Interactive perceptual and attentional limits in visual extinction

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Interactive perceptual and attentional limits in visualextinctionLilach Shalev a , Eran Chajut a & Glyn W. Humphreys ba The Open University of Israel , Ra'anna, Israelb University of Birmingham , Birmingham, UKPublished online: 16 Feb 2007.

To cite this article: Lilach Shalev , Eran Chajut & Glyn W. Humphreys (2005) Interactive perceptual and attentional limits invisual extinction, Neurocase: The Neural Basis of Cognition, 11:6, 452-462, DOI: 10.1080/13554790500376784

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Neurocase (2005) 11, 452–462 Copyright © Taylor & Francis LLC ISSN: 1355-4795 print DOI: 10.1080/13554790500376784

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Interactive perceptual and attentional limits in visual extinction

Perceptual and Attentional Factors in ExtinctionLILACH SHALEV1, ERAN CHAJUT1 and GLYN W. HUMPHREYS2

1The Open University of Israel, Ra’anna, Israel2University of Birmingham, Birmingham, UK

We report a case-study of YE, a 54-year-old person who suffered multiple shell injuries that caused a right-parietal lesion and lefthemianopia, almost 30 years ago. We conducted 4 experiments using a basic extinction paradigm in which YE had to report single stimulipresented on the left or right or two stimuli presented simultaneously in both visual fields. We show that extinction was selectively affectedboth by increasing the relative perceptual salience of the contralesional stimulus and by cueing attention to the contralesional side. Theeffects of perceptual salience and attentional cueing interacted, with cueing being more effective when the stimuli had relatively highperceptual salience. The data are consistent with attentional and perceptual factors interacting to determine the competition between left andright side stimuli that underlies extinction.

Numerous previous studies have shown that damage to theright parietal lobe is associated with problems in shiftingvisual attention to locations in space. Posner and his col-leagues (1984) reported deficits in disengaging attention fromthe ipsilesional field to detect targets presented on the con-tralesional side. Arguin and Bub (1993) showed problems inshifting attention towards the contralesional side within onehemifield. Classically, right parietal damage is associatedwith both unilateral neglect (e.g., poor report of a single itemon the left, contralesional side; Heilman and Valenstein,1979) and visual extinction (where there is good report of asingle contralesional item, but poor report of the same itemwhen it appears simultaneously with an ipsilesional stimu-lus). Extinction is often attributed to a spatial bias whencontra- and ipsilesional items compete for selection(e.g., Duncan, Humphreys and Ward, 1997; Heinke andHumphreys, 2003), while neglect can be linked to impair-ments in orienting attention to the contralesional side. Thismay be due to damage to a leftwards orienting system(Kinsbourne, 1977, 1987, 1994; Riddoch and Humphreys,1983), or to chronic orienting of attention to the ipsilesionalside (Gainotti, D’Erme, and Bratolomeo, 1991; Ladavas,Petronio, and Umilta, 1990). The right hemisphere may beparticularly implicated in such attentional deficits because ithouses neurons which produce attentional shifts to both sides ofspace, whilst the left hemisphere may only control attentionalshifts to the right side (Kinsbourne, 1987; Corbetta, 1993). Thus,damage to the right hemisphere impairs leftwards orienting,

whilst equivalent damage to the left hemisphere can be com-pensated for by the right hemisphere’s control of orienting tothe right as well as the left side of space. Alternatively, theright hemisphere may be associated with deficits such asneglect and extinction because it mediates sustained attentionas well as orienting to the opposite side of space (Robertsonet al., 1997). Poor sustained attention will exacerbate anyresidual deficit in attentional orienting.

However, although attentional deficits are implicated inboth neglect and extinction, it is possible that the problemsare at least contributed to by additional deficits in perceptualcoding of stimuli on the contralesional side. Several studiesdemonstrated positive effects of hemispatial sunglasses andeye patching on unilateral spatial neglect (i.e., Arai et al.1997; Beis et al. 1999; Butter and Kirsch, 1992). In clinicaltesting neglect of a single left stimulus can be difficult to dis-tinguish from hemianopia (though see Walker et al. 1991),especially if hemianopia is coupled with a deficit in (non-spatial) attention. Perceptual deficits may also contribute toextinction. Farah, Mondheit and Wallace (1991) simulatedextinction in normal participants by selectively masking thestimulus on one side of space. Gorea and Sagi (2000) havesimilarly shown that extinction-like results can be induced incontrols when one stimulus has higher contrast than the other.

Surprisingly, in view of the difficulty in understanding therelations between perceptual and attentional deficits inneglect and extinction, there are few studies of extinction andneglect where the investigators have examined the relationsbetween variables that putatively influence the perceptualcoding of stimuli (e.g., their eccentricity or size) and manipu-lations of attention (e.g., spatial cueing). However, a jointexamination of perceptual and attentional variables has themerit of enabling us to assess whether the variables show asimilar pattern of effects and whether they interact to jointlyinfluence performance. In the present paper we report effectsarising from manipulations of perception and attention in a

Received 24 September 2003; accepted 21 September 2005We thank YE for his cooperation and exciting insights through-

out the study. This paper was supported by grants from the BBSRC,the MRC and the Stroke Association.

Address correspondence to Lilach Shalev, Department of Educa-tion and Psychology, The Open University of Israel, 108 RavutskiSt., Ra’anana, Israel. E-mail: [email protected]

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Perceptual and Attentional Factors in Extinction 453

patient with a unilateral right parietal lesion. All the experi-ments involved different versions of a basic paradigm inwhich YE was asked to report whether a stimulus (the letter‘A’) was presented on the left (single left target), on the right(single right target) or simultaneously, in both visual fields(target on both sides). We show that manipulations of bothperceptual salience and attention can improve performanceon both single and two-item trials, with perceptual salienceand attention interacting to determine report (particularly ontwo-item trials). The data are consistent with an interactiveaccount of perceptual and attentional impairments followingright parietal damage.

Case Report

YE was 54 years old at the time of testing, and had sufferedmultiple shell injuries almost 30 years ago.

A CT scan revealed a massive right hemisphere lesionaffecting fronto-parietal areas up to midline as well as occipi-tal areas. The CT scan is shown in Figure 1. On visual fieldtesting there was a left lateral homonymous hemianopia. Thevisual field acuity test is presented in Figure 2. One problemin interpreting the visual field test is that it is notoriously dif-ficult to distinguish between visual neglect/extinction andhemianopia (Walker et al. 1991). The data reported in thepresent paper indicate that the field loss was not absolutesince YE could identify large letters that were briefly pre-sented in his left field (Experiments 1 and 2); also his perfor-mance on left field items improved under brief cueingconditions where eye movements were unlikely to beinvolved (Experiment 4). Prior to the present tests YE wasnever tested clinically for neglect. However, at the time oftesting, YE did not show unilateral neglect on a variety ofstandard clinical tests including: drawing from memory,copying, line cancellation, letter cancellation and star cancel-lation (see Table 1). The only clinical sub-test in which hisscore was below the cut-off point is the line-bisection (seeTable 1).

However, he did report some everyday situations where hefailed to notice the left-part of an object or scene. Forinstance, YE interpreted the sign saying “The shop is closed

between 13–16” as “the shop is closed between 3–16”. Healso found it extremely difficult to find his way in relativelynew areas. There were no other intellectual consequences ofthe lesion. Since the damage was sustained YE has been ableto function well in everyday life, holding down a job inhigher education. Despite this absence of neglect in standardtests with unlimited presentation conditions, our results indi-cate that YE had difficulty in detecting single stimuli pre-sented in his left visual field under brief exposure conditions.We assessed the effects of perceptual and attentional manipu-lations on this deficit and on his ability to report left andright-field stimuli under bilateral simultaneous presentationconditions (i.e., on extinction). We evaluated whether anyperceptual deficit interacted with attention to determine per-ceptual report. All the experiments reported in the presentstudy (i.e., Experiments 1–4) were administered to YEbetween August and October 2003.

Experiment 1: Manipulating the size of stimuli

The aim of the first experiment was to explore whether a rel-atively ‘pure’ perceptual manipulation (increasing the size ofthe stimuli) can improve YE’s perception of letters (presentedalone or together with a right stimulus) in his left visual field.Increasing the physical size of the letters may help to over-come a perceptual limitation in processing in YE’s left visualfield

Method

Stimuli and Apparatus

Stimulus presentation and data collection were controlled byusing Authorware pro (Macromedia) software and were pre-sented on a 14.1-in LCD monitor of an Apple PowerBook G4433 computer. The monitor was set to a resolution of1152X768 pixels. The stimuli consisted of a fixation point(subtended 0.27° of visual angle) and the target letter “A”(capital letter). In Experiment 1a the letters were presented inTimes New Roman font size 32 (subtended 0.56° in widthand 0.65° in length of visual angle). In Experiment 1b we

Fig. 1. CT scans for YE (2/1989), going from ventral (i) to dorsal (iii).

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454 Shalev, Chajut and Humphreys

Fig. 2. Visual fields acuity test of YE (9/2004).

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used a font size of 38 (subtended 0.7° in width and 0.78° inlength of visual angle). In Experiment 1c the font size was 44(subtended 0.8° in width and 0.86° in length of visual angle).The distance between the letter and the fixation point in each ofthe above font size was 7.55°, 7.48° and 7.43°, respectively.

There were three types of target displays: unilateral left orright exposures of the target letter “A” and bilateral exposuresof the same letter (simultaneously in both right and left loca-tions). All stimuli were drawn in black and appeared against awhite background. The monitor was placed 60 cm away fromYE and its center was aligned with the midsagital axis of hishead, so that 1 cm represented 0.96° of visual angle.

Procedure

In all the tasks, the displays involved presentation of the letter“A” outlined in black. There were three target conditions:unilateral right target display, unilateral left target displayand bilateral display. Each trial started with a fixation pointfor 1 sec. After the fixation point was erased, the target waspresented for 100 milliseconds in one location (left or right)or in both locations. Each trial was initialized by the experi-menter only after confirming that YE was focused at the cen-ter of the screen ready for the presentation of another trial.The experiment consisted of 75 trials. The first 15 trialsserved as an exercise (5 per target condition) followed by 60experimental trials (20 for each target condition). The targetcondition was selected randomly. Prior to each experimentalsession YE was informed about the precise nature of theexperiment (i.e., in each trial either a single letter will be pre-sented on the right/left or two stimuli will be presented simul-taneously on both sides). He was then asked to respondvocally using the words “left” for a unilateral left target dis-play, “right” for a right unilateral display, and “both” for abilateral display. YE was instructed to focus on the centre ofthe screen where the fixation point was presented and thenthe experimenter initiated the experimental session. Theexperimenter monitored YE’s eye movements; however, YEfollowed the instructions and fixated easily on the fixationpoint throughout the entire block of trials across all theexperiments. Each experimental session (i.e., Experiment 1a,

1b and 1c) lasted about 15 min. The sub-experiments wereconducted on different days.

Results and Discussion

Figure 3 shows YE’s mean percentage of correct responses asa function of the target condition and the font size.

YE had no difficulties in identifying single right targets inall font size conditions (he scored 20/20 for each condition).However, he found it impossible to perceive the single lefttarget at the smallest font size (Experiment 1a - 0/20). Onthese trials he reported that he did not see the stimulus.Increasing the physical size of the stimulus yielded a substan-tial improvement in YE’s perception of single left stimuli: InExperiment 1b, he scored 9/20, and in Experiment 1c hescored 16/20 (in both cases the improvement was significant;Fisher Exact Probability Test p = 0.0006; χ2

(1) = 5.23, p =0.022, respectively). Nevertheless, even for the largest font-size condition (44-points) YE was better at responding tosingle-right targets compared with single-left targets (20/20 vs.16/20, respectively; χ2

(1) = 4.44, p = 0.035).Interestingly no analogous effect of stimulus size on YE’s

performance was found when stimuli were shown under dou-ble simultaneous conditions (i.e., target on both sides — 1/20, 0/20 and 1/20 - 32-, 38- and 44-points of font size, respec-tively). On double simultaneous stimulation trials, YE madeerrors by reporting that only a single right target was present.

In summary, altering the physical size of the targets pro-duced a significant improvement in YE’s ability to perceivesingle-left targets. However, this perceptual manipulationwas not effective in enabling YE to perceive a left target pre-sented simultaneously with a right one. This is consistent withthe hypothesis that the poor performance in the bilateral con-dition reflects an attentional limitation in addition to any per-ceptual constraint. Note also that, although YE was informedabout the precise nature of the experiment, he did not use anyguessing or other compensatory strategy to generate aresponse on trials in which he did not detect any stimulus.There does not appear to be any contribution to performancefrom strategic guessing.

Table 1. YE’s performance on paper and pencil tests (from the BIT; October 2003)

Task Performance Cut-off norms

Line cancellation 40/40 34Star cancellation 54/55 (one miss on the right side) 51Line bisection (calculated from the midpoint) 1st line - 23 mm leftward (1 point)

2nd line - 3 mm rightward (3 points)3rd line - 17 mm rightward (2 points)Overall score: 6 7

Letter cancellation (E & R detection)

Left - 11/13Centre - 10/13Right - 13/13Overall score - 34 32

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Experiment 2: Increasing the relative size of the left-side letter

The notion that there is an attentional contribution to YE’sdeficit was tested further in Experiment 2, where we keptthe size of the letter in the right field constant and varied thesize of the letter in the left visual field in the same manneras was done in Experiment 1. If the only factor influencingYE’s performance was the absolute saliency of the left fielditem, then performance here should resemble that found inExperiment 1, on both single and double letter trials. On theother hand, if what is crucial is the relative saliency of theleft to the right letter, then a different pattern of perfor-mance should emerge. In Experiment 1, the relativesaliency of the letters was maintained even when the abso-lute size of the left letter changed, because the right letteralways matched the left letter for size. However, as the sizeof the right letter in Experiment 2 was held constant, thenthe relative saliency of the left to the right letter increased inthis study as the left letter increased in magnitude. If the rel-ative saliency matters, then, under double simultaneous pre-sentation conditions, there should be better report of the leftletter in Experiment 2 than in Experiment 1, as the left letterincreases in size. This effect of relative saliency can be con-ceptualized in terms of a ‘biased competition’ account ofvisual selection (Di Pellegrino and De Renzi, 1995; Kins-bourne, 1987; 1994). According to this idea, there is compe-tition for selection between letters in the left and rightfields, which can be spatially biased by YE’s right hemi-sphere lesion. This spatial bias leads to right field itemsbeing relatively more salient for attention than left fielditems. However, if the difference in relative saliency can be

equated, by the left side letter being relatively larger thanthe right field letter, then the bias may be overcome. Thereshould be a selective increase in report of the left field itemin the double letter condition.

Method

Stimuli and Procedure.

The stimuli, design, and procedure of each session were identi-cal to those of Experiment 1a, apart from the following excep-tion. In Experiments 2b and 2c we increased the size of the lefttarget while the right target letter remained the same size.Experiment 2a was a precise replication of Experiment 1a (thebaseline condition), in which both the left and the right stimuliwere the same size (32 points - 0.56° in width and 0.64° inlength of visual angle). In Experiment 2b the size of the left tar-get was 38 point (subtending 0.70° of visual angle in width, and0.78° of visual angle in length) and in Experiment 2c the size ofthe left target was 44-point (subtended 0.779° of visual angle inwidth, and 0.859° of visual angle in length). In Experiments 2band 2c, the right letter remained at font size 32 points.


Figure 4 shows YE’s mean percentage of correct responses asa function of the letter condition and font sizes.

As in Experiment 1, YE had no difficulties in identifyingsingle right targets at all font sizes (20/20 in each of them).With single-left targets he was poor in the baseline condition

Fig. 3. Experiment 1: YE’s total percent hits in each target type as a function of the physical size of the target.

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font size (L- left, R-right)

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single left target single right target target in both sides

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(4/20, reporting no target on 16/20 trials). However, herecovered dramatically as a result of magnifying the size ofthe left stimulus (17/20 with 38-points font size, Experiment2b and 20/20 with 44-points font size, Experiment 2c); inboth cases the improvement was highly significant (χ2

(1) =16.94, p = 0.000; χ2

(1) = 26.67, p = 0.000, respectively). Inter-estingly, a substantial improvement in YE’s performance wasobtained in the double simultaneous condition as well, butonly when the left side letter was substantially larger than theright side letter (Experiment 2c, 44-points left target vs. 32-points right target). In this condition, YE scored 18/20 (com-pared with 1/20 in the baseline with targets of equal size; χ2

(1) =16.94, p = 0.000). In Experiment 2b, where the size magnifi-cation of the left stimulus over the right one was less pro-nounced (38-points vs. 32 points, left vs. right stimulus,respectively) YE was able to perceive double simultaneoustargets on only 4 out of 20 trials (vs. 1/20 in the baseline sym-metric condition; Fisher Exact Probability Test, p = 0.171).Errors on double simultaneous trials involved YE reportingjust a single right-side stimulus.

The effect of relative letter size, on extinction, fits withthe biased competition account of visual selection. Here adeficit in perceiving a left side target under double simulta-neous conditions was overcome by increasing the relativeperceptual salience of the left field stimulus. The sizeincrease needed to overcome extinction (based on the relative

saliency of the left and right items) was greater than thatneeded to overcome any basic perceptual deficit, based onthe absolute size of the left letter, in the unilateral leftcondition.

Experiment 2 replicated the effect of physical size on thedetection of the left-side target in the unilateral presentationcondition. Performance improved with the larger letter sizes.In addition to this, however, we showed that the extinctioneffect on bilateral trials could be overcome by increasing therelative size of the letters in the left compared with the rightfield.

Experiment 3: Shifting the location of fixation

In Experiment 3 we examined the effects of another percep-tual manipulation, namely, shifting the location of the fixa-tion point to the left so that the eccentricity of the target wasaltered. Antis (1974) showed that manipulation of the eccen-tricity of a stimulus affects the detection threshold so that thegreater the eccentricity of the stimulus the higher its detectionthreshold. We tested again whether this might improve per-formance under unilateral but not bilateral presentation con-ditions or whether a relative shift in the locations of thestimuli could overcome any limitation under double simulta-neous stimulation conditions.

Fig. 4. Experiment 2: YE’s total percent hits in each target type as a function of the physical size of the left target.

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Method


This experiment consisted of 5 experimental sessions (Exper-iment 3a-3e). Each session was carried out on a different day.The stimuli, design, and procedure of each session were iden-tical to those of Experiment 1, apart from the following. InExperiment 3b-3e we shifted the position of the fixation point50 pixels (1.34° of visual angle) leftward in every experimentsession. That is, in Experiment 3b the fixation point wasshifted 50 pixels leftward from its initial position (Experi-ment 3a), in Experiment 3c the fixation point was shifted 100pixels leftward, in Experiment 3d it was shifted 150 pixelsand in Experiment 3e - 200 pixels to the left. As a result, inExperiment 3b the left target letter “A” was located 6.53° ofvisual angle to the left of the fixation point and the right tar-get letter “A” was located 9.21° of visual angle to the right ofthe fixation point. In Experiment 3c the left target waslocated 5.19° to the left of the fixation, and the right targetwas located 10.55° to the right of the fixation, and so forth inExperiment 3d and 3e. In Experiments 3b-3e the fixationpoint was located to the left of the midpoint of YE’s bodyand he was allowed to turn his head to fixate on the fixationpoint.


Figure 5 shows YE’s mean percentages of correct responsesas a function of target condition and the position of the fixa-tion point.

As in the previous experiments, YE had no difficulties inidentifying single right targets independently of the positionof fixation (19 or 20/20 in all the conditions). With single-lefttargets he was completely ‘blind’ in the baseline condition (0/20;he always reported that no stimulus was present). However,he recovered radically as a result of shifting the fixation point1.34° of visual angle to the left (18/20 - Experiment 3b; χ2

(1)= 32.73, p = 0.000). Interestingly, this manipulation onlymoderately improved the identification of double targets (5/20 vs. 1/20; Fisher Exact Probability test - p = 0.09). Simi-larly to Experiment 2, there was some recovery in YE’s per-ception of double targets once fixation was substantiallyshifted in favor of the left side target (when the left side letterwas only 2.59° of visual angle away from fixation, and theright target was some 13° away) (17/20 vs. 5/20 in Experi-ment 3e and 3b, respectively; χ2

(1) = 16.94, p = 0.000). Notethere was some non-significant variation in identifying singleleft targets in the different conditions (18/20 vs. 14/20 inExperiments 3b and 3c-d, respectively; χ2

(1) = 2.50, p = 0.11).

Fig. 5. Experiment 3: YE’s total percent hits in each target type as a function of the position of the fixation.

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All of YE’s errors on double-item trials involved his report-ing that only a single right item was presented.

Experiment 3 examined the effect of shifting the fixationpoint to the left. While a small leftward shift significantlyimproved the perception of single left targets, a much moresubstantial leftward shift was necessary in order to facilitateYE’s perception of double targets. These results replicatedthe results of Experiment 2 with a different perceptual manip-ulation. In Experiment 4 we tested whether a “pure” atten-tional manipulation can produce equivalent effects tochanging the relative perceptual salience of left and right sidetargets.

Experiment 4: Effects of cueing attention

Experiment 4 aimed to test whether YE’s extinction could beresolved by an attentional manipulation - cueing attention tothe left visual field. As was noted in the Introduction, variousprevious studies have shown that unilateral neglect andextinction can be associated with problems in shifting visualattention to locations in space (i.e. Heilman et al., 1993;Mesulam, 1985; Posner et al., 1984; Rafal, 1994). Shalev andHumphreys (2000) reported data from a patient (MB) whoseperception of left near stimuli was facilitated following cuesthat appeared at a far left but not at a near left location (i.e., atthe same location of the target). They suggested that in unilat-eral neglect, attention tends to overshoot to the right andundershoot to the left. In the present experiment we used‘standard’ spatial cues (Experiment 4b and 4c) which precuedthe location of the target to be presented and ‘overshooting’spatial cues (Experiment 4d), which precued a location far-ther to the periphery than the location of the target (i.e., thecue presented to the left of or to the right of the target). Wecompared YE’s perception following spatial cues with a con-trol condition (Experiment 4a) in which no spatial cues wereintroduced; instead, a blank interval was added after the pre-sentation of the fixation and before the onset of the target, inorder to produce temporal conditions similar to those whenthe cue was present. We predicted that spatial cues in general,and ‘overshooting’ cues in particular, would facilitate YE’sperception of left stimuli.

Method


The stimuli, design, and procedure of each experimental ses-sion were identical to those of Experiment 1, apart from thefollowing. In Experiment 4b-4d the location of the target wasprecued by a large empty circle that subtended 2.55° of visualangle and that was drawn from a 4-pixel wide line. In Experi-ment 4a, 4b and 4d we used a font of 32-points and in Exper-iment 4c we used a font of 38-points. In Experiment 4b and4c the cue was located around the target location (‘on target

cues’). In Experiment 4d the cue was located 2.55° of visualangle to the left or to the right of, respectively, the left or theright target location (‘overshoot cues’). In Experiment 4c weused the same ‘on target cues’ as in Experiment 4b, but alarger font size was employed (font size 38, for both letters).We note that YE showed a very clear extinction effect withletter font size 38 in Experiment 1, when no spatial cue wasused.

In Experiment 4a, a blank display was presented instead ofthe cue. Thus, each trial consisted of the following sequenceof events: first a fixation point was presented for 1 sec. Thecue (or a blank display) was then presented for 90 ms, ineither the left or right visual fields. This brief cue shouldinduce a shift of attention but was too brief for an eye move-ment. After the cue disappeared, the target was presented for100 ms in the same locations as in Experiment 1. A single lefttarget was always preceded by a left cue and a single righttarget was always preceded by a right cue. Half of the target-on-both-sides trials were preceded by a left cue and the otherhalf of these trials were preceded by a right cue. In order toprevent YE from responding to the cue rather than to the tar-get, in Experiments 4b-4d, in half of the trials there was notarget after the cue. Experiment 4a consisted of 75 trials: thefirst 15 trials served as practice trials (5 for each target type)and then a set of 60 experimental trials was presented (20 foreach target type). Each of the other three experimental condi-tions (Experiment 4b-d) consisted of 270 trials. The first 30trials served as practice trials (5 for each target type when thetarget appeared and 15 trials in which only the cue appearedwith no target after it). Subsequently, a set of 240 experimen-tal trials was presented: 60 trials for each cue location, 120trials when the target followed the cue and 120 trials whereno target followed the cue. Throughout all the conditionseach target-type appeared equally frequently and in randomorder. YE was informed about the precise nature of the exper-iment and was asked to try to maintain fixation throughoutthe experimental session.


Figure 6 shows YE’s mean percentage of correct responses asa function of target type (single left target, single right targetand target in both sides), cue condition (no cue, ‘on target’cue and ‘overshoot’ cue) and font size (32 and 38).

Similarly to Experiment 3, YE had no difficulties in identi-fying single right targets independently of font size and cuecondition (he was perfect in all the conditions). He also madeno errors on no-target trials. With single-left targets he wasrelatively good in the baseline condition where no cues wereintroduced (Experiment 4a - 12/20). However, YE could notperceive any of the double targets when no precues were pre-sented (0/20 - Experiment 4a; all errors involved his report-ing just the presence of a single right side item). InExperiments 4b-4d, where precues were presented before the

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target, his perception of left-single targets was improved (36/40, 33/40 and 31/40, for ‘on target’ cues font-size 32-points,‘on target’ cues font size 38-points and ‘overshoot’ cues fontsize 32-points, respectively). With double simultaneous tar-gets, there was a minimal effect of the ‘on target’ cue when32-points letters were used (4/20 - Experiment 4b). Therewas evidence for a combined effect of spatial cueing of atten-tion and for increasing the size of the letters on YE’s abilityto perceive the double simultaneous targets. Performancewith an ‘on target’ cue and larger letters (38-points in Experi-ment 4c) was better than in the no cue baseline (13/20 vs. 0/20; χ2

(1) = 19.2, p = 0.000) and than in Experiment 4b, whenthe same cue was used but with a smaller target letter size(13/20 vs. 4/20; χ2

(1) = 8.29, p = 0.004). Moreover, YE’s per-formance with double simultaneous targets using ‘on target’cues and 38-points letters (Experiment 4c) was significantlybetter than his performance in Experiment 1b where the sameletter size was used without any precues and without tempo-ral interval prior to the target (13/20 vs. 0/20, respectively;χ2

(1) = 19.2, p = 0.000). The ‘overshoot’ cue with 32-pointsletters improved performance substantially (17/20 targetsdetected on bilateral trials preceded by left overshoot precue– Experiment 4d; χ2

(1) = 16.94, p = 0.000 vs. Experiment 4b).Note that none of the above manipulations affected perfor-

mance on trials in which the double simultaneous targets fol-lowed a right cue (1/20 in Experiment 4b, 4c and 4d). Report

of the single left target tended to be better in all the cueingconditions relative to the baseline (36/40 vs. 12/20 inExperiment 4b - χ2

(1) = 7.5, p = 0.006; 33/40 vs. 12/20 inExperiment 4c - χ2

(1) = 3.6, p = 0.056; 31/40 vs. 12/20in Experiment 4d - χ2

(1) = 2.01, p = 0.16;). The cueing effectfor the single left target was much more modest than thecueing effect for the double simultaneous targets. However,the baseline of the former was much higher than the latter.

In this study, the baseline level of single left-side target(12/20) was higher than in Experiments 1a, 2a and 3a (0/20,4/20 and 0/20, respectively). We assume that at least two fac-tors contributed to this result. Firstly, in Experiment 4a, abrief empty interval of 90 msec was introduced after the off-set of the fixation cross and before the onset of the target.This interval may have positively affected YE’s ability to dis-engage his attention from fixation and shift it to the left visualfield where the left target was presented. Secondly, it is pos-sible that learning improved his baseline level of perfor-mance. Over and above any effects of learning on thebaseline condition, cueing YE’s attention to the left improvedhis detection of left-side targets. In the single left condition,there was a tendency for a benefit from both ‘on target’ and‘overshoot’ cues, both with the smaller (size 32) and largerletters (size 38). Cueing attention to the left target locationdid not improve performance under bilateral presentationconditions, however (Experiment 4b). The same cue did

Fig. 6. Experiment 4: YE’s total percent hits in each target type as a function of cue condition and physical size of the target.

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target in both sides - left cue

target in both sides - right cue

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improve bilateral performance when the target letters werelarger (Experiment 4c), and an overshoot cue to the lefthelped to resolve extinction even with smaller targets (Exper-iment 4d). Hence, there is evidence here for a ‘pure’ atten-tional benefit, but primarily when the cue overshoots the left-field letter. This replicates the data reported by Shalev andHumphreys (2000), and it is consistent with attention gener-ally undershooting to stimuli in the left field of a patient withright parietal damage. In addition, there is some suggestion ofa combined effect of attention and perceptual intensity, sincethe attentional cue (the ‘on target’ cue) reduced extinctionwhen combined with a large letter (Experiment 4c), whenperformance was compared with Experiment 1 with the samesize letters. However, our conclusions on this point should becautious because the temporal interval between the fixationand the target was introduced here compared with Experi-ment 1 (due to the presentation of the cue).

General Discussion

We have shown that, in a patient with unilateral right hemi-sphere damage:

• Gradual magnification of the font size of all stimulireduced the effects of any perceptual deficit for left-sidestimuli, whereas no reduction in extinction was recorded(Experiment 1).

• A small increase in the relative size of the left stimulus ledto a recovery of a left-side perceptual impairment with onlya very modest reduction of extinction. However, a strongermanipulation of the relative size of the left target yielded animpressive reduction of extinction (Experiment 2)

• Relatively small leftward shift of the fixation yielded a sig-nificant improvement in the effects of any perceptual defi-cit. However, a much more substantial leftward shift offixation was necessary in order to significantly reduceextinction. These results converge with the results ofExperiment 2 (Experiment 3)

• Cueing the spatial location of the left target (‘on target’cues) prior to its appearance decreased the effect of a per-ceptual deficit, but not the amount of extinction. (Experi-ment 4b)

• ‘Overshoot’ cues reduced extinction – an evidence for a‘pure’ attentional benefit. (Experiment 4c)

• A combined attentional and perceptual manipulation (‘ontarget’ cues and increased target size – Experiment 4d) pro-duced a significant reduction of extinction, while each ofthese manipulations separately did not yield a similarlystrong effect. Apparently, perceptual and attentional factorsinteract to affect extinction, in YE’s case.

These data indicate that two different manipulations canattenuate the chronic right-bias in YE’s performance: 1. Per-ceptual magnification of the left stimulus relative to the rightone, so that the left stimulus will attract more attention (or

alternatively its processing will require less attention). 2.Shifting attention to the left visual field prior to the onset ofthe target.

In Experiments 1 to 3, no catch trials were included. Theabsence of catch trials could have influenced YE to respond‘left’ each time he did not detect any stimulus, or in otherwords, to increase artificially his rate of detecting single lefttargets. However, there were no signs of such bias and therate of single left target detections was clearly affected by theexperimental manipulations in Experiments 1, 2 and 3.

In all the present experiments, recovery from the effects ofa left-side perceptual deficit was a necessary but not a suffi-cient condition to reduce extinction. In order to affect extinc-tion we also had to alter the distribution of attention in theleft relative to the right visual field. We did this by increasingthe relative perceptual saliency of the left target (Experiments2–3) and by shifting attention to the left visual field (Experi-ment 4). The recovery effects on extinction that wereobtained as a result of increasing the perceptual salience ofthe left target cannot be attributed to a perceptual deficit tiedto the absolute size of the left side letter in YE’s case sincethe very same manipulation of increasing the size of both theleft and right stimuli did not affect extinction at all.

In this study, we take the presence of poor report of singleleft-side stimuli as being indicative of a residual perceptualimpairment. Interestingly, this perceptual deficit, found evenwith single left-side items, was reduced by increasing theabsolute perceptual saliency of the stimulus (e.g., its size oreccentricity). However, the increases that improved report ofsingle left-side items did not necessarily improve extinction,when a letter was present on the right side. A possible expla-nation for the above dissociation can be the existence of twodeficits: one – hemianopia, which is a consequence of YE’soccipital lesion and the second – extinction, which is theresult of the massive parietal damage. However, further stud-ies should explore the effects of perceptual saliency and ofattention on hemianopia with patients who suffer from hemi-anopia but not from extinction. To reduce extinction, we hadto introduce a relative imbalance in the perceptual salienceand/or the degree of attention allocated to left- and right-sideitems. This suggests that extinction, in particular, reflects therelative degree of competition for selection between left- andright-side items (e.g., see Duncan et al., 1997). Parietal dam-age may yield a gradient of attention in which ipsilesionalstimuli dominate any competition for selection, and therebycontrol behavior (Marshall and Halligan, 1990; Kinsbourne,1970, 1977; Ladavas et al. 1990). A single target presented inthe contralesional visual field might be neglected eitherbecause there are not enough resources at that location orbecause of a residual perceptual deficit. Increasing the per-ceptual magnitude of the target will increase the probabilityof detection because then fewer attentional resources arenecessary and/or there is an increase in the perceptual infor-mation available. When two targets are presented simulta-neously in both visual fields, the right one attracts moreresources due to the biased rightward gradient of attention.

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As a result, left stimuli can be extinguished under brief pre-sentation conditions (see Heinke and Humphreys, 2003, forexplicit simulations). According to the above approach, inorder to overcome the chronic right-bias, we must alter theunbalanced gradient of attention. Our data show that a changecan be achieved either by a perceptual magnification of theleft stimulus relative to the right one, so that the perception ofthe former will require less attention, or by artificially shift-ing attention to the left.

These results are consistent with recent data from func-tional brain imaging of patients showing extinction.Veuilleumier and his colleuges (2002) have demonstratedthat extinguished stimuli can still activate early corticalregions in the impaired visual field. However, they do soless than single items presented to the contralesional side.Our data show that the relative increase in the perceptualsaliency of an item needs to be greater under bilateral thanunilateral presentation conditions in order to produce bet-ter perceptual report. This is consistent with perceptualsalience being decreased further under double simulta-neous stimulation, though this reduced saliency can becounter-acted through increased attention. Our findingssuggest that both perceptual and attentional factors cancontribute to extinction, and, moreover, the effects interactin modulating performance.

References

Antis SM. A chart demonstrating variations in acuity with retinal position.Vision Research 1974; 14: 579–582.

Arai T, Ohi H, Sasaki H, Bobuto H, Tanaka K. Hemispatial sunglasses:Effect on unilateral spatial neglect. Archives of Physical and MedicalRehabilitation 1997; 78: 230–232.

Arguin M, Bub D. Modulation of the directional attention deficit in neglectby hemispatial factors. Brain and Cognition 1993; 22: 148–160.

Bartolomeo P, Chokron S. Orienting of attention in left unilateral neglect.Neurosicience and Biobehavioral Reviews 2002; 26: 217–234.

Beis JM, Andre JM, Baumgarten A, Challier B. Eye patching in unilateralspatial neglect: Efficacy of two methods. Archives of Physical andMedical Rehabilitation 1999; 80: 71–76.

Butter CM, Kirsch N. Combined and separate effects of eye patching andvisual stimulation on unilateral neglect following stroke. Archives ofPhysical and Medical Rehabilitation 1992; 73: 1133–1139.

Corbetta M. Positron emission tomography as a tool to study human visionand attention. Proceeding of the National Academy of Sciences of theUnited States of America. 1993 Dec 1; 90(23); 10901–3.

Di Pellegrino G, De Renzi E. An experimental investigation on the nature ofextinction. Neuropsychologia 1995; 33: 153–170.

Duncan J, Humphreys GW, Ward R. Competitive brain activity in visualattention. Current Opinion of Neurobiology 1997; 7: 255–61.

Farah MJ, Monheit MA, Wallace MA. Unconscious perception of ‘extin-guished’ visual stimuli: reassessing the evidence. Neuropsychologia1991; 29: 949–958.

Gainotti G, D’Erme P, Bratolomeo P. Early orientation of attention towardthe half space ipsilateral to the lesion in patients with unilateral braindamage. Journal of Neurology, Neurosurgery & Psychiatry 1991; 54:1082–1089.

Gorea A, Sagi D.“Failure to handle more than one internal representation invisual detection tasks”. Proceedings of the National Academy of Sci-ences in the United States of America, 2000, 97; 12380–4.

Heilman KM, Bowers D, Valenstein E, Watson R T. Disorders of visualattention. Baillieres Clinical Neurology 1993; 2: 389–413.

Heilman KM, Valenstein E. Mechanisms underlying hemispatial neglect.Annals of Neurology 1979; 5: 166–170.

Heinke D, Humphreys GW. Attention, spatial representation, and visualneglect: simulating emergent attention and spatial memory in theselective attention for identification model (SAIM). PsychologicalReview 2003; 110: 29–87.

Kinsbourne M. A model for the mechanism of unilateral neglect of space.Transactions of the American Neurological Association 1970; 95:143–146.

Kinsbourne M. Hemi-inattention and hemispheric rivalry. In: EA., Weinstein,RP., Fridland, editors. Hemi-inattention and Hemispheric Specialization.New-York: Raven Press, 1977.

Kinsbourne M. Mechanisms of unilateral neglect. In: M., Jeannerod, editor.Neurophysiological and Neuropsychological Aspects of UnilateralNeglect. North-Holland: Amsterdam, 1987; 69–86.

Kinsbourne M. Mechanisms of neglect: Implications for rehabilitation.Neuropsychological Rehabilitation, 1994; 4: 151–153.

Ladavas E, Petronio A, Umilta C. The deployment of visual attention in theintact filed of hemineglect patients. Cortex 1990; 26: 353–366.

Marshall JC, Halligan PW. The psychophysics of visuo-spatial neglect: Anew orientation. Mdical Science Research 1990; 18: 429–430.

Mesulam MM. Attention, confusional states and neglect. In: M.M., Mesu-lam, editor. Principles of behavioural neurology. Philadelphia, PA:FA. Davis, 1985.

Posner MI, Walker JA, Friedrich FJ, Rafal RD. Effect of parietal injuryon covert orienting of attention. Journal of Neuroscience 1984; 4:1863–1974.

Rafal RD. Neglect. Current Opinion Neurobiology 1994; 4: 231–6.Riddoch MJ, Humphreys GW. The effect of cueing on unilateral neglect.

Neuropsychologia 1983; 6: 589–599.Robertson IH, Manly T, Beschin N, Daini R, Haeske-Dewick H, Homberg V,

Jehkonen M, Pizzamiglio G, Shiel A, Weber E. Auditory sustainedattention is a marker of unilateral spatial neglect. Neuropsychologia1997; 35: 1527–1532.

Shalev L, Humphreys GW. Biased attentional shifts associated with unilat-eral left neglect. Cognitive Neuropsychology 2000; 17: 339–364.

Vuilleumier P, Armony JL, Clarke K, Husain M, Driver J, Dolan RJ. Neuralresponse to emotional faces with and without awareness: event-relatedfMRI in a parietal patient with visual extinction and spatial neglect.Neuropsychologia 2002; 40: 2156–2166.

Walker R, Findlay JM, Young AW, Welch J. Disentangling neglect andhemianopia. Neuropsychologia 1991; 29: 1019–1027.

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Documents

Interactive perceptual and attentional limits in visual extinction