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EFFECTS OF OVERTRAINING ON REVERSAL AND NONREVERSAL LEARNING ON CONCURRENT DISCRIMINATIONS IN RATS.

Two experiments examined the positive extradimensional shift effect of overtraining in concurrent mixed discriminations. In Experiment 1 rats were trained on two concurrent simultaneous discriminations and then either reversed on both tasks (Group W), or reversed on one task while a second novel discrimination was introduced (Group CMS). After initial training, two novel discrimination tasks were presented to a third group of rats (Group EDS). Overtraining facilitated extradimensional shift in Group CMS, whereas it neither facilitated nor retarded shift learning in Group EDS. In Experiment 2 using two concurrent go/no-go discriminations with the same design as in Experiment 1, overtraining facilitated extradimensional shift in Group CMS, whereas it delayed the corresponding shift in Group EDS. These findings make it clear that an important factor that determines the animals performance is not only stimulus properties but also the nature of the response required.

The old dispute about whether or not overtraining facilitates reversal and nonreversal (i.e., extradimensional) shifts in a single discrimination task has generally been resolved by conceding that it facilitates reversal shift, whereas it delays or does not facilitate at least nonreversal (i.e., extradimensional) shift with a few exceptions, in which overtraining facilitates extradimensional shift (Hall, 1974; Mandler, 1968; Mandler & Hooper, 1967; Waller 1970, 1971). However, recently some studies, using a whole-partial reversal procedure, have reported that overtraining facilitated reversal shift (i.e., a positive overtraining reversal effect: positive ORE) under one training condition, say, of a whole reversal condition, in which both discrimination tasks were reversed, whereas it retarded reversal shift (i.e., a negative ORE) under the other training condition, say, of a partial reversal condition, in which only one of them was reversed, in concurrent discriminations (Nakagawa, 1978, 1986, 1992, 1998). N akagawa (1999a) has shown that rats substitute one discriminative stimulus for the other discriminative stimulus in other contexts after overtraining. Nakagawa (1992) has shown that the effects of overtraining on reversal in concurrent discriminations differed from those observed in a single discrimination. These findings suggest that it is possible that overtraining facilitates nonreversal (i.e., extradimensional) shift in concurrent discriminations differently from in a single discrimination in rats. These findings produce a specific question. Does overtraining facilitate nonreversal (i.e., extradimensional) shift in concurrent discriminations, for example? What precise condition favors the facilitation of extradimensional shift, for example? This is a very important and even fundamental issue in behavior analysis in studying the effects of overtraining on reversal and nonreversal shifts in rats and pigeons. This problem has received far too little experimental attention.

Nakagawa (1980a) tested the validity of the selective attention theory of discrimination learning, using an overtraining-shift procedure in an altered learning situation. In this experiment, 24 female rats were concurrently trained on a white-black and a horizontal-vertical stripes discrimination task to criterion or were overtrained in a Y maze, and then shifted to a circle-triangle (i.e., shape), and a large-small trapezoid (i.e., size) discrimination task in the same maze (Group S) or to successive discrimination of the shape and the size in a straight runway (Group D). Extradimensional shift was more difficult to learn in Group S than Group D, and overtraining interfered with the shift learning in both Groups S and D. These results support the selective attention theory (Mackintosh, 1965a).

Nakagawa (1978, 1986, 1992, 1998), using the whole-partial reversal procedure which compared rats' performance on whole reversal sessions (both stimulus pairs were reversed, from A+C-, B+D- to A-C+, B-D+ for example) with rats' performance on partial reversal sessions (only one pair was reversed, from A+C-, B+D- to A-C+, B+D- for example), have reported that rats formed stimulus classes between the discriminative stimuli with the same response assignment after overtraining. In a series of experiments of Nakagawa, rats were concurrently trained to criterion or were overtrained on two discriminations (A+C- and B+D- for example) in both a simultaneous (1986, 1992, 1998) and a go/no-go successive concurrent discrimination (1992, 1998) in Phase 1 training. After completing Phase 1 training, they received either a partial reversal (A-C+, B+D- or A+C-, B-D+ for example) or a whole reversal (A-C+, B-D+ for example) in Phase 2 reversal. The rats for which both discriminations were reversed took fewer days to learn th eir reversal than those for which only one discrimination of the two tasks was reversed after overtraining, but not f reversal occurred immediately upon reaching criterion. Overtraining facilitated the whole reversal, whereas it retarded the partial reversal. These findings indicate that overtraining does not always facilitate reversal shift in concurrent discriminations, and that overtraining facilitates the formation of both heterogeneous and homogeneous associations between the discriminative stimuli.

Nakagawa (1992) has examined the whole-partial reversal effect of overtraining in concurrent discriminations and assessed the effect against single discrimination training in rats. In Experiment 1 of Nakagawa (1992), overtraining facilitated reversal in Group W, in which rats were given concurrent training on two simultaneous discrimination tasks in the original learning before both tasks were reversed. By contrast, overtraining delayed reversal in Group P, in which rats were given the same training as in Group W in the original learning, but only one of the two tasks was reversed. After overtraining, Group W reversed more rapidly than Group P. Group C, in which the rats were trained in only one discrimination task before this task was reversed, also reversed more rapidly than Group P after overtraining. In contrast, after criterion training, Group P reversed more rapidly than both Groups W and C. Experiment 2 investigated the effects of overtraining on the reversal of a successive discrimination in Groups W , P, and C. In addition, the rats in a further group, Group 5, received the same concurrent training as Groups W and P before one discrimination task was omitted and the other was reversed. Overtraining facilitated reversals in Groups W, C, and S, whereas it delayed reversal in Group P. After overtraining, Group W reversed more rapidly than Groups P, C, and S. Both Groups S and C also reversed more rapidly than Group P. After criterion training, Group P reversed more rapidly than Groups W, C, and S. These findings indicate that the effects of overtraining on reversal learning in concurrent discriminations differ from those observed in single discrimination.

Nakagawa (1999a) has examined whether the members of each stimulus class between the discriminative stimuli formed during overtraining became functionally equivalent in concurrent discriminations. In Experiment 1 of Nakagawa (1999a), rats were trained on two discrimination tasks to criterion or were overtrained. Then they were tested on two new discriminations, in which the negative stimuli for the original discriminations were exchanged. This manipulation had little disruptive influence on rats' subsequent choices after overtraining, but not after criterion training. The effect of overtraining on exchanging the negative stimuli of two discriminations was replicated in Experiment 2 of the study. Experiment 2 makes it clear that the members of each pair of stimuli begin to become functionally equivalent after receiving overtraining for 10 days, and become perfectly functionally equivalent after receiving overtraining for 20 days. In Experiment 3 of the study, half of the rats were concurrently trained on two discriminations to criterion or were overtrained. The remaining rats were separately trained in one discrimination on odd days in the training phase and the other on even days to criterion or were overtrained. Then all rats received testing, in which the negative stimuli between the two discriminations were exchanged. There was no significant difference in test performance after overtraining between these two groups, which were very excellent. These findings of Experiments 1, 2, and 3 indicate that stimulus classes established during overtraining have two properties of stimulus equivalence relation: exchangeability and substitutability, in rats. These findings, especially the findings of Experiment 3, suggest that it is quite possible that overtraining facilitates nonreversal, say, of extradimensional shift.

Nakagawa (1980b) has examined the effects of overtraining on the subsequent discrimination shift learning by using existence or nonexistence of an antecedent relevant dimension in the subsequent discriminations in young children. In this experiment, young children were concurrently trained on two discrimination tasks: circle versus triangle and cross versus T-shape tasks to criterion or were overtrained. After completing Phase 1 training, they were divided into five subgroups: W, in which both tasks were reversed, P, in which only the circle-triangle task was reversed, HEDS-l, in which the circle-triangle task was not reversed and the cross-T-shape task was replaced by a new discrimination task (i.e., vertical-horizontal stripes task), HEDS-II, in which the circle-triangle task was reversed, and the cross-T-shape task was replaced by a new discrimination (i.e., vertical-horizontal stripes task), and EDS, in which both the circle-triangle and the cross-T-shape tasks were replaced by new two discriminations (i .e., vertical-horizontal stripes and colored-noncolored diamond task). Overtraining facilitated reversal learning in both Groups W and HEDS-II, whereas it delayed reversal learning in Group P. Overtraining facilitated nonreversal, say, of extradimensional shift of both Groups HEDS-I and HEDS-II, whereas it neither facilitated nor delayed nonreversal shift in Group EDS. These findings suggest that overtraining facilitated nonreversal, say, of extradimensional shift when young children were trained on a new nonreversal shift task with the discrimination task that they were trained on Phase 1 training.

On the basis of the findings in Nakagawa (1978, 1980b, 1986, 1992, 1998, 1999a), it is reasonable that overtraining facilitates nonreversal shift in concurrent discriminations.

A question that remains is whether or not overtraining facilitates nonreversal, say, of extradimensional shift in concurrent discriminations in which both an extradimensional shift task and the old discrimination task experienced in the preshift learning phase are given at the same time in rats or pigeons. Not clear from past works, however, was whether or not overtraining facilitates extradimensional shift in concurrent discriminations in rats. The present experiments were conducted to investigate whether or not overtraining facilitated reversal and nonreversal shifts in concurrent discriminations in rats. This research will make some headway towards understanding the effects of overtraining on reversal and nonreversal shifts in rats.

Experiment 1

Experiment 1 was conducted to investigate whether or not overtraining facilitated a new nonreversal, say, of extradimensional shift task that was given to rats with a reversal of simultaneous discrimination task at the same time in concurrent discriminations. Rats learned to discriminate concurrently two pairs of simple stimuli where the responses to one stimulus of each pair were rewarded and the responses to the other were not rewarded (A+C-, B+D- for example) to criterion or were overtrained. And then they were divided into three groups of whole reversal shift (W), in which the two tasks were reversed, concurrent mixed shift (CMS), in which only one of the two tasks was reversed and the other was replaced by a new discrimination, and extradimensional shift (EDS), in which the two tasks experienced in Phase 1 training were replaced by two new discrimination tasks. The expectation according to the findings of Nakagawa (1980a, 1980b, 1999a) is that overtraining facilitates reversal of both Groups W and CMS a nd a nonreversal shift, say, of extradimensional shift of Group CMS, whereas it neither facilitates nor delays extradimensional shift of Group EDS.

Method

Subjects. Thirty-six experimentally naive male Sprague-Dawley albino rats were used. They were 210 days old, with an average initial body weight of 538 g. The animals were given handling for 5 mm a day for 12 days. They were maintained on a 2-hr feeding schedule each day. 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 cage. The animals were maintained on a 8:16-hr light:dark cycle with light off at 10:00 a.m.

Apparatus. A modified Y maze shown in Figure 1 was used. The starting box measured 15 cm in height, 12 cm in width, and 25 cm in length. The distance from the starting box to the bifurcation was 25 cm. The arms of the Y maze were 85 cm in length. At the end of each arm was a goal box. Each goal box was 15 cm in height, 15 cm in width, and 25 cm in length. A guillotine door was located at the front of the starting box. At the entrance of the goal box a piece of cardboard was placed which served as a discriminandum. A gap over which animals had to jump (15 cm in depth, 12 cm in width, and 15 cm in length) was located 20 cm in front of the goal box. The apparatus was painted medium gray inside and lit throughout the experiment by two 10-W fluorescent lamps suspended horizontally 45 cm above the top of the arms.

Stimuli. Stimulus cards were 12-cm squares of cardboard. Each square was presented at the entrance of each goal box and served as an entrance door. They were arranged so that the card serving as the correct door could be pushed down easily, thus permitting the animals to gain entrance into the goal box; the card denoting an incorrect door was locked. For a white-black discrimination a white card and a black card were used. Vertically striped and horizontally striped cards were used for a vertical-horizontal stripe discrimination. Striped cards had alternating black and white lines 1 cm in width. For a triangle-circle discrimination an equilateral triangle with 10-cm sides and a circle with a diameter of 7.5 cm were used. For a [GRAPHIC EXPRESSION NOT REPRODUCIBLE IN ASCII] - [GRAPHIC EXPRESSION NOT REPRODUCIBLE IN ASCII] disc and a [GRAPHIC EXPRESSION NOT REPRODUCIBLE IN ASCII] with a figure area of 42.25 [cm.sup.2] were use figures of circle, triangle, [GRAPHIC EXPRESSION NOT REPRODUCIBLE IN ASCII], and [GRAPHIC EXPRESSION colored background.

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

Phase 1: Discrimination training. The animals were concurrently trained for 12 trials a day with two concurrent discrimination tasks: white versus black and vertical versus horizontal stripes. That is, animals were given training in a white-black discrimination on some trials and in a vertical-horizontal stripes discrimination on the other trials in a random order in the same apparatus and within each session. Training continued until a criterion had been reached of 11 correct trials out of a possible 12 for each discrimination over 2 successive days. Training continued until animals had reached this criterion on these two discriminations at the same time. A self-correction method was used in which, if the animals made an error, they were allowed to return to the choice point and select a correct stimulus. The positive and negative stimuli were counterbalanced. The order of trials with the two tasks followed four predetermined random sequences. The position of a positive stimulus also followed four predeterm ined sequences. The animals were given two 45-mg milk pellets when they made a correct response. Intertrial intervals ranged from 4 to 8 min.

After satisfying the learning criteria for Phase 1, half of the animals received an additional 20 days of training (Group OT), whereas the remaining animals received no further training in Phase 1 discrimination training once they had reached these criteria (Group NOT).

Phase 2: Shift. After completing Phase 1 training, the animals of each group were then divided into three subgroups (W, CMS, and EDS), matched with respect to the number of days to reach the criterion. Group W was run under the reversal shift condition, in which the two discriminations were reversed. Group CMS was run under the concurrent mixed shift condition, in which only the white-black task was reversed and the other was omitted and the [GRAPHIC EXPRESSION NOT REPRODUCIBLE IN ASCII] - [GRAPHIC introduced instead of the omitted task. That is, animals in Group CMS were given both reversal learning of the white-black discrimination on some trials and in the [GRAPHIC EXPRESSION NOT REPRODUCIBLE IN ASCII] - [GRAPHIC EXPRESSION NOT R in a random order within each session. Training continued until animals had reached the criterion on these two discriminations at the same time. Group EDS was run under the extradimensional shift condition, in which both the white-black and the vertical-horizontal stripes tasks were omitted and two discriminations of triangle versus circle and [GRAPHIC EXPRESSION NOT REPRODUCIBLE IN ASCII] versus [GRAPHIC EXPRESSION NOT REPRODUCIBL animals in Group EDS were given training in the triangle-circle discrimination on some trials and in the [GRAPHIC EXPRESSION NOT REPRODUCIBLE IN ASCII] - [GRAPHIC E the other trials in a random order within each session. Training continued until animals had reached the criterion on these two discriminations at the same time. All details of training on Phase 2 shift were the same as in Phase 1 training.

Results

The group mean days to criterion in Phase 1 are shown in Table 1. There was no indication of a difference between the six groups in the rate at which the animals learned in Phase 1, and this observation was supported by statistical analysis. An ANOVA using group (W vs. CMS vs. EDS), overtraining (OT vs. NOT), and task (B-W vs. H-V) revealed that neither the main effects nor the interaction was significant (all Fs [less than] 1). Thus, there were no significant differences in the rate of learning among the six groups. The standard deviations of Table 1 appeared to be larger than differences between the means. This was caused by the arrangement to equalize total number of days to criterion in the original learning for each group.

The results for each group in Phase 2 are illustrated in Figure 2. Overtraining facilitated reversals in both Groups W and CMS. Group CMS !earned the shift task (a [GRAPHIC EXPRESSION NOT REPRODUCIBLE IN ASCII]-[GRAPHIC EXPRESSION NOT An ANOVA using group (W vs. CMS) and overtraining (NOT vs. OT) was performed on the number of days to criterion on the reversed task, which revealed a significant effect of overtraining [F(1, 20) = 19.76, p [less than].001], the Group x Overtraining interaction was marginally significant [F(1, 20) = 3.76, p [less than].07], but the effect of group was not significant (F [less than] 1). Overtraining significantly facilitated reversal in Group W [F(1, 20) = 20.37, p [less than].001], and it marginally facilitated reversal in Group CMS [F(1, 20) = 3.14, p [less than] .09]. After overtraining, Group W learned reversal marginally more rapidly than Group CMS did [F(1, 20) = 3.41, p[less than] .08], but there was no significant difference in the number of days to criterion between Groups W and CMS after criterion tra ining (F[less than]1). Overtraining significantly facilitated reversal of the vertical-horizontal stripe task in Group W [mean days to criterion was 17.2 (SD = 10.8) for the OT animals and 58.7 (SD = 25.6) for the NOT animals] [t(10) = 3.34, p [less than] .01].

An ANOVA was performed on the number of days to criterion in the common shift task ([GRAPHIC EXPRESSION NOT REPRODUCIBLE IN ASCII] - [GRAPHIC EXPRESSION NOT REPRODUC effect of overtraining [F(1, 20) = 4.57, p [less than] .05]. The Group x Overtraining interaction was significant [F(1, 20) = 5.85, p [less than] .03]. Overtraining significantly facilitated shift learning in Group CMS [F(1, 20) = 10.39, p [less than] .004], whereas it neither facilitated nor retarded shift learning in Group EDS (F [less than] 1). After overtraining, Group CMS significantly learned the shift task more rapidly than Group EDS did [F(1, 20) = 4.94, p [less than] .01], whereas there was no significant difference in the number of days to criterion between Groups CMS and EDS after criterion training [F(1, 20) = 1.44]. Overtraining did not facilitate shift learning on the triangle-circle task in Group EDS [mean days to criterion was 42.7 (SD = 14.2) for the OT animals and 41.2 (SD = 13.8) for the NOT animals] (t [less than] 1).

In order to examine the tendency to adopt a position preference during Phase 2, a special criterion was devised: If the animals chose a particular side (right or left) more than 10 times out of the 12 daily trials, the day was regarded as a positional-response day. The number of these days was counted for each animal, and their means and SDs during Phase 2 are summarized in Table 2 for each group. An ANOVA revealed that the effect of overtraining was significant [F(1, 30) = 8.77, p [less than] .01], whereas the effect of group and the Group x Overtraining interaction were not significant (Fs [less than] 1).

Discussion

Overtraining facilitated reversal learning in Groups W, and it marginally facilitated reversal of Group CMS. These findings are consistent with the expectation according to the findings of Nakagawa (1980a, 1980b, 1999a). These findings are also explained by extant theories of discrimination learning: acquired cue distinctiveness theory (Lawrence, 1949, 1950), response of discrimination theory (Pubols, 1956; Reid, 1953), selective attention theory (Mackintosh, 1965a), analyzer hierarchy theory (Sutherland & Mackintosh, 1971), and response strategy or response pattern theory (Hall, 1973a, 1973b, 1974; Mandler, 1966, 1968; Mandler & Hooper, 1967). All these theories predict that overtraining should produce comparable positive ORE in Groups W and CMS, and the results were observed.

Overtraining significantly facilitated the nonreversal shift of Group CMS, whereas it neither facilitated nor delayed the nonreversal shift of Group EDS. These findings agree with the expectation according to the findings of Nakagawa (1980a, 1999a). These findings agree with the findings of Nakagawa (1980b) with young children. These findings are not readily explained by the extant theories of discrimination learning mentioned above. That is, all acquired cue distinctiveness theory, selective attention theory, and analyzer hierarchy theory predict that overtraining should neither facilitate nor delay the subsequent extradimensional shift. But these results were not observed. By contrast, response strategy or response pattern theory predict that overtraining should facilitate the subsequent extradimensional shift in both Group CMS and Group EDS. But these results were not observed.

Experiment 2

The findings in Experiment 1 indicate that overtraining facilitates reversal of both Groups W and CMS, and nonreversal of Group CMS, whereas it neither facilitates nor delays nonreversal shift of Group EDS. The present experiment was conducted to replicate the effects of overtraining on reversal and nonreversal shift among these three groups of W, CMS, and EDS using a successive discrimination procedure to test the generality of positive OREs of both Groups W and CMS, a facilitative effect of overtraining on nonreversal shift of Groups CMS, and no effect of overtraining on nonreversal shift of Group EDS observed in Experiment 1.

Method

Subjects. Forty-eight experimentally naive male Sprague-Dawley albino rats were used. They were about 180 days old, with an average initial body weight of 500 g. All details of feeding schedule and handling were the same as in Experiment 1. They were maintained on a 10:14-hr light:dark cycle with light off at 11:00 a.m.

Apparatus. A straight runway described by Nakagawa (1992) was employed.

The stimulus cards were 12-cm squares of cardboard. Each card was presented at the entrance of the goal box and served as an entrance door. They were arranged so that on rewarded trials the card serving as the correct door could be pushed down easily, thus permitting animals to gain entrance into the goal box, whereas on nonrewarded trials the card denoting the incorrect door was locked. The same stimuli as in Experiment 1 were used for the white-black, the vertical-horizontal stripe, the triangle-circle, and the [GRAPHIC EXPRESSION NOT REPRODUCIBLE IN ASC discrimination.

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

Phase 1: Discrimination training. The animals were concurrently trained to criterion in Phase 1 discrimination learning for 12 trials a day, 6 on each of the two discriminations: white versus black and vertical versus horizontal stripes. That is, animals were given training in one discrimination (i.e., white-black) on six trials and in the other discrimination (i.e., vertical-horizontal stripes) on the remaining six trials in a random order within each session. The animals were given three rewarded trials and three nonrewarded ones per day on each task. On rewarded trials the animals were given two 45-mg milk pellets as a reward in the goal box. On nonrewarded trials they were retained for 60 sec in the runway after the guillotine door was opened. The criterion in Phase 1 discrimination training was that the median of the running time on the rewarded trials was shorter than the shortest running time on the nonrewarded trials for each task for two successive days (as in Experiment 2 of Nakagawa, 1992). Traini ng continued until animals had reached this criterion on these two discriminations at the same time. The positive and negative stimuli were counterbalanced. The order of the trials with the two tasks followed four predetermined random sequences. The order of rewarded trials and nonrewarded ones followed four predetermined random sequences. lntertrial intervals ranged from 4 to 8 min.

Half of the animals received an additional 20 days of training (Group OT). The remaining animals received no further training on Phase 1 training task once they had reached the criterion (Group NOT).

Phase 2: Shift. After completing Phase 1 training, the animals of each group were then divided into three subgroups (W, OMS, and EDS), matched with respect to the number of days to reach the criterion. Group W was run under the reversal shift condition, in which the two discriminations were reversed. Group CMS was run under the concurrent mixed shift condition, in which only the vertical-horizontal stripes task was reversed and the other was omitted and the triangle-circle discrimination was newly given. Group EDS was run under the extradimensional shift condition, in which both the white-black and the vertical-horizontal stripes tasks were omitted and two discriminations of triangle versus circle and [GRAPHIC EXPRESSION NOT REPRODUCIBLE IN ASCII] versus given. All details of training on Phase 2 were the same as in Phase 1 training.

Results

The group mean days to criterion in Phase 1 are shown in Table 3. There was no indication of difference between the six groups in the rate at which they learned in Phase 1, and this observation was supported by statistical analysis. An ANOVA using group (W vs. CMS vs. EDS), overtraining (NOT vs. OT) and task (B-W vs. H-V) revealed that neither main effects nor the interaction was significant (all Fs [less than] 1). Thus, there was no significant difference in the rate of learning among these six groups.

The results for each group in Phase 2 are illustrated in Figure 3. Overtraining facilitated reversals in both Groups W and CMS. Group CMS learned the nonreversal shift task (a triangle-circle task) faster than Group EDS did after overtraining. An ANOVA using group (W vs. CMS) and overtraining (NOT vs. OT) was performed on the number of days to criterion in the vertical-horizontal stripes task, which revealed a significant effect of overtraining [F(1, 28) = 10.98, p [less than] .003], a significant effect of group [F(1, 28) = 93.57, p [less than] .001]. The Group x Overtraining interaction was significant [F(1, 28) = 27.13, p [less than].001]. Overtraining significantly facilitated reversal of both Groups W [F(1, 28) = 85.05, p [less than] .001], and OMS [F(1, 28) = 20.38, p [less than] .01]. After overtraining Group W reversed numerically faster than Group CMS, but this contrast was not statistically significant [F(1, 28) 1.72], whereas Group OMS learned their reversal more rapidly than Group W after criteri on training [F(1, 28) = 36.22, p [less than] .001]. Overtraining facilitated reversal on the white-black discrimination task in Group W [5.3 (SD = 2.3) for the OT animals and 33.9 (SD = 6.3) for the NOT animals] [t(14) = 11.28, p[less than] .001].

An ANOVA was performed on the number of days to criterion in the triangle-circle task, which revealed a significant effect of overtraining [F(1, 28) = 9.28, p [less than] .005], a significant effect of group [F(1, 28) = 30.73, p [less than] .001]. The Group x Overtraining interaction was significant [F(1, 28) = 50.80, p [less than] .001]. Overtraining significantly facilitated nonreversal shift learning of Group CMS [F(1, 28) = 8.45, p [less than] .001], whereas it significantly retarded nonreversal shift learning of Group EDS [F(1, 28) = 52.06, p [less than] .001]. After overtraining Group CMS significantly learned the nonreversal task more rapidly than Group EDS did [F(1, 28) = 80.85, p [less than] .001]. By contrast, after criterion training there was no significant difference in the number of days to criterion between Groups CMS and EDS [F(1, 28) = 1.28]. Overtraining significantly retarded shift learning on the [GRAPHIC EXPRESSION NOT REPRODUCIBLE IN AS task in Group EDS [mean days to criterion was 29.3 (SD = 6.6) for the OT animals and 15.8 (SD= 4.6) for the NOT animals] [t(14) = 4.43, p [less than] .001].

Discussion

Overtraining significantly facilitated reversal learnings in both Group Wand Group CMS. This finding is basically consistent with the findings of the first study. After criterion training Group CMS mastered their reversal learning more rapidly than Group W, whereas there was no significant difference in the rate of the reversal learning between these two groups after overtraining. These findings are not consistent with the findings of the first study.

Overtraining significantly facilitated the extradimensional shift learning of Group CMS, whereas it significantly delayed that of Group EDS. This finding of Group CMS is consistent with the finding of Group CMS in the first study, whereas the finding of Group EDS is not in line with the finding of Experiment 1. This finding is not In line with findings of Mandler (1968), Mandler and Hooper (1967), Waller (1970, 1971), and Hall (1974). Nakagawa (1979), measuring the degree of difficulty in learning among seven discrimination tasks in the rats in a successive discrimination situation, has shown that the rats took more days to criterion in the vertical-horizontal stripes discrimination task than the white-black discrimination task, that they learned the vertical-horizontal stripes discrimination more rapidly than the rough-smooth discrimination (i.e., tactual discrimination), and that there was no significant difference in the degree of difficulty in learning between the vertical-horizontal stripes task and the triangle-circle discrimination task. Thus, the significant retardative overtraining extradimensional shift effect in Group EDS in this experiment is not due to the difference in the degree of difficulty in learning between the task in the Phase 1 training and the task in the Phase 2 shift. Mandler (1968) and Hall (1974) have used the vertical-horizontal stripes discrimination as the original discrimination task and the white-black discrimination task as the shift discrimination task in a single discrimination. Thus, the positive overtraining extradimensional shift effect seen in both Mandler (1968) and Hall (1974) is due to change from a difficult task to an easy task. These positive overtraining extradimensional shift effects are readily explained by analyzer hierarchy theory (Sutherland & Mackintosh, 1971).

There is a clear difference in results between Experiments 1 and 2. In Experiment 1 overtraining did neither facilitate nor delay nonreversal shift learning (i.e., extradimensional shift learning) in Group EDS, whereas it significantly delayed the corresponding shift learning in Group EDS in Experiment 2. This difference in overtraining extradimensional shift effects between these two experiments may be due to presence of the possibility for rats to make positional responses. Indeed, rats made many positional responses in Experiment 1, and overtraining significantly decreased the numbers of positional responses. Thus, positive overtraining inhibitive effect of positional responses may cause no significant retardative extradimensional shift effect of overtraining in Group EDS in Experiment 1, whereas no possibility of positional responses may cause significant retardative extradimensional shift effect of overtraining in Group EDS in Experiment 2.

General Discussion

Two experiments have examined whether or not overtraining facilitates nonreversal shift, say, of extradimensional shift learning in concurrent mixed shift discriminations in rats. In Experiment 1 rats were trained on two concurrent simultaneous discriminations and then either reversed on both discriminations (whole reversal shift: W), or on only one, but the other discrimination was omitted and a new discrimination was given instead (concurrent mixed shift: CMS), or both discriminations were omitted and two new discriminations were given instead (extradimensional shift: EDS). Overtraining facilitated reversal shift in Group W and marginally facilitated reversal shift in Group CMS. Overtraining significantly facilitated nonreversal shift in Group CMS, whereas it did neither facilitate nor delay nonreversal shift in Group EDS. After overtraining, Group CMS learned the nonreversal shift task more rapidly than Group EDS, whereas there was no significant difference in the rate of learning between Groups CMS and E DS after criterion training. In Experiment 2 using two go/no-go concurrent discriminations, overtraining facilitated reversal earnings in both Group W and Group CMS. Overtraining significantly facilitated the subsequent extradimensional shift learning in Group CMS, whereas it significantly delayed the corresponding shift learning in Group EDS. After overtraining, Group CMS mastered the subsequent extradimensional shift learning, whereas there was no significant difference in the rate of learning between Group CMS and Group EDS after criterion training. Thus, The positive extradimensional shift effect of overtraining in Group CMS generalizes across different discrimination procedures: simultaneous and successive discrimination procedures.

These findings of the present research are not readily explained by extant theories of discrimination learning: response strategy or response pattern theory (Hall, 1973a, 1973b, 1974; Mandler, 1966, 1968; Mandler & Hooper, 1967), selective attention theory (Mackintosh, 1965a), analyzer hierarchy theory (Sutherland & Mackintosh, 1971), and nonattention theory (Anderson, Kemler, & Shepp, 1973). Response strategy or response pattern theory predict that overtraining should produce both positive overtraining reversal effects in both Groups W and CMS and positive overtraining extradimensional effects in both Groups CMS and EDS. But the results were not observed in the present study. Selective attention theory commonly assumes that during solution of a discrimination, attention to relevant dimensions is strengthened, but attention to irrelevant dimensions is weakened. Thus, selective attention theory predicts that overtraining should produce the positive overtraining reversal effect in Groups W and CMS and no facil itative overtraining extradimensional effect in both Groups CMS and EDS. But the results were not observed in the present research. Nonattention theory (Anderson et al., 1973) assumes that each dimension of the total stimulus array acquires, through training, a control strength which depends on the consistency of reinforcement of its cues. Control strengths established in the initial training transfer positively to a reversal shift, say, of intradimensional shift but negatively to an extradimensional shift. According to the nonattention theory, overtraining should facilitate reversal shift in both Groups W and CMS but delay nonreversal, say, of extradimensional shift in both Groups CMS and EDS. But the results were not observed in each experiment of the present research.

In these experiments, overtraining facilitated reversal shift in Group W, whereas it did not facilitate in Group EDS in Experiment 1 but significantly delayed extradimensional shift in Group EDS in Experiment 2. These findings are in line with the expectation according to selective attention theory (Mackintosh, 1965a) and nonattention theory (Anderson et al., 1973) but not with the expectation according to response strategy or response pattern theories (Hall, 1973a, 1973b, 1974; Mandler, 1966, 1968; Mandler & Hooper, 1967). This indicates that selective attention theory and nonattention theory are more valid as an explanatory theory for the positive overtraining reversal effect and the negative overtraining nonreversal effect than response strategy or response pattern theories.

The main finding of the present study is that overtraining facilitated both reversal and nonreversal of extradimensional shifts in Group CMS in each experiment of the present study. This finding is in line with the finding of Nakagawa (1980b) with young children. This finding makes it clear that overtraining facilitates extradimensional shift learning under only a specific training condition, in which both a new discrimination task (i.e., extradimensional shift task) and one old discrimination task experienced in the initial training phase are given at the same time in the subsequent shift learning phase. This finding is not in line with the expectation according to selective attention theory (Mackintosh, 1965a). Both Mackintosh (1962, 1964, 1965a, 1965b) and Lawrence (1949, 1950) have conceptualized the overtraining reversal and nonreversal effects as being the result of overtraining enhancing the animals' attention to the relevant dimensions of the stimulus. However, this finding of the present study sugge sts that overtraining enhances not only the animals' attention to the relevant dimensions of the stimulus but also makes the animals' acquisition another process that facilitates both reversal and extradimensional shifts under a specific training condition, say, of concurrent mixed shift condition (CMS condition). By contrast, Nakagawa (1978, 1986, 1992, 1998, 1999b, 1999c) have assumed that one stimulus has both unlearned representation of itself and the learned representation of the other stimulus with the same response assignment during enough overtraining in concurrent discriminations in rats, so that rats dissociate between stimulus dimensions, and at the same time they form stimulus classes between the discriminative stimuli with the same response assignment. Thus, Nakagawa has conceptualized the overtraining reversal effects in concurrent discriminations as being the result of overtraining enhancing the animals' formation of stimulus classes between the discriminative stimuli with the same response ass ignment. That is, Nakagawa's view has more to do with cognitive categorization than changes in selective attention to the physical dimensions of the stimuli. According to Nakagawa's view (1978, 1986, 1992, 1998, 1999a, 1999b, 1999c), during overtraining in concurrent discriminations, one stimulus has both the unlearned representation of itself and learned representation of the other stimulus with the same response assignment and at the same time, the animals begin to form stimulus classes between the discriminative stimuli with the same response following the same consequence. These stimulus classes produce an acquired equivalence effect, whereby stimuli associated with the same consequence show enhanced generalization between tasks; that is, these stimulus classes mediate the transfer of appropriate responding from one positive (or negative) stimulus to the other positive (or negative) stimulus in reversal learning.

As a result of these stimulus classes, the reversal of the one discrimination after overtraining in the whole reversal condition should exert a synergistic influence upon the reversal of the other discrimination. Each reinforcement of the new positive stimulus in one discrimination should not only enhance the strength of the approach response to this stimulus but also augment the same response to the new positive stimulus in the other discrimination via the stimulus class between the positive stimuli formed during overtraining. Correspondingly, the consequences of nonreinforcement of the new negative stimulus should also transfer between discriminations. Consequently, reversal should be faster in the whole reversal condition than the concurrent mixed shift condition in which this synergistic effect cannot operate.

The replacement of the second discrimination task for a new discrimination task, as in Group CMS in Experiments 1 and 2, removes the source of positive transfer via the stimulus classes. However, according to Nakagawa's view, the animals in Group CMS in Experiments 1 and 2 acquire such response tendencies to dissociate stimulus dimensions and to categorize the discriminative stimuli with the same response assignment during overtraining. That is, given training with the reversed discrimination (i.e., vertical-horizontal stripes discrimination) in Phase 2 shift made the animals in Group CMS persist in the required response during overtraining. This response required during overtraining will lead to facilitate the development of an approach response to the positive stimulus and of an avoidance response to the negative stimulus of the new discrimination (i.e., discrimination). Consequently, overtraining should facilitate both reversal and extradimensional shifts in Group CMS in Experiments 1 and 2. Thus, these f indings that overtraining facilitated both reversal and extradimensional shifts in Group CMS in Experiments 1 and 2 suggest that the same response to stimuli is an important factor with which the animals select a cue to solve a discrimination task.

The replacement of the two discrimination tasks for two new discrimination tasks, as in Group EDS in Experiments 1 and 2, removes the source of both positive transfer via the stimulus classes and the response tendencies after overtraining. Thus, overtraining did neither facilitate nor delay the subsequent extradimensional shift in Group EDS in Experiment 1, and it significantly delayed the corresponding shift learning in Group EDS in Experiment 2. These findings might be caused by the generalization decrement produced by the introduction of novel stimuli in Group EDS in both Experiments 1 and 2. And these findings suggest that properties of the discriminative stimuli are an important factor with which the animals select a cue to solve a discrimination task.

The two experiments make it clear that the response required is as Important as the properties of the stimuli employed: This is the novel empirical and theoretical contribution to the literature.

Correspondence concerning this article should be sent to Esho Nakagawa, Department of Psychology, Kagawa University, 1 - 1, Saiwai - Cho, Takamatsu, Kagawa, 760 - 8522, Japan. (Tel: 087-832-1533; Fax: 087-832-1417; E-mail: esho@ed.kagawa-u.ac.jp).

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Author:NAKAGAWA, ESHO
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Date:Jun 22, 2000
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