Illusory contour orientation discrimination

Vision Res. Vol. 27, No. 3, pp. 453+57. 1987 Printed in Great Britain. All rights reserved

ILLUSORY

Copyright

0

0042-6989187 $3.00 + 0.00 1987 Pergamon Journals Ltd

CONTOUR ORIENTATION DISCRIMINATION

RUFIN VOGELS*and

GUY A. ORBAN

Laboratorium vooc Nemo- en Psychofysiologie, Katholieke Universiteit te Leuven, Campus Gasthuisberg, Herestraat, B-3000 Leuven, Belgium (Received 11 November 1985; in revisedform

29 Muy 1986)

AM&act-Just noticeable differences (JNDs) in orientation for real lines and illusory contours were compared. JNDs in orientation of an illusory contour and of a real line differ by leas than a factor two. JNDs in orientation of an illusory contour showed meridional variations similar to those obtained for a real line. By scaling measurements illusory contours are equally visible at all orientations, so meridional variations in illusory orientation discrimination reflect an anisotropy in orientation processing mechanisms. JNDs in orientation measured at an oblique reference orientation improve with practice for an illusory contour as well as for a real line. However while the effect of practice transfers from an illusory to a real contour, the reverse is not true. These results suggest that there are two paths for processing orientation: one activated only by real lines, the other concemcd with both real and illusory contours. Illusory contour Orientation discrimination Meridional variations Oblique effect

Human vision

.INTRODUClTON

Training

Linking hypothesis

repsonses, suggesting that indeed these V2 cells process illusory contours. In contrast to V2 Usually the perception of a contour results from cells, none of the Vl cells they recorded from a physical discontinuity in luminance, texture or responded to the illusory contours (von der wavelength. However, under certain stimulus Heydt et al., 1984). The latter finding is very conditions contours can be perceived in the important since it enables us to test psychoabsence of a corresponding physical discon- physically the contribution of Vl and V2 to tinuity. These contours are generally known as perceptual abilities. subjective or illusory contours (for recent reIn the last decennia, hypothesises linking views see Pritchard and Warm, 1983 and Parks, visual discriminations and single cell character1984). Several theories ranging from phys- istics have been formulated (Barlow, 1972; Oriological interpretations (e.g. Smith and Over, ban, 1984; Teller, 1984). A well known example 1975) to “cognitive” explanations (Gregory, is the hypothesis linking Vl cells and line orien1972) have been put forward to explain the tation discrimination (Andrews, 1967; Orban et generation of these illusory contours. Although al., 1984; Hawken and Parker, 1984). Orban et illusory contours have been studied extensively, al. (1984) even formulated a very specific linking their origin still is a matter of debate (Pritchard hypothesis concerning line orientation discrimiand Warm, 1983). However, recent results from nation: at the level of Vl, only the S cells are physiological studies in awake monkey using supposed to carry the neural representation of illusory figures (von der Heydt et al., 1984; line orientation used in real line orientation Peterhans et al., 1984) seem to promise a major discrimination tasks. The S cells of Vl show the breakthrough in the understanding of the gene- narrowest orientation tuning width of all Vl sis of illusory contours. von der Heydt et al. and V2 cells (Kennedy et al., 1985) and further(1984) reported that about one third of V2 cells more more S cells prefer horizontal and vertical of the macaque monkey responded to the orienoblique orientations orientations than tation of illusory contours. Modifications of the (Kennedy et al., 1985; De Valois et al., 1982). illusory figures that weakened the perception of The latter so called orientation anisotropy of the the illusory contour also reduced the neuronal S cells has a perceptual analogy (Teller, 1984): line orientation discrimination is better around *Research assistant NFWO. horizontal and vertical orientations than 453

RUFIN VOGELSand CL-Y A. ORBAN

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Fig. 1. Illusorycontours usedin our experiments. In all but

linking hypothesis. This kind of study can be performed in animals (Vandenbussche et al.. 1986b) but not in humans. However. the findings of von der Heydt et al. (1984) renders it possible to do “psychophysically” in humans something similar to an ablation: by using illusory contours as stimuli we can exclude the contribution of the Vl cells, including the VI S cells, to orientation discrimination. Hence, we measured just noticeable differences (JNDs) in orientation of illusory contours at different reference orientations to determine whether orientation discrimination of illusory contours shows an oblique effect. Depending on the results, the conclusions drawn from these experiments will fall in between the following two extremes. If orientation discrimination of illusory contours shows an oblique effect which has the same properties as the oblique effect in line orientation discrimination then the hypothesis linking the oblique effect in line orientation discrimination and the orientation anisotropy of the S cells is difficult to maintain. If on the other hand orientation discrimination of illusory contours does not show an oblique effect a simple interpretation would be that indeed the oblique effect in orientation depends on the involvement of Vl S cells in the discrimination task.

one experiment illusory contour A was used. METHODS

Subjects around obliiue orientations (the oblique effect in orientation discrimination, Orban er al., 1984; Vogels et al., 1984). In our previous research we tested this linking hypothesis of the S cells and line orientation discrimination by determining whether the characteristics of the oblique effect in line oricntation discrimination were predictable from the known properties of the Vl S cells. And indeed the changes of the oblique effect with line length (Orban er al., 1984) and eccentricity (Vandenbussche et al., 1986a) fitted the corresponding changes of the S cell characteristics. In another series of studies (Vogels and Orban, 1986) we showed that the oblique e%ct in line orientation discrimination is not due to an anisotropy of decision processes. Both kinds of studies support but do not prove the linking hypothesis of line orientation discrimination and Vl S cells. A removal of the cell class or the whole area and subsequent testing of the performance would provide a critical test of such a

All 15 subjects had normal or corrected vision. None had an astigmatism larger than 0.5D. They ranged in age from 19 to 25 years. All but one subject (R.V.) were naive with respect to the aim of the experiment. Stimuli The illusory contour patterns we used for measuring orientation discrimination contained no physical orientation cue. The pattern we generally used is shown in Fig. l(A). It subtended 7” in diameter and is built up of semicircles. The two dimensional Fourier spectrum of this pattern is shown in Fig 2(B). For comparison, the two-dimensional Fourier spectrum of a line is also shown [Fig. 2(A)]. The spectra are very different: the spectrum of the illusory contour is largely isotropic. There is no energy which corresponds to the full illusory line. In fact a blow-up of the power spectrum shows that there is a relative decrease in power in the spectrum along the orientation of the

2-O FOURIER TRANSFORM A

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Fig. 2. Two-dimensional Fourier spectrum of a real line (A) and of the illusory contour of Fig. IA (B). In (C) the power spectrum of (B) has been replotted (luminance corresponds to power) on a log scale for spatial frequency after suppression of the d.c. peak. The orientation of the real line (A) and of the illusory contour (B and C) was the same (vertical). The vertical orientation corresponds to one of the axes of the spatial frequency plane (the one running from lower left to upper right in the figure).

455

Illusory contour orientation discrimination

was never presented. The subject was instructed to press one button when he perceived the increment stimulus and the other button when he perceived the decrement stimulus. He had to respond on each trial. Feedback (correct or incorrect) was given at the end of each trial. In the 2AFC design the standard stimulus was Apparatus presented on each trial. A trial consisted of a temporal sequence of two sitmuli differing in All stimuli were photographically produced on Agfa 4 paper and were attached on the rear orientation by ds deg. Two sequences had to be panel of a black-painted box. The stimuli were discriminated: in one stimulus sequence the made visible by means of two U.V. lamps. The standard stimulus was followed by the inluminance of the white lines was 1.2 cd/m* and crement stimulus while the other sequence conthe background luminance was 0.05 cd/m*. sisted of the increment stimulus followed by the Hence, the stimuli had a contrast (log AZ/Z) of standard stimulus (SIIS design). The intertrial 1.36. Only the stimuli were visible in a frameless interval was 2500 msec and the interval between surround. The viewing distance was 57 cm. On the stimuli of a sequence was 1200 msec. The the front panel of the box, an opening closed by subject had to decide whether the second stimua shutter allowed the subject to view the stimu- lus of a sequence was tilted clockwise or antilus inside the box. Only the right eye was used. clockwise with respect to the first stimulus of a The exposure time was 600 msec. The head of trial. Feedback (correct or incorrect) was given the subject was fixed in a vertical position by at the end of each trial. chin and forehead rests. This apparatus is simiOur previous research (Vogels and Orban, lar to the one used by Vogels and Orban (1985). 1986) supports the commonly held assumption The orientation of the stimuli could be that in the ID design the subject compares the changed by means of a stepper motor controlled stimulus to an internal criterion while in the by a Rockwell AIM microcomputer. The reso- particular 2AFC design used here the subject lution of the rotation was 0.06”. Responses were compares the second stimlulus of a sequence to recorded with two pushbuttons. Each trial was a memory representation of the first stimulus preceded by a warning tone and auditory feed- (also see Vogels and Orban, 1985). A comback was provided. No fixation point was avail- parison of the performance in the two designs able to the subject. However, control experi- will allow us to determine whether or not the ments with the presence of a fixation point observed effects are due to changes in decision yielded similar results. rule, The two psychophysical methods used were Procedures the Wetherill and Levitt (1965) transformed The differential thresholds were measured up-down staircase method and a signal dewith two psychophysical methods and two psy- tection method (Vogels and Orban, 1985). In chophysical designs. the staircase method an 84% correct JND in The two psychophysical designs used were an orientation was tracked. The orientation incrementdecrement (ID) design and a two difference ds was decreased by 20% after four alternative forced choice design (2AFC). The ID successive correct responses and was increased design consists of an identification task, identi- by the same factor after each incorrect response. cal to the one used by Matin and Drivas (1979) The staircase ended after 10 reversals and the and Vogels and Orban (1985). The increment geometric mean of the orientation differences of stimulus was tilted clockwise by an amount (ds) the last 3 midrun estimates (Levitt, 1971) yielded with respect to the reference orientation, the the JND in orientation. This method was used decrement stimulus was tilted anticlockwise by for the 2AFC design only. Two thresholds were the same amount. For a given orientation determined for each condition. difference ds between the reference orientation In the signal detection method each testblock and stimulus orientation, the increment and consisted of 60 trials presented with a given ds. decrement stimuli were presented sequentially in The ds used in a testblock was determined random order. The intertrial interval was during preliminary training trials with an infor2500 msec. Only one stimulus was presented mal staircase method. This d.r corresponded to

illusory contours [Fig. 2(C)]. This dip was stronger for the pattern shown in Fig. 1(B) than for the one of Fig. l(A). The illusory contour pattern shown in Fig. l(B) was used in one experiment to ascertain whether the thresholds depended on the kind of illusory contour pattern used to measure them.

during a trial and the reference orientation

457

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done using analysis of variance (ANOVA; Kirk,

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1968).Only block factorial designs (Kirk. t968) were used since they reduce the error due to individual differences in JND. The magnitude of the oblique effect was expressed by the OEI index (Orban et al., 1984) defined as follows OEl = (JND,o + JNDu,) - (JND, + JNDv) (JND, + JND,.) where the subscripts indicate the reference orientation (right oblique, left oblique. horizontal, vertical) at which the JNDsin orientation were measured. RESULTS orientation

difference

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Fig. 3. The signal detection discriminability index d’ as a function of the orientation difference (ds) for a real line and for the illusory contour of Fig. l(A) at a principal and oblique reference orientation. The lines were fitted by linear regression.

a 75% correct threshold estimation. The data of 4 or 6 (see results) testblocks were pooled to calculate the proportions of hits and false alarms, and subsequently, the d’ value (Green and Swets, 1966). From this bias free measure d’ the JND in orientation was calculated with the following formula: JND = 2*ds/d’ (Johnson, 1980; Vogeis and Orban, 1985). This yields a bias free 84% correct JND in orientation. This calculation rests on the assumption that d’ is proportional to ds. This assumption is validated by several experimental results (Orban et al., 1984; Vogels and Orban, 1986; Matin and Drivas, 1979).As an example we have plotted in Fig. 3 the d’ values as a function of the orientation difference ds for both principal and oblique orientations of an illusory contour and of a real line. Each point is based on at least 300 trials. The figure shows that for real lines as well as for illusory contours d’ is proportional to ds. The fixed stimuli signal detection method yields a more valid threshold than the staircase method since the latter procedure is not bias free. But, on the average, both methods yielded similar thresholds. The staircase method however is faster and hence was used in experiments with many experimental conditions. All JNDs were log transformed for further statistical analysis. Indeed the standard error of the JND is nearly proportional to its absolute level, and only ratios between JNDs can be interpreted. Further statistical analysis was

Experiments 1 and 2: Meridional variations in the orientation discrimination of illusory contours

Figure 4(A) shows the JNDs in orientation as a function of the reference orientation for three different stimuli: the real line (indicated by a full line), the illusory contour of Fig. l(A) (stippled line) and the illusory contour of Fig. l(B) (dotted line). The thresholds are the average of two staircase determinations in subject R.V. The JNDs in orientation are remarkably similar for both types of illusory contours. This indicates that orientation discrimination does not depend on the type of pattern used to induce the illusory contour. In addition orientation discrimination of illusory contours shows meridional variations with smaller JNDs for the principal orientations than for the oblique orientations. Figure 4(A) further suggests that these meridional variations in orientation discrimination are smaller for the illusory contours than for the real line. In four other subjects we also determined JNDs in orientation for the real line and for the illusory contour of Fig. l(A) using the same staircase method. The pooled results are shown in Fig. 4(B). For the real line (indicated by the full line) as well as for the illusory contour (shown by the stippled line) JNDs in orientation are larger for the oblique orientations than for the horizontal and vertical reference orientations. This meridional variation was highly significant [F(8,99) = 4556; P < 0.0005]. The JNDs in orientation of the illusory contour are similar to the JNDs for the real line for the reference orientations ranging from 100 to 160”. For reference orientations close to the principal orientations, the JNDs for the illusory contour are larger than for the real line. This interaction between reference orientation and stimulus was, however not

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Fig. 4. The JND in orientation as a function of the reference orientation for illusory contours and for a real line. The thresholds were determined with the staircase method. (A) JNDs in orientation for a real line (full line), for the illusory contour of Fig. l(A) (stippled tine) and for the illusory contour of Fig. l(B) (dotted line). Each datapoint is the average of two threshold determinations in subject R.V. (B) JNDs in orientation for a real line (full line) and for the illusory contour of Fig. l(A) (stippled line). Each datapoint is the average of the JNDs of five subjects. The bars represent standard errors.

In the second experiment we determined orisignificant. The results of the first experiment clearly show that (1) JNDs in orientation of entation thresholds for both types of stimuli at illusory contours are not much larger than those the two principal and the two diagonal oblique of real lines and, (2) that orientation discrimi- reference orientations. This is the classical exnation of illusory contours also shows meri- neriment to determine whether a nerceotual judgement shows an oblique effect. In twdsubdional variations.

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Fig. 5. JNDs in orkntation at four reference orientations for the illusory contour of Fig. 1A (open circks) and for the real line (solid circler). The thresholds were determined with the 6xed stimuli signal detection method. (A) Mean JNDs of two subjects tested with the 2AFC design. The vertical bars indicate the actual JNDs of both subjects. (B) Mean JNDs and standard errors of 8 subjects tested with the ID design. H: horizontah V: vertical; L.O.: kft oblique; R.O.: Right oblique.

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Fig. 6. The relationship between the magnitude of the oblique effect in orientation discrimination of an illusory contour and of a real line. The magnitude of the oblique &+I is ex@ by the oblique e&t index (OEI: see text). There was no correlation betweenbothoblique&ti i&i- as estimatedby linear regression. This may be due to the small variances in both indices.

jects we used the 2AFC design, while in the other 8 subjects the ID design was employed. The JNDs were determined with the signal detection method. Each threshold was based on 4 blocks of 60 trials. The pooled JNDs in orientation are shown in Fig. 5, the JNDs for the real line and for the illusory contour being plotted in solid and open symbols respcctiveiy. These results support and extend the data of the previous experiment in showing that for the 2AFC [Fig. 5(A)] as well as for the ID design [Fig. S(B)]orientation discrimination of illusory contours is better at the principals than at the oblique reference orientations. As in the previous experiment, the difference between the JNDs for both type of stimuli is larger at the principal reference orientations than at the oblique reference orientation. This interaction of the reference orientation and the stimulus pattern was significant [F(3,49) = 3.25; P < 0.05; this ANOVA was run on the data of the ID design only]. Thus the oblique effect is smaller for the illusory contours than for the real lines. The median OEI for the real lmt and for the illusory contour were 191 and 119 respectively. This difference was statistically significant (Wilcoxon matched pairs signed rank test, T = 5, P < 0.005). The oblique e&t index for the illusory contour is plotted as a function of the OEI for real lines in Fig, 6. This shows that the OEI for illusory contours was sma#kr than the OEI for real lines not only on average, but also for all but one subjects.

This difference in the magnitude of the oblique el%ct is not due to the presence of the inducing semicircles in the pattern with the illusory contour since in a control experilncnt we were able to show that the surround has no significant ef&ct on the orientation discrimination of a real line. Also, we found that orientation discrimination of a luminance yields very similar thresholds to orientation discrimination of a real line. Experbnent 3: Meridional variations in the visibility of illusory contours It is possible that the oblique e&t in orientation discrimination of illusory contours is due to meridional vairations in visibility of the illusory contours. To examine this explanation we determined the visibility of illusory contours of different orientations with a forced choice paired comparison technique. A trial consisted of a sequential presentation of a pair of illusory contours. The orientation of each contour could either be horizontal, vertical, left oblique or right oblique. The two stimuli of a pair always diRered in orientation. Ail 12 possible pairs were presentad in random order and the subject had to decide whether the illusory contour of the wd stixnuius of a pair was more or less visibk than the first one. The scaling tachlriquc (Coombs, 1964). This technique yields a parametric representation of the

Illusory contour orientation discrimination

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Fig. 7. The meridional variations in the visibility of the illusory contour of Fig. l(A) compared to the meridional variations in orientation discrimination of the same stimulus in two subjects. Al larger I value indicates a lower visibility and a larger JND in orientation representsa lower orientation discrimination ability. On both scales the positions of the reference orientation are indicated by arrows. H: horizontal; V: vertical; L.O.: left oblique; R.O.: right oblique.

stimuli on an interval scale. The results of two subjects are shown in Fig. 7 (left panel). The visibility decreases with increasing z value (to the right of the graph). Small meridional variations in the visibility of the illusory contour were present in both subjects. However neither of the two subjects showed an oblique effect in the visibility of illusory contours, although both of them had a typical oblique effect in the orientation discrimination of illusory contours (right panel and Fig. 7). The absence of a significant correlation of the orientation dependence of the visibility and of the orientation discrimination of illusory contours indicates that the oblique effect in the orientation discrimination of illusory contours is not due to an anisotropy in the scaled visibility of illusory contours. These results therefore suggest that the oblique effect in orientation discrimination of illusory contours is due to an anisotropy in the orientation processing mechanisms themselves. Experiment 4: Transfer of selective practice between illusory contours and lines Orientation discrimination of lines as well as of illusory contours show an oblique effect. The oblique effect obtained for illusory contours cannot be ascribed to the orientation anisotropy of the VI S cells since the latter do not respond to illusory contours (von der Heydt et al., 1984). This oblique effect is thus caused by a higher order anisotropy. Vogels and Orban (1985) showed that selective practice in line orientation judgements improves JNDs in orientation but

461

for tile oblique orientations only. This anisotropic practice effect led Vogels and Orban to postulate an anisotropy in the higher order mechanisms which compute the orientation of the real line. It could well be that the same higher order anisotropy causes the oblique effect in the orientation discrimination of illusory contours. Since this higher order mechanism improves with selective practice of the oblique reference orientations, one would expect that practising orientation judgements for a real line improves JNDs in orientation of the illusory contour, and vice versa. If on the other hand both oblique effects have no anisotropic processing stage in common, no transfer of the effect of practice from lines to illusory contours should occur. Hence we determined whether selective practice in orientation discrimination of a real line affects the non-practised orientation thresholds of an illusory contour and vice versa. The selective practice paradigm we used was the same as used by Vogels and Orban (1985). The subjects were practised in judging the orientation of left oblique stimuli, since the latter orientation is highly susceptible to selective practice (Vogels and Orban, 1985). Five subjects practised for 5000 trials orientation discrimination of a left oblique real line. The signal detection method was used. Two subjects practised with a 2AFC design while the three other subjects practised with an ID design. At the beginning, middle and end of the practice, JNDs in orientation were determined with the same method for the vertical, horizontal and right oblique reference orientation of the real line as well as for the four reference orientations of the non-practised illusory contour. Three other subjects practised orientation discrimination of a left oblique illusory contour. Also at the beginning, middle and end of the practice, thresholds for the non-practised orientations of the illusory contour and all four reference orientations of the real line were determined. These three subjects were tested with the ID signal detection method. Since for the interpretation of the results we need to know how much of the improvement of the non-practised conditions is due to the retesting instead of transfer, two other subjects were given no practice but were tested three times in succession. The time period between the testing periods was one week, which corresponds to the time period of 2500 practice trials in the other subjects. The tests were identical to the ones used in the practising subjects:

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Fig. 8. The improvement ratio of the JND in orientation after practising orientation dkcriminatior~ of a leR oblique line (above), of a kft oblique ilIus~ry contour (middle) and without selective practice (Mow). The leR panels rapnrsnt the improvement ratios in the orientation dkcrimin&0n of a real line at the four reference orkntationr, While the improvement ratios at the four nference OrientatiOnsof the illusory contour are shown in the right panel. For all conditions the median and range of the improvement ratios are indicated. The arrows point to the median values of the practised stimulus conditions.

JNDs in orientation were determined with the ID signal detection technique for the four reference orientations and the two types of stimuli (real line and illusory contour). In all subjects the thresholds were calculated on 4 blocks of 40 trials. The question we are interested in is whether the JNDs in orientation of the non-practised stimulus (e.g. the illusory contour) improve with practice of the other stimulus (e.g. the real line). To answer this question it is su#lkient to com-

pare the JNDs in orientation of the First (pmpractice) tests to the third (post-pra&ke) tests. Hence for each subject and condition an improvement ratio was calculated by divi&g the pre-practice JND by the post-pmctice JND. The median and range of these improvement ratios are

plotted in Fig. 8. The data of the subjects

ORBAN

who were tested with the 2AFC designs were pooled with the data of the three subjects which were tested with the ID design since the results of both groups of subjects were very similar. These results of the five subjects which practised orientation discrimination of a real line are shown in the upper part of Fig. 8. The results of the subjects who practised orientation discrimination of the illusory contour and of those who received no practice (controls) are shown in the middle and lower part of the figure respectively. In each part, the improvement ratios for the tests with the real and illusory contour are shown in the left and right panels respectively. The data corresponding to the practised orientation and stimulus are indicated by an arrow. The median improvement ratios of the subjects who did not receive practice (control in Fig. 8) range between 1 and 1.5. This improvement however was independent of the reference orientation (tested in an ANOVA). Hence, we will use an improvement ratio of 1.5 (indicated by the stippled lines in Fig. 8) as a criterion to decide whether a median improvement ratio reflects a genuine effect of selective practice. The selective practice of a left oblique illusory contour resulted in a significant improvement of the JND for the latter reference orientation of the practised illusory contour as well as of the non-practised real line. The improvement is larger [F(3, 14) = 7.81; P < O.OOS] for the practised reference orientations than for the non-practised reference orientations, especially the principal reference orientations. After practice the JNDs of the practised oblique reference orientation were still larger than the JNDs in orientation of the principal orientations. The average OEI for illusory contours after practice computed for the principal orientations and the practised oblique orientation was 80. These results show that selective practice of an oblique illusory contour improves orientation discrimination of both real and illusory contours of that orientation, and also affects but to a lesser extent orientation discrimination of both types of stimuli at the orthogonal oblique reference orientation. Selective practice of a real line yielded another pattern of results. Only the thresholds at the practised reference orientation of the real line showed considerable improvement. This interaction between type of stimulus and reference orientation was significant [F(3,28) = 3.88; P c 0.051. The JNDs for the orthogonal oblique reference orientations of both types of stimuli

Illusory contour orientation discrimination

also improved with practice but this effect was much smaller than the improvement for the practised reference orientation. Hence, selective practice of an oblique real line does not transfer to the illusory contour of the same orientation, though there is a small transfer to the orthogonal oblique orientation. This lack of significant transfer to the left oblique illusory contour cannot be due to a smaller effect of the selective practice for real lines compared to illusory contours since the median improvement ratio for the oblique real line was similar to the one found in the subjects who practised an illusory contour. Furthermore, the magnitude of the improvement due to selective practice of an oblique real line was similar to the one found by Vogels and Orban (1985). Also, confirming Vogels and Orban (1985), each subject showed a significant oblique effect after practice (average OEI for real lines after practice: 83). These results indicate that selective practice of orientation discrimination of an oblique illusory line transfers to orientation discrimination of a real line of the same orientation, while the reverse is not true. This in turn implies that orientation discrimination of real lines and of illusory contours is supported by partly common and partly separated mechanisms. This will be further discussed in the general discussion. GENERAL DISCUSSION

Just noticeable differences in orientation of illusory contours are remarkably small; JNDs in o~en~tion of a real line and of an illusory contour differ by a factor less than two. This indicates that the visual system has a remarkably good ability to compute the orientation of an illusory contour. These differences between the JNDs in orientation of both types of contours could at least partly be due to differences in the visibility between the real line and the illusory contour. Also, differences in more obscure factors iike attention could be involved in generating these small but nontheless significant differences in JND between illusory and real contours. Since we cannot exclude a contribution of these variables to the effect of the type of stimulus on the JND in orientation, it is presentiy not known to what degree these differences in JND in orientation reflect differences in the neural processing of the orientation of both types of stimuli. It is however noteworthy that the effect of changing from a real line to an illusory contour is similar to the

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efM of r~o~ng adequate processing of high spatial frequencies. Indeed defocusing, testing amblyopes or testing normals in their peripheral visual field all lead to a selective increase of JNDs in orientation for the principal meridians. And the latter manipulations supposedly alter the neuronal mechanisms of orientation processing. Just noticeable differences in orientation of illusory contours are larger at oblique reference orientations than at principal reference orientations. This oblique effect is unlikely to be due to decision factors since we obtained it with two psychophysical methods and two experimental designs. One could argue that the subjects used a position cue (e.g. the endpoint of one of the semicircles) rather than an orientation cue. However, position acuity in our setup is much worse than the orientation discrimination thresholds that we obtained for illusory contours and, also, position acuity does not show an oblique effect (Orban et al., 1984). The oblique effect for illusory contours is not caused by meridional variations in visibility since we found no correlation between the small meridional variations in scaled visibility and the oblique effect in orientation di~~mination of the same illusory contours. Hence, we conclude that the oblique effect in orientation discrimination of illusory contours results from an anisotropy in the orientation processing of illusory contours. The oblique effect in real line orientation discrimination is believed to depend at least partially upon the S cell o~entation anisotropy in VI, an hypothesis supported by ablation experiments (Vandenbussche et al., 1986b). Since both real lines and illusory contours show an oblique effect, the question arises whether both oblique effects have a common origin. This common origin cannot be the VI S cell orientation anisotropy, since S cells do not respond to illusory contours, but could be a computation stage common to real and illusory contours. Our finding that practice of an obliquely oriented real line does not transfer to the illusory contour of the same orientation suggests that both oblique effects result at least partially from different sources. The argument is as follows. Vogels and Orban (1985) reported that selective practice of line orientation discrimination improves the JNDs in orientation only when the practised orientation is oblique. This anisotropic practice effect led us to formulate a two stage model of orientation discrimination: an

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RUFIN VOCELSand GUY A. ORBAK

Fig. 9. The model proposed to explain the results of the experiments reported in this paper. The S cells of VI give input to higher order mechanism I. All cell types of Vl provide input to V2. The cells of V2 which respond to illusory contours and to real lines only drive mechanism II. Higher order mechanism III affects both mechanism I and II and selects the pathway (“a” or “b”) used by the decision process. Anisotropic mechanisms are indicated by an asterisk. S: S cells; C: C cells; end-stopped: end-stopped alls in VI.

anisotropic orientation filtering (subserved by oblique effect found after selective practice is the S cells) followed by an anisotropic com- due to distinct factors for real lines and for putation of the orientation using the output of illusory lines. Furthermore, it may well be that the S cells. The latter computing mechanism for the real lines the factor is the anisotropy of improves with selective practice of oblique ori- the S cells of Vl . The latter hypothesis is entations which leads to the observed reduction supported by our other psychophysical studies of the oblique effect. The results of our transfer (Orban et al., 1984; Vandenbussche et al., experiment (Experiment 4) indicates that the 1986a) and the cat ablation experiments (Vanlatter anisotropic computation stage is not denbussche et al., 1986b). However, our present shared by illusory and real lines. In fact the lack data are in agreement with both kinds of interof a transfer from real to illusory contours pretation (separate vs common sources of the suggest that at least part of the oblique effect residual oblique effect for illusory contours and obtained with both kinds of stimuli is due to for real lines) and, hence, are neutral with different mechanisms. respect to the hypothesis linking S c&s and However, our data do not rule out the possi- orientation discrimination of real lines. As discussed above, the results of the transfer bility that that part of the oblique effect that experiment indicate that the processing of real resists selective practice is due to a processing stage common to illusory contours and real lines and of illusory contours occurs at least in lines. In our previous paper (Vogels and Orban, part in parallel. However, two other results of 1985) we argued that the oblique effect which the present study render it necessary to adapt remains after selective practice of an oblique the hypothesis of a parallel processing of real reference orientation is due to the orientation and illusory contours. First, we found a strong anisotropy of the S cells. In the present study we transfer of the effect of practice of an illusory found that also the oblique effect in orientation contour to a real line indicating that there are discrimination of illusory contours resists a 5000 interactions at least in one direction between the trial long selective practice. The latter residual processing of the orientation of real lines and of oblique effect for illusory contours cannot be illusory contours. Second, when practising a due to the S cell anisotropy. Hence, it is possible real line and testing an illusory contour we that also the residual oblique e&c%- obtained found a small but significant transfer of practice after selective practice with real lines is not due for the not practised oblique reference oriento the S cell anisotropy but to an anisotropy of tation. These two points suggest that some cross a mechanism common to both kind of stimuli. talk is possible between the illusory contour On the other hand it is still possible that, just as processing mechanisms and the real line prothe anisotr6py in the computation stage which cessing mechanisms. changes with selective practice, the residual These results can be explained by means of

Illusory contour orientation discrimination

the model shown in Fig. 9. We assume here that part of the oblique effect in orientation discrimination of real lines is due to the Vl S cell anisotropy. The orientation of a real line or edge is computed with higher order mechanism I (see Fig. 9) using S cell output (pathway “a”). The orientation of an illusory contour is computed in a parallel pathway “b” with mechanism II using the output of orientation selective filters which are probably located in V2. The latter higher order mechanism II is also supposed to process real lines. The simplest assumption is that orientation filter mechanisms responsive to illusory contours are also responsive to real lines, but that the reverse is not true, that mechanisms responsive to real lines are not always responsive to illusory contours. The results of von der Heydt et al. (1984) confirm this point: the V2 cells that were responsive to illusory contours also responded to real lines while the reverse obviously was not the case. This asymmetry explains the asymmetry we found in our transfer experiment: a transfer from illusory contours to real lines but not from real lines to illusory contours. Our model thus implies that while orientation of illusory contours can only be processed by pathway “b’, orientation of real lines can be computed either along pathway “a” or “b”. The results of the practice experiment can then be explained by assuming that normally (i.e. without extensive practice of orientation discrimination of illusory contours) subjects use pathway “a” in orientation discrimination of real lines. Therefore practice with a real line will only improve mechanism I and only improves performance for real lines and not for illusory contours. When judging the orientation of an illusory contour subjects can only use pathway “b”. Practising orientation discrimination of an illusory contour will thus improve mechanism II. Our results then suggest that after training with an illusory contour, subjects use pathway “b” even for judging the orientation of the real line. This switch may be due to the fact that this pathway has become more efficient or simply because subjects continue to use the pathway they were forced to use during practice. Since in these circumstances mechanism II which has improved is used for both real lines and illusory contours, JNDs in orientation for both patterns will improve. We postulate that both mechanism I and II are anisotropic. The anisotropy of mechanism I is derived from the anisotropic practice effect reported by Vogels and Orban

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(19rP3) while the anisotropy of mechanism II explains the oblique effect in orientation discrimination of illusory contours. It is possible that in reality the processing scheme is even more complex since upon the basis of the present data we cannot exclude the possibility that before or after computation boxes I and II the information of both kinds of pattern is processed by another anisotropic mechanism (common to both illusory and real contours; see discussion above). Since, however, this additional computational box is not necessary to explain our data we did not include it in the model shown in Fig. 9. Thus far our model satisfactorily accounts for our major finding that the effect of practice transfers from an illusory contour to a real line, but not vice versa. However, our model still does not explain why we found a transfer of practice to the other non-practised (orthogonal) oblique orientations. Hence we introduce a third, higher order mechanism (III in Fig. 9) to account for these small orthogonal transfer effects. Mechanism III is suposed to facilitate mechanisms I and II during the practice in an orientation aspecific way and primarily for the non-practised obliques. Why this facilitation occurs primarily for the orthogonal obliques is presently unknown. We admit that the model shown in Fig. 9 is rather complex. But we believe it is the simplest model which can satisfactorily explain our (complex) data. Other models can be constructed to explain our data, but all possible models should contain at least the components of our model. Essock (1980) distinguished two classes of oblique effects, one related to higher cognitive and memory functions (class 2) and the other one (class 1) to a Vl anisotropy. The oblique effect in illusory contour orientation discrimination belongs to class 2 since it is not due to a Vl anisotropy. However, it is likely that Essock’s classes are not internally homogeneous. For instance, Fisher and Bornstein (1982) reported an oblique effect in the identification of symmetry of dot patterns, which followed retinal coordinates in a body tilt experiment. This identification of symmetry task requires a relatively complex processing of visual information and hence belongs to class 2 of Essock’s classification. On the other hand, the oblique effect in tilt identification reaction time can follow either gravitational or retinal coordinates (Attneave and Olson, 1967; Attneave and Reid, 1968). This suggests that

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different class 2 oblique effects exist. The same point has been made for oblique effects classified as class 1 by Essock (1980) (see Vogels and Orban, 1985). This diversity of oblique effects and of the underlying anisotropies at different levels of the visual information processing renders the heuristic value of Essock’s classification scheme small. The present evidence suggests that a preference for principal over oblique stimuli is present at different levels of visual information processing and that these anisotropies manifest themselves in different amounts in different psychophysical tasks, depending on the contribution of these processes to the different tasks. Our present results show that even in simple tasks such as orientation discrimination, higher processes are involved. This is in agreement with the findings of Wong and Weisstein (1982) who reported that line orientation discrimination is better when the line appears on what the subject perceives as a figure than when it is projected on what is perceived as background. It is very likely that higher order processes also play a role in other “simple” discrimination tasks. Experiments with stimuli that address these higher processes may resolve this issue. With respect to this point it is worth noting that illusory contours show a tilt aftereffect (Smith and Over, 1975), a tilt illusion (Smith and Over, 1977) and a motion aftereffect (Smith and Over, 1979) with similar properties as for real lines. Given the fascinating physiological results of von der Heydt et al. (1984) the use of illusory contours in psychophysical experiments opens the possibility to determine the contribution of higher mechanisms outside Vl to visual perception. In conclusion, our experiments show that fine orientation discrimination is possible with illusory contours. This indicates that orientation discrimination per se can be done without stimulating the Vl S cells. Hence, these cells are not necessary for orientation discrimination per se. In our study JNDs in orientation were lower for real than for illusory contours and this difference between both types of contour depended on the reference orientation. Although this difference could in principle be due to differences in visibility between both types of stimuli, this finding rather seems to indicate that proper stimulation of the Vl S cells is required to achieve the finest orientation discrimination. Acknowledgemenu-The authors are indebted to J. M. Sprague for critical reading of an earlier draft of this paper. We are grateful to M. Van Lammeren (Department of

Ophthalmology, K.U.L. Medical School) for performing the optometrtc examinations. The technical asststance of P. Kayenbergh, G. Vanparrijs and G. Meulemans as well as the expert typework of Y. Celis is kindly acknowledged

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