Parallel pattern processing and visual agnosia.

Parallel Pattern Processing and Visual Agnosia GLYN W. HUMPHREYS and M. JANE RIDDOCH University of Birmingham, Birmingham PHILIP T. QUINLAN University of York. York CATHY J. PRICE Hammersmith Hospital, London NICK DONNELLY University of Kent, Kent

Abstract A series of visual search experiments are reported examining pattern processing in a visual agnosic patient. We examined search for targets defined by: (1) the combination of their features relative to homogeneous distractors; (2) the combination of their features relative to heterogeneous distractors; and (3) a single feature difference relative to the distractors (their orientation). Normal subjects demonstrate evidence of spatially parallel search when combinedfeature targets are detected against homogeneous distractors, and when targets arc defined by a salient feature difference. There are non-linear effects of the number of distractors present, and absent responses can be as fast as present. In contrast, search times for combined-feature targets amongst heterogeneous distractors increase linearly with display size, with the slope for absent responses about twice that for present. The contrast between search for combined-feature targets amongst homogeneous and heterogeneous distractors can be attributed to the effects of grouping between distractors and between distractors and targets (Duncan & Humphreys, 1989, 1992; Humphreys & Muller, in press). Grouping between homogeneous distractors facilitates search. An agnosic patient, HJA, showed normal search functions for single-feature targets and for combined-feature targets amongst heterogeneous distractors. However, he was impaired at search for combinedfeature targets amongst homogeneous distractors. This suggests that HJA is selectively impaired at grouping conjunctions of form features. The relations between HJA's agnosia and his problem in the parallel grouping of form conjunctions are discussed, as arc the implications of the work for understanding normal

Resume Le document porte sur serie d'experiences dc recherche visuelle et, plus precise'ment, sur le mode dc recherche utilise par un sujet atteint d'agnosie visuelle. Nous nous sommes penches sur la recherche dc cibles selon: (1) la combinaison de traits par rapport a des distractcurs homogfenes; (2) la combinaison de traits par rapport a des distracteurs helcrogcncs et (3) un seul trait distinct par rapport aux distracteurs (I'oricntation). Chez les sujcts normaux, il est evident qu'unc recherche parallele s'effectue sur le plan spatial lorsque des cibles formees Canadian Journal of Psychology, 1992, 46:3, 377-416

378

Humphreys, Riddoch, Quinlan, Price and Donnelly

dc plusicurs traits sont reper^es parmi des distracteurs homogenes et lorsquc Ics ciblcs sont dc fin ics par un seul trait saillant. Le nombre de distracteurs presents entratne des effets non lincaircs, ct Ics rcponscs indiquant l'absence de cibles peuvent etre donnccs aussi rapidcnicnt quc ccllcs indiquant la presence de cibles. Par contraste, les delais de recherche dans lc cas dc ciblcs fonn6es de plusieurs traits et melees & des distracteurs heterogencs augmentent de fa^on lineaire avec la taillc dc I'ecran, la courbe des r6ponses indiquant I'abscncc dc ciblcs 6tant environ le double de celle enregistree pour les reponscs marquant la presence de cibles. Dans le cas de cibles formees de plusieurs traits, la difference cntre les requitals obtenus pour les distracteurs homogenes et ceux qui ont etc enrcgistres pour les distracteurs heterogenes peut etre attribute aux effets de groupement cntrc distracteurs el de goupement entre distracteurs ct ciblcs (Duncan et Humphreys, 1989, 1991; Humphreys ct Mullcr, diicument sous presse). Le groupement cntrc distracteurs homogenes facilitc la recherche. Un sujet agnosique, HJA, utilisait normalement les fonctions de recherche dans lc cas dc ciblcs form&s d'un seul ou dc plusieurs traits, melees a des distractcurs hctcrogcncs. Ccpcndant, le sujet cprouvait des difficultcs avec la recherche de cibles formees dc plusicurs traits parmi des distractcurs homogfenes. On peut done supposer quc HJA cprouve des troubles sclcctifs quant au groupement de conjonctions de traits. Tl cst question des relations cntre I'agnosie de HJA et la difficulty que celui-ci cprouve avec le groupement parallcle dc conjonctions de formes, ainsi que des repercussions du travail sur la comprehension du proccssus visuel normal.

Early stages in visual object recognition arc likely to involve the independent coding of primitive visual features at a number of spatial scales (e.g., Marr, T982; Trcisman, 1988; Watt & Morgan, T985). Such features include bars and edges of different colours, widths (spatial frequencies) and orientations, edge intersections and terminators, coded in separate retinotopically-organized 'feature maps'. Evidence for the parallel and independent coding of visual features comes from: (a)

neurophysiological studies of the stimulus preferences of cells in the primary visual cortex (e.g., Cowey, T979; Desimone & Ungerleider, 1989); (b) psychophysical studies of adaptation and masking effects selective to simple properties of patterns (such as orientation and spatial frequency; Graham, 1981); (c) studies of the ability to attend to one feature and to ignore irrelevant variation in others (e.g., Garner, T970; Pomerantz, 1983); (d) visual search for targets defined by the presence or a single feature difference relative to distractors, where search times are a non-linear function of the number of distractors present. Such nonlinear func-

Parallel PaUern Processing

379

lions are consistent with a spatially parallel search process (e.g., Treisman & Geladc, 1980); (e) studies showing the immediate and effortless segregation of texture regions defined by feature differences between the regions (e.g., Julesz, 1986; Bergen & Julesz, 1983); (f) illusory conjunctions, where subjects report incorrect combinations of features in a display under certain conditions (e.g., Treisman & Schmidt, 1982); and (g) studies of acquired perceptual deficits, where patients are selectively impaired at discriminating colour, motion or particular spatial frequencies (e.g., Bodis-Wollncr, 1972, 1976; Heywood, Wilson, Cowey, 1987; Meadows, 1974; Zihl, Von Cramon, & Mai, 1983). However, the question of how these independently processed features arc integrated to enable us to perceive coherent objects remains unresolved. One influential account, put forward by Treisman and her colleagues (e.g., Treisman, 1988, 1991; Treisman & Gormican, 1988), is that feature integration requires the operation of a spatially serial attentional process, which 'glues' together the features coded at the same location in different feature maps. Evidence consistent with this suggestion comes from studies showing that visual search for a target defined by a conjunction of features (e.g., a red o, against a background of red x's and green O's) can conform to a serial (and self-terminating) search pattern: RTS for both present and absent responses increase linearly as a function of the display size, and the slope for absent responses is about twice that for present responses (Treisman & Gelade, 1980). It is as if subjects have to attend serially to each location where features arc coded in the display in order to detect a particular featureconjunction. If subjects are prevented from attending to the locations of stimuli in a display, then it is held that features are combined either using past knowledge (e.g., an Knglish post-box shape will be combined with a red colour in a scene, and not with other colours present, because English postboxes are red), or they will be combined in a random fashion (producing illusory conjunctions; Treisman & Schmidt, 1982). Treisman's notion of a perceptual world composed of independently coded features, organized only by the application of serial attention or by the constraints of past knowledge, does not fit easily with our phenomenal experience of orderly and coherent scenes, attained even from brief glimpses (see, e.g., Biederman, 1972; Biederman, Mezzanotte, & Rabinowitz, 1982). Nor does it accord with data showing that objects containing a large number of parts are identified no more slowly than those containing few parts (Biederman, T987), or that pictures containing many perceptual features (e.g., colour and texture, as well as bounding contour) can be identified faster than those containing few (Price & Humphreys, 1989). Moreover, there are good

380


reasons to suggest that visual perception should not operate in this way. For instance, simple form conjunctions, such as corners, represent computationally informative regions of discontinuity in images, and so may be important in the recovery and coding of shape descriptions (Biederman, 1987). Our ability to identify the parts of objects defined in three-dimensional (3D) space also depends on the rapid coding of the relations between form conjunctions such as corners. Indeed, as the early computational studies of the blocks world showed, descriptions of the 3D structure of objects can be derived solely from the relations between two-dimensional (2D) corner junctions, providing assumptions arc made concerning the orthogonality of the corners etc. (Perkins, 1968). Groups of separate objects may also be segmented apart, once such 3D descriptions are constructed (e.g., Clowes, 1971). Enns and Rensink (1990, 1992, this volume) have further shown that non-linear search functions can occur for 'block figure' targets defined by a difference in their 3D orientation relative to distractors, where the 3D orientation of the figures is itself based on the relations between the 2D corner junctions. Thus it seems that at least some simple form conjunctions (e.g., corners) are coded in a spatially parallel manner, and that parallel computation of the relations between these junctions is a necessary condition for efficient object recognition. PARALLEL FEATURE COMBINATION AND GROUPING The work of Enns and Rensink (1990, 1992, this volume) fits with recent studies from our own laboratory showing that simple form conjunctions are computed in a spatially parallel manner. We (e.g., Duncan & Humphreys, 1989; Humphreys, Quinlan, & Riddoch, 1989; Humphreys, Riddoch, & Quinlan, 1985) have reported non-linear search functions for targets defined by a combination of form features when such targets are presented amongst homogeneous distractors (e.g., when the target is an inverted T and distractors are all upright T's). In contrast, with heterogeneous distractors (e.g., when distractors are a mixture of T, t and H stimuli), search times increase linearly as a function of the number of distractors present, with a slope ratio for present to absent responses of approximately 1 :i. Such search functions with heterogeneous distractors are reminisccnl of the search functions generated when subjects search for targets defined by a conjunction of two or more features from differing input domains (such as colour and form; see above). Of course, in the latter case, the distractors are again heterogeneous. The contrast between search with homogeneous and with heterogeneous distractors strongly suggests that search for feature-conjunctions is dependent on grouping within displays. When distractors are more similar to one another than they are to targets, distraclors and targets can form separate perceptual groups. In this case, search times need not be linearly related to the number of distractors present. When distractors are less similar to one another than

Parallel Pattern Processing

381

they are to targets, and when they contain some of the features that define the target (see Duncan & Humphreys, 1992), larget-distractor grouping is at least as likely as grouping between distractors. This makes search inefficient because distractors cannot be rejected as a single perceptual group. Search times increase linearly either because subjects then adopt a serial search strategy, treating each member of the display as a separate perceptual group (Treisman, 1988), or because the separate groups of items are selected serially (Humphreys & Muller, in press; see Humphreys, Freeman, & Muller, this volume). When targets and distractors do not share features grouping is less important since individual distractors will not activate a "template" for the target which may be activated by items in parallel across the visual field. Grouping distractors together may provide one means of limiting such activation since the group will be defined by new perceptual features. In the displays we have typically used, grouping operates between identical T elements (or possibly between their constituent corner junctions). For object recognition, it will also be important that grouping operates between mirrorimage, collinear junctions (see Donnelly, Humphreys, & Riddoch, 1991, for evidence). Grouping seems to operate both between identical and between collinear junctions. These results confirm that there can be parallel computation of simple form conjunctions (such as comer junctions), and that normally there is grouping between the conjunctions so computed. Grouping between such conjunctions is important for the rapid derivation of primitive shape descriptions (Donnelly et al., 1991; Enns & Rensink, this volume), and for segmenting object figures from the contextual ground. It follows that disruption of the processes that normally encode and group form conjunctions may markedly disrupt object recognition. VISUAL AGNOSIA

The term visual agnosia refers to patients who, following brain damage, have problems in recognizing visually presented objects (Farah, 1990; Humphreys & Riddoch, 1987a). The problem is one of recognition rather than naming because the patients arc unable to describe or gesture the function of the objects they cannot name. However, recognition via other modalities (hearing, touch) can be relatively intact. It seems likely that problems in visual object recognition can reflect a number of functionally different impairments (e.g., see Farah, 1990; Humphreys & Riddoch, 1987b; Humphreys, Riddoch, Donnelly, Freeman, Boucart, & Muller, 1992), possibly also including a selective problem in grouping image features. For instance, Stevens (1983) reported one visual agnosic patient who failed to report the presence of illusory contours when presented with Kanisza-like shapes. This suggests a problem in computing

382


illusory contours from 2D edges, possibly due lo a failure to group the separate edges of the figures using cues such as collinearity and continuation between end terminators. Interestingly the problem was specific to 2D edges, since the patient could report the presence of illusory contours in stereoscopically presented figures. Over the past ten years, we have been able lo study a patient with severe visual agnosia, HJA, who also seemed to have some problems in grouping visual elements in a spatially parallel manner. Boucart and Humphreys (1992) had HJA and a group of normal control subjects match pairs of fragmented line drawings of objects (formed by omitting sections of the outline contours of the objects). The fragmented forms were either 'structured' or 'unstructured', where structured means that the fragments were collinear with one another along the bounding contour of the object; in the unstructured forms, the individual fragments were rotated so that they were no longer collinear. They found that, unlike normal subjects, HJA was no better at matching 'structured' than 'unstructured' forms. HJA did not seem sensitive to local collinearity. Humphreys et al. (1985) also reported preliminary evidence on HJA's ability to detect simple form conjunctions amongst homogeneous distractors (an inverted T target amongst upright T distractors). HJA showed strong effects of the number of distractors in the displays and there was an abnormally high error rate. In addition to this, with homogeneous distractors, normal subjects can show an unusual pattern of search performance in which absent responses are faster than present (e.g., see Humphreys et al., 1985, 1989). These 'fast absent' responses seem based on the linking together of distractors into a single perceptual group—Duncan and Humphreys (1989) termed this selection at the level of the whole display. The fast absent responses also argue against performance in this homogeneous distractor search task being due to the detection of a new emergent feature when the target is present, since present responses to single-feature differences are typically fasten than responses to the absence of such features (cf. Treisman & Paterson, 1984; Treisman & Gormican, 1988). HJA showed no sign of such fast absent responses; absent responses were slower than present. From these data, Humphreys et al. (1985) argued that HJA had difficulties encoding and/or grouping even simple shape conjunctions in a spatially parallel manner. This impairment may underly his visual agnosia. In the present paper we present a more extensive investigation into HJA's ability to code and group image features. We were particularly interested in establishing whether his problem was selective to search based on the grouping of form conjunctions, and whether he could demonstrate normal search performance in other circumstances. To this end, HJA was required to perform several search tasks. In Experiment 1, he searched for a target defined by a salient feature difference relative to the distractors. Normal subjects show only minor effects of display size in this task. By having HJA


383

perform this task, we assessed whether he could show efficient visual search, with minor effects of display size, under appropriate conditions. Experiment 2 evaluated HJA'S performance when he was required to detect a form conjunction target amongst heterogeneous distractors. This is a difficult task for normal subjects, because targets and distractors do not form separate perceptual groups. This experiment examined whether HJA finds all tasks requiring the detection of conjunction targets difficult, or whether it is confined to when search is usually based on the efficient grouping and segmentation of targets and distractors (with homogeneous distractors). Experiment 3 investigated HJA'S ability to detect form conjunction targets amongst homogeneous distractors. We assessed whether his performance is changed by practice or by the type of form conjunction. We also evaluated his performance when there were two form conjunction targets present. In Experiment 3, HJA showed large slopes on his search functions for form conjunctions amongst homogeneous distractors. Given this, his search performance may be characterized as being either spatially serial, or spatially parallel but grossly inefficient. In some cases, the slopes on present and absent responses were equal, consistent with a serial, exhaustive search. The "two target" task tests this since there should be no change in performance with one and two targets if search is serial and exhaustive. Taken together, the experiments provide detailed measures of HjA's ability to process and group form information in a spatially parallel fashion. CASE DESCRIPTION

The clinical details of HJA'S case have been reported previously (Humphreys & Riddoch, 1987a; Riddoch & Humphreys, 1987). In brief he suffered a stroke in 1981 when aged 61, which produced bilateral lesions of his occipital lobes, extended forwards to the temporal lobes. Subsequently, he had marked modality-specific problems in visual perception. He had gross difficulties recognizing many real objects by sight, with his recognition of photographs and line drawings being even more impaired (Riddoch & Humphreys, 1987). He was very poor at face recognition, failing to identify even familiar family members by their faces. His word recognition skills were impaired, being reduced to a letter-by-lelter level. Drawing from memory, copying and writing skills were largely intact, as was his tactile object recognition. He has a bilateral superior altitudinal field defect, along with intact lower visual fields; saccadic and smooth pursuit eye movements, within his intact fields, are normal. There is also normal (corrected) acuity and stereopsis. In addition, for the present study, HJA was screened on a computerised version of the 'Efron shape matching test' (Efron, 1968). This requires discrimination between pairs of rectangles and squares differing in their width-length ratios. This is often taken as a standard test for adequate shape perception in brain damaged patients (e.g., Milner et al., 1991; Warrington,


384

TABLE t Single item discrimination performance Discrimination:

Age-matched

HJA

controls RT (ms)

%

RT (ms)

Errors / vs 1

±vsT lvsH 1 vs 1-

i«?

6 vs n i vs D-

(Experiinent i) (Experiments 2 and 3) (Experiment 2) (Experiment 2) (Experiments 2 and 3) (Experiment 2) (Experiment 2)

%

Errors

440 459 438

0

452

0 2

473

2-5

-5

522

5

512

0

2-5

507 500

0

522

53'

5 2-5

487 489 482

2-5 0

5

7-5

T985). Squares were 5cm high x 5cm long, and the width-length ratios ol" the rectangles were varied between .66 and .9. HJA scored 71/72, with a mean reaction time (RT) of 1401 m. A set of 10 age-matched, non-brain damaged control subjects scored between 68 and 72, with a mean RT of 1445 m. HJA does not manifest a clinical problem with simple shapes, presented in pairs, under unlimited exposure conditions. This was confirmed by a set of control experiments undertaken using the targets and distractors employed in all the visual search experiments reported in the current paper. In these control experiments, HJA was presented with a single item at fixation on a trial and he had to make a two-choice discrimination. There were 20 trials for each choice (40 trials per block). Stimuli were presented on a monitor controlled by an Apple Macintosh microcomputer, and were the same size as those used in the relevant experiments in the paper. In separate blocks of trials he had to discriminate between / and I (cf. Experiment 1), 1 and T (Experiments 2 and 3), 1- and -f (Experiment 2), • and 1 (Experiments 2 and 3), D- and HD (Experiment 2). The mean RTS and percentage errors, for HJA and for a group of 10 age-matched non-brain damaged control subjects (age range 55-70) are shown in Table 1. HJA is as efficient as the control subjects at making these single item discriminations. The present investigations were undertaken over a considerable lime period, between May 1983 and August 1990. During this time, HJA'S basic perceptual abilities remained stable (Humphreys & Riddoch, in press). Any differences between his relatively good performance on some tasks, and his poor performance on others, is unrelated to the time of testing. The tests where he performed poorly were all repeated subsequently, and they represent a stable deficit. Experiment 1: Single-feature search Humphreys et al. (1985) reported preliminary data suggesting that HJA has


385

difficulty detecting form conjunctions via spatially parallel visual search. Before proceeding to examine the nature of this problem in more detail (in Experiment 3), we believed it important to establish some boundary conditions on HJA'S visual search performance. For instance, can he conduct a normal, spatially parallel and fast search under some circumstances? Experiment 1 examined his ability to detect targets defined by the presence of a single feature difference relative to distractors: namely, its orientation. Normal subjects can detect an orientation-de fined target on the basis of a spatially parallel search process, showing minimal effects of display size on performance (Beck, 1967; Humphreys et al., 1989). The detection of orientation-defined targels is also of some interest in its own right, because line orientations may be the primitives entering into form conjunctions (though sec also Humphreys et al., 1992; Milner et al., 1991). Normal detection of line orientation, but not form conjunctions, would at least be consistent with a hierarchical model for the encoding of form conjunctions (see Humphreys et al., this volume). MF.THOD

HJA's performance was compared with that of two control groups: young and age-matched non-brain damaged subjects. The young control data arc taken from Experiment 7 in Humphreys et al. (1989); 10 young controls were run, all with either normal or corrccted-to-normal vision. There were 6 agematched control subjects, again with either normal or corrected-to-normal vision. The young controls had ages ranging between 20 and 36; the agematched controls had ages ranging between 63 and 68 years. Stimulus presentation, timing and data collection were controlled by an Apple II microcomputer. Two response keys were interfaced with the Apple and reaction times (KTs) were measured from the onset of the stimulus until a button-press response was made. Displays were plotted along the circumference of a virtual circle 4.2 cm (4 0 49') in diameter. The stimuli in each display were plotted al regular intervals so that a 4 item display comprised a square, a 6 item display a hexagon, an 8 item display an octagon and a 10 items display a decagon. The target was a single line (0.7 cm high) oriented 45 0 anticlockwise. The distractors were single upright lines, HJA and the agematched controls took part in a single experimental block in which there were 32 trials at display size 4, 36 at display size 6, 32 at display size 8, and 40 at display size 10. Different numbers of trials were necessary to ensure that each target appeared equally often in each position. The young controls only received display sizes 4, 6 and 8. There were 32 practice trials. For all subjects, the fixation cross fell at the centre of the display. When maintaining fixation, HJA reported that he was able to see all the items at the top of the display, so that items did not fall into his blind visual fields. This was confirmed by the data.

386


600 T 550 500

RT(ms)

450 400

••

350 -I-

Young controls

Age matched controls

HJA

300 250 • 200 -I

1 4

1 6

1 0

1

1 4

1 6

1 8

~t 10

1

h-—(

1-

4

8

6

10

Display size

""" present

% error

100 90 80 70 60 50 40 30 20 10

° - absent

j . -• -• -

Young controls

Age-matched controls

HJA

••

0 •8

8

10

8

10

Display size

Fig. I Mean correct Kis (ms) and % errors lor young controls, age-matched controls and HJA in Experiment i ( / vs I).

Half the control subjects (in each control group) responded 'target present' with their preferred hand, half responded absent, HJA always responded present with his preferred hand. The fixation cross appeared for 500 m, and was immediately replaced by the display which remained on for IO sec or until a response was recorded. Contingent feedback was given after each response (the word CORRECT or INCORRF.CT at the bottom left of the screen, plus a 'buzz' from the Apple on error trials). Feedback remained on for half a second and was followed by a blank inter-trial interval of 2 sec. Subjects

MeanRT Slope %V Mean SD RT Mean % E Mean SD % E

Mean RT Slope %V Mean SD RT Mean % E Mean SD % E

Experiment 2 a (_L vs T, K -i)

Experiment 2b ( i vs 9, D- vs -D )

Continued ...

Mean RT Slope %V Mean SD RT Mean 9b E Mean SD % E

Experiment r

P

412 0.0 —

97.11 416 8.0 —

—

—

95-45 313 3-2 —

92-54 329 15.0 —

51

108

3-0 1.2

4.8

1591 79 83.17 3.0 13-3 13-3

92.71

1018

1749

0.0

6.8

444 0.0

96.00

154

1737 96.75 428 15-6

72

1176

54

583 3.0

95-73

223

96.06 560 rr.8

130

3-7

3-1 1.8

3-i

67 91.88

1122

768 33 84.22 10.2 10.2

1-3

2.6

4-9

362

365

90.29

97

1382

2.4

63 3.6

99.48

-4

418

A

10.4

97-76

62

934

1.6

2.8

0.0

1105

57

1784 146 98.86

1120

—

60

3-4

4 92.31

6.6

75 0.0

9-4 67.92

421

P

106

91

92 5-2 —

0.00

521

A

Young controls

0.0

2.71

3-5 58.47

.2

i-5

2190

A 483

1577

P


505

A

L

474

P

HJA

TABLE 2 Descriptive statistics for the primary effects of target type (single-feature, form conjunction) and distractor type (homogeneous, heterogeneous)

MeanRT Slope %V Mean SD RT Mean % E Mean SD % E

Mean RT Slope % V Mean SD RT Mean % E Mean SD % E

Experiment 3 c

Experiment 3e

16.7 —

317 —

23-3 —

218

85-45 239 3.8 —

77-94

.00

.00

—

22.54 108

0.0

116 .00

5-1 .06

76.5 97 7-5 3-4

11

3.8

559 8

867 1

859

3-5 2.28

1548 62

121

.07

3-7 1.6

102

60.8

10

554

.67

2-5

116

11

68.12 83.26

565

•3

i.l

10

567

114

34 96.91

767

757 29 85.26 •05

—

112

3-9 2-5

no

-1

6.89

585

646 3 94.14

no

1054 66

29-5 —

•47

.96 .86 •5

104

103

0.0

95-64

34 9525

2-5 24.99 148

627 16

740

897

54 91.n 289 12.0

1278

3-3 —

232

203

961 69 95-83 252

995 2.14 1.88 166

3138 58 98.53

1191 18 55-82

Mean RT = Mean averaged across display size (msec) Slope = Slope of RT—display size search function (msec/item) % V = Percentage of variance in the RT—display size function attributable to a linear component Mean SD RT = Mean of individual RT SDs averaged across display size Mean % E = Mean % errors averaged across display size Mean SD % E = Mean % errors SDs across subjects, averaged across display size.

( i vs 9)

irregular display)

(-L vs T,

Mean RT Slope %V Mean SD RT Mean % E Mean SD % E

Experiment 3a (-L vs T, regular display. 250 trials practice)


389

were instructed to respond as quickly as possible whilst minimizing their errors. RBSULTS AND DISCUSSION

The mean correct RTs and percentage errors for both HJA and the two groups of control subjects are shown in Figure 1. Descriptive statistics for the search functions, based on mean RTS at each display size, arc given in Table 2, along with the average SDs (across the display sizes). For both the two sets of control subjects and HJA, there were minimal effects of display size on performance. The young controls showed flat search functions for both present and absent trials. The age-matched controls showed flat search functions for present trials along with some effects of display size on absent (a slope of 9.4 msec per item). HJA, like the young controls, showed minimal display size effects (a slope of under 5 msec per item), on both types of response. His overall RTs, SDS and error rates were also comfortably within the age-matched control range. Both HJA and the age-matched controls showed a slight 'dip' in their present RT functions at display size 6, presumably because contrasting features were closer then than at display size 4. HJA seems normally affected by such interactions between simple visual features. The data indicate that, given the task of detecting a target defined by a salient disjunctive feature, HJA performs at least as well as age-matched control subjects. Given the lack of effects of display size on performance, there are good grounds for arguing that search here was spatially parallel. Thus Experiment 1 demonstrates that, under the appropriate conditions, HJA can conduct an efficient parallel visual search within the spatial area of field in which displays were presented. Experiment 2: Conjunction search with heterogeneous distractors Experiment 2 tested whether HJA could conduct a normal visual search for conjunction targets under conditions which preclude the efficient parallel segmentation of targets and distractors into separate perceptual groups: Namely, when the distractors are heterogeneous. There were two subexperiments. In Experiment 2a, subjects searched for an inverted T target amongst heterogeneous T, 1- and H distraclors (e.g., see Humphreys et al., 1989, Experiment ib). In Experiment 2b, subjects searched for a D target amongst heterogeneous T, n - and - • dislractors (Humphreys et al., 1989, Experiment 4b). Experiment 2b replicates Experiment 2a, and generalizes the effects to non-letter stimuli.

390

Humphreys, Riddoch, Quinlan, Price and Donnelly Experiment 2a: _L vs T, I and -t METHOD

The Method was essentially the same as that used in Experiment 1, the primary difference being in the targets and distractors presented. In Experiment 2a the target was an inverted T and there were three types of distractor: T, 1- and H. The upright T was 0.7 cm high x 0.5 cm wide. There were again 4 display sizes (4, 6, 8 and 10). For display size 4 there were 2 H distractors, T T distractor and 1 1- distractor. For display size 6 each distractor occurred twice. For display size 8 there were 3 1 distraclors, 3 H distractors and 2 T distractors. For display size 10 there were 4 T distractors, 3 \- distractors and 3 H distractors. The target (1), when present, replaced a distractor whose identity was randomly determined. There were 280 experimental trials, preceded by 32 practice trials. There were 12 young controls (Humphreys et al., 1989, Experiment ib) and 10 age-matched controls (ages ranging between 55 and 71). In addition to this, data are also reported from a brain-damaged control subject: L. L'S case details have been previously reported by Bauer (1982, 1984). L was involved in a severe motorbike accident in 1980, resulting in bilateral traumatic haematomas of the occipitotemporal regions and posterior temporal lobes. He has bilateral altitudinal hemianopias with a left inferior congruous quadrantanopia. He also has a right arm monoplegia, due to a brachial plexus injury. Consequently, his responses were recorded via a voice key trigger. At the time of testing, I. was 45 years old. L is severely prosopagnosic and he also exhibits 'mild' agnosia for common objects. For instance, he is poor at identifying objects that require relatively fine 'within-category' distinctions (e.g., such as identifying types of animals or flowers). By including L in the experiment, we aimed to assess whether HJA'S pattern of data is generally characteristic of patients (a) after damage to similar brain regions, (b) with a similar visual field deficit, and (c) with a clinical problem in visual object recognition (all of these points hold for L.). RESULTS AND DISCUSSION Mean correct RTS and percentage errors for the two non-brain damaged control groups, L and IIJA, are presented in Figure 2. Table 2 gives descriptive statistics for the search functions. All the subjects showed strong effects of display size on search, the search functions were linear, and in most cases the ratios of the slopes for present: absent responses approximated 1:2. IUA'S search functions are similar to those found with age-matched controls: His search and error rales and his overall RTs are all close to the means for the controls. The present: absent slope ratio for HJA is somewhat steeper than the average for the age-matched controls (1:2.56 vs 1:2.14), n u t similar slope ratios occurred for individual control subjects (slope ratios ranged between) 1:1.7 and T : 3-' X and in any case the


AgB-matched controls

Young controls -H

1—I

4

6

1 8

39'

i—i—i—t-

r—

10

4

6

8

-i

i—i-

10

4

6

8

1

1

1 1

-qi

1*

9

g

4

ts

a

10

10

Display size

Percentage error 100 90 60 70

Young controls

Age-matched cunlr

-f 4

6

8

10

4

¥ 6

» 6

»

1

10

1 V4

6

8

10

D i s p l a y siztt

Fig. 2 Mean correct RTs (ms) and % emirs fur young controls, age-matched controls, L and HJA in Experiment 2a (_L vs H, I- and T).

steeper than average ratio for HJA was produced by relatively fast present responses rather than slow absent responses. L's RTs were somewhat slower than both the controls and HJA. Slowed RTs are likely to be a consequence of generalized effects of brain-damage. Nevertheless, the general pattern of his data is similar to the controls, demonstrating that a normal search pattern can be found even when RTs are slowed overall. The fact that HJA performs better than I. re-affirms that HJA does not have marked problems with this search task.

392

Humphreys, Riddoch, Quinlan, Price and Donnelly Experiment 2b: u , r, D- and - n METHOD

The Method was exactly the same as for Experiment 2a except that • , 1, eland -D stimuli were substituted for the 1, T, 1 and H stimuli. This was done to generalize any results across shapes. It also served another purpose. In Experiment 2a, some of the heterogeneous distractors contained a local feature that was also present in the target, namely, an upwards pointing terminator. In Experiment 3a, with homogeneous distractors, none of the distractors had this feature. It is possible that normal subjects are able to conduct an efficient parallel search with homogeneous distractors because the target can be detected from the presence of this local feature. Search for a target defined by this feature may not have been possible in Experiment 2a because the feature was present in some distractors (H and h). In Experiment 2b, none of the distractors had this feature, though it remained present in the target. Hence, if performance with homogeneous dislractors depends on this feature, search should be equivalent in Experiments 2b and 3a, and more efficient in Experiment 2b than in Experiment 2a (see Humphreys et al., 1989, for further consideration of this issue). There were 14 young control subjects (Humphreys ct al., 1989, Experiment 4b) and 4 age-matched controls. The age-matched controls had all participated in Experiment 2a, and had ages ranging between 62 and 68. We were not able to run L in the experiment. RESULTS AND DISCUSSION

Figure 3 shows the mean correct RTS and percentage errors, and Table 2 the descriptive statistics for the search functions. Although the subjects found this search task a little easier than that in Experiment 2a, the pattern of performance was essentially the same. There were marked effects of display size on search, the search functions were linear, and the present: absent slope ratios were approximately 1:2. HJA again performed similarly to the age-matched controls. The only suggestion of a difference is that HJA made slightly more errors than the controls (Figure 3). This suggests that he may have found the processing of multiple T-like stimuli more problematic than the processing of multiple T-like stimuli, perhaps because of their (slightly) increased complexity. Nevertheless, there are no major departures from the normal search pattern. Experiments 2a and 2b indicate that, when form conjunction targets cannot be segmented into separate groups from distractors (because heterogeneous distractors are used, which share features with targets), normal subjects manifest linear effects of display size, and present: absent ratios of about 1:2. Under these conditions, HJA's data cannot be distinguished from those provided by age-matched control subjects.


2500

393

T

2000 1500 RT (ms)

1000 500 0

Young controls -I

1 k—I

4

6 8 10


H.J.A.

14 6 8 10

4 6 8 10

Display size

•"- present

100 j 90

°-absent

80 ••

% error

70 60 50 40 Young controls 30 20 10 0 4

6

8 10


t£

Q—Q-

4

6

H.J.A.

8 10

4

6 8 10

Display size

Fig. 3 Mean correct RTs (ms) and % emits Tor young controls, age-matched controls and HJA in Experiment 3a (tb vs "?, D and O ).

Experiment 3: Conjunction search with homogeneous abstractors Experiments 1 and 2 have shown that HJA can carry out visual search tasks in a normal fashion under the appropriate conditions. The pattern of his search functions, his search and error rate, and his overall RT and SDs are all normal. This conclusion contrasts with that of Humphreys et al. (1985, Experiment 2), who reported preliminary data on HJA's ability to detect a form conjunction

394


target amongst homogeneous distractors. Given the same task, normal subjects show two characteristics suggestive of spatially parallel search: There are nonlinear effects of display size on RTS, and absent responses can be faster than present. Humphreys et al. (1985) noted that HJA showed marked effects of display size and slower absent than present responses. In addition, HJA's errors rates were abnormally high. They suggested that HJA was impaired at grouping form conjunctions in parallel, and hence showed abnormal search functions with homogeneous distractors, which, normally, can be rejected together as a single perceptual group. Experiment 3 sought to investigate further HJA's ability to detect form conjunction targets amongst homogeneous distractors. Experiment 3a replicated Humphreys et al. (1985) using a task in which an inverted T target had to be detected amongst a set of regularly spaced, homogeneous T distractors. It also examined the effects of giving HJA extensive practice on the task over a period of 3 days, and it included data from age-matched controls and L, as in Experiment 2a. In Experiment 3b we report data from three sessions when HJA was asked to reduce his errors in this task. This allowed HJA's RT data to be compared to the age-matched controls with the error rate equated; it also enabled us to construct a speed-accuracy trade-off function for his performance. Experiment 3c tested HJA and the age-matched controls with displays in which the stimuli were randomly spaced. It is possible that HJA's impairment is apparently only with regularly spaced items (we discuss possible reasons in more detail below); if so, a quite different pattern of performance should be evident in Experiment 3c to Experiments 3a and b. In Experiment 3d HJA carried out the same task, but in this case there were always 2 inverted T targets on present trials. This tests whether HJA adopts a serial exhaustive rather than a spatially parallel search strategy in this task. Experiment 3e attempts to replicate the results using different, nonlinguistic stimuli ( O v s r ) . Experiment 3a: 1 vs T METHOD

The Method was the same as that used in Experiment 2a, except that only upright T distractors were used. There were 10 young control subjects (ages ranging from 22 to 36) and 11 age-matched controls (ages ranging from 55 to 71; mean 65.4 years). All had either normal or corrected-to-normal vision. L (Bauer, T984) also took part in a single experimental session. There were 48 four-item displays, 48 six-item displays and 60 ten-item displays (156 trials), in each session. Half the trials had the target present, half absent. HJA was given extensive practice at the task over a three-day period. Data from Session I were recorded after 250 trials practice. The data from Session 2 were collected after 936 trials practice on each of the prior three days,

Parallel Pattern Processing 1600

395

j

1400

-

1200

-

RT (ms) 1000

-

000

-

600

-

400

200

Young controls

Ago-mulched controls

L.

H.J.A. (250 trials)

H.J.A. (3000 trials)

-

0

—I

1

1

)

4

6

10

4

-+—-| 6

)"

10

1

I -

4

6

I 10

r—I

I I 4

6

10

1

1 4

1 6

10

Display size

•*~ prosunt *-*- absont

100

-

90

-

80

-

70

- -

60

-

50

-

40

- -

30

-

20

-

10

-

Percentage error

Young controls

% orror

0 - -


H.J.A. (250 trials)

HJ.A. (3000 trials)

1 4

6

10

4

b

10

4

G

10

4

u

10

4

6

10

Display sizo

Fig. 4 Mean correct Kis (ms) and % errors for young controls, agc-matchcd controls, L and HJA in Experiment 3a (-L vs T).

followed by 156 trials practice on the third (data collection) day. Control subjects received 32 practice trials prior to the experiment. RESULTS AND DISCUSSION

The mean correct RTs (in msec) and percentage errors for the three sets of control subjects (young controls, age-matched controls and L) and for HJA are

396


shown in Figure 4. Descriptive statistics for the search functions for the primary effects are given in Table 2. Table 3 gives the descriptive statistics for the "secondary" effects, such as the effect of practice. Unlike Experiments 1 and 2, HJA showed an abnormal search function in this experiment: (1) he showed slower RTs than the age-matched control subjects, (2) the slopes of his search functions were steeper than for any of the controls, particular when the error rates are also taken into account (Tables 2 and 3), (3) unlike all of the controls, HJA's absent responses were by some margin slower than his present responses, and (4) his error rates were considerably higher than any of the controls. This was true both for the data collected after 250 practice trials and for those collected after HJA had received an extended set of nearly 3000 trials practice prior to the experiment. HJA's RTs were more than 2 sd's slower and his error rates were more than 20 sd's higher than those of the age-matched control subjects. L also responded slowly relative to the age-matched controls; nevertheless he showed a normal pattern of performance—there were relatively minor effects of display size on performance, absent responses were faster than present, and error rates were low. We take L's generally slow RTs to reflect a general reduction in his decision making and/or motor speed following brain damage (see de Haan, Young, & Newcombe, 1987, for a similar argument concerning the effects of brain damage on overall RT rale). Indeed, his RTs were slow in Experiment 2a as well, though in both cases there were no abnormalities in the pattern of his performance. L's data thus show that HJA'S slow responding on absent relative to present trials, the slopes of his search functions, and his high error rate, are not consequent on his overall slow RT rate. We note also that, in other circumstances, HJA'S RT and error rates are at least as good as those of age-matched control subjects (unlike L, see Experiment 2a). HJA has a selective problem responding to the displays used in Experiment 3a. We suggest that normal subjects respond efficiently to these displays by grouping distractors separately from targets, and by rejecting distractors as a single perceptual group. This leads to there being minimal effects of display size, and 'fast absent' responses (see Duncan & Humphreys, 1989; Humphreys et al., 1989). HJA seems impaired at search dependent on the parallel grouping of distrators'.

1 HJA's impaired search pattern in Experiment 3 cannot be attributed to abnormal eye movements. Riddoch and Humphreys (1987) showed that HJA has normal patterns of saccadic and smooth pursuit eye movements (apart from there being staircase tracking into the blind parts of his field). This is confirmed by HJA's normal search performance in Experiment 2. Note that normal subjects are more likely to make eye movements to the displays used in Experiment 2 (where there were linear effects of display size) than to those used in Experiment 3 (where there were non-linear effects of display size).

—

—

—

232

16.7

—

Mean SD RT

Mean % E Mean SD % E

218

18.7

53 98.53

Slope %V 201

—

0.0

130 —

244

—

—

IO-5

—

—

229

13-9

218

54 97-45

1278 54 91.11

—

24.3

16,0 1212

20.2

125

98.18

62

97-35

99-73 186

50

983

11

893

1

4-2 —

162

6565

32

1095

—

72.25 94 10.1

20

766

2

Number of targets

163

957 35 99-57

Strategy Least Less

1342 86 98.56

1483

0.0

25.0

1310 43 83.34

173

1002 49 99.91

165

1009 45 54-34

1318

3i-7 —

55-82 203

18

1191

Mean RT

Mean SD RT Mean % E Mean SD % E

Slope %V

Mean RT

Most

Practice : Experiment 3a Strategy : Experiment 3b Number of targets : Experiment 3d Mean RT = Mean averaged across display size (msec) Slope = Slope of RT—display size search function (msec/item) % V = Percentage of variance in the RT—display size function attributable to a linear component Mean SD RT = Mean of individual RT SDs averaged across display size Mean % E = Mean % errors averaged across display size Mean SD % E = Mean % errors SDs across subjects, averaged across display size.

Absent

Present

Practice 3000 trials trials

250

HJA

•5

74 •47

•3

1.1

112

-1

6.89

95.64

585

2-5

3-9

110

646 8 94.14

Young

627 16

.96 .86

103

34 95-25

740

Agematched

Controls

TABLE 3 Descriptive statistics for the secondary effect of practice, response strategy, and number of targets, with homogeneous distractors in Experiments 3 a, b and d


398

1600 1500 1400 RT(ms)

1300 • 1200 1100 1000 900 BOO

Most conservative

Less conservative

Leas! conservative

700 •

600

•H—

4

6

1

t-

-t

4

G

4

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6

Display sire

100

-•- present

j

o- absent

90 -• 80 - • 70 60 -

% error

50 - -

Most conservative

Less conservative

Least conservative

40 -30 20 10 0 --

Display size Fig. 5 Mean correct RTS (ms) and % errors for HJA as a function of the response criterion adopted, in Experiment 3b (_L vs T).

Experiment 3b: Speed-accuracy trade-off function We have argued thai HJA is selectively impaired at detecting a form conjunction target amongst homogeneous distractors, and we have linked this to a problem in grouping together such distractors in a spatially parallel manner. However, throughout the data collecting sessions in Experiment 3a, HJA maintained an abnormally high error rate. This error rate makes it difficult to compare both the overall RTS and the slopes of the search functions for HJA and the control subjects. In Experiment 3b, HJA carried out three experimental sessions, performed on the same day, in which we encouraged him to manipulate his strategy in order to lower his error rate.

Parallel Patlern Processing

399

1500 1400 1300 1200 1100

present

1000

absent

(ms) 900 800 700 600 80

90

100

Accuracy (% correct) Fig. 6 Relationship between speed and accuracy of response for HJA in Experiment 3b.

First, he carried out a set of 240 trials under normal instructions (see Experiment 1). Subsequently he was asked to carry out the same experiment but this time he was given 3 points for every correct RT and had 3 points deducted if he made an error. In the third session he was given 3 points for every correct RT and he had 5 points deducted if he made an error. A total of over 600 points per session qualified for a reward. MF.THOD

Experiment 3b was carried out using a Macintosh SE80 microcomputer, using the Psychlab package (Gunn & Bub, 1990). There were 3 display sizes (4, 6 and 8), with 40 present and absent trials at each display size. The size of the stimuli, the display area and arrangement were the same as in the earlier experiments. RESULTS AND DISCUSSION

Figure 5 presents the mean correct RTS and error rates for HJA in each session. Table 3 gives the summary statistics for the search functions. The data from the condition in which HJA was required to adopt a conservative response criterion, in order to minimise his error rate, can be compared with those of the age-matched controls in Experiment 3a, since their error rates were minimal in that study. HJA's mean RTS on present trials were over 2 SDs higher than the age-matched controls, and his RTS on absent trials were over 8 SDs higher. The slope for HJA's present responses was about T.3 the slope for the mean control data, and for absent responses the slope ratios were about 1:4. Thus HJA is by some margin worse than the controls even

4OO


when error rates are equated. Figure 6 presents HJA's data from Experiment 3b expressed in another way. In this figure, mean RTs for present and absent responses are given averaged over the display sizes as a function of the overall response accuracy. This shows the relationship between HJA's speed and accuracy of performance. Present responses were faster than absent, and showed less of an increase in RTs relative to response accuracy (the slopes ratio for present: absent is about 1:2.5). m ' n e accompanying paper, Humphreys et al. (this volume) report a simulation of HJA'S results using a connectionist model of visual search, in which the pattern of the speed-accuracy trade-off function generated by the model running under conditions of increased internal noise, is similar to that generated by HJA. Experiment 3c: Performance with irregularly spaced items Experiment 3b showed that HJA was impaired at detecting form conjunction targets amongst homogeneous distractors when his error rate was equated to that of control subjects. Experiment 3c further examined the generality of this result. Experiments 3a and 3b used displays with regularly spaced items, so that, considered globally, the items in each display described a simple shape (a square, a hexagon etc.). It may be that the presence of the global shape impairs HJA'S ability to group distractors and to select local targets. HJA may not be impaired when the items in the displays are irregularly spaced, eliminating possible global interference. There are some a priori grounds for this supposition, Humphreys et al. (1985) had 1UA identify either local or global compound letters (cf. Navon, 1977). Whilst his RTs to global letters were as fast as those of young nonbrain damaged control subjects, his RTs to local letters were relatively slow. This suggests that HJA is impaired at selecting local parts of a more global shape. Humphreys et al. (1985, Experiment 2) also showed that HJA was impaired at delecting form conjunctions amongst homogeneous distractors even with irregularly spaced displays. This would argue against global interference being the sole cause of the problem in the conjunction-detection task. However, in the relevant experiment reported by Humphreys et al. (1985), items in irregular displays were plotted so that none of the items were closer to fixation than the items in regular displays. This meant that the average distance between each item and fixation was greater in the irregular than the regular displays. Possibly, HJA was then worse than the controls because he showed stronger effects of distance away from fixation. This was remedied in Experiment 3c by fixing the average distance between each item and fixation in the irregular displays to be equal to that in the regular displays used in Experiment 3a.


401

1600 -p 1400

-•

1200

-

HT (ms) 1000

-•

800 600

- •

400

• HJA

Age matched controls

Young controls 200 0 - -

-+• 8

4

10

6

6

10

h 8

10

Display size

100

-p

90

-

80

--

70

-•

••- present o absent

60 - • Young controls % error


HJA

50 40 - • 30 20 10 0

=Y10

10

10

Display size

Fig. 7 Mean correct RTS (ms) and % errors for young controls, age-nialched controls and HJA in Experiment 3c (_L vs T, irregular displays).

METHOD

HJA took part in a single experimental session involving 280 trials, preceded by 140 practice trials. As in Experiments T and 2, there were 4 display sizes, involving 4, 6, 8 and 10 items per display, and there were 32, 36, 32 and 40 trials at each respective display size, for each response. Items were plotted randomly in the field with the constraint that the average distance between

402


each item and the centre of the display was the same as that for the regular displays used in Experiments i-3b. The target was an inverted T and the distractors were all upright T'S. Ten young controls and ro agc-rnatched controls (age-range between 57 and 68 years) also look part. RESULTS AND DISCUSSION

Mean correct RTs and error rates for HJA and for the controls are presented in Figure 7. Table 2 gives the summary statistics for the search functions. HJA'S present RTS were more than 2 SDS from those of the controls, and his absent RTS were more than 4.9 SOS away. HJA's error rates were over 50 SDs greater than those of the controls, HJA is clearly impaired at detecting form conjunctions amongst homogeneous distractors even with irregular spaced displays which do not configure into global shapes. Experiment 3d: Performance with two targets Experiments 3a-3c have shown that HJA is impaired at detecting a form conjunction target amongst homogeneous distractors. In this search task, HJA manifests a particular pattern of performance: There are (in contrast to normal performance) strong effects of display size, absent responses are slower than present, and there can be high error rates when he does not adopt a strict response criterion. In contrast to this, his search for form conjunction targets amongst heterogeneous distractors can be relatively normal (Experiment 3b). There are grounds for arguing that, in normal subjects, search for form conjunctions amongst heterogeneous distractors is a serial process since (1) search times increase linearly as a function of the display size, and (2) the slope for present responses is about half that for absent responses (e.g., see Treisman & Geladc, 1980, for this interpretation of search functions with these two characteristics; though sec also Humphreys & Muller, in press; Humphreys et al., this volume, for alternative arguments). Given (I) HJA's impairment in detecting form conjunctions amongst homogeneous distraciors in a normal, spatially parallel manner, and (2) his normal, possibly serial, search for the same targets amongst heterogeneous distractors, HJA might be expected to adopt a serial search in the homogeneous distractor case. However, several aspects of his performance argue against this. For instance, he manifests high error rates only with homogeneous displays—if he carried out a similar search process in all cases, his error rates would be expected to be the same. In addition, although he manifests effects of display size on search with homogeneous distractors, the slope of present to absent responses is not r:2 (Tables 1 and 3); indeed, in Experiment 3c the slopes for present and absent responses were roughly equal. Present: absent slope ratios of 1:2 would be expected if search were serial and self-terminating (Treisman & Gelade, 1980). Such considerations suggest that HJA attempts to conduct a (normal) spatially parallel search with homogeneous distractors. It is simply


403

1600 -r 1400 1200 RT 1000 800 600 400

10

100 -| • present

90 -

°- absent

80 70 60 % error

50 - • 40 -30 20 10 0 -10

4

G

H

10

Display size

Fig. 8 Mean correct KTS (ms) and % errors for HJA in Experiment 3d (2 _L or I J_ vs T), ( i _L and 2 _L signify 1 target or 2 targets on present trials).

that this search is inefficient and error prone. It is also possible, however, that roughly equal slopes on present and absent responses may be generated if HJA conducted a spatially serial but exhaustive search (Townsend, 1972). Although we would be left explaining why he does this with homogeneous but not with heterogeneous dislraclors (Experiment 3b), this remains one account of at least some of the data with homogeneous displays. This possibility was tested in Experiment 3d. HJA performed two test sessions. In one session Experiment 3c was replicated (using only one target on 'target present' trials). In a second session HJA was presented with two

404

Humphreys, Riddoch, Quintan, Price and Donnelly

targets (respond present) or no targets (respond absent). He had to respond present as soon as any one target was found. If HJA conducted a serial but exhaustive search, then having two targets on 'target present' trials should not facilitate his performance (see Pashler, 1987). MRTHOD

The Method was exactly the same as that used in Experiment 3a, except that, in one session, two targets were always presented on 'target present' trials (with one of the targets being randomly substituted for one of the distractors). For each session there were 280 trials preceded by 140 practice trials. Data were collected only from HJA, who carried oul Ihe 'single target present' session first. Note that, at this stage of testing, HJA was highly practiced at these search tasks. His performance was not greatly affected by an extra 420 trials practice. Any advantage for the two target condition is most unlikely to be due to practice within a session. RESULTS AND DISCUSSION

Mean correct RTS and error rates are given in Figure 8. Descriptive statistics are given in Table 3. Having two targets on 'target present' trials dramatically improved HJA'S performance. Averaged across the display sizes, both present and absent RTS were over 200 msec faster than when only a single target was presented; miss rates were xo% lower. The advantage for two target over single target displays was statistically significant both for RTS (/"(I,238) = 19.4 and /•"( 1,262) = 23.6, p < 0.001, for present and absent trials respectively) and for errors (x2 = 6.14, p < 0.025, summed over present and absent responses). This improvement would not be expected if HJA were conducting a serial but exhaustive search of the displays. Rather, the data arc consistent with him carrying out a parallel but abnormally inefficient parallel search with homogeneous distractors. Experiment 3c: DVS 1 Experiments 3a-3d all used letter distraclors (T'S). Although this follows a long tradition in the visual search/texture segregation literature (e.g., Beck, T967; Bergen & Julesz, 1983), it leaves open the possibility that HJA's impairment is particular to this case. For instance, HJA is impaired at reading and processes words in a piecemeal Jelter-by-lettcr fashion (Humphreys & Riddoch, 1987a). This reading impairment may reflect a problem in processing multiple letters in parallel (in the General Discussion we return to consider the relations between HJA'S reading and object recognition impairments in more detail). By using letters as dislractors, we may have inadvertently carried this problem over from reading to visual search. Experiment 3e sought to remedy this by using non-letter targets and distractors: The target


4O5

Young controls

4

6

8

10


4

6

8

10

4

HJA

6

8 10

Display sUe

100 ••- present

90 o- absent

80 70 60 % error

50 Young controls

HJA


40 30 -• 20 10 0 4

6

8

10

4

6

B

1

0

4

6

8

10

Fig. 9 Mean correcl RTs (ms) and % errors for HJA in Experiment je ( 6 vs V).

was the form tl) and the distractors were all Ts. Humphreys et al. (1989) showed that young normal subjects can conduct a spatially parallel search with these stimuli, despite targets and distractors differing only in the conjunction of their local features. Experiment 3e tested whether this also held true for HJA. METHOD The Method was the same as that in Experiment 1, except that the target was a i , and the distractors were all T. HJA took part in a single experimental session, as did 10 age-matched control subjects (ages ranging between 60 and 71), and 14 young control subjects (Humphreys et al. 1989, Experiment 4a). All the controls had either normal or corrected-to-normal vision.

406

Humphreys, Riddoch, Quinlan, Price and DonneJJy RESULTS AND DISCUSSION

Figure 9 shows the mean correct RTS and error rates for HJA and the control subjects. Table 2 gives the descriptive statistics for the search functions calculated from the mean RTS. HJA was again impaired relative to the age-matched control subjects. Averaged across the display sizes, HJA'S miss rale was over TO SDS away from the mean of these controls. His RTs on both present and absent trials were over 2 SDS away from those of the conlrols. These results mirror those found in Kxperiment 3a, with HJA being impaired when the task required the detection of a form conjunction target relative to homogeneous distractors. Kxperiment 3c shows that this holds even when non-letter stimuli are used, and that it holds under conditions in which age-matched control subjects do not manifest linear effects of display size on performance (note that linear effects were presented in the age-matched control data in Experiment 3a). General Discussion We have presented a series of detailed investigations with a patient, HJA, with marked visual agnosia. The results may be summarized as follows: 1.

2.

HJA can conduct efficient, and normal spatially parallel visual search for a line target defined by a 45 0 change in orientation relative to the background (Experiment 1); He can conduct a relatively efficient and normal search for a form conjunction target set amongst heterogeneous distractors (Experiment 2);

3.

4.

5.

6.

He is selectively impaired, relative to age-malched controls and lo another agnosic (control) subject, at delecting a form conjunction target amongst homogeneous distractors (Experiment 3); This selective deficit is not affected by giving him extensive practice on the search task (Experiment 3a) or by using irregularly spaced rather than regularly spaced items (Experiment 3c); HJA's deficit with form conjunctions amongst homogeneous distractors, is best characterized in terms of him conducting an abnormally inefficient but nevertheless spatially parallel search (Experiment 3d); The effects with form conjunctions generalize across stimuli (Experiments 2b and 3e). IMPAIRED GROUPING OF FORM CONJUNCTIONS

These results suggest that search for a form conjunction amongst homogeneous distractors depends upon particular processes that can be selectively impaired after brain damage. There are good grounds for arguing that these processes involve the grouping of distractors, and the parallel segmentation of grouped distractors from targets. For instance, consider the 'fast absent'


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responses that occur with control subjects (see also Humphreys et al., 1985, 1989). These responses cannot be due to subjects responding to some emergent local feature introduced when targets are present; (e.g., the presence of a top terminator on an inverted T target relative to upright T distractors); rather they must be due to subjects responding to a field of homogeneous distractors as a single perceptual group. This is contingent on the grouping of distractors. In the other two search conditions we have examined, involving search for a salient disjunctive feature target (Experiment 1), and search for a form conjunction amongst heterogeneous distractors sharing target features (Experiment 2), performance may not normally be based on efficient parallel grouping of distractors. Targets defined by a salient feature difference relative to distractors (such as the orientation defined target, used in Experiment 1), may be detected on the basis of activity within 'feature maps', whose units selectively respond to the presence of particular features in an image (e.g., Treisman, 1988). Activity within such a feature map may directly activate a target 'template', enabling present responses to be made. This may happen when the target and distractors have very different values within the feature domain, making it unlikely that distractor elements activate the relevant target map. When a form conjunction is presented amongst heterogeneous distraclors sharing target features, parallel grouping may not normally lead to the segmentation of separate target and nontarget groups since distractors may group with the target rather than with each other. In this case, an impairment to grouping processes may not have dramatic consequences on search efficiency. For instance, when grouping is deficient, target-distraclor linkage may break down, with there being a possible benefit to target detection. Any such benefit would be weighed against detrimental effects on search through a failure to group distractors together. Overall, the effects may be relatively small. Tn the following paper we present a computer simulation demonstrating that this is indeed the case. Significantly, HJA gave no evidence of fast absent responses to homogeneous distractors. This was not simply because he showed slow responding: The agnosic patient L showed generally slowed RTs, but still produced the effect (Experiment 3a). Nor was it because HJA simply adopts a conservative response criterion: Forcing HJA to vary his criterion does not produce a qualitative shift in his performance (Experiment 3b). Also, HJA gave no sign of adopting a conservative response criterion in the search tasks examined in Experiments I and 2, even though the search tasks in Experiment 2 are much more difficult for normal subjects than (hose for which HJA is relatively impaired. We suggest that HJA does not manifest a fast absent response because he is selectively impaired at grouping form conjunctions. HJA's problem in grouping form information is not demonstrated in single

408

Humphreys, Riddoch, Quintan, Price and Donnelly

item discrimination tests, at least when the items are presented for unlimited exposure durations (e.g., see Table i). This may be because the encoding of the relations between line strokes within a localized spatial region (e.g., with the stimuli used here) may be relatively hard-wired, constituting a separate process from grouping across broader spatial regions based on form similarity (Humphreys et a)., this volume). Impaired grouping need not thus affect single form identification. Also, where coding the relations between strokes operates across broader spatial regions (e.g., as in the Efron shape-matching task), it may be possible for a patient with impaired parallel grouping to encode simple relations serially. For parallel grouping to constrain performance, relatively more complex, multiple-item displays are needed, in which targetbackground segmentation is reliant on spatially parallel grouping processes. IMPAIRED GROUPING ALONG WITH GLOBAL FORM PERCEPTION

The present paper has been concerned with demonstrating that HJA has a selective deficit in grouping form conjunctions. We term this a deficit in parallel pattern processing since (i) the grouping processes are spatially parallel, and (2) encoding the relations between form elements is a first step in encoding visual patterns. Yet, in other studies, we have shown that HJA has relatively intact perception of global shape (Humphreys et al., 1985; Riddoch & Humphreys, 1987). For instance, HJA'S RTS to global compound letters (cf. Navon, 1977) are within the normal range (Humphreys et al., 1985), and he can make object decision discriminations to silhouettes better than he can to line drawings (Riddoch & Humphreys, 1987). How can impaired parallel pattern processing co-exist with relatively intact global form perception? We can reconcile these results by arguing that coarse global information about objects can be extracted independently of the grouping of more local form information. For instance, coarse global descriptions may be based on low spatial frequency components in the image. The grouping of form conjunctions likely depends on higher spatial frequency components. Given the parallel extraction of different spatial frequency components in the image, it may be possible to derive coarse global descriptions of objects even when local grouping processes are impaired. In the following paper (Humphreys et al., this volume) we provide a rather different account of how global form descriptions may be derived independently of local grouping processes. We suggest that global form descriptions are based on encoding the location but not the nature of form elements. Within our simulation, location coding can be relatively unaffected by 'lesioning* even when form coding is greatly impaired. Whichever account holds true, we note that global descriptions derived independently of local grouping processes are likely to be insufficient for normal object identification, particularly if local information is important for


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differentiating between members of categories with perceptually-overlapping exemplars. For instance, such descriptions may be sufficient for identifying the general category that an object belongs to, but not for determining which object within the category it is (e.g., that an object is an animal, but not which one). AGNOSIA AND PARAI.I.F.I. PATTF.RN PROCESSING

HJA is profoundly agnosic. He fails to recognize many common objects by sight, and he experiences additional difficulties interpreting photographs and line drawings (Riddoch & Humphreys, 1987). Along with his problems in visual object recognition, he is prosopagnosic, he is impaired at finding his way along routes, and his printed word identification is slow and strongly affected by word length (Humphreys & Riddoch, 1987a; Riddoch & Humphreys, 1987). The question arises as to the relation between these visual problems and his impairment in processing pattern information in parallel. In principle, we believe that a deficit in spatially parallel pattern processing should generate many visual problems, particularly when performance is time limited (and so does not allow time for anything other than spatially parallel encoding of stimuli) and when there are multiple segmentation cues available in stimuli (likely requiring parallel constraint satisfaction; see Humphreys & Bruce, 1989). A good example of this is face recognition which can normally be accomplished rapidly, despite the fact that faces, considered as 2D patterns, contain multiple cues for segmentation into separate regions. Indeed, it is the case that HJA'S face processing is particularly impaired when he is presented with photographs rather than simple line drawings, a result we attribute to the increase in segmentation cues when surface texture and shadow are added (Humphreys et al., 1992). In addition to our assertion that a deficit in parallel pattern processing should produce a myriad of visual processing problems, the argument for a causal relation would be strengthened by evidence for qualitative similarities between HJA's agnosia and his poor pattern processing. It is useful here to consider what the visual system might gain from the parallel coding of a visual pattern. One possibility is that a parallel pattern processing system could exploit mutual, high-level constraints between alternative parsings of scenes. For instance, by encoding the relations between corner junctions, a coherent 3D representation of an object can be derived and segmented from other neighbouring or even partially occluding objects (cf. Fnns & Rcnsink, 1992; Perkins, 1968). It would follow from this that someone poor at coding pattern information in parallel should have impaired perceptual organization and segmentation. HJA's problems in object recognition are consistent with this. He is considerably better at recognizing objects and line drawings seen in isolation relative to when they are overlayed (Riddoch & Humphreys, 1987), with the problem with overlayed figures being in assigning to which

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figure a given line belongs. Also his visual naming errors often reflect incorrect segmentation of an object into its parts, or a failure to group parts into a description of a single object (see Humphreys & Riddoch, 1987a, for examples). These are precisely the problems we would expect if HJA is impaired at grouping visual information in parallel across a scene. Word recognition, like object recognition, is normally based on a rapid and spatially parallel coding of form elements (line strokes and letters; e.g., see LaBerge, 1983). A deficit in encoding the relations between these elements in parallel should render word reading difficult. This difficulty may be reduced by patients limiting the number of letters being encoded, by attending to just some at a time. This would produce the cardinal symptom of letter-byletler reading, namely abnormally large effects of word length on identification time (e.g., sec Patterson & Kay, 1982; Warrington & Shallice, 1980). Nevertheless, qualitative symptoms of poor parallel pattern processing should be apparent in reading tasks. Consistent with this, Riddoch and Humphreys (1987) showed that when multiple letters were briefly presented HJA tended to make increased numbers of identification errors on all the letters. This would follow from his impaired parallel grouping of form conjunctions, which can produce interference between pattern elements (Humphreys et al., this volume), HJA'S performance in multiple letter report tasks is unlike that generated by at least some other leller-by-lettcr readers, who remain able to identify a single letter (and indeed are able only to identify a single letter) in briefly presented multi-letter displays (e.g., Kinsboume & Warrington, 1962). In IlJA's case, we suggest that there is an intimate link between his impaired parallel pattern processing and his impaired reading. However, the nature of his reading deficit would then appear to be different to that present in some of the other letter-by-leUer readers in the literature, whose problems may be in disengaging attention once it falls upon a single letter within a word (cf. Farah, r99«; Riddoch, Humphreys, Cleton, & Fery, F99o). We note here that HJA'S good search performance in Experiment 2, showing a normal linear function, betrays no sign of a problem in disengaging attention. It is also of interest that HJA is able to copy objects which he cannot identify, though he does this in a slavish line-by-line manner, and that he can draw from memory what these objects should look like (Humphreys & Riddoch, 1987a; Riddoch & Humphreys, 1987). His good drawing from memory suggests that his deficit in parallel pattern processing dissociates from his (intact) long-term memory for object forms. Further, given his good copying, and his good performance on tests of single shape perception under unlimited viewing conditions (e.g., the Efron shape-matching test; see also Table 1), it is tempting to conclude that he has normal perceptual processing (sec Humphreys et al., 1992). However, the present paper shows that this conclusion is incorrect; we should not infer normal perceptual processing from copying or from tests of single item discrimination under free viewing


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conditions. Under free viewing conditions, HJA may be able to compensate for his impaired parallel pattern processing by limiting the spatial area over which form elements are processed, and introducing a serial scan across an object. However, the serial processing of pattern information will often fail to support object recognition, because it produces incorrect segmentation and also it leads to the loss of relational and 'emergent' features. POOR PARALLEL PATTERN PROCESSING OR POOR ACCESS TO PARALLEL OUTPUTS? Recently there have been a number of interesting results showing that patients with overt visual recognition problems may in fact have covert knowledge about the objects they fail to identify overtly (e.g., Bauer, 1984; de Haan et al., 1987). One interpretation of these results is that the patients lack access to the outputs of normal object recognition processes, though the encoding of visual input maybe be preserved (e.g., Young & de Haan, 1990). It is thus relevant to ask whether HJA has normal parallel pattern processing, but with his access to this information being in some way impaired. To maintain this, we would have to make a further, and perhaps less plausible assumption; namely that the deficit is confined to the output of parallel pattern processors, but not to the output of processors of single line orientations (cf. Experiment 1). In addition to this, HJA shows no covert sign of normal pattern processing. For instance, we have failed to show covert object recognition in matching tasks using semantically related or unrelated objects (Boucart & Humphreys, in press). Also (and unlike normal subjects) there is no advantage for matching fragmented shapes whose elements configure into organised perceptual wholes over those shapes whose elements do not configure (Boucart & Hulmphrcys, 1992). These pieces of evidence are more consistent with a disorder in processing visual input, rather than in gaining normal access to the outputs of such processes. DOES HJA USE NORMAL OR REORGANIZED PERCEPTUAL PROCESSES?

The argument that detailed single-case studies are relevant to understanding normal processes can be strengthened if il can be shown that the patient is using impaired normal processing mechanisms, rather than their having adopted a new (and abnormal) processing strategy. The most relevant evidence on this point comes from Experiment 3d. In that experiment we showed that HJA was facilitated at detecting the presence of one of two form conjunction targets, relative to when only one was present. Such a gain in search efficiency would not be expected if HJA were adopting an (abnormal) serial but exhaustive search of the displays. However, it is consistent with HJA's search for conjunction targets amongst homogeneous distractors still being spatially parallel, but abnormally inefficient.

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Indeed it is even possible to argue that HJA is obliged to undertake a spatially parallel search with such displays. Such an argument can be based on the contrast between HJA'S error rale in Experiments 2 and 3. In Experiment 2 it can be proposed that HJA (and the control subjects) undertook a spatially serial search, since the search functions were linear and there was an approximately 1:2 difference in the slopes for present and absent responses (Treisman, 1988; though see Humphreys & Muller, in press; Humphreys et al., this volume). HJA's error rates were also quite low and within the normal range in that experiment. In contrast, in Experiment 3 the linear function accounted for less of the variance, present: absent slope ratios departed from 1:2, and HJA made abnormally high numbers of errors. Such errors would not be expected if HJA conducted a spatially serial search. Apparently HJA cannot refrain from grouping items in these displays, even when it might benefit him not to group items but to treat each item as a separate perceptual object. IMPLICATIONS FOR NORMAL VISION

There are several implications arising from this work for the understanding of normal vision. Hirst, and perhaps most important, is that the perceptual organization of visual form seems to involve the encoding of at least simple conjunctive relations (such as corner junctions). Disruptions to the parallel encoding and grouping of such relations seems to impair object recognition. Second, the parallel encoding of such relations seems to be an obligatory process, HJA finds it difficult not to engage in the parallel processing of multielement stimuli, even when he might benefit from not doing so. Third, the grouping of local form conjunctions seems to operate independently of the encoding of global shape. Hence local form grouping can be impaired without the necessary impainnent of the encoding of global shape (see also Humphreys et al., 1985). Fourth, in HJA's case, poor local grouping seems to co-exist with relatively intact perceptual processes—for instance, intact stereopsis (Riddoch & Humphreys, 1987). This can be taken to support the modular organization of much of early vision. The encoding of 2D shape seems to operate independently of the encoding of stereo-depth based on binocular disparities (see also Marr, 1982; Stevens, 1987). In an earlier paper on HJA'S case, we argued that his problem was one of poor integration of the parts of visual objects (Riddoch & Humphreys, 1987). In the present paper we have tried to go beyond that by attempting to document some of the processes involved in visual integration. We have suggested that the integration of the parts of objects is based on encoding the relations between local conjunctive features, which represent computationally informative regions of discontinuity, HJA'S ability to encode such relations in a spatially parallel manner seems impaired. In the following paper we go on

Parallel Paltcm Processing

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to show how a conneclionist model which encodes the relations between conjunctive features in parallel can, when lesioned, simulate the patterns of search performance found in HJA. These two lines of research converge to highlight the importance of parallel pattern processing for normal vision. An earlier version of this paper was presented to the Experimental Psychology Society, London, January 1987. The work was supported by grants from the Medical Research Council and the Science and Engineering Research Council of Great Britain to the first two authors. We thank Dr. Mary Hill and Dr. John Pattern, of Famham Road Hospital, Guildfond, for their original referral of HJA. Russell Bauer kindly gave permission for us to report the data on patient L; Andy Young suggested the collection of data on speed-accuracy trade-off functions (Experiment 3b). Finally, our special thanks to both HJA and his wife, who have put up with many sessions of tedious testing with much good humour and patience. References Bauer, R.M. (1982). Visual hypoemotionality as a symptom of visual linibic disconnection in man. Archives of Neurology, 39, 702-708. Bauer, R.M. (1984). Autonomic recognition of names and faces in prosopagnosia: A neuropsychological adaptation of the guilty knowledge test. Neuropsychologia, 22, 457-469. Beck, J. (1967). Perceptual grouping by line figures. Perception & Psychophysics, 2, 491-495Bergen, J.R., & Julesz, B. (1983). Parallel versus serial processing in rapid pattern discrimination. Nature, 303, 696-698. Bicdcrman, I. (1972). Perceiving real-world scenes. Science, 177, 77-80. Bicdcrman, I. (1987). Recognition by components: A theory of human image understanding. Psychological Review. 94. 115-145. Bicdcrman, I., Mezzanotte, R.J., & Rabinowitz, J.C. (1982). Scene perception: Detecting and judging objects undergoing relational violations. Cognitive Psychology, 14, 143-177. Bodis-Wollncr, t. (1972). Visual acuity and contrast sensitivity in patients with cerebral lesion. Science, 178. 769-771. Bodis-Wollner, I. (1976). Vulnerability of spatial frequency channels in cerebral lesions. Nature, 261, 309-311. Boucart, M., & Humphreys, G.W. (1992). The computation of perceptual structure from collincarity and closure. Neuropsychologia, JO, 527-546. Boucart, M., & Humphreys, G.W. (in press). Global shape cannot be attended without object identification. Journal of Experimental Psychology: Human Perception and Performance. Clowes, M.B. (1971). On seeing things. Artificial Intelligence, 2, 79-116. Cowey, A. (1979). Cortical maps and visual perception. Quarterly Journal of Experimental Psychology, 31, 1-17.

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Visual processing of words in a patient with visual form agnosia: a behavioural and fMRI study.

Visual agnosia in an artist.

Perception and action in 'visual form agnosia'.

Parallel Computations in Insect and Mammalian Visual Motion Processing.

When apperceptive agnosia is explained by a deficit of primary visual processing.

Modeling the role of parallel processing in visual search.

Parallel Visual Information Processing Test. An experimental assessment of alexithymia.

Visual agnosia: a case of reduced attentional "spotlight"?

Selective loss of imagery in a case of visual agnosia.

Patient DF's visual brain in action: Visual feedforward control in visual form agnosia.

A historic case of visual agnosia revisited after 40 years.

Agnosia.

Parallel processing in the brain's visual form system: an fMRI study.

Parallel-serial processing and hemispheric function.

Neurodegeneration and mirror image agnosia.

Parallel processing of face and house stimuli by V1 and specialized visual areas: a magnetoencephalographic (MEG) study.

[Pattern specific visual competition].

Audio-visual integration through the parallel visual pathways.

Visual processing: microcircuits unmasked.

Oxytocin: parallel processing in the social brain?

Parallel mechanisms for visual search in zebrafish.

Visual motion processing and visual sensorimotor control in autism.

Temporal pattern processing in songbirds.

Subcortical auditory agnosia.