals tested at the end of theirnight shift work period, i.e. in a relative state of SD (Manly et al.,2005). However, this study conducted in a natural work environ-ment makes it difficult to tease apart the potentially confoundingeffects of continued activity and accumulated fatigue (Neu et al.,2011) during a night at work, or of the use of psychostimulant sub-stances such as caffeine (Landolt, 2008) or nicotine (Thiel & Fink,2007). In this perspective, the aim of our first study was to replicateunder tightly controlled experimental conditions the possible effectof a night of SD in healthy participants on leftward bias suppres-sion in the LDM. Since results indicated inconsistent modulation ofthe pseudoneglect effects in the LDM after SD, we investigated ina second study the potential contribution of an additional variable,i.e. circadian-related changes in alertness and vigilance that maycounteract SD-related deleterious effects at specific moments ofthe day (Schmidt, Collette, Cajochen, & Peigneux, 2007), using boththe LDM and a greyscale task showing a leftward bias in luminancejudgments.

2. Study 1

2.1. Method

2.1.1. ParticipantsFourteen young healthy right-handed participants (6 males; 24.8 ± 1.6 years)

participated in this 11 days follow-up study (see Fig. 1) approved by the local EthicsCommittee. All participants were free of any known neurological and psychiatricdisorders and had good or corrected vision for the testing. They were required notto take any stimulant (e.g. caffeine or nicotine) before each testing session and dur-ing the SD period. All participants had intermediate or neutral chronotype (mean:42.42 ± 2.76), as measured by the Morningness–Eveningness Questionnaire (Horne& Ostberg, 1976): high (≥70) and low scores (≤30) identify morning- and evening-type individuals, respectively, whereas intermediate scores refer to neutral type.

2.1.2. MaterialThe LDM task was identical to the one used by Manly et al. (2005, study 2) and

consisted of 200 lines (200 mm): 100 lines evenly bisected (EBL) and 100 lines withthe vertical divider (10 × 0.2 mm) shifted 5 mm to the left or to the right (UBL). Lineswere presented one at a time for 1000 ms in a random order, using Cogent 2000and Cogent Graphics (http://www.vislab.ucl.ac.uk) on a standard PC system witha 15-in. screen display. A mask screen was presented before and after each line.Participants had to decide which section of the line was the longest (resp. short-est) according to the displayed instruction, for both line types (evenly and unevenlybisected). Consigns (i.e. “shortest?” or “longest?”) were displayed alternatively, andthe first consign was randomly chosen at the beginning of the testing. There wasno time limit for responding, but participants were encouraged to give their firstimpression whenever they experienced uncertainty. Given the absence of a hand-related effect in the Manly et al.’s study (2005), we asked participants to respondwith their right middle finger and index to indicate a left or right response, respec-tively. Responses were automatically recorded on the computer. A session lastedapproximately 15 min.

2.1.3. ProcedureThe LDM task was administered 6 times: at 21:00 on Day 1, 4, 8 and 11, and at

07:00 on Day 2 and 9 (see Fig. 1). Half of participants were sleep deprived betweenDay 1 and 2, the other half between Day 8 and 9. During SD, participants arrivedto the lab at 20:30 and left at 7:30. During the whole night, they were asked toseat and involve themselves in quiet activities (e.g. watching movies, reading, etc.),under the direct supervision of an experimenter. Participants’ subjective vigilancewas measured before each LDM session using the Karolinska Sleepiness Scale (KSS;Akerstedt & Gillberg, 1990). The KSS is a Likert-type rating scale ranging from 1(highly awakened) to 9 (highly drowsy, fighting against sleep).

The leftward attentional bias was calculated as the proportion of left longer/rightshorter responses for EBL. Additionally, a leftward response bias was also com-puted as the proportion of ‘left’ responses for each EBL no matter the instruction(i.e. deciding longest vs. shortest line).

2.2. Results

A repeated measures ANOVA on KSS scores with factors Con-dition (Sleep vs. Sleep Deprivation) and Session (21:00 vs. 07:00vs. Day + 3 at 21:00) revealed a significant Condition by Sessioninteraction (F(2,26) = 177.01; p < .001). Tukey’s post hoc analysesshowed that participants’ subjective vigilance was significantlyaltered at 07:00 after a SD night (7.28 ± 1.54) compared to the otherconditions (min: 1.64 ± 0.49, max: 2.14 ± 0.66 for the other condi-tions; all ps < .01; see Supplementary Table 1 for further details).

One sample t-tests against chance value (=50) revealed a sig-nificant leftward bias for all sessions except at 07:00 in the SDcondition (threshold for Bonferroni’s p corrected for multiple com-parisons = .008; see Table 1). In other words, our participantsjudged evenly bisected lines (EBL) as ‘left longer/right shorter’above chance level only when they were not sleep deprived. How-ever, a repeated measures ANOVA on the leftward bias score with

Table 1Mean proportions (standard deviations), p-value of t-tests against random level (=50) and Cohen’s d for the proportion of leftward responses for evenly bisected lines in theLandmark task according to the experimental condition and the time of testing. Bonferroni’s p corrected for multiple comparisons = .008 (=.05/6). Effect sizes (d) are providedwith reference to the classification proposed by Cohen (1988); i.e. small d < 0.20, medium d = 0.50, and large d > 0.80.

Regular sleep Sleep deprivation

� (sd) p d � (sd) p d

21:00 73.95 (18.45) .0003 1.30 75.34 (15.23) .00003 1.6607:00 73.53 (19.78) .0007 1.19 66.38 (21.32) .013 .7721:00 (day + 3) 71.76 (24.01) .005 .91 77.14 (14.08) .000007 1.93

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factors Condition (Sleep vs. Sleep Deprivation) and Session (21:00vs. 07:00 vs. Day + 3 at 21:00) failed to reveal any significant effect(all ps > .13), suggesting an inconsistent effect of the SD conditionon the leftward bias.

Additionally, we computed a repeated measures ANOVA onthe probability to make a correct response for unevenly bisectedlines (UBL) with within-subjects factors Condition (Sleep vs. SleepDeprivation), Session (21:00 vs. 07:00 vs. Day + 3 at 21:00), Instruc-tion (decide Longest vs. Shortest line) and Line Type (Left- vs.Right-bisected). The main effect of Line Type was significant(F(1,13) = 7.95; p < .05) revealing that UBL with a right-shifteddivider were more accurately perceived (96.06 ± 7.45%) than UBLwith a left-shifted divider (83.11 ± 19.50%). Other comparisonsfailed to reach significance.

Finally, a one-sample t-test against chance value (=50) for theproportion of ‘left’ responses was computed to evaluate the pos-sibility of a response bias during EBL judgment for each session.t-Tests did not reveal any significant difference from 50 (all ps > .19).Additionally, a repeated measures ANOVA on the proportion of ‘left’responses with factors Condition (Sleep vs. Sleep Deprivation) andSession (21:00 vs. 07:00 vs. Day + 3 at 21:00) failed to reveal anysignificant effect (all ps > .10).

2.3. Discussion study 1

In line with previous findings (Manly et al., 2005; Milner et al.,1992; Reuter-Lorenz et al., 1990; Schmitz & Peigneux, 2011), wefound a consistent attentional leftward bias in healthy young par-ticipants. Left (vs. right) sides of EBL were judged as longer (vs.shorter). Additionally, participants’ performance was worse for UBLwith the divider shifted to the left. This pattern of performance forEBL and the decrease of accuracy for leftward-bisected UBL sug-gest a less efficient processing of the RVF in pseudoneglect. Indeed,if participants misperceive their RVF (as evidenced with EBL), theymight process UBL as evenly bisected when the divider is shifted tothe left. Strengthening an attentional interpretation, no responsebias was evidenced in this study.

At variance with Manly et al. (2005), our results failed to repli-cate a rightward inversion of the attentional bias after a SD nightunder controlled conditions. To the best, a reduction of the bias’amplitude was observed at 07:00, which became non-significantafter correction for multiple comparisons. However, this differ-ence was not confirmed by the ANOVA testing more directly theconsistency of the leftward bias across the different sessions. Addi-tionally, the effect size of the t-test against chance value remainedlarge even at 07:00 after the SD (d > .75; see Table 1). The left-ward bias on UBL remained largely unaffected by SD as well. Thislatter result is also in contrast with Manly et al. (2005)’s findingof a selective impairment on UBL with a right-shifted divider insleep deprived shift-workers. A difference in vigilance levels can-not explain the discrepancy between Manly et al.’s study and thepresent one. Indeed, our results show a significant decrease in par-ticipants’ subjective vigilance at 07:00 after the SD night. Thus, ourdata are more in line with a recent study that failed to replicate avigilance effect after a 1-h session of LDM (Dufour et al., 2007).

In summary, our first study does not replicate neither a right-ward inversion of the attentional pseudoneglect bias nor a generalincrease of error rate for UBL at 07:00 after a SD night, in conditionsin which time of testing, participants’ activity and the use of psy-chostimulant substances were controlled. Nonetheless, a moderatefading of the leftward bias after SD cannot be excluded. Interest-ingly, a previous study showed that covert orienting of attentionwas markedly affected in sleep-deprived participants specificallyat 05:00 (Fimm et al., 2006), i.e. when arousal is at its minimallevel due to a conjunction between accumulated sleep pressure andmaximal circadian sleep propensity (see below). Also, participants

in this study had more facility to disengage their attention fromtheir left visual field (LVF) to orient it rightward. This effect wasinterpreted as the results of a RH visuospatial impairment that fol-lowed SD. Moreover, this attentional modulation was not presentin previous and subsequent sessions despite a prolonged SD, sug-gesting a possible circadian influence more than a sleep pressureeffect per se.

As indicated above, two interacting processes are driving tem-poral fluctuations in vigilance-related processes and alertness:homeostatically regulated sleep pressure and circadian-based vari-ations in sleep propensity (for reviews see e.g. Schmidt et al.,2007; Silver & Lesauter, 2008; Van Dongen & Dinges, 2005).Whereas homeostatic sleep pressure gradually increases with timespent awake, a nearly 24-h endogenous oscillatory variation forsleep propensity defines the circadian rhythm (Borbely, Hayaishi,Sejnowski, & Altman, 2000). The circadian process is mostly con-trolled by the suprachiasmatic nuclei located in the anteriorhypothalamus, interacting with the locus coeruleus (LC) in mod-ulating vigilance-related responses in relation with homeostaticsleep pressure (Schmidt et al., 2009). LC-noradrenergic projectionsare thought to be crucial in mediating activity in the RH alertingnetwork (Posner & Petersen, 1990), modulating itself attentionalprocesses (Coull, 1998, 2001). Circadian-based sleep propensity isat its lowest level during the evening hours (around 22:00) andreaches its maxima during early morning around 05:00 (Czeisler &Gooley, 2007). During the course a normal day, accumulating sleeppressure due to increasing time spent awake is countered by thecircadian decrease in sleep propensity, keeping alertness and vig-ilance at relatively constant levels. When circadian-related sleeppropensity starts increasing again in the late hours (after 22:00),its influence combines with accumulated sleep pressure to favoursleep initiation, eventually decreasing alertness and vigilance. Ifthe wakefulness period is prolonged over the normal day dura-tion in a night of SD, vigilance will gradually decrease to reach itsminima at the nadir of the circadian curve in the early morninghours (around 05:00), and then slightly increase again with thecircadian-based diminution of the sleep propensity signal despitecontinuously accumulating sleep pressure due to time spent awake(Van Dongen & Dinges, 2005). Hence, it might be that our failure todemonstrate an inversion of the leftward bias in the LDM after SDin our first study was due to the beneficial influence of decreasingcircadian-related sleep propensity and partial restoration of alert-ness levels at testing time set at 07:00. Our second experiment wasdesigned to test this specific hypothesis.

In addition to the LDM, we also used a greyscales task (GS;Mattingley, Bradshaw, Nettleton, & Bradshaw, 1994; Nicholls,Bradshaw, & Mattingley, 1999) in study 2. In the GS, participantsare asked to decide on the brightness of two paired, mirrored rect-angles (see Fig. 2). In this task, a leftward bias is evidenced by thefact that participants tend to choose the rectangle that is darkeron its left side, despite the fact that the two mirrored stimuli areidentical. Beyond inducing this leftward bias, performance on theGS seems to be sensitive to similar factors as compared to thoseon LDM. Indeed, neglect patient exhibit a rightward bias in theGS (Mattingley et al., 1994, 2004; Nicholls, Mattingley, Berberovic,Smith, & Bradshaw, 2004), and cueing the RVF before judging GSvanishes the leftward bias whereas left or bilateral cueing has noeffect (Nicholls & Roberts, 2002). Likewise, visuomotor prism adap-tation causing a left-shifting of the visual scene induces a rightwardbias in GS (Loftus, Vijayakumar, & Nicholls, 2009), and markersof dopaminergic activity such as eyeblinks (Slagter, Davidson, &Tomer, 2010), novelty seeking (Tomer, 2008), dopaminergic trans-porter genotype (Bellgrove, Hawi, Kirley, Gill, & Robertson, 2005)and Attention-Deficit/Hyperactivity Disorder syndrome (Klimkeit,Mattingley, Sheppard, Lee, & Bradshaw, 2003) are good predictorsof the bias directionality in GS. In line with the aim of the present

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Fig. 2. (A) Experimental design of study 2. Black and white rectangles representnights and days, respectively. The flash represents the experimental sleep depriva-tion night. All participants were tested at 21:00, 05:00 and 09:00. (B) Sample of thegreyscales used in the second experiment.

study, healthy participants exhibiting poor performance in sus-tained attention tasks demonstrate an attenuated leftward bias inGS (Bellgrove, Dockree, Aimola, & Robertson, 2004; Bellgrove et al.,2005). Surprisingly, leftward biases evidenced on LDM and GS doesnot seem to be inter-correlated (Heber, Siebertz, Wolter, Kuhlen, &Fimm, 2010), whereas intra-correlations indicate good intra-taskreliability (McCourt, 2001; Nicholls et al., 1999). The absence ofinter-tasks correlation may reflect the engagement of distinct pro-cesses.

Overall, the above data suggest that GS, likewise LDM, may besensitive to a SD protocol reducing participants’ vigilance artifi-cially. Moreover this vigilance effect on asymmetries may be mostlyevidenced at 05:00 due to circadian factors. Therefore in study2, participants were test at 21:00 at the end of a normal wakingday, and at 05:00 and 09:00 after SD to test for a potential cir-cadian effect. Both LDM and GS were administered to test for thetasks’ specific sensitivity at different timing, and to evaluate inter-and intra-correlations across different sessions. Finally, in additionto circadian typology and subjective vigilance, handedness score,sleep habits, and objective vigilance were controlled in this secondexperiment.

3. Study 2

3.1. Method

3.1.1. ParticipantsTwenty-two young right-handed healthy participants (9 males; 22.32 ± 2.82

years) participated in this study approved by the local Ethics Committee. LateralityQuotient (Oldfield, 1971) ranged from 64.7 to 100 (mean ± sd = 89.7 ± 11.1), indi-cating right-handedness. All participants had intermediate or neutral chronotypeas evaluated by the Morningness–Eveningness Questionnaire (Horne & Ostberg,1976; mean: 47.14 ± 9.30). All participants were asked to be drug free (e.g. caffeine,nicotine) before each testing session and during the SD period.

3.1.2. MaterialObjective vigilance was monitored using the Psychomotor Vigilance Task (PVT;

Dinges & Powell, 1985) in which participants were asked to press a key as fast as pos-sible whenever a millisecond countdown appeared in the middle of a black computerscreen. Stimuli appeared randomly within a time window ranging from 2 to 10 s.The task ended automatically after 10 min, and median reaction time was computedat the end of each session.

Participants performed the same LDM task used in study 1 with the exceptionthat instructions were now displayed randomly rather than in alternation to controlpossible response anticipation effects. The GS task was programmed using Cogent2000 and Cogent Graphics (http://www.vislab.ucl.ac.uk) on a standard PC systemwith a 15-in. screen display. The six types of stimulus used had all 79 pixels in heightand 320, 400, 480, 560, 640 or 720 pixels in length. Each stimulus was darker on itsleft or right side (see Fig. 2 for a sample). There were 16 stimuli for each category ofpixel length (i.e. a total of 96 stimuli). Each stimulus changed by increments of 80pixels in density from black on one side to white on the other. All stimuli were viewed

at a distance of 60 cm. Each trial consisted on one upper and one lower greyscalethat were arranged so that they were left–right reversals of one another. Thus, if theupper stimulus was darker on the right, the lower stimulus was darker on the left.Participants had to judge which of the two stimuli was the darkest. Each trial wasdisplayed until the corresponding key was pressed with the right index and middlefinger (B for “Bottom and T for “top”, respectively). Participants were not told that,at each trial, stimuli were equiluminescent. Responses were automatically recordedby the computer. A GS session lasted approximately 10 min.

3.1.3. ProcedureParticipants’ sleep quality was evaluated using the St. Mary’s Hospital sleep

questionnaire (Ellis et al., 1981) one day before the SD night. Sleep habits in thepast month were assessed using the Pittsburgh Sleep Quality Index (PSQI; Buysse,Reynolds, Monk, Berman, & Kupfer, 1989). Subjective (KSS; Akerstedt & Gillberg,1990) and objective (PVT; Dinges & Powell, 1985) vigilance was monitored beforeeach session. LDM and GS tasks were administered three times: at 21:00 and after8 and 12 h of SD (i.e. at 05:00 and 09:00; see Fig. 2). The order of the two tasks wascounterbalanced between participants during each of the three sessions. An entiresession (PVT, LDM and GS) lasted approximately 35 min, depending on participant’sspeed. As in study 1, participants were asked to engage in quiet activities duringthe SD night, under the constant supervision of an experimenter until they left thelaboratory at 10:00.

The leftward attentional bias in the LDM task was computed as the proportion ofleft longer/right shorter responses for EBL, whereas the leftward bias for GS task wascomputed as the proportion of responses for darker GS on their left side irrespectiveof their position (top or bottom). A leftward response bias was also computed in theLDM task as the proportion of ‘left’ responses for each EBL irrespective of the pro-vided instruction (decide longest vs. shortest line). The proportion of ‘top’ responsesduring the GS was also recorded irrespective of the nature of the stimulus.

3.2. Results

3.2.1. Sleep and vigilance parametersWithin the month preceding the moment they were tested, PSQI

data revealed that participants slept on average 8.02 ± .82 h; sleeplatency was 21.68 ± 12.79 min. These sleep parameters did notdiffer from those evaluated with the St. Mary’s Hospital sleep ques-tionnaire (duration: 8.19 ± 1.31 h and latency: 22.82 ± 18.64 min;ps > .56), indicating that participants kept their sleep habits duringthe night preceding the experimental testing. Participants woke upat 8.22 (±1.51) a.m., and were awake for 20.83 ± .35 h when theywere tested at 05:00.

Two separate one-way repeated measures ANOVAs revealed asignificant main effect of Session (21:00 vs. 05:00 vs. 09:00) forsubjective (F(2,42) = 43.94, p < .001) and objective (F(2,42) = 16.39,p < .001) vigilance indexes (see Supplementary Table 2). Tukey’spost hoc analyses showed that indexes recorded at 21:00 (KSS:4.14 ± 1.86; PVT: 296.05 ± 28.15 ms) were significantly lower thanthose recorded at 05:00 (7.06 ± 1.76; 332.70 ± 52.82 ms) and 09:00(7.68 ± 1.55; 348.91 ± 72.16 ms; all ps < .01) after the SD period. Vig-ilance indexes at 05:00 and 09:00 were not significantly different(all ps > .21).

3.2.2. Landmark taskOne sample t-tests against chance value (=50) revealed a sig-

nificant leftward bias for all sessions except at 05:00 withinthe SD condition (Bonferroni’s p corrected for multiple com-parisons = .017; see Table 2). Nevertheless, a one way repeatedmeasures ANOVA on the leftward bias score with factor Ses-sion (21:00 vs. 05:00 vs. 09:00) failed to reach significance(F(2,42) = 0.48, p > .61), suggesting a weak, inconsistent effect of theSD at 05:00 on the leftward bias evidenced in the LDM task.

We also computed a repeated measures ANOVA on the prob-ability to make a correct response for UBL with within-subjectsfactors Session (21:00 vs. 05:00 vs. 09:00), Instruction (decideLongest vs. Shortest line) and Line Type (Left- vs. Right-bisected).A significant main effect of Line Type (F(1,21) = 8.86; p < .01)revealed that UBL with a right-shifted divider were more accu-rately perceived (91.46 ± 8.33%) than UBL with a left-shifted divider(84.58 ± 11.79%). The main effect of Session tended to reach signif-icance (F(2,42) = 2.73, p = .076). Tukey’s post hoc analyses showed

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Table 2Mean proportions (standard deviations), p-value of t-tests against random level(=50) and Cohen’s d for the proportion of leftward responses in Landmark andGreyscales tasks according to the time of testing during a SD protocol. Bonferroni’sp corrected for multiple comparisons = .017 (= .05/3). Effect sizes (d) are providedwith reference to the classification proposed by Cohen (1988); i.e. small d < 0.20,medium d = 0.50, and large d > 0.80.

Landmark Greyscales

� (sd) p d � (sd) p d

21:00 63.05 (21.95) .011 .59 58.76 (19.05) .043 .4605:00 60.45 (22.58) .041 .46 54.12 (20.13) .348 .2009:00 61.36 (19.82) .014 .57 60.91 (17.76) .009 .61

a trend for participants to be less accurate at 09:00 after SD(86.41 ± 2.15%) than at 21:00 (90.32 ± 2.08%, p = .077). Other com-parisons failed to reach significance.

Finally, we computed one-sample t-tests against chance value(=50) for the proportion of ‘left’ responses during EBL judgmentfor each of the three sessions. t-Tests did not reveal any signif-icant response bias (all ps > .79) except at 09:00 (47.45 ± 5.06%;t(21) = −2.36, p = .028). However this probability failed to reach sig-nificance after correction for multiple comparisons (Bonferroni’sp > .017). Moreover, a one way repeated measures ANOVA on theproportion of ‘left’ responses with factor Session (21:00 vs. 05:00vs. 09:00) failed to reach significance (F(2,42) = 1.63, p > .21), con-firming the absence of SD related effect.

3.2.3. Greyscales taskOne sample t-tests against the chance value (=50) revealed a

significant leftward bias at 09:00 and a trend at 21:00 (Bonfer-roni’s p corrected for multiple comparisons = .017; see Table 2).The one way repeated measures ANOVA on the leftward bias scorewith factor Session (21:00 vs. 05:00 vs. 09:00) reached signifi-cance (F(2,42) = 4.31, p < .05). Tukey’s post hoc analyses showedthat the leftward bias at 05:00 was significantly lower comparedto the bias at 09:00 (p < .05) but not compared to the bias at 21:00(p > .13), whereas biases at 21:00 and 09:00 did not differ signifi-cantly between each other (p > .64).

One-sample t-tests against chance value (=50) for the propor-tion of ‘top’ responses failed to reveal any significant response biasall along the sessions (all ps > .13). Furthermore, a one way repeatedmeasures ANOVA on the proportion of ‘top’ responses with fac-tor Session (21:00 vs. 05:00 vs. 09:00) failed to reach significance(F(2,42) = 0.59, p > .56).

3.2.4. Correlation analysesThe consistency of the leftward attentional bias within and

between GS and LDM tasks was tested using Spearman correlations.Analyses revealed high within-subject consistency within the threesessions for the same task (all ps < .001), but failed to reveal any sta-tistically significant correlation between the GS and LDM tasks (seeFig. 3).

3.2.5. Additional analysesGiven marked differences between study 1 and study 2 regard-

ing the statistical power of the leftward bias in LDM, a t-test for twoindependent samples was computed to determine whether the left-ward bias observed at 21:00 before the SD was different betweenthe two experiments. This analysis failed to reveal significant dif-ferences between the evening (21:00) leftward bias in study 1 andstudy 2 (t(14) = 1.83, p > .17; see Tables 1 and 2).

The percentage of correct response for UBL at 21:00 before SDwas also compared between our two samples. A mixed ANOVA withwithin-subjects factors Instruction (decide Longest vs. Shortestline) and Line Type (Left- vs. Right-bisected) and with between-subjects factor Study (Study 1 vs. Study 2) revealed a highlysignificant main effect of Line Type (F(1,34) = 28.20, p < .0001)and a significant Line Type by Study interaction (F(1,34) = 4.35,p = .045). UBL with a right-shifted divider were judged moreaccurately (94.50 ± 8.38%) than UBL with a left-shifted divider(84.26 ± 13.65%). Tukey’s post hoc revealed that this differencewas significant in study 1 (96.10 ± 6.37 vs. 80.64 ± 14.50; p < .001)but only a non-significant trend in study 2 (93.43 ± 9.49 vs.86.67 ± 12.84; p = .065).

In addition, collapsing data of all participants at 21:00 beforeSD, Spearman correlation analyses were used to test the relationbetween the magnitude of the leftward bias and the percentage of

Fig. 3. Spearman correlations computed on leftward attentional bias scores within and between Landmark (LDM) and Greyscales (GS) tasks across all sessions (21:00 vs.05:00 vs. 09:00). Note the highly significant correlations within the LDM and GS (extreme left and right graphs, respectively) and the absence of significance across the twotasks.

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correct responses for UBL with left- or right-shifted dividers. Wereasoned that the more the participants would exhibit a strongperceptual leftward bias on EBL (i.e. focusing more on their LVFand/or neglecting their RVF), the better they would perform wellthe UBL with right-shifted dividers and/or less correctly the UBLwith left-shifted dividers. In line with our predictions, a clearly sig-nificant negative relationship was evidenced between the leftwardbias and UBL with a left-shifted divider (r(35) = −.46, p < .01). Addi-tionally, a trend was observed for a positive correlation betweenthe magnitude of the leftward bias and the accuracy for UBLwith a right-shifted divider (r(35) = .31, p = .074; see SupplementaryFigure).

3.3. Discussion study 2

Study 2 aimed at evaluating whether the circadian moment oftesting could explain the absence of a rightward shift of the atten-tional bias in the LDM, as may have been surmised from a previousstudy showing a modulation of RH lateralized attention at 05:00following a SD night (Fimm et al., 2006). In addition, we added theGS to the experimental protocol, and the reliability between andwithin LDM and GS was evaluated within and across sessions.

Once again our results replicated the leftward attentional biason EBL judgments with the LDM (Manly et al., 2005; Milner et al.,1992; Reuter-Lorenz et al., 1990; Schmitz & Peigneux, 2011), sinceparticipants judged EBL sides as right shorter and left longer. More-over, we replicated the effect on UBL observed in study 1: UBLwith a right-shifted divider were more correctly perceived thanUBL with a left-shifted divider. In addition, we showed that themagnitude of the attentional leftward bias partially predicts theperformance on UBL (Schmitz & Peigneux, 2011). In other words,the stronger was the leftward bias, the worse was the performanceon UBL with a left-shifted divider, and the better tended to be theperformance on UBL with a right-shifted divider. A ceiling effect onUBL with a right-shifted divider may explain the difficulty to reachsignificance for the correlation with the leftward attentional biasevidenced with EBL (see Supplementary Figure). Indeed, these UBLmight be globally easier to respond because the leftward atten-tional bias accentuates the difference between the two sectionsof the lines. Conversely, if the UBL with a left-shifted divider areperceived as EBL they might generate more variability in partic-ipants’ responses accuracy. Overall, these results are in line withthe assumption that healthy young adults misperceive their RVFand process better the LVF because of a RH visuospatial dominance(Kinsbourne, 1970; Reuter-Lorenz et al., 1990). Again, the absenceof a response bias in our second study strengthens the attentionalinterpretation of pseudoneglect.

A leftward bias during luminance judgment was also establishedin GS (Mattingley et al., 1994; Nicholls et al., 1999). Between twoequiluminescent stimuli, participants chose on average more oftenthe one that was darker on its left side. This effect was independentof the position of the stimuli. Indeed, analyses revealed that theproportion of ‘top’ responses was at random level. We confirmedalso the absence of inter-correlation between performances on GSand LDM (Heber et al., 2010). In addition, strong and highly signif-icant within-task correlations were evidenced between the threedifferent experimental sessions both for LDM and GS. Internal con-sistency of the left bias in the GS has already been shown within thesame session with split-half correlations (Nicholls et al., 1999) andbetween different sessions for the LDM (McCourt, 2001). In otherwords, our data confirm both the independence between LDM andGS, and the reliability of the leftward biases evidenced using thesetwo tasks.

Again, our data failed to disclose a clear inversion of the left-ward attentional bias in LDM at 05:00 and 09:00 after a SD night,although subjective and objective vigilance levels were signifi-

cantly impaired. Participants’ performance on UBL also tended tobe worse at 09:00 independently of the side of the divider. Thisresult argues against a specific SD-related RH processing impair-ment, for which a failure on UBL with a right-shifted divider wouldhave been expected (Manly et al., 2005). These results also argueagainst a clear-cut circadian effect acting on pseudoneglect in LDM.Additionally, the ANOVA failed to reveal an effect of session onEBL judgments and, as mentioned above, performance on UBL wasslightly impaired at 09:00 only. These results are more in line witha sleep pressure interpretation globally affecting performance onUBL.

At variance, the leftward bias evidenced in luminance judg-ments in the GS was modulated by the moment of testing. Indeed,it was obviously vanished at 05:00 to reappear at 09:00, whereasit was at an intermediate level at 21:00. This time course is fittingwith the circadian clock and melatonin secretion cycles (Czeisler &Gooley, 2007; Schmidt et al., 2007). Melatonin, a hormone endoge-nously secreted by the pineal gland, participates in the circadianeffect of sleep propensity (Czeisler & Gooley, 2007). It has beenshown, using a visual search task, increasing activity in right dorso-lateral prefrontal cortex together with a decrease in occipital cortexand thalamus activity at 22:00, i.e. when circadian sleep propen-sity is at its lower level (Gorfine & Zisapel, 2009). Additionally,right parietal cortex activation correlated with fatigue levels, andendogenous melatonin levels (one of the most accurate circadiancorrelates) predicted increased activity in the right dorsolateralPFC. Hence these data suggest that variations in the attentionalnetwork may be shaped both by the circadian clock and the homeo-static sleep pressure processes. The absence of a leftward bias in GSwas clearly demonstrated at 05:00, a period of time tightly relatedto the apex of the circadian drive for sleep propensity (Czeisler &Gooley, 2007; Schmidt et al., 2007). In line with a circadian modu-lation rather than a homeostatic sleep pressure interpretation, theleftward bias in GS was restored at 09:00 despite increased timespent awake. Indeed, in comparison to 05:00, our participants weremore sleep deprived at 09:00 but nevertheless demonstrated againa leftward attentional bias, at a time when circadian propensity forsleep is already declining (Czeisler & Gooley, 2007; Schmidt et al.,2007). Therefore, our results with the GS, but not the LDM, are inline with Fimm et al. (2006) results disclosing a specific SD effecton RH processing at 05:00.

4. General discussion

In the present study, we have replicated across two experimentsa consistent leftward attentional bias in the LDM, in line with priorliterature (Manly et al., 2005; Milner et al., 1992; Reuter-Lorenzet al., 1990; Schmitz & Peigneux, 2011). In addition, we replicatedthe finding of a correlation between the magnitude of the leftwardbias and the performance on UBL judgments (Schmitz & Peigneux,2011): the more the participants exhibited a strong leftward atten-tional bias, the more they misperceived UBL with a left-shifteddivider, and the more they tended to perceive correctly UBL witha right-shifted divider. An attentional interpretation of the left-ward bias was strengthened by the absence of response bias inour two studies. Overall, these results suggest that healthy par-ticipants focus more on their LVF and/or neglect a slight part oftheir RVF.

A leftward bias was also evidenced for the GS in our secondstudy, in line with prior findings (Mattingley et al., 1994; Nichollset al., 1999). Additionally, we confirmed the independence betweenLDM and GS, with a striking lack of inter-tasks correlation (Heberet al., 2010). Whereas independence between neural networks sub-tending these tasks is generally advocated to explain this lack ofassociation, it should be noticed that to the best of our knowl-edge no neuroimaging study has yet investigated this apparent

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neural segregation. Neuroimaging findings would be especiallyhelpful to understand the rationale of this inter-task differentia-tion, especially considering the factors affecting the leftward biasboth in LDM and GS. Our results additionally demonstrate between-sessions consistency in pseudoneglect effects observed both in LDM(McCourt, 2001) and GS (Nicholls et al., 1999), showing that thedirection of the participants’ attentional bias is highly reproducible.Given the strong reliability over time of individual attentionalbiases in healthy participants, the presence of weak intra-sessionscorrelations might reflect a greater idiosyncratic variability and beconsidered as an additional index in pathological conditions.

Our two studies consistently failed to disclose a clear modula-tion of pseudoneglect by SD in the LDM, no matter the moment oftesting (05:00, 07:00 or 09:00), at variance with the results of Manlyet al. (2005) disclosing a rightward inversion of the attentionalbias. Hence, our results obtained under controlled experimentalconditions strongly suggest that SD itself has no, or at least a veryweak impact on the leftward attentional bias as evidenced in theLDM, and is neither influenced by a circadian factor. At variance,we found in study 2 a circadian-like modulation of the leftwardbias in the GS. Indeed, the leftward bias completely disappearedat 05:00 to reappear at 09:00. Further investigations are needed totest whether a leftward bias modulation in the GS genuinely fol-lows a circadian curve, or rather, is only affected at peak pointswhen both sleep pressure and sleep propensity are maximized.It may be also hypothesized that variations in circadian markersacross different testing moments should correlate with the magni-tude of the leftward bias in GS, and/or with associated changes incerebral activity. Such cerebral functional correlations have beendemonstrated in visual search (Gorfine, Assaf, Goshen-Gottstein,Yeshurun, & Zisapel, 2006; Gorfine & Zisapel, 2009), autobiographi-cal memory (Gorfine et al., 2006; Gorfine & Zisapel, 2007) and musiclistening (Gorfine et al., 2006).

Overall, our results are in agreement with the activa-tion/orientation model (Kinsbourne, 1970). Indeed, because of theirvisuospatial nature, both the LDM and GS may activate the RH,which in turn inhibits the LH thus resulting in a better process-ing of the LVF. This pattern of RH-activation/LH-inhibition mayexplain why participants perceive the left side of a GS as darkerand the sides of EBL as left longer and right shorter, respectively,and perform better for UBL with a right-shifted divider, as wellas why the natural attentional bias revealed on EBL was predic-tive for performance on UBL. However, it must be noticed that theactivation/orientation model (Kinsbourne, 1970) is not excluding,and even may hypothesize, a potential implication of automaticleftward-biased orienting mechanisms in healthy participants if theRH is primarily involved (Bartolomeo & Chokron, 2002; Toba et al.,2011). In this perspective, an attentional orienting impairment mayexplain why the participants’ leftward bias is slightly reduced inthe LDM task and clearly suppressed in the GS task at 05.00 aftera SD night. Indeed, prior studies have indicated that SD partici-pants have more facilities to shift their attention toward the RVF atthis time of day (Fimm et al., 2006) and also fail to detect stimuliwithin the leftmost part of the visual scene (Kendall et al., 2006).Given that the time-on-day greatly affects the bias in the GS as com-pared to the LDM task, and that these two tasks are uncorrelated,it can be hypothesized that an attentional orienting mechanism ismore involved in luminance than length judgments. An alterna-tive explanation concerning the SD-related effect on GS might bethat luminance judgments further involve the processing of spa-tial frequencies (Niemeier, Stojanoski, & Greco, 2007; Niemeier,Stojanoski, Singh, & Chu 2008; Singh, Stojanoski, Le, & Niemeier,2011) that may be also supported by a primary RH processingof low frequencies during free-viewing condition (Hegde, 2008;Robertson & Ivry, 2000). Accordingly, one study showed that SDmay affect contrast sensitivity to spatial frequencies (Quant, 1992).

At least, it is worth noticing that the selective time-related effectfound in the GS, but not the LDM, may be an additional argument fora dissociation of the neural networks subtending these two tasks.Nevertheless, the lack of neuroimaging studies investigating theneural correlates of performance in the GS task or directly com-paring LDM and GS makes it difficult to state strong hypothesesconcerning this dissociation. It should be noticed that, to the best ofour knowledge, our study is the first to demonstrate a behaviouraldissociation between LDM and GS within the same experimentalprotocol and in the same subjects. Further studies are thereforeneeded to confirm these results and to test potential explanatoryhypotheses.

Acknowledgements

The authors thank Farah Ayari and Mathieu Vilain for help indata acquisition, Rachel Leproult for her careful proofreading ofEnglish language, and two anonymous reviewers for thoughtfulcomments. RS and GD are Research Fellows at the Belgian FondsNational de la Recherche Scientifique (FNRS). AM is supported byan ULB ARC grant. CU is supported by a grant from the FondationVigneron. This study has been conducted with support of FRSMGrant 3.4.594.08.F.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.neuropsychologia.2011.08.009.

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