Quarterly Journal of Experimental Psychology
ISSN: 0033-555X (Print) (Online) Journal homepage: http://www.tandfonline.com/loi/pqje19
Global stereopsis in rhesus monkeys A. Cowey , A. M. Parkinson & L. Warnick To cite this article: A. Cowey , A. M. Parkinson & L. Warnick (1975) Global stereopsis in rhesus monkeys, Quarterly Journal of Experimental Psychology, 27:1, 93-109, DOI: 10.1080/14640747508400466 To link to this article: http://dx.doi.org/10.1080/14640747508400466
Published online: 29 May 2007.
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Date: 06 November 2015, At: 01:49
Quarterly Journal of Experimental Psychology (1975)27, 93-109
GLOBAL STEREOPSIS IN RHESUS MONKEYS A. COWEY, A. M. PARKINSON AND L. WARNICK Downloaded by [Central Michigan University] at 01:49 06 November 2015
Department of Experimental Psychology, University of Oxford, England I n three separate experiments an attempt was made to demonstrate global stereopsis in two rhesus monkeys by using random dot stereograms projected and viewed through polarizing filters. Although both animals learned a number of discriminations, control tests showed that both were perceiving non-depth cues such as monocular identification of minute pattern differences or brightness differences caused by reflections of polarized light. I n a final experiment red/green anaglyph forms of the stereograms wcre viewed through red/green filters. Both monkeys, together with a third experimentally naive animal, showed incontrovertible evidence of prompt discrimination based on stereopsis. This paper makes a number of recommendations about the use of random dot stereograms to demonstrate global stereopsis in animals.
Introduction The horizontal separation of the eyes in animals with overlapping visual fields produces slight differences in the two retinal images of an object within the binocular visual field. These differences are responsible for stereoscopic vision. Although we have a little evidence of the neural mechanisms underlying stereopsis in man (Blakemore, 1970; Mitchell and Blakemore, 1970; Blakemore and Hague, 197z), the great majority of evidence on the existence of single cells in the visual cortex which register the retinal disparity of images in the two eyes comes from electrophysiological experiments on cats (see Bishop and Henry, 1971, for review). It was therefore of great importance that the demonstration of retinal disparity detectors in the prestriate cortex of monkeys (Hubel and Wiesel, 1970) was accompanied by a report of behavioural evidence for stereoscopic vision in two macaque monkeys (Bough, 1970). Bough trained his animals to discriminate between two sets of random dot stereograms of the type developed by Julesz (1964). T h e two sets differed with respect to the apparent depth of an inner square when they were viewed through Polaroid goggles. T h e monkeys learned the problem easily, transferred promptly to a new set of stereograms with a different array of random dots but the same apparent depth, and fell to chance when tested monocularly, thereby providing seemingly indisputable evidence that they were using stereoscopic depth cues to master the discrimination. This clear-cut result has been referred to by various authors (Hubel and Wiesel, 1970; Julesz, 1971; Bishop and Henry, 1971; Blakemore et al., 1972) both for its confirmation of the function of cells registering retinal disparity and because the technique of using random dot stereograms to generate apparent depth uncontaminated by non-disparity cues should be valuable
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in experiments on the mechanisms underlying stereopsis. For example, by varying the horizontal disparity in the stereograms it should be possible to determine the upper and lower disparity limits for apparent depth perception and to relate this to electrophysiological findings. By severing either the optic chiasma, or the corpus callosum, or both, it should also be possible to investigate the suggested role of the former in fine stereopsis and the latter in coarse stereopsis (Bishop and Henry, 1971). The role of the prestriate visual areas in which the disparity detectors reside could also be studied by measuring the effects of its removal. It was with such aims in mind that we performed a series of experiments using random dot stereograms to assess stereopsis in monkeys. This paper describes important and totally unexpected problems in studying stereopsis in this way.
Experiment I This was an attempt to reproduce Bough’s (1970) result by teaching two monkeys a simultaneous visual discrimination between two squares at different apparent depths from their identical surrounds.
Method The principle by which arrays of random “dots” may be used to generate an impression of depth is as follows. Two matrices of random dots are prepared, differing only in that a central region of dots in one matrix is shifted horizontally by some integral number of dots. The two matrices look identical, although they are not. If they are shown to separate eyes with the surrounds in register, the disparity caused by shifting one of the central regions is detected and the entire horizontally shifted region appears in depth, either in front of or behind the surround depending on the direction of the shift. The simplest way of ensuring that each eye sees only one matrix is to place vertical polarizing filters over one eye and one projector and horizontal polarizing filters over the other eye and projector. T o produce two separate depth stimuli simultaneously but with different apparent depths still requires only two projectors, but now two random dot arrays must be placed in each projector. However, in order to interchange the two depth stimuli on the two screens the polarizing filters in front of the projectors or the eyes must be reversed, or a second pair of projectors with the random dot matrices exchanged for left and right may be used. In Experiment I we used the latter method. T h e apparatus is shown in Fig. I . The stereograms were projected on to the two stimulus panels, 8 cm square and 8 cm apart, by means of four 35 mm projectors. The latter were mounted vertically above each other with their optical axes parallel to each other and perpendicular to the plane of the stimulus panels. I n this way optical distortion was minimized. Directly in front of and covering all four projector lenses was a Perspex holder containing polarizing filters which ensured that the beams from projectors I and z were polarized at right angles to those from projectors 3 and 4. The stimulus panels were made of frosted Perspex covered with thin glass on the side towards the monkey. They were hinged at the top outside the viewing area, and equipped with microswitches. A slight press anywhere on the panel was recorded by the automatic programming apparatus which controlled the entire experiment from a separate room. The animal started each trial by pressing the small button between the stimulus panels. Below that was a foodwell into which a peanut was delivered automatically after a correct response. Two MUCUCQ nzulatta [I male (A), I female (B)] about z years old at the start of the experiment, were used. They were fed a few hours after testing so that they were hungry at the start of each testing session.
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For each testing session the animal was placed in a primate chair. The platform around the animal’s neck prevented it from reaching its face with its hands, but a small hole in the platform immediately below the animal’s chin allowed it to pass a peanut to its mouth using finger and thumb. Goggles were attached to the animal’s head by surgical sticking tape. Each eyepiece was covered with a piece of “polaroid” material oriented at right angles to that on the other eyepiece, the absolute orientations corresponding to those used in the projector assembly.
FIG.I . TIsometric diagram of apparatus used in Experiments 1-11. text.
The parts are described in the
The stimuli were random-dot stereograms of the type developed by Julesz ( I 964). Each was made of IOO x IOO elements, half of which were filled on a random basis by dots. They were generated with a LINC-8 computer and photographed o f f a short-persistence screen using a 35 mm camera. The resulting negatives were fixed in pairs to cellophane sheets, using a light box to obtain accurate register, and reduced on to Kodalith film. Four of these film pairs were mounted on a single strip of Perspex which could be slipped into the multiple projector assembly shown in Fig. I . Once set up it was a relatively easy matter to obtain accurate register of the stimuli on the panels. The stimulus surround came to the edge of the panel, and subtended about 25’ overall, with the central square subtending 13”. The actual disparity used was 4 elements for problem DI and 6 elements for the others. The corresponding disparity angles were 80 and 120’. Luminance was about I I cd/m2. When we and several colleagues viewed these stimuli in the testing apparatus under the same viewing conditions as the monkeys the apparent depth of the central squares relative to the surround was very conspicuous. T o start a trial the monkey pressed the illuminated button. This turned on either projectors I and 3 or z and 4 and also a “white” noise. The monkey had to withhold responses for 5 s until the noise stopped. This was to encourage the animal to inspect the stimuli before responding. Presses during the noise reset the apparatus to its initial state and such trials were discarded. After the end of the noise a single press to either panel constituted a response and the stimuli were turned off and the monkey rewarded or punished depending on whether it had pressed the positive or negative stimulus. Reward was a peanut delivered into the briefly illuminated foodwell, followed by a time-out of 1 0 s , after which the monkey could start a new trial. Punishment consisted of a 10-speriod during which the houselight
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was turned off. A correction schedule was used such that the same arrangement of stimuli was used following an incorrect trial. I n the results that follow these “correction trials” are discounted. The animals were usually given IOO trials (excluding correction trials) in each daily session. The animals were trained initially on a brightness discrimination followed by a variety of pattern discriminations, during which time they were accustomed to the wearing of goggles. vs. W ; and E vs. &. The patterns were The patterns were X vs. 0 ; vs. V; T vs. ,L; I ” (2.54 cm) high and projected on to the centre of the stimulus panels. The last problem learned prior to the stereoscopic depth discrimination was a discrimination between a random dot array and a similar array with a darker central square. They were then trained on a discrimination between a stereoscopic pair of patterns giving the impression of a square in front of the background vs. a pair of identical patterns which provided no apparent depth. Finally, they were trained on several sets of patterns each involving a discrimination between a square in front of the surround vs. a square behind the surround. The stimulus showing the square in front of the surround was always positive. For every problem the criterion for learning was 90 correct trials in a daily session of I O O trials.
Results Monkeys A and B required a mean of 416 trials and 450 trials respectively to reach criterion on the five pattern discriminations and the central dark square discrimination. Both animals improved throughout the series as they learned to tolerate the goggles, and the last problem was learned by both animals in a single session. Performance on the depth/no depth discrimination is shown in Table I. Although both animals learned the discrimination, this is not adequate evidence of stereopsis for the problem could be solved on the basis of the slight pattern differences which exist along the lateral borders of the “hidden” square. T h e front/behind discrimination is not open to this objection, since ideally the positive and negative stimuli differ only in the relative planes of polarization. Both animals learned this first front/behind discrimination, but without showing transfer from the preceding depth/no depth discrimination (Table I, F-BI). Furthermore, when tested on further depth problems (F-B2,3,4) using different sets of random dot stereograms with identical disparities it is clear from Table I that transfer between successive problems was poor for monkey B and almost non-existent for monkey A, indicating that each problem was being learned by using non-depth cues. Monkey B was therefore retested on problem F-BI and after performing at 95% correct for 200 trials was tested for IOO trials on each of three different control TABLE I Results of Experiment I D-ND D FD Monkey A Monkey B
F-l3 I D FD
14
78
12
3
59
3
61 69
F-B 2 D FD
7 II
49 72
F-B 3 D FD
7 10
60 72
F-B 4 D FD -
-
10
67
Days to reach criterion (D) at IOO trials/day, and percentage correct on first day (FD) for a depth/no depth discrimination (D-ND) and four front/behind depth discriminations (F-B)
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conditions: (I) without polarizing filters in the goggles; (2)with right eye covered; (3) with left eye covered. T h e animal performed at 88, 84 and 91% correct respectively, demonstrating that the discrimination was soluble without stereopsis. Since the different pairs of random dot stereograms may have differed in brightness, either because of unequal densities introduced during their preparation, or because of differing brightness in the projectors, we carried out a final control and tested monkey B with neutral density filters covering first one random dot matrix in all four projectors, then the other. T h e effect was to noticeably darken first the positive stimulus and then the negative. T h e animal scored 96 and 89% correct in the two sessions.
Experiment I1 T h e first experiment showed that both monkeys could solve the front/behind depth discrimination without using stereopsis and that they were probably never using it, despite the prominence of the apparent depth to human observers under identical viewing conditions. What other non-depth cues were present which could enable the monkeys to solve the discrimination? There are three obvious candidates. First, specks of dust or scratches on the film may have provided minute pattern cues. Second, it was impossible to align the random dot stereograms sufficiently accurately to prevent the dots at the extreme edges of the background from providing minute differences in pattern cues along the edge of the response panels. These differences would alter from session to session when the film holder was removed from the projector but would be constant within a session. Third, and perhaps most important of all, the animal may be able to detect monocularly the lateral shift in the central square which inevitably produces a minute difference in texture at the boundaries of the square and which would always be present in identical form for a given depth problem. Experiment I1 was designed to encourage the use of stereoscopic depth cues by making it much more difficult for the animals to use any other cue. Three changes were made. First, the monkeys were trained on five different problems identical with respect to depth but with different random dot patterns. The patterns were changed every 50 trials. In addition five further sets of random dot stereograms were used to study transfer. Second, the stimuli were made smaller so that the surround no longer reached the edge of the stimulus panels, thereby eliminating pattern cues at the very edges of the panels. Third, the polarizing filters in front of the projectors were randomly moved as described below so that the same pair of stereograms could provide the positive or negative depth stimulus.
Method Ten new sets of random dot sterograms of IOO x IOO elements were prepared, but they were made smaller than in the first experiment so that the stimuli subtended an overall angle of about 10' with the central square subtending 4". Disparity angle was 25' and average luminance about 80 cd/m2. The panel containing the polarizing filters in front of the projectors was also modified by adding further filters and arranging that the entire panel could be moved up or down automatically, thus rotating the plane of polarized light from each projector through 90". Thus
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the stimulus panel on which the positive depth stimulus appeared could be altered either by using a different pair of projectors, as in Experiment I, or by moving the filter panel. It will be noted that when the latter method is used the projected patterns are unchanged and this should prevent strategies based on gross differences in pattern and brightness. However, it is still theoretically possible to detect the change in plane of polarization by virtue of the Polaroid filters in the viewing goggles, and this provides a means of solving the problem without using depth cues, as will be seen later. The same apparatus was used as in Experiment I but the monkeys were moved back slightly to a distance of 25-30 cm from the stimulus panels. The 5-s period of white noise was eliminated, and instead the animal was encouraged to inspect the stimuli carefully by arranging that the first stimulus panel to be pressed eight times activated the apparatus. The filter in front of the projectors was moved randomly between trials. The stimuli were changed every 50 trials, halfway through each daily session.
Results The performance of the two monkeys on each of the five training problems is shown in Table 11. Monkey A was tested for 7500 trials and B for 9500 trials. TABLE I1 Mean percentage correct, with standard deviations, for jive frontlbehind depth discriminations used as the training problems in Experiment 11 Problem
Monkey A s.d.
I
75'7
10.3
3 5 7 9
85.2
12.7
91.2 70.6 84.1
12.5
Monkey A was tested for
1500 trials
9'1
11.8
Monkey B s.d. 72.8 73'1 69.5 70'9 67.1
12.4 11'3
13.2
'5'3 15'7
on each problem, Monkey B for 1900.
Although monkey A reached criterion on every problem on several occasions, its average performance was better than 90% correct on only one of them. Monkey B was so poor despite scoring consistently above chance that the transfer tests to the remaining five problems were performed only on A, as follows. T h e novel set of stimuli was introduced halfway through each testing session after the monkey had exceeded 90% correct during the first half. The results are shown in Table 111. It can be seen that in almost every case transfer resulted in a significant impairment and with two problems initial performance was not better than chance. There remains the possibility that the monkey was using depth cues for those stimuli on which he showed transfer, but this possibility is remote since depth cues were the same for all stimuli. Only non-depth cues varied from one set to another, and this would seem to implicate such cues since only some of the new stimuli would be expected to share non-depth cues with the original set of five. We therefore performed various tests to determine exactly how the monkeys were solving the problem. Both monkeys were trained to a criterion of 90% correct on 2 consecutive days on
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TABLE I11 Percentage correct performance of monkey A on five sets of frontlbehind depth stimuli
Problem 2
4 6 8
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I0
Trials 1-50 per cent correct
Trials 51-100 per cent correct
Overall per cent correct
54 82
78 I00
66 91 70 75 55
60 72 56
80 78 54
Each set was presented twice, for 50 trials, after an immediately preceding session of 50 trials with a set of training stimuli identical in depth but differing in random dot matrix. The monkey average 94% correct in the immediately preceding sessions with these familiar stimuli.
their best problem (which was not the same). This gave a relatively stable baseline. All control tests were then performed for a session of 50 trials immediately following a criterion run and again for a whole session of IOO trials, giving 150 trials in all. The results are presented in Table IV. Basically two sorts of tests were performed, and these are described below. I n order to ensure that matching of patterns and movement of filter had indeed ruled out those simple strategies the monkeys had been using in Experiment I, they were tested with neutral density filters instead of Polaroid material in the goggles. Since the monkeys now have no means of resolving polarized light, the two stimuli must appear identical and the problem should be impossible. I t can be seen that both monkeys performed at chance levels under this condition. TABLE IV Percentage correct under different viewing conditions Viewing condition I
2
3
4 5
Neutral density filters Right eye seeing: no stimulus to left eye Left eye seeing: no stimulus to right eye Left eye occluded Right eye occluded
Monkey A
Monkey B
54 69" 67* 65" 75"
64 93* 43 89* 53
The animals received 150 trials on each condition, with the exception of monkey B, which received only 50 trials on the first condition. * P < 0.01, two-tailed, normal approximation to binomial test.
T h e second group of tests consisted of restricting visual input to one eye. This will, of course, abolish all depth cues and if the animal was using only such cues he should perform at chance. Strategies based on the absolute monocular identification of different patterns, however, would remain feasible. These tests were performed in two slightly different ways. Those tests described in Table IV as "right/left eye seeing" were performed by turning off on each trial the projector
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which normally illuminated a particular eye. T h e other method was to fix a piece of opaque tape to the goggles thus physically occluding one eye. T h e results for monkey B indicate strongly that it was, in fact, using only one eye (the right) to solve this problem, for it scored better than 90% correct with the right eye and was at chance levels with the left. By observing the animal’s eyes with a T V camera during testing we are certain that it was not physically closing one eye. We suppose that monkey A was using the following more general strategy. By learning to identify absolutely each pattern in each set with each eye, the monkey could solve our “depth” discrimination by attending only to the input of one eye at a time. If it knows that pattern A is coming to its left eye and pattern B to its right, then it has all the information it needs to determine whether the stimulus is positive or negative. I n practice the patterns will have common features and this will simplify the problem somewhat and lead to transfer to novel problems in some cases. We attempted to solve the various discriminations ourselves in this way, under the same testing conditions as the monkeys, by attending to one eye and searching for non-depth pattern cues. We succeeded in discriminating all five training discriminations, although the problem was not easy. One of us (A.C.) found he could attend easily with one eye, A.P. found it very difficult without physically closing an eye. No matter what detailed strategies are being used the results of Table IV indicate that both monkeys were able to solve the problem without using stereopsis. The strategy described above is quite general and also unobservable if the animal ignores the input to one eye rather than closing one eye. T h e only way of circumventing it is to change the stimuli very often, ideally on every trial, for example by using a stereo-film where the random dot matrix changes with every frame and only the depth remains constant (Julesz, 1971). However, we have seen that even cyclically changing the same five patterns every 50 trials results is an extremely difficult problem that one monkey never solved. Experiment III Our results differ from those of Bough (1970)in two main ways. His animals learned the problem very quickly, and his various tests all suggested that his monkeys were using depth cues. Let us for the moment accept that Bough’s monkeys were using depth cues. What could account for our different results? There are three likely possibilities. First, there may have been something peculiar to our monkeys that rendered them incapable of discriminating the patterns. Not all people can perceive the depth in random-dot stereograms (Richards, 1970) and we may have been extraordinarily unlucky in the monkeys we chose. Second, Rough used a successive discrimination, which would tend to make the discrimination of minute pattern differences even more difficult. Third, his animals were much further from the stimuli. This is a likely explanation since the depth impression in these patterns was easier to obtain at greater distances, possibly as a result of lesser conflict between accommodation and convergence. Our third experiment examines the second and third possibilities.
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In this experiment the depth stimuli were presented successively on a single stimulus panel, instead of simultaneously, in order to make comparison of minute pattern differences very difficult if not impossible.
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Method The same two monkeys were used. The apparatus was derived from that used in the simultaneous experiments and used the same projector assembly. The stimuli were now projected on to a single 13-cm square screen located centrally 30 cm behind the response panel. The response panel was made of clear glass 7.5 cm square, and the monkey viewed the stimulus by looking through this panel and down a tunnel painted matt black inside. Once again the panel could be pressed anywhere. A button to start each trial and a foodwell were provided as in the previous experiments. The monkeys were now about 60 cm from the stimuli. The stimuli were prepared in a slightly different way for this experiment. A 16-mm cine camera was used to photograph the computer screen and the actual film taken, after reversal processing, was used in the projectors. As a result quality was more consistent. T h e stimulus subtended an angle of 5 . 5 " overall and 2' for the central square. Disparity angle was 15' and luminance 50 cd/m2. Although four projectors were used, only two are necessary for the front/behind discriminations since the change from positive to negative can be achieved by moving the Polaroid filters. We took advantage of this by using all four projectors to provide two sets of stimuli, identical in depth but differing in pattern and presenting them concurrently within each testing session. T o start a trial the monkey pressed the button and the stimulus came on. If the stimulus was positive (central square in front of surround) the animal was rewarded for making 10 presses within 10s. Fewer than 1 0 presses in this period produced no reward and the house light was turned off. The monkey was rewarded if it made fewer than five presses to the negative stimulus (centre square behind the surround) and punished if it made five or more. The monkeys were accustomed to the new apparatus and the fixed ratio reinforcement schedule by presenting two preliminary discriminations: (I) a set of random dots vs. an evenly illuminated screen; ( 2 ) two sets of random dots one of which contained a darker central square. They were then given a depth/no depth discrimination, before going on to discriminate a square in front and a square behind the background as before. I n the latter discrimination two sets of positive and negative stimuli, identical in depth but differing in random dot matrix, were presented randomly.
Results T h e results of the depth discriminations are presented in Table V. Monkey B failed to learn either the depth/no depth discrimination or the frontlbehind problem. Since there was no evidence of stereopsis the animal was not tested further. TABLE Q Results of Experiment 111 Depth/no depth Front/behind I Front/behind 2 Best Best Best Sessions score Sessions score Sessions score MonkeyA MonkeyB
25
34
89 62
Table shows total number of daily sessions percentage correct on any session.
6 I4
91 64
(100trials/session)
0.5
-
89
-
on each problem and highest
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Monkey A performed consistently above 80% correct on the depth/no depth problem and showed good transfer to the frontlbehind depth discrimination. At this point it was transferred for 60 trials to two new sets of random dot stereograms with the same disparities in the central squares and performed at 89% correct. We then conducted control tests, as in Experiment 11, to determine whether the monkey was using stereopsis. T h e results are shown in Table VI. TABLE VI Control tests on monkey A in Experiment III Downloaded by [Central Michigan University] at 01:49 06 November 2015
Viewing condition Right eye seeing: no stimulus to left eye Left eye seeing: no stimulus to right eye No goggles Neutral density filters in goggles
Percentage correct 61 49
68 82
Two features of Table VI are especially noteworthy. Restricting the stimuli to one eye by turning off one projector resulted in chance performance but the animal scored 82% correct when the polarizing filters in the goggles were replaced with neutral density filters. Therefore the demonstration that occluding one eye results in chance performance cannot be taken as conclusive evidence for stereopsis. Of course the control tests were given in the order shown in Table VI and because of order effects they do not demonstrate that monocular viewing is necessarily worse than binocular viewing through ND filters. However, the important question is how could the animal solve the problem without polarizing filters in the goggles? We have previously argued that this discrimination should be impossible, since the only difference between positive and negative stimuli is now the plane of polarization of the light and we assumed that the animal had no means of determining this. By testing ourselves in the apparatus we found that our assumption was wrong, as follows. When light is reflected obliquely from a non-metallic surface the reflected light is partially plane-polarized (more correctly, elliptically polarized). This is known as the Brewster effect, and the brightness of the reflection depends on the angle of incidence. Similarly, if a plane polarized beam is so reflected the brightness of the reflected beam additionally depends on the angle between the reflecting surface and the plane of polarization of the incident light. I n our experiment we were projecting, from two projectors on each trial, light of different angles of plane polarization and different angles of incidence on to reflecting surfaces. If instead of looking at the stimulus screen the monkey looks at a suitable reflection in the tunnel leading to the screen the brightness of the reflection will depend on which projectors and polarizing filters are being used. When we tested ourselves in the apparatus we observed that there were reflections on all four walls of the black tunnel joining the clear glass response panel to the stimulus panel behind it. Although the brightness of the reflections varied with the different combinations of stimuli, projectors, and polarizing filters in front of them, we observed that the reflections from a positive stimulus were brighter than those from a negative. By
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attempting to judge the brightness of the reflections and in particular by noting whether they increased or decreased from one trial to the next we were able to perform at 100% correct on the problem without looking at the stimulus panel at all. We therefore retested monkey A on the same frontlbehind depth problem then abolished the reflections by removing the tunnel. T h e animal’s performance fell from close to 90% correct to chance, but then rose to 70% correct after 12 daily sessions. We then observed that less conspicuous but nevertheless discriminable reflections were still present along the painted metal edge of the frame of the glass response panel and that by removing the tunnel we had allowed light to reach other reflecting surfaces such as the tube from the feeder to the food well. Since these reflections are almost impossible to eliminate we ceased testing. Experiment IV I n the first three experiments the stereograms were projected through vertically and horizontally polarized filters and the animal wore corresponding filters in front of its eyes. Unfortunately any lateral head rotation degrades this correspondence, introduces “crosstalk” between the two channels, and can reduce the depth effect. Since we did not prevent the animal from rotating its head laterally we performed a final experiment with a technique which is much less vulnerable to head rotation. This involved the use of red/green anaglyph forms of the random dot stereograms. Here the random dots are printed with red or green dye. When viewed with a green filter over the left eye and a red filter over the right eye the green dots are invisible to the left eye and the red dots appear black. T h e right eye does not see the red dots and the green dots appear black. Overlapping dots also appear black to each eye. If the anaglyph is printed such that left and right images are horizontally shifted the central hidden figure is now seen, in depth, through the coloured filters. A large number of red/green anaglyph forms of random dot stereograms are provided by Julesz (1971). In each the left and right images are shifted horizontally and nasally by one or two picture elements when the green filter is over the left eye. However, if the anaglyph is turned upside down the nasal shift is converted to a temporal one and the depth effect reversed. By using two identical anaglyphs and inverting one of them it should be possible to measure discrimination between a central figure in front of the random dot surround and the same central figure behind the surround. T h e apparent depth can also be reversed of course by exchanging the coloured filters over the eyes. T h e purpose of the experiment was therefore to determine whether monkeys could perceive the apparent depth in a variety of redlgreen random dot anaglyphs, each of which contained a central square apparently in front of or behind the surround, as in the earlier and unsuccessful experiments.
Method Three monkeys were used, the previous two together with an experimentally naive juvenile female (monkey C). Discrimination testing took place in a Wisconsin general test apparatus, with the animal
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in a primate chair as before. When the opaque screen in front of the monkey was raised the animal faced, beyond its reach, a transparent Perspex board 56 cm wide by 30 cm high, and reclining q 0from the vertical so that stimulus plaques could be placed on the narrow lip along the lower edge. Two foodwells, 3ocm apart, were cut into the Perspex. This testing board was mounted on a heavy rectangular base which could be pushed 15 cm along a fixed track towards the animal by means of a rod extending from the back and beneath the one-way vision screen in front of the experimenter. Since the entire testing display was transparent, apart from the stimuli over the foodwells, the animal was visible throughout a trial. The testing display was illuminated from directly above by a 40 W fluorescent "daylight" tube and from above and behind the animal by a IOO W tungsten bulb in a reflector. Each pair of stimuli consisted of two identical anaglyphs, one of which was rotated through 180"to reverse the apparent depth of the central square. The stimuli were taken from two separate copies of the book by Julesz (1971) and will be referred to by the code used in the source. Each anaglyph was mounted between two pieces of clear Perspex, 125 mm square by 1.8 mm thick, held firmly together by grey plastic "grip-strip" along all four sides. The anaglyph, measuring approximately 86 mm square, occupied the centre of this Perspex plaque. Each anaglyph was carefully trimmed in a guillotine so that the edges of each member of a pair were identical. I n addition the narrow monochromatic bands at the side edges, which could provide non-depth cues, were removed. With one exception every pair of anaglyphs contained a hidden and central 34-mm square that appeared in depth when viewed through red/green filters. But the various pairs differed conspicuously in non-depth features such as the size, shape, number and arrangement of the random coloured elements. For a precise description of each pair the reader is referred to the appendix of Julesz (1971). I n order to obtain a clear impression of depth, the anaglyphs must be viewed through red and green filters which are matched with the red and green dyes of the anaglyphs. Although a set of filters is provided with the book by Julesz, we needed to replace any filters that became marked. We therefore measured the absorption spectrum of the filters provided by Julesz, using a scanning photoelectric monochromotor. The transmission curves for the red and green filters were very close to Kodak Wratten 26 and 55 respectively. The Polaroid filters in the goggles used in Experiments 1-111 were therefore replaced with a green Wratten 55 in front of the left eye and a red Wratten 26 in front of the right eye. These filters were interchangeable, which reverses the depth of any particular stimulus, and they were so exchanged in certain of the control procedures. Before being tested with any anaglyph stimuli each monkey learned a real-depth discrimination. The positive stimulus was a black/white random dot pattern with a central 34-mm square of random dots mounted on a 2-cm stalk in front of the 86-mm square background. T h e negative stimulus consisted of the same random dot surround with a central square cut out to reveal a random dot pattern 2 cm behind it. Each animal was taught to take peanuts from the open foodwells and then to displace black and white stimuli mounted between Perspex and covering the foodwells. T h e animal was then accustomed to waiting 5 s after the opaque screen was raised before the display trolley was pushed 15 cm forward from its initial position, which was out of reach. I n the initial position the stimuli were approximately 40-45 cm from the animal's eyes. As soon as the animal displaced one stimulus, and retrieved any reward, the trolley was withdrawn. A re-run correction procedure was used except where otherwise stated. I n all testing the positive and negative stimuli were presented randomly left and right, with the constraint that not more than five identical trials should occur consecutively. Superimposed on this random schedule was another, governing the orientation of the anaglyphs. On half the trials the two anaglyphs were oriented so that one showed the central figure in front of the surround and the other showed i t behind. On the remaining trials the two anaglyphs were rotated through 180"so that the apparent depth in each was reversed. Each anaglyph was therefore used to provide the positive and the negative stimulus randomly so that any non-depth features such as slightly different colour rendering were irrelevant.
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GLOBAL STEREOPSIS I N RHESUS MONKEYS
10.5
Each animal was tested for 5 0 trials/day on the following problems. With the exception of the final reversal problems the square apparently in front of the background was positive. (I) T h e animal learned the real depth discrimination to a criterion of 90 correct responses in IOO consecutive trials, excluding correction trials. T h e red/green goggles were then introduced and the animal retrained to the same criterion. The coloured goggles are of course irrelevant to the solution of this problem. (2) T h e animal was transferred to anaglyphs 5'5-4 (Julesz, 1971),in which a central square was conspicuous to human observers. Disparity angle was approximately 5 5 min. After reaching the same criterion the following controls were carried out on successive days and for 5 0 trials each. (a) The Perspex holders and plastic frames of the two stimuli were interchanged to control for any clues they might have provided. (b) The red/green filters in the goggles were interchanged. The effect of this is to reverse the apparent depth in each stimulus and the animal should now choose the previously negative stimulus panel. Both stimuli were baited on this control. (c) The animal was tested without the red/green goggles, which abolishes apparent depth and should make the discrimination impossible. (d) The animal was retrained under the initial conditions. (3) The animal was transferred to the new anaglyphs 2.4-1(Julesz, 1971)and the four controls repeated. Disparity angle was again approximately 5 5 min. (4) T h e animal was transferred to new anaglyphs 3.10-5 and the controls repeated. Disparity angle was approximately 38 min. (5) T h e animal was transferred to new anaglyphs 5.4-3, and the controls repeated. Disparity angle was approximately 45 min. (6) Problem 5'4-3 was presented again with the stimuli just within reach, and stationary, to compare performance with the earlier experiments where the projected stimuli were always stationary. (7) Problem 5'4-3 was repeated with monocular viewing. This abolishes any apparent depth, as in the previous control of removing the goggles, but might allow the animal to continue to use any monocular clue which depends on having a coloured filter over the eye. The animal was tested over 3 days as follows: (a) left eye covered with opaque filter, right eye with usual red filter; (b) right eye obscured, green filter over left eye as usual; (c) retrain. (8) Problem 5'4-3 was repeated for 5 0 trials with the stimuli rotated through 90". This converts the disparity fron horizontal to vertical. The animal was retrained the following day. (9) T h e animal was tested for 50 trials with new anagIyphs 4.5-2 (Julesz, 1971)but on reversal, i.e. the central square behind the background was positive. T o prevent unnecessarily long runs of correction trials the animal was immediately allowed to displace the other stimulus after making an error. Disparity angle was approximately 25 min. (10)A second reversal discrimination was performed with new anaglyphs (Fig. A, Julesz, 1971). The central figure was now a diamond instead of a square. Disparity angle was approximately 30 min.
Results T h e number of trials to criterion for the real depth discrimination are shown in Table VII. Only monkey C transferred with few errors to the first anaglyph problem. Monkeys A and B performed at chance initially but learned in 71 and 63 trials respectively. All three monkeys subsequently showed excellent transfer to three different anaglyph versions of the central square discrimination. I n every case they performed at criterion in the 50 transfer trials, and in six out of nine examples made no errors. This result alone is very strong evidence that the monkeys were perceiving a central square in depth, and the results of the controls carried out with each anaglyph confirm this. Exchanging the Perspex holders and plastic frames around the stimuli (control i), or exchanging the red/green viewing
Real depth
T
i
2
11
111
haglyph 5.5-4 .. ... iv
T i 11
111
3 Anaglyph 2 '4.. I ...
iv
T i
11
111
4 haglyph 3'10-5 .. ...
iv
T
5 AnaglYPh 5 '4-3 .. ... i 11 111
iv
Trials to criterion on a real depth discrimination (column I ) followed by total correct out of 50 trials on transfer to each of four different anaglyph problems (columns z T , 3 T , 4T and ST). The columns i-iv show the scores out of 50 on the control tests carried out after transfer to each anaglyph and described in the test. On transfer to the first anaglyph (column 2T) no animal reached criterion in 50 trials and for this problem only the animals were given further trials to criterion before being tested on the four controls.
Monkey
I
TABLE VII Experiment IV
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filters (control ii) made no difference to the animal’s choice of the square in front of the surround, whereas removing the coloured goggles resulted in chance performance (control iii). Several other controls were performed with anaglyph 5-4-3, and results are shown in Table VIII. Keeping the stimuli just in reach and stationary did not affect performance, whereas covering each eye in turn resulted in chance performance, as did rotating the stimuli through 90’ to convert the retinal disparity from horizontal to vertical.
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TABLE VIII Experiment IV 6
7
8
AnaglYPh 5.4-3
AnaglYPh 5 ‘4-3
Anaglyph 5.4-3
9
I0
Anaglyph Anaglyph 5.4-2 “A”
Monkey Stationary A
B C
50 50 50
Right Left eye eye
28
30
22
28
25
29
Both eyes
Vert. disp.
Horiz. disp.
50 50 50
25
50
3
27 24
49
I2
21 21
50
7
5
Reversal
Reversal
Number correct in 50 trials in conditions 6-10 described in the text.
The final controls in which the animals were tested on reversal with new anaglyphs show that they performed at below 50% correct in every case. Although animals A and B both scored 21 out of 50 on the final reversal with a diamond shape in the centre, animal A got the first 24 trials incorrect and then successfully reversed, and animal B adopted a position habit after the first few incorrect trials.
Discussion Our main purpose in initiating this study was to replicate an earlier finding (Bough, 1970)on the discrimination of apparent depth in random dot stereograms and to use the technique to investigate disparity limits and their possible alteration by various surgical procedures. The use of random dot stereograms is particularly attractive since cues provided by pattern and head movements can in theory be eliminated and relevant cues such as disparity angle, number and proportion of elements in binocular correspondence, and brightness gradients can all be precisely controlled and varied. However, we found no evidence of stereopsis in three experiments using random dot stereograms projected and viewed through polarizing filters yet were able to demonstrate it easily with red/green anaglyph versions of the stereograms which are unfortunately less useful for precise studies because more difficult to prepare. They also, in theory, contain many more non-depth cues than black/white random-dot stereograms. We have therefore confirmed Bough’s conclusion about stereopsis in monkeys but it must be asked why we failed to do so with what should be a superior method.
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A. COWEY, A. M. PARKINSON AND L. WARNICK
T h e answer does not lie in disparity angle. Although Bough (1970)did not state the disparity angle he used it can be measured on his published stimuli and we estimated it to be about 40 min of arc, which is within the range we used in both our successful and unsuccessful experiments. Nor can we now conclude that the monkeys failed to perceive the depth reliably in Experiments 1-111 because of the “crosstalk” between left- and right-eye channels when the animal wears vertical and horizontal polarizing filters and rotates its head. Although Bough avoided this problem by using circular polarizers for projection and viewing, three pieces of evidence indicate that it should not have been a problem in our experiments. ( I ) We observed both monkeys on a T V screen during testing and they characteristically kept the head upright. (2) We carefully measured the extent of lateral head rotation consistent with stereopsis in ourselves, using all 1 0 stimulus pairs from Experiment 11. We were able to rotate the head +_ 27’ before abolishing stereopsis. (3) Head rotation degrades not only the depth effect but the non-depth pattern and brightness cues which the two animals were demonstrated to be using in the first three experiments. There are other differences between Bough’s experiment and ours. For example, Bough’s monkeys observed the stimuli by reflected light and not by transillumination, and of course we used reflected light in our final successful experiment. His animals responded by pressing a lever remote from the stimulus screen, whereas ours pressed the stimuli directly, which may have encouraged them to look intently at the stimuli and to notice pattern and brightness differences. Lastly, our first two monkeys were extensively pre-trained on a variety of pattern discriminations and may have been expecting, and searching for, pattern cues. Unfortunately there are no good grounds for accepting or rejecting the importance of these differences without further lengthy experiments. Instead of attempting to explain why our results differ from Bough’s, it is possible to argue that they do not differ and that his animals had solved his discrimination by non-depth cues. Certainly we must point out that some of the alternative strategies used by our animals were available to his. For example, he presented no controls for brightness differences in the patterns themselves or in reflections from them, and we found in Experiment 111, which resembles Bough‘s most closely, that one animal was discriminating the successive brightness changes in the reflected polarized light from positive and negative stimuli. And although both of Bough’s animals showed good transfer from the first to the second set of stereograms, we found in Experiment I1 that transfer was good to some stereograms and poor to others. Had we used only one set of transfer stimuli we might have erroneously concluded that we had demonstrated stereopsis. I n Experiment 111, one animal did show excellent transfer to the new stereograms, but was subsequently shown to be discriminating reflections. Lastly, Bough found that his animals scored at chance when tested monocularly and after surgically-induced convergent strabismus. Unfortunately his paper does not state how long the animals were tested under these conditions, and we found that abolishing stereopsis cues sometimes had significant but transient effects on performance, as in Experiment 111. Whatever the explanation for the discrepancies between Bough’s experiments and
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ours one thing is clear: monkeys can detect features other than disparity in projected random-dot stereograms. Unless the random dot matrix is changed on every trial and differential reflections from plane-polarized light are eliminated, or deliberately varied to make them irrelevant, animals which have learned to use disparity information may conceal the effect of a lesion by relying on the other cues postoperatively.
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This research was supported by Medical Research Council Grant G97/397/B. It is a pleasure to thank Dr B Julesz for his comments on the manuscript and for his invaluable suggestions about experimental techniques.
References BARLOW, H. B., BLAKEMORE, C. and PBTTIGREW, J. D. (1967). The neural mechanism of binocular depth discrimination. Journal of Physiology (London), 193,327-42. BISHOP,P. 0. and HENRY, G. H. (1971). Spatial vision. Annual Review of Psychology, 22, I I 9-60. BLAKEMORE, C. (1970). Binocular depth perception and the optic chiasm. Vision Research, 10943-7. BLAKEMORE, C. and HAGUE,B. (1972). Evidence for disparity detecting neurones in the human visual system. Journal of Physiology (London), 225, 437-55. BLAKEMORE, C., FIORENTINI, A. and MAFFEI,L. (1972). A second neural mechanism of binocukdr depth discrimination. Journal of Physiology (London), 226, 725-49. BOUGH, E. W. (1970). Stereoscopic vision in the macaque monkey: a behavioural demonstration. Nature (London), 225, 42-4. HUBEL,D. H. and WIESEL,T. N. (1970). Stereoscopic vision in macaque monkey. Nature (London), 225, 41-2. JULESZ,B. (1964). Binocular depth perception without familiarity cues. Science, 145, 356-62. JULESZ,B. (197 I). Foundations of Cyclopean Perception. Chicago: University of Chicago Press. MITCHELL, D. E. and BLAKEMORE, C. (1970). Binocular depth perception and the corpus callosum. Vision Research, 10,49-54. RICHARDS, W. (1970). Stereopsis and stereoblindness. Experimental Brain Research, 10, 380-8. Received 11 February 1974
ISSN: 0033-555X (Print) (Online) Journal homepage: http://www.tandfonline.com/loi/pqje19
Global stereopsis in rhesus monkeys A. Cowey , A. M. Parkinson & L. Warnick To cite this article: A. Cowey , A. M. Parkinson & L. Warnick (1975) Global stereopsis in rhesus monkeys, Quarterly Journal of Experimental Psychology, 27:1, 93-109, DOI: 10.1080/14640747508400466 To link to this article: http://dx.doi.org/10.1080/14640747508400466
Published online: 29 May 2007.
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Date: 06 November 2015, At: 01:49
Quarterly Journal of Experimental Psychology (1975)27, 93-109
GLOBAL STEREOPSIS IN RHESUS MONKEYS A. COWEY, A. M. PARKINSON AND L. WARNICK Downloaded by [Central Michigan University] at 01:49 06 November 2015
Department of Experimental Psychology, University of Oxford, England I n three separate experiments an attempt was made to demonstrate global stereopsis in two rhesus monkeys by using random dot stereograms projected and viewed through polarizing filters. Although both animals learned a number of discriminations, control tests showed that both were perceiving non-depth cues such as monocular identification of minute pattern differences or brightness differences caused by reflections of polarized light. I n a final experiment red/green anaglyph forms of the stereograms wcre viewed through red/green filters. Both monkeys, together with a third experimentally naive animal, showed incontrovertible evidence of prompt discrimination based on stereopsis. This paper makes a number of recommendations about the use of random dot stereograms to demonstrate global stereopsis in animals.
Introduction The horizontal separation of the eyes in animals with overlapping visual fields produces slight differences in the two retinal images of an object within the binocular visual field. These differences are responsible for stereoscopic vision. Although we have a little evidence of the neural mechanisms underlying stereopsis in man (Blakemore, 1970; Mitchell and Blakemore, 1970; Blakemore and Hague, 197z), the great majority of evidence on the existence of single cells in the visual cortex which register the retinal disparity of images in the two eyes comes from electrophysiological experiments on cats (see Bishop and Henry, 1971, for review). It was therefore of great importance that the demonstration of retinal disparity detectors in the prestriate cortex of monkeys (Hubel and Wiesel, 1970) was accompanied by a report of behavioural evidence for stereoscopic vision in two macaque monkeys (Bough, 1970). Bough trained his animals to discriminate between two sets of random dot stereograms of the type developed by Julesz (1964). T h e two sets differed with respect to the apparent depth of an inner square when they were viewed through Polaroid goggles. T h e monkeys learned the problem easily, transferred promptly to a new set of stereograms with a different array of random dots but the same apparent depth, and fell to chance when tested monocularly, thereby providing seemingly indisputable evidence that they were using stereoscopic depth cues to master the discrimination. This clear-cut result has been referred to by various authors (Hubel and Wiesel, 1970; Julesz, 1971; Bishop and Henry, 1971; Blakemore et al., 1972) both for its confirmation of the function of cells registering retinal disparity and because the technique of using random dot stereograms to generate apparent depth uncontaminated by non-disparity cues should be valuable
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A. COWEY, A. M. PARKINSON A N D L. WARNICK
in experiments on the mechanisms underlying stereopsis. For example, by varying the horizontal disparity in the stereograms it should be possible to determine the upper and lower disparity limits for apparent depth perception and to relate this to electrophysiological findings. By severing either the optic chiasma, or the corpus callosum, or both, it should also be possible to investigate the suggested role of the former in fine stereopsis and the latter in coarse stereopsis (Bishop and Henry, 1971). The role of the prestriate visual areas in which the disparity detectors reside could also be studied by measuring the effects of its removal. It was with such aims in mind that we performed a series of experiments using random dot stereograms to assess stereopsis in monkeys. This paper describes important and totally unexpected problems in studying stereopsis in this way.
Experiment I This was an attempt to reproduce Bough’s (1970) result by teaching two monkeys a simultaneous visual discrimination between two squares at different apparent depths from their identical surrounds.
Method The principle by which arrays of random “dots” may be used to generate an impression of depth is as follows. Two matrices of random dots are prepared, differing only in that a central region of dots in one matrix is shifted horizontally by some integral number of dots. The two matrices look identical, although they are not. If they are shown to separate eyes with the surrounds in register, the disparity caused by shifting one of the central regions is detected and the entire horizontally shifted region appears in depth, either in front of or behind the surround depending on the direction of the shift. The simplest way of ensuring that each eye sees only one matrix is to place vertical polarizing filters over one eye and one projector and horizontal polarizing filters over the other eye and projector. T o produce two separate depth stimuli simultaneously but with different apparent depths still requires only two projectors, but now two random dot arrays must be placed in each projector. However, in order to interchange the two depth stimuli on the two screens the polarizing filters in front of the projectors or the eyes must be reversed, or a second pair of projectors with the random dot matrices exchanged for left and right may be used. In Experiment I we used the latter method. T h e apparatus is shown in Fig. I . The stereograms were projected on to the two stimulus panels, 8 cm square and 8 cm apart, by means of four 35 mm projectors. The latter were mounted vertically above each other with their optical axes parallel to each other and perpendicular to the plane of the stimulus panels. I n this way optical distortion was minimized. Directly in front of and covering all four projector lenses was a Perspex holder containing polarizing filters which ensured that the beams from projectors I and z were polarized at right angles to those from projectors 3 and 4. The stimulus panels were made of frosted Perspex covered with thin glass on the side towards the monkey. They were hinged at the top outside the viewing area, and equipped with microswitches. A slight press anywhere on the panel was recorded by the automatic programming apparatus which controlled the entire experiment from a separate room. The animal started each trial by pressing the small button between the stimulus panels. Below that was a foodwell into which a peanut was delivered automatically after a correct response. Two MUCUCQ nzulatta [I male (A), I female (B)] about z years old at the start of the experiment, were used. They were fed a few hours after testing so that they were hungry at the start of each testing session.
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For each testing session the animal was placed in a primate chair. The platform around the animal’s neck prevented it from reaching its face with its hands, but a small hole in the platform immediately below the animal’s chin allowed it to pass a peanut to its mouth using finger and thumb. Goggles were attached to the animal’s head by surgical sticking tape. Each eyepiece was covered with a piece of “polaroid” material oriented at right angles to that on the other eyepiece, the absolute orientations corresponding to those used in the projector assembly.
FIG.I . TIsometric diagram of apparatus used in Experiments 1-11. text.
The parts are described in the
The stimuli were random-dot stereograms of the type developed by Julesz ( I 964). Each was made of IOO x IOO elements, half of which were filled on a random basis by dots. They were generated with a LINC-8 computer and photographed o f f a short-persistence screen using a 35 mm camera. The resulting negatives were fixed in pairs to cellophane sheets, using a light box to obtain accurate register, and reduced on to Kodalith film. Four of these film pairs were mounted on a single strip of Perspex which could be slipped into the multiple projector assembly shown in Fig. I . Once set up it was a relatively easy matter to obtain accurate register of the stimuli on the panels. The stimulus surround came to the edge of the panel, and subtended about 25’ overall, with the central square subtending 13”. The actual disparity used was 4 elements for problem DI and 6 elements for the others. The corresponding disparity angles were 80 and 120’. Luminance was about I I cd/m2. When we and several colleagues viewed these stimuli in the testing apparatus under the same viewing conditions as the monkeys the apparent depth of the central squares relative to the surround was very conspicuous. T o start a trial the monkey pressed the illuminated button. This turned on either projectors I and 3 or z and 4 and also a “white” noise. The monkey had to withhold responses for 5 s until the noise stopped. This was to encourage the animal to inspect the stimuli before responding. Presses during the noise reset the apparatus to its initial state and such trials were discarded. After the end of the noise a single press to either panel constituted a response and the stimuli were turned off and the monkey rewarded or punished depending on whether it had pressed the positive or negative stimulus. Reward was a peanut delivered into the briefly illuminated foodwell, followed by a time-out of 1 0 s , after which the monkey could start a new trial. Punishment consisted of a 10-speriod during which the houselight
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was turned off. A correction schedule was used such that the same arrangement of stimuli was used following an incorrect trial. I n the results that follow these “correction trials” are discounted. The animals were usually given IOO trials (excluding correction trials) in each daily session. The animals were trained initially on a brightness discrimination followed by a variety of pattern discriminations, during which time they were accustomed to the wearing of goggles. vs. W ; and E vs. &. The patterns were The patterns were X vs. 0 ; vs. V; T vs. ,L; I ” (2.54 cm) high and projected on to the centre of the stimulus panels. The last problem learned prior to the stereoscopic depth discrimination was a discrimination between a random dot array and a similar array with a darker central square. They were then trained on a discrimination between a stereoscopic pair of patterns giving the impression of a square in front of the background vs. a pair of identical patterns which provided no apparent depth. Finally, they were trained on several sets of patterns each involving a discrimination between a square in front of the surround vs. a square behind the surround. The stimulus showing the square in front of the surround was always positive. For every problem the criterion for learning was 90 correct trials in a daily session of I O O trials.
Results Monkeys A and B required a mean of 416 trials and 450 trials respectively to reach criterion on the five pattern discriminations and the central dark square discrimination. Both animals improved throughout the series as they learned to tolerate the goggles, and the last problem was learned by both animals in a single session. Performance on the depth/no depth discrimination is shown in Table I. Although both animals learned the discrimination, this is not adequate evidence of stereopsis for the problem could be solved on the basis of the slight pattern differences which exist along the lateral borders of the “hidden” square. T h e front/behind discrimination is not open to this objection, since ideally the positive and negative stimuli differ only in the relative planes of polarization. Both animals learned this first front/behind discrimination, but without showing transfer from the preceding depth/no depth discrimination (Table I, F-BI). Furthermore, when tested on further depth problems (F-B2,3,4) using different sets of random dot stereograms with identical disparities it is clear from Table I that transfer between successive problems was poor for monkey B and almost non-existent for monkey A, indicating that each problem was being learned by using non-depth cues. Monkey B was therefore retested on problem F-BI and after performing at 95% correct for 200 trials was tested for IOO trials on each of three different control TABLE I Results of Experiment I D-ND D FD Monkey A Monkey B
F-l3 I D FD
14
78
12
3
59
3
61 69
F-B 2 D FD
7 II
49 72
F-B 3 D FD
7 10
60 72
F-B 4 D FD -
-
10
67
Days to reach criterion (D) at IOO trials/day, and percentage correct on first day (FD) for a depth/no depth discrimination (D-ND) and four front/behind depth discriminations (F-B)
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conditions: (I) without polarizing filters in the goggles; (2)with right eye covered; (3) with left eye covered. T h e animal performed at 88, 84 and 91% correct respectively, demonstrating that the discrimination was soluble without stereopsis. Since the different pairs of random dot stereograms may have differed in brightness, either because of unequal densities introduced during their preparation, or because of differing brightness in the projectors, we carried out a final control and tested monkey B with neutral density filters covering first one random dot matrix in all four projectors, then the other. T h e effect was to noticeably darken first the positive stimulus and then the negative. T h e animal scored 96 and 89% correct in the two sessions.
Experiment I1 T h e first experiment showed that both monkeys could solve the front/behind depth discrimination without using stereopsis and that they were probably never using it, despite the prominence of the apparent depth to human observers under identical viewing conditions. What other non-depth cues were present which could enable the monkeys to solve the discrimination? There are three obvious candidates. First, specks of dust or scratches on the film may have provided minute pattern cues. Second, it was impossible to align the random dot stereograms sufficiently accurately to prevent the dots at the extreme edges of the background from providing minute differences in pattern cues along the edge of the response panels. These differences would alter from session to session when the film holder was removed from the projector but would be constant within a session. Third, and perhaps most important of all, the animal may be able to detect monocularly the lateral shift in the central square which inevitably produces a minute difference in texture at the boundaries of the square and which would always be present in identical form for a given depth problem. Experiment I1 was designed to encourage the use of stereoscopic depth cues by making it much more difficult for the animals to use any other cue. Three changes were made. First, the monkeys were trained on five different problems identical with respect to depth but with different random dot patterns. The patterns were changed every 50 trials. In addition five further sets of random dot stereograms were used to study transfer. Second, the stimuli were made smaller so that the surround no longer reached the edge of the stimulus panels, thereby eliminating pattern cues at the very edges of the panels. Third, the polarizing filters in front of the projectors were randomly moved as described below so that the same pair of stereograms could provide the positive or negative depth stimulus.
Method Ten new sets of random dot sterograms of IOO x IOO elements were prepared, but they were made smaller than in the first experiment so that the stimuli subtended an overall angle of about 10' with the central square subtending 4". Disparity angle was 25' and average luminance about 80 cd/m2. The panel containing the polarizing filters in front of the projectors was also modified by adding further filters and arranging that the entire panel could be moved up or down automatically, thus rotating the plane of polarized light from each projector through 90". Thus
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the stimulus panel on which the positive depth stimulus appeared could be altered either by using a different pair of projectors, as in Experiment I, or by moving the filter panel. It will be noted that when the latter method is used the projected patterns are unchanged and this should prevent strategies based on gross differences in pattern and brightness. However, it is still theoretically possible to detect the change in plane of polarization by virtue of the Polaroid filters in the viewing goggles, and this provides a means of solving the problem without using depth cues, as will be seen later. The same apparatus was used as in Experiment I but the monkeys were moved back slightly to a distance of 25-30 cm from the stimulus panels. The 5-s period of white noise was eliminated, and instead the animal was encouraged to inspect the stimuli carefully by arranging that the first stimulus panel to be pressed eight times activated the apparatus. The filter in front of the projectors was moved randomly between trials. The stimuli were changed every 50 trials, halfway through each daily session.
Results The performance of the two monkeys on each of the five training problems is shown in Table 11. Monkey A was tested for 7500 trials and B for 9500 trials. TABLE I1 Mean percentage correct, with standard deviations, for jive frontlbehind depth discriminations used as the training problems in Experiment 11 Problem
Monkey A s.d.
I
75'7
10.3
3 5 7 9
85.2
12.7
91.2 70.6 84.1
12.5
Monkey A was tested for
1500 trials
9'1
11.8
Monkey B s.d. 72.8 73'1 69.5 70'9 67.1
12.4 11'3
13.2
'5'3 15'7
on each problem, Monkey B for 1900.
Although monkey A reached criterion on every problem on several occasions, its average performance was better than 90% correct on only one of them. Monkey B was so poor despite scoring consistently above chance that the transfer tests to the remaining five problems were performed only on A, as follows. T h e novel set of stimuli was introduced halfway through each testing session after the monkey had exceeded 90% correct during the first half. The results are shown in Table 111. It can be seen that in almost every case transfer resulted in a significant impairment and with two problems initial performance was not better than chance. There remains the possibility that the monkey was using depth cues for those stimuli on which he showed transfer, but this possibility is remote since depth cues were the same for all stimuli. Only non-depth cues varied from one set to another, and this would seem to implicate such cues since only some of the new stimuli would be expected to share non-depth cues with the original set of five. We therefore performed various tests to determine exactly how the monkeys were solving the problem. Both monkeys were trained to a criterion of 90% correct on 2 consecutive days on
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TABLE I11 Percentage correct performance of monkey A on five sets of frontlbehind depth stimuli
Problem 2
4 6 8
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I0
Trials 1-50 per cent correct
Trials 51-100 per cent correct
Overall per cent correct
54 82
78 I00
66 91 70 75 55
60 72 56
80 78 54
Each set was presented twice, for 50 trials, after an immediately preceding session of 50 trials with a set of training stimuli identical in depth but differing in random dot matrix. The monkey average 94% correct in the immediately preceding sessions with these familiar stimuli.
their best problem (which was not the same). This gave a relatively stable baseline. All control tests were then performed for a session of 50 trials immediately following a criterion run and again for a whole session of IOO trials, giving 150 trials in all. The results are presented in Table IV. Basically two sorts of tests were performed, and these are described below. I n order to ensure that matching of patterns and movement of filter had indeed ruled out those simple strategies the monkeys had been using in Experiment I, they were tested with neutral density filters instead of Polaroid material in the goggles. Since the monkeys now have no means of resolving polarized light, the two stimuli must appear identical and the problem should be impossible. I t can be seen that both monkeys performed at chance levels under this condition. TABLE IV Percentage correct under different viewing conditions Viewing condition I
2
3
4 5
Neutral density filters Right eye seeing: no stimulus to left eye Left eye seeing: no stimulus to right eye Left eye occluded Right eye occluded
Monkey A
Monkey B
54 69" 67* 65" 75"
64 93* 43 89* 53
The animals received 150 trials on each condition, with the exception of monkey B, which received only 50 trials on the first condition. * P < 0.01, two-tailed, normal approximation to binomial test.
T h e second group of tests consisted of restricting visual input to one eye. This will, of course, abolish all depth cues and if the animal was using only such cues he should perform at chance. Strategies based on the absolute monocular identification of different patterns, however, would remain feasible. These tests were performed in two slightly different ways. Those tests described in Table IV as "right/left eye seeing" were performed by turning off on each trial the projector
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A. COWEY, A. M. PARKINSON AND L. WARNICK
which normally illuminated a particular eye. T h e other method was to fix a piece of opaque tape to the goggles thus physically occluding one eye. T h e results for monkey B indicate strongly that it was, in fact, using only one eye (the right) to solve this problem, for it scored better than 90% correct with the right eye and was at chance levels with the left. By observing the animal’s eyes with a T V camera during testing we are certain that it was not physically closing one eye. We suppose that monkey A was using the following more general strategy. By learning to identify absolutely each pattern in each set with each eye, the monkey could solve our “depth” discrimination by attending only to the input of one eye at a time. If it knows that pattern A is coming to its left eye and pattern B to its right, then it has all the information it needs to determine whether the stimulus is positive or negative. I n practice the patterns will have common features and this will simplify the problem somewhat and lead to transfer to novel problems in some cases. We attempted to solve the various discriminations ourselves in this way, under the same testing conditions as the monkeys, by attending to one eye and searching for non-depth pattern cues. We succeeded in discriminating all five training discriminations, although the problem was not easy. One of us (A.C.) found he could attend easily with one eye, A.P. found it very difficult without physically closing an eye. No matter what detailed strategies are being used the results of Table IV indicate that both monkeys were able to solve the problem without using stereopsis. The strategy described above is quite general and also unobservable if the animal ignores the input to one eye rather than closing one eye. T h e only way of circumventing it is to change the stimuli very often, ideally on every trial, for example by using a stereo-film where the random dot matrix changes with every frame and only the depth remains constant (Julesz, 1971). However, we have seen that even cyclically changing the same five patterns every 50 trials results is an extremely difficult problem that one monkey never solved. Experiment III Our results differ from those of Bough (1970)in two main ways. His animals learned the problem very quickly, and his various tests all suggested that his monkeys were using depth cues. Let us for the moment accept that Bough’s monkeys were using depth cues. What could account for our different results? There are three likely possibilities. First, there may have been something peculiar to our monkeys that rendered them incapable of discriminating the patterns. Not all people can perceive the depth in random-dot stereograms (Richards, 1970) and we may have been extraordinarily unlucky in the monkeys we chose. Second, Rough used a successive discrimination, which would tend to make the discrimination of minute pattern differences even more difficult. Third, his animals were much further from the stimuli. This is a likely explanation since the depth impression in these patterns was easier to obtain at greater distances, possibly as a result of lesser conflict between accommodation and convergence. Our third experiment examines the second and third possibilities.
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In this experiment the depth stimuli were presented successively on a single stimulus panel, instead of simultaneously, in order to make comparison of minute pattern differences very difficult if not impossible.
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Method The same two monkeys were used. The apparatus was derived from that used in the simultaneous experiments and used the same projector assembly. The stimuli were now projected on to a single 13-cm square screen located centrally 30 cm behind the response panel. The response panel was made of clear glass 7.5 cm square, and the monkey viewed the stimulus by looking through this panel and down a tunnel painted matt black inside. Once again the panel could be pressed anywhere. A button to start each trial and a foodwell were provided as in the previous experiments. The monkeys were now about 60 cm from the stimuli. The stimuli were prepared in a slightly different way for this experiment. A 16-mm cine camera was used to photograph the computer screen and the actual film taken, after reversal processing, was used in the projectors. As a result quality was more consistent. T h e stimulus subtended an angle of 5 . 5 " overall and 2' for the central square. Disparity angle was 15' and luminance 50 cd/m2. Although four projectors were used, only two are necessary for the front/behind discriminations since the change from positive to negative can be achieved by moving the Polaroid filters. We took advantage of this by using all four projectors to provide two sets of stimuli, identical in depth but differing in pattern and presenting them concurrently within each testing session. T o start a trial the monkey pressed the button and the stimulus came on. If the stimulus was positive (central square in front of surround) the animal was rewarded for making 10 presses within 10s. Fewer than 1 0 presses in this period produced no reward and the house light was turned off. The monkey was rewarded if it made fewer than five presses to the negative stimulus (centre square behind the surround) and punished if it made five or more. The monkeys were accustomed to the new apparatus and the fixed ratio reinforcement schedule by presenting two preliminary discriminations: (I) a set of random dots vs. an evenly illuminated screen; ( 2 ) two sets of random dots one of which contained a darker central square. They were then given a depth/no depth discrimination, before going on to discriminate a square in front and a square behind the background as before. I n the latter discrimination two sets of positive and negative stimuli, identical in depth but differing in random dot matrix, were presented randomly.
Results T h e results of the depth discriminations are presented in Table V. Monkey B failed to learn either the depth/no depth discrimination or the frontlbehind problem. Since there was no evidence of stereopsis the animal was not tested further. TABLE Q Results of Experiment 111 Depth/no depth Front/behind I Front/behind 2 Best Best Best Sessions score Sessions score Sessions score MonkeyA MonkeyB
25
34
89 62
Table shows total number of daily sessions percentage correct on any session.
6 I4
91 64
(100trials/session)
0.5
-
89
-
on each problem and highest
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A. COWEY, A. M. PARKINSON AND L. WARNICK
Monkey A performed consistently above 80% correct on the depth/no depth problem and showed good transfer to the frontlbehind depth discrimination. At this point it was transferred for 60 trials to two new sets of random dot stereograms with the same disparities in the central squares and performed at 89% correct. We then conducted control tests, as in Experiment 11, to determine whether the monkey was using stereopsis. T h e results are shown in Table VI. TABLE VI Control tests on monkey A in Experiment III Downloaded by [Central Michigan University] at 01:49 06 November 2015
Viewing condition Right eye seeing: no stimulus to left eye Left eye seeing: no stimulus to right eye No goggles Neutral density filters in goggles
Percentage correct 61 49
68 82
Two features of Table VI are especially noteworthy. Restricting the stimuli to one eye by turning off one projector resulted in chance performance but the animal scored 82% correct when the polarizing filters in the goggles were replaced with neutral density filters. Therefore the demonstration that occluding one eye results in chance performance cannot be taken as conclusive evidence for stereopsis. Of course the control tests were given in the order shown in Table VI and because of order effects they do not demonstrate that monocular viewing is necessarily worse than binocular viewing through ND filters. However, the important question is how could the animal solve the problem without polarizing filters in the goggles? We have previously argued that this discrimination should be impossible, since the only difference between positive and negative stimuli is now the plane of polarization of the light and we assumed that the animal had no means of determining this. By testing ourselves in the apparatus we found that our assumption was wrong, as follows. When light is reflected obliquely from a non-metallic surface the reflected light is partially plane-polarized (more correctly, elliptically polarized). This is known as the Brewster effect, and the brightness of the reflection depends on the angle of incidence. Similarly, if a plane polarized beam is so reflected the brightness of the reflected beam additionally depends on the angle between the reflecting surface and the plane of polarization of the incident light. I n our experiment we were projecting, from two projectors on each trial, light of different angles of plane polarization and different angles of incidence on to reflecting surfaces. If instead of looking at the stimulus screen the monkey looks at a suitable reflection in the tunnel leading to the screen the brightness of the reflection will depend on which projectors and polarizing filters are being used. When we tested ourselves in the apparatus we observed that there were reflections on all four walls of the black tunnel joining the clear glass response panel to the stimulus panel behind it. Although the brightness of the reflections varied with the different combinations of stimuli, projectors, and polarizing filters in front of them, we observed that the reflections from a positive stimulus were brighter than those from a negative. By
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attempting to judge the brightness of the reflections and in particular by noting whether they increased or decreased from one trial to the next we were able to perform at 100% correct on the problem without looking at the stimulus panel at all. We therefore retested monkey A on the same frontlbehind depth problem then abolished the reflections by removing the tunnel. T h e animal’s performance fell from close to 90% correct to chance, but then rose to 70% correct after 12 daily sessions. We then observed that less conspicuous but nevertheless discriminable reflections were still present along the painted metal edge of the frame of the glass response panel and that by removing the tunnel we had allowed light to reach other reflecting surfaces such as the tube from the feeder to the food well. Since these reflections are almost impossible to eliminate we ceased testing. Experiment IV I n the first three experiments the stereograms were projected through vertically and horizontally polarized filters and the animal wore corresponding filters in front of its eyes. Unfortunately any lateral head rotation degrades this correspondence, introduces “crosstalk” between the two channels, and can reduce the depth effect. Since we did not prevent the animal from rotating its head laterally we performed a final experiment with a technique which is much less vulnerable to head rotation. This involved the use of red/green anaglyph forms of the random dot stereograms. Here the random dots are printed with red or green dye. When viewed with a green filter over the left eye and a red filter over the right eye the green dots are invisible to the left eye and the red dots appear black. T h e right eye does not see the red dots and the green dots appear black. Overlapping dots also appear black to each eye. If the anaglyph is printed such that left and right images are horizontally shifted the central hidden figure is now seen, in depth, through the coloured filters. A large number of red/green anaglyph forms of random dot stereograms are provided by Julesz (1971). In each the left and right images are shifted horizontally and nasally by one or two picture elements when the green filter is over the left eye. However, if the anaglyph is turned upside down the nasal shift is converted to a temporal one and the depth effect reversed. By using two identical anaglyphs and inverting one of them it should be possible to measure discrimination between a central figure in front of the random dot surround and the same central figure behind the surround. T h e apparent depth can also be reversed of course by exchanging the coloured filters over the eyes. T h e purpose of the experiment was therefore to determine whether monkeys could perceive the apparent depth in a variety of redlgreen random dot anaglyphs, each of which contained a central square apparently in front of or behind the surround, as in the earlier and unsuccessful experiments.
Method Three monkeys were used, the previous two together with an experimentally naive juvenile female (monkey C). Discrimination testing took place in a Wisconsin general test apparatus, with the animal
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A. COWEY, A. M. PARKINSON AND L. WARNICK
in a primate chair as before. When the opaque screen in front of the monkey was raised the animal faced, beyond its reach, a transparent Perspex board 56 cm wide by 30 cm high, and reclining q 0from the vertical so that stimulus plaques could be placed on the narrow lip along the lower edge. Two foodwells, 3ocm apart, were cut into the Perspex. This testing board was mounted on a heavy rectangular base which could be pushed 15 cm along a fixed track towards the animal by means of a rod extending from the back and beneath the one-way vision screen in front of the experimenter. Since the entire testing display was transparent, apart from the stimuli over the foodwells, the animal was visible throughout a trial. The testing display was illuminated from directly above by a 40 W fluorescent "daylight" tube and from above and behind the animal by a IOO W tungsten bulb in a reflector. Each pair of stimuli consisted of two identical anaglyphs, one of which was rotated through 180"to reverse the apparent depth of the central square. The stimuli were taken from two separate copies of the book by Julesz (1971) and will be referred to by the code used in the source. Each anaglyph was mounted between two pieces of clear Perspex, 125 mm square by 1.8 mm thick, held firmly together by grey plastic "grip-strip" along all four sides. The anaglyph, measuring approximately 86 mm square, occupied the centre of this Perspex plaque. Each anaglyph was carefully trimmed in a guillotine so that the edges of each member of a pair were identical. I n addition the narrow monochromatic bands at the side edges, which could provide non-depth cues, were removed. With one exception every pair of anaglyphs contained a hidden and central 34-mm square that appeared in depth when viewed through red/green filters. But the various pairs differed conspicuously in non-depth features such as the size, shape, number and arrangement of the random coloured elements. For a precise description of each pair the reader is referred to the appendix of Julesz (1971). I n order to obtain a clear impression of depth, the anaglyphs must be viewed through red and green filters which are matched with the red and green dyes of the anaglyphs. Although a set of filters is provided with the book by Julesz, we needed to replace any filters that became marked. We therefore measured the absorption spectrum of the filters provided by Julesz, using a scanning photoelectric monochromotor. The transmission curves for the red and green filters were very close to Kodak Wratten 26 and 55 respectively. The Polaroid filters in the goggles used in Experiments 1-111 were therefore replaced with a green Wratten 55 in front of the left eye and a red Wratten 26 in front of the right eye. These filters were interchangeable, which reverses the depth of any particular stimulus, and they were so exchanged in certain of the control procedures. Before being tested with any anaglyph stimuli each monkey learned a real-depth discrimination. The positive stimulus was a black/white random dot pattern with a central 34-mm square of random dots mounted on a 2-cm stalk in front of the 86-mm square background. T h e negative stimulus consisted of the same random dot surround with a central square cut out to reveal a random dot pattern 2 cm behind it. Each animal was taught to take peanuts from the open foodwells and then to displace black and white stimuli mounted between Perspex and covering the foodwells. T h e animal was then accustomed to waiting 5 s after the opaque screen was raised before the display trolley was pushed 15 cm forward from its initial position, which was out of reach. I n the initial position the stimuli were approximately 40-45 cm from the animal's eyes. As soon as the animal displaced one stimulus, and retrieved any reward, the trolley was withdrawn. A re-run correction procedure was used except where otherwise stated. I n all testing the positive and negative stimuli were presented randomly left and right, with the constraint that not more than five identical trials should occur consecutively. Superimposed on this random schedule was another, governing the orientation of the anaglyphs. On half the trials the two anaglyphs were oriented so that one showed the central figure in front of the surround and the other showed i t behind. On the remaining trials the two anaglyphs were rotated through 180"so that the apparent depth in each was reversed. Each anaglyph was therefore used to provide the positive and the negative stimulus randomly so that any non-depth features such as slightly different colour rendering were irrelevant.
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Each animal was tested for 5 0 trials/day on the following problems. With the exception of the final reversal problems the square apparently in front of the background was positive. (I) T h e animal learned the real depth discrimination to a criterion of 90 correct responses in IOO consecutive trials, excluding correction trials. T h e red/green goggles were then introduced and the animal retrained to the same criterion. The coloured goggles are of course irrelevant to the solution of this problem. (2) T h e animal was transferred to anaglyphs 5'5-4 (Julesz, 1971),in which a central square was conspicuous to human observers. Disparity angle was approximately 5 5 min. After reaching the same criterion the following controls were carried out on successive days and for 5 0 trials each. (a) The Perspex holders and plastic frames of the two stimuli were interchanged to control for any clues they might have provided. (b) The red/green filters in the goggles were interchanged. The effect of this is to reverse the apparent depth in each stimulus and the animal should now choose the previously negative stimulus panel. Both stimuli were baited on this control. (c) The animal was tested without the red/green goggles, which abolishes apparent depth and should make the discrimination impossible. (d) The animal was retrained under the initial conditions. (3) The animal was transferred to the new anaglyphs 2.4-1(Julesz, 1971)and the four controls repeated. Disparity angle was again approximately 5 5 min. (4) T h e animal was transferred to new anaglyphs 3.10-5 and the controls repeated. Disparity angle was approximately 38 min. (5) T h e animal was transferred to new anaglyphs 5.4-3, and the controls repeated. Disparity angle was approximately 45 min. (6) Problem 5'4-3 was presented again with the stimuli just within reach, and stationary, to compare performance with the earlier experiments where the projected stimuli were always stationary. (7) Problem 5'4-3 was repeated with monocular viewing. This abolishes any apparent depth, as in the previous control of removing the goggles, but might allow the animal to continue to use any monocular clue which depends on having a coloured filter over the eye. The animal was tested over 3 days as follows: (a) left eye covered with opaque filter, right eye with usual red filter; (b) right eye obscured, green filter over left eye as usual; (c) retrain. (8) Problem 5'4-3 was repeated for 5 0 trials with the stimuli rotated through 90". This converts the disparity fron horizontal to vertical. The animal was retrained the following day. (9) T h e animal was tested for 50 trials with new anagIyphs 4.5-2 (Julesz, 1971)but on reversal, i.e. the central square behind the background was positive. T o prevent unnecessarily long runs of correction trials the animal was immediately allowed to displace the other stimulus after making an error. Disparity angle was approximately 25 min. (10)A second reversal discrimination was performed with new anaglyphs (Fig. A, Julesz, 1971). The central figure was now a diamond instead of a square. Disparity angle was approximately 30 min.
Results T h e number of trials to criterion for the real depth discrimination are shown in Table VII. Only monkey C transferred with few errors to the first anaglyph problem. Monkeys A and B performed at chance initially but learned in 71 and 63 trials respectively. All three monkeys subsequently showed excellent transfer to three different anaglyph versions of the central square discrimination. I n every case they performed at criterion in the 50 transfer trials, and in six out of nine examples made no errors. This result alone is very strong evidence that the monkeys were perceiving a central square in depth, and the results of the controls carried out with each anaglyph confirm this. Exchanging the Perspex holders and plastic frames around the stimuli (control i), or exchanging the red/green viewing
Real depth
T
i
2
11
111
haglyph 5.5-4 .. ... iv
T i 11
111
3 Anaglyph 2 '4.. I ...
iv
T i
11
111
4 haglyph 3'10-5 .. ...
iv
T
5 AnaglYPh 5 '4-3 .. ... i 11 111
iv
Trials to criterion on a real depth discrimination (column I ) followed by total correct out of 50 trials on transfer to each of four different anaglyph problems (columns z T , 3 T , 4T and ST). The columns i-iv show the scores out of 50 on the control tests carried out after transfer to each anaglyph and described in the test. On transfer to the first anaglyph (column 2T) no animal reached criterion in 50 trials and for this problem only the animals were given further trials to criterion before being tested on the four controls.
Monkey
I
TABLE VII Experiment IV
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filters (control ii) made no difference to the animal’s choice of the square in front of the surround, whereas removing the coloured goggles resulted in chance performance (control iii). Several other controls were performed with anaglyph 5-4-3, and results are shown in Table VIII. Keeping the stimuli just in reach and stationary did not affect performance, whereas covering each eye in turn resulted in chance performance, as did rotating the stimuli through 90’ to convert the retinal disparity from horizontal to vertical.
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TABLE VIII Experiment IV 6
7
8
AnaglYPh 5.4-3
AnaglYPh 5 ‘4-3
Anaglyph 5.4-3
9
I0
Anaglyph Anaglyph 5.4-2 “A”
Monkey Stationary A
B C
50 50 50
Right Left eye eye
28
30
22
28
25
29
Both eyes
Vert. disp.
Horiz. disp.
50 50 50
25
50
3
27 24
49
I2
21 21
50
7
5
Reversal
Reversal
Number correct in 50 trials in conditions 6-10 described in the text.
The final controls in which the animals were tested on reversal with new anaglyphs show that they performed at below 50% correct in every case. Although animals A and B both scored 21 out of 50 on the final reversal with a diamond shape in the centre, animal A got the first 24 trials incorrect and then successfully reversed, and animal B adopted a position habit after the first few incorrect trials.
Discussion Our main purpose in initiating this study was to replicate an earlier finding (Bough, 1970)on the discrimination of apparent depth in random dot stereograms and to use the technique to investigate disparity limits and their possible alteration by various surgical procedures. The use of random dot stereograms is particularly attractive since cues provided by pattern and head movements can in theory be eliminated and relevant cues such as disparity angle, number and proportion of elements in binocular correspondence, and brightness gradients can all be precisely controlled and varied. However, we found no evidence of stereopsis in three experiments using random dot stereograms projected and viewed through polarizing filters yet were able to demonstrate it easily with red/green anaglyph versions of the stereograms which are unfortunately less useful for precise studies because more difficult to prepare. They also, in theory, contain many more non-depth cues than black/white random-dot stereograms. We have therefore confirmed Bough’s conclusion about stereopsis in monkeys but it must be asked why we failed to do so with what should be a superior method.
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A. COWEY, A. M. PARKINSON AND L. WARNICK
T h e answer does not lie in disparity angle. Although Bough (1970)did not state the disparity angle he used it can be measured on his published stimuli and we estimated it to be about 40 min of arc, which is within the range we used in both our successful and unsuccessful experiments. Nor can we now conclude that the monkeys failed to perceive the depth reliably in Experiments 1-111 because of the “crosstalk” between left- and right-eye channels when the animal wears vertical and horizontal polarizing filters and rotates its head. Although Bough avoided this problem by using circular polarizers for projection and viewing, three pieces of evidence indicate that it should not have been a problem in our experiments. ( I ) We observed both monkeys on a T V screen during testing and they characteristically kept the head upright. (2) We carefully measured the extent of lateral head rotation consistent with stereopsis in ourselves, using all 1 0 stimulus pairs from Experiment 11. We were able to rotate the head +_ 27’ before abolishing stereopsis. (3) Head rotation degrades not only the depth effect but the non-depth pattern and brightness cues which the two animals were demonstrated to be using in the first three experiments. There are other differences between Bough’s experiment and ours. For example, Bough’s monkeys observed the stimuli by reflected light and not by transillumination, and of course we used reflected light in our final successful experiment. His animals responded by pressing a lever remote from the stimulus screen, whereas ours pressed the stimuli directly, which may have encouraged them to look intently at the stimuli and to notice pattern and brightness differences. Lastly, our first two monkeys were extensively pre-trained on a variety of pattern discriminations and may have been expecting, and searching for, pattern cues. Unfortunately there are no good grounds for accepting or rejecting the importance of these differences without further lengthy experiments. Instead of attempting to explain why our results differ from Bough’s, it is possible to argue that they do not differ and that his animals had solved his discrimination by non-depth cues. Certainly we must point out that some of the alternative strategies used by our animals were available to his. For example, he presented no controls for brightness differences in the patterns themselves or in reflections from them, and we found in Experiment 111, which resembles Bough‘s most closely, that one animal was discriminating the successive brightness changes in the reflected polarized light from positive and negative stimuli. And although both of Bough’s animals showed good transfer from the first to the second set of stereograms, we found in Experiment I1 that transfer was good to some stereograms and poor to others. Had we used only one set of transfer stimuli we might have erroneously concluded that we had demonstrated stereopsis. I n Experiment 111, one animal did show excellent transfer to the new stereograms, but was subsequently shown to be discriminating reflections. Lastly, Bough found that his animals scored at chance when tested monocularly and after surgically-induced convergent strabismus. Unfortunately his paper does not state how long the animals were tested under these conditions, and we found that abolishing stereopsis cues sometimes had significant but transient effects on performance, as in Experiment 111. Whatever the explanation for the discrepancies between Bough’s experiments and
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ours one thing is clear: monkeys can detect features other than disparity in projected random-dot stereograms. Unless the random dot matrix is changed on every trial and differential reflections from plane-polarized light are eliminated, or deliberately varied to make them irrelevant, animals which have learned to use disparity information may conceal the effect of a lesion by relying on the other cues postoperatively.
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This research was supported by Medical Research Council Grant G97/397/B. It is a pleasure to thank Dr B Julesz for his comments on the manuscript and for his invaluable suggestions about experimental techniques.
References BARLOW, H. B., BLAKEMORE, C. and PBTTIGREW, J. D. (1967). The neural mechanism of binocular depth discrimination. Journal of Physiology (London), 193,327-42. BISHOP,P. 0. and HENRY, G. H. (1971). Spatial vision. Annual Review of Psychology, 22, I I 9-60. BLAKEMORE, C. (1970). Binocular depth perception and the optic chiasm. Vision Research, 10943-7. BLAKEMORE, C. and HAGUE,B. (1972). Evidence for disparity detecting neurones in the human visual system. Journal of Physiology (London), 225, 437-55. BLAKEMORE, C., FIORENTINI, A. and MAFFEI,L. (1972). A second neural mechanism of binocukdr depth discrimination. Journal of Physiology (London), 226, 725-49. BOUGH, E. W. (1970). Stereoscopic vision in the macaque monkey: a behavioural demonstration. Nature (London), 225, 42-4. HUBEL,D. H. and WIESEL,T. N. (1970). Stereoscopic vision in macaque monkey. Nature (London), 225, 41-2. JULESZ,B. (1964). Binocular depth perception without familiarity cues. Science, 145, 356-62. JULESZ,B. (197 I). Foundations of Cyclopean Perception. Chicago: University of Chicago Press. MITCHELL, D. E. and BLAKEMORE, C. (1970). Binocular depth perception and the corpus callosum. Vision Research, 10,49-54. RICHARDS, W. (1970). Stereopsis and stereoblindness. Experimental Brain Research, 10, 380-8. Received 11 February 1974
