Speeding up visual discrimination learning in cats by differential exposure of positive and negative stimuli

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Behavioural Brain Research, 36 (1990) 1-12 Elsevier BBR 00980

Research Reports

Speeding up visual discrimination learning in cats by differential exposure of positive and negative stimuli P. d e W e e r d , E. V a n d e n b u s s c h e a n d G . A . O r b a n Laboratory for Neuro- and Psychophysiology, Leuven Catholic University, Leuven (Belgium) (Received 10 January 1989) (Revised version received 20 April 1989) (Accepted 10 May 1989)

Key words: Training; Threshold; Orientation discrimination; Cortical lesion; Illusory contour; Cat

We have developed an adaptive training method which considerably reduces the total time required to train cats to threshold in an orientation discrimination task. During training, the animals are given greater exposure in time to the positive stimulus compared to the negative one. Therefore, this method has been coined the differential exposure method (DEM). The greater exposure to the positive stimulus reduces the number of errors an animal commits during training and thereby enhances speed of learning. Indeed, with the DEM, 34 daily sessions sufficed to train cats to threshold for 2 different reference orientations. Furthermore, the DEM was effective not only for simple stimuli such as real bars but also for complex stimuli such as illusory contours. Finally, the DEM was equally effective for training naive cats which had undergone large visual cortical lesions as it was for normal animals.

INTRODUCTION

The vast amount of anatomical and physiological knowledge concerning the cat's visual system (for review see ref. 15) which has accumulated the last 20 years, makes the cat an apt subject for visual behavioural experiments. However, this animal is notoriously difficult to train in such experiments ~. Initially, investigators faced great difficulties with the training of cats, since the powerful operant conditioning techniques proved difficult to apply in this species. Investigators have used mazes, shuttle boxes or monkey-testing apparatuses (for review see ref. 1) adapted to the cat, with varying degrees of success. Only after the development of training and test apparatuses specially designed for the cat, did operant con-

ditioning paradigms become an efficient tool for investigating its perceptual abilities. The jumping stand 1°'13"27the go/no-go runway z2 and the twochoice r u n w a y 4-6 have been used successfully for operant conditioning of visual discriminations. In these devices, the discrimination behaviour closely matches the natural behaviour of the cat, and this certainly contributes to easier conditions of visual discriminations. Unfortunately, test procedures are time-consuming, permitting only a limited number of daily trials to be administered. Furthermore, such testing may lack standardization. To the contrary, the use of a testing chamber with an automatic food dispenser 1 restricts the behaviour of the cat but allows the administration of a large number of daily trials, without interference of the experimenter. The efficiency of

Correspondence: G.A. Orban, Laboratorium voor Neuro- en Psychofysiologie, Katholieke Universiteit te Leuven, Campus Gasthuisberg Herestraat, B-3000 Leuven, Belgium. 0166-4328/90/$03.50 © 1990 Elsevier Science Publishers B.V. (Biomedical Division)

this apparatus in visual discrimination learning resides in the close temporal and spatial contiguity of ~timulus presentation and reinforcement. An additional advantage is the closer control of viewing distance. Procedures such as conditioned visual discrimination 3'9"16"18'19"23'24 and conditioned suppression 7,s have been used in association with this testing box. Despite the progress in the technology of behavioural testing of the cat, some discriminations remain difficult to acquire for this animal, especially if one wishes the subject to achieve differential thresholds. Differential threshold measurements in animals are particularly interesting since they provide an efficient tool for investigating the neural substrates of visual behaviour 2. For example, the effect of visual cortical lesions on visual behaviour of animals is assessed most accurately by comparing threshold performance before and after a lesion in a visual discrimination task. However, training an animal to threshold in a difficult discrimination task can take up to 250 daily sessions of 300 trials and the training of a lesioned animal can be even more prolonged. For most visual scientists, such an investment in time and effort may entail more cost than benefit. Consequently, a training method which pushes the animal to reach threshold in a minimum of dally sessions would overcome a major obstacle in this field of research. Fading in the negative stimulus, by increasing the contrast until it equals the contrast of the positive stimulus, can be helpful in particular visual discrimination tasks, such as shape discriminations I and discrimination between horizontal and vertical gratings ~~.12. However, in our experience, this method has not proved efficient in training cats in line orientation discrimination, even for an orientation difference as large as 30 degrees. We found no difference in training duration between cats trained with the fading procedure and cats which had to discriminate without assistance. Furthermore, in our experiments it is impossible to begin with a larger orientation difference, since the animals have to discriminate orientations around different references. Indeed, overly large orientation differences lead to interference between negative and positive stimuli of

different reference orientations (see Materials and Methods). Therefore, ideally, an efficient training method should teach the cat to discriminate the initial 30-degree orientation difference in a minimum number of sessions. Furthermore, it should be possible to train an animal for increasingly small orientation differences, until the threshold is reached. This method must be effective not only for normal cats but for lesioned cats as well. Finally, discrimination of complex stimuli, such as orientation discrimination of illusory contours, should be taught as efficiently as the discrimination of simple stimuli. We present here a method which fulfills these requirements. It follows the principle of differential exposure of positive and negative stimuli. We developed the differential exposure method (DEM) with 4 cats performing in a line orientation discrimination task. Afterwards, we applied the DEM to the training of 2 lesioned cats and to the training of 2 normal cats working with illusory contours.

MATERIALS AND METHODS

Testing apparatus During an initial shaping procedure24. cats learn to operate a discrimination apparatus designed after BerkleyL Briefly, this apparatus consists of a translucent screen, with one or more slide projectors on one side and a box housing the cat on the other. The cat views 2 stimuli projected onto the screen, through Ple~igtas nosekeys. A separator prevents the eat from seeing both stimuli together. The animal receives a food reward for pushing the nosekey behind which the positive stimulus is projected. In every trial, the positive stimulus is projected behind the left or the right nosekey randomly. A trial starts with the intertdal interval (5 s), followed by a response delay period (RDP) of 0.35 s to enhance attention. The stimuli are presented at the start of the RDP but responses are ignored until after the RDP. Response latencies are measured from the end of the RDP. A trial is completed upon the cat's response, after which the stimuli disappear.

tive stimulus ( S - ) deviates clockwise or anticlockwise from the horizontal and right oblique reference respectively. Except for the decision to conclude training, the same method has been applied in all cases, regardless of stimulus type and whether the animal is lesioned or not. The D E M begins with a block of a fixed num-

Training with the DEM After the shaping procedure, the D E M is used to train the cat in orientation discrimination around the horizontal and the right oblique reference axes. Both real bars and illusory contours are used as stimuli. The reference orientation serves as positive stimulus (S + ). The nega-

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Fig. 1. Sequence of immediate occlusion (A), delayed occlusion (B) and no occlusion (C) discrimination trials at a given orientation difference between the positive (S + ) and the negative (S - ) stimuli. These trials have been taken from the first 3 blocks of the fourth training-session for the horizontal reference orientation of cat 60, depicted in Fig. 3. Every trial starts with the intertrial interval (ITI) during which no stimuli are present. At the end of the ITI, both S + and S - are presented (S) but responses are ignored during 0.35 s. This period is the response delay period (RDP). From the end of the RDP, responses are allowed. The response latencies (LAT) equal the elapsed time between the end of the R D P and the moment at which the cat responded. Correct responses are indicated with R + , incorrect ones with R - . Upon a response, the stimuli disappear. In immediate occlusion trials (A), the S - is occluded (OC) 0.1 s after the end of the RDP. For the most part, the cat will attend the occlusion of S - and respond correctly (trials 17, 18 and 20). Responding before occlusion often leads to incorrect responses (trial 19). Assuming that the animal reached a 75 % correct performance in the immediate occlusion block, 20 delayed occlusion trials will be administered (B), using the same orientation difference. In these trials, the occlusion of S - occurs after a period equal to twice the response latency (LAT) of the previous trial. Therefore, at trial 1 (B), the time period between the end of the R D P and the moment of occlusion (OC) equals twice the response latency (LAT) of trial 20 (A). Again, at trial 2 (B), the time period between the end of the R D P and occlusion equals twice the response latency (LAT) of trial 1 (B), etc. In trials 1 and 2, the cat waits for the occlusion before responding, but the rapidly increasing waiting periods cause the animal to respond before occlusion in trial 3. After reaching a 75 % correct discrimination performance on 20 delayed occlusion trials, 20 trials without occlusion of S - are administered, still without changing the orientation difference. If the criterion is reached again, the orientation difference between S - and S + is reduced and the whole sequence is repeated.

ber of trials, in which S + and S - differ by 30 degrees in orientation and are shown for different durations. In the present study, all blocks consisted of 20 trials and a single session consisted of 15 blocks. However, the number of trials per block can be adjusted, if required, by the behaviour of the animal being tested or the sort of discrimination task. During the first block, the stimuli are presented simultaneously, but S - is occluded after a short period of time, leaving only S + in view. The cat is then administered a block of trials in which the moment of occlusion is gradually delayed. Finally, a block without occlusion of S - is given, after which the orientation difference is reduced and the same sequence is repeated. This sequence assumes that the cat steadily reaches a 75~o correct discrimination performance. Before elaborating upon the sequence of a complete training session, the different stages in delaying the moment of occlusion shall be presented in more detail. As indicated above, the D E M training uses 3 different types of discrimination trials. The first type is a discrimination trial with immediate occlusion of S - (Fig. 1A): both S + and S - are presented for 0.45 s, after which S disappears so that only S + remains present. As in all trials, responses falling within the R D P are ignored. Since the R D P lasts 0.35 s, the delay between the end of the R D P and the moment of occlusion equals 0.10 s. Because the cat has not yet identified S +, a response before the occlusion of S - will have a low probability of being correct (trial 19 in Fig. 1A). Delaying its response until after the occlusion of S - , however, offers the cat an easy discrimination, and therefore has a high probability of being correct (trials 17, 18 and 20 in Fig. 1A). In this way, all cats used in the present study learned rapidly to look through the left and right key in order to see both stimuli and to wait for the cue which permits identification of S +. Cats having difficulties to attend the occlusion-cue may benefit from a shortened delay between the occlusion of S - and the end of the RDP. The second type of discrimination trial is a trial with delayed occlusion (Fig. 1B) of S - . Here, the occlusion at trial N occurs after a period of time

equal to twice the response latency of the previous trial (trial N-l). Again, the cat either waits until Sdisappears or responds before S is occluded. In the former case however, the advantage of an easy discrimination is offset by the disadvantage of rapidly increasing waiting periods (trials 1 and 2 in Fig. 1B). In the latter case, the cat has the advantage of getting rewarded quickly, but at the price of correctly solving a more difficult discrimination (trial 3 in Fig. 1B). Hungry cats are pushed to choose the second alternative, and therefore quickly learn the discrimination. For slow learners, delaying the moment of occlusion at a slower rate than described above can be helpful. The third type of discrimination trial is a trial without occlusion of S - (Fig. 1C). Here the animal must discriminate without any assistance. Summarizing, the D E M comprises 3 types of discrimination trials: (1) discrimination trials with immediate occlusion, which provide the cat with a cue allowing easy identification of S + ; (2) discrimination trials with delayed occlusion, which push the cat to learn the discrimination; (3) discrimination trials without occlusion, which assess whether the cat has effectively mastered the discrimination task. During a training session, the type of discrimination trial as well as the orientation difference are determined by the performance, according to the following rules (Fig. 2). During one block of trials, the orientation difference and the type of discrimination trial remain fixed. In the first block of a session, as after every block following a change in orientation difference, immediate occlusion is administered. If the cat gets a 75~o or better performance in the immediate occlusion block, a block with delayed occlusion follows. Typical for a delayed occlusion block are the long response latencies (1-2 s). Upon reaching a 75 ~o correct or better performance again, a block without occlusion is given. If the 75~o correct criterion is attained in this block without occlusion, the orientation difference is reduced. If the cat reaches an orientation difference which is too difficult, the animal will first oscillate between immediate and delayed occlusion, then fall into quickly guessing. As a consequence, the 75 f'~;,cot-

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I" Fig. 2. Block diagram, showingthe rules governing the DEM training. Every session starts (START) with a block of 20 trials at a particular difference (DIFF X). Every one of the 5 stages shown in the figure refers to a block of 20 trials. A descending series (DIFF X - AX) occurs if the pathway bearing 3 consecutive plus-signs is followed ( + : 75 % correct criterion reached). An ascending series (DIFF X + AX) occurs if the pathway marked by 3 consecutive minus-signs is followed ( - : 75 % correct criterion not reached). In the former case, the orientation difference (DIFF X) will be reduced, in the latter case, it will be increased. The rule for determining AX is described in the text.

Fig. 3. Results obtained from a single DEM training session (session 4 in Fig. 4B). Orientation differences and type of discrimination trial (1: immediate occlusion; 2: delayed occlusion and 3: no occlusion) are plotted as a function of the successive blocks of the session. Closed symbols refer to blocks in which the 75 % correct criterion was reached. Open symbols refer to blocks in which the 75 % correct criterion was not reached.

rect criterion will not be reached, even in blocks with immediate occlusion. If this is the case during 3 consecutive immediate occlusion blocks, the orientation difference is increased. The last orientation difference at which the cat fails to reach the 75% correct criterion without occlusion will be the first orientation difference o f the next session. The very first session of the training of a reference orientation always starts at an orientation difference of 30 degrees. Finally, it must be mentioned that the adaptation o f the orientation difference depends on the current value of the orientation difference. The current orientation difference ( D I F F c ) is increased or decreased by respectively multiplying or dividing D I F F c by the factor (1 + 0 . 5 / x / ( D I F F c ) ) . This rule avoids too large absolute changes in orientation difference during the first training sessions and too small changes during the last sessions, while approaching the threshold. Fig. 3 gives an example o f training results obtained from a single session.

In our experience, the training with immediate and delayed occlusion can be stopped after 2 consecutive sessions without improvement at an orientakion difference of 8 degrees or smaller for normal (Figs. 4 and 5) and lesioned cats (Fig. 6A) working with a bar. Since progress near threshold in orientation discrimination training with illusory contours was supposed to be slower than for real lines, we only stopped the training after 4 consecutive sessions without improvement (Fig. 6B). After stopping the training with immediate and delayed occlusion, a large number o f no occlusion trials are administered to assess the effectiveness o f the previous training. This is accomplished by means of threshold measurements with a 73.5 % correct Wetherill and Levitt 26 staircase. Training was considered complete when thresholds remained stable during 5 sessions for both the horizontal and the right oblique reference orientations. Notice that the staircase threshold measurements during the final training phase can

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compensate for halting the training with immediate and delayed occlusion too early (Fig. 4C). In the first test following training, 75~o correct thresholds for both references are determined with the method of constant stimuli. These thresholds closely match the plateau of staircase thresholds reached during the final training stage (Fig. 4D). We introduced a Wetherill and Levitt 26 staircase procedure during the final training phase since a staircase allows one to track small daily improvements in threshold performance for which the constant stimuli method is insensitive (De Weerd, Vandenbussche and Orban, in preparation). In the staircase, the orientation difference is adapted to the performance according to the following 3 rules: (1) The orientation difference is increased after an incorrect response, a correct response followed by an incorrect response and 2 correct responses followed by 2 incorrect responses. (2)The orientation difference is decreased after 3 correct responses or after 2 correct responses, followed by an incorrect response and a correct response. (3) The magnitude of the increase or decrease of the orientation difference is calculated by respectively multiplying or dividing the difference with a factor 1.2. This factor was experimentally determined to be optimal. The obtained thresholds track the 73.5~o correct level, as predicted by Wetherill and Levitt 26. We chose this criterion since less stringent criteria give less reliable thresholds in the cat. On the other hand, we had to avoid too-stringent criteria as well. Lesioned cats are often unable to reach performance levels much above 75 ~o, even if the stimulus difference is maximized. Therefore, using staircases converging upon performance levels above 75~o makes little sense, at least if one wishes the method to be appropriate for both normal and lesioned cats. The staircase procedure is used during the final training phase as follows: during a single session, two thresholds are measured, one for each reference orientation. One threshold measurement takes 150 trials and starts at the threshold level determined in previous sessions. Each pair of thresholds for one reference orientation is preceded by a block of 20 trials with a fixed

orientation difference, 1.5 times larger than the expected threshold. Because of the proportional rule for adapting the orientation difference to performance, the geometrical mean of all reversals is used in calculating the threshold. The constant stimuli testing procedure In the constant stimuli testing session, a single threshold is determined for both reference orientations. For each reference, 5 orientation differences are centered around the expected threshold, as determined with the staircase method during the final training stage. Therefore, one session consists of 10 blocks of 30 trials, every block presenting a fixed orientation difference from a particular reference orientation. The differences for the two references are interleaved. From session to session, the sequence of blocks is changed following the rules of a Latin square, to exclude order effects. Thresholds (75~o correct) are calculated by linear regression after a Z-transformation of the proportions of correct responses.

RESULTS Eight cats were trained in orientation discrimination with the DEM. Four of them were normal cats, trained in line orientation discrimination. Two naive cats, having no experience with the test situation, underwent a large visual cortical extrastriate lesion and were then trained in line orientation discrimination with the DEM. Finally, two normal cats were trained with the DEM in orientation discrimination of illusory contours. Training normal cats in line orientation discrimination Fig. 4 shows the results of cat 60, our fastestlearning subject so far. This animal needed a total of only 20 daily sessions to achieve threshold for both the horizontal and the right oblique reference. The total duration of training includes training with the D E M for both references (Fig. 4A,B) as well as the staircase training sessions required to achieve stable performance at threshold level (Fig. 4C). The DEM learning curves in Figs. 4A and 4B show for each session the smallest orientation difference at which the cat

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Fig. 4. Time course for the training of cat 60. This normal cat was trained in real line orientation, first for the right oblique (RO) and afterwards for the horizontal (H) reference orientation. The stimulus consisted of a 12-degrees long and 0.2degrees wide bar of 15 cd/m2 luminance in all experiments. Background luminance was maintained at 0.5 cd/m2. All luminances were measured using a Minolta luminance meter. Squares represent performance level for H orientation discrimination; circles represent performance level for RO orientation discrimination. In both Fig. A and B, the last orientation difference of every session at which the 75~o correct criterion was reached, is plotted against session number. In C, the just noticeable differences (JNDs) for H and RO reference orientations, obtained in an interleaved fashion during the final training phase with the staircase procedure, are plotted as a function of session number. Figure D shows the mean of 5 interleaved JNDs for both references, obtained during the constant stimuli test. Error bars indicate standard deviations. reached a 75~o correct performance in a block of trials without occlusion, plotted as a function of session number. The rapidity of learning during the first few sessions is striking: the orientation difference could be reduced by more than 20 degrees in 6 sessions. The learning curves of Fig. 4 show slowing d o w n only for differences below 10 degrees and suggest that the real threshold training begins only at relatively small orientation differences. Learning difficulties above orientation differences o f 10 degrees probably reflect problems with discrimination learning per se, rather than sensorial shortcomings. Although not all cats learned as quickly as cat 60, the above observations hold for other cats, as attested by Fig. 5. Fig. 5 shows the D E M learning curves for normal cats, for the reference orientation which was trained first. N o n e o f the 4 cats

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Fig. 5. DEM training curves for the first trained reference in line orientation discrimination in 4 normal cats. The final staircase training phase is omitted. Same conventions as in Fig. 4. needed more than 8 sessions to reach a 15-degree orientation difference, although 2 o f the 4 cats required 5 sessions to master the initial 30-degrees difference. The rapid improvement of the first 4 - 8 sessions is then followed by a slower improvement for orientation differences smaller than 10-15 degrees. In normal cats, there is no significant difference in training duration between the reference orientation which is trained first, and the reference orientation which is trained second. Taking together the results o f the 4 normal cats, trained in line orientation discrimination, 3 general observations emerge. Consider first the total training duration required to obtain stable performance at threshold level. With the D E M , an average of only 34 sessions is needed to train normal cats working with a bar to threshold for horizontal and right oblique reference orientations. The mean thresholds of both reference orientations range between 4.3 and 12.3 degrees. Second, the range of training durations extends only from 20 to 47 sessions. This range is small enough to permit speed o f learning to become a measurable parameter. This is particularly valuable in lesion studies, since this allows one to assess the effect of visual cortical lesions not only on performance level, but also on speed o f learning. Third, the learning curves produced by the D E M have a standard shape, consisting of 3 parts. The first part is a region o f no improvement (plateau) which extends from 1 to 5 sessions in

normal cats. The second part is a region of rapid improvement during the next few sessions, followed by the third part which is a region of asymptotic improvement near threshold. For a bar, the transition between the last two regions occurs in the range of 10-15 degrees orientation difference. Fig. 5 also demonstrates that the quicker normal cats working with a bar attain the 15-degrees orientation difference, the more rapidly they reach the learning curve asymptote. This characteristic of the D E M offers the possibility of screening a large number of cats in 3 or 4 preliminary sessions, then selecting the best of the group for experimentation. Furthermore, changes of the standard shape of the learning curves themselves can be the subject of study.

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Training lesioned cats in line orientation discrimination The standard properties of the D E M training invites one to study the effect of visual cortical lesions in terms of other parameters in addition to performance level. Not only can the final postoperative threshold be studied, but also the postoperative training curves required to achieve these thresholds can be investigated. Therefore, a lesion effect can be described in terms of threshold magnitude, training duration, and learning curve shape for both references. This has been confirmed by the learning curves of 2 naive cats (cats 61 and 62) which were trained after a large (bilateral) cortical ablation, involving areas 7, 19, 20, 21 and the 3 medial suprasylvian areas (VLS, AMLS, PMLS ; ref. 17). As shown in Fig. 6A, the D E M learning curves for the reference orientation which was trained first are shifted to the right, due to the slower learning during the first 10 sessions. Total training duration, however, is within the normal range. Since the cats are still being tested, we do not yet have histological control of the lesions. Training normal cats in illusory contour orientation discrimination The D E M is effective not only for simple stimuli such as bars, but also for very complex stimuli such as illusory contours. Indeed, we were successful in training two cats very rapidly in

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orientation discrimination of 2 types of illusory contours comparable to those used by Vogels and Orban 25 in humans. In one stimulus the end points of the inducing circle hafts are separated by a 1.2-degree wide gap. In the other stimulus, the endpoints of the inducing circle halfs are shifted along the illusory contour. For a d e t ~ d description of the stimuli used in the cats, see ref. 14. The complete training of one cat trained with the second type of illusory c o n t o ~ is presented in Fig. 6B. Noteworthy is the fact that with the

classic fading in and out techniques, cat 54 was unable to learn orientation discrimination of this type of illusory contour 14.

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DISCUSSION The DEM places a new tool at the disposal of animal psychophysicists. Before 1970, training cats in visual discriminations was difficult, since no testing apparatus was then available to train these animals in an operant discrimination paradigm efficiently. The development of such apparatuses, especially a visual discrimination box 1 adapted to the cat, proved to be a major step forward in the training and testing of these animals. Discriminations were learned more quickly and more daily trials could be administered in a standardized manner. The DEM extends this line of improvement, allowing quick visual discrimination learning following a standardized program. The contribution of the DEM resides in the efficient teaching of difficult visual discriminations. Visual discriminations can be difficult either because of the nature of the task or because of the state of the animal itself. Orientation discrimination at the threshold level is a difficult task, especially if complex stimuli are used, and even more so if the animal has been lesioned. The power of the D E M is clearly illustrated by comparing its results with training results previously obtained in our laboratory ~6'23'24. The old training method is briefly described in order to further appreciate the progress offered by the DEM. As in the DEM, training started at a 30-degrees orientation difference. Both stimuli remained present until a response occurred. Therefore, this method will be referred to as the Equal Exposure Method (EEM) throughout the remainder of the discussion. When the cat reached an 80% correct performance in a 300-trial session, the orientation difference was decreased in the next session. The magnitude of the adaptations was 2 degrees for orientation differences above 10 degrees and 1 degree for differences below 10 degrees. When the animal failed to reach this criterion during 3 consecutive sessions, the orientation difference was increased to the smallest difference at which the cat had previously

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number

Fig. 7. Justnoticeabledifferences(JNDs) as a functionofthe total number of training sessions for DEM (stippled area) and EEM (hatchedarea). For the DEM,this numberincludes the training sessions for each reference and the complete interleaved staircase measurements of the final training phase, required to obtain stable thresholds (see parts A, B and C in Fig.4). For the EEM, this number includes the training for each reference and, in most instances, 10 constant stimuli sessions required to obtain a first stable estimate of the thresholds for both references. For both DEM and EEM, the threshold corresponding to every training duration is the mean threshold for the horizontal and right oblique references. These thresholds are constant stimuli thresholds obtained in the first test after the training had been completed. Notice that for cat 58, the threshold increased relative to the training curve asymptotein Fig. 5. Indeed, during the final trainingphase, thresholds of cat 58 increased rather than decreased as for cat 60 (Fig.4). See text for abbreviations. reached the 80% correct criterion. Training stopped if the animal reached the same asymptotic orientation difference 3 times. Aside from the absence of a cue for identifying S +, the slow adaptation of the orientation difference to the performance is an additional major difference with the DEM. The mean number of sessions required to reach threshold in real line orientation discrimination with the EEM amounted up to 170 sessions of 300 trials. This number includes the sessions to train both the horizontal and right oblique reference orientations and 10 constant stimuli sessions required to obtain stable thresholds. With the DEM stable thresholds are achieved for both references after a mean number of only 34 sessions. Therefore, the training to threshold in real line orientation discrimination was reduced by a factor of 5 in normal animals. The power of the D E M is further illustrated by

10 its efficiency in training animals which are even more difficult to train than normals. Indeed, the training of lesioned cats, having received no preoperative training whatsoever, was completed within the training duration range of normal animals. This is surprising in light of the size of the lesion, involving areas 7, 19, 20, 21 and the areas of the medial bank of the lateral suprasylvian sulcus 17. Training a naive animal with such a lesion would prove a near-impossible task. Yet, the DEM allows doing just that. The quick and standardized training of both normal and lesioned animals with the DEM, offers new possibilities in evaluating the effect of visual cortical lesions. The effect of a lesion now can be assessed not only on the final performance level, but also on speed of learning and on the shape of the learning curve. Our results suggest that the effect of visual cortical lesions on performance level within one animal will also be measured more efficiently, especially in cases where considerable retraining is necessary. In such a case, the relatively rapid retraining will allow one to measure thresholds during recovery. Before, recovery and retraining were confounded because of the lengthy retraining. Thus with the DEM, pre- and postoperative training curves can be compared, with respect to length and shape, as well as pre- and postoperative performance level during and after recovery. Finally, the DEM is very valuable for investigating the visual discrimination capacities of cats with more complex stimuli. With the DEM, we succeeded in training cats in illusory contour orientation discriminations in which the classic fading in and out procedures had failed TM. The 'working substance' of the DEM is the occlusion of S - . Trying to point the animal's attention to S + first and gradually bringing in S - , is an approach previously tried out by others 1'~a2. Terrace 2°'21 used different procedures to train pigeons at color (and other) discriminations, of which one resembles the DEM. The task of the pigeon was to discriminate between 2 discriminanda, each of them presented successively for 60 s in the final testing situation. In a first training phase, however, only S + was present. A second training phase introduced S -

with gradually increasing intensity and duration. The third phase included unassisted discrimination between the successively presented discriminanda, being of equal duration and intensity. This procedure was compared to a training method in which S + and S - were presented successively with equal duration and intensity from the beginning. In the latter case, any response to S - was punished by prolonging the duration of S - by 30 s. The former method results in errorless discrimination learning so that S - is a neutral stimulus. The latter method entails numerous errors, so that S - may become aversive. Together with the different generalization curves of S + and the differential influence of drugs upon discrimination performance21, this suggests that both training methods initiate different types of learning. There are more differences between the DEM and Terrace's errorless discrimination learning2°,2~ than just the difference between simultaneous and successive stimulus presentation. Firstly, the D E M does not use an increasing contrast of S - . In our experience, an initial difference in contrast between the discriminanda does not enhance discrimination learning. Secondly, in the DEM both S + and S - are presented from the beginning, though initially the exposure of S - is very short. In the DEM, presentation of S + alone is insufficient to speed up subsequent discrimination learning. Indeed, although Terrace 2°'2~ reports that errorless discrimination learning causes better stimulus control, he does not report increases of learning speed. Thirdly, the increase of exposure duration of S - , is determined by the animal's performance, whereas Terrace 2°'2~ used predefined schedules. Finally, contrary to Terrace's 2°m errorless discrimination learning, the DEM is used to train the animal until the threshold is reached. The initially differential exposure of S + and S - is repeated after every performance-dependent change in orientation difference, which facilitates rapid discrimination learning. Despite these differences, some of Terrace's 2°'21 observations help to understand why the DEM is so effective. The clear 'instruction' as to which stimulus is S + by means of occlusion

11 trials, prevents the cats from making excessive errors. Furthermore, the adaptive nature of the DEM, which adapts the difficulty of the task to the current performance level, in itself limits the number of errors an animal commits during training. Conversely, the unassisted training with the EEM necessarily causes a large number of errors each time the orientation difference is decreased. As a consequence, S - is a neutral stimulus in the D E M but may be rather aversive in the EEM. In the EEM, emotional reactions and lack of attention due to the large number of errors might hinder progress during training. These factors are absent in the DEM. The success of the DEM with complex stimuli points to another characteristic of the learning with this method. The rapid training in illusory contour orientation discrimination shows that the different sorts of occlusion trials are sufficient for the cat to find the characteristic that distinguishes the discriminanda. Hence, fading procedures, which aim to teach the animal with simple stimuli the dimension on which the in-fading complex stimuli will differ, are no longer required. The D E M proved to be much more efficient than the fading in and out techniques TM. Training procedures in animal psychophysics must provide the animal with unambiguous information as to which stimulus is S + : if this information is clear enough, the animal will go on to apprehend how the discriminanda differ. In conclusion, although we still do not understand completely by what mechanism the DEM works, the method is clearly effective. This has been demonstrated by the rapidity with which both normal and lesioned cats were trained to threshold in a line orientation discrimination task. Additionally, normal animals learned illusory contour orientation discriminations at an unprecedented rate. These results promise that the D E M could become a very useful new tool in animal psychophysics. ACKNOWLEDGEMENTS

We are very grateful to J.M. Sprague, for making the lesions in cat 61 and 62 and for comments on earlier versions of the manuscript. We also

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