A two-stage account of computing and binding occluded and visible contours: evidence from

visual agnosia and effects of lorazepam

Giersch, A. (1), Humphreys, G.W. (2), Barthaud, J.C. (1), Landmann, C. (1)

(1) INSERM U405, Hôpitaux Universitaires de Strasbourg, France

(2) Behavioural Brain Sciences Centre, School of Psychology, University of

Birmingham, United Kingdom

Suggested running head: ‘Two stages for completing occluded contours’

Requests for reprints should be sent to A. Giersch, Hôpitaux Universitaires de Strasbourg, INSERM

U405, Département de Psychiatrie I, 67091 Strasbourg Cedex, France. Tel: 0033 (0) 3 88 11 64 71.

Email: giersch@alsace.u-strasbg.fr.

Aknowledgements The authors thank Dr. M. Welsch for medical examination of the healthy

volunteers, and HJA for his kind participation in the study. This work was supported by the University

Hospital of Strasbourg, by INSERM, and by an MRC grant (UK) to G.W. Humphreys.

2

ABSTRACT

Previous work has shown that HJA, a patient suffering from visual agnosia, can complete

occluded contours whilst being impaired at assigning contours to foreground and background

figures (Giersch, Humphreys, Boucart, & Kovács, 2000). Here we tested whether completed

contours are automatically bound with visible contours, after being derived from them. HJA,

lorazepam-treated and non-treated healthy participants were asked to match a first reference

line with an equal or longer line of identical orientation included in one of two lateral figures.

The target line was in the foreground or the background of the figures. The distractor picture

included two short collinear line-segments belonging to two different figures, so that

participants had to process the occluded parts to discriminate the target from the distractor

line. When the target line was in the background, both HJA and lorazepam-treated

participants were faster when the length of the reference line corresponded to the length of the

occluded part of the target line, relative to when it corresponded to the length of the occluded

part plus a visible contour. In contrast, control participants tended to show an advantage for

matching a reference line whose length was the same as the visible contours plus the occluded

part. However, when the stimuli were displayed for 50 msec only and then masked, controls

showed the same results as HJA. These results suggest that responses in the matching tasks

are biased by the existence of an early completed occluded line that remains isolated from real

contours.

3

INTRODUCTION

Occluded contours are very common in the visual environment. They occur each time

an object is partially occluded by other objects in the foreground. Recent papers have shown

that occluded contours are completed at an early level of processing, before edges are

attributed to foreground and background figures (Giersch, Humphreys, Boucart, & Kovács,

2000; Rensink & Enns, 1998). In the present paper, we present novel evidence that, in vision,

the occluded lines are computed prior to being bound to other (visible) contours, even if the

occluded and visible contours are part of the same edge of an object. The binding between the

occluded and visible parts of an edge is a secondary process, separable from a first stage of

computing the occluded edge from the visible part-edges. Our argument is based on evidence

that the secondary process can be disrupted by brain damage (in agnosia), by selective effects

of a drug (lorazepam) that affects contour binding in vision, and by using short, masked

presentations of stimuli for normal observers. These results are used to reconcile prior

findings showing (1) that contours are completed before the assignment of contours to

foreground and background figures, and (2) that the perception of occluded contours is

sensitive to figure-ground organization.

Completing occluded contours

The level of processing at which occluded contours are completed is still a matter of

debate. Some studies have shown that familiarity of the occluded object influences the way

occluded contours are completed, suggesting a role for top-down processes occurring at a late

stage of processing (Peterson & Gibson, 1994; Peterson & Kim, 2001; Wagemans &

d’Ydewalle, 1989). Similarly, several authors have proposed that the completion of occluded

contours follows the segregation of figures and ground (Anderson, Singh & Fleming, 2002;

Gillam & Nakayama, 2002; Grossberg, 1994; Wouterlood & Boselie, 1992). This argument is

4

consistent with data showing that (1) depth and display layout plays an important role in the

interpolation of incomplete contours (Anderson et al., 2000; Gillam & Nakayama, 2002), (2)

completion takes some time (Murray, Sekuler & Bennett, 2001; Sekuler & Palmer, 1992) and

(3) a representation in which parts are not completed appears to precede a later representation

where modal completion takes place (Rauschenberger & Yantis, 2001).

In contrast, other results suggest that contours are completed early-on, prior even to

processes such as figure-ground assignment. Kellman and Shipley (1991) proposed a model

of early completion, based on the ‘interpolation’ between visible segments. In their model,

completion is based on the properties of local oriented operators, governed by factors such as

the relatability of visible segments. Numerous neuropsychological investigations have

confirmed the existence of an early completion process operating between collinear contours

(Gilchrist, Humphreys, & Riddoch, 1996; Humphreys, 1998; Mattingley, Davis & Driver,

1997). For example, Humphreys (1998) and Mattingley et al. (1997) showed that the presence

of an occluder separating two collinear contours affected the phenomena of extinction in

patients with parietal damage. In the study of Humphreys (1998), the occluder reduced the

effects of distance between the collinear segments so that extinction increased less as the

stimuli were placed further apart. In Mattingley et al. (1997), there was less extinction when

collinear elements appeared to fall behind rather than in front of a cube. This indicates that

occlusion relations facilitated grouping by collinearity between the edges. Since the patients

in these studies were unable to perceive edges on the contralesional side unless grouping took

place, the results suggest that grouping between the occluded edges occurred pre-attentively.

Investigations conducted in normal participants also suggest that occluded contours are

completed early-on in stimulus processing. Rensink and Enns (1998) used a visual search task

and showed that the presence of an occluder prevented pop-up based on a gap between the

collinear parts of the occluded object.

5

The two views (for early or late completion processes) can be reconciled, however, by

proposing that there is a two-step process of completion, in which completion is computed

early-on between oriented elements, but then the completed edge is coded with respect to the

more global context. For example, the time taken for completion to take place in some

situations, and the role of depth and the spatial layout of stimuli, may come about because of

the influence of higher-level processes which utilise the information computed at the earlier

stage. Within this second stage, occluded contours may not be treated in the same manner as

visible contours, leading to some differences in tasks requiring responses to both types of

stimuli (Kellman, Guttman & Wickens, 2001).

Results obtained recently with the agnosic patient HJA (Giersch, Humphreys, Boucart

& Kovacs, 2000) are consistent with the hypothesis that occluded contours are computed

early, but then integrated at a second stage into a more global context. HJA was selectively

impaired at matching occluded relative to superimposed shapes. Interestingly, however, the

impairment increased as the occluded contours became shorter, and thus easier to compute.

Short occluded contours also impaired the discrimination of occluding shapes, falling in front

of the occluded edges. This suggests that HJA may have computed the occluded edges, but

that the computed edges then disrupted shape coding. Further evidence for this came from

studies using copying and similarity judgements, where HJA made copies and choices in

which the occluded edge was sometimes represented as a visible edge in the foreground of the

image. Apparently occluded edges were computed early on in this patient, but they were not

integrated appropriately with the other (visible) edges present.

On the above view, occluded edges may be computed in a rapid, first-pass of the image,

but (normally) are only assigned a preliminary status (cf. Lesher, 1995). Subsequently, a

secondary ‘confirmation’ process may be used to verify whether such edges are indeed

6

visible, and how the various edges are inter-related. This might involve re-entrant activation

from higher-order visual areas that are impaired in a patient like HJA.

In the present study, we asked whether a similar two-stage account is needed to explain

binding between an occluded edge and visible edges that connect to it. To the best of our

knowledge, this issue has not been investigated before. This may be because, at first sight, it

seems counter-intuitive to suggest that occluded contours computed from two edges must be

specifically integrated with them in a representation of the whole contour. Why should there

be this secondary integration process once a contour is computed? However, a specific

process of integration is suggested by the neuropsychological evidence on agnosia. Consider

the finding that a patient can misattribute an occluded edge to part of the foreground. Here the

patient computes the occluded edge but does not integrate it with the other background

contours (Giersch et al., 2000). This proposal was put to a more detailed test here.

We take a three-way approach, investigating the processing of occluded contours in an

agnosic patient (HJA again), in normal observers treated with lorazepam, and in non-treated

healthy volunteers. Lorazepam modulates the fixation of GABA on the GABAA receptor, and,

at the dose used here, it has been shown to specifically facilitate the detection of a

discontinuity between collinear contours (Beckers, Wagemans, Boucart, & Giersch, 2001,

Giersch, 1999, 2001). This effect results in impaired integration of local visual information

into a global configuration (Giersch, Boucart & Danion, 1997; Giersch & Lorenceau, 1999).

This may be due to an imbalance between (i) integration processes that tend to bind two

collinear segments (Kovács & Julesz, 1993) and (ii) segmentation processes that allow the

detection of the discontinuity, in particular the two line-ends composing the discontinuity

(Giersch & Fahle, 2001). If the occluded line needs to be bound to the visible contours,

lorazepam may impair this second stage of line completion. In contrast, no effect should be

observed if occluded lines are automatically bound to visible contours as they are completed.

7

Similarly, in HJA, the ability to integrate visible and occluded parts of the line may be

disrupted due to brain damage, in addition to the processes involved in integrating computed

contours into more wholistic shapes. Finally, we tried to disrupt the second stage of line

completion in non-treated healthy volunteers by reducing the duration of stimulus

presentation and by masking the stimuli.

We used a two-alternative forced-choice matching task in which a reference line was

presented and followed by two pairs or triplets of overlapping shapes (the target plus a

distractor). The task was to judge which of the overlapping pairs contained the line matching

the reference line. The critical line in the shapes could be visible or occluded, and it could

either match in length the reference line or differ in length. In particular, the reference line

could be short or long. When the critical line was occluded, the short reference line matched

the length of the occluded contour and the long reference line matched the length of the

occluded contour plus the visible parts it connected to. When the contour was visible (part of

the front rather than the background shape), the long reference line matched the full contour

in the shape. The short reference line then matched part of the contour (the part that

intersected with the second shape in the background). Examples of the stimuli are given in

Figures 1 and 2. We assumed that it would be easier to match the reference and target lines

when they were the same perceived length, relative to when they were different perceived

lengths. Thus when the critical contour was visible in the overlapping shapes, it should be

easier for matches to be made with the long relative to the short reference line. The situation

may differ when the critical contour is occluded, especially in participants who have difficulty

in integrating the occluded part with the other parts of the contour. Here, the short reference

line matches the length of the occluded contour. If this contour is not integrated with the

visible contour parts, then performance may be better with the short reference line. In

contrast, if the occluded contour is integrated with the visible part-contours, then performance

8

may be better (again) with the long reference line. Finally, if both integrated and non-

integrated representations are generated, then matching may be equally good with the short

and long reference lines We report data from HJA and from controls who either were or were

not given a dose of lorazepam. The findings indicate that the occluded contour is not well-

integrated with the surrounding visible contour parts, in these cases.

Case Report

HJA was formerly an executive in charge of the European office of an American firm.

In 1981, obstruction of the posterior cerebral artery, occurring post-operatively, induced a

bilateral ischemia of the occipital cortex, extending anteriorly towards the temporal cortex

(see Riddoch, Humphreys, Gannon, Blott, & Jones, 1999, for an MRI scan). Subsequently he

suffered from visual agnosia, with several visual processing deficits including: object agnosia

(he fails to recognize many everyday objects by sight), prosopagnosia, alexia (he reads letter-

by-letter), topographical impairments and achromatopsia (see Humphreys & Riddoch, 1987;

Riddoch & Humphreys, 1987; Riddoch et al., 1999; for full case descriptions). There is an

upper altitudinal defect of both visual fields (Goldman and Octopus perimetry). Visual acuity

is preserved. Although profoundly impaired in object recognition, HJA is able to copy

drawings (slowly); he can match some photographs of objects shown from unusual views

(when distinctive features are available) and he can perform the Efron-shape discrimination

task normally (Humphreys, Riddoch, Quinlan, Price, & Donnelly, 1992). On initial testing his

long-term memory for objects was largely preserved (though see Riddoch et al., 1999).

Riddoch and Humphreys (1987) offered a diagnosis of integrative agnosia since his

impairment was most pronounced when contour integration processes were needed for task

performance (e.g. with line drawings and overlapping figures, rather than with silhouettes).

HJA was 78 years old at the time of testing.

9

GENARAL METHOD

Participants

In addition to HJA, young and elderly control participants, placebo- and lorazepam-

treated participants were evaluated. There were 10 young and elderly control participants

tested in the same experiment as HJA (Experiment 1). They were 24 per group in the

pharmacological study (Experiments 2), and 12 in Experiment 3. Young participants were

students attending the University of Strasbourg or employees of the University, and were aged

between 20 and 40 years. The elderly control subjects (7 women, 3 men) ranged in age from

69 to 82 years (mean age: 74.6 years). Half of the elder subjects had a tape cover over glasses

to simulate the upper altitudinal defect of both visual fields observed in HJA. There was no

difference in performance between the subjects with a tape cover and without a tape cover.

The results are thus presented averaged over all elderly control subjects.

Control participants had no medical illness or history of alcoholism, drug abuse or

tobacco consumption of more than 10 cigarettes/day. They had not taken any concomitant

medication for at least 15 days. They all had normal or corrected-to-normal vision and were

naive as to the precise aim of the study.

In the pharmacological study, each participant was randomly assigned to one of two

parallel groups, a placebo group and a lorazepam 0.038 mg/kg group. The drug tablet was

given orally using a double-blind procedure. Participants were instructed to abstain from

beverages containing caffeine or alcohol for 24h prior to the study. They were tested in the

morning, after an overnight fast, and between 2h00 and 3h00 after the intake of the drug. All

experiments were conducted with a monocular presentation of the visual stimuli, to avoid any

contamination of the results by diplopia (Giersch, Boucart, Speeg-Schatz, Muller-Kauffmann,

& Danion, 1996). The protocol was approved by the Faculty Ethics Comittee. All participants

gave written informed consent.

10

Stimuli

< Insert Figure 1 about here>

Examples of the stimuli with two shapes present are given in Figure 1, and for the

stimuli with three shapes see Figure 2. When the stimulus with a long oblique contour was in

the foreground, the critical contour (to be matched to the reference line) was visible. When the

critical figure was in the background, the oblique contour was partially occluded (and the

external segments of the whole contour were visible, each segment being 12 pixels (0.7°)

long). The distractor stimulus was the mirror version of the target. One of the shapes in the

distractor had an oblique line that was orthogonal to the critical line in the target. The

distractor stimulus also included two collinear line segments in the same orientation as the

critical contour in the target, but which belonged to different shapes. These segments were

matched in length to the visible parts of the critical occluded contour. The length of the

occluded contour (in the target) was equal to the distance between the collinear segments

belonging to different shapes in the distractor (43 pixels, 2.5°). The three shape stimuli were

similar to the two shape items, but with a small, third shape added, making the stimuli slightly

more complex. At a distance of 60 cm, the angular size of the stimuli was 4.5°x 3°. The

stimuli were displayed at an angle of either 0, 90, 180 or 270°, to reduce effects of shape

familiarity.

< Insert Figure 2 about here>

The oblique reference lines were drawn in the same gray as the figures. They were tilted

right or left, and their length, long or short, corresponded respectively to the total length of the

long oblique contour (occluded part and visible segments, 67 pixels) or to the length of the

occluded part of this oblique contour, without the visible parts (43 pixels).

11

Apparatus

The stimuli were displayed on a color video monitor (Sony), screen resolution 640x480

pixels. They were generated through a pentium micro-computer equipped with a sVGA

graphic card. The stimuli were presented in white on a black background. A fixation point

subtending 0.04° was centrally displayed. Responses were given by pressing on the mouse.

Procedure

A fixation point was displayed for 500 msec in the centre of the upper half screen. It

was followed after a delay of 500 msec by an oblique line, tilted right or left, which was either

short (corresponding to the length of the occluded part of the long oblique contour) or long

(corresponding to the total length of the long oblique contour). The oblique line remained on

the screen throughout the trial. After a delay of 300 msec, a fixation point was also displayed

in the centre of the lower half screen. It was followed after a delay of 500 msec by two lateral

pictures, the target and the distractor. The distractor was the mirror image of the target. The

task was to decide which lateral picture contained a line matching the orientation of the

reference line. The target line could be longer but not shorter than the reference line, so the

response could not be based on the presence of the short oblique segments alone. Participants

gave their response by pressing a response key located on the side of the target. They were

instructed to answer as quickly as possible without making errors.

The onset of the pictures activated the computer's clock, which was stopped when the

participant pressed a key. There was no feedback on accuracy. The intertrial interval was

fixed at 1000 msec after the execution of the response.

Before the experiments, participants were shown examples on the screen and were

given a practice session of 40 trials to ensure that they all understood the task requirements.

One experimental session, with pair and triplet stimuli, involved respectively 64 and 128 trials

12

(16 and 32 trials in each condition). HJA carried out 3 blocks of trials in each experiment,

starting with the stimuli shown in pairs prior to the trials with triplets. Controls and

participants treated with lorazepam carried out 2 blocks of trials with the stimuli shown in

pairs and 1 block with triplets. The different properties of the stimuli were randomly and

equally represented in all experiments: the right or left location of the target, the angle of

presentation of the pictures (0°, 90°, 180°, 270°), and the orientation of the reference line.

RESULTS

Experiment 1 – HJA

HJA

The mean correct RTs and errors, for HJA and the controls, are shown in Figure 3 and

4. Analyses of variance were conducted on RTs, with each RT treated as an independent

observation.. There were four between-subjects variables, experiment (with stimuli shown in

pairs vs. triplets), session (first, second and third session), length of the reference line (short or

long), and type of target line (visible or occluded). Chi square tests were conducted on errors.

The mean RT was 1785 msec and mean error rate was 18.4%. RTs and errors decreased

across sessions (F[2, 446]=2, ns for RTs and χ2(2, N=576)=6, p<0.05 for errors). RTs and

errors decreased respectively by 57 msec and 8% from the first to the second session (F[1,

291]=3.6, p=0.06 for RTs and χ2(2, N=384)=3.7, p=0.056 for errors) and then remained stable

(F<1 and χ2<1). This variable did not interact with any other experimental factor. The results

are averaged over the experimental sessions.

RTs and errors were higher when the stimuli were shown in triplets than in pairs, by 810

msec and 7.3% (F[1, 446]=228, p<.001 for RTs and χ2(1, N=576)=4.5, p<.05 for errors). This

variable did not interact significantly with any other experimental factor.

13

RTs and errors were higher when the line was occluded than when it was visible, by 306

msec and 9.9 % (F[1, 446] = 27.8, p<.001 for RTs and χ2(1, N=576)=9.1, p< .005 for errors).

These effects did not differ between experimental blocks (F[1, 446]=2.2, ns). In both

experiments, RTs increased by 19% when the line was occluded compared to when it was

visible (¨+219 msec, F[1, 154]=10.9, p<.001 for RTs and +10.4%, χ2(1, N=192)=4.4, p< .05

for errors when stimuli were shown in pairs, vs. +368 msec, F[1, 292]=25.6, p<.001 for RTs

and +9.4%, χ2(1, N=384)=5.1, p< .05 for errors when stimuli were shown in triplets)..

When the critical line was visible and stimuli shown in pairs, RTs and errors were lower

when the reference line was long than when it was short, by 173 msec and 4.2% (F[1,

82]=4.4, p<.05 for RTs and χ2<1 for errors). This effect was lost when stimuli were shown in

triplets: there was a non significant reverse effect of 41 msec and 7.3% (F<1 for RTs and χ2(1,

N=192)=1.9, ns for errors). When data were put together, there was no overall effect of

length, although there was an advantage of 58 msec and a disadvantage of 3.1% for the long

versus the short reference line (F<1 and χ2<1).

In contrast, when the line was occluded, RTs were longer and errors higher, by 239

msec and 9.4%, when the reference line was long than when it was short (F[1, 209]=12.3,

p<.001 for RTs, χ2=(1, N=320)=3.3, p=0.07 for errors). These effects did not differ between

experiments, in the analysis on RTs (F<1). In both experiments, RTs increased by around

17% when the reference line was long as compared to when it was short (+217 msec, F[1,

72]=5.5, p<.05 for pairs, and +343 msec, F[1, 137]=10.1, p<.005 for triplets). However, errors

increased only when stimuli were shown in triplets, by 17.7% (χ2=(1, N=192)=7.9, p<0.005).

Errors decreased slightly when stimuli were shown in pairs, by 4.1% (χ2<1).

These effects resulted in a lack of a significant advantage for visible vs. occluded target

lines when the reference line was short (165 msec, F[1, 233]=2.3, ns for RTs and 9.4% χ2(1,

N=320)=1.4, ns for errors). In contrast, the advantage for visible vs. occluded target lines was

14

very large when the reference line was long (462 msec F[1, 213]=34.6, p<.001 and 11.9%

χ2(1, N=320)=5.4, p<.05). These effects led to a significant interaction on RTs between the

length of the reference line (short vs. long) and the type of target line (visible vs. occluded)

(F[1, 446]=9.9, p<.005). This interaction was significant in both experiments (F[1, 154]=10,

p<.005 for pairs and F[1,292]=4, p<.05 for triplets).

< Insert Figures 3 and 4 about here >

Young vs. elderly control participants

Two analysis of variance were conducted, on the mean correct response times (RTs) and

on the mean error rate, with participants as random variable. There were two between-

participant variables, age and experiment (with stimuli shown in pairs vs. triplets), and two

within-participant variables, the length of the reference line (short or long), and the type of

target line (visible or occluded).

Elderly controls were slower and less accurate than young control participants, by 440

ms, F[1,36]=31.1, p<.001 and by 10.7%, F[1,36]=24.9, p<.001. The mean RT and the mean

error rate were respectively 783 msec and 3.2 % in young controls, vs. 1222 msec and 13.9 %

in elderly controls. Except when otherwise stated, there was no effect of age, and data are

averaged over age.

Performance did not differ signifcantly between stimuli shown in triplets and in pairs

The data are averaged over the two experiments.

Similarly to HJA, RTs were longer when the line was occluded than when it was

visible, by 157 msec (F[1, 36] = 103, p<.001). Errors were also higher for occluded than for

visible lines, by 5.1%, F[1, 36]=11.9, p<.005). The advantage for visible over occluded lines

was larger in elderly than in young control participants, in the analysis on errors (by 50 ms,

F[1,36]=2.6, ns for RTs, and by 8.4 % F[1,36]=7.9, p<.01 for errors.

15

RTs and errors were lower when the reference line was long than when it was short, by

38 msec (F[1,36]=24.8 , p<.001) and 1.1 % (F[1,36]=5.4 , p<.05). This effect did not differ

significantly with the type of target, visible or occluded (45 msec and 1.6% vs. 30 msec and

0.7 %, Fs<1). However, the advantage for long reference lines, as compared to short reference

lines, was larger for figures shown in triplets than in pairs, in the analysis on RTs (by 46 ms,

F[1, 36]=9.4, p<.005 for RTs, and smaller by 0.9 %, F<1, for errors).

Discussion

The results show that both HJA and control participants were much slower when the

target line was occluded than when it was visible. These effects with normal observers were

exaggerated with elderly control participants and with HJA. The effect of occlusion with the

controls appears to contradict other studies that essentially show no differences in

performance for visible and occluded contours, in normal observers (Gerbino & Salmaso,

1987; Giersch et al., 2000; van Lier, Leuwenberg, & van der Helm, 1995). However, here, the

use of a reference line may have enabled subjects to use a physical-match strategy when the

target contour was visible, and this may have facilitated performance. Indeed, in terms of

visible contours, target and distractor stimuli clearly differed when the target contour was

visible: only the target picture included a visible line identical to the reference line. In

contrast, when the target contour was occluded, visible contours matching the reference line

were identical in both the target and distractor stimuli.

The most striking result, though, is that HJA was affected by whether the reference line

matched the length of the target: he was faster when the reference and target lines were the

same length than when they differed in length. This effect was clearest with the occluded

target. It was observed in both experiments, and with different types of figures. The higher

RTs observed in HJA for figures shown in triplets suggests greater difficulties in the second

16

experiment. Nevertheless, the effect of line length with occluded targets was similar to the

length effect observed with figures shown in pairs. In contrast, the controls tended to be faster

for long reference lines. The effect of line length, for HJA, suggests that he matched the

occluded contour directly, without necessarily integrating the occluded and visible contours

together. Controls, on the other hand, appear to respond to an integrated representation in

which the occluded contour is bound to the visible parts (matching the long reference line).

This advantage for longer lines was even larger for elderly relative to young control

participants, despite the fact that they were slower than young controls and were

disadvantaged for occluded lines, like HJA. This shows that the length effect in HJA is not

due to the effect of age or a non-specific effect, such as a slowing of RTs.

Experiment 2 - Effect of lorazepam

Method.

The method was the same as indicated above, except that participants treated with

lorazepam or placebo were run with stimuli shown in triplets.

Results

The results are displayed Figure 5.

The mean RT and the mean error rate were respectively 812 msec and 4.2% in placebo-

treated participants vs. 1084 msec and 4% in lorazepam-treated participants. Participants

treated with lorazepam were slower by 272 msec than participants treated with placebo (F[1,

44]=13.9, p<.001), but error rates were equivalent (F<1).

In both groups, RTs and errors were lower when the target line was visible than when it

was occluded, by 180 msec (F[1, 22]=17.8, p<.001) and 4.4% (F[1, 22]=22.8, p<.001) in the

17

placebo group and by 256 msec (F[1,22]=27.6, p<.001) and 2.2% (F[1, 22]=4.4, p<.05) in the

lorazepam group. There was also an interaction between the type of target line (visible vs.

occluded), the length of the reference line (long vs. short) and the drug treatment (lorazepam

vs. placebo; F[1, 44]=7.2, p<.01).

< Display Figure 5 about here >

When the target line was occluded, RTs were shorter by 38 msec when the reference

line was short than when it was long, but only in the lorazepam group (F[1, 22]=6.2, p<.05).

In contrast, in the placebo group, RTs tended to be longer, by 62 msec, when the reference

line was short than when it was long (F[1, 22]=3, p=0.097). There was no significant effect of

length when the target line was visible (Fs<2.5).

Discussion

The results again demonstrate that the task was more difficult when the target line was

occluded than when it was visible. This effect was not reliably increased after lorazepam

compared with the placebo. There was also a general slowing of RTs in the lorazepam group,

which is likely a non-specific effect (e.g., including some degree of sedation). More

interestingly, there was a selective effect of lorazepam as a function of the length of the

reference line and whether the target line was occluded or visible. The result matched that

found in HJA, with performance being facilitated with the short relative to the long reference

line, but only when the target was occluded. As with HJA, this suggests that matches were

based on the occluded contour alone, without it being integrated with the visible contour it

connected to. The same does not hold for the controls, where performance was best if the

reference line matched the length of the occluded contour plus the visible connecting parts.

We suggest that this is because the binding between the occluded contour and its connecting

18

parts was disrupted by the drug. We note that this drug effect, in controls, was much less

pronounced than in the agnostic patient, HJA.

The results observed in lorazepam-treated participants were qualitatively similar to

those of HJA, suggesting that completion is not the result of some neuronal rearrangement in

a brain-lesioned patient, but can also be observed in healthy volunteers. We explored this

possibility further in the next experiment, by testing healthy volunteers only, without drug

administration, and by reducing the duration of the stimuli presentation.

Experiment 3 – Non-treated healthy volunteers

The experiment was designed to verify whether non-treated healthy volunteers also

complete occluded contours in two processing stages. To do this, we attempted to limit

performance by using: (1) reduced presentation durations, (2) a masking procedure, (3) only

targets where the critical line was occluded, (4) and a limited training period. Several studies

have concluded that the time to complete occluded lines takes place across a 75 msec to 200

msec time window (Murray et al., 2001; Sekuler & Palmer, 1992). We suspect that this

duration includes the two processing stages of line completion (i.e. the production of the line

and its binding with visible contours). Hence, we used two durations, 50 msec and 200 msec,

expecting that the occluded line may be bound to the visible contours by 200 msec but not

after only 50msec.

Method

Participants were 12 new students or employees in the University of Strasbourg. They

had not participated in any other experiment with the same stimuli.

Participants were run with stimuli shown in triplets. Only a short training session was

given, with only 20 trials, during which the stimuli stayed on the screen until the participant

19

responded. After that, participants were trained with stimulus presentations of 50 or 200 msec,

for 48 trials. This training session was immediately followed by the test session with 80 trials

(20 trials per condition). The critical line was always occluded and the stimuli were masked at

the end of their presentation (the mask is illustrated Figure 6). To avoid too much complexity

in the task, the stimuli were not rotated and they were always shown as illustrated in Figure 1.

The side of the target and distractor, the orientation of the reference line, and the presentation

duration were randomized across trials.

< Display Figure 6 about here >

Results

The results are displayed Figure 7. The mean RT was 905 msec and mean error rate was

29.8%. Error rates were lower by 6.7% when the presentation duration was 200 msec than

when it was 50 msec (F[1,11]=5.3, p<.05). RTs were lower, by 47 msec, for the 200 relative

to the 50 msec duration, but this was not reliable (F[1, 11]=1.4, ns).

When the presentation duration was 50 msec, RTs and errors were lower, by 92 msec

and 2.9%, when the reference line was short compared with when it was long (F[1,11]=9.8,

p<.01 for RTs and F<1 for errors). When the presentation duration was 200 msec, RTs and

errors were only slightly and non significantly lower, by 27 msec and 0.4%, when the

reference line was short than when it was long (F[1,11]=1, ns for RTs and F<1 for errors).

These effects resulted in a significant interaction between the duration of the stimulus (50 vs.

200 msec) and the length of the reference line (short vs. long): F[1,11]=6, p<.05 for RTs and

F<1 for errors).

< Display Figure 7 about here >

20

Discussion

When the presentation duration was 50 msec, the results were qualitatively similar to

those observed in HJA and the participants who had taken lorazepam, with RTs being shorter

when the reference line was short than when it was long. No significant effect of the length of

the reference line was observed when the presentation duration was 200 msec. The high error

rates observed when the stimuli were displayed for 50 msec confirm that subjects only barely

see the stimuli in these conditions. The fact that responses are still faster when the reference

line matches in length the occluded contour suggests that the occluded line has been

completed but is not yet bound to the visible contours. The lack of an effect at 200 msec may

be explained by the fact that the occluded line has by then been bound to the visible contours,

and/or that the duration of presentation was enough to allow a strategy based on a more top-

down recognition process.

General Discussion

The results showed that HJA, lorazepam-treated participants, and normal participants

given short stimulus presentations, matched a reference line with an occluded target line more

efficiently when (i) the length of the reference line matched the length of the occluded contour

than (ii) when it matched the length of the occluded contour linked to the connecting collinear

visible contours. Such an effect of length was never observed when the target line was in the

foreground. In contrast, all participants tended to show an advantage for the long reference

line when the target was in the foreground. When the presentation duration was long enough,

the controls also tended to show a benefit for the long reference line in the occluded

condition. Apparently, for the controls, matches were easiest if the reference line was the

21

same length as the occluded contour plus the connected visible parts, provided the stimuli

were displayed for more than 200 msec,.

These effects are consistent with the hypothesis that occluded contours are completed

early and are subsequently bound with real contours at a second stage of visual processing.We

suggest that occipito-temporal brain lesions (in the case of HJA) and changes to GABAA

transmission (after taking lorazepam) can alter this second stage of contour integration, even

though a first stage of computing occluded contours remains relatively intact. Indeed, in a

study measuring thresholds to detect collinearity between contour elements, Giersch et al.

(2000) reported that HJA was intact. This is consistent with the disruption occurring primarily

when computed contours must be integrated, and assigned their appropriate role in foreground

and background representations. Our proposal is that this secondary, integration process

requires an interaction between early and later visual processes that pool information across

more global areas in a scene. This interaction can be selectively disrupted. The fact that it can

be disrupted in all participants reinforces the idea that this two-stage account holds even for

non-treated and healthy participants.

The two-stage account we propose is consistent with neurophysiological evidence for

early grouping based on collinearity between edge segments (Gilbert, Ito, Kapadia &

Westheimer, 2000) and for late-acting influences on these first processing stages as figure-

ground formation processes take place (Roelfsema, Lamme & Spekreijse, 2000). In a patient

such as HJA, damage to the secondary, integration stage can lead to occluded contours not

being assigned their correct representation in the background, but instead being assigned to

the foreground (Giersch et al., 2000). There are consequently detrimental consequences on

object recognition, particularly in complex scenes with many occluded objects (Humphreys &

Riddoch, 1987). Relating to this point, it is also of interest that HJA was strongly affected

here by the complexity of the stimuli. Both his RTs and his errors increased substantially

22

when he was presented with triplets rather than pairs of overlapping figures, and this effect

was longer than could be predicted from any general slowing in his RTs (there was a

proportional increase of 45 % on RTs to triplets relative to the mean RTs when stimuli were

presented in pairs; for controls there was a 11 % increase). We suggest that, due to his

problems with integrating contour information across a scene, he is abnormally sensitive to

increases in stimulus complexity.

A two-stage account of completion may help to reconcile findings that appear to

contradict each other. (1) Our results and others (Rensink & Enns, 1998) confirm the earlier

interpolation theory proposed by Kellman and Shipley (1991). (2) However, several findings

have shown a critical influence of stereoscopic depth information and figure-ground

organization on the perception of illusory contours (Anderson et al., 2002; Gillam &

Nakayama, 2002). For example, Gillam and Nakayama (2002) used (1) sets of lines that had

all the same stereoscopic depth and composed a plane, and (2) sets of lines that varied in

depth and did not belong to the same plane. In both cases, line-ends were aligned. When the

two sets were presented, one abutting the other, a subjective contour was perceived along the

abutment, at least in monocular vision. However, stereoscopically, the perception of the

subjective contour depended on the depth ordering of the two sets of line. It was visible only

when the set of lines composing a plane was in the foreground, and not when it was in the

background. These results show that the perception of an occluding contour does not depend

only on local signals like the alignment of line-ends, but also a 3D analysis of the scene

layout. Similarly, the way a horizontal bar is perceived in a cross stereogram depends on the

depth differences between the vertical and horizontal bars (horizontal bar in front vs. behind

the vertical bar, Anderson & Julesz, 1995, Anderson et al., 2002). Gillam and Nakayama

(2002) and Anderson et al. (2002) argue that, if completion occurs before the assignment of

contours to foreground or background figures, then it should not be modified by figure-

23

ground organization. An influence of the scene layout suggests thus that completion occurs

after the assignment of contours to foreground and background figures. However, one of the

main differences between the two series of experiments is that the studies showing an

influence of depth information have typically explored the conscious perception of occluding

contours. In contrast, studies suggesting early completion (Giersch et al. 2000, Kellman et al.,

2001; Rensink & Enns, 1998) have explored the influence of unconsciously perceived

contours on performance. Thus, as suggested by Kellman et al. (2001, pp227-229),

discrepancies may be resolved by distinguishing a first stage of completion that would not be

accessible consciously, and a second stage that would lead to the conscious perception of

illusory contours, maybe involving the deletion of early completed contours. Still, Gillam and

Nakayama (2002) argue that, according to the principle of minimal commitment, it makes no

sense to produce a line and then to inhibit it. Interestingly, our results suggest a possible

means that would eliminate this inhibition stage. If occluded contours remain separated from

visible contours, they would be used and lead to a conscious perception only if they are later

integrated with visible contours. If not they would be produced but remain inaccessible

consciously. There would be no need to inhibit them.

In summary, according to our results, a line may be computed at the location of an

occluded contour, probably based on the relatability of the visible contours and the presence

of T-junctions (Kellman & Shipley, 1991). However, this line is not automatically bound to

the visible contours present, being only preliminary, as suggested by Lesher (1995). This

computation helps the visual system to derive a fast and ‘dirty’ representation of objects

(Rensink & Enns, 1998). The line may not be accessible consciously, however, consistent

with the distinction made by Kellman, Guttman & Wickens (2001) that not all interpolated

contours reach consciousness. What we propose in addition is that the binding of this

occluded line with visible contours is a necessary step for the occluded contour to reach

24

consciousness. This binding step may then be sensitive to other factors like depth or figure-

ground organization, which together constrain the final representation. In a sense, the whole

completion process may indeed be late operating (cf. Anderson et al., 2002; Gillam &

Nakayama, 2002) since the binding stage occurs along with the attribution of contours to

figure and ground objects.

In addition, computational models such as that offered by Grossberg may help to refine

further our understanding of the processes involved in the secondary integration stage.

Grossberg (1994; see also Kelly & Grossberg, 2000) proposed that the filling-in of contours

was prevented by the presence of T-junctions. According to these authors, the attribution of

contours to foreground and background figures would allow the filling-in of the foreground

figure. This would allow in turn the inhibition of T-junctions signals, finally allowing the

filling-in of occluded contours. This model may be consistent with our proposal, if one admits

that a contour is produced at an earlier stage but is bound with visible contours only when T-

junctions are inhibited. This proposal is supported by the results observed with lorazepam,

even if such interpretations are to be taken with caution. It has been proposed that lorazepam

alters integration and segmentation processes by impairing the inhibition of segmentation cues

like line-ends signals, and maybe T-junctions (Giersch, 2001). If lorazepam prevents the

inhibition of T-junctions, it would impair the integration of early completed occluded

contours with visible contours. The results showing that lorazepam disrupts this stage are thus

consistent with the idea that occlusion cues may play a role in the binding of occluded with

visible contours.

A two-stage account may seem costly. However, neuropsychological data and other

studies suggest that completion starts at an early level. Such early completion involves the

risk of confounding real with visible contours. Keeping early completed contours isolated and

not consciously accessible may represent a way to avoid the risk of confounding real and

25

visible contours, and to avoid the necessity of deleting previously produced occluded

contours. Even if early completed contours are not accessible consciously, they may

accelerate access to the form of the occluded object. The production of an isolated occluded

line may then represent an intermediary stage that allows access both to a complete

representation of the occluded object and to the perception of an occluder.

26

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31

Figure captions

Figure 1: Illustration of the stimuli shown in pairs. The first reference line had to be

matched with the lateral picture that included a line was identical in orientation and at least as

long as the reference line. The critical line could be in the background object (left panels) or

an object in the foreground (right panels). When the critical line was in the background, the

length of the reference line corresponded to the length of the occluded part of the target

(upper panels) or to the length of the occluded line bound with the visible contours (lower

panels).

Figure 2: Illustration of the stimuli shown in triplets. As for the stimuli shown in pairs,

the critical line could be in the background object (left panels) or in a foreground object (right

panels). When the critical line was in the background, the length of the reference line

corresponded to the length of the occluded part of the target (upper panels) or to the length of

the occluded line bound with visible contours (lower panels).

Figure 3: Mean correct RTs (left panels) and percentage errors (right panels) with

standard errors (averaged across participants in the group of controls) in Experiment 1, for

stimuli shown in pairs. The data are presented as a function of the length of the reference line

and of the type of target (line in the background vs. in the foreground), for HJA (upper

panels), young and elderly controls (lower panels).

Figure 4: Mean correct RTs (left panels) and percentage errors (right panels) with

standard errors (averaged across participants in the group of controls) in Experiment 1, for

stimuli shown in triplets. The results are broken down as a function of the length of the

32

reference line and the type of target (line in the background vs. in the foreground), for HJA

(upper panels), young and elderly controls (lower panels).

Figure 5: Mean correct RTs (left panels) and percentage errors (right panels) with

standard errors (averaged across participants) in Experiment 2. The data are broken down as a

function of the length of the reference line and the type of target (line in the background vs.

the foreground), in lorazepam-treated participants (upper panels) and in placebo-treated

participants (lower panels).

Figure 6: Illustration of the mask used in Experiment 3.

Figure 7: Magnitude of the advantage for short relative to the long reference lines for

each participant at each duration in Experiment 3, for RTs (left panel) and percentage errors

(right panel) . Only two subjects did not show an RT advantage for the short reference line at

50 ms presentation duration.

33

Stimuli shown in pairs

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34

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35

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