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Acquired equivalence of discriminative stimuli following two concurrent discrimination learning tasks as a function of overtraining in rats.

There are many studies on stimulus classes formation in pigeons and in rats. They make it clear that both pigeons and rats have an ability to form stimulus classes or stimulus-stimulus associations between stimuli. Especially, some studies have reported, using a whole-partial reversal procedure, that both rats and pigeons form stimulus classes between discriminative stimuli that signal either reward or nonreward during overtraining in two concurrent discriminations (Delius et al., 1995; Nakagawa, 1978, 1986, 1992, 1998; Zentall et al., 1991). A specific question, however, remains. Are stimulus classes between the discriminative stimuli which both rats and pigeons established during overtraining valid? This is a very important and even fundamental issue in behavior analysis and one that has received far too little experimental attention.

Edwards, Jagielo, Zentall, and Hogan (1982) have reported that pigeons are capable of acquiring an arbitrary stimulus class. In this experiment, pigeons were given two stimulus matching-to-sample tasks: shape matching-to-sample task on some trials and hue matching-to-sample task on other trials in a random order within session, in which correct matches of one hue and one shape were followed by corn and correct matches of the other hue and the other shape were followed by wheat. On a transfer test, when the hue samples were presented with shape comparisons, and vice versa, pigeons chose the comparisons associated with the same outcome as the sample. This finding indicates that a stimulus class consists of arbitrary stimuli, related to each other through their association with a same outcome in pigeons.

Urcuioli, Zentall, Jackson-Smith, and Steirn (Experiment 2 in 1989), using a design analogous to that used by Spradlin, Cotter, and Bexley (1973), trained pigeons on a conditional discrimination involving hue and line samples in Phase 1 training (red-vertical line and green-horizontal line associations for example). In Phase 2 training, pigeons were trained on a second task involving the hue samples from Phase 1 training and a new pair of comparison stimuli (red-vertical line and green-horizontal line associations for example). In Phase 3 test, pigeons matched the line samples from Phase 1 training to the comparisons from Phase 2 training. Urcuioli et al. (1989) found that when Phase 3 associations were consistent with the hypothesized stimulus classes, pigeons performed well above chance, whereas when the associations were inconsistent with the stimulus classes, performance levels were below chance. This finding indicates that pigeons form stimulus classes on a basis of reinforcement concordance.

Vaughan (1988) demonstrated that pigeons could form stimulus classes using a different procedure. In Vaughan's experiment, pigeons were trained to respond to a set of 20 randomly selected slides (S+s) and to not respond to a second set of 20 stimuli (S-s). After pigeons completed acquisition, the contingencies associated with both sets were reversed, and then reversed again repeatedly. After a large number of reversals, pigeons responded correctly to stimuli presented later in a session following presentation of the first few stimuli in each set. Apparently, pigeons had formed a stimulus class for each of the two stimulus sets.

Zentall et al. (1991) showed that after many-to-one overtraining in which a red (R) and vertical-line (V) sample were associated with a circle (C) comparison and a green (G) and horizontal-line (H) sample were associated with a dot (D) comparison, pigeons acquired their reversal learning faster when both sets of associations were reversed than when only one set was reversed. This finding makes it clear that pigeons form stimulus classes.

Nakagawa (1978, 1986, 1992, 1998), using a whole-partial reversal procedure which compared rats' performance on whole reversal sessions (both stimulus pairs reversed, from A+C-, B+D- to A-C+, B-D+ for example) with rats' performance on partial reversal sessions (only one pair reversed, from A-C+, B-D+ to A+C-, B-D+ for example), reported that rats could form stimulus classes. In a series of Nakagawa's experiments, rats were trained to criterion or overtrained on two concurrent discriminations (A+C-, B+D-) in both simultaneous (1986, 1998) and go-no go successive concurrent discriminations (1992, 1998) in Phase 1 training. After completing Phase 1 training, they received either partial reversal (A-C+, B+D-, or A+C-, B-D+) or whole reversal (AC+, B-D+) in Phase 2 reversal. The rats for which both discriminations were reversed took fewer days to acquire their reversal learning than those for which only one discrimination of the two tasks was reversed after overtraining, but not after if reversal occurred immediately upon reaching criterion in the original learning. Overtraining facilitated the whole reversal, whereas it retarded the partial reversal. These findings make it clear that stimulus classes between the discriminative stimuli with the same outcome can be formed during overtraining in rats.

Delius et al. (1995), using the full-half reversal procedure in their Experiment 3, have shown that pigeons take fewer trials to reach near-asymptote performance (85.5% correct) in the full-reversal condition than in the half-reversal condition, and that additional prereversal training promotes the occurrence of the full-reversal versus the half-reversal advantage in pigeons.

Edwards et al. (1982), Urcuioli et al. (1989), Nakagawa (1978, 1986, 1992, 1998), Vaughan (1988), Zentall et al. (1991), and Delius et al. (1995) indicate that both pigeons and rats form stimulus classes between stimuli.

One way to demonstrate that members of each stimulus class between the discriminative stimuli become functionally equivalent is the whole-partial reversal procedure in concurrent discriminations reported by Nakagawa (1978, 1986, 1992, 1998). The other is the serial concurrent reversal technique reported by Vaughan (1988). Not clear from the past works using either the whole-partial reversal procedure or the serial concurrent reversal technique, however, was that stimulus classes that these animals, especially rats, formed during overtraining were valid. The present experiments were conducted to investigate directly whether or not the members of each stimulus class between the discriminative stimuli with the same response assignment became functionally equivalent. There are two separate definitions of a stimulus class: The first one pertains to functional equivalence, the second pertains to the control of a specific response by one member of the stimulus class. Goldiamond (1962) argues that both are necessary for a set of stimuli to be considered a stimulus class. Thus, a primary definition of functional equivalence of stimuli is that each member of a stimulus class controls the same response. Sidman and Tailby (1982) indicate that stimulus equivalence has three properties of an equivalence relation: reflexivity, symmetry, and transitivity. The present study, however, begins with some simpler facts. In order to demonstrate the establishment of functional equivalence of the discriminative stimuli, it is necessary to confirm both exchangeability and substitutability between the discriminative stimuli of the stimulus class after training in concurrent discriminations.

Experiment 1

Experiment 1 was conducted to investigate whether or not the members of each stimulus class between the discriminative stimuli that the animals formed during overtraining in two concurrent two-choice discriminations became functionally equivalent. According to a cue-associations theory of stimulus classes formation mechanism (Nakagawa, 1978, 1986, 1992), 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. They also form associations between discriminative stimuli with the same response assignment during overtraining in concurrent discriminations, and these stimulus associations (i.e., cue associations) produce an acquired equivalence effect. As the cue-associations theory postulates, if overtraining does result in the formation of associations between the two positive or two negative stimuli, then this should result in the members of each pair becoming functionally equivalent. The exchange, say, of the two negative stimuli, after a period of overtraining should then have little disruptive influence on the animals' subsequent choices. On balance, if these stimuli were to be exchanged without overtraining, then they should not be functionally equivalent, and accuracy on the novel tasks would be poor. In order to test these predictions, animals were concurrently trained to discriminate four stimuli where responses to two stimuli were rewarded and responses to the other stimuli were not rewarded (A+B+C-D-) to reach a criterion or overtraining. After completing the original training, they were tested on two new discrimination tasks, in which the negative stimuli for the original discrimination tasks were exchanged (from A+C-, B+D- to A+D-, B+C-for example). The expectation from the cue-associations theory is that discrimination performance of the overtrained animals would be superior to those of the not overtrained ones on the test.

Four additional groups were included in the present study in order to examine the empirical questions as to whether discrimination performance was based on approach to positive stimuli, withdrawal from negative stimuli, or both, and whether associations were formed between both types of stimulus. If overtraining does result in an acquired distinctiveness of stimulus, as advocated by Lawrence (1949, 1950), or increment attentions to a relevant analyzer, as proposed by Mackintosh (1965a), then this should result in the members of each pair, or a relevant analyzer, becoming salient. The expectation according both Lawrence (1949, 1950) and Mackintosh (1965a) was that the replacement, say, of two old positive stimuli (A+B+) with new stimuli (E and F) or, say, of two old negative stimuli (C-D-) with new stimuli (E and F) after a period of overtraining should then have little disruptive influence on the animals' subsequent choices, and when these stimuli were to be replaced after criterion training, then they should not be salient, and accuracy on the novel tasks (either E+C-, F+D- or A+E-, B+F- for example) would be poor.

Method

Subjects

Forty-eight experimentally naive Sprague-Dawley rats (16 females and 32 males) were used. The female rats were about 90 days old with an initial average body weight of 170 g, and the male ones were about 170 days old with an initial average body weight of 260 g. The animals were handled for 5 min a day for 12 days. They 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 cages. The animals were maintained on a 12:12-hr light:dark cycle, with light off at 11:00 p.m.

Apparatus

An apparatus (a modified Y maze) was the same as in Nakagawa (1986, 1992, 1998).

Stimuli

Stimulus cards were 12-cm squares of cardboard. Each square was presented at the entrance of each goal box and served as entrance door. They 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, that is, animals required only little force to push down the door which weighed 12 g, sized 169 [cm.sup.2] (13 cm x 13 cm) and was 0.9 mm thick; the card denoting an incorrect door was locked. For a white-black discrimination a white card and a black one 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. The additional new stimuli used in Phase 2 test were an equilateral triangle (with 10-cm sides) and a circle (with a diameter of 7.5 cm).

Procedure

The animals were pretrained 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 enter the goal box to obtain food for 10 daily trials. The gap between the arms and goal boxes 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 animals jumped over the 15-cm gap. They were given the same number of trials on each arm during pretraining. Medium-grey stimulus cards were used during this period.

Phase 1: Discrimination training (Original learning). The animals were concurrently trained to a criterion in the original learning for 12 trials a day on two discrimination tasks: white versus black and vertical versus horizontal stripe, six on each of the two discrimination tasks. A criterion was 11 correct trials out of a possible 12 for each discrimination over 2 successive days combined. 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 predetermined random 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 reaching the criterion in Phase 1 training, half of the animals were given overtraining on the original discrimination tasks for 20 days (Group OT). The remaining animals received no further training on those tasks once they had reached the criterion (Group NOT). The animals of Group NOT were tested 12 times under a given test condition on the next day when they completed Phase 1 training.

Phase 2: Test. After completing Phase 1 training, the animals of each group were divided into three groups: IDS (interchanging the discriminative stimuli), PC (positive stimuli change), and NC (negative stimuli change). They were then tested 12 times under a given test condition. Under the IDS condition, the animals were run on two new discriminations for 12 trials, in which the negative stimuli between the original discrimination tasks were exchanged. That is, they were presented as white versus horizontal stripe and vertical stripe versus black. Under the PC condition, the animals were run on two new discrimination tasks, in which both the positive stimuli of the initial two tasks were replaced with new stimuli of the equilateral triangle and the circle; whereas the negative stimuli of the initial discriminations remained unchanged. Under the NC condition, the animals were run on two new tasks, in which both negative stimuli of the initial two tasks were replaced with new stimuli of the equilateral triangle and the circle, whereas the positive stimuli of the initial tasks remained unchanged. All details of training on test trials were the same as in Phase 1 training.

Results

The group mean days-to-criterion in Phase 1 training are shown in Table 1. There was no indication of a difference among six groups in the rate at which they learned in Phase 1 training. An ANOVA using overtraining (OT vs. NOT) and group (IDS vs. PC vs. NC) and task (B-W vs. H-V) revealed that neither main effects nor interactions were significant (all Fs [less than] 1). The percentage of errors during overtraining was 4.0%.
Table 1

Means and Standard Deviations of Number of Days to Criterion
in Phase 1 Training and Standard Errors of Mean in Experiment 1

 B-W H-V
Group/Task
 M SD SE M SD SE

IDS/OT 10.75 3.67 1.38 11.38 3.84 1.45
IDS/NOT 11.75 5.09 1.92 11.75 5.12 1.93
PC/OT 12.63 5.79 2.18 12.50 5.92 2.23
PC/NOT 8.75 2.91 1.10 9.13 2.32 0.88
NC/OT 12.00 5.39 2.03 11.63 5.45 2.06
NC/NOT 10.38 2.45 0.92 11.00 3.12 1.18

Note. W = white stimulus, B = black stimulus, H = horizontal stripe
stimulus, V = vertical stripe stimulus.


The results of test performance for each group are illustrated in Figure 1. An ANOVA using overtraining (OT vs. NOT) and group (IDS vs. PC vs. NC) was performed on the number of correct responses on the test, shown in Figure 1, which revealed that overtraining increased the number of correct responses on the test in all groups of IDS, PC, and NC [F(1,42) = 27.07, p [less than] .001].

The results of errors on the test for each group are as follows: percentage of errors was 7.0% for Group IDS-OT, 36.0% for Group IDS-NOT, 15.8% for Group PC-OT, 31.0% for Group PC-NOT, 14.0% for Group NC-OT, and 31.0% for Group NC-NOT. Group IDS-OT made no more errors on the test than that during overtraining [[x.sup.2](1) [less than] 1], whereas Group PC-OT made significantly more errors [[x.sup.2](1) = 23.8 1, p [less than] .001], and Group NC-OT also made more errors ([x.sup.2](1) = 15.30, p [less than] .01] than that during overtraining. After overtraining, Group IDS made fewer errors than either Group PC [[x.sup.2](1) = 4.33, p [less than] .05], or Group NC [[x.sup.2](1) = 3.85, p [less than] .05], whereas there was no significant difference in the number of errors between Groups PC and NC [[x.sup.2](1) [less than] 1]. By contrast, there was no significant difference in the number of errors among these three groups after criterion training [[x.sup.2](1) [less than] 1].

Discussion

Overtraining significantly increased the number of correct responses on the test in Group IDS. Also the percentage of errors on the test for the overtrained animals in Group IDS was not inferior to that during overtraining, whereas one for the counter-partner, the nonovertrained animals, was significantly inferior to that during overtraining. These results indicate that the manipulation of exchanging the negative stimuli between the two discrimination tasks has a disruptive influence on performance, but only when animals were not overtrained on the original discrimination tasks. These results are in line with the findings of Nakagawa (1986). These results make it clear that the members of each pair of the two discriminations become functionally equivalent after overtraining in two concurrent discriminations.

The reacquisition results for Group PC-OT and Group PC-NOT were the same as for Groups NC-OT and NC-NOT. This result implies that animals depended to a similar extent on the positive and negative stimuli when they solved the original discriminations. The fact that overtraining facilitated learning of the new discriminations in both Group PC and Group NC is consistent with the cue-associations theory. As a result of the cue associations established during overtraining, the learning of the one discrimination after overtraining should exert a synergistic influence upon that of the other discrimination. Each reinforcement of a 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 a positive stimulus in the other discrimination via the cue association between the positive stimuli formed during overtraining. Correspondingly, the consequences of nonreinforcement of a negative stimulus should also transfer between the two discriminations. This additional opportunity for learning is not available to the nonovertrained animals, and they learn both tasks at a correspondingly slower rate.

Overtraining significantly increased the number of correct responses on the test for Group PC and Group NC. There was no significant difference in discrimination performance, measured in terms of percentage of errors, between Group PC and Group NC after overtraining, although discriminative performance of these two groups were inferior to that during overtraining. This result suggests that after overtraining both the positive and the negative discriminative stimuli gain control over responding. This indicates that overtraining results in an acquired distinctiveness of cues, as advocated by Lawrence (1949, 1950).

After overtraining, percentages of errors on the test in both Groups PC and NC were significantly inferior to one of Group IDS. This difference may be caused by a generalization decrement produced by the introduction of novel stimuli in Groups PC and NC.

Alternatively, these results in this experiment are exactly interpreted by a simple discrimination theory (e.g., Spence, 1936, 1937). Let us consider the effect of replacing the negative stimuli in single discrimination after either criterion training (NOT) or overtraining (OT). According to simple conditioning theory (e.g., Spence, 1936, 1937) discriminative performance is determined by the strength of approach conditioned to the positive stimuli (App) and the strength of avoidance conditioned to the negative stimuli (Av). Within this framework, let us assume that App and Av each have values of 5 units after NOT but 10 units after OT so that the net conditioned tendencies to choose the positive stimuli are 10 units after NOT and 20 units after OT. Replacing the trained negative stimuli by novel ones will, of course, reduce the Av values to 0 in both cases and thus reduce the net conditioned tendencies to 5 units in the case of the NOT condition, but to 10 units in the case of the OT condition. Thus, the simple discrimination theory would predict that performance on test trials should be better following overtraining. Analogous explanations can be given for the effects of replacing the positive stimuli and of interchanging the negative stimuli.

Experiment 2

The findings in Experiment 1 indicate that the members of each stimulus class between the discriminative stimuli which animals have formed during overtraining become functionally equivalent. Thus, the findings in Experiment 1 support the notion that "functional equivalence" or "stimulus classes" can be established in rats.

Nakagawa (1978, 1986, 1992, 1998), Zentall et al. (1991), and Delius et al. (1995) suggest that stimulus classes formation is evident only after animals receive overtraining on the prerequisite discriminations. Nakagawa (1998) suggests that 20 days of overtraining of preceding discriminations is an obvious operational precondition for the occurrence of stimulus classes formation in two concurrent discriminations in rats, and that animals begin to form stimulus classes between the discriminative stimuli. Not clear from the past works, however, was the degree of overtraining required for the occurrence of exchangeability of the members of the stimulus class.

The present experiment conducted a limited parametric study of the overtraining variable. The animals 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-). After reaching a criterion of the original discrimination learning, they were then concurrently overtrained in the two discrimination tasks for either 0 day, 2 days, 10 days, or 20 days. After completing overtraining, the animals were tested on two new discrimination tasks, in which the negative stimuli for the original discrimination tasks were exchanged (from A+C-, B+D- to A+D-, B+C- for example). The expectations from both the cue-associations theory and the findings of Nakagawa (1998) are that discrimination performance on the test of the 20 days-overtrained animals would be superior to those of either the 10 days-overtrained animals, the 2 days-overtrained ones, or the 0 day-overtrained ones, and be excellent, and that discrimination performance on the test of the 10 days-overtrained animals would also be superior to those of both the 2 days-overtrained animals and the 0 day-overtrained ones.

Method

Subjects

Thirty-two experimentally naive Sprague-Dawley rats (12 females and 20 males) were used. They were about 120 days old with an initial average body weight of 275 g. All details of feeding schedule, handling, and apparatus were the same as in Experiment 1. The animals were maintained on a 14:10-hr light:dark cycle with light off at 11:00 p.m.

Stimuli

The same stimuli as in Experiment 1 were used for the white-black and the vertical-horizontal stripe discriminations.

Procedure

All details of pretraining and discrimination training (original learning) were the same as in Experiment 1.

After reaching the criterion in Phase 1 training, the animals were then divided into four groups: OT-0, OT-2, OT-10, OT-20, matched with respect to the number of days to criterion. The animals in Group OT-0 received no further training on the original discrimination tasks after reaching the original learning criterion. The animals in Groups OT-2, OT-10, and OT-20 were continued to training on the original discrimination tasks for 2 days (24 trials), 10 days (120 trials), and 20 days (240 trials) after reaching the original learning criterion, respectively.

Phase 2: Test. After completing Phase 1 training, all animals were tested 12 times under a test condition, in which the negative stimuli were exchanged. That is, they were tested on two novel discrimination tasks (white vs. horizontal stripe and vertical stripe vs. black for example). All details of training on test trials were the same as in Phase 1 training.

Results

The group mean days-to-criterion on each discrimination task in Phase 1 training for each group are shown in Table 2. There was no indication of difference among four groups in the rate at which they learned in Phase 1 training, and this observation was supported by statistical analysis. An ANOVA using Group (OT-0 vs. OT-2 vs. OT-10 vs. OT-20) and task (W-B vs. H-V) revealed that neither main effects nor the interaction were significant (all Fs [less than] 1). The standard deviations of Table 2 appeared to be larger than the difference between means. This was caused by the arrangement to equalize total number of days to criterion in the original learning for each group. The percentage of errors during overtraining was 3.7%.
Table 2

Means and Standard Deviations of Number of Days to Criterion
in Phase 1 Training and Standard Errors of Mean in Experiment 2

 B-W H-V
Group / Task
 M SD SE M SD SE

OT-0 13.13 5.86 2.21 12.75 5.72 2.16
OT-2 12.38 4.27 1.61 13.00 5.34 2.02
OT-10 10.88 3.30 1.25 10.88 3.41 1.29
OT-20 11.00 5.07 1.91 11.38 4.72 1.78

Note. W = white stimulus, B = black stimulus, H = horizontal stripe
stimulus, V = vertical stripe stimulus.


The results for each group in Phase 2 test are illustrated in Figure 2. An ANOVA using group (OT-0 vs. OT-2 vs. OT-10 vs. OT-20) was performed on the number of correct responses on the test, which revealed a statistically significant effect of group [F(3, 28) = 14.02, p [less than] .001]. A Scheffe test was run to analyze differences in the number of correct responses among the four groups. The animals in Group OT-20 made more correct responses on the test than both Groups OT-2 [F(1, 28) = 22.41, p [less than] .01] and OT-0 [F(1, 28) = 40.76, p [less than] .01], whereas there was no significant difference in the number of correct responses between Groups OT-20 and OT-10 [F(1, 28) = 2.72]. The animals in Group OT-10 also made more correct responses than both Groups OT-2 [F(1, 28) = 7.14, p [less than] .01] and OT-0 [F(1, 28) = 18.16, p [less than] .01].

Percentage of errors on the test was 34.4% for Group OT-0, 26.0% for Group OT-2, 12.5% for Group OT-10, and 2.1% for Group OT-20. The animals in Group OT-20 made no more errors on the test than that during overtraining [[x.sup.2](1) [less than] 1], whereas errors during overtraining were significantly fewer than those on the test for either Group OT-10 [[x.sup.2](1) = 9.30, p [less than] .05], Group OT-2 [[x.sup.2](1) = 51.60, p [less than] .001], or Group OT-0 [[x.sup.2](1) = 82.10, p [less than] .001]. The animals in Group OT-20 made fewer errors on the test either Group OT-10 [[x.sup.2](1) = 7.70, p [less than] .01], Group OT-2 [[x.sup.2](1) = 22.80, p [less than] .001], or Group OT-0 [[x.sup.2](1) = 33.58, p [less than] .001]. The animals in Group OT-10 also made fewer errors on the test than both Group OT-2 [[x.sup.2](1) = 5.66, p [less than] .02] and Group OT-0 [[x.sup.2](1) = 12.80, p [less than] .001]. There was no significant difference in the number of errors on the test between Groups OT-2 and OT-0 [[x.sup.2](1) = 1.58].

Discussion

The experiment essentially replicated the pattern of results of Group IDS seen in the first study: As the amount of overtraining increased, correct responses on the test increased. Discrimination performance on the test for Group OT-20, measured in terms of percentage of errors, was not inferior to that during overtraining, which was very excellent, whereas one of Group OT-10 was inferior to that during overtraining, which was good (87.5% correct response). Discrimination performance of Group OT-20 was significantly superior to that of Group OT-10, which was superior to those of both Groups OT-2 and OT-0, which were at chance level. These results indicate that the manipulation of exchanging the negative stimuli between the two discrimination tasks has no disruptive influence on the animals' discrimination performance after 20 days overtraining, whereas it has a little disruptive influence on the animals' discrimination performance after 10 days overtraining, and a disruptive influence on the animals' performance after either 2 days overtraining or criterion training. These results indicate that it is necessary for the occurrence of exchangeability of the members of the stimulus class to receive overtraining for 10 days or more. That is, these results make it clear that the members of each pair of stimuli start to become functionally equivalent after receiving overtraining for 10 days, and become perfectly functionally equivalent after receiving overtraining for 20 days, this is a new contribution. These results are in line with the finding of Nakagawa (1998).

Experiment 3

The findings in Experiment 2 indicate that the members of each pair of stimuli in two concurrent discrimination tasks start to become functionally equivalent after receiving overtraining for 10 days (120 trials), and become perfectly functionally equivalent after receiving overtraining for 20 days (240 trials).

Many studies, using the whole-partial reversal procedure in concurrent discriminations, in which animals are given training in a discrimination task A on some trials and in a discrimination task B on other trials in random order within in each session, show that both pigeons and rats form stimulus classes between the discriminative stimuli during overtraining (Delius et al., 1995; Nakagawa, 1978, 1986, 1992, 1998; Zentall et al., 1991). These findings make it clear that when the discriminative stimuli of two discrimination tasks were presented within one stimulus context, animals form stimulus classes between the discriminative stimuli during overtraining. A specific question, however, remains. When discriminative stimuli of two discrimination tasks are presented within different stimulus contexts, do animals form stimulus classes or stimulus associations between the discriminative stimuli of two discrimination tasks? That is, does substitutability of the members of the stimulus class occur after overtraining in two discrimination tasks in rats? This is a very important and even fundamental issue to analyze the second property of functional equivalence (i.e., substitutability) which animals form during overtraining in two discrimination tasks.

The present experiment examined, if the discriminative stimuli of two discrimination tasks were presented within different stimulus contexts, whether or not animals could substitute a discriminative stimulus of one discrimination task presented in one stimulus context for a discriminative stimulus of the other discrimination task in the other stimulus context. Half of the animals learned to discriminate concurrently two pairs of simple stimuli to criterion or overtraining. The remaining animals learned to discriminate separately one pair of simple stimuli on odd days in the training phase and the other pair of simple stimuli on even days to criterion or overtraining. After completing Phase 1 training, the animals received a test in which the two negative stimuli between the two discrimination tasks were exchanged. The expectation from the cue-associations theory is that there would be no significant difference in discrimination performance on the test between the animals concurrently overtrained on two discrimination tasks and the animals separately overtrained.

Method

Subjects

Thirty-two experimentally naive male Sprague-Dawley rats were used. They were about 180 days old with an initial average body weight of 420 g. All details of feeding schedule, handling, and apparatus were the same as in Experiment 1. The animals were maintained on a 10:14-hr light: dark cycle with light off at 9:00 p.m.

Stimuli

The same stimuli as in Experiment 1 were used for the white-black and the vertical-horizontal stripe discriminations.

Procedure

All details of pretraining were the same as in Experiment 1.

Phase 1: Discrimination training (Original learning). Half of the animals were concurrently trained to a criterion in the original learning for 12 trials a day on two discrimination tasks: white versus black and vertical versus horizontal stripes (Group C). All details of Phase 1 discrimination training for animals in this group were the same as in Experiment 2. The remaining animals were trained to a criterion in the original learning on the white-black discrimination task for 12 trials a day on odd days in the discrimination training phase session, and on the vertical-horizontal stripe discrimination task for 12 trials a day on even days in the discrimination training phase session, and vice versa (Group A). Other aspects of the procedure were the same as in Group C.

After reaching the criterion in Phase 1 training, half of the animals of each group received overtraining on the original discrimination tasks for an additional 20 days (Groups C-OT and A-OT), whereas the remaining animals received no further training in the original discrimination tasks once they had reached this criterion (Groups C-NOT and A-NOT). The animals of both Groups C-NOT and A-NOT were tested 12 times under a test condition on the next day when they completed Phase 1 training.

Phase 2: Test. After completing Phase 1 training, all the animals were tested 12 times under a test condition, in which the negative stimuli between the two discrimination tasks were exchanged. That is, the animals were tested on two novel discrimination tasks (white vs. horizontal stripe and vertical stripe vs. black for example). Other aspects of the procedure were the same as in Phase 1 training.

Results

The group mean days-to-criterion in Phase 1 training are shown in Table 3. There was no indication of difference among four groups in the rate at which they learned in Phase 1 training. An ANOVA using overtraining (OT vs. NOT) and group (A vs. C) and task (W-B vs. H-V) was performed on the number of days to criterion on the two tasks, which revealed that neither main effects nor interactions were significant (all Fs [less than] 1). The standard deviations of Table 3 appeared to be larger than the difference between means. This was caused by the arrangement to equalize total number of days to criterion in the original learning for each group. The percentage of errors during overtraining was 4.8%.
Table 3

Means and Standard Deviations of Number of Days to Criterion
in Phase I Training and Standard Errors of Mean in Experiment 3

 B-W H-V
Group/Task
 M SD SE M SD SE

C/OT 9.75 5.65 2.13 10.63 5.98 2.26
C/NOT 11.00 4.18 1.58 10.25 4.52 1.71
NOT 10.25 3.31 1.25 10.00 3.46 1.31
A/NOT 12.00 6.21 2.34 11.13 6.68 2.52

Note. W = white stimulus, B = black stimulus, H = horizontal stripe
stimulus, V = vertical stripe stimulus.


The results for each group in the Phase 2 test are illustrated in Figure 3. An ANOVA using overtraining (OT vs. NOT) and group (A vs. C) was performed on the number of correct responses on the test, which revealed only effect of overtraining was significant [F(1, 28) = 51.30, p [less than] .001], whereas neither effect of group (F [less than] 1) nor the interaction was significant (F [less than] 1).

The results of errors on the test for each group are as follows: Percentage of errors was 1.0% for Group C-OT, 24.0% for Group C-NOT, 4.2% for Group A-OT, and 21.1% for Group A-NOT. There was no significant difference in the percentage of errors on the test between Groups C-OT and A-OT [[x.sup.2](1) [less than] 1]. Either Group C-OT or Group A-OT made no more errors on the test than that during overtraining [[x.sup.2](1) = 2.94, [x.sup.2](1) [less than] 1, respectively].

Discussion

The experiment essentially replicated the pattern of results seen in both Experiments 1 and 2: Effects of interchanging the discriminative stimuli were observed in Group C, in that discrimination performance on the test was very excellent as well as that during overtraining, and overtraining improved test performance in Group C, whereas test performance in Group C-NOT was very poor.

The main purpose of the present experiment was to investigate the substitutability of the members of the stimulus class established as a result of overtraining in rats. In the present experiment, discrimination performance on the test for the animals in Group A-OT was very excellent as well as those of Group C-OT. Furthermore, overtraining improved discrimination performance on the test for the animals in Group A. These results indicate that the animals can substitute one discriminative stimulus for the other discriminative stimulus in other contexts after overtraining.

These results of the present experiment make it clear that stimulus classes between the discriminative stimuli which the animals established during overtraining have two properties of functional equivalence: exchangeability and substitutability.

General Discussion

Three experiments reported here examined whether or not the members of each stimulus class between the discriminative stimuli established during overtraining became functionally equivalent. In Experiment 1, the animals were trained on two concurrent discrimination tasks to criterion or overtrained. After completing Phase 1 training, they were tested on two new discrimination tasks, in which the negative stimuli between the two original discrimination tasks were exchanged. This manipulation had a disruptive influence on discrimination performance on the test, but only when the animals were not overtrained on the original discriminations. Overtraining facilitated discrimination performance on the test. These results make it clear that exchangeability of the members of the stimulus class occur after overtraining in two concurrent discriminations in rats. The effect of overtraining on exchanging the negative stimuli of two concurrent discrimination tasks was replicated in Experiment 2. In addition, Experiment 2 indicates that it is necessary for occurrence of exchangeability of the members of the class to receive overtraining for 10 days or more. That is, results of Experiment 2 makes it clear that the members of each pair of stimuli start to become functionally equivalent after the animals receive overtraining for 10 days (120 trials), and the stimuli become perfectly functionally equivalent after 20 days (240 trials). In Experiment 3, half of the animals were concurrently trained on two discrimination tasks to criterion or overtrained as in Group IDS in Experiment 1. The remaining animals were trained to discriminate separately one pair of simple stimuli on odd days in the training phase and the other pair on even days to criterion or overtraining. After completing Phase 1 training, the animals received a test, in which the two negative stimuli between the two discrimination tasks were interchanged. There was no significant difference in discrimination performance on the test after overtraining between the animals trained concurrently on two discrimination tasks and the animals trained separately on two tasks, measured in both correct response and percentage of errors, which were very excellent, whereas their discrimination performance on the test after criterion training were at chance level. These results indicate that substitutability of the members of the stimulus class occur after overtraining in two discrimination tasks within different stimulus contexts. These findings of all three experiments indicate that stimulus classes between the discriminative stimuli established during overtraining in concurrent discriminations have two primary properties of functional equivalence relation: exchangeability and substitutability. Thus, they provide strong evidence that stimulus classes between the discriminative stimuli established during overtraining in two concurrent two-choice discriminations are valid.

The present research has examined the effects of overtraining on nonreversal shift in two concurrent discrimination tasks. Mackintosh (1962, 1964, 1965a, 1965b) and Lawrence (1949, 1950) also have examined the effect of overtraining on reversal and nonreversal shift in a single discrimination task. Both Mackintosh (1962, 1964, 1965a, 1965b) and Lawrence (1949, 1950) have conceptualized the overtraining reversal effects as being the result of overtraining enhancing the animals' attention to the relevant dimensions of the stimulus. That is, either Mackintosh's view or Lawrence's view has to do with changes in selective attention to the physical dimensions of the stimuli. By contrast, Nakagawa (1992) has shown that the effects of overtraining on reversal learning in concurrent discriminations differ from those observed in a single discrimination. Nakagawa (1992, 1998) makes it clear that the animals form stimulus-stimulus associations or stimulus classes between the discriminative stimuli with the same response assignment during overtraining in concurrent discriminations, but not after criterion training. Nakagawa (1978, 1986, 1992, 1998) has assumed that one stimulus has both the 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. That is, 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 assignment. That is, Nakagawa's view has more to do with cognitive categorization than changes in selective attention to the physical dimensions of the stimuli. However, Nakagawa's view is not necessarily in conflict with either Mackintosh's view or Lawrence's. Conversely, it seems likely that in nature both principles may often act in consonance. That is, little overtraining results in increment attentions to the relevant dimensions of the stimulus as proposed by Mackintosh (1965a), or an acquired distinctiveness of cues as proposed by Lawrence (1949, 1950), whereas enough overtraining results in categorization of the discriminative stimuli with the same response assignment in concurrent discriminations.

Functional Equivalence vs. Exchangeability vs. Substitutability

There is growing evidence that when rats are overtrained on concurrent discrimination tasks in which more than one stimulus is associated with the same outcome (e.g., food or no food), these stimuli become functionally equivalent. Thus, a concept of functional equivalence is a synonym for a concept of stimulus classes. A primary definition of functional equivalence of stimuli is that each member of a stimulus class controls the same response. Such concept of functional equivalence has two primary properties: exchangeability and substitutability. Exchangeability is that stimuli, which are associated with the same consequence during overtraining in concurrent discriminations, are interchangeable with one another within the stimulus context. By contrast, substitutability is that they are interchangeable for one another in other contexts. Substitutability is a more important property of functional equivalence of stimuli than is exchangeability.

The results of the present experiments, together with my earlier works (Nakagawa, 1978, 1986, 1992, 1998), support the notion that "functional equivalence" (Shipley, 1935) or "stimulus classes" (Spradlin & Saunders, 1986) can be established in rats. Although this phenomenon does not meet the more rigorous criterion of equivalence described by Sidman (1986), the apparent development of new, derived relations between the discriminative stimuli may involve underlying processes in the formation of so-called perceptual concepts.

The results of the present experiments reported here make it clear that the postulate from cue-associations theory (Nakagawa, 1992), that is, that animals form stimulus-stimulus associations or stimulus classes between the discriminative stimuli with the same response assignment, and the stimulus-stimulus associations produce an acquired equivalence effect, is valid; this is the novel empirical and theoretical contribution to the literature.

References

DELIUS, J. D., AMELING, M., LEA, S. E. G., & STADDON, J. E. R. (1995). Reinforcement concordance induces and maintains stimulus associations in pigeons. The Psychological Record, 45, 283-297.

EDWARDS, C. A., JAGIELO, J. A., ZENTALL, T. R., & HOGAN, D. E. (1982). Acquired equivalence and distinctiveness in matching to sample by pigeons: Mediation by reinforcer-specific expectances. Journal of Experimental Psychology: Animal Behavior Processes, 8, 244-259.

GOLDIAMOND, I. (1962). Perception. In A. J. Bachrach (Ed.), Experimental foundations of clinical psychology (pp. 280-340). New York: Basic Books.

LAWRENCE, D. H. (1949). Acquired distinctiveness of cues: I. Transfer between discriminations on the basis of familiarity with the stimulus. Journal of Experimental Psychology, 39, 770-784.

LAWRENCE, D. H. (1950). Acquired distinctiveness of cues: II. Selective association in a constant stimulus situation. Journal of Experimental Psychology, 40, 175-188.

MACKINTOSH, N. J. (1962). The effect of overtraining on a reversal and a nonreversal shift. Journal of Comparative and Physiological Psychology, 55, 555-559.

MACKINTOSH, N. J. (1964). Overtraining and transfer within and between dimension in the rat. Quarterly Journal of Experimental Psychology, 16, 250-256.

MACKINTOSH, N. J. (1965a). Selective attention in animal discrimination learning. Psychological Bulletin, 64, 124-150.

MACKINTOSH, N. J. (1965b). Overtraining, reversal, and extinction in rats and chicks. Journal of Comparative and Physiological Psychology, 59, 31-36.

NAKAGAWA, E. (1978). The effect of overtraining on discrimination learning in white rats (in Japanese). Japanese Journal of Psychology, 49, 70-77.

NAKAGAWA, E. (1986). Overtraining, extinction and shift learning in a concurrent discrimination in rats. Quarterly Journal of Experimental Psychology, 38B, 213-226.

NAKAGAWA, E. (1992). Effects of overtraining on reversal learning by rats in concurrent and single discriminations. Quarterly Journal of Experimental Psychology, 44B, 37-56.

NAKAGAWA, E. (1998). Stimulus classes formation process in concurrent discriminations in rats as a function of overtraining. The Psychological Record, 48, 537-560.

SHIPLEY, W. C. (1935). Indirect conditioning. Journal of General Psychology, 12, 337-357.

SIDMAN, M. (1986). Functional analysis of emergent verbal classes. In T. Thompson & M. D. Zeiler (Eds.), Analysis and integration of behavioral units (pp. 213-245). Hillsdale, NJ: Erlbaum.

SIDMAN, M., & TAILBY, W. (1982). Conditional discrimination vs. matching to sample: An expansion of the testing paradigm. Journal of the Experimental Analysis of Behavior, 37, 25-44.

SPENCE, K. W. (1936). The nature of discrimination learning in animals. Psychological Review, 43, 427-449.

SPENCE, K. W. (1937). The differential responses in animals to stimuli varying within a single dimension. Psychological Review, 44, 430-444.

SPRADLIN, J. E., COTTER, V. W., & BAXLEY, N. (1973). Establishing a conditional discrimination without direct training: A study of transfer with retarded adolescents. American Journal of Mental Deficiency, 7, 556-566.

SPRADLIN, J. E., & SAUNDERS, R. R. (1986). The development of stimulus classes using match-to-sample procedures: Analysis and Intervention in Developmental Disabilities, 6, 41-58.

URCUIOLI, P. J., ZENTALL, T. R., JACKSON-SMITH, P., & STEIRN, J. N. (1989). Evidence for common coding in many-to-one matching: Retention, intertrial interference, and transfer. Journal of Experimental Psychology: Animal Behavior Process, 15, 264-273.

VAUGHAN, W., Jr. (1988). Formation of equivalence sets in pigeons. Journal of Experimental Psychology: Animal Behavior Processes, 14, 36-42.

ZENTALL, T. R., STEIRN, J. N., SHERBURNE, L. M., & URCUIOLI, P. J. (1991). Common coding in pigeons assessed through partial versus total reversals of many-to-one conditional and simple discriminations. Journal of Experimental Psychology: Animal Behavior Processes, 17, 194-201. , & LEINENWEB
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Author:Nakagawa, Esho
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Date:Mar 22, 1999
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