The Effects of Intertrial and Feature–Target Intervals on Operant Serial Feature Negative Discrimination Learning

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LEARNING AND MOTIVATION ARTICLE NO.

27, 21–42 (1996)

0002

The Effects of Intertrial and Feature–Target Intervals on Operant Serial Feature Negative Discrimination Learning PETER C. HOLLAND

AND JAVIER

R. MORELL

Duke University The effects of intertrial interval (ITI) and feature–target interval (FTI) on learning of discrete-trial operant serial feature negative (target+/feature → target−) discriminations were examined in two experiments with rats. In Experiment 1, the FTI was 10 s and the ITIs were 0.5, 1, 2, 4, or 8 min, and in Experiment 2, the FTI was 20 s and the ITIs were 1, 2, 4, 8, or 16 min. Discrimination performance was acquired more rapidly with longer ITIs and with shorter FTIs. The best predictor of acquisition performance was the ratio of ITI and FTI. This predictive relation broke down at the smallest ratio value, which did not support discrimination learning when the FTI was 20 s. Transfer of the feature’s inhibitory control to a separately trained target cue was minimal in all conditions, regardless of ITI. The results were discussed in the context of occasion setting. © 1996 Academic Press, Inc.

The intertrial interval (ITI) has often been shown to be an important variable in determining excitatory conditioning: spaced practice is frequently found to be superior to massed practice (e.g., Gormezano & Moore, 1969; Spence & Norris, 1950). More interesting, some investigators (e.g., Gibbon, Baldock, Locurto, Gold, & Terrace, 1977) have found the ITI to affect the within-trial interstimulus interval (ISI) function: When the ITIs are relatively long, conditioning can occur with longer intervals between the onsets of the conditioned stimulus (CS) and unconditioned stimulus (US) than when the trials are more closely spaced. Indeed, Gibbon et al. (1977) found that acquisition of simple conditioning was a function of the ITI/ISI ratio, regardless of the absolute values of those intervals. The ITI is likely to play similar roles in the learning of more complex conditioning tasks as well. For example, many investigators (e.g., Bowers & Richards, 1990; Grant, 1975) have found that longer ITIs improve performance in delayed matching-to-sample discrimination tasks, in which a sample cue signals which of two choice cues presented after an intervening delay interval is the This research was supported in part by grants from the National Science Foundation. We thank Marie Crock, David Jones, and Stephanie Nevels for technical assistance. Address reprint requests to Peter C. Holland at Department of Psychology: Experimental, Box 90086, Duke University, Durham, NC, 27708-0086, E-mail: [email protected]. 21 0023-9690/96 $18.00 Copyright © 1996 by Academic Press, Inc. All rights of reproduction in any form reserved.

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correct target of responding. Furthermore, reminiscent of Gibbon et al.’s (1977) findings, Roberts and Kraemer (1982) found that the use of longer ITIs extended the delay intervals over which subjects could maintain information within a trial in delayed matching-to-sample procedures: discrimination accuracy was a simple function of the ratio of ITI and delay interval, regardless of the particular values of those intervals (at least within a moderate range of delay intervals). Similarly, Holland (1995) found that rats’ acquisition of operant serial feature positive (feature → target + /target−) discriminations was facilitated by the use of longer ITIs. Interestingly, longer ITIs also apparently encouraged the rats’ use of a particular solution strategy described as occasion setting (Holland, 1983, 1992), whereby the feature controls conditioned responding by modulating the action of the association between the target and the US. Moreover, the use of the occasion setting strategy seemed best predicted by the ratio of the ITI and a within-trial interval, that between the feature and the target. The experiments described in this article complemented Holland’s (1995) study of ITI effects in feature positive discriminations by examining the effects of variations in the ITI on rats’ learning of operant serial feature negative (FN) discriminations (A+ / X → A−). Previous reports from this laboratory (e.g., Holland, 1989, 1991; Holland & Coldwell, 1993; Holland & Lamarre, 1984) showed that the temporal arrangement of elements within a compound stimulus can affect the way rats solve FN discriminations. Those reports claimed that when X and A are presented simultaneously on compound trials, solution of the discrimination chiefly involves the acquisition of inhibitory associations between X and the US. But when X precedes A on compound trials, it also comes to modulate the action of the A–US association, setting the occasion for the nonreinforcement of A. The distinction between these two solution strategies was supported by several kinds of data (see Holland, 1992, for a review). For example, reinforced presentations of the X feature alone after simultaneous FN discrimination training abolished X’s ability to serve as an inhibitor, indeed enhancing responding during the XA compound to a level greater than that previously shown during A alone, but comparable reinforced X-alone presentations after serial FN training did not destroy X’s ability to suppress responding to A (Holland, 1985, 1989, 1991). Similarly, the range of transfer of X’s putative occasion setting powers is typically more limited than that of X’s simple inhibitory control. Although X’s inhibition reduces the net excitatory strength of any target cue, its negative occasion setting powers are often limited to excitatory targets that were explicitly trained as targets of other negative occasion setters (e.g., Holland, 1989; Holland & Coldwell, 1993; Lamarre & Holland, 1987). Although there has been considerable investigation of the effects of ITI on the development of inhibitory stimulus control in backward, discriminative, and explicitly unpaired conditioning procedures (e.g. Ewing, Larew, & Wagner, 1985; Kaplan, 1984; LoLordo & Fairless, 1985; Morris, 1974; Weisman & Litner, 1971; Williams & Overmier, 1988), there has been little study of the

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effects of variations in ITIs on the acquisition of inhibitory control in serial FN discriminations. Informal comparisons of the acquisition rates in operant serial FN discriminations in our laboratory suggest that longer ITIs facilitate that learning. For example, in two otherwise identical experiments that used 10-s feature– target intervals (FTIs), acquisition to a common discrimination criterion took about four times as many trials when 1-min ITIs were used (Holland, 1991, Experiment 2) as when 4-min ITIs were used (unpublished research). The experiments reported here used a discrete-trial operant procedure to investigate the effects of intertrial interval (ITI) on the acquisition of serial feature negative discriminations. In order to consider the effects of the ITI/FTI ratio, we also used two different FTIs. Unpublished research in our laboratory (described briefly by Lamarre & Holland, 1985) found that longer FTIs slowed FN discrimination learning in conditioned suppression procedures. In Experiment 1, the FTI was 10 s and the ITIs were 0.5, 1, 2, 4, or 8 min, and in Experiment 2 the FTI was 20 s and the ITIs were 1, 2, 4, 8, or 16 min. Thus, as in Gibbon et al’s (1977) and Roberts and Kraemer’s (1982) studies, performance at equivalent ratios of intertrial and within-trial intervals, with different constituent intervals, could be compared. The existing data from our laboratory (described earlier) suggest that the use of longer ITIs and shorter FTIs should facilitate the acquisition of the serial FN discriminations. In an attempt to distinguish between interval effects on simple conditioned inhibition and those on occasion setting, the present experiments also examined the effects of ITI on transfer of conditioned responding to compounds in which the original target cue was replaced by a separately trained cue. Because typically we have found relatively small amounts of transfer of occasion setting to separately trained target cues, but substantial transfer of simple inhibitory conditioning (e.g., Holland, 1992, noted above), we entertained the possibility that variations in transfer responding might mirror variations in the level of simple inhibitory conditioning of the feature cue. Using a similar strategy with serial feature positive discriminations, Holland (1995) found evidence suggesting that the contribution of occasion setting to the solution of FP discriminations was greater with longer ITIs but that of simple conditioning was greater with shorter ITIs: despite superior performance on the original FP discrimination with longer ITIs, transfer of the feature’s stimulus control to a separately trained cue was greater with shorter ITIs. Thus, in the present experiments, we considered whether a feature’s inhibitory powers might transfer to other target cues more readily after training with shorter ITIs, despite superior acquisition with longer ITIs. EXPERIMENT 1 In Experiment 1, five groups of rats were trained to discriminate between an auditory cue presented alone, during which lever pressing was reinforced, and presentations of the auditory target within a serial visual → auditory compound, during which reinforcement was not available. Then, all rats were trained to lever

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press during another auditory cue. Finally, to compare the breadth of transfer of the visual feature’s inhibitory stimulus control to the separately trained target, responding controlled by the two auditory target cues alone, and by the serial compounds of the visual feature with each of the two auditory targets, was examined in a test phase. Method Subjects and apparatus. The subjects were 23 male and 14 female CD-strain albino rats, bred from Charles River (Raleigh, NC) stock in a Duke University Department of Psychology: Experimental facility. At the beginning of the experiment they were 110–130 days old and experimentally naive. The rats were maintained at 80% of their ad lib body weights throughout the experiment by restricting their access to food. Water was available at all times in their individual home cages. There were eight identical experimental cambers, each 22.9 × 20.3 × 20.3 cm. The front and back walls of each chamber were aluminum; the side walls and top were clear acrylic. A food cup was recessed behind a 5 × 5 cm opening in the front wall; the bottom of the opening was 2 cm from the floor, and its center was 2 cm to the right of the center of the front wall. A 6-W jeweled lamp (“panel light”) was centered on the front wall, 4 cm above the top of the food cup opening. Except when the panel light was illuminated as a signal, the chambers were dark. A 2 × 2 cm lever was mounted 3 cm above the floor, 4 cm left of the food opening. The floor of the chamber was composed of 0.48-cm stainless steel rods spaced 1.9 cm apart. Each of the chambers was enclosed in a sound-attenuating shell. Two speakers for delivering the auditory CSs were mounted on one wall of each shell, level with the top of the chamber, and 2 cm in front of and 10 cm to the left of the front wall of the chamber. One stimulus, a 72-dB (A) square wave, 1500-Hz tone, was presented through one speaker, and a 72-dB white noise was presented through the other speaker. Finally, each shell was enclosed in another sound-attenuating box. Constant background noise (62.5 dB) was provided by a ventilating fan on each box. Procedure. The rats were first trained to drink from the food cup and to press the lever. Each rat received one 1-h session in which 0.3-ml deliveries of 0.2 M sucrose were delivered on a VT 2-min schedule during the first 40 min, and each lever press was reinforced with a similar sucrose delivery. Then, the rats received two additional sessions in which each lever press was reinforced, but no response-independent sucrose was delivered. Each of these sessions was terminated for each rat as soon as it made 50 presses. Next, all rats were trained to respond during an auditory (noise) cue but not during the intertrial interval (ITI) in three 60-min sessions. In each of these sessions, there were 60 presentations of the noise, during which each lever press produced sucrose delivery. The duration of each noise presentation was 30, 15, and 5 s in the first, second, and third sessions, respectively.

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Four or five male and two or three female rats were then randomly assigned to each of five groups, and serial feature negative discrimination training was begun. In each of the first two sessions of this phase, the rats in all groups received eight 5-s presentations of the 1500-Hz tone (A+), during which each lever press was reinforced, randomly intermixed with eight X → A− presentations, in which a 5-s illumination of the panel light feature (X) was followed, after a 5-s empty interval, by a 5-s tone target cue (A), during which lever pressing was not reinforced. For all groups, the feature–target interval was 10 s. The groups differed in their average ITI, which was 0.5 min in Group 0.5–10 (n 4 8), 1 min in Group 1–10 (n 4 7), 2 min in Group 2–10 (n 4 8), 4 min in Group 4–10 (n 4 7), and 8 min in Group 8–10 (n 4 7). In each group, the ITIs ranged from one-half to twice the mean ITI, rectangularly distributed. Session durations ranged from 8 min in Group 0.5–10 to 128 min in Group 8–10. Thus, the groups were matched with respect to the number of trials in each session, but not session duration. In each discrimination training session after the first 2, the rats received 4 reinforced A+ trials and 12 nonreinforced X → A− trials, randomly intermixed. The rats in the various groups were given different numbers of discrimination training sessions in an attempt to equalize performance levels before proceeding to the transfer test. The rats in Group 8–10 received 15 discrimination training sessions, the rats in Group 4–10 received 40 sessions, and the rats in Groups 2–10, 1–10, and 0.5–10 received 80 sessions. Then, the rats received retraining of the cue to be used as the transfer target, B. In a single session, each rat received four 5-s presentations of the noise cue that was used in preliminary conditioning. During noise presentations, each lever press was reinforced. The ITIs were the same as those used in discrimination training. Finally, the rats received two test sessions, which examined X’s ability to modulate responding to its original target (A) and to the separately trained transfer target (B). In each of the two sessions there were four of the original X → A compound trials, four presentations of the transfer X → B compound, two presentations each of the two targets, A and B, alone, during which reinforcement was available, and two presentations each of A and B during which reinforcement was unavailable. The mean ITIs were those used in training; to avoid confounding differences in ITI with trial type in these test sessions, the ITIs were fixed at the mean value in each group. Data analysis. We recorded the rate of lever pressing, the percentage of trials on which at least one response occurred, and the latency of the first response during each 5-s interval of CS presentations, empty trace intervals, and the 5-s pre-CS interval. However, response rates did not provide an appropriate measure of performance on reinforced trials: response rates were artificially suppressed on those trials because the rats did not press for several seconds after sucrose was delivered. Neither the latency nor percentage of trials with a response measures was affected in this manner. Those two measures were closely related throughout

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the experiment and led to identical conclusions. We selected latency to first response as the primary measure in the present experiments because it was less affected by ceiling effects and seemed especially appropriate for studying the effects of temporal manipulations. All three measures were analyzed, however. A simple index of discrimination performance was calculated by subtracting the latency of responding on target-alone trials from the latency of responding to the target on compound trials. Acquisition of the discrimination was also evaluated by examining the number of sessions required to reach arbitrarily selected criteria of discrimination difference scores (0.3, 0.5, or 1.0 s) over three sessions of discrimination training. Untransformed latencies were subjected to analyses of variance (ANOVAs). Between-group effects of ITI were assessed by analyses of orthogonal polynomial trends. The criterion of statistical significance adopted was p < .05. Results Except for the first few conditioning sessions, the rats seldom pressed during pre-CS intervals (fewer than 5% of those intervals). We do not discuss pre-CS responding further. Figure 1 shows the acquisition of the feature negative discrimination in Experiment 1. Rats trained with longer ITIs acquired the discrimination more rapidly than those trained with shorter ITIs, primarily because the rats trained with longer ITIs were better at learning to withhold responding on the nonreinforced compound trials. Separate groups by sessions ANOVAs were performed for discrimination difference scores, for latencies to respond on reinforced targetalone trials, and for latencies to respond to the target on nonreinforced compound trials. Separate analyses were performed for sessions 1–15 (which all five groups received), sessions 16–40 (which excluded Group 8–10), and sessions 41–80 (which included only Groups 2–10, 1–10, and 0.5–10). In addition, one-way ANOVAs were performed for the number of sessions necessary to reach 0.3-, 0.5-, and 1.0-s discrimination difference score criteria for three consecutive sessions. In those analyses, subjects in Groups 0.5–10 and 1–10 that failed to reach a criterion were assigned the 80-session maximum number of sessions. We present only the results of this analysis for the 0.3-s criterion, the highest criterion reached by some rats in all groups of both experiments. All rats in Experiment 1 reached that criterion. The top panel of Fig. 1 shows the discrimination difference scores. Over the first 15 sessions, the effects of groups, F(4,32) 4 31.61, sessions, F(14,448) 4 22.99, and the groups by sessions interaction, F(56,448) 4 6.10, were all reliable. Discrimination performance was better in groups that were trained with longer ITIs: the linear trend across the groups was reliable, F(1,32) 4 122.55. Newman–Keuls comparisons showed performance of Group 8–10 to be reliably better than that of any other group, and Group 4–10’s performance was significantly better than that of Groups 1–10 and 0.5–10. Over sessions 16–40, the

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FIG. 1. Performance during serial feature negative discrimination training in Experiment 1. The bottom panel portrays the mean latencies to respond to the target cue when it was preceded by the feature cue (S−), the middle panel shows the mean latencies to respond to the target cue when it was presented alone (S+), and the top panel shows a mean difference score measure calculated by subtracting the latency to respond on S+ trials from that on S− trials. The group designations indicate first the intertrial interval in minutes and then the feature–target interval in seconds.

effects of groups, F(3,29) 4 17.89, sessions, F(24,624) 4 4.07, and the groups by sessions interaction, F(72,624) 4 2.11, were all reliable, but the only reliable difference among individual groups was Group 4–10’s superiority to the other groups. Finally, over sessions 41–80, the effects of groups, F(2,20) 4 47.14, sessions, F(39,780) 4 18.96, and the sessions by groups interaction, F(78,780) 4 1.79, were reliable. The linear trend among the groups was reliable, F(1,20) 4 96.86, and Newman–Keuls comparisons showed each of the three remaining groups (2–10, 1–10, and 0.5–10) to differ significantly from each other. Similar conclusions were reached by analyzing the numbers of sessions to reach the various discrimination criteria. For example, the mean numbers of sessions to reach the 0.3-s difference criterion were 5, 8, 21, 37, and 48 in Groups 8–10, 4–10, 2–10, 1–10, and 0.5–10, respectively. A one-way ANOVA of those

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scores showed a reliable effect of groups, F(4,32) 4 31.36, and a reliable linear trend across groups, F(1,32) 4 119.98. (The number of sessions needed to meet several discrimination criteria in this phase is indicated by the filled circles in the various panels of Fig. 5, which also shows the performance of subjects in Experiment 2). The center panel of Fig. 1 shows the latency to respond on reinforced targetalone trials. None of the ANOVAs (comparable to the ones as just described for the difference scores) yielded significant effects or interactions. The bottom panel of Fig. 1 shows the latency to respond to the tone target on nonreinforced compound trials. This measure showed a pattern similar to that of the discrimination difference scores (top panel). Over the first 15 sessions, the effects of groups, F(4,32) 4 35.10, and the groups by sessions interaction, F(56,448) 4 4.44, were reliable. Response latencies were longer in groups that were trained with longer ITIs: the linear trend across the groups was reliable, F(1,32) 4 40.08. Newman–Keuls comparisons showed the latencies in Group 8–10 to be reliably higher than those in any other group, and Group 4–10’s and Group 2–10’s latencies were significantly longer than those of Groups 1–10 and 0.5–10. Over sessions 16–40, the effects of groups, F(3,29) 4 20.10, sessions, F(24,624) 4 4.29, and the groups by sessions interaction, F(72,624) 4 3.96, were all reliable, but the only reliable difference among individual groups was Group 4–10’s displaying longer latencies than the other groups. Finally, over sessions 41–80, the effects of groups, F(2,20) 4 36.84, and sessions, F(39,780) 4 16.27, were reliable. The linear trend among the groups was reliable, F(1,20) 4 78.76, and Newman–Keuls comparisons showed each of the three remaining groups (2–10, 1–10, and 0.5–10) to differ significantly from each other. The results of the transfer test are shown in Fig. 2. Response latencies on X → A original compound trials, A-alone trials, X → B transfer compound trials, and B-alone trials are shown in the top panel, and discrimination difference scores (X → A minus A original discrimination, and X → B minus B transfer discrimination) are shown in the bottom panel. Although all rats responded more rapidly on A-alone trials than on X → A trials, there was little transfer of X’s suppressive powers to the separately trained B target, and the magnitudes of any such transfer did not differ among the groups. Response latencies on the individual trial types did not differ across groups: none of the effects of groups or of linear trends across groups was reliable. Similarly, although the effect of trial type was highly reliable, F(3,96) 4 54.64, that factor’s interaction with group was not significant, F < 1. A contrast of responding on A and X → A trials confirmed the accurate original discrimination performance, F(1,32) 4 18.81, and the X → A minus A original discrimination difference scores showed a reliable linear trend across groups, F(1,32) 4 4.33, with larger differences the longer the ITI. A similar contrast of responding on B and X → B trials was not reliable, F(1,32) 4 1.99, and analysis of the X → B minus B difference scores showed no reliable main effect of group, F(4,32) 4 1.31, or of linear trend across groups, Flin(1,32) 4 0.76. Finally, a comparison

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FIG. 2. Performance during the transfer test of Experiment 1. The groups, indicated by their training ITIs, are arrayed along the abscissa. The top panel shows the mean latencies to the first response to the original (A) and transfer (B) targets presented alone or when preceded by the X feature. The bottom panels show, for both the original, A+ / X → A− (ORIG) and transfer, B+ / X → B− (XFER) discriminations, the mean differences between the latencies to the first response to a target when it was preceded by the feature (X) and when it was presented alone.

of the original and transfer discrimination difference scores also showed a reliable linear trend across groups, F (1,32) 4 4.20, supporting the impression given in the bottom panel of Fig. 2 that the difference in performance between the original and transfer discriminations was greater with longer ITIs. However, given the previous analyses, it is likely that this difference more reflects the levels of original discrimination performance than the amounts of transfer. Discussion Unlike in Gibbon et al.’s (1977) experiments, there was no reliable effect of ITI on the acquisition or performance of conditioning (as measured by reduced response latency) to the reinforced target cue within FN discrimination training.

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However, it should be pointed out that the present experiment differed from those of Gibbon et al. (1977) in many ways. Besides using an operant conditioning preparation with rats rather than pigeon autoshaping, several aspects of the training procedures may have biased the outcomes of the present experiment. Prior to the serial conditioning phase the rats were trained to lever press during another auditory cue, with identical ITIs in all groups, which may have affected acquisition performance. Furthermore, the frequent nonreinforcement of responding during the target cues on serial compound trials surely influenced responding to those same cues on target-alone trials. Thus, comparison of these target-alone data with those from simple nondiscriminative conditioning experiments seems inappropriate. Nevertheless, learning of the FN discrimination was substantially affected by ITI. The acquisition of discrimination performance was more rapid the longer the ITI, reflecting more rapid loss of responding during nonreinforced compound trials. Thus, although acquisition of conditioning to S+ was apparently unaffected by ITI, loss of conditioning during S− was enhanced with longer ITIs. Holland (1995) found a similar effect of ITI on the acquisition of serial FP discriminations: although the latencies of responding on reinforced compound trials were unaffected by ITI, the latencies to respond on nonreinforced target-alone trials were longer the longer the ITI. Unlike in Holland’s (1995) studies of FP learning, however, there was little evidence for an effect of ITI on transfer performance, and hence on the rats’ selection of occasion setting or simple conditioned inhibition strategies. Rather, there was only minimal transfer in any of the ITI conditions. One interpretation of this finding is that performance in all groups primarily reflected occasion setting. EXPERIMENT 2 Experiment 2 was similar to Experiment 1, except that the feature–target interval was 20 s rather than 10 s, and the ITIs used were 1, 2, 4, 8, and 16 min. These values preserved Experiment 1’s ITI/FTI ratios of 3, 6, 12, 24, and 48, despite the altered FTI. Method Subjects and apparatus. The subjects were 39 CD-strain rats, bred from Charles River (Raleigh, NC) stock in a Duke University Department of Psychology: Experimental facility. The first replication of the experiment used 16 experimentally naive female rats, 120 days old at the beginning of the experiment. The second replication used 21 female and 2 male rats about 180 days old at the beginning of the experiment. Four of those female rats were experimentally naive; the remaining rats had received prior exposure to variable interval or variable ratio schedules of lever pressing. The rats were maintained at 80% of their ad lib. body weights throughout the experiment by limiting their access to food. Water was available at all times in their individual home cages.

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The apparatus included four chambers similar in every dimension to those used in Experiment 1. Procedure. The experiment was conducted in two replications. The rats were assigned to groups so that each group comprised 4 experienced and 4 naive rats (except Group 16–20, which included 4 naive and 3 experienced rats). All subjects in Group 16–20 were run in the second replication; otherwise, the 4 naive rats in each group were run in the first replication and the 4 experienced rats in the second. The 20 experimentally naive rats were first trained to drink from the food cup and to press the lever. Each rat received one 1-h session in which 0.3-ml deliveries of 0.2 M sucrose were delivered on a VT 2-min schedule during the first 40 min, and each lever press was reinforced with a similar sucrose delivery. Then, the rats received two additional sessions in which each lever press was reinforced, but no response-independent sucrose was delivered. Each of these sessions was terminated for each rat as soon as it made 50 presses. The 19 experienced rats received the second of these sessions. Next, all 39 rats were trained to respond during an auditory (noise) cue but not during the intertrial interval (ITI) in three 60-min sessions. In each of these sessions, there were 60 5-s presentations of the noise, during which each lever press produced sucrose delivery (note that in the corresponding phase of Experiment 1, the duration of the noise cue was reduced gradually from 30 to 5 s). Serial feature negative discrimination training was then begun. In each session, the rats in all groups received four 5-s presentations of the 1500-Hz tone (A+), during which each lever press was reinforced, randomly intermixed with 12 X → A− presentations, in which a 5-s illumination of the panel light feature (X) was followed, after a 15-s empty interval, by a 5-s tone target cue (A), during which lever pressing was not reinforced. For all groups, the feature–target interval was 20 s. The groups differed in their average ITI, which was 1 min in Group 1–20, 2 min in Group 2–20, 4 min in Group 4–20, 8 min in Group 8–20, and 16 min in Group 16–20. In each group, the ITIs ranged from one-half to twice the mean ITI, rectangularly distributed. Session durations ranged from 16 min in Group 1–20 to 256 min in Group 16–20. The rats in the various groups were given different numbers of discrimination training sessions in an attempt to equalize performance levels before proceeding to the transfer test. The rats in Groups 16–20 and 8–20 received 25 discrimination training sessions, whereas the rats in the other groups received 100 discrimination training sessions. Then, the rats received training designed to establish responding during a noise cue to be used as the transfer target, B, and (unlike in Experiment 1) also to extinguish responding to the original tone target, A. Rescorla and Holland (1977) argued that this procedure might minimize the contribution of generalization between the original and transfer targets to any transfer observed. In each of these sessions, each rat received four 5-s presentations of the noise, during which each lever press was reinforced, and 12 presentations of the 5-s tone target, during which reinforcement was not available. The rats in the various groups were given

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different numbers of sessions in an attempt to equalize performance levels before proceeding to the transfer test. The rats in Groups 16–20, 8–20, and 4–20 received 15 sessions, and the rats in Groups 2–20 and 1–20 received 38 sessions in this phase. Finally, all subjects received a series of three test sessions. Test 1 examined transfer of the panel light feature cue’s ability to modulate responding to the noise target cue. All subjects received a single session that included eight presentations of the noise alone (B trials) and eight presentations of the 5-s panel light, followed, after a 15-s empty trace interval, by a 5-s presentation of the noise (X → B trials). Reinforcement was available on half of the B-alone trials, but not on any of the other trials. Reinforced B trials served as “reminder” trials; the test session began with a reminder trial, and each block of four test trials included one reminder trial, one nonreinforced B trial, and two nonreinforced X → B trials. In each group, the ITI was held constant at the mean value used in training in that group. Test 2 examined performance on the original discrimination in a single session, identical to Test 1 except that the noise transfer target was replaced by the original tone target on each trial. Finally, Test 3 examined the effects of altering the ITI on performance on the original discrimination. The rats in Groups 8–20 and 16–20, together with half of the rats in each of Groups 2–20 and 4–20, were tested with constant 1-min ITIs, and the rats in Group 1–20, along with the other half of the rats in Groups 2–20 and 4–20, were tested with constant 8-min ITIs. Except for the ITIs, Test 3 was identical to Test 2. Test 3 permitted determining whether the differential effects of the 1-min and 8-min ITIs observed in the previous phases were primarily performance effects or reflected differences in learning. Results Except for the first few conditioning sessions, the rats seldom pressed during pre-CS intervals (fewer than 5% of those intervals). We do not discuss pre-CS responding further. Figure 3 shows the acquisition of the feature negative discrimination in Experiment 2. As in Experiment 1, rats trained with longer ITIs acquired the discrimination more rapidly than those trained with shorter ITIs, primarily because the rats trained with longer ITIs were better at learning to withhold responding on the nonreinforced compound trials. Separate replication by groups by sessions ANOVAs were performed for discrimination difference scores, for latencies to respond on reinforced target-alone trials, and for latency to respond on nonreinforced compound trials. (Neither the main effects of replication nor any of its interactions was reliable for any of these measures, so we do not mention those effects further.) Separate analyses were performed for sessions 1–25 (which all five groups received) and sessions 26–100 (which included only Groups 4–20, 2–20, and 1–20.) A one-way ANOVA was performed for the

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FIG. 3. Performance during serial feature negative discrimination training in Experiment 2. The bottom panel portrays the mean latency to respond to the target cue when it was preceded by the feature (S−), the middle panel shows the mean latency to respond to the target cue when it was presented alone (S+), and the top panel shows a mean difference score measure calculated by subtracting the latency to respond on S+ trials from that on S− trials. The group designations indicate first the intertrial interval in minutes and then the feature–target interval in seconds.

number of sessions necessary to reach a 0.3-s discrimination difference score criterion (the highest criterion reached by rats in all groups in both experiments). In those analyses, subjects (in Groups 1–20 and 2 –20 only) that failed to reach a criterion were assigned the 100-session maximum number of sessions. The top panel of Fig. 3 shows the discrimination difference scores. Over the first 25 sessions, the effects of groups, F(4,34) 4 14.62, sessions, F(24,816) 4 9.70, and the groups by sessions interaction, F(96,816) 4 3.87, were all reliable. Newman–Keuls comparisons showed the performances of Groups 8–20 and

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16–20 to be reliably better than those of any other group. Over sessions 26–100, the effects of groups, F(2,21) 4 35.81, sessions, F(74,1554) 4 15.80, and the sessions by groups interaction, F(148,1554) 4 1.72, were reliable. The linear trend among the groups was reliable, F(1,20) 4 79.42, and Newman–Keuls comparisons showed Group 4–20 to differ significantly from each of the other two groups. The numbers of sessions to reach various discrimination criteria (filled squares in Fig. 5, which also shows the performance of subjects in Experiment 1) showed similar patterns. A one-way ANOVA of the number of trials to reach a 0.3-s difference criterion (bottom panel of Fig. 5) showed a reliable effect of groups, F(4,34) 4 94.94, and a reliable linear trend across groups, F(1,34) 4 314.38. The center panel of Fig. 3 shows the latency to respond on reinforced targetalone trials. None of the ANOVAs (comparable to the ones as just described for the difference scores) yielded significant effects or interactions. The bottom panel of Fig. 3 shows the latency to respond to the tone target on nonreinforced compound trials. This measure showed a pattern similar to that of the discrimination difference scores (top panel). Over the first 25 sessions, the effects of groups, F(4,34) 4 30.14, sessions, F(24,816) 49.10, and the groups by sessions interaction, F(96,816) 4 3.01, were all reliable. Newman–Keuls comparisons showed the performances of Groups 8–20 and 16–20 to be reliably better than those of any other group. Over sessions 26–100, the effects of groups, F(2,21)431.16, sessions, F(74,1554) 4 8.91, and the sessions by groups interaction, F(148,1554) 4 4.29, were reliable. The linear trend among the groups was reliable, F(1,20) 4 37.14, and Newman–Keuls comparisons showed Group 4–20 to differ significantly from each of the other two groups. After serial FN discrimination training, the rats were trained to respond to a new target cue (B) and to withhold responding to the original target cue (A). Table 1 shows the mean latencies to respond on the final session of that phase in each group. Analysis of variance showed no effect of group on either measure. The results of Test 1 (transfer target) and Test 2 (original target) are shown in Fig. 4. Response latencies on X → A original compound trials, A-alone trials, X

TABLE 1 Mean Latency (s) to Respond to A and B Prior to Testing in Experiment 2 Group

Session No.

B+

A−

16–20 8–20 4–20 2–20 1–20

15 15 15 38 38

1.77 1.97 1.59 1.28 2.11

3.90 4.07 3.48 3.96 3.09

Note. Data are from the session number specified (the last session of this phase for each group). A was the original, training excitor, during which lever press responses were not reinforced (−), and B was the transfer excitor, during which lever presses were reinforced (+).

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FIG. 4. Performance during the first transfer test of Experiment 2, in which the intertrial intervals (ITIs) were the mean of the original training intervals. The groups, indicated by their training ITIs, are arrayed along the abscissa. The top panel shows the mean latencies to the first response to the original (A) and transfer (B) targets presented alone or when preceded by the X feature. The bottom panel shows, for both the original, A+ / X → A− (ORIG) and transfer, B+ / X → B− (XFER) discriminations, the mean difference between the latency to the first response to a target when it was preceded by the feature (X) and when it was presented alone.

→ B transfer compound trials, and B-alone trials are shown in the top panel, and discrimination difference scores (X → A minus A original discrimination, and X → B minus B transfer discrimination) are shown in the bottom panel. Note that these two tests came after extensive training of responding during the transfer target (B) and extinction during the original target (A). Nevertheless, the A+ reminder trials in Test 2 were apparently sufficient to reinstate reasonably rapid responding to A. Original discrimination performance was maintained in all groups that showed it initially, but transfer performance was small. An overall group by trial type ANOVA showed reliable effects of group, F(4,34) 4 3.23, trial type, F(3,102) 4 55.29, and of the group by trial type interaction, F(12,102) 4 5.15. A contrast

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of responding on A and X → A trials confirmed the accurate original discrimination performance, F(1,34) 4 14.88, and the X → A minus A original discrimination difference scores differed reliably across groups, F(4,34) 4 5.87, and showed a reliable linear trend across groups, F(1,34) 4 19.86, with larger differences the longer the ITI. Response latencies on X → A trials and A-alone trials individually also differed across groups, Fs (4,34) 4 7.45 and 2.78, respectively. However, whereas response latencies on X → A trials increased with increasing ITIs, Flin(1,34) 4 25.34, latencies on A-alone trials decreased with increasing ITI, Flin(1,34) 4 6.00. A contrast of responding on B and X → B trials was not reliable, F(1,34) 4 1.89, and the X → B minus B transfer discrimination difference scores failed to showed an overall difference among the groups, F(4,34) 4 1.67, or a reliable linear trend across groups, Flin(1,34) 4 2.98. Nevertheless, the response latencies on X → B trials differed among the groups, F(4,34) 4 8.80, increasing with increases in the ITI, F(1,34) 4 30.32. Latencies on B-alone trials did not differ among the groups, F(4,34) 4 1.71. Despite the lack of a reliable trend in the transfer discrimination difference scores, and the presence of such a trend with the original discrimination difference scores, a comparison of the original and transfer discrimination difference scores failed to show an overall difference among the groups, F(1,34) 4 1.43, or a reliable linear trend, F(1,34) 4 2.60. Thus, a conservative conclusion from the results of Tests 1 and 2 is that transfer was small and related to the level of performance on the original discrimination. Test 3 examined performance on the original A-alone versus X → A discrimination when the ITIs were shifted to either 1 or 8 min. Performance in Test 3 was evaluated by comparing the A versus X → A difference scores in Test 3 with those in Test 2, in which the ITIs were the intervals used in training. As in the previous phases of this experiment, and in Experiment 1, between-group variation in difference scores (as well as any differential changes in those scores) reflected almost entirely variations in responding on serial compound trials. Reduction of the ITI to 1 min reduced discrimination performance in every subject in Groups 16–20 (mean change 4 −1.76 s, n 4 7), 8–20 (−1.84 s, n 4 8), and 4–20 (−1.52 s, n 4 4), but in only one of the four rats in Group 2–20 (+0.15). However, despite these reductions, each of these groups showed better discrimination performance than Group 1–20 (in which the rats were trained with 1-min ITIs) when that group was tested with 1-min ITIs in Test 1, Fs(1,34) > 3.97. Thus, the use of 1-min ITIs apparently disrupted both the learning and performance of serial feature negative discriminations in Experiment 2. In contrast, an increase in the ITI to 8 min had no consistent effect in any of the groups: the mean changes in difference scores were +0.05, +0.15, and −0.08 in Groups 1–20 (n 4 8), 2–20 (n 4 4), and 4–20 (n 4 4), respectively, and each of those three groups showed poorer discrimination performance in Test 3 than did Group 8–10 (in which the rats were trained at 8-min ITIs) when that group was tested with 8-min ITIs in Test 1.

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Discussion As in Experiment 1, learning of the FN discrimination was substantially affected by ITI. The acquisition of discrimination performance was more rapid the longer the ITI, reflecting more rapid loss of responding during nonreinforced compound trials. Also as in Experiment 1, there was little evidence for an effect of ITI on transfer performance; rather, there was only minimal transfer in any of the ITI conditions. Furthermore, the use of 1-min ITIs disrupted both the learning and performance of serial feature negative discriminations. The performance deficit is demonstrated by the reduced performance of rats trained with longer ITIs (4–16 min) when tested with 1-min ITIs. Reduced learning with short training ITIs is indicated both by the observation that extending the ITIs to 8 min in testing did not enhance performance of rats trained with shorter ITIs and by the finding that when all groups were tested with 1-min ITIs, the rats trained with longer ITIs showed performance superior to that of rats trained with 1-min ITIs. GENERAL DISCUSSION Two experiments showed consistent effects of ITI on the rate of acquisition of serial FN discriminations: the longer the ITI, the faster the acquisition. Furthermore, in both experiments, differences in discrimination performance were the consequence of variations in responding on nonreinforced compound trials, rather than on the reinforced target-alone trials. Finally, transfer of the features’ inhibitory stimulus control to a separately trained target cue was minimal in all conditions, consistent with previous observations of transfer after serial FN learning, and was not reliably affected by variations in the ITI. The two experiments used different FTIs, chosen so that the ITI/FTI ratios were identical across experiments. Otherwise, the procedures of the discrimination training phases of the two experiments were identical, except for the number of training sessions used (which differed so that subjects in all of the groups in both experiments would enter the test sessions with similar levels of responding to the original cues). Consequently, a statistical comparison of the number of sessions needed to reach acquisition criteria across both studies seemed appropriate. Figure 5 shows the mean number of sessions needed to reach various criteria of discrimination performance in all of the groups in both experiments. We selected the number of sessions needed to reach the highest discrimination difference score criterion reached by subjects in most of the groups, 0.3 s, as our dependent measure in a stepwise linear regression analysis, with ITI/FTI ratio, FTI, and ITI as variables. Even that criterion was never met by five of the eight rats in Group 1–10 in Experiment 2, so data from that group were excluded. Of the three variables, the only significant contributor to performance was the ITI/ FTI ratio, r2 4 0.45, F(1,66) 4 54.14. Further analyses showed the best twovariable (ITI/FTI ratio and FTI) and three-variable regression models (r2s 4 0.46) to be no better than the one-variable model.

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FIG. 5. Mean number of sessions (on a log scale) before reaching a criterion of three successive sessions with a difference between the latency of responding to the target cue on nonreinforced and reinforced trials greater than 1.0 s (top), 0.5 s (middle), or 0.3 s (bottom), in both Experiments 1 and 2. The abscissa shows the ratio of the intertrial to the feature–trial interval, on a log scale, of the various groups, the parameter distinguishes rats trained with the 10-s (Experiment 1) and 20-s (Experiment 2) feature–target intervals, and the point labels indicate the mean intertrial interval used in the groups represented by each point.

Thus, among the nine groups of rats that learned the serial FN discrimination, the ITI/FTI was the best predictor of acquisition performance. These data then join those of Gibbon et al. (1977) and others that show the ratio of between- and within-trial time intervals to be an important determinant of the rate of learning. However, it is important to note that the necessity to exclude the data from Group 1–20 of Experiment 2, whose subjects failed to acquire the discrimination within the 100-session maximum, indicates that this predictivity breaks down with small ITI/FTI ratios and long FTIs. Indeed, with the inclusion of the data from Group 1–20, a stepwise regression analysis yielded a model that included ITI and FTI

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as significant variables, r 4 0.54, F(2,73) 4 42.95. Although the best onevariable model was again one that included ITI/FTI ratio, r2 4 0.44, F(1,74) 4 59.31, the best two-variable model was the one that included ITI and FTI. Further investigation of interval effects with smaller ITI/FTI ratios and longer FTIs seems in order. Setting aside for a moment concerns about the contribution of the ITI/FTI ratio to performance, inspection of Fig. 5 shows that for equal ITIs, performance was better with the 10-s FTI than with the 20-s FTI. Thus, as in serial feature positive discrimination learning (e.g., Holland, 1992), the feature–target interval was also an important determinant of discrimination performance. A two-way ANOVA of the data from groups with ITIs used in both experiments, with factors of FTI (10 or 20 s; i.e., Experiment 1 vs. Experiment 2) and ITI (1, 2, 4, and 8 min) showed reliable effects of FTI, F(1,53) 4 132.06, ITI, F(3,53) 4 132.77, and their interaction, F(3,53) 4 28.49. Individual contrasts showed that for each ITI except 8-min, acquisition was more rapid with the shorter FTI, Fs (1,53) $ 4.11. It is worth noting again that the interval effects observed here were attributable to differences in subjects’ withholding responding on nonreinforced trials: we failed to find effects of the ITI on the acquisition of responding on reinforced trials. Holland (1995) made a similar observation in an investigation of the effects of FTI and ITI in the acquisition of serial feature positive discriminations: ITI and ITI/FTI ratio effects on acquisition were noted only in the loss of responding on nonreinforced target-alone trials, and not on responding during reinforced compound trials. Although these failures in both the present and Holland’s (1995) studies might be attributable to the very rapid acquisition of responding in all conditions or to the prior training of another excitor with the same ITI in all groups, it is also worth considering that, within the range of ISI and ITIs examined here, simple conditioning in this operant conditioning preparation is not affected by those factors. It should be recognized that demonstrations of invariance of conditioning with constant ITI/FTI ratios have used almost exclusively the pigeon autoshaping preparation (see also Coleman, Hemmes, & Brown, 1986). Holland (1995) found that larger ITI/FTI ratios or ITIs, in addition to producing more rapid acquisition of feature positive discrimination learning, were more likely to generate patterns of data usually thought to characterize occasion setting rather than simple conditioning. That is, with larger ITI/FTI ratios or ITIs, there was less transfer of performance to a compound of the feature and a separately trained and extinguished cue, and there was more resistance of compound responding to the effects of nonreinforced feature-alone presentations. Thus, temporal relations affected the nature of learning as well as its rate. No such differences were observed in the present experiments: transfer of the features’ inhibitory stimulus control was small in all groups and was unaffected by ITI or ITI/FTI ratio. Regression analyses and ANOVAs performed on the levels of transfer over all conditions in these two experiments revealed no reliable variables or effects. The general lack of transfer observed here, taken in the context

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of the results of previous studies that compared serial and simultaneous feature negative discrimination learning (e.g., Holland, 1991; Holland & Coldwell, 1993), suggests that the rats used occasion setting strategies under all conditions in the present experiments. However, in the absence of any comparison condition to indicate how much transfer would be anticipated with simple inhibitory learning with different ITIs, or another assessment of occasion setting (e.g., resistance to counterconditioning), this conclusion cannot be made with confidence. Nevertheless, the observation of faster acquisition of serial feature negative discriminations with longer ITIs or ITI/FTI ratios is consistent with a casual gestalt notion previously introduced (Holland, 1983) to describe conditions that favor the establishment of occasion setting. Holland (1983, 1985) suggested that occasion setting would be induced in feature negative discrimination learning whenever some perceptual discontinuity between the feature and other elements in the stream of events encouraged parsing those events as feature → [target–no US], rather than say, feature → no US (as in simple inhibitory conditioning) or [feature–target] → no US (as in configural conditioning, e.g., Pearce, 1987). The use of lengthy feature–target intervals would encourage the former parsing and discourage the latter two. Within this perspective, temporal relations between trials might also be expected to play an important role in the acquisition of occasion setting. Although temporal separation of the feature from the [target–(no US)] pair would encourage treating the target–US inhibitory association as a unit, it is nevertheless essential that the feature be linked with that unit at some level. Thus, it is likely that the temporal isolation of feature → [target–(no US)] sequences from other events in the conditioning session by using relatively lengthy intertrial intervals (ITIs) would also encourage the abstraction of the hierarchical relation underlying occasion setting. In contrast, the use of ITIs that are short relative to the feature– target interval might make it difficult to hierarchically group the feature with the [target–(no US)] unit, rather than with a preceding target or US presentation. The present experiments are not informative about possible mechanisms for interval effects nor do they identify which of several intertrial temporal relations contribute to the effects observed: the interval between any two trials, the interval between deliveries of the food USs, the interval between reinforced serial compound trials, the interval between nonreinforced target-alone trials, the interval between trials of different types, and so forth. Elsewhere, many of these intervals have been shown to influence the rate and nature of learning. For example, Schachtman and Reilly (1987) found that inserting a nonreinforced cue between conditioning trials substantially reduced the advantage of longer ITIs for simple conditioning, suggesting that the interval between any two trials may contribute. Similarly, many studies (e.g., Mustaca, Gabelli, Papini, & Balsam, 1991) have indicated that higher rates of US presentation (as occurred in our shorter ITI conditions) encourage greater context conditioning, which might in turn influence the effectiveness of various trial types in generating discrimination learning. Others (e.g., Tiffany, Maude-Griffin, & Dorbes, 1991) have found no influence

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of interpolated, unsignaled US presentations on ITI effects, suggesting that the interval between conditioning trials, rather than between USs, might be critical. And, in an “ambiguous feature” discrimination procedure (X → A+ / A− / X → B− / B+) related to the feature negative discriminations used here, Holland and Gallagher (1996) noted an interaction between ITI and trial sequence that was consistent with the suggestion that longer ITIs enhance discrimination learning by reducing proactive interference from preceding trials (Kehoe, Kool, & Gormezano, 1991). With short (1-min) ITIs, they found substantial decrements in discrimination performance when target-alone trials were preceded by feature → target trials, compared to when they were preceded by target-alone trials. In contrast, with long (8-min) ITIs, trial sequence was irrelevant. Related outcomes in delayed matching-to-sample (Hogan, Edwards, & Zentall, 1981) and nonmatching-to-sample (Pontecorvo, 1983) procedures have been reported. Clearly, further study is needed to uncover mechanisms that may underlie the contribution of ISI and ITI to feature negative discrimination performance. REFERENCES Bowers, R. L., & Richards, R. W. (1990). Pigeons’ short-term memory for temporal and visual stimuli in delayed matching-to-sample. Animal Learning & Behavior, 18, 23–28. Coleman, D. A., Hemmes, N. S., & Brown, B. L. (1986). Relative durations of conditioned stimulus and intertrial interval in conditioned suppression. Journal of Experimental Analysis of Behavior, 46, 51–66. Ewing, M. F., Larew, M. B., & Wagner, A. R. (1985). Distribution-of-trials effects in Pavlovian conditioning: An apparent involvement of inhibitory backward conditioning with short intertrial intervals. Journal of Experimental Psychology: Animal Behavior Processes, 11, 537–547. Gibbon, J., Baldock, M. D., Locurto, C. M., Gold, L., & Terrace, H. S. (1977). Trial and intertrial durations in autoshaping. Journal of Experimental Psychology: Animal Behavior Processes, 3, 264–284. Gormezano, I., & Moore, J. W. (1969). Classical conditioning. In M. Marx (Ed.), Learning processes London: Macmillan, & Co. Grant, D. S. (1975). Proactive interference in pigeon short-term memory. Journal of Experimental Psychology: Animal Behavior Processes, 1, 207–220. Hogan, D. E., Edwards, C. A., & Zentall, T. R. (1981). Delayed matching in the pigeon: Interference produced by the prior delayed matching trial. Animal Learning & Behavior, 9, 395–400. Holland, P. C. (1983). Occasion-setting in Pavlovian feature positive discriminations. In M. L. Commons, R. J. Herrnstein, & A. R. Wagner (Eds.), Quantitative analyses of behavior: Discrimination processes (Vol. 4, pp. 183–206). New York: Ballinger. Holland, P. C. (1985). The nature of conditioned inhibition in serial and simultaneous feature negative discriminations. In R. R. Miller and N. E. Spear (Eds.) Information processing in animals: Conditioned inhibition (pp. 267–297). Hillsdale, NJ: Erlbaum. Holland, P. C. (1989). Transfer of negative occasion setting and conditioned inhibition across conditioned and unconditioned stimuli. Journal of Experimental Psychology: Animal Behavior Processes, 15, 311–328. Holland, P. C. (1991). Acquisition and transfer of occasion setting in operant feature positive and feature negative discriminations. Learning and Motivation, 22, 366–387. Holland, P. C. (1992). Occasion setting in Pavlovian conditioning. In D. Medin (Ed.), The psychology of learning and motivation (Vol. 28, pp. 69–125). San Diego: Academic Press. Holland, P. C. (1995). The effects of intertrial and feature-target intervals on operant serial feature positive discrimination learning. Animal Learning & Behavior, 23, 411–428.

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Holland, P. C., & Coldwell, S. E. (1993). Transfer of inhibitory stimulus control in operant feature negative discriminations. Learning and Motivation, 24, 345–375. Holland, P. C., & Gallagher, M. (1996). Intertrial interval determines the effects of hippocampal lesions on conditional discrimination performance, submitted for publication. Holland, P. C., & Lamarre, J. (1984). Transfer of inhibition after serial and simultaneous feature negative discrimination training. Learning and Motivation, 15, 219–243. Kaplan, P. S. (1984). The importance of relative temporal parameters in trace autoshaping: From excitation to inhibition. Journal of Experimental Psychology: Animal Behavior Processes, 10, 113–126. Kehoe, E. J., Cool, V., & Gormezano, I. (1991). Trace conditioning of the rabbit’s nictitating membrane response as a function of CS-US interstimulus interval and trials per session. Learning and motivation, 22, 269–290. Lamarre, J., & Holland, P. C. (1985). Acquisition and transfer of feature negative discriminations. Bulletin of the Psychonomic Society, 23, 71–74. Lamarre, J., & Holland, P. C. (1987). Acquisition and transfer of serial feature negative discriminations. Learning and Motivation, 18, 319–342. LoLordo, V. M., & Fairless, J. L. (1985). Pavlovian conditioned inhibition: The literature since 1969. In R. R. Miller & N. E. Spear (Eds.), Information processing in animals: Conditioned inhibition (pp. 1–49). Hillsdale, NJ: Erlbaum. Morris, R. G. (1974). Pavlovian conditioned inhibition of fear during shuttlebox avoidance behavior. Learning and Motivation, 5, 424–447. Mustaca, A. E., Gabelli, F., Papini, M. R., & Balsam, P. (1991). The effects of varying the interreinforcement interval on appetitive contextual conditioning. Animal Learning & Behavior, 19, 125–139. Pearce, J. M. (1987). A model for stimulus generalization in Pavlovian conditioning. Psychological Review, 94, 61–75. Pontecorvo, M. J. (1983). Effects of proactive interference on rats’ continuous nonmatching-tosample performance. Animal Learning & Behavior, 11, 356–366. Rescorla, R. A., & Holland, P. C. (1977). Associations in Pavlovian conditioned inhibition. Learning and Motivation, 8, 429–447. Roberts, W. A., & Kraemer, P. J. (1982). Some observations of the effects of intertrial interval and delay on delayed matching to sample in pigeons. Journal of Experimental Psychology: Animal Behavior Processes, 8, 342–353. Schachtman, T. R., & Reilly, S. (1987). The role of local context in autoshaping. Learning and Motivation, 18. Spence, K. W., & Norris, E. B. (1950). Eyelid conditioning as a function of the inter-trial interval. Journal of Experimental Psychology, 40, 716–720. Tiffany, S. T., Maude-Griffin, P. M., & Drobes, D. J. (1991). Effect of interdose interval on the development of associative tolerance to morphine in the rat: A dose–response analysis. Behavioral Neuroscience, 105, 49–61. Weisman, R. G., & Litner, J. S. (1971). Role of intertrial interval in Pavlovian differential conditioning of fear in rats. Journal of Comparative and Physiological Psychology, 74, 211–218. Williams, D. A., & Overmier, J. B. (1988). Backward inhibitory conditioning with signaled and unsignaled unconditioned stimuli: Distribution of trials across days and intertrial interval. Journal of Experimental Psychology: Animal Behavior Processes, 14, 26–35. Received January 26, 1995 Revised April 28, 1995