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Overtraining, extinction, and shift learning in matching-to-sample discriminations in rats.

There are many studies on stimulus class formation in pigeons and rats. They make it clear that both species treat stimuli as being equivalent if they signal the same outcome in either concurrent discriminations (Delius, Ameling, Lea, & Staddon, 1g95; Dube, Callahan, & McIlvane, 1993; Nakagawa, 1978, 1986, 1992a, 1998, 1999a, 1999b, 1999c, 1999d, 2000b, 2001a) or matching- (or nonmatching)-to-sample discriminations (Aggleton, 1985; Mumby, Pine, & Wood, 1990; Nakagawa, 1992b, 1993a, 1993b, 1999b, 2000a, 2000c, 2001b, 2001c; Rothblat & Hayes, 1987; Urcuioli, 1977; Urcuioli & Nevin, 1975; Zentall & Hogan, 1974, 1975, 1976; Zentall, Sherburne, Steirn, Randall, Roper, & Urcuioli, 1992; Zentall, Steirn, Sherburne, & Urcuioli, 1991).

Nakagawa (2001c) has examined effects of overtraining on nonshift and shift (i.e., reversal shift) learning in a matching- (or nonmatching)-to-sample discrimination. Nakagawa (2001c) has reported that overtraining facilitated both nonshift and reversal shift (i.e., shift-1) learnings, and that discrimination performance of Group Nonshift was superior to that of Group Reversal (i.e., Shift-1) after overtraining. In Experiment 1 of Nakagawa (2001c), rats were trained to criterion or were overtrained with a matching- (or nonmatching)-to-sample discrimination. And they were then given either nonshift problem (Group Nonshift), in which the rule was not changed from Phase 1 training but novel stimuli were used, shift problem (Group Shift) (i.e., Shift-2), in which the rule was changed from Phase 1 training and novel stimuli were used, or reversal shift (Group Reversal) (i.e., Shift-1), in which the rule was changed but the stimuli were not changed. Overtraining facilitated shift learnings of both Groups Nonshift an d Reversal shift. Discrimination performance of Group Nonshift was superior to that of Group Reversal. From these findings of Nakagawa (2001c), two specific questions arise. Why do facilitative effects of overtraining on nonshift and reversal shift earnings take place? And what accounts for the superiority of Group Nonshift to Group Reversal in discrimination performance?. According to the theory of Nakagawa (1978, 1986, 1992a, 1993b, 1998, 1999a, 1999c, 1999d), during the original training animals learn a connection between a positive stimulus and an approach response as well as a connection between a negative stimulus and an avoidance response for each discrimination task in concurrent discriminations. But during overtraining they also form associations between the discriminative stimuli with the same response assignment. Thus, animals learn to associate the two positive stimuli as well as to associate the two negative stimuli of the two independent discriminations. The associations between the two positive stimuli and between the two negative stimuli are called "cue associations." These cue associations produce "an acquired equivalence" effect, whereby stimuli associated with the same consequence show enhanced generalization; that is, cue associations mediate the transfer of appropriate responding from one positive (or negative) stimulus to the other positive (or negative) stimulus in reversal learning (Nakagawa, 1992a). So animals form a concept of matching or nonmatching based on common response (e.g., choosing a certain goal box): animals associate configurations of stimuli with lever pressing responses (or goal box choices) (Nakagawa, 1993b). Furthermore, overtraining in a matching- (or nonmatching)-to-sample discrimination results in the development of common response to the configurations of stimuli with the same response assignment. As a consequence following overtraining, this common response to the configurations of stimuli directly transfers to subsequent shift learning. Consequently, overtraining fa cilitates shift learnings in a matching- (or nonmatching)-to-sample discrimination. These proposals are supported by the findings of Nakagawa (2001c).

Why is the performance of nonshifted animals superior to the data obtained with shifted animals (i.e., reversal shift) in a matching- (or nonmatching)-to-sample discrimination performance? One possibility for the superiority of Group Nonshift to Group Shift (i.e., Reversal shift) is that rats in Group Shift have to extinguish the rule acquired in Phase 1 training to resolve their shift problems in Phase 2 shift because the rule to resolve their problems in Phase 2 shift is different from that in Phase 1 training, whereas ones in Group Nonshift do not need to extinguish the rule acquired in Phase 1 training to resolve their shift problems in Phase 2 shift because the rule to resolve their shift problems in Phase 2 shift is the same as that in Phase 1 training. This question is a very important and even fundamental issue in behavior analysis in studying a mechanism of concept formation in matching- (or nonmatching)to-sample discriminations in rats and pigeons. This problem has received far too little experiment al attention in matching- (or nonmatching)-to-sample discriminations. There are no studies on the cause of the superiority of Group Nonshift to Group Shift (i.e., reversal shift).

The present paper reports an attempt to investigate the questions of whether or not superiority of Group Nonshift to Group Reversal in discrimination performance is due to extinction of the rule acquired in Phase 1 training. Rats were trained with a matching-to-sample discrimination to criterion or were overtrained. After completing Phase 1 training, they received extinction training, and then finally were transferred to either nonshift or reversal shift. The expectation according to the above account is that overtrained rats learn both nonshift and reversal shift in Phase 3 more rapidly than nonovertrained ones, and that there is no significant difference in the rate of learning in Phase 3 shift between nonshift and reversal shift after both criterion training and overtraining.

Method

Subjects

Thirty-two experimentally naive male Sprague-Dawley rats were used. They were about 270 days old with an initial average body weight of 550 g. Animals were handled for 5 mm a day for 12 days and were maintained on a daily 2-hr feeding schedule prior to the experiment. The amount of food in the daily ration was gradually reduced until the body weight of each animal reached 80% of the baseline weight at the start of the experiment. Water was always available for animals in their individual home cages. Animals were maintained on a 12:12-hr light:dark cycle, with lights off at 11:30 a.m.

Apparatus

A three-stimulus presentation T-type jumping stand shown in Figure 1 was used. It consisted of a runway (15 cm high, 12 cm wide, and 25 cm long) with a start box (15 cm high, 12 cm wide, and 20 cm long), two goal boxes (15 cm x 20 cm x 25 cm) and a center box (15 cm x 20 cm x 25 cm). A guillotine door was located 20 cm from the front of the start box. A piece of cardboard was placed at the entrance of the goal boxes and the center box, each 12 cm square, 15 cm from the floor, 5 cm apart from edge to edge. A jumping stand (platform) measured 54 cm wide, and 12 cm long. A gap over which animals had to jump (15 cm deep, 56 cm wide and 15 cm long) was located 20 cm from the front of a goal box. The apparatus was painted medium gray inside and was lit throughout the experiment by a 10-W fluorescent lamp suspended 40 cm above the top of the center of the apparatus.

Stimuli

Stimulus cards were 12-cm squares of cardboard. A sample stimulus was presented at the entrance of the center box. A comparison stimulus card was presented at the entrance of each goal box and served as an entrance door. The comparisons were arranged so that the card serving as the correct door could be pushed down easily, thus permitting animals to gain entrance into the goal box, whereas the card denoting the incorrect door was locked. Four stimulus cards were used: white stimulus card, black stimulus card, vertically striped stimulus card, and horizontally striped stimulus card. The vertically or horizontally striped stimulus card had alternating black and white lines 1 cm in width. For a white-black stimulus set, a white 12-cm square and a black 12-cm square of cardboard were used. A vertical stripe stimulus and a horizontal stripe stimulus were used for a vertical-horizontal stripe stimulus set.

Procedure

Animals were given pretraining for 8 days prior to the beginning of a matching-to-sample discrimination training. On Day 1, animals were allowed to explore the apparatus for two periods of 7 and 5 mm. From Day 2 to Day 4, they were trained to push down a stimulus card and enter the goal box to obtain food for 10 daily trials. The gap was not present for this stage of the experiment. For Day 5 to Day 8, they were trained to jump over the gap for 10 trials a day. On the last day, all animals jumped over the 15-cm gap. They were given the same number of trials on each goal during this pretraining. Medium-gray stimulus cards were used during this period.

Phase 1 Matching-to-Sample Discrimination Training

Animals were trained for 12 trials a day with a matching-to-sample discrimination. Training continued until a criterion had been reached of 11 correct trials out of a possible 12 per day. Half of the animals were trained with the white-black stimulus set, and the remaining animals were trained with the vertical-horizontal stripe stimulus set. On the white-black stimulus set, a white card was a sample stimulus on some trials and a black card on the other trials in random order within each session. On the vertical-horizontal stripe stimulus set, a vertically striped card was a sample stimulus on some trials and a horizontally striped one on the other trials in random order within each session. A self-correction method was used, in which, if animals made an error, they were allowed to return to the platform and select the correct stimulus. The position of a positive stimulus followed four predetermined random sequences. Animals were given two 45-mg milk pellets when they made a correct response. An intertrial in terval ranged from 4 to 8 min.

Half of the animals received the same training for an additional 20 days after reaching the original learning criterion (Group OT), whereas the remaining animals received no further training in the original tasks once they had reached this criterion (Group NOT).

Phase 2 Extinction

After completing original training, animals were given the same extinction training as in Nakagawa (1986, 1999c) to criterion. That is, for animals, both doors were blocked on all trials on a matching-to-sample discrimination task, although positive comparison stimuli were still changed from side to side. Extinction continued for all animals until both positive and negative comparison stimuli were chosen equally often over 12 successive trials on each task.

Phase 3 Shift Learning

After reaching the extinction criterion, animals were divided into two groups: nonshift and reversal, matched with respect to the number of days to criteria in both the original learning and extinction. Group Nonshift were run under a nonshift condition where animals received a matching-to-sample discrimination training with novel stimuli. Group Reversal were run under a reversal shift condition in which animals received a nonmatching-to-sample discrimination training with the original stimulus set. Other aspects of the procedure were the same as during the original training.

Results

Phase 1 Training

Acquisition of Phase 1 training by a nonshift group was compared with acquisition of the corresponding training in a reversal group. These data are summarized in Table 1. A two-way analysis of variance (ANOVA) using overtraining (OT vs. NOT) and group (nonshift vs. reversal) was performed on the number of days to criterion, which revealed neither significant main effects nor significant interaction between overtraining and group (all Fs < 1). Thus, there were no significant differences in the rate of learning among four groups. The percentage of error during overtraining was 5.7%.

Phase 2 Extinction

Acquisition of Phase 2 by a nonshift group was compared with acquisition of the corresponding training in a reversal group. These data are summarized in Table 1. A Welch's method was performed on the number of days to criterion in extinction, which revealed that overtraining significantly facilitated extinction, t(14) = 4.07, p < .003.

Phase 3 Shift

Acquisition of Phase 3 shift by a nonshift group was compared with acquisition of the corresponding training in a reversal group. These data are illustrated in Figure 2. A two-way analysis of variance (ANOVA) using overtraining (OT vs. NOT) and group (nonshift vs. reversal) was performed on the number of days to criterion, which revealed a significant effect of overtraining, F(1, 28) = 48.01, p < .001. Overtraining significantly facilitated shift learnings in both Group Nonshift, F(1, 14) = 33.39, p < .001, and Group Reversal, F(1, 14) = 23.73, p < .001.

To make clear whether these present data represent a facilitation of learning for Group Reversal or a retardation of learning for Group Nonshift, acquisitions in Phase 3 by both a nonshift group and a reversal group were compared with acquisitions in Phase 2 by both a nonshift group and a reversal group in Experiment 1 of Nakagawa (2001c) without the extinction procedure. Acquisition data in Experiment 1 of Nakagawa (2001c) are summarized in Table 2. A three-way analysis of variance (ANOVA) using group (nonshift vs. shift), procedure (extinction vs. no extinction), and overtraining (OT vs. NOT) was performed on the number of days to criterion presented in Figure 2 and Table 2. The analysis revealed significant main effects of group, F(1, 56) = 28.28, p < .001, procedure, F(1, 56) = 33.47, p < .001, and overtraining, F(1, 56) = 80.08, p < .001, and a significant Group x Procedure interaction, F(1, 56) = 30.82, p < .001, a significant Group x Overtraining interaction, F(1, 56) = 4.69, p < .05, and a significant Procedure x Overtraining, F(1, 56) = 8.17, p < .01. The extinction procedure facilitated learning for Group Reversal, F(1, 60) = 25.86, p < .001, whereas it did neither facilitate nor retard learning for Group Nonshift, F(1, 60) < 1. Group Nonshift mastered their shift learning more rapidly than Group Reversal under the no extinction condition, F(1, 60) = 23.77, p < .001, but not under the extinction condition, F(1, 60) <1.

To examine transfer of the initial learning to both the subsequent matching and the nonmatching tasks in Phase 3 shift, performance on the first trial in Phase 3 shift was analyzed. Wright (1991) has proposed criteria for concept learning: (1) Stimuli: all the transfer stimuli on a transfer trial should be novel and the differences among the stimuli (transfer and learning) should be large enough that the discrimination confusion is unlikely. (2) Testing frequency: transfer testing should be limited to the first presentation of each novel stimulus, so that the results will not be confounded by a history of reinforcement and subsequent learning. (3) Performance: transfer performance should be as good as baseline performance and both should be at a good performance level (Wright, 1991, p. 252). Therefore, performance on the first trial in the shift learning was used as a measure of transfer of rule learned in Phase 1 training. The results were as follows: 75% of the animals of Group Nonshift-OT responded correct ly, and 62.5% of those in Group Nonshift-NOT responded correctly. By contrast, 50% of the animals of either Group Reversal-OT or Group Reversal-NOT responded correctly.

In order to examine whether or not stimulus-stimulus associations between the discriminative stimuli formed in Phase 1 training remain effective intact after extinction, a special measure was devised: The measure was the number of rats that had made more than 11 correct trials out of a possible 12 on the first day in Phase 3 shift (non-error learners). The number of non-error learners was as follows: 5 rats for Group Nonshift-OT, 0 for Group Nonshift-NOT, 4 rats for Group Reversal-OT, and 0 for Group Reversal-NOT. Overtraining significantly increased the number of non-error learners in both Group Nonshift, [chi square](1) = 7.27, p <.01, and Group Reversal, [chi square](1) 5.33, p < .05. There was no significant difference in the number of non-error learners between Group Nonshift and Group Reversal after overtraining, [chi square](1) < 1.

Individual scores of days to criteria in original, extinction, and shift learnings are summarized in Table 3. Inspection of Table 2 indicated that overtraining facilitated extinction in either Group Nonshift or Group Reversal. Furthermore, Table 3 made it clear that discrimination performance of 3 rats (i.e., 1, 6, and 8) in Group Nonshift-OT were more inferior to others of that group in Phase 3.

Discussion

With regard to the number of days to criterion in extinction, overtrained animals took fewer days to criterion in extinction than did nonovertrained animals. This result makes clear that overtraining facilitates extinction in matching-to-sample discrimination learning. This finding was consistent with findings of Nakagawa (1986).

On the first trial in Phase 3 shift, 75% of the animals of Group Nonshift-OT and 62.5% of those in Group Nonshift-NOT responded correctly. These results are consistent with those of two nonshift groups (i.e., Group MM and Group NN) in Nakagawa (2001 c), in which animals were not given extinction training prior to shift learning. Thus, the findings of Groups Nonshift-OT and Nonshift-NOT suggest that rats choose between a novel pair of stimuli in accordance with the rule that they learned in Phase 1 training after extinction. This indicates that the novel configuration of stimuli and the stimuli appearing in Phase 3 shift have rats make the common response after extinction. These results suggest that the rules formed in Phase 1 training remain effective intact after extinction. These results are consistent with the findings of Nakagawa (1986, 1999d) using two concurrent discriminations. This suggests that the mechanism of the formation of concepts of matching and nonmatching between configurations of stimuli in a matching-to-sample discrimination is the same as that of the formation of stimulus classes between the discriminative stimuli during overtraining in two concurrent discriminations. This is supported by Nakagawa (1 999b). Nakagawa (1999b) makes clear that transfer effects between two concurrent and matching- (or nonmatching)-to-sample discriminations in rats are governed by the same mechanism for the formation of association between stimuli (see Nakagawa, 1999b).

There was no significant difference in the rate of learning between Groups Nonshift and Reversal after criterion training and overtraining in Phase 3. These findings were consistent with the expectation according to the account mentioned in the introduction. These findings provide stronger evidence that the superiority of Group Nonshift to Group Reversal (i.e., Group Shift-1) in discrimination performance found in Experiment 1 of Nakagawa (2001c) is due to extinction of the rule acquired in Phase 1 training.

What the extinction procedure might accomplish in the context of a matching-to-sample discrimination learning is dissociation of a sample stimulus-comparison stimulus association (or connection) but not a common response to configurations of stimuli. Thus, animals of Group Nonshift could perform well when novel configuration of stimuli appeared in Phase 3. Therefore, they would acquire easily a subsequent shift learning. By contrast, animals of Group Reversal performed at chance level when the same configuration of stimuli as in Phase 1 appeared in Phase 3. To master a reversal learning in Phase 3, animals of Group Reversal had to extinguish the sample stimulus-a comparison stimulus connection acquired in Phase 1 and then to reacquire a new sample stimulus-a comparison stimulus connection. As animals of Group Reversal were experienced the extinction training in Phase 2, they would not need to dissociate the sample stimulus-comparison stimulus connection acquired in Phase 1 and so they had only to acquire a ne w stimulus-comparison stimulus connection in Phase 3. Furthermore, animals of Group Reversal would have common response to configurations of stimuli so that they would acquire easily a new sample stimulus-comparison stimulus connection in Phase 3. Common response mediates concepts of matching or nonmatching to subsequent problems. Thus, common response would facilitate the subsequent shift learning for Group Reversal. This was supported by the finding that the extinction procedure facilitated the subsequent shift learning for Group Reversal. Therefore, there was no difference in the rate of the subsequent shift learning in Phase 3 between Group Nonshift and Group Reversal.

Overtraining facilitated the subsequent shift learning in both Group Nonshift and Group Reversal. These findings were consistent with findings of Experiment 1 of Nakagawa (2001c). Nakagawa (1993b) has asserted that rats form a concept of matching or nonmatching by common response (e.g., choosing a certain goal box): Rats associate configurations of stimuli with goal box-choosing responses. For example, in a case of a MTS discrimination, rats learn to associate one configuration of stimuli (i.e., AAB and BBA) with choosing the left goal box followed by a reward and the other (i.e., BAA and ABB) with choosing the right goal box followed by a reward, in which the two side letters refer to the comparison stimuli and the center refers to the sample stimulus. They then form associations between the configurations with the same response assignment. Configurations of stimulus pairs, to which common responses are made, tend to become functionally equivalent in evoking further responses. The common response mediates co ncepts of matching and nonmatching to subsequent shift problems. Furthermore, Nakagawa (2001c) has asserted that rats have not yet enough established common response to the configurations of stimuli after criterion training, whereas they have steadily established common response to the configurations of stimuli during overtraining. Consequently, overtraining should facilitate subsequent shift learning of Group Nonshift. These proposals are supported by the findings of the present experiment. By contrast, Lawrence's theory (1949, 1950) and Mackintosh's theory (1965) could not readily explain the findings of the present experiment. According to these two theories, overtraining should not facilitate the shift learning in Group Nonshift, because the stimuli employed in the shift phase were quite different from those in Phase 1 training. Indeed, this result was not observed in the present experiment.

The finding that overtraining significantly increased the number of non-error learners in Group Nonshift indicates that common responses to the configurations of stimuli with the same response assignment steadily acquired during overtraining remain effective intact after extinction, and they reinstate their functions when food reinforcement was reintroduced again in Phase 3 shift training. This finding is in line with the findings of Nakagawa (1986, 1999d) that an effective interchange of the discriminative stimuli between two discrimination tasks is possible after extinction training. See also Nakagawa (1986, 1999d).

Overtraining facilitated learning on Phase 3 shift of Group Reversal. Both Lawrence's theory (1949, 1950) and Mackintosh's theory (1965) could readily explain this result. This finding suggests that overtraining resulted in an acquired distinctiveness of stimuli or an increment of attention to a relevant analyzer in a matching-to-sample in which just one stimulus set was used, and an acquired distinctiveness of stimuli or an increment of attention to a relevant analyzer as a result of overtraining remained effective intact after extinction training, so that overtraining facilitated subsequent reversal shift in a matching-to-sample discrimination.

From inspection of Table 2, discrimination performances of 3 rats (i.e., 1, 6, and 8) in Group Nonshift-OT were inferior to those of the remaining rats of the group in Phase 3. This result might be due to generalization decrement of common response to the configuration of stimuli produced by the introduction of the novel stimuli.

This experiment made it clear that common response to configuration of stimuli with the same response assignment formed in Phase 1 training remains effective intact after extinction, and that superiority of Group Nonshift to Group Reversal (i.e., Shift-i) in matching- (or nonmatching)to-sample discrimination performance is due to extinction of the rule acquired in Phase 1 training. This is the novel empirical and the theoretical contribution to the literature.

[FIGURE 2 OMITTED]
Table 1

Means and Standard Deviations of Number of Days to Criteria in Both
Initial Learning and Extinction

 Initial Learning Extinction
Group /
 Mean SD Mean SD

Nonshift - NOT 13.88 3.56 1.75 0.46
Nonshift - OT 14.25 3.99 1.00 0.00
Reversal - NOT 14.63 4.53 1.50 0.76
Reversal - OT 14.25 4.56 1.00 0.00
Table 2

Means and Standard Deviations of Number of Days to Criterion in Phase 2
Shift in Experiment 1 of Nakagawa (2001c)

Group / Mean SD

Nonshift-OT 1.38 0.70
Nonshift-NOT 5.50 2.18
Reversal-OT 5.00 2.14
Reversal-NOT 13.50 4.00
Table 3

Individual Scores of Days to Criteria in Original, Extinction, and Shift
Learning

Group Rats Original Extinction Shift

NNOT 1 11 2 7
 2 13 2 5
 3 14 2 5
 4 18 1 4
 5 9 2 5
 6 13 2 4
 7 13 2 5
 8 20 1 4

RNOT 1 11 2 7
 2 13 1 6
 3 16 1 3
 4 21 1 4
 5 11 2 6
 6 9 3 7
 7 15 1 4
 8 21 1 2

NOT 1 13 1 2
 2 13 1 1
 3 16 1 1
 4 17 1 1
 5 10 1 1
 6 10 1 3
 7 13 1 1
 8 22 1 4

ROT 1 11 1 1
 2 11 1 2
 3 14 1 2
 4 23 1 2
 5 9 1 2
 6 12 1 1
 7 17 1 1
 8 17 1 1

Note. NNOT=Group Nonshift-NOT; NOT=Group Nonshift-OT; RNOT=Group
Reversal-NOT; ROT=Group Reversal-OT.


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Requests for reprints should be sent to E. Nakagawa, Department of Psychology, Kagawa University, 1-1 Saiwai-Cho, Takamatsu, Kagawa, 760-8522, Japan. (E-mail: esho@ed.kagawa-u.ac.jp).
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Date:Mar 22, 2002
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