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Effect of location of the conditional cue on conditional discrimination learning by monkeys (Macaca mulatta)
LEARNING
Effect
AND
MOTIVATION
of Location
Discrimination
(1970) 1, 207-217
of the Conditional Learning
by Monkeys
ALLAN Brown
Cue on Conditional (Macaca
mulatta)’
M. SCHRIER University
Monkeys were trained on conditional discrimination problems consisting of a pair of objects differing in form, or, depending on the problem, in size. The conditional cue was color of the test tray for half the Ss and color of the pair of objects for the other half. The discriminative response was displacement of one of the objects. Original and reversal learning were generally better when the conditional cue was color of the objects. This agrees with previous findings that monkeys are more likely to attend to stimuli that they touch when responding than those they do not touch.
Warren (1960, 1964) studied conditional discrimination learning by rhesus monkeys in the Wisconsin General Test Apparatus (WGTA) and noted, among other things, that the learning rates were considerably faster than were found in previous studies with monkeys (Chow, 19.52; Pribram & Mishkin, 1955) and with children (Gollin & Liss, 1962). Warren thought that these differences might have been based on the fact that his Ss were experimentally “sophisticated,” that is, that they had had a great deal of training on other types of discrimination tasks, or that they were trained on all components of the task from the beginning of training. However, the difference in results may have another basis. In Warren’s studies the conditional, or successive, cue was the color of the objects used as the simultaneous cues (“objects-conditional” procedure), so that if both objects were one color, object A was correct, but if both were another color, object B was correct. With this procedure, Ss were forced to touch both the conditional and the simultaneous cues when responding. In contrast, in earlier studies the typical procedure was such that Ss touched either none of the cues when responding (Gollin & Liss, 1962) or ’ This research was supported by research grants from the National Science Foundation (GB-7590) and the National Institute of Mental Health (MH-15389 and MH-07136), U. S. Public Health Service. The author gratefully acknowledges the assistance of Gertrude A. Trudel and David Henshaw in carrying out Experiment 1 of this research. Requests for reprints should be sent to the author, Department of Psychology, Brown University, Providence. Rhode Island 029 12. 207
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M. SCHRIER
only the simultaneous cues (Chow, 1952; Pribram & Mishkin, 1955). For example, in a number of studies done in the Wisconsin General Test Apparatus (Chow, 1952) the conditional cue was the color of the test tray, so that if the test tray was one color, object A was correct, but if it was another color, object B was correct. With this procedure, Ss touched only the simultaneous cues. There is now a great deal of evidence from experiments with monkeys (Meyer, Treichler, & Meyer, 1965; Stollnitz, 1965) and children (Wunderlich, Nazzaro, & Youniss, 1968) demonstrating that spatial separation of the stimulus and the response, even when as little as + in., retards discrimination learning. Furthermore, it appears that the reason for this is that these Ss are more likely to attend to those parts of visual stimuli that they touch when responding than to parts they do not touch. If, then, Ss are more likely to attend to conditional cues if they touch them, then the faster learning rates obtained by Warren might also have been the result of his use of the objects-conditional procedure in contrast to the use of the tray-conditional procedure, or the equivalent, in the studies which preceded his. The primary concern in the two experiments reported here was a comparison of the effects of the tray-conditional and objects-conditional procedures on conditional discrimination learning by monkeys. The prediction was that the objects-conditional procedure would facilitate learning, Experiments 1 and 2 differed primarily in the order in which the conditional discrimination problems were presented and in the number of problems. EXPERIMENT
1
Method Subjects The Ss were 14 wild-born rhesus monkeys (Mucaca mulatta). Six of these (Group 1) were between 5 and 6 yr of age and had a mean weight of 13 lb. These Ss had previously been given extensive object-discrimination learning-set training (6600 trials) and training (600 trials) on repeated reversals of a single object-discrimination problem (Schrier, 1966a,b). Eight of the Ss (Group 2) were about 4 yr of age and had a mean weight of 12 lb. This group had also previously been given object-discrimination learning-set training (2300 trials; Vaughan & Schrier, 1970). Apparatus The apparatus, a WGTA with motor-operated screens, has been described before (Schrier, 1961). The test tray had two foodwells, 6 in.
CONDITIONAL
DISCRIMINATION
LEARNING
209
apart center to center, with a 6-in.-high clear-plastic partition between them. The partition was designed to reduce the possibility of response correction and to prevent collisions between objects when one was displaced. Test tray color could be altered by changing Masonite inserts all of which had holes corresponding to the two foodwells and were about 13.5 in. wide and 7.75 in. deep. These test-tray inserts were painted red, green, yellow, blue, or gray. Three pairs of objects, all cut out of OS-in.thick wood, were used for the first conditional problem (circle-triangle problem); one member of each pair was a 2-in.-diameter circle and the other was an equilateral triangle with 1.5-in. sides; one pair of objects was painted red, a second pair green, and the third gray. Three pairs of squareshaped objects, also cut out of 0.5-in.-thick wood, were used for the second conditional problem (large-small problem); one member of each pair had 3-in. sides and the other had 2-in. sides; one pair of objects was painted blue, a second pair yellow, and the third gray. One Noyes 75-mg sucrose pellet was used as the reinforcer on each trial. Procedure Pretraining. The general conditions of testing in the present experiment were much like they were in previous studies in which the Ss of Group 1 were used, so they required little pretraining. Their pretraining consisted of a different 30-trial junk-object discrimination problem each day for the 5 days preceding the experiment using standard learning-set procedures (Schrier, 1966a). All Ss were responding to objects quickly and consistently by the end of this pretraining. The prior learning-set training of Group 2 was not as extensive as that of Group 1 and was carried out under conditions which required either a somewhat different mode of responding to the objects or a different locus of presentation of the reinforcer than was to be used in the present experiment. Therefore, the Ss of this group were given four 1O-trial object discrimination problems per day for the 18 days preceding the experiment, using the standard procedures. By the end of pretraining, all of these Ss were also responding quickly and consistently. Their learning-set performance was still well below that of Group 1, but it would have taken many weeks of further training to bring them up to the same level. It should be emphasized that the lower level of performance was primarily a matter of their having had a smaller amount of training on the learning-set task. Training. The Ss in each group were divided into two equal subgroups, one of which (the objects-conditional subgroup) was given conditional discrimination training with the conditional cue being color of the objects, whereas the other (the tray-conditional subgroup) was given this training with1 the conditional cue being color of the test tray. The subgroups of
210
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M.
SCHRIER
Group 1 were matched on the basis of their learning-set performance in the previous experiments (Schrier, 1966a, 1966b). During the last 5 weeks of this previous training, the objects-conditional subgroup made 96% correct responses and the tray-conditional subgroup made 97%. The subgroups of Group 2 were matched on the basis of their learning-set performance during pretraining. Both subgroups made 87% correct responses during pretraining. The Ss of each group were first trained on the circle-triangle problem. If the conditional cue was red the circle was correct, but if it was green the triangle was correct. For the objects-conditional subgroups, both objects were either red or both were green and the test tray was always gray, whereas for the tray-conditional subgroups, the test tray was either red or green and both objects were always gray. All Ss were given 40 trials a day, 5 days a week. Each of the four possible configurations-red or green conditional cue with the positive object on the left or right side-was used on 10 trials each day in a random order that varied daily. No correction was allowed; the intertrial interval was I5 sec. After 10 days of this training, it was clear that there was no significant difference in the performance of the subgroups, so it was decided to reverse the conditional discrimination, with red now signaling that the triangle was positive and green that the circle was positive. Following 10 days of this reversal training, the large-small problem was presented for 10 days. A blue conditional cue indicated that the large square was positive, whereas a yellow cue indicated that the small square was positive. All other conditions were the same as they were for the circle-triangle problem. Because the performance of the tray-conditional subgroup was now found to be significantly lower than that of the objects-conditional, the former subgroup was given an extra 5 days of training in order to bring its performance level up to that of the objects-conditional subgroup. Both subgroups were then given 10 days of reversal training. One assistant tested two Ss in each subgroup of Groups 1 and 2 and a second assistant tested the remaining Ss. RESULTS
The percentage of correct responses made by the two subgroups during the first 10 days of training is shown separately for each problem and each group in Fig. 1. Reversal performance is similarly shown in Fig. 2. The general picture is one of poorer performance by the tray-conditional subgroup than by the objects-conditional. For purposes of statistical analysis, an arc sine transformation was performed on the proportion of correct responses made by each S each day. The transformed scores for the first 10 days on each problem and on its reversal were analyzed in four mixed within-Ss-between-Ss analyses of
CONDITIONAL
DISCRIMINATION
CIRCLE-TRIANGLE
I
2 3 4
5 6
211
LEARNING
LARGE-SMALL
7 6 9 IO I
2 3 4
5 6
7 6
9 IO
DAYS FIG. 1. Learning of the circle-triangle (top- and bottom-left panels) and large-small (topand bottom-right panels) conditional discrimination problems by the tray-conditional and objects-conditional subgroups of Groups 1 and 2.
variance in which the main effects groups), and Practice (days). In order N per subgroup, one S, chosen at subgroup of Group 2. Cue Location
were Groups, Cue Location (subto meet the requirement of an equal random, was eliminated from each (df= A) was not significant for the
FIG. 2. Learning of the reversals of the circle-triangle (top- and bottom-left panels) and large-small (top- and bottom-right panels) conditional discrimination problems by the trayconditional and objects-conditional subgroups of Groups 1 and 2.
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SCHRIER
original learning of the circle-triangle problem, but was highly significant for the reversal of this problem (F = 48.8, p < .OOl) and for the original learning (F = 50.5, p < .OOl) and reversal (F = 15.8, p < .Ol) of the large-small problem. Practice (df = A) was significant in all four analyses (all Fs > 25.8, all ps < .OOl). A significant interaction of Practice and Cue Location was obtained for the reversals of both the circle-triangle (F = 2.00, df= A, p < .05) and large-small problems (F = 2.87, df = &, p < .Ol), reflecting a tendency for the differences between the subgroups to diminish with practice. This seemed to occur more quickly in Group 1 than in Group 2 and, indeed, the triple interaction, which would test such an outcome, was significant for the reversal of the circletriangle problem (F = 2.56, df = &, p < .02), though it was not for the reversal of the large-small problem. The Groups term was not significant in any of the analyses, indicating that the over-all performance of the two groups was similar. The interaction of Cue Location and Groups was significant for reversal learning of the circle-triangle problem (F = 7.29, df= +, p < .05) and original learning of the large-small problem (F = 9.12, df = i, p < .02). Both of these interactions indicate that the difference in performance of the two subgroups was reliably smaller for Group 1 than in Group 2. Nevertheless, as follow-up analyses of variance conducted on the transformed data of the individual groups showed, the smaller as well as the larger difference between subgroups was significant for both reversal learning of the circle-triangle problem (Group 1: F = 17.5, df = 4, p < .02; Group 2: F = 24.3, df = Q, p < .Ol) and original learning of the large-small problem (Group 1: F = 18.0, df = i, p < .02; Group 2: F = 58.3, df= Q,p < .OOl). Additional analyses of variance, in which the main effects were Cue Location, Practice, and Problems (circle-triangle vs. large-small), were conducted on the transformed data for the two groups combined. There was no significant difference in over-all performance on the two problems or in rate of learning them during either original learning or reversal learning. There was no significant interaction of Cue Location and Problems during either original learning or reversal learning. EXPERIMENT
2
One question raised by the results of Experiment 1 was why cue location had a reliable effect on original learning of the large-small problem, but not the circle-triangle problem. The problems were learned at about the same rate, so problem difficulty did not seem to be important. Perhaps order of presentation of the problems influenced the outcome. Since cue location influenced reversal of the circle-triangle problem, which immedi-
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DISCRIMINATION
LEARNING
213
ately preceded the large-small problem, it is possible that there was some carry-over of the effect from the reversal. It is also possible that the difference in outcome for the original learning of the two problems had something to do with the particular stimuli employed. Both of these possibilities were investigated in Experiment 2. The two problems used in the first experiment were presented in the reverse order to a new group of animals with the conditions otherwise the same. In addition, a second conditional form-discrimination problem was presented with the conditional-cue colors as well as the form cues different than those for the first problem. Method Subjects
The Ss were six wild-born rhesus monkeys, about 3.5 to 4 yr of age with a mean weight of 10.8 lb. Each S had been given a series of 30-trial form- and color-discrimination problems (two problems per day) until it reached a criterion of 90% or more correct responses on 5 out of 6 consecutive days on each type of problem. This required a mean of 2882 trials including those on criterion days. Apparatus
In addition to the two problems used in Experiment 1, a third conditional discrimination problem (rectangle-oval problem) was presented in Experiment 2. The objects, ovals and rectangles, were again cut out of 0.5-in.-thick wood and had an area of approximately 4 sq in. The conditional-cue colors were brown and lavender. Procedure
The Ss were first given 4 weeks. 5 days per week, of learning-set training, with junk objects used as the stimuli. The conditions were the same as those for the pretraining of Group 2 in Experiment 1. Training. The Ss were equally divided into an objects-conditional subgroup and a tray-conditional subgroup (the term, “subgroup,” is used for the sake of consistency with Experiment l), which were matched on the basis of pretraining performance. Both subgroups made slightly more than 86% correct responses during pretraining. All Ss were trained successively on the large-small problem, a reversal of this problem, the circle-triangle problem, the rectangle-oval problem, and a reversal of this last problem. Each training condition was continued until S made at least 95% correct responses on 4 consecutive days. The Ss were tested by one Pretraining.
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ALLAN
M. SCHRIER
assistant who did not test in Experiment 1. All other conditions were the same as they had been in Experiment 1.
of training
RESULTS
The results are summarized in Table 1 which shows the percentage of correct responses made by the subgroups during the first 5 days of training under each training condition. None of the Ss completed a training condition in less than 5 days, counting the criterion days. The results were generally quite similar to those of Experiment 1. Analyses of variance in which the main effects were Cue Location and Practice (Days l-5) were conducted on the arc sine-transformed proportions of correct responses. The performance level of the objects-conditional subgroup was significantly higher than that of the tray-conditional during the first 5 days of both the original learning (F = 29.8, d’ = 4, p < .Ol) and the reversal (F = 27.8, df = 4, p < .Ol) of the large-small problem. Practice (df = &) was highly significant for original (F = 20.1, p < .OOl) and reversal learning (F = 62.3, p < .OOl) of this problem, but, as in Experiment 1, the interaction of Practice and Cue Location was significant only for reversal learning (F = 12.1, df = &, p < .OOl). This interaction again reflected a diminishing effect of cue location with practice. Only the Practice term was significant (F = 10.5, df = &,p < .OOl) in the analysis of the results for the learning of the circle-triangle problem. As in Experiment 1, the difference in performance of the subgroups. on the circle-triangle problem was in the expected direction. A f test conducted on the combined data of the animals of both experiments for the first 5 days on this problem fell just short of significance (r = 1.89, df = 18, p < .lO). The results for the rectangle-oval problem were about the same as those in Experiment 1 for the circle-triangle problem. The difference in TABLE 1 Percentage of Correct Responses (%) Made by the Tray-Conditional and Objects-Conditional Subgroups During Days l-5 on Each Training Condition in Experiment 2 Problem= Group Tray Objects
L-S 78 91
n L-S = large-small;
L-S Reversal 59 87 C-T = circle-triangle;
C-T
R-O
a7 94
93 94
and R-O = rectangle-oval.
R-O Reversal 80 90
CONDITIONAL
DISCRIMINATION
LEARNING
215
correct responses made by the two subgroups during original learning was not significant, but the difference during reversal learning was significant (F = 14.9, df= 3, p < .025). Practice (df= &) was significant during both original (F = 45.8, p < .OOl) and reversal learning (F = 79.1, p < .OOl), but the interaction of Practice and Cue Location was significant only for reversal learning (F = 3.95, df= &-, p < .025). The arc sine-transformed scores for original learning of each problem were compared with the corresponding scores for each of the other problems in separate analyses of variance in which the main effects were Cue Location, Practice (Days l-5), and Problems. As in Experiment 1, in no case was there a significant interaction of Practice and Problems, indicating that there was no significant difference in rate of learning of the large-small problem and either of the form problems. However, the main effect of Problems was significant when the large-small problem was compared with both the circle-triangle problem (F = 24.5, df = 4, p < .Ol) and with the rectangle-oval problem (F = 19.9, df= a, p < .025). In both instances, performance was poorer on the large-small problem than on the form problem. However, in both cases the difference in performance on the two problems was considerably greater for the tray-conditional subgroup than for the objects-conditional (Table 1). This suggests an interaction of Cue Location and Problems and this was confirmed in the analyses (for the large-small vs. circle-triangle problem comparison, F = 8.72, df = f, p < .05, and for the large-small vs. rectangle-oval problem comparison, F = 7.81, df= S, p < .05). DISCUSSION
The results of both of the experiments reported here indicate that monkeys are more likely to attend to conditional cues when they are spatially contiguous with the locus of response than when they are not. Thus, the same rule seems to apply to placement of conditional cues as to placement of the simultaneous cues in simple two-choice discriminations (Meyer, Treichler, & Meyer, 1965; Stollnitz, 1965) and, presumably, also of the simultaneous cues in conditional discriminations. The superiority of the objects-conditional procedure is especially striking considering that the area devoted to the conditional cue was at least six times greater with the tray-conditional procedure. Also, the present results reemphasize what the studies involving two-choice discriminations have shown-that even small differences in spatial separation of cue and response can have a marked negative effect on performance. Differences due to cue location were consistently reliable for reversal learning but not for original learning, in which case the difference was
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SCHRIER
significant for the large-small problem but not for the circle-triangle problem. It is not clear what the basis is for this outcome, though differences in problem difficulty and order of presentation of the problems seem to be ruled out. The fact that cue location had no reliable effect on the original learning of either of the form problems might suggest that this represents the general case for this type of problem. Such a conclusion would, of course, require considerably more data than was provided by the present experiment, especially because no striking differences in saliency of form and size cues have been reported for monkeys as yet (Meyer, Treichler, & Meyer, 1965). Also, it would not appear reasonable to discount altogether the cue location variable in the case of original learning of conditional form-discrimination problems, because the difference between subgroups was in the expected direction in all instances and, overall, was very close to significance for original learning of the circle-triangle problem. A number of studies (French, 1965) carried out some years ago showed that some monkeys and apes can, after extensive training, learn higherorder conditional problems, such as Weigl-type oddity. Perhaps these tasks would have been less formidable for such animals if an objects-conditional procedure and other procedures which have subsequently been found to facilitate conditional-discrimination learning (Warren, 1964) had been employed. Experiment 1 suggested that conditional-discrimination performance was influenced by prior training, though the results were not clear-cut in this respect. The difference between subgroups was larger for Group 2, but this was significant only for original learning of the large-small problem and for the reversal of the circle-triangle problem. Also, the difference between subgroups tended to persist longer over the course of reversal learning by Group 2, but this was significant only with the circletriangle problem. Group 1 had greater prior training on the learning-set task, as well as training on the repeated-reversal task, two tasks which seem to have some common properties (Schrier, 1966b). Perhaps with a greater difference in prior training, the difference between groups would have been found more consistently. Of course, the Ss of Group 1 were also older than those of Group 2, but the younger Ss had already reached sexual maturity and the available evidence (Zimmerman & Torrey, 1965) strongly suggests that developmental changes in learning ability are complete by then. REFERENCES CHOW, K. L. Further studies visually mediated behavior. 1952,45, 109-l 18.
on selective ablation of associative cortex Journal of Comparative and Physiological
in relation to Psychology,
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FRENCH, G. M. Associative problems. In A. M. Schrier, H. F. Harlow. & F. Stollnitz (Eds.). Behavior ofnonhuman primates. Vol. I. New York: Academic Press, 1965. Pp. 167-209. GOLLIN, E. S., & Lrss, P. Conditional discrimination in children. Jourrrtrl ~!~Comptrvoti~~c, and
Physiological
Psychology,
1962,
55,
850-855.
MEYER, D. R., TREICHLER. F. R., & MEYER, PATRICIA M. Discrete-trial training techniques and stimulus variables. In A. M. Schrier, H. F. Harlow, & F. Stollnitz (Eds.). Erhtr~ior of nonhumun primates. Vol. I. New York: Academic Press, 1965. Pp. l-49. PRIBRAM, K. H., & MISHKIN, M. Simultaneous and successive visual discrimination by monkeys with inferotemporal lesions. Jo~rncrl of Comptrrc7ri1~c~ and Phy.sio/ogic~tr/ Psychology, 1955, 48, 198-202. SCHRIER, A. M. A modified version of the Wisconsin General Test Apparatus. lortrntrl f!/ Psychology. 1961, 52, 193-200. SCHRIER, A. M. Learning-set formation by three species of macaque monkeys. Jorrrntrl of Comparutive and Physiological Psychology, 1966. 61, 490-492. (a) SCHRIER, A. M. Transfer by macaque monkeys between learning-set and repeated-reversal tasks. Perceptual and Motor Skills, 1966, 23, 787-792. (b) STOLLNITZ. F. Spatial variables, observing responses, and discrimination learning sets. PI?chological
Review,,
1965,
12, 247-261.
VAUGHAN, J., & SCHRIER,A. M. Effect of locus reinforcment on learning-set formation by rhesus monkeys. Learning and Moti\wtion, 1970. in press. WARREN, J. M. Solution of sign-differentiated object and positional discriminations by monkeys. Journal of Genetic Psycho/ogy. 1960, 96, 365-369. WARREN, J. M. Additivity of cues in conditional discrimination learning hy rhesus monkey\. Journal of Compctrati\re and Physiological Psychology, 1964. 58, l24- 126. WUNDERLICH. R. A., NAZZARO, JEAN, & YOUNISS, J. Stimulus, response, and reward contiguity in pattern discrimination by children. Journcd c?f‘E~p-pc,ri,ilrilrctl Child Ps~c~l70/0~~, 1968, 6, 556-562. ZIMMERMAN, R. R., & TORREY, C. C. Ontogeny of learning. In A. M. Schrier. H. F. Harlow, & F. Stollnitz (Eds.), Behavior of nonhuman primules. Vol. II. New York: Academic Press, 1965. Pp. 405-447. (Received. July 10, 1969)
Effect
AND
MOTIVATION
of Location
Discrimination
(1970) 1, 207-217
of the Conditional Learning
by Monkeys
ALLAN Brown
Cue on Conditional (Macaca
mulatta)’
M. SCHRIER University
Monkeys were trained on conditional discrimination problems consisting of a pair of objects differing in form, or, depending on the problem, in size. The conditional cue was color of the test tray for half the Ss and color of the pair of objects for the other half. The discriminative response was displacement of one of the objects. Original and reversal learning were generally better when the conditional cue was color of the objects. This agrees with previous findings that monkeys are more likely to attend to stimuli that they touch when responding than those they do not touch.
Warren (1960, 1964) studied conditional discrimination learning by rhesus monkeys in the Wisconsin General Test Apparatus (WGTA) and noted, among other things, that the learning rates were considerably faster than were found in previous studies with monkeys (Chow, 19.52; Pribram & Mishkin, 1955) and with children (Gollin & Liss, 1962). Warren thought that these differences might have been based on the fact that his Ss were experimentally “sophisticated,” that is, that they had had a great deal of training on other types of discrimination tasks, or that they were trained on all components of the task from the beginning of training. However, the difference in results may have another basis. In Warren’s studies the conditional, or successive, cue was the color of the objects used as the simultaneous cues (“objects-conditional” procedure), so that if both objects were one color, object A was correct, but if both were another color, object B was correct. With this procedure, Ss were forced to touch both the conditional and the simultaneous cues when responding. In contrast, in earlier studies the typical procedure was such that Ss touched either none of the cues when responding (Gollin & Liss, 1962) or ’ This research was supported by research grants from the National Science Foundation (GB-7590) and the National Institute of Mental Health (MH-15389 and MH-07136), U. S. Public Health Service. The author gratefully acknowledges the assistance of Gertrude A. Trudel and David Henshaw in carrying out Experiment 1 of this research. Requests for reprints should be sent to the author, Department of Psychology, Brown University, Providence. Rhode Island 029 12. 207
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only the simultaneous cues (Chow, 1952; Pribram & Mishkin, 1955). For example, in a number of studies done in the Wisconsin General Test Apparatus (Chow, 1952) the conditional cue was the color of the test tray, so that if the test tray was one color, object A was correct, but if it was another color, object B was correct. With this procedure, Ss touched only the simultaneous cues. There is now a great deal of evidence from experiments with monkeys (Meyer, Treichler, & Meyer, 1965; Stollnitz, 1965) and children (Wunderlich, Nazzaro, & Youniss, 1968) demonstrating that spatial separation of the stimulus and the response, even when as little as + in., retards discrimination learning. Furthermore, it appears that the reason for this is that these Ss are more likely to attend to those parts of visual stimuli that they touch when responding than to parts they do not touch. If, then, Ss are more likely to attend to conditional cues if they touch them, then the faster learning rates obtained by Warren might also have been the result of his use of the objects-conditional procedure in contrast to the use of the tray-conditional procedure, or the equivalent, in the studies which preceded his. The primary concern in the two experiments reported here was a comparison of the effects of the tray-conditional and objects-conditional procedures on conditional discrimination learning by monkeys. The prediction was that the objects-conditional procedure would facilitate learning, Experiments 1 and 2 differed primarily in the order in which the conditional discrimination problems were presented and in the number of problems. EXPERIMENT
1
Method Subjects The Ss were 14 wild-born rhesus monkeys (Mucaca mulatta). Six of these (Group 1) were between 5 and 6 yr of age and had a mean weight of 13 lb. These Ss had previously been given extensive object-discrimination learning-set training (6600 trials) and training (600 trials) on repeated reversals of a single object-discrimination problem (Schrier, 1966a,b). Eight of the Ss (Group 2) were about 4 yr of age and had a mean weight of 12 lb. This group had also previously been given object-discrimination learning-set training (2300 trials; Vaughan & Schrier, 1970). Apparatus The apparatus, a WGTA with motor-operated screens, has been described before (Schrier, 1961). The test tray had two foodwells, 6 in.
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apart center to center, with a 6-in.-high clear-plastic partition between them. The partition was designed to reduce the possibility of response correction and to prevent collisions between objects when one was displaced. Test tray color could be altered by changing Masonite inserts all of which had holes corresponding to the two foodwells and were about 13.5 in. wide and 7.75 in. deep. These test-tray inserts were painted red, green, yellow, blue, or gray. Three pairs of objects, all cut out of OS-in.thick wood, were used for the first conditional problem (circle-triangle problem); one member of each pair was a 2-in.-diameter circle and the other was an equilateral triangle with 1.5-in. sides; one pair of objects was painted red, a second pair green, and the third gray. Three pairs of squareshaped objects, also cut out of 0.5-in.-thick wood, were used for the second conditional problem (large-small problem); one member of each pair had 3-in. sides and the other had 2-in. sides; one pair of objects was painted blue, a second pair yellow, and the third gray. One Noyes 75-mg sucrose pellet was used as the reinforcer on each trial. Procedure Pretraining. The general conditions of testing in the present experiment were much like they were in previous studies in which the Ss of Group 1 were used, so they required little pretraining. Their pretraining consisted of a different 30-trial junk-object discrimination problem each day for the 5 days preceding the experiment using standard learning-set procedures (Schrier, 1966a). All Ss were responding to objects quickly and consistently by the end of this pretraining. The prior learning-set training of Group 2 was not as extensive as that of Group 1 and was carried out under conditions which required either a somewhat different mode of responding to the objects or a different locus of presentation of the reinforcer than was to be used in the present experiment. Therefore, the Ss of this group were given four 1O-trial object discrimination problems per day for the 18 days preceding the experiment, using the standard procedures. By the end of pretraining, all of these Ss were also responding quickly and consistently. Their learning-set performance was still well below that of Group 1, but it would have taken many weeks of further training to bring them up to the same level. It should be emphasized that the lower level of performance was primarily a matter of their having had a smaller amount of training on the learning-set task. Training. The Ss in each group were divided into two equal subgroups, one of which (the objects-conditional subgroup) was given conditional discrimination training with the conditional cue being color of the objects, whereas the other (the tray-conditional subgroup) was given this training with1 the conditional cue being color of the test tray. The subgroups of
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Group 1 were matched on the basis of their learning-set performance in the previous experiments (Schrier, 1966a, 1966b). During the last 5 weeks of this previous training, the objects-conditional subgroup made 96% correct responses and the tray-conditional subgroup made 97%. The subgroups of Group 2 were matched on the basis of their learning-set performance during pretraining. Both subgroups made 87% correct responses during pretraining. The Ss of each group were first trained on the circle-triangle problem. If the conditional cue was red the circle was correct, but if it was green the triangle was correct. For the objects-conditional subgroups, both objects were either red or both were green and the test tray was always gray, whereas for the tray-conditional subgroups, the test tray was either red or green and both objects were always gray. All Ss were given 40 trials a day, 5 days a week. Each of the four possible configurations-red or green conditional cue with the positive object on the left or right side-was used on 10 trials each day in a random order that varied daily. No correction was allowed; the intertrial interval was I5 sec. After 10 days of this training, it was clear that there was no significant difference in the performance of the subgroups, so it was decided to reverse the conditional discrimination, with red now signaling that the triangle was positive and green that the circle was positive. Following 10 days of this reversal training, the large-small problem was presented for 10 days. A blue conditional cue indicated that the large square was positive, whereas a yellow cue indicated that the small square was positive. All other conditions were the same as they were for the circle-triangle problem. Because the performance of the tray-conditional subgroup was now found to be significantly lower than that of the objects-conditional, the former subgroup was given an extra 5 days of training in order to bring its performance level up to that of the objects-conditional subgroup. Both subgroups were then given 10 days of reversal training. One assistant tested two Ss in each subgroup of Groups 1 and 2 and a second assistant tested the remaining Ss. RESULTS
The percentage of correct responses made by the two subgroups during the first 10 days of training is shown separately for each problem and each group in Fig. 1. Reversal performance is similarly shown in Fig. 2. The general picture is one of poorer performance by the tray-conditional subgroup than by the objects-conditional. For purposes of statistical analysis, an arc sine transformation was performed on the proportion of correct responses made by each S each day. The transformed scores for the first 10 days on each problem and on its reversal were analyzed in four mixed within-Ss-between-Ss analyses of
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I
2 3 4
5 6
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7 6 9 IO I
2 3 4
5 6
7 6
9 IO
DAYS FIG. 1. Learning of the circle-triangle (top- and bottom-left panels) and large-small (topand bottom-right panels) conditional discrimination problems by the tray-conditional and objects-conditional subgroups of Groups 1 and 2.
variance in which the main effects groups), and Practice (days). In order N per subgroup, one S, chosen at subgroup of Group 2. Cue Location
were Groups, Cue Location (subto meet the requirement of an equal random, was eliminated from each (df= A) was not significant for the
FIG. 2. Learning of the reversals of the circle-triangle (top- and bottom-left panels) and large-small (top- and bottom-right panels) conditional discrimination problems by the trayconditional and objects-conditional subgroups of Groups 1 and 2.
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original learning of the circle-triangle problem, but was highly significant for the reversal of this problem (F = 48.8, p < .OOl) and for the original learning (F = 50.5, p < .OOl) and reversal (F = 15.8, p < .Ol) of the large-small problem. Practice (df = A) was significant in all four analyses (all Fs > 25.8, all ps < .OOl). A significant interaction of Practice and Cue Location was obtained for the reversals of both the circle-triangle (F = 2.00, df= A, p < .05) and large-small problems (F = 2.87, df = &, p < .Ol), reflecting a tendency for the differences between the subgroups to diminish with practice. This seemed to occur more quickly in Group 1 than in Group 2 and, indeed, the triple interaction, which would test such an outcome, was significant for the reversal of the circletriangle problem (F = 2.56, df = &, p < .02), though it was not for the reversal of the large-small problem. The Groups term was not significant in any of the analyses, indicating that the over-all performance of the two groups was similar. The interaction of Cue Location and Groups was significant for reversal learning of the circle-triangle problem (F = 7.29, df= +, p < .05) and original learning of the large-small problem (F = 9.12, df = i, p < .02). Both of these interactions indicate that the difference in performance of the two subgroups was reliably smaller for Group 1 than in Group 2. Nevertheless, as follow-up analyses of variance conducted on the transformed data of the individual groups showed, the smaller as well as the larger difference between subgroups was significant for both reversal learning of the circle-triangle problem (Group 1: F = 17.5, df = 4, p < .02; Group 2: F = 24.3, df = Q, p < .Ol) and original learning of the large-small problem (Group 1: F = 18.0, df = i, p < .02; Group 2: F = 58.3, df= Q,p < .OOl). Additional analyses of variance, in which the main effects were Cue Location, Practice, and Problems (circle-triangle vs. large-small), were conducted on the transformed data for the two groups combined. There was no significant difference in over-all performance on the two problems or in rate of learning them during either original learning or reversal learning. There was no significant interaction of Cue Location and Problems during either original learning or reversal learning. EXPERIMENT
2
One question raised by the results of Experiment 1 was why cue location had a reliable effect on original learning of the large-small problem, but not the circle-triangle problem. The problems were learned at about the same rate, so problem difficulty did not seem to be important. Perhaps order of presentation of the problems influenced the outcome. Since cue location influenced reversal of the circle-triangle problem, which immedi-
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ately preceded the large-small problem, it is possible that there was some carry-over of the effect from the reversal. It is also possible that the difference in outcome for the original learning of the two problems had something to do with the particular stimuli employed. Both of these possibilities were investigated in Experiment 2. The two problems used in the first experiment were presented in the reverse order to a new group of animals with the conditions otherwise the same. In addition, a second conditional form-discrimination problem was presented with the conditional-cue colors as well as the form cues different than those for the first problem. Method Subjects
The Ss were six wild-born rhesus monkeys, about 3.5 to 4 yr of age with a mean weight of 10.8 lb. Each S had been given a series of 30-trial form- and color-discrimination problems (two problems per day) until it reached a criterion of 90% or more correct responses on 5 out of 6 consecutive days on each type of problem. This required a mean of 2882 trials including those on criterion days. Apparatus
In addition to the two problems used in Experiment 1, a third conditional discrimination problem (rectangle-oval problem) was presented in Experiment 2. The objects, ovals and rectangles, were again cut out of 0.5-in.-thick wood and had an area of approximately 4 sq in. The conditional-cue colors were brown and lavender. Procedure
The Ss were first given 4 weeks. 5 days per week, of learning-set training, with junk objects used as the stimuli. The conditions were the same as those for the pretraining of Group 2 in Experiment 1. Training. The Ss were equally divided into an objects-conditional subgroup and a tray-conditional subgroup (the term, “subgroup,” is used for the sake of consistency with Experiment l), which were matched on the basis of pretraining performance. Both subgroups made slightly more than 86% correct responses during pretraining. All Ss were trained successively on the large-small problem, a reversal of this problem, the circle-triangle problem, the rectangle-oval problem, and a reversal of this last problem. Each training condition was continued until S made at least 95% correct responses on 4 consecutive days. The Ss were tested by one Pretraining.
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assistant who did not test in Experiment 1. All other conditions were the same as they had been in Experiment 1.
of training
RESULTS
The results are summarized in Table 1 which shows the percentage of correct responses made by the subgroups during the first 5 days of training under each training condition. None of the Ss completed a training condition in less than 5 days, counting the criterion days. The results were generally quite similar to those of Experiment 1. Analyses of variance in which the main effects were Cue Location and Practice (Days l-5) were conducted on the arc sine-transformed proportions of correct responses. The performance level of the objects-conditional subgroup was significantly higher than that of the tray-conditional during the first 5 days of both the original learning (F = 29.8, d’ = 4, p < .Ol) and the reversal (F = 27.8, df = 4, p < .Ol) of the large-small problem. Practice (df = &) was highly significant for original (F = 20.1, p < .OOl) and reversal learning (F = 62.3, p < .OOl) of this problem, but, as in Experiment 1, the interaction of Practice and Cue Location was significant only for reversal learning (F = 12.1, df = &, p < .OOl). This interaction again reflected a diminishing effect of cue location with practice. Only the Practice term was significant (F = 10.5, df = &,p < .OOl) in the analysis of the results for the learning of the circle-triangle problem. As in Experiment 1, the difference in performance of the subgroups. on the circle-triangle problem was in the expected direction. A f test conducted on the combined data of the animals of both experiments for the first 5 days on this problem fell just short of significance (r = 1.89, df = 18, p < .lO). The results for the rectangle-oval problem were about the same as those in Experiment 1 for the circle-triangle problem. The difference in TABLE 1 Percentage of Correct Responses (%) Made by the Tray-Conditional and Objects-Conditional Subgroups During Days l-5 on Each Training Condition in Experiment 2 Problem= Group Tray Objects
L-S 78 91
n L-S = large-small;
L-S Reversal 59 87 C-T = circle-triangle;
C-T
R-O
a7 94
93 94
and R-O = rectangle-oval.
R-O Reversal 80 90
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correct responses made by the two subgroups during original learning was not significant, but the difference during reversal learning was significant (F = 14.9, df= 3, p < .025). Practice (df= &) was significant during both original (F = 45.8, p < .OOl) and reversal learning (F = 79.1, p < .OOl), but the interaction of Practice and Cue Location was significant only for reversal learning (F = 3.95, df= &-, p < .025). The arc sine-transformed scores for original learning of each problem were compared with the corresponding scores for each of the other problems in separate analyses of variance in which the main effects were Cue Location, Practice (Days l-5), and Problems. As in Experiment 1, in no case was there a significant interaction of Practice and Problems, indicating that there was no significant difference in rate of learning of the large-small problem and either of the form problems. However, the main effect of Problems was significant when the large-small problem was compared with both the circle-triangle problem (F = 24.5, df = 4, p < .Ol) and with the rectangle-oval problem (F = 19.9, df= a, p < .025). In both instances, performance was poorer on the large-small problem than on the form problem. However, in both cases the difference in performance on the two problems was considerably greater for the tray-conditional subgroup than for the objects-conditional (Table 1). This suggests an interaction of Cue Location and Problems and this was confirmed in the analyses (for the large-small vs. circle-triangle problem comparison, F = 8.72, df = f, p < .05, and for the large-small vs. rectangle-oval problem comparison, F = 7.81, df= S, p < .05). DISCUSSION
The results of both of the experiments reported here indicate that monkeys are more likely to attend to conditional cues when they are spatially contiguous with the locus of response than when they are not. Thus, the same rule seems to apply to placement of conditional cues as to placement of the simultaneous cues in simple two-choice discriminations (Meyer, Treichler, & Meyer, 1965; Stollnitz, 1965) and, presumably, also of the simultaneous cues in conditional discriminations. The superiority of the objects-conditional procedure is especially striking considering that the area devoted to the conditional cue was at least six times greater with the tray-conditional procedure. Also, the present results reemphasize what the studies involving two-choice discriminations have shown-that even small differences in spatial separation of cue and response can have a marked negative effect on performance. Differences due to cue location were consistently reliable for reversal learning but not for original learning, in which case the difference was
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significant for the large-small problem but not for the circle-triangle problem. It is not clear what the basis is for this outcome, though differences in problem difficulty and order of presentation of the problems seem to be ruled out. The fact that cue location had no reliable effect on the original learning of either of the form problems might suggest that this represents the general case for this type of problem. Such a conclusion would, of course, require considerably more data than was provided by the present experiment, especially because no striking differences in saliency of form and size cues have been reported for monkeys as yet (Meyer, Treichler, & Meyer, 1965). Also, it would not appear reasonable to discount altogether the cue location variable in the case of original learning of conditional form-discrimination problems, because the difference between subgroups was in the expected direction in all instances and, overall, was very close to significance for original learning of the circle-triangle problem. A number of studies (French, 1965) carried out some years ago showed that some monkeys and apes can, after extensive training, learn higherorder conditional problems, such as Weigl-type oddity. Perhaps these tasks would have been less formidable for such animals if an objects-conditional procedure and other procedures which have subsequently been found to facilitate conditional-discrimination learning (Warren, 1964) had been employed. Experiment 1 suggested that conditional-discrimination performance was influenced by prior training, though the results were not clear-cut in this respect. The difference between subgroups was larger for Group 2, but this was significant only for original learning of the large-small problem and for the reversal of the circle-triangle problem. Also, the difference between subgroups tended to persist longer over the course of reversal learning by Group 2, but this was significant only with the circletriangle problem. Group 1 had greater prior training on the learning-set task, as well as training on the repeated-reversal task, two tasks which seem to have some common properties (Schrier, 1966b). Perhaps with a greater difference in prior training, the difference between groups would have been found more consistently. Of course, the Ss of Group 1 were also older than those of Group 2, but the younger Ss had already reached sexual maturity and the available evidence (Zimmerman & Torrey, 1965) strongly suggests that developmental changes in learning ability are complete by then. REFERENCES CHOW, K. L. Further studies visually mediated behavior. 1952,45, 109-l 18.
on selective ablation of associative cortex Journal of Comparative and Physiological
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FRENCH, G. M. Associative problems. In A. M. Schrier, H. F. Harlow. & F. Stollnitz (Eds.). Behavior ofnonhuman primates. Vol. I. New York: Academic Press, 1965. Pp. 167-209. GOLLIN, E. S., & Lrss, P. Conditional discrimination in children. Jourrrtrl ~!~Comptrvoti~~c, and
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MEYER, D. R., TREICHLER. F. R., & MEYER, PATRICIA M. Discrete-trial training techniques and stimulus variables. In A. M. Schrier, H. F. Harlow, & F. Stollnitz (Eds.). Erhtr~ior of nonhumun primates. Vol. I. New York: Academic Press, 1965. Pp. l-49. PRIBRAM, K. H., & MISHKIN, M. Simultaneous and successive visual discrimination by monkeys with inferotemporal lesions. Jo~rncrl of Comptrrc7ri1~c~ and Phy.sio/ogic~tr/ Psychology, 1955, 48, 198-202. SCHRIER, A. M. A modified version of the Wisconsin General Test Apparatus. lortrntrl f!/ Psychology. 1961, 52, 193-200. SCHRIER, A. M. Learning-set formation by three species of macaque monkeys. Jorrrntrl of Comparutive and Physiological Psychology, 1966. 61, 490-492. (a) SCHRIER, A. M. Transfer by macaque monkeys between learning-set and repeated-reversal tasks. Perceptual and Motor Skills, 1966, 23, 787-792. (b) STOLLNITZ. F. Spatial variables, observing responses, and discrimination learning sets. PI?chological
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VAUGHAN, J., & SCHRIER,A. M. Effect of locus reinforcment on learning-set formation by rhesus monkeys. Learning and Moti\wtion, 1970. in press. WARREN, J. M. Solution of sign-differentiated object and positional discriminations by monkeys. Journal of Genetic Psycho/ogy. 1960, 96, 365-369. WARREN, J. M. Additivity of cues in conditional discrimination learning hy rhesus monkey\. Journal of Compctrati\re and Physiological Psychology, 1964. 58, l24- 126. WUNDERLICH. R. A., NAZZARO, JEAN, & YOUNISS, J. Stimulus, response, and reward contiguity in pattern discrimination by children. Journcd c?f‘E~p-pc,ri,ilrilrctl Child Ps~c~l70/0~~, 1968, 6, 556-562. ZIMMERMAN, R. R., & TORREY, C. C. Ontogeny of learning. In A. M. Schrier. H. F. Harlow, & F. Stollnitz (Eds.), Behavior of nonhuman primules. Vol. II. New York: Academic Press, 1965. Pp. 405-447. (Received. July 10, 1969)