Brain and Cognition 62 (2006) 128–133www.elsevier.com/locate/b&c
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Right hemispheric dominance in gaze-triggered reXexive shift of attention in humans
Takashi Okada a,b,¤, Wataru Sato c, Motomi Toichi b,d
a Department of Neuropsychiatry, Faculty of Medicine, Kyoto University, Japanb Health and Medical Services Center, Shiga University, Japanc Department of Psychology, Faculty of Letters, Kyoto University, Japand School of Health Sciences, Faculty of Medicine, Kyoto University, Japan
Accepted 7 April 2006Available online 30 May 2006
Abstract
Recent Wndings suggest a right hemispheric dominance in gaze-triggered shifts of attention. The aim of this study was to clarify thedominant hemisphere in the gaze processing that mediates attentional shift. A target localization task, with preceding non-predicativegaze cues presented to each visual Weld, was undertaken by 44 healthy subjects, measuring reaction time (RT). A face identiWcation taskwas also given to determine hemispheric dominance in face processing for each subject. RT diVerences between valid and invalid cueswere larger when presented in the left rather than the right visual Weld. This held true regardless of individual hemispheric dominance inface processing. Together, these results indicate right hemispheric dominance in gaze-triggered reXexive shifts of attention in normalhealthy subjects.© 2006 Elsevier Inc. All rights reserved.
Keywords: Gaze; Attention; Unilateral visual Weld presentation; Hemispheric dominance
1. Introduction
Eye gaze is a powerful communicative medium inhuman interaction. We have a tendency to direct attentionto where another person is looking (Kingstone, Friesen, &Gazzaniga, 2000), and recent psychological studies haveshown that this tendency—joint attention—occurs reXex-ively and automatically even if the gaze direction does notpredict any relevant events in the environment (Driveret al., 1999; Friesen & Kingstone, 1998; Langton & Bruce,1999).Neurophysiological and neuropsychological studieshave identiWed neural systems specializing in gaze process-ing (Emery, 2000). A single-cell recording in monkeys (Per-rett et al., 1985) and lesion studies in monkeys and humans(Campbell, Heywood, Cowey, Regard, & Landis, 1990;
Heywood & Cowey, 1992) have suggested involvement ofthe superior temporal sulcus (STS) in the perception ofgaze direction. Another neurophysiological study showedinvolvement of the temporo-parietal projection from theSTS to the intraparietal sulcus (IPS) in these processes(Harries & Perrett, 1991), while others have reported ondefects in discrimination of gaze direction in a bilaterallyamygdala-damaged patient (Young, Aggleton, Hellawell,Johnson, & Broks, 1995). Finally, recent neuroimagingstudies have demonstrated involvement of neural networksincluding the STS (Puce, Allison, Bentin, Gore, & McCar-thy, 1998; Wicker, Michel, HenaV, & Decety, 1998), IPS(HoVman & Haxby, 2000), and amygdala (Kawashimaet al., 1999) in gaze perception.A variety of human cognitive functions, such as lan-guage processing, are dominantly processed in a singlehemisphere. Such lateralization has been suggested regard-ing shift of attention in response to gaze direction by arecent study on two split-brain patients (Kingstone et al.,2000). Functional neuroimaging studies also suggest that
* Corresponding author. Fax: +81 75 751 3246.E-mail address: tksokada@kuhp.kyoto-u.ac.jp (T. Okada).
T. Okada et al. / Brain and Cognition 62 (2006) 128–133129
changes in gaze direction preferentially activates the rightSTS in comparison to the left, although the diVerence didnot reach signiWcance (Puce et al., 1998; Wicker et al., 1998).These preliminary Wndings suggest that gaze perceptionmay be a single hemisphere-dominant process in humans.In order to investigate this possibility in gaze processingmediating reXexive shifts of spatial attention, this studyexamined the reaction times (RTs) of healthy subjects inlocalizing targets with preceding non-predictive gaze cuespresented to each visual Weld.An additional purpose of this study was to investigatethe relationship between gaze-triggered attentional shiftand face perception. Kingstone et al. (2000) examined twosplit-brain patients and reported that the reXexive atten-tional shift is lateralized to the single hemisphere process-ing face perception. If this is the case with subjects havingintact brain function, it would indicate that the dominanthemisphere for gaze-triggered reXexive shift of attention islateralized to that involved with face processing. A faceperception task was conducted in order to test thishypothesis.
2. Method
2.1. Subjects
Experiments were conducted at the Health and MedicalServices Center of Shiga University. Forty-four healthysubjects initially participated in the study. The subjectswere on no medication, and had no history of psychiatric,neurological, or ophthalmologic illness. All subjects wereright-handed, as assessed by the Edinburgh HandednessInventory (OldWeld, 1971), and had normal or corrected-to-normal visual acuity. Average age was 19.0-year-old (SD0.7, range 18–21). Informed consent was obtained after theprocedure had been fully explained, but without revealingthe goal of the experiments or the nature of the experimen-tal conditions. The study protocol was approved by theinstitutional review board of the center.
2.2. Apparatus
Stimuli were presented on a Xat type 19-in. CRT moni-tor. The refresh rate of the monitor was set to 100 Hz. Theresolution of the monitor was 1024 £ 768 pixels. The pre-sentation of stimuli was controlled by SuperLab Pro ver.2.0 (Cedrus, San Pedro, CA, USA) software. This softwareallowed for stimulus presentation within one screen refreshcycle (i.e. 10 ms) by setting up a new graphic page in thebackground of the screen. The RTs and accuracy measureswere based on responses through a Cedrus RB-400Response Box (Cedrus, San Pedro, CA, USA). Subjectswere seated approximately 57.3 cm from the monitor, rest-ing their heads on a head-rest to keep their heads Wxed, withtheir bodies centered with respect to the monitor and thekeys of the switch box. Conformity with these conditionswas conWrmed by the experimenter.
2.3. Gaze processing task
Valid (i.e. gaze direction toward targets) or invalid (i.e.gaze direction away from targets) gaze cues were randomlypresented to either the left or the right visual Weld (LVRand RVF, respectively), alongside presentation of astraight-gazing face in the other visual Weld. A schematicrepresentation of the face was adopted, as in previous stud-ies (Friesen & Kingstone, 1998; Kingstone et al., 2000), tominimize extraneous complexities associated with real faces(e.g. face asymmetry, hair, gender, etc.). The face displayconsisted of a white background with a black line drawingof two round faces subtending 3.6°, located 3.9° away fromthe vertical axis of the screen.The experiments were performed individually. Subjectswere seated in an armchair and instructed to look at themonitor situated in front of them. The sequence of stimuli ineach trial is illustrated in Fig. 1. The start of a trial was sig-naled by a warning alarm and the two faces with blank eyeswere presented simultaneously on the CRT monitor. After675ms, the pupils in either the LVF or RVF were presentedlooking up or down at random. After 200ms, the two faceswere replaced by two target circles presented above or belowuntil a response was made. The stimulus onset asynchrony(SOA) of 200ms was adopted for maximum stability in per-formance as predicted from the results of previous studiesusing diVerent SOAs (Driver et al., 1999; Friesen & King-stone, 1998; Langton & Bruce, 1999), and because this SOAwas appropriate for the unilateral visual Weld presentationparadigm. The inter-trial interval was 675ms.The subjects were instructed to indicate whether the tar-gets appeared above or below the faces by pressing theupper or lower key on the switch box with either the left orright index Wnger. The position of hands in responding wascounterbalanced among subjects. RTs were timed from theonset of target presentation, and measured in milliseconds.At the beginning of the experiments, the subjects weregiven 16 practice trials. After the practice trials, Wve blocksof eight test trials were conducted twice. Then, subjectswere requested to change hands for responding, and givenanother 16 practice trials. After the second block of practicetrials, Wve blocks of eight test trials were conducted twice(all together 160 test trials). The order of the test trials wasrandomized within each block. There was a short break ofabout 15s between blocks of the test and practice trials.Before beginning the test, subjects were informed that itwas important to Wxate their eyes on the central Wxationcross while it was presented, and that the gaze direction wasnot predictive of the location of the targets. They were alsoinstructed to respond as quickly and accurately as possibleto the targets.
2.4. Face perception task
The visual Weld demonstrating superiority in face pro-cessing was examined in each subject. The tasks werealmost identical to those of Gazzaniga and Smylie (1983).
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The stimuli were 20 unfamiliar Japanese faces (10female, 10 male), presented in the LVF or RVF. The faceswere oval shaped, minimizing extraneous clues (e.g., style ofhair, outline of face). Pictures were devoid of apparent dis-tinguishable features such as glasses or facial hair. Eachface subtended 4°£ 8° of visual angle. The nearest edge was3° away from the central Wxation.The subjects, seated in front of a CRT monitor, wereinstructed to look at the Wxation cross, which was black forthe Wrst 2000ms, before turning red 500 ms before stimuluspresentation. Then, the stimulus face was presented for120ms in either the left or right visual Weld at random. Fol-lowing this stimulus presentation, a panel was presentedwith 10 diVerent faces of the same gender. Subjects wereinstructed to select the same face as the stimuli from the setof 10 pictures and responses were recorded. The subjectswere informed that each face might be projected more than
once. They were instructed not to respond when they wereunable to select any face on the panel.Following 10 practice trials, the subjects were given 20test trials. After a rest of about 30 s, another 20 test trialswere conducted. The order of stimulus types was random-ized in each block. Rates of correct response higher bymore than 20% in presentations to one hemisphere than theother was regarded as demonstration of hemispheric supe-riority in face processing.
2.5. Data analysis
All data were analyzed with SPSS ver. 11.0J software(SPSS Japan Inc., Tokyo, Japan). Concerning the gaze pro-cessing task, mean RT of correct responses was Wrst calcu-lated for each experimental condition, excludingmeasurements beyond the mean §2 SD as artifacts. Then,
Fig. 1. Sample sequence of stimuli presented in the gaze-processing task. In this Wgure, the valid gazing-up cue is presented in the left visual Weld.
T. Okada et al. / Brain and Cognition 62 (2006) 128–133131
RT diVerences between valid and invalid conditions werecalculated as the measure of the shift of attention. The RTdiVerences were analyzed using a 2 £ 2 £2 repeated-mea-sures ANOVA with hand of response (left-hand up/right-hand up), visual Weld (LVF/RVF), and position of targets(above/below) as within-subject factors.To conWrm the eVect of hemispheric diVerence on RT,we subsequently analyzed mean RT using a 2 £ 2 repeated-measures ANOVA with visual Weld (LVF/RVF), and valid-ity (valid/invalid) as within-subject factors.Concerning the face processing task, performance forLVF and RVF was compared using Student’s paired t teston the number of correct responses calculated for eachvisual Weld. Then, the degree of hemispheric diVerence wasobtained expressed as follows: (the number of correctresponses in LVF¡the number of correct responses inRVF)/(the total number of correct responses in LVF andRVF)£ 100 (%). The value of this measure ranges from100% to ¡100%. Positive values indicate right hemisphericdominance, and negative values indicate left hemisphericdominance. As stated before, right- and left-hemispheresuperiority groups were deWned as subjects exhibiting val-ues exceeding 20% or less than ¡20% on this measure,respectively.To examine whether the gaze-triggered reXexive shift ofattention was inXuenced by hemispheric dominance in faceprocessing, RT data was re-analyzed using face-processinghemispheric dominance as an additional between-subjectfactor. RT diVerences were analyzed using a 2£ 2 £ 2 £ 2ANOVA with hand of response (left-hand up/right-handup), visual Weld (LVF/RVF), and position of targets (above/below) as within-subject factors, and face-processing hemi-spheric dominance (left hemispheric dominance/right hemi-spheric dominance) as a between-subject factor.An analysis of covariance (ANCOVA) was also con-ducted. The RT diVerences were analyzed using a2 £2 £ 2 £ 2 ANCOVA with hand of response (left-handup/right-hand up), visual Weld (LVF/RVF), and position oftargets (above/below) as within-subject factors, and theface-processing hemispheric-dominance score as the covari-ate.
3. Results
Of the 44 subjects who participated in the study, Wvewere excluded from analysis because of non-compliance tothe instructions (in one), a signiWcant diVerence in visionbetween eyes (in one), and exceptionally slow responses (i.e.beyond mean + 2SD of the whole data set, in three sub-jects). As a result, data from the remaining 39 (7 males and32 females) subjects were analyzed. Preliminary analysesrevealed no signiWcant eVect or interactions regarding gen-der. Therefore, the following analyses were conducted usingdata collapsed across gender.The mean RT Wndings are shown in Fig. 2. Subjectsresponded about 17 and 11 ms faster when the gaze cue waspresented in the LVF and RVF, respectively, in the valid
than invalid condition, which is consistent with results fromprevious studies using a similar paradigm. Error rates werevery small (less than 1%), and there was no apparent speed-accuracy trade-oV. The ANOVA on RT diVerencesrevealed a signiWcant main eVect of visual Weld (F(1,38) D 4.4,p < .05). There were no other signiWcant main eVects orinteractions with visual Weld. These results indicate thatLVF presentation produced larger RT diVerences betweenvalid and invalid stimuli than RVF presentation.The subsequent 2 (visual Weld: LVF/RVF)£ 2 (validity:valid/invalid) ANOVA on mean RT revealed signiWcantinteraction of visual Weld and validity (F(1,38) D 4.4, p < .05),conWrming the above analysis on RT diVerences. The maineVect of validity was also signiWcant (F(1,38)D 134.8,p < .001).In the face processing task, the mean number of correctresponses were 4.0 (SD 2.3) and 4.0 (SD 2.2) for the RVFand LVF presentations, respectively. DiVerence betweenvisual Welds in terms of correct recognition was not signiW-cant (t(38) D 0.1, p > .1, n.s.). Regarding hemispheric domi-nance in face perception (based on the face identiWcationtask), the left and right dominant groups consisted of 9 and11 subjects, respectively, with the remaining 19 showing noapparent hemispheric dominance. Data was then re-ana-lyzed for the 20 subjects with clear hemispheric dominance.The mean RT (SD) was 291.7 (22.6), 306.4 (22.4), 297.9(18.3), and 303.6 (25.2) for valid LVF, invalid LVF, validRVF, and invalid RVF presentations, respectively, in theleft dominance group, and 303.4 (35.8), 320.8 (39.3), 307.8(36.8), 312.8 (31.8), respectively, in the right dominancegroup.The 2 (hand of response: left-hand up/right-hand up)£2(visual Weld: LVF/RVF)£2 (position of targets: above/below)£2 (face-processing hemispheric dominance: lefthemispheric dominance/right hemispheric dominance)ANOVA for RT diVerences showed that the main eVect ofvisual Weld remained signiWcant (F(1,18)D7.4, p<.05) evenwith addition of the extra between-subject factor of face-pro-cessing hemispheric dominance, with no interaction between
Fig. 2. Mean reaction times (with standard error) to valid and invalid cuesin the gaze-processing task.
LVFRVF
mean reaction times (ms) 
validinvalid
132T. Okada et al. / Brain and Cognition 62 (2006) 128–133
visual Weld and face-processing hemispheric dominance. Noother main eVects or interactions were signiWcant.The 2 (hand of response: left-hand up/right-hand up)£ 2(visual Weld: LVF/RVF)£ 2 (position of targets: above/below) ANCOVA for RT diVerences using face-processinghemispheric dominance scores as the covariate showed thatonly the main eVect of visual Weld remained signiWcant(F(1,37) D 4.2, p < .05).
4. Discussion
The current study investigated hemispheric dominancein the reXexive shift of attention in response to perceivedgaze directions, using a target localization task in an RTparadigm. The result showed that the cuing eVect was sig-niWcantly larger for the LVF than RVF presentation, whichwas not attributable to a speed-accuracy trade-oV. Theseresults indicate a right hemispheric dominance in gaze-trig-gered attentional shifts. Right hemispheric superiority ingaze processing has been suggested by previous neuropsy-chological (Campbell, Landis, & Regard, 1986) and neuro-imaging studies (Puce et al., 1998; Wicker et al., 1998). Theresults from those studies, however, were inconclusive, andno previous studies have speciWcally examined hemisphericdominance in the gaze-triggered reXexive shift of attentionin normal subjects. To our knowledge, the present study isthe Wrst to demonstrate right hemispheric dominance in thegaze-triggered reXexive shift of attention in a normalhuman population.It is of interest that the main eVect of visual Weldremained signiWcant even when taking face-processinghemispheric dominance into consideration, indicating righthemispheric dominance in the gaze-triggered reXexive shiftof attention regardless of the hemispheric dominance inface perception. This suggests that the cerebral regions thatprocess gaze directions may not be entirely the same asthose involved in the face processing demanded by the faceidentiWcation task. This appears consistent with recent Wnd-ings that suggest the involvement of diVerent neural sub-strates in gaze processing and face perception. For example,an fMRI study revealed that the fusiform gyrus wasinvolved in face perception (Kanwisher, McDermott, &Chun, 1997) and another fMRI study showed that the STSregion was more active in response to averted gazes thanstraight gazes, whereas the fusiform gyrus did not diVeren-tiate these conditions (HoVman & Haxby, 2000). Such neu-roimaging Wndings (e.g., HoVman & Haxby, 2000) raise thepossibility that the primary neural substrate may be theSTS for gaze processing and the fusiform gyrus for faceperception.There is a study examining two split-brain patientsreporting that the processing of gaze directions and uprightfaces were lateralized to the same hemisphere (Kingstoneet al., 2000). In the study, while gaze directions both withand without other face components produced gaze-trig-gered shift of attention in one, but not the other, hemi-sphere, gaze directions lost their eVect when presented as
part of an inverted face. These Wndings provide evidencepointing to a common laterality for gaze and face process-ing, as well as the import of face components on gaze pro-cessing. Our study, on the other hand, suggests righthemispheric dominance in gaze processing even in (right-handed) subjects who process faces primarily in the lefthemisphere. One possible explanation for this inconsistencyin results of the two studies might be the diVerence in typeof face processing between subjects. In other words, thestrategy for face discrimination might depend, at least inpart, on cultural factors and other demographic variables.Another reason may be the lack of dominance in face iden-tiWcation in a considerable proportion of our subjects (i.e.19 out of 39). This seems to suggest the possibility that,unlike gaze processing of faces, the strategies of face identi-Wcation may be heterogeneous among individuals. There-fore, the Wndings from the present study do not necessarilyexclude the possibility of their portraying a relationshipbetween gaze-triggered attentional shift and a speciWc typeof face perception. In monkeys, neuroanatomical studieshave found a connection between the STS and inferior tem-poral regions (Perrett, Oram, & Ashbridge, 1998; Seltzer &Pandya, 1978), which have been suggested as being homol-ogous to the human STS and fusiform gyrus, respectively.In humans, similar neural or functional connections mightexist between brain regions for processing gaze and otherface components.Our results from the face perception task did not revealright hemispheric dominance. This seems inconsistent withprevious experimental studies on normal subjects (e.g.,Moscovitch & Klein, 1980) and neuropsychological pros-opagnosic patients (e.g., Benton, 1980) reporting righthemispheric dominance in facial recognition. However,other studies have reported bilateral hemispheric involve-ment in face recognition (e.g., Magnussen, Sunde, & Dyr-nes, 1994; Hamsher, Levin, & Benton, 1979), calling forfurther research to resolve such inconsistencies regardinghemispheric asymmetry in face processing.While the current study focused on reXexive responses togaze stimuli, previous functional imaging studies have pri-marily examined the non-reXexive components of process-ing facial parts including the eyes. A lack of joint attentionis a well-documented feature of autism, a pathological con-dition in which abnormality in neural development hasconsistently been noted in the amygdala-limbic regions(Bauman & Kemper, 1985, 1994). Considering these Wnd-ings on autism, there is a possibility that subcortical struc-tures other than the STS and fusiform gyrus (such as theamygdala) may be involved in the reXexive or automaticprocessing of gaze stimuli, warranting functional imagingstudies to clarify the neural mechanisms that mediate gaze-triggered reXexive shifts of attention.
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