Visual Neglect can be Object-Based or Scene-Based Depending on Task Representation

INTRODUCTION

Visual neglect is a relatively common deficitfollowing unilateral brain damage (Bisiach andVallar, 1988; Halligan and Marshall, 1993; Vallarand Perani, 1986). A patient with this syndromewill fail to notice, or respond to, items on the sideopposite to their lesion. Thus a person with damageto their right hemisphere may not shave the leftside of their face, or may not respond to a personapproaching them from their left. This syndromemay result from unilateral damage to manydifferent parts of the brain, including the frontallobe, basal ganglia, and thalamus (Damasio et al.,1980; Halligan and Marshall, 1993; Watson andHeilman, 1979; Watson et al., 1981; Weinstein,1994). Most commonly, however, lesions to theparietal lobe, typically involving the temporo-parietal junction are associated with visualhemineglect (Baylis et al., 1993, 2001; Freidrich etal., 1998; Karnath et al., 2001; Marshall et al.,2002).

An important insight into the attentional natureof visual neglect came from Bisiach and Luzzatti(1978), suggesting that visual neglect may apply torepresentation rather than sensation, perception, ormemory. We propose that the notion ofrepresentational neglect provides a framework forunderstanding visual neglect, and resolving animportant controversy in research on visual neglect- what is the reference frame of neglect? This keyquestion concerns what defines the spatial extent ofthe neglected region. Do patients with lesions totheir right hemisphere neglect everything to the lefttheir body? Or is it information to the left of thehead, or to the left of gaze? In fact evidence for allof these influences have been found. For example,it has been shown (Karnath et al., 1991) that thepositions of targets with respect to the retinal

midline, the head midline and the trunk axis allaffected target detection latency. Thus the longestreaction times were found for targets that were tothe left of all of these midlines, and the shortest fortargets to the right of all midlines. Although thesedifferent reference frames were centered differently,they were all oriented similarly in the upright axis.In other words all these reference frames could beconsidered as obeying a gravitationally definedupright axis.

Farah and colleagues (1990; see also Ladavas,1987) investigated whether the axis defining thereference frame of neglect was always upright (i.e.,defined by gravity). They dissociated the axis ofthe patient’s body from a gravity-defined uprightby having patients lie on one side, and dissociatedboth of these from object-based axes by rotatingobjects with respect to the patient. Their resultssuggest that targets to the left as defined by agravitational reference-frame were missed more.However, probably the most important factor indetermining neglect was whether items were on theleft of the patient, regardless of the patient’sorientation. Further studies have confirmed theimportance of gravity-based reference frames inneglect (Ladavas, 1987). Thus evidence exists tosuggest that neglect can represent a deficit within ascene-based, or various body-based referenceframes.

Conversely, evidence for object-based visualneglect has been found in a number of studies. Forexample, patients with neglect tended to misstargets on the left side of objects, even when thosetargets were in the right visual hemifield (Driverand Halligan, 1991). Similarly, a patient with leftneglect showed intact visual parsing throughout thevisual scene, and thus could detect figures on theleft of the patient’s midline (Driver et al., 1992).This patient’s neglect applied to the left side of the

Cortex, (2004) 40, 237-246

VISUAL NEGLECT CAN BE OBJECT-BASED OR SCENE-BASED DEPENDING ON TASK REPRESENTATION

Gordon C. Baylis1, Leslie L Baylis1 and Christopher L. Gore2

(1University of South Carolina; 2Indiana State University)

ABSTRACT

Three patients with visual neglect were tested on their ability to detect target letters at ipsilesional and contralesionallocations on a monitor, and at different locations within large shapes on the monitor. When patients were asked to detecttargets within the entire monitor, they showed neglect for all the contralesional hemifield. In contrast when they wereasked to detect targets within a particular object, they showed object-based neglect. In these two conditions the displays,the targets and the response were identical, with the only difference being the space that is represented for the task. Theseresults show that the reference frame of visual neglect may be altered by task-instructions changing how a structured visualscene is represented, with neglect applying to the contralesional side of this represented space.

Key words: Target detection, object-based neglect, scene-based neglect, task instructions, top-down control

figures that were created by his intact visualparsing. This study highlights the fact that visualparsing must be largely intact if neglect is trulyobject-centered (but see also Driver et al., 1994).

Tipper and Behrmann (1996; Behrmann andTipper, 1999) showed that object-based and scene-based neglect could occur in the same patient atthe same time. In this study, patients had to detecta target that could appear in either end of adumbbell-shaped object. Not surprisingly, thesepatients with right hemisphere lesions showed anelevated reaction time (RT) to detect targets in theleft end of the dumbbell. Such neglect might beobject-based (on the left side of the dumbbell) orscene-based (on the left side of the display).However, in half of the trials, the dumbbell rotatedthrough 180 degrees. In this condition, RTs wereelevated for targets in the end of the dumbbell thatwas originally on the left (and was now on theright side of the patient due to the rotation). Thisneglect must be object-based and not scene-basedsince the targets were now on the right in scene-based coordinates. Interestingly, this object-basedreference-frame of neglect could be removedsimply by removing the central bar of thedumbbells – now neglect became entirely scene-based. This study shows that very subtle changes instimuli could change neglect from scene-based toobject-based.

The multitude of different reference frames forneglect in different studies seems surprising -evidence for almost any reference frame has beenfound. One possibility is that different patientshave different areas of damage, and hence differentspatial representations being compromised.However, Tipper and Behrmann (1996) show thatthe apparent reference-frame seen may be alteredby subtle changes to the stimuli. This means that inany patient, it may not be reasonable to assert thatneglect occurs in a single spatial reference frame.Moreover, a given reference-frame has always beenfound when workers have tested patients in tasksdesigned to demonstrate it. It seems unlikely that asingle compromised reference-frame, appropriate toeach patient’s lesion site could have always beenfound so fortuitously. More likely is the possibilitythat the apparent deficit varies according to themethod of testing.

According to this view, the task given to anobserver will itself lead to the representation ofsome defined area within which the observer is toperceive and act. The task requirements thereforelead the observer to load different aspects or partsof the visual world into their spatial representationsystem. If neglect applies to the representationalsystem (as the work of Bisiach and Luzzatti, 1978suggests), then neglect will apply to thecontralesional side of whatever is loaded into therepresentational system. Suppose a task where aperson is asked to make a response when they seea red light anywhere in the room. The observer

knows that the entire room is potentially relevant,and must represent the whole room. In contrast, anobserver might be asked to look for a red lightappearing somewhere on an object, say a clock.Here it is not reasonable for the observer todistribute their attention over the entire room.Rather it is more efficient to define only the object(here the clock) as task-relevant and to representonly the clock. Now suppose that our observer hasa lesion in the system that holds a representation oftask-relevant space. In one case we might supposethat they had, say, neglect for the left side of theroom, and in the other case they might demonstrateneglect for the left of the clock. Thus the samepatient might have their deficit labeled as “scene-based” neglect or “object-based” (i.e., clock-based)neglect. The deficit that was demonstrated woulddepend crucially on the task used.

This task-relevance view of neglect does, ofcourse, entail that the extent of the task-relevantregions of the visual scene can still be determined– likely given that patients with neglect are able toparse the entire visual scene (Driver et al., 1992;Grabowecky et al., 1993), and that visual parsing isdissociable from visual neglect (Baylis and Baylis,1997; Kartsounis and Warrington, 1991). In orderto effectively test this task-dependence view, it isnecessary to define the pattern of response timesand errors that would be expected in object-basedand scene-based neglect. For simplicity we willconsider just the case of a patient with a right brainlesion leading to left neglect (Figure 1).Performance is assessed at four points within thevisual scene, spaced such that two are to either sideof fixation (see the four Xs in each part of thisfigure). The nature of the underlying asymmetry ofattention induced by this right hemisphere lesionmay be of two broad types – scene-based (Figures1a and 1b) and object-based (Figure 1c). Inaddition, scene-based inattention can becharacterized either in terms of a sharp breakbetween an attended ipsilesional and an unattendedcontralesional hemifield (Figure 1a), or else as agradient of attention (see Kinsbourne, 1993)declining from the ipsilesional side (Figure 1b).

In Figure 1a, we show how a sharply definedscene-based inattention will lead to equally highdifficulty for both contralesional points. If thesedata are subjected to a two-way analysis of variance(ANOVA), with factors of visual hemifield, and partof visual hemifield, we would expect a clear effectof the former, no effect of the latter, and nointeraction. Conversely, a gradient of inattention(Figure 1b) would create data affected by both thevisual hemifield, and the part of the visualhemifield, leading to significant effects of bothfactors. Finally, if inattention is object-based (Figure1c), no effect of visual hemifield will be seen, butthere will be an effect of part of the visual hemifield(here the side of an object). The crucial differencebetween scene-based and object-based neglect is

238 Gordon C. Baylis and Others

that scene-based neglect will always lead to aneffect of the side of the display. An effect of part ofvisual hemifield (or side of object) will always beseen in object-based neglect but may also be seenwith a graded scene-based neglect (Figure 1b).

In order to test whether neglect could be bothobject-based or scene-based, depending on theparticular representation used, we examined theeffect of changing the task instructions on theperformance of patients with neglect, whilekeeping both the stimuli and responses constant.Thus any modulation in neglect could not be dueto stimulus-based factors, however subtle (cf.Behrmann and Tipper, 1999), but only on what thepatient placed in their (damaged) representationalsystem. Patients looked for target letters that laywithin large simple shapes (triangles, squares,circles, and diamonds) colored to contrast witheach other and the background on a computerdisplay. The target letters could appear at one offour possible locations in the display thatcorresponded to the left and right sides of the twosimple shapes (see Figure 2). We measured boththe reaction time to correctly detect a target, andthe proportion of targets missed.

There were a total of three different tasksemployed in this study. An initial screening task(task 1) was used where patients had to detecttargets in a display that contained no coloredobjects. In task 2 (the scene-based task) patientswere asked to detect a target anywhere in thedisplay that contained two objects (for an exampledisplay see Figure 2). Here they would be asked,for example, “Is there an E in the display?”. Intask 3 (the object-based task) the targets andstimuli were exactly the same as in task 2 butpatients were asked to determine if a target waspresent within a specified one of the large shapes –for example: “Is there an E in the triangle?” Trialsof tasks 2 and 3 were randomly intermingled.

MATERIALS AND METHODS

Subjects

There were three participants who wererecruited while they were inpatients in HealthSouthRehabilitation Hospital, Columbia, SC. TheInstitutional Review Boards of the rehabilitation

Changing the Reference Frame of Neglect 239

Fig. 1 – Schematic of the predicted patterns of data that will be seen with two different types of scene-based neglect (a and b), andobject based neglect (c). If brain damage leads to a particular gradient of attention across the visual field (top line), and four points inthe visual field are tested (second line), one would predict the pattern of data in the third line. When these data are subjected to a two-way ANOVA, each pattern leads to a different prediction of the factors expected to be significant and non-significant (final lines of eachpart of the figure).

hospital and The University of South Carolinaapproved all procedures. All participants gaveinformed consent to participate. Reconstructions ofthe extent of their lesions are given in Figure 3(see Rorden and Brett, 2000).

JJ was a 58 year-old Caucasian female whosustained a cerebrovascular accident (CVA) to herleft parietal and temporal lobe. Initially shepresented as mute, with severe paresis of her rightarm. At the time of testing she had recovered mostof her language function with 90% of her speechintelligible, and no memory or other cognitivedeficits. She showed no evidence of reducedsomatosensation, but showed mild weakness of herright arm. She displayed moderate right visualneglect, as assessed by a battery of tests thatcomprised cancellation of stars within distractors,line bisection, clock drawing and copying ofpictures. JJ showed right-sided errors on all of thesetasks, with no left-sided errors. JJ showed visualextinction to clinical confrontation and using acomputerized task of Baylis et al. (1993). Perimetryshowed no evidence of any visual field defects.

JL was a 54 year-old Caucasian male with aCVA to the right hemisphere. A follow-up CTshowed that this extended across parts of the rightparietal and frontal lobes. At the time of testing hewas lethargic, generally pleasant and cooperative,but with occasional incongruent diatribes. He waswell oriented and showed no cognitive deficitsoutside of visual attention. Testing also showed noevidence of deficits of sensation or attention inauditory or tactile modalities. He showed left

neglect, showing left-sided errors with linecancellation, cancellation with distraction, clockdrawing and visual copying. Perimetry showedonly one small visual field defect in the lower leftquadrant. When JL fixated the center of themonitor, this small defect lay approximately 6inches beyond the lower left corner of the monitor.JL showed visual extinction by clinicalconfrontation and by computerized task. Finally, JLshowed a marked right gaze preference whenrelaxing, but was fully able to maintain centralfixation when instructed to do so.

WK was a 68 year-old African-American malewho had sustained small infarcts to the right basalganglia and thalamus two years previously, andwas admitted following a large CVA in the rightparietal lobe. He was alert and oriented with nomemory or other impairments, but showed slightweakness of the entire left hemibody, includingsome left facial droop. He showed no evidence ofany sensory loss in auditory or tactile modalities. Abattery of tests for neglect showed left errors online cancellation, clock drawing and picturecopying, and a right-deviated line bisection,together clearly indicative of visual neglect. Heshowed visual extinction to clinical confrontationand by computerized task, but showed no visualfield defects by perimetry.

Apparatus and Stimuli

The experiment was conducted on a DellPentium microcomputer with color VGA graphics.


Fig. 2 – Example of a display used in tasks 2 and task 3, with a letter at every possible screen location, although actual displayscould only have a single target letter present. Displays for task 1 had letters of the same size in the same locations, but presented on ablank field.

Display onsets and offsets were provided byaltering the color lookup table to ensure that theyoccurred within a single frame. Stimuli werepresented on a color 21-inch monitor, and thepatients sat approximately 80 cm from thismonitor.

Target letters were either E or O and werelocated at one of four locations spaced with theirtops on the horizontal meridian of the display, andcentered 5 or 12 cm to the left or the right offixation. Displays were created by placing twolarge shapes within the monitor such that each wascentered in one side of the display, and thusoverlapped two of the possible target letterlocations. Figure 2 shows the screen locations andexample shapes drawn approximately to scale.There were four possible shapes – a square, circle,triangle and diamond of approximately equal area.These were selected at random except that both ofthe two shapes in any display were never the same.Placing these two shapes in the display led to thecreation of three areas – the area contained withineach shape and the background. These three areaswere colored three different colors selected atrandom from the following six colors: blue, yellow,red, green, magenta and cyan. The target letters

were presented in a bold font approximately 2.5cm high, in black. The example displays in figure2 are drawn approximately to scale.

Design

The design was within-subject in all tasks.Task 1. The display contained a target letter on

80% of all trials, and contained a distractor on thereminder of trials. A target letter was equally likelyto occur at each of the four screen locations.

Tasks 2 and 3. These two tasks were interleavedrandomly within each block of 44 trials such thatan equal number were the object-based task andthe scene-based task. For the 22 trials of task 2(the scene-based task), 16 trials contained a targetletter, 4 contained a non-target letter, and 2 did notcontain any letter. Both target and non-target letterswere equally likely to occur at each of the fourpossible screen locations. For the 22 trials of task 3(object-based task), 16 contained the target withinthe specified large shape, 4 contained the targetwithin another shape, and 2 contained a non-targetletter. The fact that targets were present much moreoften than not was not apparent to the participants.At debriefing, all of them said that they believed


Fig. 3 – Reconstructions of the lesions in the three patients based on CT scans. These reconstructions superimpose the extent of thelesions on a high resolution MRI of a typical (normal) brain, according to the method of Rorden and Brett (2000).

that there were an approximately equal number oftrials with and without targets.

Treatment of Data

Reaction time data were only analyzed for thosetrials in which a target was correctly detected, as allpredictions refer to these trials. Trials where a targetwas present and yet the patient did not detect itwere counted as errors. Response times andaccuracy for trials without a target were notanalyzed, as no predictions can be made in thiscase. Reaction time data were summarized bycalculating a median RT for each condition for eachpatient, and these data are shown in Figures 4, 5 and6. RT data were also analyzed for each subject withtwo separate analyses of variance for the twodifferent tasks. For each ANOVA there were twonested factors of visual field and part of visual field.

Procedure

After participants had given their informedconsent to take part in this study they were showna diagram of typical displays, similar to Figure 2(for tasks 2 and 3), and told that they were to lookfor the presence of a particular target letter.Examples of the two letters in the same font usedwere shown to the participants. It was explainedthat two possible questions could be asked, eitherof the form “Is there an O in the display?” or “Isthere an O in the triangle?” It was emphasized thatfor the display questions that had to verify that thespecified letter was present in the display, and thatthe presence of any other letter, or no letter at allshould be treated as a “no” response. For theobject-based task (task 3), they were to verify thatthe correct letter was in the specified shape, andthat the presence of any letter outside of the shapeshould be treated as a “no” response, as should adifferent letter in the correct shape.

Participants were given 3-5 blocks of task 1, and5-7 blocks of the interleaved tasks 2 and 3. Testingwas carried out over 3-4 testing sessions, in whichpatients were allowed to rest as they wishedbetween trials, although participants rarely chose todo so. Each trial began with a question being writtenon the screen such as “Is there an O in the display”(task 1 or task 2), or “Is there an E in the triangle?”(task 3). The experimenter read the question, thenturned to face the patient and asked “Ready?”.When the patient indicated that they were ready, theexperimenter pressed any key to begin thepresentation of a blinking fixation point at thecenter of the screen for 500 ms, followed by thedisplay with the target. The patient’s task was torespond verbally to a question as to the presence ofa particular target letter. The experimenter hit the<space> bar as soon as the patient responded, thusproviding an estimate of response time, and thenentered the patient’s response into the computer for

subsequent scoring. Eye movements weremonitored by the experimenter who was facing thepatient throughout each trial. A sample of trials werealso videotaped; subsequent analysis of these tapesverified that the experimenter had monitored eyemovements correctly. If the patient made an eye-movement before making their verbal response, theexperimenter keyed this into the computer, the trialwas rejected from analysis, and was replaced laterin the block. The experimenter provided no explicitfeedback to the participants except to providecontinuing encouragement and general remarksabout them performing well. At the end of all thetesting, the experimenter provided a brief summaryof the findings to each participant and explained theimplications of visual hemineglect in everyday life.

Results – Task 1

When participants were shown letters on anempty colored field, reaction times and error rateswere elevated for both locations in thecontralesional hemifield compared to the reactiontimes in the ipsilesional hemifield. Note that forpatients JL and WK with lesions in the righthemisphere, elevated reactions times were seen fortargets in the left visual hemifield, whereas acomplementary pattern of right neglect was seenwith JJ, who had a left hemisphere lesion. Thepattern of reaction times and error rates of thesethree patients are shown in Figure 4.

These reaction data were examined using anANOVA on the reaction times of individual trialsfor each patient. Two factors were employed, thehemifield in which the target was presented, andthe part of the hemifield in which the target waslocated. In each patient a significant effect ofhemifield was found [F (1, 74) = 37.6, p < 0.001,for JJ; F (1, 27) = 6.8, p < 0.02, for JL; F (1, 71)= 30.9, p < 0.001, for WK]. However, there was noeffect of part of hemifield for any patient [F (1, 74)= 0.3, n.s. for JJ; F (1, 27) = 0.3, n.s. for JL; F (1,44) = 0.1, n.s. for WK]. Finally there was nointeraction of hemifield and side of hemifield forany patient [F (1, 74) = 0.6, n.s. for JJ; F (1, 27) =2.9, n.s. for JL; F (1, 44) = 2.3, n.s. for WK]. Theerror data closely follow the reaction time data, ascan be seen in Figure 4, demonstrating that aspeed-accuracy trade-off cannot account for thereaction time data.

These results can be simply characterized asshowing neglect for the contralesional side of thevisual scene. Thus, targets in the contralesionalside of the visual scene are detected much moreslowly, and are more likely to be missed thantargets on the ipsilesional side of space.

Results – Task 2

When patients were performing the scene-basedtask, their RTs were elevated for all items on the


contralesional side of the display (see Figure 5). Inorder to analyze these data, we carried out two-waywithin-subject ANOVAs on the RTs of each patient.Again all patients showed a significant effect ofhemifield in which the target was presented [F (1,71) = 6.0, p < 0.02, for JJ; F (1, 74) = 9.4, p <

0.01, for JL; F (1, 82) = 13.0, p < 0.001, for WK].For JJ, there was no effect of part of hemifield [F(1, 71) = 0.0, n.s.]. However, for the other twopatients there was a small effect of the part ofhemifield [F (1, 74) = 4.3, p < .05. for JL; F (1, 82)= 4.7, p < .05. for WK]. There was no interaction


Fig. 4 – Results for task 1. The median reaction times (left) and error rates (right) for each of the three patients on task 1.

Fig. 5 – Results for task 2. The median reaction times (left) and error rates (right) for each of the three patients on the scene-basedtarget detection task (task 2).

of hemifield and side of hemifield for any patient[F (1, 71) = 1.8, n.s. for JJ; F (1, 74) = 2.1, n.s. forJL; F (1, 82) = 0.4, n.s. for WK].

These data are similar to those shown in Figure4 for the empty scene detection task (task 1), andagain can be characterized as showing neglect forthe contralesional side of the visual scene. Thus,targets in the contralesional side of the visual sceneare detected much less slowly than targets on theipsilesional side of space. The fact that JL and WKshow a small effect of the part of the hemifieldsuggests that neglect for the scene may representmore of a gradient of attention (see Kinsbourne,1993), as schematized in Figure 1b, than a sharplydefined step function (Figure 1a). One slightdifference between the results for this task and theresults for the empty scene task is the slightincrease in RTs and error rates, especially forcontralesional targets. It is likely that this is due tothe fact that in tasks 2 and 3, the patients had toremember (i.e., hold in working memory) whattheir task was. This would increase the difficulty ofthe task, and lead to a modest increase in RT anderror rate. Most important is that the overall patternof data remained the same.

Results – Task 3

A different pattern of data was seen for theobject-based task, as can be seen in Figure 6. Onceagain we analyzed these data with two-wayANOVAs on the RTs of each patient. This timepatients showed no effect of hemifield in which the

target was presented: [F (1, 69) = 2.2, n.s. for JJ; F(1, 72) = 3.1, n.s. for JL; F (1, 83) = 0.0, n.s. forWK]. However, there was a highly significanteffect of part of hemifield [F (1, 69) = 11.1, p <0.001, for JJ; F (1, 72) = 18.4, p < 0.001, for JL; F(1, 83) = 6.7, p < 0.01, for WK]. There was nointeraction of hemifield and side of hemifield forany patient [F (1, 69) = 2.2, n.s. for JJ; F (1, 72) =2.1, n.s. for JL; F (1, 83) = 1.0, n.s. for WK]. Thesedata can best be seen as reflecting neglect operatingwithin the reference frame of the individual objects(large shapes) in which the targets were located.

In order to be sure that the difference betweenthe results of these two tasks was significant, anANOVA was conducted across all the data fromboth tasks, with conditions of patient, task,hemifield (= object), and side of hemifield (= sideof object). First, the interaction between taskinstructions and side of hemifield was alsosignificant [F (1, 443) = 4.1, p < .05]. This resultshows that there was a greater effect of side ofhemifield (part of object) in task 3. However, thecrucial comparison (see also Figure 1) is thepresence or absence of an effect of hemifield in thetwo tasks. The interaction between task instructionsand hemifield was significant [F (1, 443) = 4.0, p < .05], showing that there was a different effectof hemifield in the two different tasks. This showsthat the finding of an effect of hemifield inexperiment 2 and the lack of a similar effect inexperiment 3 represented a significant contrast,supporting the notion of different effects in the twodifferent task instructions.


Fig. 6 – Results for task 3. The median reaction times (left) and error rates (right) for each of the three patients on the object-basedtarget detection task (task 3).

DISCUSSION

It is well established that patients with visualhemineglect fail to attend to information on theside opposite to their brain lesion. Inherent in anynotion of one-sided inattention is the spatialreference frame of neglect. Considerable debate hasconsidered whether this is centered on the observer,the scene, or visual objects (Driver et al., 1992;1994; Driver and Halligan, 1991; Farah et al.,1990; Karnath et al., 1991; Marshall and Halligan,1994). A few studies have suggested that multiplereference-frames can coexist at one time(Behrmann and Tipper, 1999; Farah et al., 1990;Tipper and Behrman, 1996). However, no study hasattempted a rapprochement between the contrastingclaims of different reference frames for neglectseen in different studies of neglect (but seeCaramazza and Hillis, 1990, Hillis et al., 1999).Reconciling the different possible reference framesof neglect is possible, using the notion ofrepresentational neglect (Bisiach and Luzzatti,1978) as a starting point.

The present study demonstrates for the firsttime that the reference-frame of visual neglect canbe modified by task instructions alone. The twodifferent tasks used the same stimulus materials,and the same target-detection task, and the sameresponse. Yet the neglect was either object-based orscene-based dependent only on the taskinstructions. These instructions presumably led tothe definition of a task-relevant region in thedisplay – either the entire display or a single objectwithin the display. When the patient generated arepresentation of this region, they had a deficit inthe contralesional side of this representation. Theresult was that patients neglected the contralesionalside of whatever was task relevant. The task-relevant region was apparently selectedsuccessfully even when it lay on the contralesionalside of the display. This supports previous claims(e.g., Driver et al., 1992; Grabowecky et al., 1993)that visual parsing is intact throughout the entirevisual field of patients with neglect. The resultsfound here have profound implications for theassessment, rehabilitation and management ofvisual neglect, because the deficit demonstratedmay depend on how the patient defines theparticular task, or daily life activity. Rehabilitationstrategies could be devised that enable patients touse the lability of their neglect to their advantage,and allow them to become aware of hithertoneglected items.

The results of this study suggest that patientswith neglect may use task instructions, along withintact visual parsing to correctly determine the task-relevant region. It is of interest to note that thesame pattern of data was seen in patients with bothleft and right brain lesions, suggesting that thedependence of observed neglect from taskinstructions is not specific to right brain damage.

Since the task-relevant region varies from trial totrial in out study, the brain must have arepresentational system that can be flexibly used tostore a representation of the currently relevantregion of space. This view suggests that theprocessing of contralesional space is not theprimary deficit, but rather the representation of thecurrently relevant region for the direction ofdecisions and responses. This view follows fromthe notion of representational neglect pioneered byBisiach and Luzzatti (1978), and explains why somany different “reference frames” for neglect havebeen described. Our patients, and indeed a largeproportion of patients displaying neglect havesustained lesions to their parietal lobe. It istempting to suggest, therefore, that the parietalcortex, rather than computing a single invariantrepresentation of space, instead represents task-relevant space “on-line” in a rapid and flexiblemanner.

Acknowledgements. GC Baylis and LL Baylis weresupported by a grant from the National Institutes of Health(1 R01 NS042047-01) and by generous intramural fundingfrom the University of South Carolina.

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Gordon Baylis, Attention and Perception Laboratory, Department of Psychology,University of South Carolina, Columbia, SC 29208. E-mail: [email protected].


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Visual Neglect can be Object-Based or Scene-Based Depending on Task Representation