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Impaired orientation discrimination andlocalisation following parietal damage: Onthe interplay between dorsal and ventralprocesses in visual perceptionM. Jane Riddoch a , Glyn W. Humphreys a , Sarah Jacobson a , GrahamPluck a , Andrew Bateman a & Martin Edwards aa University of Birmingham , Birmingham, UKPublished online: 13 May 2010.

To cite this article: M. Jane Riddoch , Glyn W. Humphreys , Sarah Jacobson , Graham Pluck , AndrewBateman & Martin Edwards (2004) Impaired orientation discrimination and localisation followingparietal damage: On the interplay between dorsal and ventral processes in visual perception , CognitiveNeuropsychology, 21:6, 597-623, DOI: 10.1080/02643290342000230

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IMPAIRED ORIENTATION DISCRIMINATION AND

LOCALISATION FOLLOWING PARIETAL DAMAGE:ON THE INTERPLAY BETWEEN DORSAL AND

VENTRAL PROCESSES IN VISUAL PERCEPTION

M. Jane Riddoch, Glyn W. Humphreys, Sarah Jacobson,Graham Pluck, Andrew Bateman, and Martin Edwards

University of Birmingham, UK

We report the case of a patient with left parietal damage (MH) who is selectively impaired at bothdetecting and localising targets defined in terms of their orientation. Performance was relatively good inother tasks where the target was defined either by a single feature (colour) or an orientation conjunction.The results are consistent with the idea that the dorsal stream supports some aspects of basic visualperception (i.e., the discrimination and localisation of orientation-defined targets). The effect of aparietal deficit may be to stress processing via the intact ventral stream, which groups information forobject recognition, making differences in orientation less salient. Consistent with this, MH performedbetter when localising targets at a larger display size, where elements were less likely to group into afamiliar shape and where local texture-based processes can come into play. In addition, MH’s ability tolocalise a target was more impaired than his ability to detect a target, supporting the argument thatorientation discrimination precedes (or operates independently of) feature localisation.

INTRODUCTION

Ungerleider and Mishkin (1982) carried out aclassic series of lesion studies with primates, inwhich they showed the following: If a task requiresthat two visually presented forms be discriminated,the ventral visual pathway is critical; in contrast, ifthe task requires discrimination between differentspatial locations, the integrity of the dorsal visualpathway was necessary. More recently the differentroles of the dorsal and ventral visual pathways have

been distinguished in terms of whether visualinformation is processed for perception or for theprogramming and on-line control of visuallyguided movement (Goodale, Milner, Jakobson, &Carey, 1991; Milner et al., 1991). The case of DFhas been used as a striking illustration of the disso-ciation of these two visual pathways (see Milner etal., 1991). DF suffered anoxia as a result of fumesleaking from a faulty gas water heater, which causeda severe impairment of visual object recognition(she could recognise few real objects and no line

COGNITIVE NEUROPSYCHOLOGY, 2004, 21 (6), 597–623

� 2004 Psychology Press Ltdhttp://www.tandf.co.uk/journals/pp/02643294.html DOI:10.1080/02643290342000230

597

Correspondence should be addressed to Jane Riddoch, Behavioural Brain Sciences, Hills Building, School of Psychology,University of Birmingham, Birmingham B14 2TT (Email: M.J.Riddoch@bham.ac.uk).

We are indebted to MH and JH for their involvement in this project. The first and second authors were supported in this work bygrants from the Medical Research Council and the Stroke Association. The first, second, and fourth authors were supported in thiswork by the Wellcome Trust. The first author is grateful for a travelling fellowship awarded by La Trobe University, Melbourne, whichalso supported the report. We acknowledge with thanks the help of Luke Donnelly and Alec Critchlow in collecting and collatingsome of the control data.

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drawings). Her copying was poor and she was atchance at making perceptual judgements evenabout the basic dimensions of form (see Milner etal., 1991). Despite this, DF was able to reachappropriately to objects. Her grasp aperture andhand orientation were tuned to the size and orien-tation of stimuli and the kinematics of hermovements were normal. Milner and Goodale(1995) argue that DF’s lesion affected the ventralrather than the dorsal visual system, compromisingperceptual judgements about even the basic dimen-sions of form. However, the dorsal visual streamremained intact. Thus, Milner and Goodaleconcluded that the dorsal stream not only codesstimulus location (cf. Ungerleider & Mishkin,1982), but also provides visual information forprehensile actions such as reaching and grasping.Orientation and size information can be coded bythe dorsal stream and used for action, even if codingthe same information in the ventral stream isimpaired. On this view, the ventral stream (forobject recognition) and the dorsal system (forlocation coding and action) are separated even fromearly stages of cortical coding. Consequently,patients such as DF can have a deficit in early shapecoding in the ventral system along with intact earlyshape coding in the dorsal system.

Cases such as DF raise the question of whatkinds of form information can be used by the dorsalstream, in order that actions be made to visualstimuli. In addition, we may ask: Is form informa-tion coded by the ventral object recognition systemin a manner that is qualitatively different from theway it is coded in the dorsal stream?

Additional studies with DF provide someindications that the information used by the dorsalstream is relatively limited. For example, when DFhad to code the relations between stimuli, thenimpairments were apparent even on action tasks.Goodale, Jakobson, Milner, Perrett, Bewon, andHietanen (1994) reported that, in contrast to whenshe had to post a letter (coding a single orientation),DF was poor at posting a T-shaped object into a

T-shaped aperture. This deficit, when severalorientations must be coded, extended beyondorientation information to include the coding ofrelative locations of other simple visual tokens.Murphy, Carey, and Goodale (1998), for example,found that DF was unable to judge whether arrayscontaining up to five tokens of different colours hadthe same configuration. Dijkerman, Milner, andCarey (1998) also demonstrated that DF wasunable to orient her hand correctly to put herfingers in three holes in a Plexiglass disk (as in a ten-pin bowling ball), though she could place twofingers correctly when there were just two holes(perhaps coding a single orientation between theholes). Thus, to judge from DF, the dorsal systemseems limited at coding spatial relations based onform information.

In contrast to patients such as DF, patients withdorsal (parietal) lesions can be poor even at codingsingle line orientations.1 Damage to the left parietallobe is linked to the disorder of constructionalapraxia (De Renzi, 1982), where patients can beimpaired at representing line orientations correctlyin copying and drawing. In some cases, this can evenextend to simple perceptual tasks. We (Riddoch &Humphreys, 1988) reported one patient (LM) witha left parietal lesion who had clear difficulties withleft–right oriented stimuli. The problem was foundin action tasks (e.g., in copying lines and arm move-ments, in block construction tasks, etc.) and also inperceptual judgement tasks (when asked to decidewhether an oriented line and its mirror reflectionwere the same). This problem, with oriented lines,might reflect the classic “oblique effect” in percep-tion (Appelle, 1972), with oblique stimuli beingcoded more coarsely at a neuronal level than stimuliin cardinal horizontal and vertical orientations(Wang, Ding, & Yunokochi, 2003). However, anearly locus to the effect seems unlikely given thatLM had intact object recognition. Thus, evenoblique edge orientations were coded and usedimplicitly in object processing. Explicit judgementsof line orientation, however, may require coding

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1 Similar effects have been observed in animal studies. Eacott and Gaffan (1991) report impaired orientation discriminationrelative to shape discrimination after inferior parietal lobule lesions in the monkey (including the lateral bank of the inferiorparietal sulcus).

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concurrent variation along horizontal (x) and vertical(y) coordinates within a frame based on the patient’sbody, a process that seems to be impaired bydamage to the dorsal visual stream. The importanceof the parietal lobes for orientation coding is alsosuggested by several recent reports of patients withparietal lobe damage and with impairments in tasksrequiring mirror-image discrimination of picturedobjects (Davidoff & Warrington, 2001; Priftis,Rusconi, Umiltà, & Zorzi, 2003; Turnbull &McCarthy, 1996; Warrington & Davidoff, 2000).Again, coding of object orientation in relation to aexplicit coordinate frame seems crucial for suchdiscriminations. This process seems disrupted byparietal damage. Other patients with lesions to thedorsal visual stream have been shown to be impairedat making both absolute and relative location judge-ments to visual stimuli. Absolute judgements, aboutthe positions of single stimuli in space in relationto the patient, are often poor—for both action(e.g., pointing) and perceptual judgement tasks(Humphreys & Price, 1994; Perenin & Vighetto,1988; Ratcliff & Davies-Jones, 1972). In addition,Shalev and Humphreys (2002) have recently shownthat judgements about the parts within novelstimuli can be difficult. Their patient, GK, sufferedbilateral parietal damage and Bálint’s syndrome.He was, for example, unable to judge whether dotsfell towards the top or bottom of an oval surround-ing shape. Nevertheless, he could perform the tasksuccessfully if primed to treat the oval as either anupright or inverted face (eyes top and bottom res-pectively). Again, visual information (in this case,relative coding of the spatial positions of the parts)was assimilated implicitly, for object recognition,but was not available for an explicit spatial judge-ment task. Thus, the dorsal system seems to domore than use visual information for action; it alsomodulates explicit perceptual judgements involvinglocation as well as orientation information.

In the present paper we report data on a patientwith posterior parietal damage who shows impairedperceptual judgements as well as actions toboth line orientations and their locations. Theseimpaired judgements to line orientations contrastwith formally equivalent judgements using colour,which are relatively spared (including localisation).

Thus the type of information leading to the judge-ment, and not the judgement itself, is critical. Wealso show that the patient, MH, is detrimentallyaffected when oriented lines appear to grouptogether. When presented with an orientation-defined target in the context of lines of anotherorientation, MH would sometimes detect thatthere was a line with an odd orientation present, buthe was extremely impaired at localising this item.This was particularly the case when the lines fell atlocations that defined a known shape (e.g., a squareor a diamond), and the elements were spaced suffi-ciently apart to prevent any contribution from localtextural discontinuities to contribute to perfor-mance (Nothdurft, 1991, 1992). The case conse-quently raises questions about (1) the implicitcoding of line orientation within its spatial context,a process we suggest is conducted within a ventralobject-recognition stream, and (2) explicit spatialjudgements to the same input, a process supportedby the dorsal visual stream. We suggest that, in apatient with a “weakened” dorsal stream, explicitjudgements can be disrupted because they are“over-influenced” by the more spared ventralstream. We discuss the implications for under-standing orientation and location coding in thebrain, for understanding the relations betweenorientation coding within and outside spatialcontexts, and, more generally, for interpretingneuropsychological data. In MH’s case, we suggestthat outputs from an impaired system are maskedby outputs from a more preserved system. We donot witness a normal system minus a particularcomponent but a system changed by altering thebalance between two separate processes thatnormally code line orientations for differentpurposes.

BACKGROUND INFORMATION

Case history

MH suffered an anoxic incident when aged 42. Atthe time he was a garage manager, and his previousoccupations included having been a bricklayer, awelder, and a chef. Following the incident he was

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admitted to hospital, with some right-side muscleweakness and raised sensory thresholds on theright, but in all other respects the neurologicalexamination was unremarkable. His wife reportedthat while he had no difficulty in walking or in usinghis arms, he had extreme difficulty in all activities ofeveryday living (ADL). He was unable to dress, orto use a knife and fork. He could not place the mugon the table without it tipping over. Over time,performance improved but there were still someaspects of ADL that were problematic. He couldnot fasten a necktie or shoelaces, and had difficultyputting on his socks. He could not coordinate aknife and fork when eating, and after cutting hisfood would eat using a fork only. Writing was veryimpaired and he could only produce upper- but notlower-case letters (see Figure 1).2 Figure 2 illus-trates MH’s attempts to reproduce figures from theWechsler Memory Scale (Wechsler, 1987).

The CT-scan report stated that there wasincreased signal in the lentiform nucleus bilaterally,slightly more pronounced on the left than the right.There was some involvement of the heads of thecaudate nuclei bilaterally. There was some increasedsignal in the subcortical and cortical regions of thehemispheres, particularly in the left posteriorparietal region, but also to a lesser extent in the leftfrontal region. The EEG report indicated an excessof slow wave activity over the right hemisphere.The present experimental tests were conductedover a 3-year period, starting 1 year after theincident. Performance was stable during this time.

Neuropsychological performance

Visual neuropsychological ability was assessedusing the Visual Object and Space PerceptionBattery (VOSP; Warrington & James, 1991) , once1 year post incident and once 3 years later. Thescores are shown in Table 1, along with data fromother neuropsychological tests. Essentially, MH

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2 MH’s letters were clumsy and ill formed. He showed particular difficulties with letters with oriented lines; for instance, he wasunable to determine the slope of the individual lines making up the letter W (immediately above the letter Z in Figure 1), and he foundit easier to produce a + sign than an X. This was particularly striking as, due to his inability to write, he used an X as a signature and hadmore practice with this letter than with other letters. He also had some difficulties with mirror-image letters (e.g., the misorientation ofthe letter C in Figure 1).

Table 1. MH’s performance on basic neuropsychological tests ofobject and space perception tested at 1 and 4 years post lesion

Time post lesion

Test 1 year 4 years

VOSPScreen

Shape detection 16/20a 19/20Incomplete letters 19/20 20/20

Object recognitionSilhouettes (object naming) 17/30 19/30Object decision 19/20 19/20Progressive silhouettes 10 9

Space perceptionDot counting 3/10a 4/10a

Position discrimination 17/20a 19/20Number location 5/10a 2/10a

Cube analysis 2/10a 3/10a

BORBPerception

Circle size (same/different)b 15/30*Line length (same/different) 21/30*

Object recognitionPerceptual categorisation 24/25Association match 30/30Picture naming 76/76

BITStar cancellation 47/52c

Letter cancellation 38/40c

VOSP: Visual Object and Space Perception Battery(Warrington & James, 1991); BORB: Birmingham ObjectRecognition Battery (Riddoch & Humphreys, 1993); BIT:Behavioural Inattention Test (Wilson, Cockburn, &Halligan, 1987).

a Impairment relative to the published control data (2 SD fromthe control mean).

b MH was unable to perform this test, saying that all thecircles appeared to be the same size even on “different”trials.

c MH made few omissions on the cancellation tests; whereomissions occurred, they tended to be on the left (3/5 in thestar cancellation test, 2/2 on the letter cancellation task).

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performed the tests of object recognition at controllevels, but he was impaired on all tests of spaceperception. On the BORB (Birmingham ObjectRecognition Battery; Riddoch & Humphreys,1993), MH again did well at object recognitiontests but had difficulty on tests of perceptual

judgement based on basic visual attributes (e.g.,circle size, line length). The spatial difficultiesapparent in MH were not due to unilateral neglect.He showed no neglect on selected subtests of theBIT (Behavioural Inattention Test; Wilson,Cockburn, & Halligan, 1987; see Table 1).

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Figure 1. Illustrating MH’s attempts to write letters of the alphabet from memory. The task took 15 minutes. Note the particular difficultywith letters requiring an oriented line component.

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MH’s sensitivity to mirror-rotated objects wasalso assessed. Test items were six objects depictedwith either a long horizontal axis (elephant, zebra),a long vertical axis (kangaroo, kettle), or a longoblique axis (pliers, axe). In addition, two geomet-ric figures were presented, a right-angle and a 60°angle. A trial consisted of three pictures of the sameobject, one of the three being the mirror image ofthe other two. The position of the odd-man-outwas randomised across trials. There were a total of48 trials (each stimulus triad being repeated sixtimes, the stimuli randomised over the session).MH scored 40/48. He made 1 error each withstimuli with a long horizontal or a long vertical axis(22/24 correct), 2 errors with objects with anoblique axis of orientation (10/12 correct), and4 errors with the geometric figures (8/12 correct).There was no major indication here of an “orienta-tion agnosia” in discriminating mirror images ofobjects, and indeed if anything his performance

tended to be better with known objects than withgeometric figures. It is interesting to note that atleast one “orientation agnosic” patient has beenreported in whom mirror-image discriminations oflines was preserved relative to objects (Davidoff &Warrington, 2001) and that in another, perfor-mance was more impaired for those stimuli she wasable to identify than those she was not (Warrington& Davidoff, 2000).

To assess MH’s spatial difficulties in moredetail he was given sets of same–different matchtasks, matching-to-sample, and construction tasksusing arrays with either two lines joined at oneend to form an angle, or arrays constructed fromsquares.

Line stimuliIn the same–different matching task MH had toindicate whether two angles placed side-by-sidehad the same or different angular displacement. For

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target copycopy

Figure 2. MH’s attempts to copy items from the Wechsler Memory Scale.

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‘different’ trials there were three levels of difficulty(with 18 trials at each level): small difference (5º),medium difference (10º), and large difference (15º).There were equal numbers of same and differenttrials. MH scored 57/108 (52.8%) correct (he had atendency to answer “same” scoring 48/54 and 9/54correct on same and different trials respectively). Inthe matching-to-sample task MH was asked toindicate which of two different angles matched atarget angle. He scored 14/24 (58.3%) correct.Three age-matched controls performed these taskseffortlessly.

Squares stimuliIn the matching-to-sample tasks MH was askedto indicate which of two arrays of blocks matcheda target stimulus. The target array was placed in aframe and situated below the other two stimuli.The arrays could consist of two or three blocks thatwere aligned horizontally, vertically, or angled (seeFigure 3), although on any trial targets and dis-tractors consisted of the same number of blocks.Target arrays were always angled. Performance wasnot time limited. With the two-block arrays therewere 16 trials each with horizontal and vertical

distractors and 16 with an angled distractor. Withthree-block arrays there were 8 trials each with ahorizontal and vertical distractor, and 31 trials withangled distractors (16 trials when the to-be-matched item and distractor item were mirrorimages of each other, and 15 when they were not,see Figure 4).

While MH’s performance was good when thedistractor was either horizontal or vertical (47/48correct), performance deteriorated significantly

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Two-block arrays

Three-block arrays

horizontal angledvertical horizontal angled

horizontal angledvertical horizontal angled

Figure 3. Orientations of two- and three-block arrays.

Three-block arrays with angled distractors

mirror-image arrays

asymmetric arrays

Figure 4. Match-to-sample test using three-block arrays.

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with angled distractors (37/47correct), Fisher’sExact p-value = .004. With three-block arraysperformance tended to be poorer with the asym-metric arrays (10/15 vs. 14/16 correct). Again,controls were at ceiling.

A similar pattern of performance was shown inconstruction tasks. When MH was asked toconstruct an array with bricks that was identical to atarget array, he performed well when the targetarray consisted of a horizontal or vertical lineconstructed with two or three bricks (19/20correct). However, when he was asked to constructan angled array (similar to those displayed inFigures 1 and 2) performance deteriorated signifi-cantly (17/30 correct), �2(1) = 8.7, p < .003. ThusMH has an impaired ability to code orientation,found in perceptual and construction tasks. Inparticular, he seemed to have a deficit in the abilityto code the “handedness”of visual stimuli (e.g., inmaking judgements to stimuli with left–rightorganisation of parts; see also patient LM, Riddoch& Humphreys, 1988).

Given his problems in everyday life (e.g., pickingup a mug) we were interested to examine whetherMH could correctly orient his hand to match theorientation of a slot (cf. DF, Milner et al., 1991).We therefore examined MH’s ability to post atablet (95 mm × 41 mm × 3 mm) through a slotteddisc (dimensions of the slot: 98 mm × 44 mm ×5 mm) using either his left or right hand. MH’s wifeacted as a control. The slot in the disc was posi-tioned at one of four different orientations: vertical,horizontal, diagonally oriented with the top onthe left, or diagonally oriented with the top on theright. Infra-red markers were attached to the hand,allowing movement to be detected by two infra-redsensitive cameras (MacReflex 2.5); kinematicvariables were extracted using Wingz software.Movement time was taken as the time taken to pickup the tablet and to post it through the slot (posi-tioned 50 cm in front of MH). There were threetrials per condition.

The results are shown in Figure 5. The orien-tation of the slot had little effect on movementtime for the control subject, who also showed nodifference in the time taken with left or right hands;however, the position of the slot did affect MH’s

performance. Overall, he was much slower than thecontrol subject, and this effect was particularlymarked for his right hand. In addition, with bothleft and right hands, movement times were longerwhen the slot was oriented left or right relative towhen it was horizontal or vertical. MH also madefrequent errors when performing this task, ofteninitially placing the tablet at the mirror imageorientation to the orientation of the slot. He wouldthen use tactile contact with the slot to reorient thetablet so that it eventually went in.

We have demonstrated on a range of clinicaltests that MH’s performance is particularly im-paired when judgements of orientation have to bemade, whether these are perceptual tasks (such asthe match-to-sample test) or whether an action hasto be initiated. In contrast, his object recognitionabilities are spared. The particular difficulty shownby MH with orientation perception, and the diffi-culties shown in correctly orienting the hand inorder to post a tablet, led us to perform a detailedseries of investigations in order to pinpoint thelocus of his difficulties.

EXPERIMENTAL INVESTIGATIONS

MH’s perceptual judgements to line orientation,and to the locations of line orientations, wereexamined more formally using visual search andtexture judgement tasks. Performance when targetswere defined by orientation was contrasted withperformance when targets were defined by a colourdifference relative to distractors.

In studies of orientation perception in normalsubjects it can be argued that horizontal and verticalorientations serve as “standards,” and that obliquelines are coded in terms of their divergence fromthese standards (see Treisman, 1985; Treisman &Gormican, 1988, for evidence from search asym-metries). Hence we contrasted MH’s perception of“standard” (horizontal and vertical) orientationswith his perception of oblique orientations, andin particular we used judgements requiring the left–right coding of oblique orientations (e.g., target /,distractors \). We examined performance whensingle line orientations had to be coded, and when

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stimuli were defined by conjunctions of line orien-tations (T elements—see Goodale et al., 1994).Since actions based on orientation conjunctionswere problematic for DF, their coding may dependto some degree on processing within the ventralvisual system. We also varied whether MH had todetect a target defined by line orientation or colour,and whether a localisation judgement had to bemade to the target. Finally the spatial contextsurrounding the target was varied, to assess if thecontext “masked” the location of the target for MH.

Experiment 1: Visual search for targetsdefined in terms of a single feature(colour or orientation)

Experiment 1a: Detection of colour

Method. In Experiment 1 the target was a redoriented line; the distractors were blue orientedlines. Each line was 9 × 1 mm. Each block consistedof 60 trials, 10 target-present and 10 target-absent

for each display size. The display sizes were 4, 8,and 12. The display area for 12 elements was 17 cmin width, 12 cm in height, for 8 elements it was 13 ×9 cm, and for 4 elements 6 × 6 cm. The differentdisplay areas for the different display sizes ensuredthat the density of the arrays was relatively constantover display size (the lines covered respectively10.0%, 6.1%, and 5.2% of the display area, fordisplay sizes 4, 8, and 12). The positions of thedistractors (and the target, when present) variedrandomly on each trial. MH’s performance wascontrasted with that of eight age-matched controls.MH performed two blocks of trials while thecontrols performed one. The experiment was runusing V-search software (Enns, Ochs, & Rensink,1990) on a Macintosh Powerbook computer.Responses were made by button press using the Xand C buttons pressed by the middle and indexfingers of the left hand.

Results and discussion. The mean correct reactiontimes (RTs) are shown in Figure 6. The mean

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Figure 5. The time taken (in seconds) for (a) MH and (b) a control to post a tablet through an aperture set at different orientations.

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percentage correct errors are shown in Figure 7.The number of errors were small for both MH andthe control subjects (the numbers of errors for MHfell within 2 SDs of the mean errors for the controlsfor all display sizes and for both present and absenttrials). MH’s RTs were inflated in all conditionsrelative to those of the controls, by an average 194msec. MH’s data were analysed by treating each RTas a separate subject (see De Haan, Young, &Newcombe, 1987). There was a significant maineffect of target present/target absent, F(1, 111) =29.1, p < .0001, indicating that MH was faster ontarget present trials. There was also a significanteffect of display size, F(2, 111) = 3.6, p < .03.The interaction was not significant, F(2, 111) =0.8, p > .05. For the controls the main effectof target present/target absent was not reliable,

F(1, 7) = 2.66, p > .05, but there was a significanteffect of display size, F(1, 7) = 42.50, p < .0001.The interaction was not significant, F(1, 7) = 4.08,p > .05.

The values for the intercepts, slopes, and per-centage of variance due to linearity were computedfor present and absent trials for both MH and thecontrols. The values (with standard deviations inbrackets) are given in Table 2. The values obtainedfor MH fall within 2 SDs of the control perfor-mance in nearly all instances (the exception beingthe intercept on target absent trials).

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Figure 7. Percentage correct for MH and the controls inExperiment 1.

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The results indicate that although MH wasslower to respond than controls, particularly ontarget absent trials, the slopes for both targetpresent and target absent trials fell within 1 SD ofthe control mean. This suggests that the basicprocesses underlying search for a coloured targetwere relatively intact. On absent trials, MH mayhave adopted a relatively conservative responsecriterion, generating relatively slow RTs (Chun &Wolfe, 1996; Humphreys & Müller, 1993).

Experiment 1b: Detection of an “easy” orientationdifference

Method. In Experiment 1b the target consisted of ahorizontal line with vertical line distractors. Thelines were depicted in black against a white back-ground. In all other respects the procedure wasidentical to that described for Experiment 1a. MHand the controls undertook four and two blocks oftrials respectively.

Results and discussion. As in Experiment 1a, fewerrors were made in this experiment (in fact, MHmade no errors; see Figure 7). The mean correctRTs are presented in Figure 6. The disparity inoverall RTs between MH and the controls was

greater than in Experiment 1a. While RTs weresimilar in Experiments 1a and 1b for the controls(537 ms in Experiment 1a vs. 524 ms in Experiment1b), the same was not true for MH, whose perfor-mance was relatively depressed in Experiment 1b(731 ms in Experiment 1a vs. 1132 ms in Experi-ment 1b; MH’s mean RT in Experiment 1b wasmore than 2 SDs from his mean RT in Experiment1a). For MH, there was a significant effect of targetpresent/target absent, F(1, 234) = 32.7, p < .0001,indicating that he was faster on target present trials.There was a significant effect of display size,F(2, 234) = 6.6, p < .002, and the interaction wasalso reliable, F(2, 234) = 4.1, p < .02. There was agreater effect of display size on absent than onpresent trials. For the controls there was a signifi-cant effect of display size, F(1, 7) = 8.8, p < .02, butneither the main effect of target present/absent northe interaction between present/absent and displaysize proved reliable (both p > .05). The values forthe intercepts, slopes, and percentage of variancedue to linearity are given in Table 3. The valueof the intercept for MH was doubled on bothabsent and present trials relative to that of thecontrols. While similar slopes obtained on targetpresent trials, the value on target absent trialswas dramatically increased in MH (where his

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Table 2. Intercepts, slopes, and percentage of variance due to linearity in the colour detection task (SDs are given inbrackets), Experiment 1a

MH Controls

Target present Target absent Target present Target absent

Intercept (ms) 589 738 506 (94) 458 (54)Slope (ms/item) 9.1 7.6 4.8 (5.1) 9.0 (3.1)r2 .8 .5 .3 .6

Table 3. Intercepts, slopes, and percentage of variance due to linearity in the horizontal line detection task (SDs aregiven in brackets), Experiment 1b

MH Controls

Target present Target absent Target present Target absent

Intercept (ms) 1037 1005 514 (100) 489 (62)Slope (ms/item) 2.8 25 1.3 (1.6) 4.7 (6.0)r2 .5 1.0 .2 .3

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RTs increased linearly with display size). For thecontrols, little of the variance on either targetpresent or target absent trials could be attributed tolinearity.

The pattern of performance of the controlsubjects in the colour (Experiment 1a) and easyorientation (Experiment 1b) tasks was very similar.In contrast, MH found the detection of orientationmuch more difficult than the detection of colour.In our initial investigations, it had seemed thatMH had little difficulty distinguishing betweenhorizontal and vertically oriented stimuli (e.g.,matching-to-sample, and construction tasks);however, when using a more sensitive indicator ofperformance (RT as opposed to error rate withunlimited viewing), we have shown that MH is alsoimpaired in the detection of an “easy” orientationdiscrimination. The implication here is thatperformance should be even more impaired with adifficult orientation discrimination tasks (e.g.,detection of an oriented line among lines of theopposite orientation), as we had demonstrated inthe initial clinical investigations. This was tested inExperiment 1c.

Experiment 1c: Detection of a “difficult” orientationdifference

Method. In Experiment 1c the target consisted of aright-oriented line against left-oriented distractors(all lines were oriented at 45º with respect to thevertical). The procedure was otherwise identical tothat described for Experiment 1a. MH and thecontrols performed four and two blocks of trialsrespectively.

Results and discussion. Error rates remained low (seeFigure 7). The number of errors made by MH fellwithin 2 SDs of the mean of the controls’ errors foreach display size, and for both target present andtarget absent trials. The mean correct RTs are givenin Figure 6. The analysis of MH’s data revealed asignificant effect of target present/target absent,F(1, 449) = 24.0, p < .0001, indicating that he wasfaster on target present trials. There was also asignificant effect of display size, F(2, 449) = 8.0, p <.0004, and a Present/Absent × display size inter-action, F(2, 449) = 3.8, p < .02. In this case, presenttrials were more affected by the display size thanabsent trials. Descriptive statistics for the searchfunctions are shown in Table 4. No reliable effectswere obtained with the control subjects (all p > .05).

Performance in terms of absolute RTs did notdiffer dramatically for the control subjects inExperiment 1c relative to Experiment 1a (meanRTs = 547 ms here vs. 537 ms in Experiment 1a).Also, the slopes of their search functions were lessthan 5 ms/item, within the benchmark typicallytaken to indicate spatially parallel search. However,MH’s RTs were 1.73 times slower compared withExperiment 1a (1261 ms vs. 731 ms, and more than2 SDs from the mean RT in that study) and theywere some 129 ms slower than even Experiment 1b(easy orientation search). MH also showed a largeslope on present RTs (32 ms/item). There was areduced slope on his absent responses. However,MH’s RTs on absent trials were very inflated andthe fact that the slope was less than that for targetpresent trials may simply indicate that he operated acut-off criterion in his search for the target (onabsent trials, the strategy might be: Search for a setperiod based, perhaps on the time taken to detect a

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Table 4. Intercepts, slopes, and percentage of variance due to linearity in the oriented line detection task (SDs are givenin brackets), Experiment 1c

MH Controls

Target present Target absent Target present Target absent

Intercept (ms) 1099 1451 533 (104) 525 (66)Slope (ms/item) 31.6 6.5 0.4 (3.5) 4.7 (5.2)r2 1.0 .8 .4 .3

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target at the largest display size). With both targetpresent and target absent trials the vast majority ofthe variance for MH may be attributed to linearity.Thus, relative to the control subjects, MH clearlyfound the detection of a target defined in terms oforientation a much harder task than the detectionof a target defined in terms of colour. This wasparticularly the case with oriented rather thanhorizontal and vertical stimuli (in Experiment 1cvs. Experiment 1b).

Experiment 1d: Detection of “difficult” orientationusing T stimuliThis experiment was similar to Experiment 1cabove, but now oriented Ts were used rather thansingle line orientations. In order to detect thetarget, subjects now must compute a particularfeature conjunction. Previous visual search studieswith normal participants indicate that simple formconjunctions may be computed in a spatiallyparallel manner, giving rise to efficient search whenthe distractors are homogeneous and group together(Duncan & Humphreys, 1989; Humphreys,Quinlan, & Riddoch, 1989; Humphreys, Riddoch,& Quinlan, 1985). Moreover, search for form con-junction targets can be selectively disrupted afterlesions to the ventral visual system (Humphreys,Müller, & Freeman, 1992). If form conjunctionsare computed in the ventral visual system, and ifresponses to such targets are based on outputs fromthe ventral stream (and so are sensitive to group-ing), then MH may even perform relatively well.On the other hand, given that the relations betweenline orientations must be computed, both withineach item and then in comparison across items, wemay predict that performance should generally bemore difficult.

Method. The method was the same as in Experi-ments 1a–c except that oriented T elements wereused. The target was right oriented (at 45º) and thedistractors left oriented (45º). Each line component

of the T was 9 × 1mm in length and width. MH andthe controls performed four and two blocks of trialsrespectively.

Results and discussion. Both MH and the controlsfound the task relatively difficult when Ts ratherthan oriented lines were used. The error ratesincreased (see Figure 7), and RTs were slower(overall: 823 ms for controls and 1864 ms for MH).The number of errors made by MH still fell within2 SDs of the mean of the controls’ errors for eachdisplay size, and for both target present and targetabsent trials. In addition, the relative increase inRTs for MH and the controls was approximatelythe same, relative to when single lines of the sameorientation were used (in Experiment 1c; forcontrols, RTs were 1.5 times slower here; for MH,RTs were 1.48 times slower).3 Analysis of MH’sRT data indicated a significant effect of targetpresent/target absent, F(1, 204) = 115.0, p <.0001:MH was faster on target present than target absenttrials. There was a significant effect of display size,F(1, 204) = 26.4, p < .0001, but the interaction wasnot reliable (p > .05). The controls only showed aneffect of display size, F(1, 7) = 41.0, p < .0001; themain effect of target present/absent and the inter-action proved unreliable (both p > .05). The meancorrect RTs are presented in Figure 6. For bothMH and the controls, most of the variance wasaccounted for by a linear trend in the data (seeTable 5).

Both MH and the controls found it relativelydifficult to search for a right-oriented T amongstleft-oriented T distractors. This is not too surpris-ing given that the line orientations making up thetargets and distractors had to be conjoined in orderto distinguish one shape from the other. Conjunc-tion search is typically more difficult that search fortargets defined by differences in a single featurerelative to distractors (Treisman & Gelade, 1980).More noteworthy, MH’s search was not sloweddisproportionately (relative to when single oriented

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3 When RTs in Experiment 1c were subtracted from those in Experiment 1d, MH’s RTs fell within 2 SDs of the control means forall display sizes on target present trials and for set size 4 on target absent trials. His RTs fell outside 2 SDs of the control mean for setsizes 8 and 12 on target absent trials, which may again reflect a conservative response criterion being adopted.

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lines were used as stimuli). Also, although MH’serror rates were raised, they still fell within thecontrol range. Thus, while MH clearly found thistask more difficult than the controls, it was no moredifficult than when he had to search through singleoriented lines.

Discussion: Experiment 1The data from Experiment 1 show contrastingpatterns of search performance for MH, accordingto whether targets are defined by colour or orienta-tion relative to distractors. MH’s search for acolour-defined target was relatively efficient, andthe slopes of his search functions did not differ fromthose found with controls. However, he wasimpaired on orientation search tasks. Some impair-ment was apparent even in an easy orientationsearch task (Experiment 1b, horizontal target vs.vertical distractors). Relative to the colour searchtask, MH’s RT increased by 1.55 times whereascontrols actually improved on the easy orientationtask (the ratio of RTs was 0.98 in Experiment 1a vs.Experiment 1b). When oriented line stimuli wereused, MH’s performance deteriorated even further.With single oriented lines, control performancediffered little from that found with horizontal andvertical lines, and little from colour search. MH’ssearch was 1.73 times slower than with colour search,and a further 129ms slower than with horizontaland vertical line stimuli. We conclude that MH hada selective deficit in detecting targets defined byorientation differences relative to distractors.

In Experiment 1d we used targets defined by aconjunctive relationship between line orientations.

Both MH and the controls found these stimulimore difficult than the stimuli used in earlier exper-iments; however, there was little evidence for adisproportionate decrease in MH’s performance.Dealing with conjunction stimuli did not seemparticularly difficult for MH. This contrasts withthe performance of patients with ventral lesions,who can find form conjunctions disproportionatelydifficult (Humphreys et al., 1992). The results areconsistent with form conjunctions being codedwithin the ventral visual system, and with thiscoding being relatively preserved in MH.

During testing MH remarked on several occa-sions that, though he had “the feeling” that anorientation-defined target was present, he wasfrequently unsure of where it was in the display.This was examined more formally in Experiment 2,where MH was required to localise a target definedby a colour or orientation difference relative todistractors.

Experiment 2: Localisation of targetsdefined by colour or orientation

Experiment 2a: Colour-defined targets

Method. The method for Experiment 2 was thesame as for Experiment 1, except that now MH wasasked to localise the target on the VDU. Hewas asked to do this as quickly and accurately aspossible. On absent trials he was asked simply to say“no target.” RTs were recorded by a stopwatch,4

which was started when stimuli were presented andstopped when MH touched the screen. MH began

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Table 5. Intercepts, slopes, and percentage of variance due to linearity in the oriented T detection task (SDs inbrackets), Experiment 1d

MH Controls

Target present Target absent Target present Target absent

Intercept (ms) 1082 1545 655 (152) 540 (105)Slope (ms/item) 55.6 81.9 25 (8.2) 36 (18.8)r2 .9 1.0 .8 .9

4 Though this may introduce some error into the recordings, MH’s RTs were so long in the critical conditions that any recordingerrors are unlikely to be an important factor.

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each trial with his finger placed on a standard posi-tion on the table directly in front of the computer.In Experiment 2a MH was asked to point to a redoriented line target, when it was present. Thedistractors were blue oriented lines. There was atotal of 120 trials (presented in two blocks), 20target present and 20 target absent for each displaysize. The display sizes were 4, 8, and 12 and thepositions of the distractors (and the target, whenpresent) varied randomly on each trial. Controlswere not tested here and our interest concerns therelative differences between MH’s performancewith the different stimuli. Control data are reportedin Experiment 3, where RTs were recorded bycomputer on similar localisation tasks.

Results and discussion. Only RTs on present trialswere recorded: Mean RTs and percentage errors areshown in Figure 8. MH’s RTs were relatively slowhere compared with Experiment 1a, when the samestimuli were used but targets had simply to bedetected (943 ms vs. 731 ms in Experiment 1).Since RTs here included the time to move to thetarget, the increase in latency is not surprising.There was also some evidence of an increased slope(see the descriptive statistics in Table 6 relative toTable 2), although the effect of display size was notreliable here, F(2, 119) = 2.30, p > .05. These data,on the localisation of a colour-defined target, serveas a baseline for the subsequent tasks involvinglocalisation of an orientation defined target.

Experiment 2b: Easy orientation target

Method. The method matched that in Experiment1b, except that in Experiment 2b MH was asked topoint to a horizontal line target when it was presentamongst vertical distractors. The stimuli wereblack, on a white background. There was a total of240 trials (presented in two blocks), with a total of40 target present and 40 target absent trials for eachdisplay size.

Results and discussion. The mean correct RTs andpercentage errors are shown in Figure 8. MH’s RTswere some 432 ms slower here than in Experiment2a (an increase of 1.45 in ratio terms). Thus there

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Table 6. Intercepts, slopes, and percentage of variance due tolinearity, for Experiments 2a–d

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2a 2b 2c 2d

Intercept (ms) 844 1357 4840 1365Slope (ms/item) 12.5 1.0 –146.0 113.9r2 1.0 .02 .6 .9

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Figure 8. RT (ms) for MH on target present trials inExperiment 2.

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was a relative increase in difficulty in this taskcompared with when MH had to localise a colour-defined target. As in Experiment 2a, the effect ofdisplay size was not reliable (F < 1.0). MH made noerrors (see Table 6).

Experiment 2c: Difficult orientation target

Method. The method was the same as in Experi-ment 1c except that MH had to point to an orientedline target (when present). The target was right-oriented at 45º, and the distractors left-oriented at45º. There were 480 trials (four blocks of 120 trials).

Results and discussion. The mean correct RTs andpercentage errors are shown in Figure 8. MHclearly had difficulty on this task, and his overallRTs were a full 2.7 s longer than in the colourlocalisation task. He also made vastly more errors(44.6% here compared with 3.3% in Experiment2a). There was also an interesting negative slope onhis RTs, with performance being faster at the largerdisplay sizes than at display size 4: the main effect ofdisplay size, F(2, 35) = 11.44, p < .01, with a signifi-cant, negative linear component, F(1, 33) = 14.39,p < .001.

This last result suggests that the contextualrelationship between the stimuli might have dis-rupted MH’s localisation of targets at display size 4,since it is difficult to account for the improvementin performance at the larger target display sizesin terms of factors such as limited processingresources. Errors were also increased at set size 4, sothe result was not due to a speed–accuracy trade-off. When errors occurred, MH typically pointed toa distractor adjacent to the target.

In Experiment 2d, we examined whether MHcontinued to show this pattern of performancewhen presented with oriented T stimuli.

Experiment 2d: Oriented Ts

Method. The method was exactly the same as inExperiment 2c except that MH was asked to pointto a right-oriented T target (when present) whichfell amongst left-oriented T distractors (all stimuliwere oriented 45º). The stimuli were the same as in

Experiment 1d. There were 60 trials, performed ina single block.

Results and discussion. The mean correct RTs anderrors are given in Figure 8. MH’s overall RTs were2275 ms which, whilst still slow, were neverthelessfaster than in Experiment 2c (single oriented lines)by some 1400 ms. In addition, errors were drasti-cally reduced (10% overall).The improvement inboth RTs and errors was reliable, F(1, 170) = 36.7,p < .0001 for RTs, �

2(1) = 9.51, p < .01 for errors. InExperiment 2d there remained a reliable overalleffect of display size, F(2, 51) = 8.5, p > .001, butunlike Experiment 2c, this was due to slow RTsat the larger rather than the smaller display sizes(p < .05 for comparisons between display sizes 8 and12 and display size 4).

The improvement with oriented T stimuli,compared with single oriented lines, is intriguing,given that search to detect an oriented T wasrelatively difficult for both MH and the controlsubjects (Experiment 1d). This points to a dissocia-tion between the factors leading to target detectionand those underlying localisation based on orienta-tion information. MH had relatively poor detectionof targets defined by orientation differences relativeto distractors (Experiment 1). Over and above this,though, he showed poor localisation of targets,particularly at set size 4 with simple oriented lines.We propose that this reflects a form of contexteffect in which the relations between all the itemspresent “masks” the detection and (especially) thelocalisation of the target. When there are fourelements present, the stimuli fall in locations thatalways define a simple identifiable shape, whereasthis is not the case with random positioning ofitems at larger display sizes (see Mandler & Shebo,1982, for a discussion of this in the context ofsubitisation effects in normal observers). When thetarget was positioned so that it was a part of an iden-tifiable shape, any “weak” signal for an orientationdisparity may have been less salient than the shapeinformation. Consequently, if attention becamefocused on the overall shape, target detection andtarget localisation may have become difficult.

It is also important to note here that the inter-element spacings (the density of the display) were

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held roughly constant over the display sizes. Hence,the improvement at the larger display sizes wasunlikely to reflect a contribution from anotherfactor, such as the presence of local orientationdiscontinuities when there are more elementspresent (as would have occurred if the overalldisplay area was held constant, Nothdurft, 1991,1992).

Discussion: Experiment 2MH was able to localise a colour-defined target buthe was strikingly poor at localising targets definedby an orientation difference relative to distractors.Relative to the colour-target condition, perfor-mance with oriented targets was overall impaired,but this difference was particularly marked withsingle oriented line stimuli shown amongst a smallset of mirror-oriented distractors (Experiment 2c).Furthermore, MH was worse at relatively smalldisplay sizes compared with when there were moreelements in the field (although the inter-elementspacing was roughly constant, and large, across thedisplay sizes). We attribute this to MH showingpoor detection and localisation when the target iscoded as a part of a more global (and identifiable)shape. With relatively few elements present, thestimuli typically describe a familiar shape, atendency that decreases with display size (Mandler& Shebo, 1982). The familar shape may be moresalient to MH than an orientation disparity basedon mirror image reflections (when the orientationsignal is weak). As a consequence, MH may attendto the overall shape and not to the individual targetelement, making target detection and localisationdifficult.

Experiment 3: Forced-choice localisationof orientation-defined targets

In Experiment 3, we incorporated a more formalassessment of MH’s ability to localise orientation-defined targets. To do this, we presented him withmulti-item displays composed of single orientedlines in which a target was always present. Thedisplays were presented briefly (3000 ms) and werefollowed by a probe display in which a “?” symbol

fell either at the location of the target, or at thelocation of an adjacent distractor. MH (andcontrols) had to decide whether or not the probefell at the location of the target. In Experiment 3awe used symmetrical arrays and varied the spacingbetween elements. Any effects due to maskingfrom the context provided by the other elementsmay be changed as these variables are manipu-lated. Grouping effects may be stronger withreduced inter-element spacing (see Pomerantz &Schaitzberg, 1975). In Experiment 3b we usedasymmetrical displays, which may also reducegrouping effects.

Experiment 3a: Symmetrical displays

Method. Stimuli were presented on an AppleMacintosh Powerbook, having been drawn inMacDraw and transferred to VScope software(Enns et al., 1990). Two display sizes were used,one with four elements (set size 4) and one witheight elements (set size 8). The items for displaysize 4 also appeared across two display areas (“close”being 9 cm2 and “spaced” being 16 cm2), whilstdisplay size 8 had a display area 25 cm2 but the crit-ical items fell at the same locations for the spaceddisplay size 4 and for display size 8. Thus, here, theelements covered 40, 25, and 23% of the totaldisplay areas for the close 4, 8, and spaced 4displays, respectively. The minimal separationsbetween the centres of each item were 3 cm (close4), 2.06 cm (8), and 4 cm (spaced 4) respectively.The displays were bilaterally symmetrical aroundfixation. Set size 8 displays were created by addingfour elements to set size 4 displays so that the addi-tional elements were adjacent to the originalelements. There were two types of display: a targetdisplay and a probe display. Probe displays con-tained black dots and a “?” symbol and immediatelyfollowed target displays (any probe display con-sisted of the same number of elements as thepreceding target array). The elements in targetarrays consisted of oriented lines (the lines inthe individual stimuli were 13 × 1 mm, and wereoriented from bottom left to top right; each arrayhad a single target line that was oriented orthogo-nally to the other elements). Lines were replaced by

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dots in the same locations in probe displays. Onvalid probe trials a question mark was located in thespatial position of the target element in thepreceding target array. On invalid trials the ques-tion mark replaced one of the other elements. Alllocations were probed equally often on invalidtrials. Examples of the target and probe displays areshown in Figure 9.

The location of the target was manipulatedacross trials and all positions were equally sampled.

With set size 4 the target appeared at each locationeight times for each of the two sizes of display.With set size 8 only target locations that werecongruent with the locations sampled on four-itemtrials contained targets and each of these fourlocations was sampled 16 times. This created a totalof 128 trials, 64 at each display size, half with a validprobe and half with an invalid probe. The order ofthe conditions was fully randomised, with the trialspresented in blocks of 64.

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(a) Target display

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Figure 9. Examples of the displays used in Experiment 3a.

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Trials were initiated by a central fixation cross(duration 150 ms) followed by a target array(duration 3000 ms) and then the subsequent probearray, which appeared for a maximum duration of7500 ms (the offset of this display was triggered bythe subject’s key press response). Participants wereasked to respond as quickly and accurately aspossible using either the M or the N key on thekeyboard (to signal when they thought the targetwas in the location of the “?”, and when theythought the “?” was not in the target’s location,respectively). Subjects were familiarised in a seriesof 32 practice trials and they received feedbackafter each response (+ for a correct and – for anincorrect response). There was a response deadlineof 7500 ms.

Along with MH, an age- and education-matched control (JH) and 10 undergraduatestudents (ages 18–28) were tested.

Results and discussion. The mean correct RTs andpercentage errors are presented in Figure 10. Forthe control subjects and for JH error rates were low(an average of 5.7 and 8 errors respectively). Incontrast, MH failed to perform above chance at thesmaller display sizes, summing across valid andinvalid probes, both �

2 < 1.0 relative to chance.MH’s performance was somewhat more accuratefor display size 8, �

2(1) = 33.1 and 28.7 for displaysize 8 relative to spaced and close items at displaysize 4, both p < .001.

Given MH’s low accuracy at display size 4, it isdifficult to analyse his RTs, though we note thathe was overall slower than the controls, particularlyat display size 4. There was no evidence for a speed–accuracy trade-off.

JH’s RTs were analysed separately in an inde-pendent factors ANOVA, treating each data pointas a separate subject. There was no main effect ofdisplay size, F(2, 115) = 1.6, p > .05, but there wasa main effect of probe validity, F(1, 115) = 34.7, p <.0001. The interaction between display size andvalidity of probe was not reliable, F(2, 115) = 0.5,p > .05.

The mean scores for each younger controlsubject were entered into a repeated measuresANOVA. The main effect of display size was not

significant, F(2, 18) = 1.2, p > .05, nor was the inter-action, F(2, 18) = 0.08, p > .05, but there was a maineffect of probe validity, F(1, 18) = 7.2, p < .025.Thus, for the controls, there was no evidence thatdisplay size 4 was particularly difficult.

The data here confirm those we observed inExperiment 2c. MH was strikingly poor at making

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localisation judgements to a 45º-oriented targetpresented amongst mirror-image distractors. Thedeficit was greater for display size 4, and this heldwith both close and spaced configurations.

Across the different displays here, the overalldensity of the display and the minimal inter-element distance varied orthogonally. The close 4display had a higher overall density than the displaywith 8 elements, but the minimal inter-elementdistance was smaller for the 8-element display. Thedata suggest that MH was better able to detect andlocalise the target when the minimal inter-elementdistance was small (display size 8). This in turnindicates that there may be two factors at play. Oneis the strength of an orientation disparity signal,contingent on local inter-element spacing. Theother is whether the elements fell at locations thatdefine an identifiable shape. Target detection andlocalisation is good when there is a strongorientaion disparity signal. However, when thissignal is relatively weak, and the elements define anidentifiable shape, then he tends to attend to theshape. Target detection and localisation suffer mostunder this circumstance.

However, one problem in comparing acrossdisplay sizes 4 and 8 here is that, on invalid trials atdisplay size 8, probes appeared at four locationswhere targets were never presented. If MH learnedthis, his performance at display size 8 could haveimproved because responses on some invalid trialswould not necessitate target localisation. There areseveral reasons to think that this factor was notcritical. For example, there was little differencebetween MH’s performance on valid and invalidtrials. Also, for the control participants, probevalidity did not interact with any other factor, sothere did not appear to be differential effects oninvalid trials at display size 8. Nevertheless, inExperiment 3b this potential problem wasremedied by only presenting probes at locationswhere targets could appear, across all display sizes.In Experiment 3a we used symmetrical displays,and in Experiment 3b we used asymmetricaldisplays. With asymmetrical displays there may be areduced tendency to code the whole display as asingle object, in which case MH’s performanceshould improve.

Experiment 3b: Asymmetrical displays

Method. The stimuli for this experiment werecreated by manipulating the original stimuli fromExperiment 1. The positions of the original stimulichanged by moving individual elements to createrandom and asymmetric displays (we attempted tomaintain the total area of both set size 4 and set size8 displays) matching those used for the spaced 4and display size 8 conditions in Experiment 3a.For both display sizes, four different arrays wereproduced.

With set size 4, each location was sampled fourtimes for each array, both for valid and invalid trials(making a total of 32 trials at each location, 128trials altogether).

With set size 8 arrays, target elements only everoccupied one of the four central locations, matchedto the positions of the targets at display size 4. Eachof these locations was again sampled four times foreach array, for both valid and invalid trials, againcreating 128 trials in total.

The trials for set sizes 4 and 8 were randomisedand presented in two blocks of 128 trials each. Thedurations of the different displays were the same asin Experiment 3a. The sizes of the stimuli were alsothe same as in Experiment 3a. Examples of thestimuli are shown in Figure 11.

Results and discussion. The mean correct results andpercentage errors for the different display sizes arepresented in Figure 12. In contrast to Experiment3a, MH performed at above chance at display size 4,�

2(1) = 47.5 vs. chance, p < .0001. He also per-formed better at this display size than in Experi-ment 3a, �

2(1) = 4.0, p < .04. Localisation accuracyof stimuli at display size 8 was above chance, �

2(1) =91.1, p < .0001, and did not differ from that foundin Experiment 3a, �

2(1) = 1.7, p > .05. However, itremained better than his discrimination at displaysize 4 �

2 = 9.5, p < .002. MH’s response latenciesshowed a significant effect of display size,F(1, 194) = 166.0, p < .0001, and an effect of valid/invalid probe, F(1,194) = 8.58, p < .004; there wasno significant interaction between display size andprobe validity (p > .05). Overall, his RT’s were208 ms faster than in Experiment 3a.

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For JH there was no effect of display size,F(1, 242) = 0.084, p > .05, but there was a signifi-cant effect of validity of the probe, F(1, 242) =10.15, p < .0016. The interaction was unreliable,F(1, 242) = 2.9, p < .09. Similarly, the youngercontrols also showed no effect of display size,F(1, 9) = 4.6, p > .05, a reliable effect of validity,F(1, 9) = 15.24, p < .0036, but no reliable inter-action, F(1, 9) = 0.36, p > .05.

Further analyses were performed contrastingMH’s RTs in Experiments 3a and 3b (see Figure

13). The factors entered into the analysis wereExperiment (Experiment 3a vs. Experiment 3b),Display Size (4 vs. 8), and Validity (valid vs.invalid probe) Significant main effects wereobtained for experiment and display size, indicatingbetter performance in Experiment 3b, F(1, 339) =9.3, p < .003, F(1, 339) = 279.3, p < .0001 forexperiment and display size respectively) but themain effect of validity was not reliable (p > .05).Two of the 2-way interactions were significant:Experiment × Validity, F(1, 339) = 12.5, p < .0005;

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Figure 11. Examples of the displays used in Experiment 3b.

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and Display × Validity, F(1, 242) = 11.4, p < .0008,but the interaction between experiment and displaysize failed to reach significance, (p > .05). The 3-way interaction was also found to be reliable,Experiment × Display Size × Validity, F(1, 339) =17.88, p < .0001. MH improved at the small displaysize in Experiment 3b, particularly on valid trials.

The results show that MH found it easierto localise the target in an asymmetric display(Experiment 3b), relative to when it appeared in asymmetric display (Experiment 3a). Indeed, it wasonly with asymmetric displays that MH performedabove chance at the smaller display size. This resultprovides direct evidence that MH was particularlydisrupted by cues that led him to code the elementsas a single object, including overall positionalsymmetry. The controls too found it easier tolocalise the target in an asymmetric display, butthe effect was much less dramatic (an overall RTgain of 67 ms for the young controls, and no effectfor JH). We conclude that MH was abnormallysensitive to the contextual relations between targetand nontarget elements when the task required

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selective attention and localisation of an orientedtarget.

GENERAL DISCUSSION

We have reported a series of studies examiningdetection and localisation of orientation andcolour-defined targets in a patient with left parietalbrain damage, MH. MH was relatively good atboth detecting and localising a colour-definedtarget. However, in comparison with the controls,he was impaired at all tasks with targets defined bytheir orientation. This impairment was apparenteven with a simple orientation discrimination task(target = horizontal line, distractors = vertical lines),but it became more pronounced as the dis-crimination increased in difficulty (target = /,distractors = \ ) and when localisation was required(Experiments 2 and 3). Interestingly, performancewas not impaired disproportionally relative tocontrols when orientation conjunctions were used(target = right, 45º-oriented T , distractors left,45º-oriented Ts), and indeed MH’s localisa-tion of orientation conjunctions was better thanhis of single-line oriented targets (Experiment 2).

These results are consistent with the proposalthat the dorsal visual stream is not only involved in“vision for action,” but also that it supports someaspects of basic visual perception—in particular thediscrimination and localisation of orientation-defined targets. The deficit for MH was mostpronounced with 45º-oriented stimuli, wheretargets and distractors were mirror images of eachother. We may think of this in various ways. Onepossibility is that the parietal lobe is important forcoding the “handedness” of visual stimuli, so thattheir left–right orientation is made explicit(Riddoch & Humphreys, 1988). Another possi-bility is that, following his parietal lesion, MHis oversensitive to information coded by hisventral visual system, which responds in arotation-invariant manner to stimuli (Cooper &Humphreys, 2000; Turnbull, Beschin, & DellaSala, 1997; Turnbull, Laws, & McCarthy, 1995).As we discuss below, there was evidence for strongeffects on MH of contextual relations between the

stimuli, consistent with him being over-affected bya ventral pattern-recognition system. However, hisrelatively better performance with oriented Ts,compared with oriented single lines, does not fitwith this proposal. For example, a rotation-invariant ventral system should have just as muchdifficulty discriminating rotated Ts as rotated lines.The relative improvement with oriented Ts mayinstead be attributed to their discrimination beingsupported by conjunctive coding and groupingwithin the ventral visual system, which segmentsthe target and distractor into separate groups(Humphreys & Müller, 1993; Humphreys et al.,1992). In contrast to this, single lines of differentorientations can group into a single object, whichmakes it difficult to discriminate the target.Whichever account is maintained, MH did havesome problems even with horizontal and verticalorientation discriminations (Experiment 1a), so thedifficulty was not confined to left–right reflectedstimuli. Neurophysiological evidence indicatesthat, in the monkey, cells in the anteriorintraparietal sulcus are sensitive to orientationinformation (e.g., Sakata, Taira, Kusunoki,Murata, Tanaka, & Tsutsui, 1999), though it is notclear whether these cells support orientationdiscrimination as opposed to the visual guidance ofhand actions. FMRI data also indicate that theparietal cortex is activated in orientation discrimi-nation tasks, with the anterior intraparietal sulcusparticularly activated when orientation discrimina-tion is contrasted with imagery for orientationinformation (Faillenot, Sunaert, Van Hecke, &Orban, 2001; Shikata et al., 2003). Indeed, Shikataet al. demonstrate that the caudal part of theintraparietal sulcus was activated more strongly inan orientation discrimination task than whensurface orientation was used to guide reaching. InExperiments 1 and 3 here MH was not asked to actto the stimulus but to show a discriminationresponse based on an orientation disparity. Theclear deficit in his case indicates that orientationdiscrimination is supported by the parietal lobe.Note also that MH showed good performance withcolour-defined targets. There was not a generalproblem in feature discrimination, but a morespecific deficit within one feature domain.

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The interplay between dorsal and ventralstreams

There are two pieces of evidence indicating thatMH was overly sensitive to pattern coding in theventral visual stream. One is that his localisationperformance was strikingly poor at display size 4compared with when larger display sizes were used.Oriented lines at display size 4 are likely to con-figure into a familiar shape (a square or rectangle).The ventral pattern recognition system shouldrespond most strongly to familiar shapes. In thesearch displays used in Experiments 1 and 2, withrandomly placed elements, stimuli at display size 8may not be coded as a familiar figure. In Experi-ment 3, the stimuli at both display sizes 4 and 8 fellin locations corresponding to parts of a familiarfigure (e.g., a diamond for display size 8). However,the minimal inter-element distance was smaller atdisplay size 8. It is possible that, with a small inter-element distance, targets are detected from localorientation disparities (Nothdurft, 1991, 1992),operating in a texture discrimination system. Inwork with normal observers, Wolfe (1992) hasdemonstrated that texture-based segmentation canbe distinguished from orientation discrimination invisual search. There are also neurophysiologicaldata indicating that texture discrimination can bebased on cells in early visual areas (V1, Knierim &Van Essen, 1992). Thus MH may have other infor-mation available to support his discrimination atthe larger display sizes in Experiment 3.

The second piece of evidence on the effects ofventral coding came in Experiment 3, where wefound that MH was better at localising stimuli inasymmetrical displays than in symmetrical displays(in Experiment 3b compared with Experiment 3a).Again this fits with the idea that MH is disruptedwhen stimuli can be responded to as a singlepattern. These data, where the grouped wholeseems to “mask” part-information, match resultsfrom normal participants reported by Pomerantzand colleagues (Pomerantz & Garner, 1973;Pomerantz, Sager, & Stoever, 1977; Pomerantz &Schaitzberg, 1975), where again factors such assymmetry made it difficult to detect a “part” targetin a “whole” grouped array. In MH, however, the

“masking” effect is abnormally strong, and leads tohim sometimes not being able to localise targetseven with unlimited presentations (Experiment 2).It should also be noted that a form of cognitiverather than perceptual masking seems to operate. InExperiments 1 and 2 in particular, we used low-density displays where it is highly unlikely thatperceptual masking took place. Instead of percep-tual masking, we suggest that MH tended to attendto familiar shape information when it was present.When attention was allocated to the whole ratherthan the part, detection and localisation of a localtarget part was disrupted.

In cognitive neuropsychology, the effects ofbrain damage are frequently interpreted in terms ofthe loss/impoverishment of a particular processingmodule. In MH’s case, however, we suggest thatperformance is not simply impaired by impover-ished coding of line orientation but also by outputfrom a separate (ventral) system. We propose thatorientation information is coded by both dorsal andventral visual systems in parallel, but for differentpurposes. The ventral system is concerned withorganising orientations implicitly into shapes forpattern recognition. The dorsal system is concernedwith explicit coding of orientation. Normally wemay gain access to each type of information,depending upon task demands. However, damageto the dorsal system can mean that orientationinformation is not simply lost, but coded in adifferent way (more strongly weighted to theventral stream). The system is not simply reduced,but operates in a qualitatively different manner.This possibility, of neural damage producing aqualitative change and not just a reduction inperformance, needs to be taken into account whenreviewing neuropsychological patients.

Feature detection and localisation

MH was typically able to detect orientation-defined targets, albeit slowly. In contrast, he wasmarkedly inaccurate at localising the stimuli, evenwhen unlimited presentations were used (Experi-ment 1). Localisation was more impaired thandiscrimination. In studies of feature detection innormal participants there has been some debate

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over the relations between detection and localisa-tion performance. Some authors have argued thatfeatures can be detected prior to localisation(Treisman & Gelade, 1980), whilst others haveargued that coarse discrimination can be achievedprior to feature discrimination (Atkinson &Braddick, 1989; Donk & Meinecke, 2001) or thatdiscrimination and localisation occur together(Green, 1992; Johnston & Pashler, 1990). MH’sdata are relevant to this debate, since his localisationwas worse than his discrimination performance.This is consistent with orientation discriminationpreceding (or at least operating independently of)feature localisation. Note, though, that MH didhave coarse location information available. Forexample, when pointing he typically made near-neighbour errors. This was not due to inaccuracy inmaking the motor response, since he was certainlyable to point to colour-defined targets (Experiment2a). Thus feature discrimination can be dissociatedfrom (relatively) fine-grain localisation. Featuredisparities can be computed, but their explicit, fine-grained localisation is disrupted following damageto the parietal lobe. Nevertheless, given that MHhad spared object recognition, we assume thatfeature locations are coded implicitly within theventral object recognition system.

Manuscript received 20 August 2002Revised manuscript received 30 May 2003Revised manuscript accepted 30 May 2003

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