humpreys- binding and brian

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ipsilesional stimuli precedes a contralesional one when they are presented simultaneously (RordenMattingley Karnath amp Driver 1997) Report of briefly presented stimuli was better with simulta-neous than with staggered presentations eventhough the contralesional stimulus had to be pre-

sented first in order for the stimulus onsets to beequated phenomenally Hence GKrsquos conscious

judgements of temporal order dissociate from theeffects of temporal simultaneity on stimulus report

There is unconscious binding of visual events by time to facilitate perceptual report

GENERAL METHOD

For Experiments 1ndash3 5 and 6 the stimuli werepresented on an Apple MacIntosh computer usingeither V -scope (Experiments 1 3 5 and 6) orPsyscope (Experiment 2) For Experiment 4 thestimuli appeared on an IBM pentium computerusing MEL software Letters were presented 7 mmfrom their centre to fixation and they were 6 mm x6 mm drawn in Times Roman font The viewingdistance was approximately 80 cm

For Experiment 1 there were four types of dis-play blank trials (no letters present) one-left one-right and two-letters (bilateral one left and oneright) On two-letter displays letters were neverrepeated Letters were drawn from the set A B Cand D Each letterappeared randomly to the left orright of fixation an equal number of times The dif -ferent stimulus durations were presented in sepa-rate trial blocks with there being 72 trials in oneblock 24 two-letter 12 one-left 12 one-right and24 blank trials The trials were randomised within

each block For six stimulus durations there werefourblocks of trials (75300450 600750and 900ms respectively) there were six blocks of trials witha stimulus duration of 150 ms The order of thestimulus durations was randomised GK sat at a

viewing distance of about 80 cm Each trial began with a fixation asterisk for 1500 ms in the centre of the screen GK reported when he was fixated

There followed a blank interval of 150 ms and thenthe stimulus display GK made a verbal letter iden-tification response (either ldquonothingrdquo or the identity

of one or two letters) The experimenter wrotedown the responses and initiated the next trial by pressing the space bar

The procedure for Experiment 2a was the sameexceptthat prior to thetarget display twosymmet-rical figure-of -eight masks appeared along with the

fixation asterisk each centred 7mm from fixation The target letters were L H T or F which couldbe formed by removing contours from the pre-masks The durations were the same as for Experi-ment 1 and there were two blocks of 72 trials foreach duration except 75 ms for which there werefour trial blocksBlockswere presentedin a randomorder For Experiment 2b there was no pre-mask but stimulus displays were followed by a figure-of -eight post-mask With this post-mask the con-

tours of letters (when present) remained in the fieldand were added to by the contours of the mask Theletters were the same as in Experiment 2a but thetask was changed from letter identification todetection GK had to decide whether two one orno letters were presented There was just one stim-ulus duration 150 ms There were three blocks of 72 trials

Experiment3 contained two subexperiments InExperiment 3a the letters A B C and D were

again used There were two task conditions Sametask (identify the letters) and Different task (iden-tify any letters on the left and detect any letters onthe right) There were two blocks of 72 trials foreach task condition (24 blank 24 single-itemmdashoneleft or one rightmdashand 24 two-item trials) with theblocks presented in ABBA order The stimulusduration was 150 ms and no masks were employedIn Experiment 3b the procedure was the sameexcept that only the letters X and O were used rep-licating Goodrich and Ward (1997) On two-itemtrials the letters were identical half the time (eitherOO or XX) and differed on the remaining trials(OX or XO) for both the Same and the Differenttask conditions

In Experiment 4 the procedure was the same asin Experiment 1 except that there were just twodurations 180 ms (five blocks of 72 trials) and 450ms (four blocks of 72 trials) The blocks were pre-sentedin random orderUnlikein theother studiesGKrsquos eye movements were recorded using an

COGNITIVE NEUROPSYCHOLOGY 2002 19 (4) 363

TRANSIENT BINDING BY TIME



ISCAN remote pupil tracker system (resolution03deg of visual angle) Prior to each trial block GKrsquoseye movementswere calibrated with dynamic stim-uli (a circle containing a letter that moved from onelocation to the next GK had to identify the letterpresent when the circle was stationary)

In Experiment 5 the fixation display waschanged to contain three black figure-of -eightpre-masks one at the centre and the other twoeach separated by one letter width from the centralmask in the left and right visual fields respectivelyGK was asked to fixate the central mask After1500 ms contours in the central mask offset tocreate a black target letter Simultaneous with theoffsets in the central pre-mask 0 1 or 2 ldquoonsetrdquotarget letters were presented in the gaps between

the three pre-masks The target lettersweredrawnat random from the set L H T and F In Experi-ment5atherewerefourblocksof72trialsinwhichthere were 24 displays with no new onsets 12 withone onset letter in the left field 12 with one onsetletter in the right field and 24 with two onset let-ters (one right and one left) In blocks 1 and 4 theonset letters appeared in red to enhance theirgrouping In blocks 2 and 3 the onset letters wereblack There was a central offset letter on all trials

The letter display appeared for 300 ms and GK was asked to identify all the letters present irre-spective of their colour or how they were created(by an offset or onset) In Experiment 5b theonset letters were always red and the central lettercreated by the offset of contours from a mask wasagain black In this experiment GK was asked justto identify the black letter This provided a test of

whether poor report of the central target inExperiment 5a was due to lateral masking from

the onset lettersExperiment 6 examined two tasks report of the

temporal orderof redand green lettersand reportof the colours of the letters present For each task asingle block of trials contained three different timerelations between the stimuli when two letterswerepresent In one blockthe letterseither (1) appearedsimultaneously or (2) the left preceded the right by 450 ms or (3) the right preceded the left by 450 msIn theotherblock the stimuli again appearedsimul-taneously or either (1) the left led the right by 720

ms or (2) the right led the left by 720 ms In thetemporal judgement task only displays with twoletters were presented and there were 24 trials foreach of the sequential exposures and 48 trials withsimultaneous presentations The left and right let-ters always differed in colourand they wereeach red

on half the trials and green on the remaining In thecolour identification task the two-letter trials wereaccompanied by 24 one-left and 24 one-right lettertrials with the single target letter being red on half the trials and green on the other The blocks with a450 ms stimulus onset asynchrony (SOA) were runin one session and the blocks with a 720 ms SOA

were run inanotherWithin each sessionthereweretwoblocks for each task (temporal order judgementand colour identification) presented in an ABBA

order Letters were presented following the expo-sure of a central fixation cross for 1500 ms and thepresentation time for the letters was always 345 ms

There was no mask

CASE REPORT

GK was 60 years old at the time of testing Con-secutive strokes in 1986 produced occipito-parietal damage in the right hemisphere and bilat-eral parietal-temporal damage An MRI scan isprovided in Boutsen and Humphreys (2000) GK has a number of neuropsychological deficitsincluding attentional dyslexia (Hall Humphreysamp Cooper 2001) and Balintrsquos syndrome(simultanagnosia and optic ataxia Cooper amp Humphreys 2000 Humphreys Romam OlsonRiddoch amp Duncan 1994) Although there is

generally poor selection of multiple visual stimuliperformance is worse on his left-side There is left

visual neglect and left-sideextinction with relativelong stimulus exposures (Boutsen amp Humphreys2000 Gilchrist et al 1996 Humphreys 1998)

The left-side extinction effect is modulated by grouping based on the form and common surfaceproperties of items in the left and right fields (seeHumphreys 1998 for a review) Despite thesedifficulties GK is well-oriented in time and placeand he has good comprehension

HUMPHREYS ET AL

364 COGNITIVE NEUROPSYCHOLOGY 2002 19 (4)



EXPERIMENT 1 FROM ANTI-EXTINCTION TO EXTINCTION

Experiment 1 examined the time course of extinc-tionand anti-extinctioneffectsby varying theexpo-sure duration of contra- and ipsilesional stimuli in

an identification task On each trial either no oneor two letters were presented with single lettersappearing equally often on the contra- andipsilesional sides

Results and discussion

The results are given in Figure 1 For stimulusexposures under 300 ms there were significantanti-extinction effects There was poor report of

single contralesional letters and better report of

contralesional letters on two-item trials c2(1) =714 501 and 394 all p lt 05 for exposure dura-tions of 75 150 and 300ms respectivelyFor dura-tions of 600 ms and above the opposite patternoccurred There was better report of singlecontralesional letters than of contralesional letters

ontwo-item trials c2

(1) =725148 and 5555all p lt 01 for exposure durations of 600750and 900ms respectively) On two-item trials performanceacross durations 75ndash300 ms was better than thatacross durations 450ndash900 ms c2(1) =552 p lt 02On trials with no letters there were only 2 falsealarms (708720 correct) On trials with singleipsilesional letters there were 21 errors acrossexposures On two-item trials 625 of the errors

were due to report of the right (ipsilesional) letter

onlyOn all trials where two letters were identified



Figure 1 (a) Example displays from Experiment 1 (b) The percentage correct letter identification responses made by GK on trials withone -left one -right or two (bilateral) letters being presented as a function of the exposure durations of the letters



the right was reported first This held across all theexperiments

Experiment1 demonstratestwo clearpatternsof performance which changed as a function of theexposuredurationof target letters With briefexpo-sures (of up to 300 ms or so) there was an anti-

extinction effect There was poor identification of single left-side letters but report of the same lettersimproved when a second ipsilesional (right) stimu-lus was presented simultaneously This mirrors theresults originally reported by Goodrich and Ward(1997) However as the stimulus durationincreased then the pattern of anti-extinctionaltered to one of extinction There was then rela-tively good identification of single stimuli pre-sented in theleftfieldbut poor identification of the

same items when an ipsilesional item appeared atthe same time Theresults at longer durations repli-cate the extinction effect previously documented

with this patient (Boutsen amp Humphreys 2000Gilchrist et al 1996 Humphreys 1998) IndeedGKrsquos performance on two-item trials decreased asthestimulus exposureduration increasedmdasha highly unusual pattern of performance

Prior studieswithnormal observers demonstratethat stimulus onsets can serve as strong cues for

visual attention (Yantis 1998 Yantis amp Jonides1984 1990) In Experiment 1 the stimuli weredefined by simultaneous onsets and it may be thatthese transient signals led to the anti-extinctioneffect by cueing GKrsquos attention to the region occu-pied by both stimuli on a proportion of two-itemtrials There remains to explain why extinctionoccurred subsequently as the stimuli remained inthe field and we will return to this point in Experi-ment 4 Experiment2 however examined whether

common onsets were important by (1) presentingthe stimuli bymeans ofoffsets rather than onsets soremoving any effects due to onsets (Experiment2a) and (2) defining the stimuli by onsets but fol-lowing them with masks designed so that the con-toursof the letter targets did notoffset (Experiment2b) We ask does the absence of common onsetsdisrupt the effect (ie are common onsets neces-sary) and does the effect still arise when the stim-uli do not offset together (ie are common offsetsnecessary)

EXPERIMENT 2 THE EFFECTS OFCOMMON OFFSET

Experiment 2a Stimuli defined by offsets

Method

The method was the same as for Experiment 1except that the stimuli were defined by offsetsrather than by onsets The procedure is outlined inFigure 2a


The results are presented in Figure 2b There wasno evidence for anti-extinction for durations below 300ms c2 lt 10 forall comparisons between singleleft and two-item trials In contrast there was a

clear extinction effect at longer exposures (450 msand above) c2(1) =1033701484and 1997all p lt 05 for durations 450 600 750 and 900 msrespectivelyOn two-item trials GK identified only the right-side letter on 73 of the trials On no-item trials there were just 1 false alarms (380384correct)

Unlike in Experiment 1 there was no clear evi-dence for anti-extinction at the shorter stimulusexposuresherePerformance on two-itemtrialswas

no longer better than on trials with single left tar-gets This suggests that common onsets may benecessary to generate anti-extinction (in Experi-ment 1 but not Experiment 2a) In contrast to thiscommonoffsets seem notto be effective in isolation(commonoffsets are not sufficient) Experiment2bsought to test the effects of stimulus offset in moredetail by presenting new stimuli again that weredefined by common onset (as in Experiment 1) butnow replacing these stimuli with post-display

masks The post-display masks contained the fea-tures present in the letters plus additional featuresthat fell around the same spatial area When suchmasks are used there is no offset of contours thatuniquely make up each letter However it is thecase that the post-masks used here make perform-ance more difficult relative to when no masks areemployed (Experiments 1 and 2a) Consequentlythetask changed from letter identification to detec-tion (2 vs 1 vs 0 items on a trial) If the anti-extinction effect is due to stimulus offsets occurring

HUMPHREYS ET AL




close in time to onsets (under the short exposureconditions of Experiment 1) then theeffect shouldnot be found here Just one short exposure duration

was used 150 ms

Experiment 2b Eliminating common offsets

Method

The procedure is outlined in Figure 3a


The data are given in Figure 3b There was a reli-able anti-extinction effect single left-side items

were detected worse than both letters on two-itemtrials c2(1) =567 p lt 025 On single-right itemtrials there were 14 errors all due to no-itemresponses There were 6 false alarms on blank trials

Experiment 2b confirmed the anti-extinctioneffect with shortdurations in a detectiontask whentarget letters onset but did not offset togetherFrom this we conclude that common offsets are notnecessary for anti-extinction (though they still play a contributory role when common onsets are pres-ent) In contrast to this Experiment 2a demon-strated that the effect was eliminated when stimuli

were defined by offsets The results suggest thatcommon onsets are critical

Goodrich and Ward (1997) when first definingthe anti-extinction phenomenon did not considerstimulus factors such as common onsets in greatdetail Instead they suggested that the effect wascaused by response priming report of thecontralesional stimulus was supported by the sameresponse being made to the ipsilesional item Wetested this in Experiment 3 GK either had to iden-



Figure 2 (a) Example displays from Experiment 2a (targets defined by offsets) (b) The percentage correct letter identifications made by GK in Experiment 2a



tify the letters on a trial (same task for both letterson two-item trials) or he had to identify the left-side letter and detect the right-side one (present orabsent different task for both letters on two-itemtrials) According to the response priming accountanti-extinction should occur in the same-task butnot the different-task condition

EXPERIMENT 3 THE EFFECTS OFRESPONSE LINKAGE

Experiment3 contained two substudies In Experi-ment 3a we used the same letters as in the earlierstudies with two different letters always beingemployed on each trial In Experiment 3b weemployed a procedure more directly modelled onGoodrich and Wardrsquos (1997) study where on two-item trials the letterswere eitherthesame or differ-

ent (XX or OO vs OX or XO) Goodrich and Ward reporteddifferent effects of item similarity inthe Same and Different task conditions In theSame task condition (identify both letters) perfor-mance was better when the letters were different(OX or XO) than when they were the same (OO or

XX see also Baylis Driver amp Rafal 1993) In theDifferent task condition (detect right identify left

letter) item similaritydidnotmatterThecontrast-ing effects of item similarity provides convergingevidence for contrasting response strategies in theSame and Different task conditions

Experiment 3a Different letters

Method

The method was the same as in Experiment 1except for the change to a detection response forright-side item in the differentresponse condition

HUMPHREYS ET AL


Figure 3 (a) Example displays from Experiment 2b (targets followed by masks to prevent contour offsets) (b) Percentage correct detectionresponses in Experiment 2b



Results

The results are illustrated in Figure 4a The data were subjected to log-linear analysis with the fac-tors being response (same vs different) number of items (one-left vs two-items) and accuracy (num-ber correct vs error) There were proportionately

more correct to error responses on two-item com-pared with one-left trials c2(1) = 705 p lt 01

There was also a trend for proportionately morecorrect responses when different tasks were per-formed on left- and right-field stimuli relative to

whenthe same task was performed c2(1) =124 p gt05 There was no evidence of an interactionbetween number of items and the task factor or forathree- wayinteraction (bothc2 lt 10) Single-rightletters were both identified (same task condition)

and detected accurately (different task condition) There were 2 false alarmson blank trials (9496)

Experiment 3b Same and different letters

Method This was the same as for Experiment 3a exceptthat on two-item trials the letters were the sameon half the trials (OO or XX) and different on theremainder (OX or XO)

Results

The results are presented in Figure 4b The data were again subject to a log linear with the factors

being response (same vs different) number of items (one- left vs two-items) and accuracy (cor-rect or error) There was a reliable interactionbetween number of items and the proportion of correct to error responses c2(1) =949 p lt 01 anda trend for an interaction between response andaccuracy c2(1) = 235 p gt 010 There were rela-tively more correct to error responses on two-itemtrials compared with single-left trials There alsotended to be proportionately more correct

responses on different- than on same-response tri-als There was no evidence for an interactionbetween response and number of items or for athree- way interactionbetween response numberof items and accuracy (both c2 lt 10) The advantagefor two-item over single-left trials held irrespectiveof whether the same or a different response was car-ried out on letters in the left and right fields There

was no difference in the accuracy of responses tosingle-right letters in the two response conditions

(c2 lt 10) There was just one false alarm on blank trials (9596 correct)

Performance on two-item trials was brokendown according to whether the letters were thesame (OO or XX) or different (OX or XO) Onsame-response trials there were 324 correctresponses to two identical letters and 1524 to twodifferent letters On different-response trials there

were 1124 correct responses on trials with identicalletters and 1324 correct responses on trials withdifferent letters The effect of item similarity



Figure 4 (a) Percentage correct responses in Experiment 3a In the same task condition GK made identification responses to left and right -side letters In the different task condition GK identified anyleft -side letters and made a detection response to any right -side letters The letters on each trial were always different (b)Percentage correct responses in Experiment 3b The procedure was

the same as in Experiment 3a except that the letters were identical on half the trials and different on the remaining



tendedtobelargeronsame-response trials (identify both the left and right letters) than on different-response trials (identify the left letter and detect theright) c2(1) =275 p gt 05 On same-response tri-alsperformancewasbetterwith different than withidentical letters c2(1) = 1076 p lt 01 On differ-

ent-response trials letter identity had no significanteffect (c2 lt 10) On different-response trials 19errors were due to GK only detecting the right-sideletter on the remaining 5 trials he detected theright-side letter and assigned the wrong identity tothe left letter (3 onsame-identity trials and 2 on dif -ferent-identity trials) On same-response trials GK made 20 errors by identifying only the right letteron the remaining 10 trials he identified the right-side letter and misidentified the left letter There

were nine such misidentifications on same-identity trials and one on different-identity trials This sug-gests that GK was biased to guess that the identity of the left letterwas different to that of the right let-ter when he had to assign identities to both items(on same-response trials)

Discussion

In both Experiments 3a and 3b we observed ananti-extinction effect left items were identified

more accurately on two-item trials than on single-left trials Also in both experiments this effect wasalmost identical whenthe same (identification) task

was performed on left- and right-field items as when different tasks were used (identify left anddetect right) There was no evidence for a responsepriming effect In Experiment3a it mightbearguedthat GK adopted the same response strategy irre-spective of whether different tasks were formally requiredto left-and right-sidestimuliHowever in

Experiment3b we found contrasting effects of itemsimilarity on same- and different-response trials

with item similarity only affecting performance onsame-response trials This provides converging evi-dence that GK did adopt different response proto-colsunder thetwotask conditionsThefinding thatidentification performance was betterwhen the let-ters differed relative to when they were the samematches the pattern of data reported previously by

Baylis et al (1993) and GoodrichandWard (1997)Baylis et al interpreted their results as indicatingimpaired formation of ldquoobject tokensrdquo when stim-uli have the same response-relevant feature How -ever the error patterns shown by GK suggest thathis results could reflect a response bias away from

giving letters the same identity Nevertheless thisbias was not apparent in the different-task condi-tion There was a shift in the response strategy ondifferent- compared with same-response trials

So far wehave presented evidence that two stim-ulus factors are important for the anti-extinctioneffect with GK stimulus exposure and commononset of the items There was no effect of responselinkage The effects of common onsets may arisebecause of at least three factors (1) the onsets may

cue attention to a spatial area subtended by the let-ters enabling left- as well as right-side items to bereported (Yantis amp Jonides 1990) (2) the onsetsmay increase arousal and this enables both lettersto be reported more accurately than when only asingle-left stimulus appears (cf Robertson amp Manley 1999)or (3) lettersthanonset togetherarebound by their common temporal signal (Singer amp Gray 1995) and this form of grouping enables theleft as well as the right letter to be identified In

Experiment 5 we return to provide an explicit testbetween thesedifferent accounts In Experiment4though we explore one factor that might underliethe effects of stimulus exposure on GKrsquos perfor-mance Whether identification changes at longerexposure durations because GK then makes an eyemovement to the right item (on two-item trials) Itis possible that eye movements are responsible forthis shift from anti-extinction to extinction Forexample GK may show extinction at long stimulus

durations because of a bias to make a right eyemovementwhich then reduceshis identificationof left-side items when two letters are present Shortexposuresmay reduce the likelihoodthat eye move-ments are made enabling an anti-extinction effectto emerge To test this in Experiment 4 GK received blocks of trials with stimulus durations of 180 and 450 ms during which we recorded his eyemovements

HUMPHREYS ET AL




EXPERIMENT 4 THE EFFECTS OFEYE MOVEMENTS

Method

The method was the same as that employed inExperiment 1 except that GKrsquos eye movements

were recorded whilst he undertook the task Letteridentification performance was scored according to

whether a rightwards eye movement was made whilst the stimulus was displayed


Data were summed across the two exposure dura-tions since they were similar in both cases In Fig-

ure 5 we present the number of correct letteridentifications as a function of trials where either arightwards eye movement occurred (Figure 5a) orthere was no eye movement (Figure 5b) There wasa rightwards eye movement during the stimuluspresentation on 106540 (20) of the trials and noeye movement on 409 trials (76) There were just25 trials where a left eye movement occurred (lessthan 5 of the time) Trials with left eye move-ments were not analysed further

There was a reliable anti-extinction effect bothon trials where no eye movement occurred and ontrials where a rightwards eye movement was initi-ated For trials without an eye movement c2(1) =452 for one-left trials (685) vs two-item correcttrials (28156) For trials with a rightwards eyemovement Fisher exact probability = 03 for one-left (017) vs two-item correct trials (1149) Onblank trials there were 9 falsealarm errorson trials

without an eye movement (7380 correct) and 15

errors (1720 correct) when a rightwards eyemove-ment occurred On one-right trials performance

was slightly better when there was a rightwards eyemovement relative to when no eye movement took place (1220mdash60 vs 4180mdash51)

The results were quite clear Theanti-extinctioneffect survives even when an eye movement is initi-ated to the right during stimulus presentationHence the time course of performance the change

fromanti-extinctiontoextinctionisnotsimplydueto a change in the probability of an eye movementoccurring Whatever the factor is that determinesthe better reportof the left letteron two-item trialsit survives an eye movement being made to theright

EXPERIMENT 5 TEMPORALBINDING ATTENTIONAL CUEINGOR AROUSAL

Experiments 1ndash4 reveal a reliable anti-extinctioneffect when left- and right-field stimuli onsettogether and are presented for a short duration Wehave discussed three accounts of the onset effectthat it is due to (1) cueing attention to a common



Figure 5 (a) Percentage correct letter identifications made ontrials when a rightwards eye movement occurred when letters were present (b) Percentage correct letter identifications made on trials

when no eye movements occurred whilst the letters were exposed





central item to suffer more on two-item than onone-right trials More critical is to assess report of the central letter on two-item and one-left trials

when the left onset letter was identified Theldquospreading attentionrdquo and arousal accounts main-tain that the central lettershouldtypically be identi-fied prior to the left-side letter hence report of the

central letter should be good on trials on which theleft onset letter is identified This should hold forone- and two-item trials alike Contrary to thisaccount identification of the central letterremained better on one-item than on two-itemtrials even when theleft letterwas identified (88vs

233 trials Fisher exact probability p = 000) Onthe one-item trials when the left letter was identi-fied GK always reported the central letter first (88) On the two-items trials when the left letter wasidentified he never identified the central letterbefore the left letter (033) On two-item trialsreportof thecentral letterdidnotdiffer whether theonset letterswere red (1 correct)orblack (1 correct)

with the left letter identified The data indicate that on one-item trials GK

identified the central letter on all occasions wherehe able to name the left-side target Also on all of these trials GK identified the central letter beforethe left letterwasreportedIncontrast on two-itemtrials GK was poor at reporting the central offsetletter even when he identified the left onset lettercorrectly He then also always reported the left let-ter before thecentral oneThese data are consistent

with the binding account rather than theattentional cueing and arousal accounts According

to the binding account the anti-extinction effectoccurs because the left and right letters are groupedby common onset GK is biased to select therightmost items for report and in this case there isselection of the onset group prior to the central let-tersince members of theonset groupfall in his rightfieldThis leads to the left-side letter being selectedprior to the central letter and to it being identifiedeven when the central letter is not reported In con-trast there was no evidence for attention spreading

across the spatial region covering the two onsets(due either to cueing visual attentionor to increasedarousal) If attention simply spread across theregion GKrsquos tendency to report from right-to-left

wouldlead himto reportingthe central letterbeforethe left On two-item trials where he identifies theleft letter he should also typically identify the cen-tral letter He did notmdasheven though this was thepattern on single-left trials

Other accounts of the data from Experiment 5aare not easy to sustain One possibility is that GK



Figure 6 (a) Example displays from Experiment 5 with ldquooffsetrdquo as well as ldquoonsetrdquo target letters (b) Percentage correct identificationof the onset letters (c) Percentage correct identification of the central offset letter as a function of the whether no one -left one -right or two onset letters were presented



opted to report the peripheral onsets before thecentral offset letter perhaps because of anattentional bias to stimuli created by onsets ratherthan offsets (eg Yantis 1998) However we

would expect this bias to hold across the one- andtwo-item trials here which occurred randomly

This was not the case GK did not identify a singleleft onset letter before the central offset letter Amore viable possibility is that the central letter wasdifficult to report on two-onset trials because of masking from the onsets1 A single left onset may not generate such a strong masking effect enablingthe central letter then to be reported before the leftflanker To test this we ran Experiment 5b Thisstudy was the same as Experiment 5a except thatGK was asked just to report the central letter To

help himselect thecentral item it always differed incolour from theflanking onset letters The maskingeffect should be caused by the presentation of theonsets and so it should occur evenwithout reportof the flankers On the other hand if the relatively poor report of the central letter was due to GK selecting the two flankers for report then this effectshould be lessened here

Experiment 5b Reporting only the central black letter

Method

This was the same as for Experiment 5a except thatthe central letter was always black and the flankersred

Results

Figure 7 shows the number of correct reports of thecentral black letter as a function of whether no

(blank) one-leftone-rightor twoonsetsoccurredUnlike Experiment5a there was only a small non-significant trend for identification of thecentral let-ter to be worse on two onset trials relative to trials in

which only a single left flanker was presented (c2 lt10) On one-left trials GK made 114 error by reporting the left letter rather than the right letterOn one-right trials there were 719 errors due toreporting the right letter rather than the left letter

and there were reports of the right rather than thecentral letter on 1036 of the errors on trials with

two flankers On all such occasions GK maintainedthat he reported the black letter Prior studies havedemonstrated that GK is susceptible to illusory conjunctions of letters and colour (HumphreysCinelWolfe Olson amp Klempen 2000) and theseincorrect letter reports may reflect illusory bindingof the central colour with the identity of a flankerletter More importantly for our present purposesthe decreased report of the central item was lessapparent here than when flanker letters had to be

selected for report (Experiment 5a) The decreasedreport on the central letter in the two-item condi-tion of Experiment 5a does not seem to be due tolateral masking

EXPERIMENT 6 TEMPORAL ORDER JUDGEMENTS AND SEQUENTIALLETTER PRESENTATIONS

Experiments 1ndash5 have demonstrated an anti-extinction effect where under brief exposure con-ditions report of a left-side letter was facilitated by simultaneouspresentation of a right-sideletterWehave proposed that anti-extinction is caused by temporal binding based on common onset of theletters According to this account the stimuli may need to occur simultaneously in order for the anti-extinction effect to occur We examined this possi-


HUMPHREYS ET AL

Figure 7 The percentage of correct identifications of the central offset letter as a function of whether no one -left one -right or twoonset letters occurred in Experiment 5b

1 Our thanks to Rob Ward for this suggestion



bility in Experiment 6 In so doing we also evalu-ated therelations between theanti-extinction effectand GKrsquos conscious judgements of when onsets

were simultaneous To do this we compared GKrsquosidentification and temporal order judgementsRorden et al (1997) demonstrated that patients

with extinction frequently show biased temporalorder judgements so that when ipsi- andcontralesional stimuli are presented simulta-neously they judge that the ipsilesional item pre-cedes the contralesional one In order for ipsi- andcontralesional events to be judged as simultaneousthe contralesional event needs to lead theipsilesional oneby some time periodthe phenome-non of ldquoprior entryrdquo If GK manifested prior entrythen we expected him to be poor at judging that

simultaneously occurring letters did onset at thesame time and that we would need to present theleft letter first for judgements of simultaneity to bemade In contrast the report of briefly presentedleft letters may be best under simultaneous presen-tations There may be a dissociation between GKrsquosconscious perceptionof simultaneity and theeffectsof simultaneous onsets on letter identification This

would suggest that GK is not conscious of temporalbinding even though it affects his conscious letter

report

Method

The procedure is outlined in the General MethodFor the temporal order judgements GK was pre-sented with one red and one green letter on eachtrial and he had to decide which colour appearedfirst We required report of the temporal order of the colours rather than of the left and right letters

in order to eliminate possible response biasesfavouring theright-side letterThe letterswerepre-sented simultaneously or they were staggered intimemdashthe left preceding or following the right by 450 ms or by 720 ms (presented in separate trialblocks) Tomatchtheattributesbeing reported theidentification task required GK to report thecolours present The two-item trials were the sameas for the temporal order judgement task but forthe identification task there were also single colourtrials (one-left or one-right) Within a block one

third of the trials had simultaneous presentationson another third the left letter preceded the rightone and on the remaining third the right precededthe left


The results for the temporal order judgements arepresented in Figure 8a and Figure 8b gives the per-centage of correct colour identifications For colouridentification there was an anti-extinction effect

when the stimuli appeared simultaneously (reportontwo-item trials was better thanon one-lefttrialsc2(1) = 671 p lt 01 There was no reliable effect

when the onsets of the letters were staggered (c2 lt10 for left-first and for right-first) In the temporal

order judgement task though GK showed a strongeffect of ldquoprior entryrdquo judging that the right letterappeared before the left on the vast majority of tri-als This tendency varied across the different tem-poral presentation conditions but even when theleft letter led the right one by 720 ms GK reportedthat the right came first on over half the trials

Experiment 5 shows that the anti-extinctioneffect occurred when right and left letters appearedsimultaneously but not when they were staggered

in time In contrast under these same conditionsGK consistently judged that the right letter waspresented first and the left letter had to precede by at least 720 ms for the onsets to be judged as simul-taneous The effects of time on anti-extinction dis-sociated from the effects on conscious temporalorder judgements

Across Experiments 1ndash5 when the onset lettersalways had the same colour the advantage variedfrom 12ndash22 for the identification of left letters on

two-item relative to one-left trials In Experiment6 when the onset letters differed in colour theadvantage was 145 The colour differencebetweenthelettershadlittleinfluenceontheeffect

GENERAL DISCUSSION

We have demonstrated a reliable anti-extinctioneffect when left and right stimuli are presentedsimultaneously for brief exposure durations in





HUMPHREYS ET AL


Figure 8 (a) Percentage of trials on which GK reported that the letter on the right appeared first as a function of the stimulus onset asynchrony (SOA) between the left and right letters (b) Percentage of correct colour identifications across the different SOAs and on one -left and one -right letter trials + indicates that the left letter preceded the right letter ndash indicates the opposite



patient GK who has a bias favouring right-sidestimuli after bilateral parietal damage The effect isdependent on the stimuli having simultaneousonsets (Experiments 2 and 6) and is unaffected by eliminating the common offset of the stimuli(Experiment2b)by varying the tasks performedon

left and right-side items (Experiment 3) and by arightwards eye movement occurring whilst stimuliare exposed (Experiment 4) The anti-extinctioneffect reduced as the time since stimulus onsetincreased being replaced by left-side extinction atlonger stimulus durations (Experiments 1 and 2) Italso dissociated fromGKrsquosconscious judgementsof temporal order the anti-extinction effect occurred

when letters appeared simultaneously whereas leftletters had to precede right letters by 720 ms or

more before GK tended to judge that the lettersappeared simultaneously (Experiment 6)

Experiment 5 provided evidence that the anti-extinction effect was due to temporal bindingrather than to increased arousal or due to a spreadof spatial attention across a region activated by theleft and right stimuli In that study we presented acentral letter defined by an offset between the leftand right letters which were defined by onsets

We found that on two-item trials where GK

reported the left as well as the right letter he waspoor at identifying the central letter On one-lefttrials however he could report the central letter inaddition to the left one (indeed he identified thecentral one first) This result was found both whenthe onset and offset letters had the same coloursand when they had different colours so that col-our grouping did not appear important We con-clude that GK did not attend to the regionsubtended by the left and right onset letters and

report the letters there in right to left orderInstead the left and right letters were grouped by common onset and reported together prior to thecentral letter being selected Consistent with thisthere was not a reliable drop in performance ontwo-item relative to one-left trials when only thecentral letter had to be identified when selectionof the flankers was not required (Experiment 5b)Identification of the central letter dropped partic-ularly when onset letters were selected for aresponse

These results suggest that there can be groupingbased on common onset between visual stimuli

This form of binding dissociates from GKrsquos con-scious judgements about the temporal order of occurrence of the stimuli (Experiment 6) Thus thetemporal code that ldquobindsrdquo the left and right letters

was not consciously available to GK The findingsare consistent with the notion that that temporalcodesare important for binding in vision (Eckhorn1999 Singer amp Gray 1995) and with one signalfor binding being based on actual (rather than per-ceived) synchronous onsets (Elliott amp Muller1998 Fahle 1993) Once bound both left andright stimuli could be selected for report even whenan eye movement was first made to the item on theright side

Our evidence alsoindicatesthatbindingbycom-mon onset produces only a transient influence on

visual selection Thus the anti-extinction effectlasted for stimuli presented for up to 400ndash500 msbefore being replaced by extinction as the exposuredurationofthestimuliincreasedWithlongerexpo-sures single left letters couldbe identified but there

was then biased selection favouring the right letterontwo-item trials How can we explain this shift toan extinction effect at longer stimulus durations

To account for the effects of stimulus duration we suggest that there is a decay in the temporal tagthat binds the right and left stimuli even thoughthelettersmay remain present in thefieldThis ideais illustrated in Figure 9 in which we separate twoforms of binding a temporary effect based on co-occurrence of a transient signal generated by theonsetsof thestimuli (perhaps also reinforced by off -sets when offsets occur relatively close in time tothe onsets) and a longer-lasting effect based on

sustained (form and colour) information derivedfrom stimuli In other studies with the patienttestedhere GKwe have shownthat the magnitudeof extinction (at longer stimulus durations) can bemodulated by grouping based on the form and sur-face properties of objects (Boutsen amp Humphreys2000 Gilchrist et al 1996 Humphreys 1998)

The anti-extinction effect though was littleaffected by whether onset letters had the same ordifferent colours (Experiment 1ndash5 vs Experiment6) This is consistent with different forms of bind-





ing between visual stimuli operating across con-trasting time courses There is transient bindingbetween stimuli thathave synchronous onsets Thismay provide the visual system with a ldquoquick butdirtyrdquo representation of the world useful perhapsfor fast actions However since stimuli that onsettogethermay have different form and surface prop-erties binding by common onset may beof littleusefor object recognition In order to construct a moreaccurate representation the transient binding may be replaced over time by a more sustained bindingprocess sensitive to form and surface-based simi-larities between stimuli

To account for the extinction effect at longerduration we propose that GK has a spatial bias inusing the sustained information for selectionThusif two stimuli are presented bilaterally for relatively long durations (long enough to establish the

sustained signal) GKtends to select theitemontheright before the one on the left generating anextinction effect The exception to this is if thestimuli group by form or surface property in whichcase they may be selected together (Boutsen amp Humphreys 2000 Gilchrist et al 1996Humphreys 1998)Due to this spatial bias inusing

sustained information from a stimulus the anti-extinctioneffect is replacedover time byextinction

If there is temporary binding based on commononsets of stimuli we might ask why anti-extinctioneffects have not been found more frequently inpatients Theremay be several reasons for this Oneis that in patients showing poor report of singlecontralesional items experimenters may not exam-ine performance with double simultaneous stimu-lation due to an assumption that two-item trials

would in any case be moredifficult than single-item

HUMPHREYS ET AL


Figure 9 An illustration of two proposed neural signals for binding in vision a transient signal that links stimuli having common onsetsand a more sustained signal determined by the form and surface details of objects Note that the strength of the transient signal decreases over time even if the stimulus remains in the field



trials As we have demonstrated this assumption isnot necessarily valid A second reason relates to themagnitude of the effects The differences betweentwo-item and one-left trials reported here (and alsoby Goodrich amp Ward 1997)are quite subtle with abenefit of about 15 on two-item trials It is possi-

ble that such a small difference may not be detected when small numbers of trials are collected A thirdpossibility is that other patients may have morerapid development of sustained information fromthe stimulus than is the case for GK and they toomay have a spatial bias in using the sustained infor-mationConsequently theremaynotbea sufficienttime window for anti-extinction to be demon-strated before extinction effects emerge A fourthsuggestion is that such effects depend on

subcortical structures sensitive to dynamic changesin visual stimulation Effects at this level may only become apparent in patients like GK with rela-tively large lesions who are impaired at respondingusing cortical mechanisms at least under brief exposure conditions In the only other case of anti-extinction reported previously Goodrich and

Ward (1997) argued that the effect was caused by responsepriming The contralesional item could berecoveredif its responsewas primedby the response

to the ipislesional item However we found no evi-dence for this (Experiment 3) It may be that differ-ent types of anti-extinction exist though we notethat Goodrich and Wardrsquos patient continued toshow a trend for anti-extinction even on trials

wheredifferent responses were requiredto ipsi-andcontralesional stimuli so that factors additional toresponse priming may also have played a roleClearly future work needs to assess the factors thatcan determine anti-extinction in other patients

Interestingly Rorden Simon Gore and Baylis(2001) have recently reported another dissociationbetween perceptual report and conscious identifi-cation of the temporal relations between stimuli inthe phenomenon of extinction They found thatextinction was maximal when stimuli appearedsimultaneously and decreased as the temporalinterval between ipsi- and contralesional itemsincreased (see also Di Pellegrino Basso amp Frassinetti 1997) However all the patients testedalso showed a prior entry effect with the

contralesional item having to precede theipsilesional one in order for them to be judged asoccurring simultaneously It may be that any com-petition for selectionbetween developing sustainedcodes for ipsi- and contralesional stimuli is maxi-mised when the codes begin to develop at the same

time (with simultaneous stimulus exposures) or itmay be that this competition is pronounced whenthere is an initial temporal tag to bind the stimuli

Whichever is the case their result suggests againthat the effect of time on perceptual report is inde-pendent of conscious awareness of the temporalrelations between events

Manuscript received 5 June 2001Revised manuscript received 7 November 2001

Revised manuscript accepted 21 November 2001

REFERENCES

Baylis G Driver J amp Rafal RD (1993) Visualextinction and stimulus repetition Journal of Cogni -tive Neuroscience 5 453ndash466

Boutsen L amp Humphreys GW (2000) Axis-based

grouping reduces visual extinction Neuropsychologia38 896ndash905

Cooper ACG amp Humphreys GW (2000) Codingspace within but not between objects Evidence fromBalintrsquos syndrome Neuropsychologia 38 723ndash733

Di Pellegrino G Basso A amp Frassinetti F (1997)Extinction to double asynchronous stimulation Neuropsychologia 35 1215ndash1223

Duncan J Humphreys GW amp Ward R (1997)Competitive brain activity in visual attention Current Opinion in Neurobiology 7 255ndash261

Eckhorn R (1999)Neural mechanisms of visual featurebinding investigated with microelectrodes and mod-els Visual Cognition 6 231ndash266

Elliott MA amp Muller HM (1998) Synchronousinformation presented in 40Hz flicker enhances visual feature binding Psychological Science 9 277ndash283

Fahle M (1993) Figure-ground discrimination fromtemporal information Proceedings of the Royal SocietyB254 199ndash203

Gilchrist I Humphreys GW amp Riddoch MJ(1996) Grouping and extinction Evidence for low -





level modulation of selection Cognitive Neuro- psychology 13 1223ndash1256

GoodrichSJ amp Ward R (1997)Anti-extinction fol-lowing unilateral parietal damage Cognitive Neuropsychology 14 595ndash612

Hall DA Humphreys GW amp Cooper ACG(2001)Neuropsychologicalevidence for case-specificreading Multiletter units in visual word recognitionQuarterly Journal of Experimental Psychology 54A 439ndash467

Heinke D amp Humphreys GW (in press) Attentionspatial representation and visual neglect Simulatingemergent attention and spatial memory in theSelective Attention for Identification Model(SAIM) Psychological Review

Humphreys GW (1998) Neural representation of objects in space A dual coding account Philosophical

Transactions of the Royal Society B353 1341ndash1352Humphreys GW Cinel C Wolfe J Olson A amp Klempen N (2000) Fractionating the binding pro-cess Neuropsychological evidence distinguishingbinding of form from binding of surface featuresVision Research 40 1569ndash1596

Humphreys GW Romani C Olson A RiddochMJ amp Duncan J (1994) Non-spatial extinctionfollowing lesions of the parietal lobe in humans Nature 372 357ndash359

Karnath H-O (1988)Deficitsof attention in acute and

recovered visual hemi-neglect Neuropsychologia 2627ndash43

Mozer MC amp Behrmann M (1990) On the interac-tion of selective attention and lexical knowledge Aconnectionist account of neglect dyslexia Journal of Cognitive Neuroscience 2 96ndash123

Robertson IH amp Manley T (1999) Sustained atten-tion deficits in time and space In GW Humphreys J Duncan amp A Treisman (Eds) Attention space and action Studies in cognitive neuroscience (pp 297ndash310)Oxford Oxford University Press

Rorden C Mattingley JB Karnath HO amp Driver J (1997) Visual extinction and prior entry Impairedperception of temporal order with intact motion per-ception after parietal injury Neuropsychologia 35 421ndash433

Rorden C Simon SL Gore C amp Baylis GC(2001) Visual attention A biased competition Paperpresented to the South Carolina Symposium on

Attention University of South Carolina USAMaySinger W amp Gray CM (1995)Visual feature integra-tion and the temporal correlation hypothesis Annual Review of Neuroscience 18 555ndash586

Yantis S (1998) Objects attention and perceptualexperience In RD Wright (Ed) Visual attention(pp 187ndash214) Oxford Oxford University Press

Yantis S amp Jonides J (1984) Abrupt visual onsets andselective attention Evidence from visual search Jour -nal of Experimental Psychology Human Perception and Performance 10 601ndash621

Yantis S amp Jonides J (1990) Abrupt visual onsets andselective attention Voluntary versus automatic allo-cation Journal of Experimental Psychology HumanPerception and Performance 16 121ndash134

HUMPHREYS ET AL









GENERAL METHOD




























There was no mask

CASE REPORT





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humpreys- binding and brian

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