Journal of Experimental Psychology: a 1998, Vol. 24, No. 2, 123-135
Copyright 1998 by the American PsychologicalAssociation,Inc.
S o m e Factors That Determine the Influence of a Stimulus That Is Irrelevant to a Discrimination Edward S. Redhead and John M. Pearce University of Wales, Cardiff In 5 autoshaping experiments pigeons received 3 stimuli, A, B, and C, for a discrimination in which food was presented after the simultaneous compounds AC and BC, but not after the simultaneous compound ABC. The ease with which this discrimination was mastered was facilitated by presenting C continuously throughout each session (Experiment 1), by presenting C by itself for nonreinforced trials (Experiment 2), and by pairing C by itself consistently with food (Experiment 3). Presenting C by itself and pairing it with food according to a partial reinforcement schedule had no significant influence on the acquisition of the discrimination (Experiments 4 and 5). The results are consistent with a configural theory of associative learning that suggests that experience with a stimulus alters its salience.
with a number of different theories of associative learning. For example, both an elemental theory of associative learning, such as the Rescorla-Wagner (1972) model, and a configural theory of learning, such as that offered by Pearce (1987, 1994), would predict that leaving C on permanently should reduce the associative strength of C and make an A C + , BC+, ABCo discrimination more difficult. If our results were to reveal the opposite outcome, then they would indicate a need for a revision of these theories. One obvious way in which the above-mentioned theories might be revised is to take account of the possibility that our experimental manipulations could modify the salience of, or the attention paid to, the irrelevant stimulus. Leaving C on permanently throughout an experimental session might reduce the attention paid to it and lessen its disruptive influence on the discrimination. If this is correct, then by examining the factors that determine the disruptive influence of C on the acquisition of an AC +, BC +, ABCo discrimination, it should be possible to gain a deeper understanding of the rules that determine the extent to which animals pay attention to a stimulus during the course of a discrimination. For the sake of convenience, rather little will be said about the significance of our fndings for theories of attention and discrimination learning until the general discussion.
The presence of an irrelevant stimulus on every trial of a discrimination can disrupt the acquisition of that discrimination. A demonstration of this effect has been reported by Pearce and Redhead (1993), who studied negative patterning. For one group of pigeons, food was presented after two stimuli when they were presented alone but not when they occurred together, A + , B + , ABo. A second group received the same training except that an additional stimulus, C, was present on every trial, creating an A C + , BC+, ABCo discrimination. The second group acquired the discrimination more slowly than the first (see also Redhead & Pearce, 1995; Rescorla, 1972). The overall concern of this article is to examine whether it is possible to alter the disruptive influence of an irrelevant stimulus by manipulating the manner in which it is presented. For example, in Experiment 1 a group of pigeons was trained with an A C + , BC+, ABCo discrimination, but C remained on permanently throughout the experimental session. The question of interest was whether this training with C would have any impact on the ease with which the discrimination was mastered. One reason for wishing to conduct these studies relates to the theoretical analysis of the way animals solve discriminations. The results described by Pearce and Redhead (1993) can hardly be described as surprising. After all, few would disagree with the proposal that the presence of C should enhance the similarity of the signals for reward and nonreward, and that this enhancement should make the discrimination more difficult for the second group. But, as we shall show in the general discussion, the effects of the manipulations that we employed with C are rather difficult to explain
Experiment l Three groups of pigeons received autoshaping in which the stimuli consisted of numerous small, colored rectangles presented on a television screen behind a response key. Group Simple received an A + , B + , ABo discrimination whereby food was presented after trials in which all the rectangles on the screen were of one color, A, or another color, B, but no food was presented when rectangles of both colors were presented simultaneously on the screen, AB. As a result of this training, a high rate of pecking on the response key was eventually expected on trials with A and B alone, but not during the compound. Group Common received much the same training as Group Simple, except that every presentation of A, B, and AB was accompanied by
This research was supported by Grant 72/504241 from the United Kingdom Biotechnology and Biological Sciences Research Council. Correspondence concerning this article should be addressed to Edward S. Redhead or John M. Pearce, School of Psychology, University of Wales, Cardiff, Cardiff CF1 3YG, Great Britain. Electronic mail may be sent to
[email protected] or pearcejm @cardiff.ac.uk, respectively. 123
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the presentation of rectangles of a third color, C. Thus, these animals received an A C + , B C + , ABCo discrimination. According to the findings by Pearce and Redhead (1993), the discrimination between the reinforced and nonreinforced trials should have been acquired more readily by Group Simple than by Group Common. The third group, Group Continuous, received the same discrimination training as Group Common, but the common element, C, was left on permanently throughout the experimental session. The question of interest was whether this change in procedure would have any influence on the ease with which the discrimination was acquired.
Method Animals. The animals were 33 adult homing pigeons (Columba livia) that had previously participated in an autoshaping experiment that used stimuli unrelated to those in this study. They were housed in pairs and maintained at 80% of their free-feeding weights by being fed a restricted amount of food after each experimental session. The pigeons were maintained in a lightproof room in which the lights were switched on at 8:00 a.m. for 14.5 hr each day. Apparatus. The experimental apparatus consisted of four pigeon test chambers (30 × 33 × 35 cm), each containing a threekey pigeon panel (Campden Instruments Ltd., Loughborough, England). The central key was replaced by a clear Perspex panel 4.5 cm wide and 5.0 cm high that was hinged at the top. The center of the panel was 24 cm above the floor of the chamber. Pecks on the panel were detected by a reed relay that was operated whenever its lower edge was displaced by a distance greater than 0.7 mm. A Panasonic microcolor television with a screen 5.5 cm × 4.4 cm was located 4.0 cm behind the Perspex panel. Food was delivered through the operation of a grain feeder (Campden Instruments Ltd.), which was illuminated whenever food was presented. The chambers were permanently illuminated during all experimental sessions by a 2.8-W bulb, operated at 24 V, located 2.5 cm above the top of the Perspex panel. An Archimedes 5000 computer (Acorn Computers Ltd., Cambridge, England) programmed in ARACHNID (Paul Fray Ltd., Cambridge, England) was used for the control of events, the recording of responses, and the generation of stimuli on the televisions. The stimuli consisted of small colored rectangles (. 13 cm × . 14 cm) that were randomly located from trial to trial in a matrix that could accommodate a maximum of 42 rectangles horizontally and 32 rectangles vertically. There were no gaps between the edges of the rectangles that occupied adjacent cells in the matrix. The television screen was dark in the regions between the rectangles. The color of the rectangles was red for A, green for B, and white for C. For Group Simple, 100 red or 100 green rectangles were presented on the screen for trials with A or B alone; for trials with A and B together there were 50 red and 50 green rectangles. For Groups Common and Continuous, trials with AC and BC consisted of 100 white rectangles together with 100 rectangles of either red or green; trials with ABC used 100 white rectangles, 50 red rectangles, and 50 green rectangles. For every session of the experiment, the television screen was dark during the intervals between trials for Groups Simple and Common, but it displayed 100 white rectangles during these intervals for Group Continuous. The location of the rectangles was changed randomly for each trial, and when more than one color was presented on the screen the different types of rectangle were intermixed randomly. For Group Continuous, the position of the white rectangles was changed immediately after every trial.
Procedure. The animals were randomly assigned in equal numbers to the three groups. Because of their previous training, the animals did not require magazine training. Every group received four pretraining sessions prior to the introduction of the discrimination. In each pretraining session, there were eight trials with A and eight trials with B for Group Simple; Groups Common and Continuous received a similar number of trials with AC and BC. Stimulus C was continuously present throughout every intertrial interval (ITI) for Group Continuous. For all animals every trial was followed by the delivery of food at this stage. The mean ITI was 180 s (range = 60-300 s). The training that has just been described continued for the subsequent seven sessions of discrimination training, with the addition of 32 nonreinforced trials in each session. The additional trials were with AB for Group Simple and ABC for Groups Common and Continuous. The ITI was 60 s (range = 20-100 s). Throughout the experiment the stimuli were presented for 10 s, and, when appropriate, food was presented for 4 s immediately after the offset of a stimulus. Trials were presented in a random sequence, with the constraint that no more than three trials of the same type could occur in succession. The number of pecks on the response key made by individual birds during the 10 s prior to each trial and during each trial was recorded. It seemed plausible that the presence of C between successive trials might encourage the Group Continuous birds to peck more vigorously on the response key during the ITI than the birds in the other two groups; to take account of any effect such a difference might have on the response rates during trials, a difference score was used to represent the response rate for each trial for this experiment. This score was computed by subtracting the rate of responding during the 10 s prior to each trial from that during the trial. Results and Discussion All statistical tests were evaluated with respect to an alpha level of .05. For the final session of pretraining the mean difference scores in responses per minute for both types of trial combined were 71.8 for Group Simple, 75.6 for Group Common, and 88.2 for Group Continuous. A one-way analysis of variance (ANOVA) indicated that these differences among the group were not significant (F < 1). The equivalent results for the 10-s intervals before each trial in this session were 0.6 for Group Simple, 0.9 for Group Common, and 2.7 for Group Continuous. These differences among the groups were not significant, F(2, 30) = 2.0. The mean difference scores for each type of trial in every session of discrimination training are shown for the three groups in the three panels of Figure 1. The figure indicates that the discrimination between the reinforced and nonreinforced trials was acquired most readily by Group Simple, at an intermediate rate by Group Continuous, and most slowly by Group Common. To simplify the comparison of the rates at which the different groups acquired the discrimination, we computed ratios for every bird for every session. The ratios were based on the mean difference scores for the nonreinforced trials in each session, X, and the mean differences scores for both types of reinforced trial in a session, Y. The ratio was of the form XI(X + Y). A ratio of .50 indicated a failure to discriminate between the reinforced and nonreinforced tri-
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als, whereas a ratio of .00 indicated a perfect discrimination between these trials. The mean discrimination ratios for the three groups are presented in Figure 2, which again demonstrates that Group Simple found the discrimination easier than Group Continuous, which, in turn, found it easier than Group Common. For an analysis of the discrimination ratios to be meaningful, the response rates during the reinforced trials should not differ among the groups. A three-way ANOVA was therefore performed on individual mean difference scores for the two different types of trial that signaled food for the three groups for the seven sessions of the discrimination. There was no significant effect of group or trial type, and none of the interactions involving these factors were significant, all Fs < 1, except for the Group × Trial Type interaction, F(2, 30) = 1.9. The effect of session was significant, F(6, 180) = 7.2. In view of the results of the foregoing analysis, it was
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deemed justifiable to compare the performance of the three groups by using discrimination ratios. A two-way ANOVA of individual mean discrimination ratios for the three groups for the seven sessions of training revealed significant effects of group, F(2, 30) = 18.9, and of session, F(6, 180) = 57.6, and a significant interaction between these effects, F(12, 180) = 2.6. Simple main effects analyses then revealed a significant difference among the groups on Sessions 2 through 7, Fs(2, 210) > 3.8. Comparisons of the group means for these sessions, made by using the Newman-Keuls technique, revealed that the discrimination ratios for Group Simple were smaller than for Group Common on every session, and smaller than for Group Continuous on Sessions 2 and 3. The discrimination ratios for Group Continuous were smaller than for Group Common on Sessions 3, 4, 6, and 7. The results from Group Simple and Group Common replicate our earlier findings (Pearce & Redhead, 1993) by showing that the presence of an irrelevant stimulus can hinder the solution of a negative patterning discrimination through use of autoshaping. The novel finding from this experiment is that this disruptive influence of the irrelevant stimulus can be reduced by presenting it throughout the entire experimental session. One explanation for the results of Group Continuous is
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that the protracted presentation o f C resulted in habituation to this stimulus so that it was, in effect, ignored on every trial of the discrimination. Wagner (1976, 1981) has proposed, for example, that when a stimulus is left on for a long time it loses effectiveness through a process o f self-generated priming. If this possibility is accepted, then the question is raised whether the only mechanism for reducing the disruptive influence o f C in an A C + , B C + , A B C o discrimination is through habituation based on its continuous exposure. The next experiment was designed to answer this question. Experiment 2 There were two groups in Experiment 2, both of which received an A C + , BC + , A B C o discrimination. This was the only training that was given to Group Common, whereas Group Co received additional trials in which C was presented by itself in the absence of food for 10 s at a time. If Group Co should acquire the discrimination more readily than Group Common, we reasoned, then it would indicate that extended exposure to C is not the only way o f reducing its disruptive influence.
no more than three trials of the same type could occur in succession. The trials for Group Co were presented with an ITI of 63 s. For Group Common the ITI was 123 s (range = 63-189 s). The order and time of occurrence of the trials with AC, BC, and ABC were the same in every session for both groups. Procedural details that have not been reported were the same as for Experiment 1.
Results a n d Discussion In the final pretraining session, the mean number o f responses per minute for both reinforced trials combined was 79.2 for Group C o m m o n and 69.0 for Group Co. The rate of responding during the trials with C by itself was 0.4 responses per min. The mean rate of responding for all pigeons in Group Co was slower on trials with C than on either type o f reinforced trial. A one-way A N O V A of individual mean rates of responding for the two types of reinforced trial combined failed to reveal a significant difference between the groups ( F < 1). The mean rates of responding during the discrimination training are shown in the upper panel o f Figure 3 for the three types o f trial given to Group C o m m o n and in the lower panel for the four types of trial given to Group Co. Figure 4
Me~od 150-
Animals.
The 21 pigeons were from the same stock and maintained in the same manner as those in the previous experiment. They had previously taken part in an autoshaping experiment that used visual stimuli that were different in size and color from those used in the present experiment. Apparatus. The experimental apparatus consisted of eight pigeon test chambers (32.0 × 35.0 × 35.0 cm). The front panel of each chamber contained a clear Perspex panel that was 10.0 cm wide and 8.0 cm high. The center of the panel, which was hinged at the top, was 22.5 cm above the floor of the chamber. A Roadster television with a screen 10.0 cm × 9.0 cm was located 4.0 cm behind the Perspex panel. Food was delivered via a grain feeder (Colbourn Instruments, Allentown, Pennsylvania). The lower edge of the 5.0 × 6.0 cm opening to the feeder was 5.0 cm above the floor of the chamber. The feeder was illuminated whenever food was presented. The stimuli consisted of colored rectangles (0.2 × 0.22 cm). The color of the rectangles was red for stimulus A, green for stimulus B, and white for stimulus C. All other details concerning the apparatus and the stimuli were the same as for Experiment 1. Procedure. The pigeons were randomly divided into two groups, Group Common (n = 10) and Group Co (n = 11). In each of the six sessions of the prelxaining stage of the experiment, both groups received six trials in which AC was followed by food and six trials in which BC was followed by food. The mean ITI for Group Common was 166 s (range = 126-252 s). Group Co received a further 24 trials in each session, in which C was presented by itself and not followed by food. The mean ITI for Group Co was 83 s (range = 63-126 s). The trials were presented in a random sequence, with the constraint that no more than three trials of the same type could occur in succession. The timing of the trials with AC and BC for Group Common coincided with that for Group Co. The training that has just been described continued for the next 20 sessions of discrimination training, with the addition in each session of 12 nonreinforced trials with ABC. The trials for both groups were presented in random sequence, with the constraint that
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IRRELEVANTSTIMULUS 0.6"
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Group mean discrimination ratios for each session for the two groups in Experiment 2.
presents the results from the discrimination training in the form of discrimination ratios. The ratios were computed in the same way as for Experiment 1, except that the values of X and Y were determined by the response rates during the various trials rather than by difference scores. From both figures it is apparent that the discrimination was mastered more readily by Group Co than by Group Common. A three-way ANOVA was conducted that used individual mean response rates for the two types of reinforced trial for the 20 sessions of discrimination training for both groups. There was no significant effect of group or trial type, and none of the interactions involving these factors were significant (all Fs < 1), with the exception of the Group × Session interaction, F(19, 399) = 1.3. The effect of session was significant, F(19, 399) = 3.2. A two-way ANOVA of individual mean discrimination ratios for each of the 20 sessions revealed a significant effect of group, F(1, 21) = 4.6, a significant effect of session, F(19, 399) = 14.3, and a significant interaction, F(19, 399) = 5.9. Subsequent tests of simple main effects indicated that the difference between the groups was significant from Session 14 onward. The results show that pigeons' acquisition of an A C + , B C + , ABCo discrimination can benefit from the inclusion of additional trials in which C is presented by itself in the absence of food. This finding is similar to that attained in Experiment 1, in which C remained present throughout the experimental session. It therefore seems that it is by no means essential for there to be prolonged periods of exposure to C to reduce the disruptive influence of this stimulus on the acquisition of an A C + , B C + , ABCo discrimination. A comparison of the results from Group Common in Figure 1 with those for the equivalent group in Figure 3 indicates that the discrimination was acquired more readily
by the former group. Although both groups were trained with the same discrimination, the difference in their performance is probably due to the greater frequency with which the nonreinforced trials with ABC were presented in Experiment 1. Experiment 3 In Experiments 1 and 2 a reduction in the disruptive influence of C on the acquisition of the A C + , B C + , ABCo discrimination was achieved by presenting this stimulus by itself and without food. The purpose of Experiment 3 was to determine how important it is for the independent presentations of C to take place in this way. There were three groups in the experiment. Group Common and Group Co were treated in much the same way as the groups with the same names of Experiment 2. Hence, both groups received an A C + , B C + , ABCo discrimination, and Group Co received additional nonreinforced trials in which C was presented by itself for 10 s at a time. The third group, Group C + , received the same training as Group Co, but each presentation of C by itself was followed by the delivery of food. If the independent presentations of C had to take place without food reinforcement for its disruptive influence to be reduced, we reasoned, then acquisition of the discrimination by Group C + would progress more slowly than by Group Co. There were theoretical reasons for believing that this would indeed be the outcome of the experiment. According to several theories of attention (e.g. Mackintosh, 1975; Sutherland & Mackintosh, 1971), animals pay more attention to stimuli that predict events of significance, such as food, than to stimuli that predict nothing of consequence. Thus pairing C with food in Group C + seemed likely to result in this group paying considerably more attention to C than would Group Co, for which many trials with C were followed by nothing.
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On the trials of the discrimination, therefore, the presence o f C for Group C + might distract attention away from A and B, and the solution to the discrimination would emerge only slowly. In contrast, Group Co might pay considerable attention to A and B and learn the discrimination rapidly. There was a relatively minor, but nonetheless interesting, difference between the methods used for Group Co in this experiment and those used for the groups that received independent exposure to C in the previous experiments. In Experiments 1 and 2, exposure to C by itself was given prior to as well as during the acquisition o f the discrimination. It therefore seemed possible that exposure to C before discrimination training was entirely responsible for the effects seen during discrimination training. To test this possibility, we did not give Group Co any independent exposure to C until the outset o f the discrimination. If the discrimination should be acquired more readily by this group than by Group Common, we reasoned, then it would indicate that nonreinforced exposure to C by itself during the course o f the discrimination is sufficient to attenuate its disruptive influence.
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In the final session of pretraining, the mean number of responses per minute during the trials with A C and BC combined was 74.7 for Group Common, 75.7 for Group Co, and 49.8 for Group C + . A one-way A N O V A o f individual mean rates of responding for the two types o f trial combined indicated that this difference among the groups was not significant ( F < 1). The course of acquisition o f the discrimination for the three groups is shown in the three panels o f Figure 5. Figure 6 shows the group mean discrimination ratios for the 20 sessions o f training. The ratios were computed in the manner described for Experiment 2. The results shown in both figures indicate that Group C o m m o n acquired the discrimination more slowly than either Group Co or Group C + . A
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Me~od Animals and apparatus. The 33 pigeons, which were randomly assigned to the three groups in equal numbers, were from the same stock and housed in the same manner as in the previous experiments. They had taken part in an autoshaping experiment in which the size and color of the stimuli were different from those in the present study. The apparatus was the same as for Experiment 2. Procedure. The three groups received seven pretraining sessions, each of which consisted of six trials with AC and six trials with BC. Food was delivered at the end of every trial in these sessions. The trials were presented with a mean ITI of 240 s (range = 60-360 s). In each of the 20 sessions of discrimination training, Group Common received 6 trials with AC, 6 trials with BC, and 12 trials with ABC. Food was presented after AC and BC, but not after ABC. Group Co was given the training that has just been described, together with 24 trials in which C was presented in the absence of food. The treatment for Group C+ was the same as for Group Co, except that the individual presentations of C were always followed by food. All other procedural details were the same as for Experiment 2.
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Figure 5. Mean rates of responding, for each session of discrimination training, during the reinforced trials with AC and BC and during the nonreinforced trials with ABC for the three groups of Experiment 3. Also shown are the mean rates of responding for the independent presentations of C, which were paired with food for Group C+ but not for Group Co.
further feature of interest in Figure 5 is that the training with C by itself was effective in both groups. Thus, throughout the discrimination training, responding on the trials with C in Group C + was at much the same rate as during either A C or BC. In contrast, responding during this stimulus in Group Co was relatively slow, particularly toward the end o f the experiment. The results shown in Figure 5 indicate that the three groups responded similarly during the trials with A C and BC. A three-way A N O V A was conducted o f individual mean response rates for the trials, with AC and BC treated separately, for the three groups throughout the 20 sessions o f the discrimination. The effects o f group and of trial and all the interactions involving these factors were not significant (Fs < 1), but there was a significant effect of session, F(19, 570) = 8.5. A two-way A N O V A of individual discrimination ratios for the three groups over the 20 sessions of the discrimination revealed that the effect of group was not significant, F(2, 30) =
IRRELEVANTSTIMULUS 0,6
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Group mean discrimination ratios for each session for the three groups in Experiment 3.
3.0, but there was a significant Group × Session interaction, F(38, 570) = 1.8, and the effect of session was also significant, F(19, 570) = 15.6. Tests of simple main effects then revealed significant differences among the groups on Sessions 13, 15, 16, 18, and 19, Fs(2, 600) > 3.0. A comparison of group means for these sessions by use of the Newman-Keuls procedure indicated that the discrimination ratios for Group Co were significantly less than those for Group Common on Sessions 15, 16, 18, and 19; likewise the ratios for Group C + were significantly smaller than those for Group Common on Sessions 13, 15, and 16. The difference between Groups Co and C + was not significant on any session. Once again, the results show that independent presentations of C during the course of an A C + , B C + , ABCo discrimination facilitate its acquisition. The results from the present experiment go beyond those of the previous studies by showing that exposure to C is just as effective when it is presented alone as when it is followed by food. Experiment 4 In the introduction to Experiment 3 we suggested that pairing C with food in Group C + might result in considerable attention being paid to C. This high level of attention was then expected to be sustained when C was accompanied by A, B, and AB and thus make it difficult for Group C + to solve its discrimination. In contradiction of our expectations, the results of the experiment show that pairing C with food facilitated the acquisition of the discrimination to the same degree as when C was presented by itself in the absence of food. On the basis of these results it is tempting to abandon the idea that independent exposure to C is effective because it reduces the attention the stimulus receives. There
is, however, one way in which an attentional explanation for the results from Experiments 1 and 2 can be reconciled with the findings from Experiment 3. Pearce and Hall (1980; see als0 Pearce & Hall, 1992; Pearce, Kaye, & Hall, 1982) have proposed that the attention paid to a stimulus is governed by how accurately the events that follow it can be predicted: If a stimulus can be used to predict these events reliably, then the attention paid to it will decline. In Experiment 3, Groups Co and C + received many independent presentations of C in each experimental session. Because this stimulus by itself was always followed by the same event--nothing for Group Co, food for Group C + - - i t is possible that attention to C declined in both groups as learning progressed about the relationship between C alone and the event that followed it. If so, then C should receive little attention when it is accompanied by other stimuli and its impact on the acquisition of the AC +, B C + , ABCo discrimination should be slight for both groups. Two of the three groups in Experiment 4 were treated in the same way as those used in Experiment 3. Group Common received an A C + , B C + , ABCo discrimination, and Group Co received this discrimination with additional nonreinforced trials with C. The third group, Group C+/o, received the same training as Group Co, except that a randomly selected half of the trials with C were followed by food and the remainder were followed by nothing. According to the theory of Pearce and Hall (1980), the partial reinforcement schedule associated with C should mean that Group C + / o would not be able to predict with consistent accuracy the event that would follow an independent presentation of C. As a consequence, attention to C should be sustained at a relatively high level in this group. If C
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should therefore receive considerable attention during the trials of the negative patterning discrimination, we reasoned, its presence might be more disruptive for Group C + / o than for Group Co, whose training could be expected to result in C being ignored.
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Results a n d Discussion The mean number o f responses per minute in the final session o f pretraining for the trials with AC and BC combined was 53.1 for Group Common, 56.1 for Group Co, and 65.7 for Group C + / o . A one-way ANOVA showed that this difference among the groups was not significant (F < 1). Figure 7 shows the mean rates of responding during the various types of trial in the 20 sessions of discrimination training. Figure 8 shows the group mean discrimination ratios, calculated in the manner described in Experiment 2, for each session of the discrimination stage. The impression gained from both figures is that the discrimination was acquired most readily by Group Co and that Groups C o m m o n and C + / o found the discrimination equally difficult. Figure 7 also shows that the partial reinforcement schedule associated with C resulted in a level of responding by Group C + / o that was similar to that shown on the continuously reinforced trials with AC and BC. In general, the results in Figure 7 show that the three groups responded at similar rates during the reinforced trials and that within each group, responding during AC was much the same as during BC. In support of these observations, a three-way ANOVA, based on individual mean response rates for each of the 20 sessions for the trials, with AC and BC treated separately, revealed that the effects of group and trial type, and the interactions involving these factors were not significant (all Fs < 1), except for the Group × Session interaction, F(38, 551) = 1.1. The effect of session was significant, F(19, 551) = 8.2. A two-way ANOVA that used individual discrimination ratios for each of the discrimination training sessions revealed a significant Group × Session interaction, F(38, 551) = 1.5. The effect of session, F(19, 551) = 9.6, but not
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Me~od Animals and apparatus. The 32 pigeons were from the same stock and housed in the same manner as in the previous experiments. They had taken part in an autoshaping experiment that used stimuli that were different in size and color from those used in the present study. The apparatus was the same as for Experiment 2. Procedure. At the start of the experiment the pigeons were allocated at random to the three groups, Group Common (n = 11), Group Co (n = 10), and Group C+/o (n = 11). The three groups all received six sessions of pretraining in which AC and BC were paired with food in the manner described in Experiment 3. For the 20 sessions of discrimination training, Group Common received trials in which AC and BC were paired with food and ABC was followed by nothing. Group Co was treated in the same way except that there were additional trials in which C was presented by itself in the absence of food. The details of this training were the same as for the equivalent groups in Experiment 2. Group C+/o was treated in a similar way except that a randomly selected half of the trials with C by itself resulted in the delivery of food.
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of group, F(2, 29) = 3.2 was also significant. Analysis of the interaction that used tests of simple main effects indicated a significant difference among the groups on Sessions 9, 12, 14, 18, 19, and 20, Fs(2, 580) > 3.1. Comparisons of the group means by use of the Newman-Keuls procedure then revealed that the discrimination ratios for Group Co were significantly lower than for Group C + / o on Sessions 12, 14, 18, and 19. In addition, the ratios for Group Co were significantly lower than for Group Common on Sessions 9, 12, 14, 18, 19, and 20. The difference between Groups C + / o and Common was not significant in any session. The results demonstrate that the effects of exposure to C by itself on the acquisition of an A C + , B C + , ABCo discrimination can be influenced by the events that follow C. When C is consistently followed by nothing, then exposure to this stimulus alone can facilitate discrimination learning. But this beneficial effect of exposure to C is abolished if it is followed by food presented according to a partial reinforcement schedule.
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Experiment 5 If the ease with which an A C + , B C + , ABCo discrimination is acquired is influenced by the amount of attention paid to C, then the results from the previous experiment are entirely in keeping with the Pearce and Hall (1980) theory. According to this theory, consistently presenting the same outcome after each trial with C by itself should reduce attention to this stimulus and facilitate the discrimination. On the other hand, if C is followed by an unpredictable event, such as the intermittent delivery of food on a random schedule, then attention to C should be sustained and make the discrimination difficult. Thus a prediction from the Pearce and Hall (1980) theory is that the discrimination should be easier when C is paired by itself with food according to a continuous rather than a partial reinforcement schedule. A comparison of the results for Group C + in Experiment 3 with those for Group C + / o in Experiment 4 suggests that this prediction is correct. However, such a between-study comparison is not ideal; Experiment 5 was devised so that the effects of different reinforcement schedules could be compared directly.
Method Animals and apparatus. The 34 experimentally naive pigeons were from the same stock and housed in the same manner as in the previous experiments. The apparatus was the same as for Experiment 4. Procedure. The birds were allocated at random into two groups in equal numbers. Both groups received six sessions of pretraining in which AC and BC were paired with food in the manner described in Experiment 4. Both groups next received 20 sessions of discrimination training. Group C+ received trials in which AC, BC, and C were paired with food and ABC was followed by nothing. Group C+/o was treated in the same way except that food
followed C trials according to a random partial reinforcement schedule. The details of this training were the same as for the equivalent groups in Experiment 3 and 4.
Results and Discussion In the final session of pretraining, the mean number of responses per minute for trials with AC and BC combined was 50.4 for Group C + and 66.0 for Group C+/o. A one-way ANOVA showed that this difference between the groups was not significant (F < 1). The mean rates of responding per minute to all of the stimuli by the two groups during the discrimination stage are shown in Figure 9. Figure 10 shows the discrimination ratios, calculated in the manner described in Experiment 2, over the same period for the two groups. Both figures suggest that Group C + solved the discrimination faster than Group C+/o. Responding during trials of AC and BC in Figure 9 seem marginally higher for Group C + than for Group C+/o; however, this difference proved not to be significant. A three-way ANOVA of individual mean response rates for each of the 20 sessions for the trials, with AC and BC treated separately, revealed that there was no significant group difference (F > 1); nor were there any significant interactions involving this factor (Fs > 1). The effect of trial type was not significant, F(1, 32) = 1.7, but the Trial Type × Session interaction was, F(19, 608) = 2.5. Further analysis of simple main effects showed that responding to AC was significantly higher than to BC on sessions 13 and 15, Fs(1, 640) > 4.2. The effect of session was also significant, F(19, 608) = 4.8. A two-way ANOVA that used individual discrimination ratios for each of the 20 discrimination training sessions revealed significant effects of group, F(1, 32) = 4.2, and
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Figure 9. Mean rates of responding, for each session of discrimination training, during the reinforced trials with AC and BC and during the nonreinforced trims with ABC for the two groups of Experiment 5. Also shown are the mean rates of responding for the independent presentations of C, which were intermittently paired with food for Group C +/o and always paired with food for Group C +. session F(19, 608) = 11.7, and a Group x Session interaction F(19, 608) = 1.8. Analysis of the interaction by tests of simple main effects revealed that the discrimination ratios of Group C + were significantly lower than those of Group C + / o on sessions 14, 15, 17, 19, and 20, Fs(1,640) > 4.3. The results confirm that when C is always followed by food its disruptive influence on solving an A C + , B C + , ABCo discrimination is reduced in comparison with when it is followed by food according to a partial reinforcement schedule. These results are entirely consistent with the findings from Experiments 3 and 4, where the effects of the different reinforcement schedules were compared with a condition in which C was followed by nothing. Taken together, the results from the last three experiments are also consistent with the predictions derived from the Pearce and Hall (1980) theory. General Discussion In each of five experiments a group of pigeons was trained with a negative patterning discrimination in which an
irrelevant stimulus, C, was present on all trials, A C + , B C + , ABCo. Experiment 1 demonstrated that the presence of C disrupted the acquisition of the discrimination, and the purpose of this article has been to identify some of the factors that reduce this disruptive influence. Three factors have been identified: (a) leaving C permanently on throughout the experimental session, (b) repeatedly presenting C by itself for brief periods during the course of the discrimination, and (c) repeatedly pairing C with food during the course of the discrimination. In contrast, presenting C by itself and pairing it with food according to a partial reinforcement schedule did not lessen its disruptive power. For some of the groups in the experiments the proportions of reinforced and nonreinforced trials were the same, whereas for other groups there was a marked imbalance in the proportions of these trials. Consider Experiment 2, for example. There were 12 reinforced trials and 12 nonreinforced trials for Group Common, whereas Group Co received 12 reinforced trials and 36 nonreinforced trials. After any reinforced trial, therefore, the probability of the next trial not being reinforced was greater for Group Co than for Group Common. Perhaps the discrimination was easier for Group Co than for Group Common because the birds belonging to the former group used the outcome of one trial to predict the outcome of the next one. In fact, there are several reasons for not taking this suggestion too seriously. First, it implies, for Experiment 2, that responding on the reinforced trials should have been weaker for Group Co than for Group Common. An inspection of Figure 3 reveals that this was not the case, as does the statistical analysis of Experiment 2. Second, when this explanation is applied to Experiment 5, it suggests that responding during ABC should have been more vigorous by Group C + than by Group C+/o, but, if anything, the opposite pattern of responding was observed. Finally, an examination of the results for individual trials with ABC in the last session of Experiment 4, selected arbitrarily from all of the sessions in the experiments in which there was a significant difference among the discrimination ratios, reveals that the mean rates of responding during ABC after previously reinforced and previously nonreinforced trials, in responses per minute, were 45.0 and 44.7, respectively, for Group C+/o, 44.7 and 44.5, respectively, for Group Common, and 15.0 and 16.2, respectively, for Group Co. The similarity of these values for each group strongly suggests that the outcome of the immediately preceding trial had very little influence on responding during ABC. Exposure to C by itself in each of the five experiments provided the opportunity for the associative strength of this stimulus to be altered. We must therefore consider whether changes in the associative properties of C were by themselves directly responsible for all of our experimental findings. There are at least two reasons for believing that this was not the case. First, we found that manipulations designed to leave C either with little associative strength or with high associative strength (e.g., Groups Co and C + of Experiment 3) had the same effect, of facilitating the acquisition of the AC +, BC +, ABCo discrimination. On the other hand, manipulations that left C with intermediate
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associative strength (e.g., Group C +/o of Experiment 4) had no effect on discrimination learning. It is very difficult to understand how these different associative strengths could have directly influenced the solution of the discriminations in the ways that were observed. The second reason for believing that something other than changes in the associative strength of C was responsible for our results has its roots in the discussion below. This discussion makes it evident that by referring solely to changes in associative strength it is difficult for any current theory of associative learning to explain all of our results. We consider first the configural model of conditioning developed by Pearce (1987, 1994). A fundamental assumption of this theory is that compound conditioning results in the growth of a single association between a representation of the compound and the outcome of that trial. Because both excitation and inhibition are assumed to generalize from one configuration to another, a direct prediction of this model is that the ease with which a discrimination is acquired is influenced by the similarity of the signals for reward and nonreward. Any manipulation that enhances the similarity of these signals, such as adding a common cue, is predicted to make the discrimination more difficult. Configural theory would therefore predict, for example, that the discrimination should be acquired more slowly by Group Common than by Group Simple in Experiment 1 (see Pearce, 1994; Pearce & Redhead, 1993). Unfortunately, it is not so easy to derive additional predictions from this theory concerning the other experimental manipulations that we have adopted. For example, if nonreinforced trials with C by itself are presented during the course of the discrimination, then C acquires a measure of inhibition that generalizes to AC, BC, and ABC, but it is not immediately obvious how this inhibition affects the course of the discrimination. Accordingly, we conducted a series of computer simulations based
on equations presented in Pearce (1994). For these simulations the intensities of A, B, and C were equal, the value of h was 1 on reinforced and 0 on nonreinforced trials, and the learning rate parameter was set at .2. All of the simulations took account of the training with AC and BC that was given in the above experiments prior to the discrimination. The results from the simulations revealed that the Pearce model would predict the A C + , B C + , ABCo discrimination to be made more difficult by the addition of the Co, C + , and C + / o trials. In fact the experiments showed that the first two of these treatments made the discrimination easier. Configural theory therefore had to be modified to explain all of our results. We noted earlier that according to configural theory the presence of C should make the A C + , B C + , ABCo difficult because it enhances generalization between the compounds that signal the presence and absence of food. In theory, therefore, if attention to C were to decline, relative to attention to A and B, then so would its disruptive influence. To confirm that this prediction from configural theory was justified, we conducted a further set of simulations. These manipulations were identical to those just described except that the salience of C was reduced to one fifth that of A or B. More specifically, the theory of Pearce (1994) was presented as a connectionist network. The input unit that was assigned to C for the simulations could be activated only to a value of 0.2, whereas the input units assigned to A and B were activated to their maximum value of 1 by these stimuli. The simulations revealed that independent presentations of C, in the form of Co, C + , or C+/o, would now be predicted to facilitate the AC +, BC +, ABCo discrimination. Thus, to explain our findings it is necessary for the main assumptions of the Pearce-Hall (1980) to be incorporated into configural theory: Presenting C by itself must be assumed to reduce its salience if it is consistently followed by either food or nothing. Such an assumption allows
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configural theory to predict the beneficial influence of the Co and C + trials on the discrimination. On the other hand, if the theory is to explain the failure of the C + / o trials to have any impact on the A C + , B C + , ABCo discrimination, then it must adopt the additional assumption of the Pearce-Hall (1980) theory that this treatment sustains the salience of C. Before leaving the Pearce-Hall (1980) theory, some further comment is needed concerning the effects of trials with Co or C + . Both these treatments have been claimed to reduce the salience of C by making it an accurate predictor of the event that follows it. Strictly, this claim is unjustified, because the trials of the discrimination ensured that C was always followed by food on some occasions and by nothing on others. Thus, in none of our experimental conditions can it be said that C was consistently followed by the same outcome. It is worthy of mention, however, that there were many more trials with C alone than of any other type in each of the experiments. As a consequence, both the Co and C + trials would be predicted by the Pearce-Hall (1980) theory to result in a more rapid loss of salience by C than by either A or B (see also Pearce et al., 1982). Once the salience of C is less than those of A and B, then configural theory would predict that generalization among AC, BC, and ABC should be reduced and the discrimination easier to solve. Perhaps a more serious issue for our theoretical proposals derives from the results of experiments by Darby and Pearce (1995). Rats were trained with an A C + , B C + , Co discrimination before being presented with the compound ABC. Whenever C was presented to one group its duration was 10 s, but for a second group C was present throughout the experimental session. The strength of responding during ABC was signifcantly stronger for the test trials with the first than with the second group. According to configural theory, the strength of responding during ABC should be directly related to the salience of C. In that case, the findings by Darby and Pearce (1995) can be said to be entirely consistent with the proposal that the salience of a stimulus should be greater when it is presented for short periods rather than throughout each session. However, an additional finding by Darby and Pearce (1995) does not fit so comfortably with the theoretical proposals that have just been developed. Two groups of rats were exposed to alternating periods of light and dark, each of which lasted between 2 and 6 min. Conditioning trials with A and B were presented during the light, C, for both groups, and one group also received nonreinforced trials with A and B in the dark. Test trials with ABC then revealed substantially stronger responding by the group that received the nonreinforced trials with A and B. The implication of this finding is that the salience of the light was greater when it was of value for predicting whether food would follow A and B than when A and B by themselves accurately signaled food. The salience of the light in this experiment by Darby and Pearce thus appears to have been determined by factors other than those specified by the theory of Pearce and Hall (1980). Obviously, there is a need for caution when drawing conclusions from experiments that have used different species and different procedures. However, when the results from the experiments by Darby and Pearce are viewed in the company of those
described in the present article, they indicate that the attentional processes of animals operate in a more complex manner than we have suggested. We turn now to explore the significance of our findings for an elemental theory of conditioning: the Rescorla-Wagner (1972) model. The first obstacle for this theory is that it predicts that the presence of a common element should facilitate rather than hinder the solution to a negative patterning discrimination. The rationale behind this prediction is presented in detail elsewhere (Pearce, 1994; Pearce & Redhead, 1993; Redhead & Pearce, 1995), and we shall not pursue it further here. However, even if this erroneous prediction by the theory is ignored, there remain additional aspects of our results that are difficult for it to explain. For example, the theory predicts that any manipulation that is designed to reduce the associative strength of C, such as presenting it by itself in the absence of food, should make the A C + , B C + , ABCo discrimination more difficult. The results from the experiments described above contradict this prediction by consistently showing the opposite outcome. Moreover, it does not seem that allowing the salience of C to change as a result of its independent exposure improves matters for the model. If the training given to Group Co in Experiment 3 is assumed to reduce the salience of C, the discrimination would still be incorrectly predicted to be acquired more slowly by this group than by a group trained with just A C + , B C + , ABCo. Another theoretical possibility is that different types of training result in compound stimuli being treated in different ways. Williams, Sagness, and McPhee (1994) have suggested that, depending on their prior experience, humans treat compounds in either an elemental or a configural manner. It is tempting to apply this suggestion to the above experiments. For instance, nonreinforced presentations of C by itself might encourage pigeons to treat it as an element that is distinct from the compounds to which it belongs. If this were true, then, in terms of configural theory, generalization among the components of the negative patterning discrimination would be reduced, and the problem would be easier to solve. An obvious obstacle for this type of analysis is that the treatment given to Groups C + / o in Experiments 4 and 5 would be expected to encourage pigeons to treat C as an element that did not belong to any configuration. The slow rate at which the discrimination was mastered by the birds in these groups suggests, however, that this was not the case. Thus this type of explanation for our results should perhaps not be taken too seriously, until there is convincing evidence that animals treat compounds in an elemental fashion on some occasions and in a configural fashion on others. We close this discussion by considering the implications of the conclusions that we have drawn for two other experimental designs. First, Rescorla (1991) has reported a series of experiments with autoshaping among pigeons that used either a serial feature positive design (AX+, Xo) where A was followed by X and then food, and X was presented alone, or a serial feature negative design (AXo, X+), where food was presented after X but not after AX. With both designs, Rescorla found that the discrimination was acquired
IRRELEVANTSTIMULUS more readily when additional trials were included in which A was followed immediately by food. Collins and Pearce (1985) have argued that it is the accuracy with which the immediate consequences of a stimulus can be predicted that determines the attention it receives. If this is correct, then for both discriminations, the Pearce-Hall (1980) theory would predict that the inclusion o f trials in which A is paired with food should ensure that it is an inaccurate predictor of the events that follow it and that attention to it is high. In the absence of the extra trials with A, this stimulus would always be followed by the same outcome, and attention to it would eventually be low. These differences in attention to A could be important because the successful solution of both discriminations may depend on animals' being able to remember during X whether or not A has just been presented. If the amount o f attention that is paid to A determines the ease with which it can be recalled during X, then it follows that pairing A directly with food should facilitate the acquisition of both discriminations. In a rather different vein, Aitken, Bennett, McLaren, and Mackintosh (1996) considered the factors that determine the ease of a discrimination between two complex stimuli. If the stimuli share common elements, they argued, the discrimination is facilitated by ensuring that the associative strength of the common elements is kept to a minimum. Our finding that nonreinforced exposure to C by itself facilitated the acquisition o f an A C + , B C + , ABCo discrimination is consistent with this proposal. But clearly our additional finding that independently pairing C with food also facilitated the discrimination is not in keeping with their proposal. In this latter instance, we ensured that the associative strength of C was high, and yet the discrimination was mastered with relative ease. If this finding should prove to have some generality, then it will cast doubt on the theoretical analysis of discrimination learning offered by Aitken et al. References Aitken, M. R. E, Bennett, C. H., McLaren, I. E L., & Mackintosh, N. J. (1996). Perceptual differentiation during categorization learning by pigeons. Journal of Experimental Psychology: Animal Behavior Processes, 22, 43-50. Collins, L., & Pearce, J. M. (1985). Predictive accuracy and the effects of partial reinforcement on serial autoshaping. Journal of Experimental Psychology: Animal Behavior Processes, 11, 548564. Darby, R. J., & Pearce, J. M. (1995). Effects of context on responding during a compound stimulus. Journal of Experimentai Psychology: Animal Behavior Processes, 21, 143-154. Mackintosh, N. J. (1975). A theory of attention: Variations in the associability of stimuli with reinforcement. Psychological Review, 82, 276-298.
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Pearce, J. M. (1987). A model of stimulus generalization for Pavlovian conditioning. Psychological Review, 94, 61-73. Pearce, J. M. (1994). Similarity and discrimination: A selective review and a connectionist model. Psychological Review, 101, 587--607. Pearce, J. M., & Hall, G. (1980). A model for Pavlovian learning: Variations in the effectiveness of conditioned but not unconditioned stimuli. Psychological Review, 87, 532-552. Pearce, J. M., & Hall, G. (1992). Stimulus significance, conditionability, and the orienting response in rats. In B. A. Cambell, H. Hayne, & R. Richardson (Eds.), Attention and information processing in infants and adults: Perspectives from human and animal research (pp. 137-160). Hillsdale, NJ: Edbaum. Pearce, J. M., Kaye, H., & Hall, G. (1982). Predictive accuracy and stimulus associability: Development of a model for Pavlovian conditioning. In M. L. Commons, R. J. Herrnstein, & A. R. Wagner (Eds.), Quantitative analyses of behavior (Vol. III, pp. 241-255). Cambridge, MA: Ballinger. Pearce, J. M., & Redhead, E. S. (1993). The influence of an irrelevant stimulus on two discriminations. Journal of Experimental Psychology: Animal Behavior Processes, 19, 180-190. Redhead, E. S., & Pearce, J. M. (1995). Similarity and discrimination learning. Quarterly Journal of Experimental Psychology, 48B, 46-66. Rescorla, R. A. (1972). "Confignral" conditioning in discrete-trial bar pressing. Journal of Comparative and Physiological Psychology, 79, 307-317. Rescorla, R. A. (1991). Separate reinforcement can enhance the effectiveness of modulators. Journal of Experimental Psychology: Animal Behavior Processes, 17, 259-269. Rescorla, R. A., & Wagner, A. R. (1972). A theory of Pavlovian conditioning: Variations in the effectiveness of reinforcement and nonreinforcement. In A. H. Black & W. F. Prokasy (Eds.), Classical conditioning II: Current research and theory (pp. 64-99). New York: Appleton-Century-Crofts. Sutherland, N. S., & Mackintosh, N. J. (1971). Mechanisms of animal discrimination learning. New York: Academic Press. Wagner, A. R. (1976). Priming in STM: An information-processing mechanism for self-generated or retrieval-generated depression in performance. In T. J. Tighe & R. N. Leaton (Eds.), Habituation: Perspectives from child development, animal behavior, and neuropsychology (pp. 95-128). Hillsdale, NJ: Erlbaum. Wagner, A. R. (1981). SOP: A model of automatic memory processing in animal behavior. In N. E. Spear & R. R. Miller (Eds.), Information processing in animals: Memory mechanisms (pp. 5-47). Hillsdale, NJ: Erlbaum. Williams, D. A., Sagness, K. E., & McPhee, J. E. (1994). Configural and elemental strategies in predictive learning. Journal of Experimental Psychology: Learning, Memory, and Cognition, 20, 694-709.
Received December 4, 1996 Revision received May 21, 1997 Accepted May 28, 1997 •
