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Determinants of serial discrimination learning by squirrel monkeys
LEAHNINC:
AKD
AlOTI\‘ATION
Determinants
(
1971)
2, 246-254
of Serial
Discrimination
by Squirrel
Mon keys1
JAMES University
E.
Learning
KING
of Arizona
Squirrel monkeys were presented multiple serial discriminations 1, 2, 4, and 8 problems long. They were then presented problems designed to separate the effects of within-list associative interference from the effects of within-problem intertrial interval as list length was increased. The Ss committed consistently fewer errors after Trial 1 reward than after Trial 1 nonreward and showed strong stimulus perseveration. An increase in within-problem intertrial interval from 30 set to 4 min had no effect whereas the associative interference resulting from increased problem length caused a small but significant performance decrement. Old and new problems had about equal effects on serial discrimination. The findings indicated that squirrel monkeys are relatively insensitive to within-problem associative interference.
The typical serial or concurrent discrimination task is constructed from a series or list of paired discrimination stimuli. These stimuli are presented, one pair at a time, until the S has responded to each pair once. The entire list is then presented again in the same manner repeatedly until each pair has been presented for a predetermined number of trials. Thus, unlike a successive discrimination in which all trials with each stimulus pair are given before presentation of the next problem, a serial discrimination requires that consecutive trials of each pair be separated by one-trial presentations of all other pairs in the series. Serial discrimination procedures are thus somewhat similar to verbal discrimination and verbal maze procedures used in human verbal learning research. King and Goodman ( 1966), on the basis of an increase in stimulus perseveration errors as list length was increased up to four problems, suggested that the learning derived from nonreward trials decreased as list length increased. Bessemer (1968) found that rhesus monkeys showed *Preparation of this manuscript was completed during the author’s tenure as PHS fellow ( 1 F03 HD-42963) at the Yerkes Regional Primate Research Center. The author wishes to express his gratitude to Louise Chernetz for her assistance in testing. Requests for reprints should be sent to the author at the Department of Psychology, University of Arizona, Tucson, Arizona 85721. 246
SERIAL
DISCRIMISATIOS
24;
significant retention loss of successively prcscnted object discriminations after as short a delay as 2 min, but that the loss was almost entirely confined to problems in which the object preferred on Trial 1 was incorrect. Therefore, since stimulus perseveration is by definition limited to those problems in which the stimulus chosen on Trial 1 is nonrewarded and since the intertrial interval within-problems increases with list length. one might expect that as list length is increased performance following Trial 1 nonreward should become increasingly poorer relative to performance following Trial 1 reward. This possibility was tested in Experiment 1 in which squirrel monkeys learned serial lists of object discriminations containing 8, 4, and 2 problems each as well as successivcl discriminations (list length 1). In order to allow determination of the interaction of series length with Trial 1 reward and nonreward, the stimulus chosen on Trial 1 was correct in half of the problems and incorrect in the other half. Past research on squirrel monkeys (King & Goodman, 1966), rhesus monkeys (Darby & Riopelle, 1955) and chimpanzees (Hayes, Thompson, & Hayes, 1953) has amply demonstrated an increase in errors during learning as list length is increased. There are two obvious possible causes for the positive relationship between errors and list length. First, in previous experiments where list length was varied, the time between SUCCMsive presentations of each particular discrimination necessarily increascxd as list length increased. Second, the interval between successive presentations of any problem in a serial d;scrimination contains presentations of all other discriminations in the list thereby allowing the possibility of associative interference among the different discriminations. Presumably, the longer the list length the greater the amount of such interference, Experiment 2 was designed to clarify the relationship between list length and performance by separating the effects of lengthened time between trials of the same problem from the effects of associative interference among the discriminative stimuli in serial discrimination lists. METHOD
Subjects. The Ss were four male and five female adult squirrel monkeys. All Ss were feral and had previously received extensive discrimination learning-set experience. Eight Ss served in Experiment 1. Subsequently, one female died and was replaced by another female in Experiment 2. Apparatus and General Procedure. Testing was conducted in a modified Wisconsin General Test Apparatus (WGTA). The stimulus presentation tray measured 36 cm by 15 cm, was manually operated,
248
JAMES
E.
KING
and contained two foodwells located 18 cm apart, center to center. A pully-operated opaque screen was interposed between the S’s test cage and the stimulus presentation tray during the intertrial interval. E observed S and the stimulus tray from behind a fixed one-way screen. The stimulus objects were constructed from junk and hardware items differing in multiple dimensions and mounted on heterogeneously colored wooden bases measuring 44.4 X 44.4 X 6.4 mm. The following procedure was used in presenting each trial of all discrimination problems. While the opaque screen was down, E baited the positive foodwell with a small bit of raisin and placed the two discrimination objects in slides covering the foodwells. E then raised the opaque screen and pushed the tray slowly forward. After the S’s response, the opaque screen was lowered and the tray withdrawn. Correction was not allowed. Except for Trial 1, the position of the positive object was determined by a Gellermann sequence. Experiment 1. During pretraining all Ss were trained on six discrimination problems. The problems were presented successively and were each learned to a criterion of nine out of ten correct responses. Immediately after pretraining, all Ss received eight IO-trial discrimination problems each test day for 32 days. The eight problems given during each session were presented under one of the following four conditions: (a) All 10 trials of each discrimination were presented before going on to the next discrimination. Although such problems are typically called successive discriminations, for purposes of clarity they will be referred to here as serial problems of length 1. (b) The eight problems were divided into four serial lists of two problems each. After the first two problems were presented serially, the next two problems were presented and so on until all eight discriminations had been presented. (c) The eight problems were divided into two serial lists of four problems each with the two lists presented in succession. (d) All eight problems were combined into a single serial list. The above four conditions thus defined serial lists composed of 1, 2, 4, and 8 problems, The presentation order for the four list lengths during the first 16 days was determined by a balanced set of four 4 x 4 Latin squares with two Ss randomly assigned to each row and successive days assigned to columns. This presentation order was replicated during the second 16 days. On half of the test davs for each of the above four list lengths both objects were baited on Trial 1 of each discrimination thereby insuring that the object chosen by the S on that trial was rewarded. The chosen object was then correct on the remaining trials of the discrimination. During the other half of the test days, neither object was baited on Trial 1
SERIAL
DISCRlhilNATlON
3-N
of each discrimination and the chosen object on that trial was incorrect on the remaining trials. For half of the Ss the object chosen on Trial I was correct on even numbered test days and incorrect on odd numbered test days whereas the reverse order obtained for the other half of the Ss. Experiment 2. Following the completion of Experiment 1, the SS lvert’ trained on seven discrimination problems over a 5-day period. The proI)lems were presented serially for a total of 42 trials per day. The purpos(\ of this training was to train the Ss thoroughly on seven discriminations to be used in the Old-Problems condition described below. The Ss were then given further discrimination trials for a 64 day period under each of the following four conditions. One test condition prevailed during each test day for each S. (a) Short Zntervub. A single 7-trial discrimination was presented with 30-WC intervals between the onsrts of successive trials. (b) Delay. A single 7-trial discrimination was presented with a 4-min intertrial interval. (c) New-Problems. Eight ‘i-trial discrirninations were presented in a single serial list with the onsets of succclssivcb trials separated by 30 sec. The interval between successive trials of any partic&r discrimination was therefore 4 min, the same as for the single problem presented in the Delay condition. A new set of discrimination objects was used each day the New-Problems condition was in effect. (d) Old-Problems. Eight 7-trial discrimination problems were presented serially in the same manner as in the New-Problems condition. Seven of the problems, however, were always those discriminations learned just prior to the beginning of Experiment 2. The rtxward values of the objects in these recurring discriminations remained constant. Problem 8 was formed from new objects and, over all testing days. appeared equally often at all positions in the seria1 list. The presentation order of the above four conditions within succcssivc 18day blocks was determined by a balanced set of four 4 x 4 Latin squares, with a different set in effect for each block. Two Ss wercl therefore tested under each condition on each test day. Furthermore, on each test day, the object pair used for the new discrimination problem in the> Old-Problems condition was also used for the single discrimination in th(x Short Interval and Delay conditions as well as one discrimination in th
250
JAMES
E.
KING
RESULTS
Experiment 1. Figure 1 depicts the percentage of correct responses as a function of list length, Trial 1 outcome, and intraproblem trial blocks. Clearly, performance was higher following Trial 1 reward than following Trial 1 nonreward (F( 1,7) = 30.42, p < 901) and declined systematically as list length increased from one to eight problems (F( 3,21) = 7.49, p < 905). In addition, performance increased over intraproblem trial blocks (F( 2,14) = 22.70, p < .OOl) but at nearly equivalent rates for all list lengths. The only significant interaction was preference X list length X trials (F( 6,42) = 2.41, p < .05).
1&5 ,,..... A-..-.60- ,,=‘::.‘: /‘: ,_,.._ . ..“’ ......’
5ow TRIALS FIG. 1. Percentage of correct responses in Experiment length, Trial 1 outcome, and intraproblem trial blocks.
1 as a function
of list
A further analysis was made for the stimulus perseveration error factor (Harlow, 1950) according to the technique described in detail by Leary ( 1958) and by King and Goodman ( 1966). Stimulus perseveration occurs when an S makes continued responses to an incorrect stimulus and therefore commits an excessive number of errors prior to the first correct response in each problem, More specifically, stimulus perseveration is said to be present when the number of consecutive errors prior to the first correct response of each problem exceeds the number of such consecutive errors that would be expected based upon the overall error rate for each trial. To measure stimulus perseveration, the predicted total number of repeated consecutive errors on Trials 2-6 was determined using error rates for unique combinations of S, trial, and list length. The total stimulus perseveration was then defined as the observed minus the
SERIAL
251.
DISCRlMIiYATIO~
predicted consecutive errors. The proportion of stimulus perseveration errors was calculated by dividing the number of stimulus perseveration errors on each trial by the total number of errors occurring on that trial. The differences among the percentages of stimulus perseveration errors for the 1, 2, 4, and 8 problem lists were small and nonsignificant (.061. .054, .077, and .091, respectively). Similarly there was little differenct among the proportions of stimuh perseveration errors occurring on Trials 2-6 averaged over all list lengths (.071, .084, .084, .066, and .047. respectively). Experiment 2. Figure 2 depicts the percentage of correct responses following rewarded ( + ) and nonrewarded ( - ) first trials on thci discrimination problems common to each of the four experimental conditions. As in Experiment I, performance was substantially higher following rewarded Trial 1 than following nonrewarded Trial I (F( 1,7) = 66.60. p < .OOl) and there was marked intraproblem learning (F( I,7) = 37.02. p < .OOl) . An analysis of variance revealed a significant difference among the four experimental conditions (F( 3,21) = 3.88, p < .05). None of the interactions among the three main effects approached statistical significance.
‘“Oo
--
+ IMMEDIATE
NEW
-
,_ +
-
OLD
FIG. 2. Percentage of correct responses in Experiment 2 as a function condition and Trial 1 outcome (reward = +, nonreward = -- ).
of testing
The differences among the experimental conditions were analyzed further by three a priori comparisons. First, the difference between the Immediate and the Delay conditions was negligible and not significant thereby indicating that successive discrimination learning is not substantially affected by an increase in intertrial interval from 30 SW to 4
252
JAMES
E.
KING
min. The second comparison between the Delay condition and the combined New- and Old-Problems conditions was significant (F( 1,21) = 4.48, p < .05) and indicated that for a constant 4-min intertrial interval within a particular discrimination problem, performance is lower when the Ss received one-trial presentations from seven other discriminations during that interval than when the Ss simply remained in the test cage and received no intervening trials. The final comparison between the New- and Old-Problems conditions was not significant. Stimulus perseveration was calculated as in Experiment 1. The mean proportion of stimulus perservation errors, averaged over Trials 2-6 of each problem, was .147, .231, .126, and .264 for the Immediate, Delay, Old- and New-Problems conditions, respectively. The corresponding proportions for Trials 2-6 averaged over all conditions were ,154, .258, .144, .177, and .152, respectively. The differences among the proportions were not significant in either of these two series. However, the overall proportion of stimulus perseveration errors was significantly greater in Experiment 2 than in Experiment 1 (t( 6) = 3.27, p < 62). DISCUSSION
The experiments described here demonstrated the expected significant decrease in performance as list length was increased from one to eight problems; however, the magnitude of this decrease, about 8% in both experiments, was remarkably small. Apparently, squirrel monkeys are able to learn at least eight discrimination problems simultaneously with almost the same efficiency as they learn such problems successively. King and Goodman (1966) previously observed that an increase in list length from one to four problems also had a relatively small effect upon discrimination learning by previously untrained squirrel monkeys and rock squirrels. In both experiments, performance was consistently higher following rewarded Trial 1 than following nonrewarded Trial 1. Naive rhesus monkeys typically make the fewest errors following rewarded Trial 1, but with continued discrimination training the difference diminishes and is reversed in highly sophisticated animals which typically commit the fewest errors following nonrewarded Trial 1 (Harlow & Warren, 1952; Riopelle, 1953, 1955; Behar, 1961). Clearly, the higher performance of the squirrel monkeys following rewarded Trial 1 is much more resistant to practice effects than that of rhesus monkeys since the squirrel monkeys in this experiment had extensive previous experience on multiple discrimination learning, and their superior performance following rewarded Trial 1 was even slightly greater in Experiment 2 than in Experiment 1. This result may be attributed to the squirrel monkeys’ relatively
SERlAL
DISCHIMINATJO\
453
strong tendency to make repeated responses to incorrect stimuli chosen on Trial 1. Direct evidence of this tendency was provided by the increase in the proportion of stimulus perseveration errors from Experiment 1 to Experiment 2 and the consistency of this proportion over Trials 2-6, which was especially evident in Experiment 2. This persistence of stimulus perseveration occurred despite the fact that the overall error rate decreased over Trials 2-6 and remained about the same in Expcriment 2 as in Experiment 1. In contrast, stimulus perseveration in rhesus monkeys decreases over Trials 2-6 and becomes negligible in highly trained animals (Harlow, 1950; Leary, 1958). In both experiments the squirrel monkeys showed no consistent intcraction between list length and Trial 1 outcome. This result fails to support King and Goodman’s ( 1966) earlier supposition that the amount of learning derived from nonrewarded trials decreases as list length increases. The virtually identical performance under the Immediate and Delay conditions in Experiment 2 is evidence that an increase from 30 set to 4 min in the interval between successive trials of a discrimination problem is not appreciably responsible for whatever performance decrt>ment accompanies an increase in list length from one to eight problems. Similarly, other research with rhesus monkeys has indicated that variation of the intertrial interval has small and inconsistent effects upon performance (Harlow & Warren, 1952; Riopelle & Churukian, 1958; Fletcher & Cross, 1964). The squirrel monkeys committed more errors under the Delay condition than under the combined New- and OldProblems conditions. Since the within-problems intertrial interval was 3 nun under all of these conditions, the lower performance under the New- and Old-Problems conditions may be attributed to intralist associative interference occasioned by interpolation of extraneous discrimination problems between successive trials of a given discrimination problem. However, since increasing list length caused only a small performance decrement in the performance of the squirrel monkeys in this experiment as well as those tested by King and Goodman ( 1966) it appears that highly trained as well as naive squirrel monkeys arc’ relatively resistant to intralist interfcrcncc while learning serial discriminations. The inconsequential diffcrencc bctwecn the New- and Old-problems conditions showed that both new and old, well-learned discriminations have about ctruivalent disruptive cfl’ects within a serial discrimination. Since the familiar problems in the Old-Problems condition were well learned, thev presumably cans4 little intralist interference. The low degree of similarity among the new stinriilns objects used in this cx-
254
JAMES
E. KING
periment apparently minimized the intralist confusion of objects in the New-Problems condition as well and therefore failed to depress performance below that in the Old-Problems condition. The lack of a significant performance difference under these two conditions further emphasizes the squirrel monkeys’ marked insensitivity to intralist associative interference. REFERENCES Learned avoidance of nonreward. Psychological Reports, 1961, 9, 43-52. D. W. Retention of object discriminations by learning set experienced monkeys. Dissertation Abstracts, 1968, 28, 4771. DAHBY, C. L., & RIOPELLE, A. J. Differential problem sequences and the formation of learning sets. Journal of Psychology, 1955, 39, 105-108. FLETCHER, H. J., & CROSS, H. A. Effects of Trial 1 reward contingency, intertrial interval, and experience on intraproblem discrimination performance of monkeys. Journal of Comparative and Physiological Psychology, 1964, 57, 318-320. HAHLOW, H. F. Analysis of discrimination learning by monkeys. Journal of ExperiBEHAR, I. BESSEMER,
mental HARLOW,
Psychology,
1950,
H. F., & WARREN,
46, 26-39.
and transfer of discrimination learning sets. Journal of Comparative and Physiological Psychology, 1952, 45, 482-489. HAYES, K. J., THOMPSON, R., & HAYES, C. Concurrent discrimination learning in chimpanzees. Journal of Comparative and Physiological Psychology, 1953, 46, 105-108. KING, J. E., & GOODMAN, R. R. Successive and concurrent discrimination by rock squirrels and squirrel monkeys. Perceptual and Motor SkiZ.?.s, 1966, 23, 703-710. LEARY, R. W. Analysis of serial discrimination by monkeys. Journal of Comparative and Physiological Psychology, 1958, 50, 581-584. RIOPELLE, A. J. Transfer suppression and learning sets. journal of Comparative and Physiological Psychology, 1953, 46, 108-114. RIOPELLE, A. J. Rewards, preferences, and learning sets. PsychoZogicaZ Reports, 1955, 1, 167-173. RIOPELLE, A. J., & CHURUKIAN, G. A. The effect of varying the intertrial interval in discrimination learning by normal and brain-operated monkeys. Journal of Comparative and Physiological Psychology, 1958, 51, 119-125. (Received July 31, 1970)
J. M. Formation
AKD
AlOTI\‘ATION
Determinants
(
1971)
2, 246-254
of Serial
Discrimination
by Squirrel
Mon keys1
JAMES University
E.
Learning
KING
of Arizona
Squirrel monkeys were presented multiple serial discriminations 1, 2, 4, and 8 problems long. They were then presented problems designed to separate the effects of within-list associative interference from the effects of within-problem intertrial interval as list length was increased. The Ss committed consistently fewer errors after Trial 1 reward than after Trial 1 nonreward and showed strong stimulus perseveration. An increase in within-problem intertrial interval from 30 set to 4 min had no effect whereas the associative interference resulting from increased problem length caused a small but significant performance decrement. Old and new problems had about equal effects on serial discrimination. The findings indicated that squirrel monkeys are relatively insensitive to within-problem associative interference.
The typical serial or concurrent discrimination task is constructed from a series or list of paired discrimination stimuli. These stimuli are presented, one pair at a time, until the S has responded to each pair once. The entire list is then presented again in the same manner repeatedly until each pair has been presented for a predetermined number of trials. Thus, unlike a successive discrimination in which all trials with each stimulus pair are given before presentation of the next problem, a serial discrimination requires that consecutive trials of each pair be separated by one-trial presentations of all other pairs in the series. Serial discrimination procedures are thus somewhat similar to verbal discrimination and verbal maze procedures used in human verbal learning research. King and Goodman ( 1966), on the basis of an increase in stimulus perseveration errors as list length was increased up to four problems, suggested that the learning derived from nonreward trials decreased as list length increased. Bessemer (1968) found that rhesus monkeys showed *Preparation of this manuscript was completed during the author’s tenure as PHS fellow ( 1 F03 HD-42963) at the Yerkes Regional Primate Research Center. The author wishes to express his gratitude to Louise Chernetz for her assistance in testing. Requests for reprints should be sent to the author at the Department of Psychology, University of Arizona, Tucson, Arizona 85721. 246
SERIAL
DISCRIMISATIOS
24;
significant retention loss of successively prcscnted object discriminations after as short a delay as 2 min, but that the loss was almost entirely confined to problems in which the object preferred on Trial 1 was incorrect. Therefore, since stimulus perseveration is by definition limited to those problems in which the stimulus chosen on Trial 1 is nonrewarded and since the intertrial interval within-problems increases with list length. one might expect that as list length is increased performance following Trial 1 nonreward should become increasingly poorer relative to performance following Trial 1 reward. This possibility was tested in Experiment 1 in which squirrel monkeys learned serial lists of object discriminations containing 8, 4, and 2 problems each as well as successivcl discriminations (list length 1). In order to allow determination of the interaction of series length with Trial 1 reward and nonreward, the stimulus chosen on Trial 1 was correct in half of the problems and incorrect in the other half. Past research on squirrel monkeys (King & Goodman, 1966), rhesus monkeys (Darby & Riopelle, 1955) and chimpanzees (Hayes, Thompson, & Hayes, 1953) has amply demonstrated an increase in errors during learning as list length is increased. There are two obvious possible causes for the positive relationship between errors and list length. First, in previous experiments where list length was varied, the time between SUCCMsive presentations of each particular discrimination necessarily increascxd as list length increased. Second, the interval between successive presentations of any problem in a serial d;scrimination contains presentations of all other discriminations in the list thereby allowing the possibility of associative interference among the different discriminations. Presumably, the longer the list length the greater the amount of such interference, Experiment 2 was designed to clarify the relationship between list length and performance by separating the effects of lengthened time between trials of the same problem from the effects of associative interference among the discriminative stimuli in serial discrimination lists. METHOD
Subjects. The Ss were four male and five female adult squirrel monkeys. All Ss were feral and had previously received extensive discrimination learning-set experience. Eight Ss served in Experiment 1. Subsequently, one female died and was replaced by another female in Experiment 2. Apparatus and General Procedure. Testing was conducted in a modified Wisconsin General Test Apparatus (WGTA). The stimulus presentation tray measured 36 cm by 15 cm, was manually operated,
248
JAMES
E.
KING
and contained two foodwells located 18 cm apart, center to center. A pully-operated opaque screen was interposed between the S’s test cage and the stimulus presentation tray during the intertrial interval. E observed S and the stimulus tray from behind a fixed one-way screen. The stimulus objects were constructed from junk and hardware items differing in multiple dimensions and mounted on heterogeneously colored wooden bases measuring 44.4 X 44.4 X 6.4 mm. The following procedure was used in presenting each trial of all discrimination problems. While the opaque screen was down, E baited the positive foodwell with a small bit of raisin and placed the two discrimination objects in slides covering the foodwells. E then raised the opaque screen and pushed the tray slowly forward. After the S’s response, the opaque screen was lowered and the tray withdrawn. Correction was not allowed. Except for Trial 1, the position of the positive object was determined by a Gellermann sequence. Experiment 1. During pretraining all Ss were trained on six discrimination problems. The problems were presented successively and were each learned to a criterion of nine out of ten correct responses. Immediately after pretraining, all Ss received eight IO-trial discrimination problems each test day for 32 days. The eight problems given during each session were presented under one of the following four conditions: (a) All 10 trials of each discrimination were presented before going on to the next discrimination. Although such problems are typically called successive discriminations, for purposes of clarity they will be referred to here as serial problems of length 1. (b) The eight problems were divided into four serial lists of two problems each. After the first two problems were presented serially, the next two problems were presented and so on until all eight discriminations had been presented. (c) The eight problems were divided into two serial lists of four problems each with the two lists presented in succession. (d) All eight problems were combined into a single serial list. The above four conditions thus defined serial lists composed of 1, 2, 4, and 8 problems, The presentation order for the four list lengths during the first 16 days was determined by a balanced set of four 4 x 4 Latin squares with two Ss randomly assigned to each row and successive days assigned to columns. This presentation order was replicated during the second 16 days. On half of the test davs for each of the above four list lengths both objects were baited on Trial 1 of each discrimination thereby insuring that the object chosen by the S on that trial was rewarded. The chosen object was then correct on the remaining trials of the discrimination. During the other half of the test days, neither object was baited on Trial 1
SERIAL
DISCRlhilNATlON
3-N
of each discrimination and the chosen object on that trial was incorrect on the remaining trials. For half of the Ss the object chosen on Trial I was correct on even numbered test days and incorrect on odd numbered test days whereas the reverse order obtained for the other half of the Ss. Experiment 2. Following the completion of Experiment 1, the SS lvert’ trained on seven discrimination problems over a 5-day period. The proI)lems were presented serially for a total of 42 trials per day. The purpos(\ of this training was to train the Ss thoroughly on seven discriminations to be used in the Old-Problems condition described below. The Ss were then given further discrimination trials for a 64 day period under each of the following four conditions. One test condition prevailed during each test day for each S. (a) Short Zntervub. A single 7-trial discrimination was presented with 30-WC intervals between the onsrts of successive trials. (b) Delay. A single 7-trial discrimination was presented with a 4-min intertrial interval. (c) New-Problems. Eight ‘i-trial discrirninations were presented in a single serial list with the onsets of succclssivcb trials separated by 30 sec. The interval between successive trials of any partic&r discrimination was therefore 4 min, the same as for the single problem presented in the Delay condition. A new set of discrimination objects was used each day the New-Problems condition was in effect. (d) Old-Problems. Eight 7-trial discrimination problems were presented serially in the same manner as in the New-Problems condition. Seven of the problems, however, were always those discriminations learned just prior to the beginning of Experiment 2. The rtxward values of the objects in these recurring discriminations remained constant. Problem 8 was formed from new objects and, over all testing days. appeared equally often at all positions in the seria1 list. The presentation order of the above four conditions within succcssivc 18day blocks was determined by a balanced set of four 4 x 4 Latin squares, with a different set in effect for each block. Two Ss wercl therefore tested under each condition on each test day. Furthermore, on each test day, the object pair used for the new discrimination problem in the> Old-Problems condition was also used for the single discrimination in th(x Short Interval and Delay conditions as well as one discrimination in th
250
JAMES
E.
KING
RESULTS
Experiment 1. Figure 1 depicts the percentage of correct responses as a function of list length, Trial 1 outcome, and intraproblem trial blocks. Clearly, performance was higher following Trial 1 reward than following Trial 1 nonreward (F( 1,7) = 30.42, p < 901) and declined systematically as list length increased from one to eight problems (F( 3,21) = 7.49, p < 905). In addition, performance increased over intraproblem trial blocks (F( 2,14) = 22.70, p < .OOl) but at nearly equivalent rates for all list lengths. The only significant interaction was preference X list length X trials (F( 6,42) = 2.41, p < .05).
1&5 ,,..... A-..-.60- ,,=‘::.‘: /‘: ,_,.._ . ..“’ ......’
5ow TRIALS FIG. 1. Percentage of correct responses in Experiment length, Trial 1 outcome, and intraproblem trial blocks.
1 as a function
of list
A further analysis was made for the stimulus perseveration error factor (Harlow, 1950) according to the technique described in detail by Leary ( 1958) and by King and Goodman ( 1966). Stimulus perseveration occurs when an S makes continued responses to an incorrect stimulus and therefore commits an excessive number of errors prior to the first correct response in each problem, More specifically, stimulus perseveration is said to be present when the number of consecutive errors prior to the first correct response of each problem exceeds the number of such consecutive errors that would be expected based upon the overall error rate for each trial. To measure stimulus perseveration, the predicted total number of repeated consecutive errors on Trials 2-6 was determined using error rates for unique combinations of S, trial, and list length. The total stimulus perseveration was then defined as the observed minus the
SERIAL
251.
DISCRlMIiYATIO~
predicted consecutive errors. The proportion of stimulus perseveration errors was calculated by dividing the number of stimulus perseveration errors on each trial by the total number of errors occurring on that trial. The differences among the percentages of stimulus perseveration errors for the 1, 2, 4, and 8 problem lists were small and nonsignificant (.061. .054, .077, and .091, respectively). Similarly there was little differenct among the proportions of stimuh perseveration errors occurring on Trials 2-6 averaged over all list lengths (.071, .084, .084, .066, and .047. respectively). Experiment 2. Figure 2 depicts the percentage of correct responses following rewarded ( + ) and nonrewarded ( - ) first trials on thci discrimination problems common to each of the four experimental conditions. As in Experiment I, performance was substantially higher following rewarded Trial 1 than following nonrewarded Trial I (F( 1,7) = 66.60. p < .OOl) and there was marked intraproblem learning (F( I,7) = 37.02. p < .OOl) . An analysis of variance revealed a significant difference among the four experimental conditions (F( 3,21) = 3.88, p < .05). None of the interactions among the three main effects approached statistical significance.
‘“Oo
--
+ IMMEDIATE
NEW
-
,_ +
-
OLD
FIG. 2. Percentage of correct responses in Experiment 2 as a function condition and Trial 1 outcome (reward = +, nonreward = -- ).
of testing
The differences among the experimental conditions were analyzed further by three a priori comparisons. First, the difference between the Immediate and the Delay conditions was negligible and not significant thereby indicating that successive discrimination learning is not substantially affected by an increase in intertrial interval from 30 SW to 4
252
JAMES
E.
KING
min. The second comparison between the Delay condition and the combined New- and Old-Problems conditions was significant (F( 1,21) = 4.48, p < .05) and indicated that for a constant 4-min intertrial interval within a particular discrimination problem, performance is lower when the Ss received one-trial presentations from seven other discriminations during that interval than when the Ss simply remained in the test cage and received no intervening trials. The final comparison between the New- and Old-Problems conditions was not significant. Stimulus perseveration was calculated as in Experiment 1. The mean proportion of stimulus perservation errors, averaged over Trials 2-6 of each problem, was .147, .231, .126, and .264 for the Immediate, Delay, Old- and New-Problems conditions, respectively. The corresponding proportions for Trials 2-6 averaged over all conditions were ,154, .258, .144, .177, and .152, respectively. The differences among the proportions were not significant in either of these two series. However, the overall proportion of stimulus perseveration errors was significantly greater in Experiment 2 than in Experiment 1 (t( 6) = 3.27, p < 62). DISCUSSION
The experiments described here demonstrated the expected significant decrease in performance as list length was increased from one to eight problems; however, the magnitude of this decrease, about 8% in both experiments, was remarkably small. Apparently, squirrel monkeys are able to learn at least eight discrimination problems simultaneously with almost the same efficiency as they learn such problems successively. King and Goodman (1966) previously observed that an increase in list length from one to four problems also had a relatively small effect upon discrimination learning by previously untrained squirrel monkeys and rock squirrels. In both experiments, performance was consistently higher following rewarded Trial 1 than following nonrewarded Trial 1. Naive rhesus monkeys typically make the fewest errors following rewarded Trial 1, but with continued discrimination training the difference diminishes and is reversed in highly sophisticated animals which typically commit the fewest errors following nonrewarded Trial 1 (Harlow & Warren, 1952; Riopelle, 1953, 1955; Behar, 1961). Clearly, the higher performance of the squirrel monkeys following rewarded Trial 1 is much more resistant to practice effects than that of rhesus monkeys since the squirrel monkeys in this experiment had extensive previous experience on multiple discrimination learning, and their superior performance following rewarded Trial 1 was even slightly greater in Experiment 2 than in Experiment 1. This result may be attributed to the squirrel monkeys’ relatively
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strong tendency to make repeated responses to incorrect stimuli chosen on Trial 1. Direct evidence of this tendency was provided by the increase in the proportion of stimulus perseveration errors from Experiment 1 to Experiment 2 and the consistency of this proportion over Trials 2-6, which was especially evident in Experiment 2. This persistence of stimulus perseveration occurred despite the fact that the overall error rate decreased over Trials 2-6 and remained about the same in Expcriment 2 as in Experiment 1. In contrast, stimulus perseveration in rhesus monkeys decreases over Trials 2-6 and becomes negligible in highly trained animals (Harlow, 1950; Leary, 1958). In both experiments the squirrel monkeys showed no consistent intcraction between list length and Trial 1 outcome. This result fails to support King and Goodman’s ( 1966) earlier supposition that the amount of learning derived from nonrewarded trials decreases as list length increases. The virtually identical performance under the Immediate and Delay conditions in Experiment 2 is evidence that an increase from 30 set to 4 min in the interval between successive trials of a discrimination problem is not appreciably responsible for whatever performance decrt>ment accompanies an increase in list length from one to eight problems. Similarly, other research with rhesus monkeys has indicated that variation of the intertrial interval has small and inconsistent effects upon performance (Harlow & Warren, 1952; Riopelle & Churukian, 1958; Fletcher & Cross, 1964). The squirrel monkeys committed more errors under the Delay condition than under the combined New- and OldProblems conditions. Since the within-problems intertrial interval was 3 nun under all of these conditions, the lower performance under the New- and Old-Problems conditions may be attributed to intralist associative interference occasioned by interpolation of extraneous discrimination problems between successive trials of a given discrimination problem. However, since increasing list length caused only a small performance decrement in the performance of the squirrel monkeys in this experiment as well as those tested by King and Goodman ( 1966) it appears that highly trained as well as naive squirrel monkeys arc’ relatively resistant to intralist interfcrcncc while learning serial discriminations. The inconsequential diffcrencc bctwecn the New- and Old-problems conditions showed that both new and old, well-learned discriminations have about ctruivalent disruptive cfl’ects within a serial discrimination. Since the familiar problems in the Old-Problems condition were well learned, thev presumably cans4 little intralist interference. The low degree of similarity among the new stinriilns objects used in this cx-
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periment apparently minimized the intralist confusion of objects in the New-Problems condition as well and therefore failed to depress performance below that in the Old-Problems condition. The lack of a significant performance difference under these two conditions further emphasizes the squirrel monkeys’ marked insensitivity to intralist associative interference. REFERENCES Learned avoidance of nonreward. Psychological Reports, 1961, 9, 43-52. D. W. Retention of object discriminations by learning set experienced monkeys. Dissertation Abstracts, 1968, 28, 4771. DAHBY, C. L., & RIOPELLE, A. J. Differential problem sequences and the formation of learning sets. Journal of Psychology, 1955, 39, 105-108. FLETCHER, H. J., & CROSS, H. A. Effects of Trial 1 reward contingency, intertrial interval, and experience on intraproblem discrimination performance of monkeys. Journal of Comparative and Physiological Psychology, 1964, 57, 318-320. HAHLOW, H. F. Analysis of discrimination learning by monkeys. Journal of ExperiBEHAR, I. BESSEMER,
mental HARLOW,
Psychology,
1950,
H. F., & WARREN,
46, 26-39.
and transfer of discrimination learning sets. Journal of Comparative and Physiological Psychology, 1952, 45, 482-489. HAYES, K. J., THOMPSON, R., & HAYES, C. Concurrent discrimination learning in chimpanzees. Journal of Comparative and Physiological Psychology, 1953, 46, 105-108. KING, J. E., & GOODMAN, R. R. Successive and concurrent discrimination by rock squirrels and squirrel monkeys. Perceptual and Motor SkiZ.?.s, 1966, 23, 703-710. LEARY, R. W. Analysis of serial discrimination by monkeys. Journal of Comparative and Physiological Psychology, 1958, 50, 581-584. RIOPELLE, A. J. Transfer suppression and learning sets. journal of Comparative and Physiological Psychology, 1953, 46, 108-114. RIOPELLE, A. J. Rewards, preferences, and learning sets. PsychoZogicaZ Reports, 1955, 1, 167-173. RIOPELLE, A. J., & CHURUKIAN, G. A. The effect of varying the intertrial interval in discrimination learning by normal and brain-operated monkeys. Journal of Comparative and Physiological Psychology, 1958, 51, 119-125. (Received July 31, 1970)
J. M. Formation