The Effects of Feature Identity in Operant Serial Feature-Negative Discriminations

LEARNING AND MOTIVATION ARTICLE NO.

28, 577–608 (1997)

LM970978

The Effects of Feature Identity in Operant Serial Feature-Negative Discriminations Murray J. Goddard University of New Brunswick, Saint John, New Brunswick, Canada

and Peter C. Holland Duke University, Durham, North Carolina The effects of feature identity in an operant serial feature-negative discrimination (F1 c T10, T1/) were examined in two experiments with rats. In Experiment 1, rats were trained with two operant serial feature-negative discriminations in which different operants were reinforced during two auditory target cues (T1 and T2). The features (F1 and F2) were two neutral cues (visual or auditory stimuli), two motivationally significant cues (flavored sucrose solutions, also used as the operant reinforcers), or one neutral and one motivationally significant cue. Experiment 1 showed that discrimination acquisition, transfer performance, and feature–target interval testing were facilitated with a flavored sucrose feature. Experiment 2 showed that flavored sucrose-alone presentations, more than flavored sucrose trained in a pseudodiscrimination (F1 c T1/, T1/), shared several similarities with a standard flavored sucrose feature. The results suggest flavored sucrose rapidly acquires inhibitory properties, which facilitates operant serial feature-negative discrimination performance. q 1997 Academic Press

In occasion-setting, an organism must learn that a particular target conditioned stimulus (CS) will, or will not, normally be followed by an unconditioned stimulus (US) when preceded by a feature cue. For example, in positive occasion-setting, subjects receive F c T/, T0 trials and must

This research was supported in part by a grant from the National Science Foundation to P.C.H. and by a Natural Sciences and Engineering Research Council of Canada grant to M.J.G. Requests for reprints should be sent to M.J. Goddard, Psychology Department, University of New Brunswick, Saint John, New Brunswick, Canada, E2L 4L5, or P.C. Holland, Psychology Department, Duke University, Durham, NC, 27708-0086 (E-mail: [email protected] or pch@ acpub.duke.edu). 577 0023-9690/97 $25.00 Copyright q 1997 by Academic Press All rights of reproduction in any form reserved.

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learn that a target cue is only reinforced when preceded by a particular feature cue. In negative occasion-setting, subjects receive F c T0, T/ trials and must learn that a target cue is only reinforced when not preceded by a particular feature cue. The characteristics of positive and negative occasion-setting have been investigated extensively (for reviews see Holland, 1992; Swartzentruber, 1995). In almost all of these studies, initially neutral stimuli were used as the feature cues. Less attention, however, has been paid to the characteristics of positive and negative occasion-setting when biologically significant events are used as features (but see Bottjer & Hearst, 1979; Holland & Forbes, 1982; Reberg & Memmott, 1979); further, there has been little comparison of occasion-setting using relatively neutral, versus motivationally significant, cues. Recently, however, Goddard and Holland (1996) explicitly compared positive occasion-setting with visual, CS, and sucrose, US, features. (Although the visual cue was trained as an occasion-setter, rather than as a CS, and sucrose itself can serve as a CS in conditioning experiments, for example, in taste aversion learning, the terms CS and US were used to simplify discussion.) In Goddard and Holland’s study (1996, Exp. 2), each subject received two operant serial feature-positive discriminations (F1 c T1/, T10 and F2 c T2/, T20) in which one response (R1) was reinforced on F1 c T1/ trials and a different response (R2) was reinforced on F2 c T2/ trials. In separate groups of subjects, F1 and F2 were two US features, two CS features, or one US and one CS feature. Results showed that discrimination learning proceeded at roughly the same rate in all conditions and that, in many respects, operant serial feature-positive discrimination performance with US features resembled that with CS features. For example, a US feature’s occasion-setting ability transferred to a target cue that had been trained with another US feature. Similarly, extinction of a US feature did not importantly reduce original occasion-setting performance or transfer performance to a target cue that had been trained with another US feature. However, occasion-setting transfer was poor when a US feature was combined with a target cue that had been originally trained with a CS feature; similarly, occasion-setting transfer was also poor when a CS feature was presented with a target cue that had been originally trained with a US feature. Further, the inclusion of pseudodiscrimination control groups showed that greater transfer of occasionsetting with same modality features was not due solely to feature generalization. Goddard and Holland’s (1996) finding that US and CS features failed to exhibit mutual transfer, despite robust transfer within each modality, provided an important constraint for theories designed to explain the frequent observation of greater occasion-setting transfer to targets of other occasionsetters than to targets with other training histories (Honey & Hall, 1989; Lamarre & Holland, 1987; Schmajuk, Lamoureux, & Holland, 1994). The experiments reported here extended Goddard and Holland’s work

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(1996) to compare negative, rather than positive, occasion-setting using US and CS features. In the present study, the apparatus, the feature and target cues, and the original training and testing parameters were kept as similar as possible to those used by Goddard and Holland (1996), with the exception that an operant serial feature-negative, rather than positive, discrimination was studied. Two important questions were addressed in the present study. First, would operant serial feature-negative learning proceed at roughly the same rate with US and CS features, as it did in operant serial feature-positive learning and, second, would occasion-setting transfer within, but not between, modalities, as was the case in operant serial feature-positive learning? EXPERIMENT 1

In Experiment 1, all subjects received two operant serial feature-negative discriminations (F1 c T10, T1/ and F2 c T20, T2/) in which one response (R1) was reinforced with one flavored liquid (Rf1) on T1/ trials and a different response (R2) was reinforced with another flavored liquid (Rf2) on T2/ trials. In separate groups of subjects, F1 and F2 were two US features (Group US–US), two CS features (Group CS–CS), or one US and one CS feature (Group US–CS). The US features, F1 and F2, were identical to the two reinforcers, Rf1 and Rf2. In addition to examining original discrimination performance with US and CS features, Experiment 1 compared within- and between-modality transfer of occasion-setting. Furthermore, Experiment 1 examined the ability of a US feature to inhibit performance of a new response (R3), which was separately trained to a new target cue (T3). Finally, Experiment 1 explored the ability of a US or CS feature to inhibit responding to its original target when the feature–target interval was systematically manipulated. Holland, Hamlin, and Parsons (1997) found that rats represented the feature–target interval in serial feature positive learning: performance was degraded when the test interval was either longer or shorter than the interval used in training. The feature– target interval test in Experiment 1 examined the temporal specificity of serial feature negative learning. Method Subjects. The subjects were 24 female Sprague-Dawley rats obtained from Charles River, Inc. (Raleigh, NC). They were about 100 days old at the beginning of the experiment and had not been involved in previous research. The rats were maintained at about 85% of their ad libitum body weights by measured feedings at the end of a session. Water was available at all times in their individual home cages. Apparatus. There were four identical chambers, each 22.9 1 20.3 1 20.3 cm. The front and back walls of each chamber were aluminum; the side walls and top were clear acrylic. The floor of each chamber was composed of 0.48cm stainless-steel rods spaced 1.9 cm apart. A food cup centered on the front

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wall was recessed behind a 5 1 5-cm opening and the bottom of the food cup opening was 2 cm from the floor. There were two plastic dispensers behind the front wall; each dispenser held one of two flavored sucrose solutions. When a solenoid valve was activated, there was an audible click and approximately 0.3 ml of 0.2 M flavored sucrose solution was delivered into the food cup. A 6-W, 110-VAC jeweled lamp (panel light) was centered on the front wall 4 cm above the top of the food cup opening. A 2 1 2-cm lever was mounted 3 cm above the floor, 4 cm to the left of the food cup opening. A chain was suspended from the ceiling, in line with the center of the lever and 5 cm from the front wall. Finally, a paddle was positioned behind a 3cm-diameter circular opening, 2.5 cm from the right edge of the front wall and 9 cm from the floor. Each of the chambers was enclosed in a sound-attenuating shell. A 6-W 110-VAC lamp was mounted on the door of each shell (door light), just above the center of one acrylic side wall of the chamber. Constant dim background illumination was provided by a third 6-W, 110-VAC lamp, mounted on the shell wall opposite the door light. This lamp was operated at 70 VAC, and was covered by a red lens assembly. A TV camera was mounted outside of each shell so that its lens protruded into the shell just below the red lens assembly. Video recorders were programmed to record the rats’ behavior 10 s before and during trials in the final phase of the study. A speaker for delivering the auditory cues was mounted next to the red lens assembly, level with the top of the chamber and 2 cm in front of and 10 cm to the left of the front wall. Constant background noise (72 dB) was provided by a ventilating fan on each box. Procedure. The rats were first familiarized with the two KOOL-AID solutions, which served as the features and reinforcers. The solutions were made by dissolving 50 g sucrose and 1 g unsweetened KOOL-AID in 1 liter of tap water. On the first day, approximately 5 ml of grape KOOL-AID was placed in each magazine and subjects were simply left in the chambers for 30 min with the levers and chains removed from the boxes (the paddle was physically present but remained immoveable in Experiment 1, except when otherwise specified). The same procedure was used on the second day with orange KOOL-AID. Magazine training was conducted over the next 2 days. On the first day, subjects received twenty 0.3-ml deliveries of orange KOOL-AID on a variable-time 2-min schedule; the same procedure was used on the second day with grape KOOL-AID. The levers and chains continued to be absent from the boxes. The subjects were then trained to press the lever and pull the chain. On the first 3 days, each lever-press or chain pull (counterbalanced) was reinforced with grape KOOL-AID; shaping was used if necessary. Only one response manipulandum was available and subjects were removed from the chambers after making approximately 50 responses. The same procedure was used on the next 2 days except the other response (chain pulling or lever-

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pressing, counterbalanced) was reinforced with orange KOOL-AID. Note that, for an individual subject, lever-pressing was reinforced with one solution (grape or orange, counterbalanced) while chain pulling was reinforced with the other solution (orange or grape, counterbalanced). The next 13 sessions placed lever-pressing and chain pulling under the control of the auditory targets to be used later in operant serial featurenegative training. For half of the rats, each lever press was reinforced during the tone and each chain pull was reinforced during the noise. For the other half of the rats, those contingencies were reversed. In all sessions, tone and noise presentations were randomly intermixed. The response (lever pressing or chain pulling), target (tone or noise) and reinforcer (grape or orange) were completely counterbalanced. The first two sessions were 30 min long and contained thirty 30-s tone trials with only the manipulandum appropriate for tone responding available. The same procedure was used in the next two sessions except the noise was used and only the manipulandum appropriate for noise responding was available. In the fifth and sixth session, both manipulanda were available and subjects received fifteen 30-s tone trials and fifteen 30-s noise trials in a 30-min session. The cues were then shortened to 15 s (sessions 7 and 8) and the session length extended to 40 min (session 9), 50 min (session 10), and 60 min (session 11). In session 12, subjects received ten 10-s tone, trials and ten 10-s noise trials in a 60-min session; finally, in session 13, subjects received eight 10-s tone trials and eight 10-s noise trials in a 64-min session. Subjects were then assigned to groups (n Å 8), maintaining complete counterbalancing of response, target, and reinforcer, and matching response levels as closely as possible. Table 1 presents an outline of these and all subsequent procedures of Experiment 1. Operant serial feature-negative training was conducted next. Subjects in Groups US – US, CS – CS, and US – CS received training on two operant serial feature-negative discriminations. In Group US – US, the features were grape or orange KOOL-AID; in Group CS – CS, the features were a panel light and a clicker; and, in Group US – CS, the US feature was either grape or orange KOOL-AID and the CS feature was either a panel light or a clicker. In each 64-min session, all subjects received four T1/ trials (each R1 response during T1 was reinforced), four F1 r T10 trials (responding was not reinforced), four T2 / trials (each R2 response during T2 was reinforced), and four F2 r T20 trials (responding was not reinforced). All cues were 10-s in duration (except for US delivery, which was 0.5 s) and the interval between feature onset and target onset was 30 s in both serial compounds. The identities of the feature cues were included in the counterbalancing as much as possible, with two exceptions. First, with a US feature, the feature and the reinforcer associated with its target were identical. Second, in Group US – CS, only 8 of the 16 possible combinations were represented. Because the operant serial feature-negative discrimination was dramatically more rapid with a US, in comparison to a

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US1 r T1, T1R1/ US2 r T2, T2R2/ CS1 r T1, T1R1/ CS2 r T2, T2R2/ US1 r T1, T1R1/ CS2 r T2, T2R2/

US1 r CS1, US2 r CS1, CS1 US2 r CS2, US1 r CS2, CS2 — — US1 r CS1, CS2 r CS1, CS1

CS1R3/ CS2R3/ — — CS1R3/

— — CS1 r T1, T1R1/ CS2 r T2, T2R2/ US1 r T1, T1R1/ CS2 r T2, T2R2/

Discrimination 40 ses

Discrimination 2 ses

US1 r T1, US2 r T1, T1, US1 US2 r T2, US1 r T2, T2, US2 CS1 r T1, CS2 r T1, T1, CS1 CS2 r T2, CS1 r T2, T2, CS2 US1 r T1, CS2 r T1, T1, US1 CS2 r T2, US1 r T2, T2, CS2

Transfer test 1 2 ses

Transfer test 3 1 ses

US1 r T1, T1R1/ US2 r T2, T2R2/ CS1 r T1, T1R1/ CS2 r T2, T2R2/ US1 r T1, T1R1/ CS2 r T2, T2R2/

Discrimination 10 ses

Train R3 7 ses

T1R1/ T2R2/ T1R1/ T2R2/ T1R1/ T2R2/

Training 13 ses

US1 r x r T1, US1 r T1, T1R1/ US2 r x r T2, US2 r T2, T2R2/ CS1 r x r T1, CS1 r T1, T1R1/ CS2 r x r T2, CS2 r T2, T2R2/ US1 r x r T1, US1 r T1, T1R1/ CS2 r x r T2, CS2 r T1, T2R2/

Interval tests 12 ses

— — CS1 r T1, CS2 r T1, T1, CS1 CS2 r T2, CS1 r T2, T2, CS2 US1 r T1, CS2 r T1, T1, US1 CS2 r T2, US1 r T2, T2, CS2

Transfer test 2 2 ses

Note. CS1/2, panel light and clicker, counterbalanced; US1/2, grape and orange KOOL-AID features, counterbalanced; T1/T2, tone and noise target stimuli, counterbalanced; R1/R2, reinforced lever-press and chain pull operant responses, counterbalanced; R3, paddle push operant response; /, availability of KOOL-AID reinforcer with an appropriate operant response; r, serial relation with trace interval; ses, session(s); x, variable test trace interval (see text).

US–CS

CS–CS

US–US

Group

US–CS

CS–CS

US–US

Group

TABLE 1 Outline of Procedures in Experiment 1

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CS feature, Group US – US received 10 training sessions whereas Groups CS – CS and US – CS required 50 training sessions. Following 10 sessions of operant serial feature-negative training in all groups (Test 1), and again after 50 sessions in Groups CS–CS and US–CS (Test 2), the rats received two sessions to evaluate feature transfer. In each 64-min test session, there were two presentations of each of eight trial types. The eight trial types included the two features alone (F1 and F2), the two targets alone (T1 and T2), the two original serial compounds (F1 r T1 and F2 r T2), and, of most interest, the two novel transfer compounds (F1 r T2 and F2 r T1). Responding was not reinforced during the test sessions. Following Test 2, the rats in Groups US – US and US – CS were trained to make a new response (paddle-push) in the presence of a panel light and/or a clicker. During training, the paddle was made operative and the levers and chains were removed from the boxes. In the first 30-min session, the rats in Group US – US received fifteen 30-s panel light and fifteen 30-s clicker presentations. During the panel light, each paddle-push was reinforced with one KOOL-AID solution (grape or orange, counterbalanced) and, during the clicker, each paddle-push was reinforced with the other KOOL-AID solution (orange or grape, counterbalanced). The cues were then shortened to 15 s (session 2), and the session length extended to 40 min (session 3), 50 min (session 4), and 60 min (session 5). In session 6, subjects received ten 10-s panel light and ten 10-s clicker presentations in a 64-min session; session 7 was similar to session 6 except that 8 (rather than 10) presentations of each cue were given. The rats in Group US – CS received similar training, except that only the cue that had not been used previously as a feature (clicker or panel light) was trained. Consequently, in Group US – CS, each session was one-half as long and included half as many total CS presentations as in Group US – US. The rats in Groups US–US and US–CS then received one test session to evaluate transfer to the separately trained cues. In Group US–US, there were two presentations of each of six trial types in a 64-min test session. The six trial types included the two targets alone (panel light and clicker), the two US r target compounds in which the US feature and the target’s reinforcer were identical, and the two US r target compounds in which the US feature and the target’s reinforcer were different. In Group US–CS, the test session was 32 min long, and included only three trial types, panel light or clicker target alone, US1 r target, and US2 r target. Paddle-pushing was not reinforced in the test session and the levers and chains continued to be absent from the boxes. Next, all subjects received two retraining sessions on the original operant serial feature-negative discrimination. For the rats in Group CS–CS, retraining followed Test 1; for the rats in Groups US–US and US–CS, retraining followed paddle-push training and testing. In retraining, the levers and chains were present in the chambers but the paddle was again immoveable. Finally,

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the features’ ability to inhibit responding at different feature–target intervals was tested. In each of twelve 64-min sessions, the rats received four T1/ trials (each R1 response during T1 was reinforced), two F1 r T10 trials (responding was not reinforced), four T2/ trials (each R2 response during T2 was reinforced), and two F2 r T20 trials (responding was not reinforced). Of most interest, however, were two additional F1 r T10 test trials and two F2 r T20 test trials in which the interval between feature onset and target onset was systematically varied across sessions; this interval was 4 min (sessions 1 and 12), 2 min (sessions 2 and 11), 1 min (sessions 3 and 10), 45 s (sessions 4 and 9), 30 s (sessions 5 and 8), or 15 s (sessions 6 and 7). Note that at 4 min the interval between feature onset and target onset was equivalent to the original ITI and that at 30 s, the interval between feature onset and target onset matched that during original training. Data analysis. We recorded the percentage of trials on which at least one response occurred during a cue, response rates during that cue, and the latency to the first response to that cue; in addition, we recorded those same measures in a 10-s interval before the cue. Response rate was the primary index of performance in the test sessions when reinforcement was unavailable. However, response rate was inappropriate when reinforcement was available. Typically, rats ceased responding immediately after a reinforced response and collected the available KOOL-AID. Therefore, the percentage of trials on which at least one response occurred (acquisition), or the latency to the first response (interval testing), was used as the dependent variable when reinforcement was available. Finally, the behavior of the rats was recorded on video tapes during the final interval test phase. The behavior of the rats in Group US–CS was scored from the video tapes, using the methods described by Holland (1977). Paced by auditory signals recorded on the tapes, the behavior of each rat was evaluated every 1.25 s during the 10 s prior to and the 120 s after the delivery of each feature. The behaviors reported here were ‘‘magazine behavior,’’ scored whenever a rat’s head or nose was within the recessed food cup, and ‘‘rear,’’ scored whenever a rat was standing with both front feet off the floor, but not grooming. The number of observations on which that behavior was scored was divided by the total number of observations made to obtain the measure ‘‘% total behavior.’’ Nonparametric inferential statistics were used throughout; a criterion level of statistical significance of p õ .05, two tailed, was adopted. Results During initial training, the rats learned to perform one response during one target cue and a different response during the other target cue. By the final session, across all groups, the rats made at least one correct response on 92% of trials and at least one incorrect response on 20% of trials. The choice of

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response (lever-pressing or chain pulling), target (tone or noise), or reinforcer (grape or orange) did not affect initial training. Discrimination training. Figure 1 shows the percentage of trials with a correct target-appropriate response (i.e., R1 during T1 and R2 during T2) during the first 10 sessions of discrimination training (the data in session 5 were lost due to a recording failure). Discrimination training was clearly more rapid with a US feature than with a CS feature. For example, in session 10, discrimination difference scores (target-appropriate responding on target trials minus that on feature r target trials) were significantly higher in Group US– US than Group CS–CS [U(8,8) Å 0]. In Group US–CS, discrimination difference scores were significantly higher with the US feature than with the CS feature [T(8) Å 2]. By the end of this phase, inappropriate responses (R2 during T1 and R1 during T2) occurred on fewer than 6% of the trials. Because comparable low levels of inappropriate responding were maintained throughout the remainder of the experiment, that responding is not discussed further. Group US–US showed some differential responding on target and UStarget trials in the first session, suggesting some performance component may have been involved prior to discrimination learning (for example, subjects may still have been drinking when the target was presented on US r target trials). To check this possibility, the trial by trial data from the first session of discrimination training were examined. Results are shown in Fig. 2, collapsed across feature (grape or orange) and target (tone or noise) identity in Groups US–US and CS–CS. On the first trial, subjects in Group US–US responded on 88% of target trials, and 81% of US r target trials. Differential responding, however, did develop in the first session; averaged over the first four trials, subjects responded significantly more on target, compared to US r target, trials [T(6) Å 0]. However, there was no evidence of differential responding in the first session for the US [T(2) Å 3] or CS [T(3) Å 3] feature in Group US–CS, or the CS features in Group CS–CS [T(7) Å 11]. Test 1. Figure 3 shows the results of Test 1, administered after 10 sessions of discrimination training. Performance on the original discriminations established with US features was maintained in Test 1, but, as in training, discrimination performance was not evident with CS features. The rats in Group US– US showed significantly more R1 responding on T1-alone trials than on US1 r T1 trials, and significantly more R2 responding on T2-alone trials than on US2 r T2 trials [all T’s(8) Å 0]. The rats in Group CS–CS did not show these differences [all T’s ú 8]. The rats in Group US–CS showed significantly more R1 responding on T1-alone trials than on US r T1 trials [T(8) Å 0], but did not show more R2 responding on T2-alone trials than on CS r T2 trials [T(6) Å 9]. Also of interest was responding during the novel transfer compounds. The rats in Group US–US showed reliable transfer, emitting significantly more R1 responding on T1-alone trials than on US2 r T1 trials, and significantly more R2 responding on T2-alone trials than on US1 r T2 trials [all T’s(8)

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FIG. 1. Percentage of trials on which an appropriate response occurred during the target cues in the first 10 sessions of operant serial feature-negative discrimination training in Experiment 1. Group US–US (top) received two US features, Group US–CS (center) received one US and one CS feature, and Group CS–CS (bottom) received two CS features. Due to a recording failure, the data from session 5 were lost.

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FIG. 2. Percentage of trials on which an appropriate response occurred during the target cues in the first session of operant serial feature-negative discrimination training in Experiment 1. Group US–US (top) received two US features, Group US–CS (center) received one US and one CS feature, and Group CS–CS (bottom) received two CS features.

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FIG. 3. Mean response rates in Test 1, which followed 10 sessions of operant serial featurenegative discrimination training in Experiment 1. The abscissae labels identify the features (first line) and the targets (second line) for each type of test trial. In training, Group US–US (top) received two US features, Group US–CS (center) received one US and one CS feature, and Group CS–CS (bottom) received two CS features.

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Å 0]. In contrast, the rats in Group CS–CS showed no evidence of transfer; there were no significant differences in R1 responding on T1-alone trials, compared to CS2-T1 trials, or in R2-responding on CS1-T2 trials, relative to T2-alone trials [Ts(7) Å 7]. This lack of transfer in Group CS–CS is not surprising given the absence of reliable performance on the original discrimination. Finally, the rats in Group US–CS showed transfer with the US feature, but not with the CS feature. The rats in Group US–CS emitted significantly more R2 responding on T2-alone trials than on US r T2 trials [T(8) Å 0], but showed a nonsignificant trend toward less R1 responding on T1-alone trials than on CS r T1 trials [T(7) Å 4.5]. Again, as in Group CS–CS, the failure to observe transfer with the CS feature is not surprising, given the lack of reliable performance on the original discrimination with that feature. After Test 1, discrimination training was continued for a total of 50 sessions in Groups US–CS and CS–CS. Figure 4 shows the percentage of trials with a correct target-appropriate response over five-session blocks. By the end of training, all of the discriminations were learned. In the last five-session block, rats in Group US–CS showed significantly more R1 responding on T1-alone trials than on US r T1 trials [T(8) Å 0] and significantly more R2 responding on T2-alone trials than on CS r T2 trials [T(8) Å 3]. The rats in Group CS– CS showed significantly more R1 responding on T1-alone trials than on CS1 r T1 trials [T(7) Å 1] and showed significantly more R2 responding on T2-alone trials than on CS2 r T2 trials [T(7) Å 0]. Also of interest was that discrimination difference scores with CS features were not different in Groups US–CS and CS–CS [U’s(8,8) § 24]. Test 2. Figure 5 shows the results of Test 2, which followed the 50 discrimination training sessions in Groups US–CS and CS–CS. Performance on the original discriminations was maintained in all cases. The rats in Group US–CS showed significantly more R1 responding on T1-alone trials than on US r T1 trials [T(8) Å 0] and significantly more R2 responding on T2-alone trials than on CS r T2 trials [T(7) Å 0]. The rats in Group CS–CS showed significantly more R1 responding on T1-alone trials than on CS1 r T1 trials [T(7) Å 3] and more R2 responding on T2-alone trials than on CS2 r T2 trials [T(5) Å 1]. Transfer to the novel compounds was observed with US features, but not with CS features. The rats in Group US–CS showed significantly more R2 responding on T2-alone than on US r T2 trials [T(8) Å 0] but showed no more R1 responding on T1-alone trials than on CS r T1 trials. The rats in Group CS–CS showed no significant differences in R1 responding between T1-alone and CS2 r T1 trials [T(7) Å 5.5], or significant differences in R2 responding between T2-alone and CS1 r T2 trials [T(8) Å 10]. Although occasion-setting transfer is typically shown to targets of other occasion-setters, the generally poor discrimination performance likely hindered the ability to show occasion-setting transfer in Group CS–CS. Paddle-push training and testing. Following Test 2, the rats in Group US– US and US–CS were trained to make a new response (paddle-push), in the

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FIG. 4. Percentage of trials on which an appropriate response occurred during the target cues in the 50 sessions of operant serial feature-negative discrimination training for Groups US– CS (top) and CS–CS (bottom) in Experiment 1. Group US–CS received one US and one CS feature and Group CS–CS received two CS features.

presence of a panel light and/or a clicker, and the ability of a US, or CS, feature to inhibit paddle-push responding was then examined. Paddle-push training proceeded rapidly; in the last session, the rats in Group US–US responded on 97% of panel light trials and 98% of clicker trials; in Group US–CS, both the rats trained with the clicker and those trained with the panel light responded on 100% of the trials. The ability of the US or CS features to inhibit paddle-push responding was then examined in a single test session. The US features modulated performance to a separately trained cue but the CS feature did not. The rats showed significantly more responding to the target presented alone than to the US r target compounds in both Group US – US [M, 23.8 vs

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FIG. 5. Mean response rates in Test 2 which followed 50 sessions of operant serial featurenegative discrimination training in Groups US–CS (top) and CS–CS (bottom) in Experiment 1. The abscissae labels identify the features (first line) and the targets (second line) for each type of test trial. Group US–CS received one US and one CS feature and Group CS–CS received two CS features in training.

5.5 responses/min, T(8) Å 0], and Group US – CS [M, 20.9 vs 5.5 responses/min, T(8) Å 0]. However, the rats in Group US – CS did not show significantly more responding on target-alone trials than on CS r target trials [M, 20.9 vs 21.0 responses/min, T(8) Å 21]. Feature–target interval tests. Figure 6 shows response latencies during the feature r target interval tests, combined over the two sessions with each test interval. The identity of the feature (grape or orange), target (tone or noise), or position of the test interval session (first half or second half of the 12session sequence) did not affect responding, so the data for all statistical tests were collapsed across those factors as well. The original discriminations were maintained throughout the test procedure: the response latency on reinforced target-alone (T/) trials was significantly shorter than the response latency on the original, 30-s interval feature r target- trials for each pair of test

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FIG. 6. Response latencies during the feature-target interval tests in Experiment 1. Group US–US (top) received two US features, Group US–CS (middle) received one US and one CS feature, and Group CS–CS (bottom) received two CS features in training.

sessions in each group [T’s (8) õ 5], except for the 15-s test interval sessions in Group CS–CS [T(7) Å 6.5]. Responding on test trials varied with the feature–target interval with both CS and US features: the longer the feature-target interval, the less effective the feature

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was in suppressing responding to the target. The US feature remained effective at surprisingly long feature–target intervals. In Group US–US, the response latencies on test trials were significantly longer than those on T/ trials with 120-, 60-, 45-, 30-, and 15-s test intervals [T’s(8) õ 4]. At the same time, test trials with 120-, 60-, and 45-s intervals generated response latencies that were significantly shorter than the latencies on the original 30-s interval US r T0 trials [T’s(8) õ 4]. The response latencies on test trials with 30-s test intervals did not differ from those on the US r T0 trials, and the latencies on test trials with the 15-s interval were longer than those on US r T0 trials [T(7) Å 1]. Finally, with the longest test interval, 240 s, the response latencies were shorter on test trials than on both T/ and US r T0 trials [T’s(8) Å 0]. In Group US–CS, the response latencies on test trials with the US feature were significantly longer than the response latencies on the comparable targetalone trials (T1/) with 60-, 45-, 30-, and 15-s intervals. The US feature was less effective (shorter response latencies) on test trials with the 45-, 60-, 120-, and 240-s test intervals than it was on US r T10 trials [all T’s (8) õ 2.5], equally effective on test trials with 30-s test intervals [T(7) Å 19], and more effective (longer latencies) on test trials with 15-s test intervals [T(8) Å 3]. The CS feature was not as effective as the US feature in Group US–CS. The response latencies on test trials with the CS feature were significantly longer than the response latencies on T2/ trials only with the 15-, 30-, and 45-s test intervals (T’s õ 5.) The response latencies on test trials with 60and 240-s intervals were reliably shorter than those on CS r T2- trials (T’s õ 4). Similarly, the response latencies on the original, 30-s interval feature r target trials were always higher for the US feature than for the CS feature (T’s Å 0). Likewise, the response latencies on test trials with a US feature were higher than those on test trials with a CS feature at the 15-, 30-, and 45-s test intervals (T’s õ 3). These findings provide further evidence that in Group US–CS, the rats showed better discrimination performance with the US feature than with the CS feature, despite having received 50 sessions of operant serial feature-negative discrimination training with both features. The CS features were also minimally effective in Group CS–CS: the response latencies on test trials were longer than those on T/ trials only with the 15-s and 30-s test intervals [T’s(8) Å 2]. Test trial latencies were significantly higher than those on CS r T0 trials with the 15-s test interval [T(8) Å 4], and significantly lower than those on CS r T0 trials with 60-, 120-, and 240-s intervals. Figure 7 shows food cup behaviors during and after the delivery of CS and US features in Group US–CS on the 120-s test trials of Test 3, as well as the pre-CS, baseline levels of food cup entry. Not surprisingly, during the first 20 s after presentation of the US feature, food cup behavior constituted a greater proportion of the rats’ behavior than during the pre-CS periods [T’s(8) Å 0]. However, in the sixth through eighth 5-s post-US intervals,

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FIG. 7. Frequency of food cup behavior in Group US–CS during the 120-s interval test sessions in Experiment 1. The line parallel to the abscissa indicates the mean response levels that occurred during all pretrial intervals. Percentage total behavior was computed by dividing the number of observations on which a food cup response was scored by the total number of observations made during that interval.

food cup behavior was suppressed relative to pre-CS periods [T’s(7) õ 4]. This suppression of food cup behavior soon after the delivery of the US feature is consistent with data reported by Goddard (1995). In contrast, food cup behavior was suppressed during each of the first three 5-s intervals after the onset of the 10-s CS feature [T’s(8) õ 4]. This suppression may reflect active withdrawal from the site of food delivery (e.g., Farwell & Ayres, 1979) or competition from orienting behavior generated by the visual feature: During the first of these intervals, rear behavior (which occurs as an orienting response to visual cues, Holland, 1977) was elevated to a level of 24% of the rats’ behavior, compared to 6% during the pre-CS intervals [T(7) Å 2]. Discussion In Experiment 1, the effects of feature identity in operant serial featurenegative discriminations were examined. The results differed considerably from the results of Goddard and Holland’s (1996) analogous studies of feature positive discrimination learning. First, whereas Goddard and Holland (1996) found that the acquisition of operant serial feature-positive discrimination performance was approximately equivalent with US and CS features, in the present study, operant serial feature-negative discrimination performance was significantly superior with US features. Second, whereas Goddard and Holland (1996) showed occasion-setting transfer within, but not between, modalities in operant serial feature-positive discriminations, the present study showed robust occasion-setting transfer with US features, even to a separately trained

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cue, but no transfer with CS features in operant serial feature-negative discriminations. In addition, the present study showed that a feature’s ability to inhibit target responding was a function of the feature–target interval at the time of performance. That is, a feature’s inhibitory power was generally impaired as the feature–target interval was systematically lengthened, and enhanced when that interval was shortened. On the surface, the results obtained with the CS feature generally corresponded to previous occasion-setting research using a CS feature. First, serial feature-negative discrimination learning is often slow with a CS feature and does not typically show transfer to a separately trained cue (e.g., Holland & Lamarre, 1984; Lamarre & Holland, 1987). Second, the lack of transfer to a target cue that had been trained with a US feature could be considered analogous to Goddard and Holland’s (1996) observation of no transfer across CS and US features in operant serial feature-positive discriminations. However, these similarities are hard to interpret because, unlike in previous research (e.g., Holland & Coldwell, 1993), there was no transfer across targets that were both trained with CS features. It is likely that the lack of transfer was the consequence of the low level of original discrimination performance. What was especially noteworthy in Experiment 1, however, were the results obtained with the US feature. Operant serial feature-negative discrimination performance was remarkably rapid with a US feature: Subjects showed evidence of discrimination learning in the first session of training. Also, in contrast to Goddard and Holland’s (1996) findings, the US feature inhibited responding to a transfer target originally trained with a CS feature, and, in contrast to typical occasion-setting results, the US feature inhibited responding to a separately trained cue. It is difficult to argue that the substantive differences in results between the US feature in the present study and the US feature in Goddard and Holland’s (1996) study arose from differences in the apparatus, the feature or target cues, or the training and testing parameters, since explicit efforts were made to keep these factors as constant as possible. Nor does the one significant difference between their procedures and the present ones, the amount of training, seem critical. The performances of Groups US–US and US–CS in the present study were similar with the US feature, and the former group received less training than Goddard and Holland’s (1996) rats, and the latter group received more training. It would appear, therefore, that in Experiment 1 the US feature showed unique inhibitory properties: very rapid acquisition and broad transfer. An issue of immediate importance then is what factors were responsible for the development of these unique inhibitory properties. The simplest account, that with US features the rats simply remained in the food cup, interfering with the display of operant responding, is shown to be false by the food cup observational data. Another possibility is suggested by a recent study by

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Goddard (1995). In Goddard’s (1995) experiments, rats received temporally separated USs in an appetitive conditioning preparation. With temporally separated USs, subjects showed a significant decline in magazine entries following a US, and conditioning to a CS was significantly more rapid, in comparison to appropriate control groups, when subjects received a CS–US pairing immediately following a US. These results suggested that post-US inhibition develops (or subjects learn a ‘‘US–no US association’’) when subjects receive USs that are temporally separated. In Experiment 1 of the present study, subjects also received US features that were temporally separated and any post-US inhibition may have significantly facilitated operant serial feature-negative discrimination performance. As further support for this possibility, subjects in Holland and Forbes (1982) and Reberg and Memmott (1979) also received US features that were temporally separated and showed more rapid feature-negative, rather than feature-positive, performance with a US feature (but see Bottjer & Hearst, 1979). However, even if the above analysis is correct, it is still unclear why Goddard and Holland (1996) did not find impaired operant serial featurepositive discrimination performance with a US, in comparison to a CS, feature. Because the rats in Goddard and Holland’s (1996) experiments also received US features that were temporally separated, any post-US inhibition would presumably have impaired feature-positive discrimination performance with a US feature. Of course, it is possible that the greater salience of US cues promoted more rapid learning than observed with CS features, which compensated for the inhibitory effects of the US feature. EXPERIMENT 2

Experiment 2 further explored the inhibitory properties of US features in operant serial feature-negative discriminations. In Experiment 2, a US feature in a standard operant serial feature-negative discrimination (US1 r T10, T1/) was either trained in conjunction with a US-alone control cue and a separately trained stimulus (US20, T2/) or in conjunction with a US used as a ‘‘pseudofeature’’ in a pseudodiscrimination (US2 c T2/, T2/). Of interest was whether a US-alone cue or a pseudofeature would be as effective as a true US feature in modulating performance to another target, or separately trained, cue. Presumably, if a US feature in an operant serial feature-negative discrimination acquires inhibition because USs are temporally separated, a US-alone cue would also acquire such inhibition, but a US pseudofeature would not, because it is followed shortly by another US. In Experiment 2, two groups of rats received an operant serial featurenegative discrimination (US1 c T10, T1/) in which one response (R1) was reinforced on T1/ trials. In Group USa, the rats also received US2-alone trials on which no response-contingent reinforcement was available, and T2/ trials, on which a second response (R2) was reinforced. In Group USpd, in addition to the serial feature negative discrimination, the rats received

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pseudodiscrimination trials (US2 c T2/, T2/) in which R2 was reinforced during T2 on both US2 c T2/ and T2/ trials,. As in Group US–US in Experiment 1, the features were two flavored sucrose solutions. After training, all subjects received US1 c T2 and US2 c T1 transfer tests. Of most interest was the US2 c T1 transfer test. If the US1 feature acquired inhibitory properties solely because of the explicit serial feature negative discrimination contingency, then US2 should not inhibit responding to T1 in either group. In contrast, if mere temporal separation of USs is sufficient, then US2 should inhibit T1 responding in Group USa but not in Group USpd. Experiment 2 also examined the ability of a US feature and US pseudofeature to inhibit responding to a separately trained cue. Finally, Experiment 2 explored the ability of the true feature, the US-alone cue, and the pseudofeature to modulate responding when the US – target intervals were systematically varied. Method Subjects. The subjects were 16 female Sprague-Dawley rats obtained from Charles River, Inc. (Raleigh, NC). They were between 130 and 180 days old at the beginning of the experiment and had not been involved in previous research. The rats were maintained at about 85% of their ad libitum body weights by measured feedings at the end of a session. Water was available at all times in their individual home cages. Apparatus. The apparatus was identical to that used in Experiment 1. Procedure. The rats were first familiarized with the two KOOL-AID solutions using the identical procedure as used in Experiment 1. Magazine training was somewhat more extensive in Experiment 2 than in Experiment 1. On the first day, subjects received twenty 0.3-ml deliveries of grape KOOL-AID, on a variable-time 2-min schedule, with the levers and chains removed and the paddle secured. The same procedure was used on the second and third day with orange KOOL-AID and on the fourth day with grape KOOL-AID. The subjects were then trained to press the lever and pull the chain (as in Experiment 1, the paddle remained secured unless otherwise specified). On the first 2 days, lever-pressing or chain pulling (counterbalanced) was reinforced with grape KOOL-AID; shaping was used if necessary. Only one response manipulandum was available and subjects were removed from the chambers after making approximately 50 responses. The same procedure was used on the next 2 days except the other response (chain pulling or leverpressing, counterbalanced) was reinforced with orange KOOL-AID. As in Experiment 1, an individual subject was reinforced with one solution (grape or orange, counterbalanced) for lever-pressing and the other solution (orange or grape, counterbalanced) for chain pulling. The next 10 sessions placed lever-pressing and chain pulling under the control of the auditory targets to be used later in operant serial feature negative

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training. For half of the rats, each lever press was reinforced during the tone and each chain pull was reinforced during the noise. For the other half of the rats, those contingencies were reversed. In all sessions, tone and noise presentations were randomly intermixed. As in Experiment 1, the response (lever pressing or chain pulling), target (tone or noise), and reinforcer (grape or orange) were completely counterbalanced. The first session was 30 min long and contained thirty 30-s tone trials with only the manipulandum appropriate for tone responding available. The same procedure was used in the next session except the noise was used and only the manipulandum appropriate for noise responding available. In the third session, both manipulanda were available and subjects received fifteen 30-s tone trials and fifteen 30-s noise trials in a 30-min session. The cues were then shortened to 15 s (session 4), and the session length was extended to 40 min (session 5), 50 min (session 6), and 60 min (session 7). In session 8, subjects received ten 10-s tone trials and ten 10-s noise trials in a 60-min session; finally, in sessions 9 and 10, subjects received eight 10-s tone trials and eight 10-s noise trials in a 64-min session. Subjects were then assigned to groups (n Å 8), maintaining complete counterbalancing of response, target, and reinforcer, and matching response levels as closely as possible. Table 2 shows an outline of these and all subsequent procedures of Experiment 2. Operant serial feature-negative training was conducted over the next 10 sessions. Subjects in Groups USa and USpd received training on one operant serial feature negative discrimination with grape or orange KOOL-AID as the US feature. In each 64-min session, all subjects received four T1/ trials (each R1 response during T1 was reinforced) and four US1 r T10 trials (responding was not reinforced). Subjects in Groups USa and USpd differed, however, in the training received with the other US feature, US2. In Group USa, subjects received four US2-alone trials (responding was not reinforced) and four T2/ trials (each R2 response during T2 was reinforced). In Group USpd, subjects received a pseudodiscrimination with four US2 r T2/ (each R2 response during T2 was reinforced), and four T2/ trials (each R2 response during T2 was reinforced). All cues were 10 s in duration (except for US delivery, which was 0.5 s) and the interval between feature onset and target onset was 30 s in all serial compounds. The identities of the feature cues were included in the counterbalancing as much as possible with the exception that the US feature and the reinforcer associated with its target were identical. Test 1 followed operant serial feature-negative training. In each of two 64min test sessions, there were two presentations of each of eight trial types. The eight trial types included the two features alone (US1 and US2), the two targets alone (T1 and T2), the original serial feature negative discrimination compound (US1 r T1), a US2 r T2 compound (which was novel in Group USa and was the pseudodiscrimination compound in Group USpd), and the two novel transfer compounds (US1 r T2 and US2 r T1). As in Experiment 1, responding was not reinforced during the two test sessions.

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Train R3 7 ses — US1 r CS1, US2 r CS1, CS1 US2 r CS2, US1 r CS2, CS2

— US1 r T1, T1R1/ US2 r T2, T2R2/

Discrimination 2 ses

US1 r T1, T1R1/ US2, T2R2/ US1 r T1, T1R1/ US2 r T2/, T2R2/

T1R1/ T2R2/ T1R1/ T2R2/ Transfer Test 2 1 ses

Discrimination 10 ses

Training 10 ses

r r r r

US2 US1 US2 US1

r r r r

T1, T2, T1, T2, Interval tests 12 ses

T1, T2, T1, T2,

T1, T2, T1, T2,

US1 US2 US1 US2

— US1 r x r T1, US1 r T1, T1R1/ US2 r x r T2, US2 r T2, T2R2/

US1 US2 US1 US2

Transfer Test 1 2 ses

Note. CS1/2, panel light and clicker, counterbalanced; US1/2, grape and orange KOOL-AID features, counterbalanced; T1/T2, tone and noise target stimuli, counterbalanced; R1/R2, reinforced lever-press and chain pull operant responses, counterbalanced; R3, paddle push operant response; /, availability of KOOL-AID reinforcer with an appropriate operant response; r, serial relation with trace interval; ses, session(s); x, variable test trace interval (see text).

USa USpd

Group

USpd

USa

Group

TABLE 2 Outline of Procedures in Experiment 2

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The rats in Group USpd were then trained to make a new response (paddlepush) in the presence of a panel light and a clicker (separately) and the ability of the feature and pseudofeature to modulate responding to the separately trained cues was examined. Training and testing were identical to that received by Group US–US in Experiment 1. Finally, rats in Groups USa and USpd received two retraining sessions in which the levers and chains were present in the chambers but the paddle was immoveable. After retraining, the ability of US1 and US2 to modulate responding when the US1–T1 and US2–T2 intervals were systematically manipulated was explored. Feature–target interval testing was identical to that in Experiment 1. Results During initial training, the rats learned to perform one response during one target cue and a different response during the other target cue. By the final session, across all groups, the rats made at least one correct response on 93% of trials and at least one incorrect response on 16% of trials. The choice of response (lever-pressing or chain pulling), target (tone or noise), or reinforcer (grape or orange) did not affect initial training. Discrimination training. Figure 8 shows the percentage of trials with a correct, target-appropriate response during the 10 sessions of discrimination training (as in Experiment 1, by the end of this phase, inappropriate responding occurred on fewer than 10% of the trials, so it is not described further). In both Groups USa and USpd, R1 responding on US1 r T1 trials was not significantly lower than R1 responding on T1/ trials in session 1 but was significantly lower in all other sessions [all T’s(8) Å 0]. Therefore, as in Experiment 1, acquisition of operant serial feature-negative discrimination performance was remarkably rapid with a US feature. Furthermore, acquisition of the discrimination was somewhat more rapid with the true US feature (US1) in Group USa than in Group USpd. For example, responding on US1 r T10 trials was significantly lower in Group USa than in Group USpd in sessions 1, 3, and 4 [all U’s(8,8) õ 13]. Therefore, serial feature negative discrimination performance was acquired more rapidly when discrimination trials were intermixed with presentations of another US alone than with another US presented in pseudodiscrimination trials. Note that if a US feature acquires unique inhibitory properties, inhibition might be strengthened by generalization from another US that also signaled nonreinforcement (Group Usa), rather than another US that signaled reinforcement (Group USpd). Because the rats in Group USa showed some discrimination between F1 r T10 and T1/ trials in the first session, it would have been of interest to examine responding over trials in the first session (as was done in Experiment 1). However, due to a programming error, the trial by trial data from the first session were not available. R2 responding occurred at a high level during T2 on T2-alone trials in

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FIG. 8. Percentage of trials on which an appropriate response occurred during the target cues in operant serial feature-negative discrimination training in Experiment 2. All subjects received one operant serial feature-negative discrimination with a US feature (T1/ and US1T10 trials). Group USa (top) also received nonreinforced US2-alone trials and reinforced trials with another target (T2/ trials). Group USpd (bottom) received an additional pseudodiscrimination (T2/ and US2-T2/ trials).

both groups and on US2 r T2 trials in Group USpd. In Group USpd, R2 responding on T2 and US2 r T2 trials did not differ reliably in any of the training sessions. Test 1. Figure 9 shows the results of Test 1. The rats in Group USa showed significantly more R1 responding on T1-alone trials than on US1 r T1 trials, and significantly more R2 responding on T2-alone trials than on US2 r T2 trials [T’s(8) Å 0]. Note that in Group USa, although the rats had never received US2 r T2 trials, the US-alone feature was able to inhibit responding to the separately reinforced T2. Rats in Group USpd also showed significantly

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FIG. 9. Mean response rates in Test 1 of Experiment 2. The abscissae labels identify the features (first line) and the targets (second line) for each type of test trial. In training, all subjects received one operant serial feature-negative discrimination with a US feature. Group USa (top) also received nonreinforced US2-alone trials and reinforced trials with another target (T2/ trials). Group USpd (bottom) received an additional pseudodiscrimination (T2/ and US2-T2/ trials).

more R1 responding on T1-alone trials than on US1 r T1 trials [T(7) Å 0], but did not show more R2 responding on T2-alone trials than on US2 r T2 trials [T(6) Å 10.5]. During the novel transfer compounds, the rats in Group USa showed significantly more R1 responding on T1-alone trials than on US2 r T1 trials [T(7) Å 0], and significantly more R2 responding on T2-alone trials than on US1 r T2 trials [T(7) Å 0]. Therefore, for the rats in Group USa, a US-alone was as effective as a true US feature in modulating performance to a target or a separately trained cue. This observation supports the hypothesis that simple presentation of temporally separated USs may be sufficient to endow USs with inhibitory properties that facilitate operant serial feature-negative discrimination performance. The rats in Group USpd showed significantly more R1 responding on T1-

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alone trials than on US2 r T1 trials [T(7) Å 0], but did not show significantly more R2 responding on T2-alone trials than on US1 r T2 trials. The ability of the pseudofeature, but not the feature, to modulate performance to another target cue was unexpected (and is discussed later). Paddle-push training. The rats in Group USpd were trained to make a new response (paddle-push), in the presence of a panel light and clicker (separately). Paddle-push training proceeded rapidly; in the last session, the rats in Group USpd responded on 97% of panel light trials and 98% of clicker trials. The ability of the US feature (US1) or pseudofeature (US2) to inhibit paddle-push responding was then examined. Subjects showed significantly more responding on target-alone trials (M, 31.5 responses/min) than on either US-target trials [M, 11.1 responses/min, T(7) Å 0] or pseudofeature-target trials [M, 5.4 responses/min, T(7) Å 0]. Therefore, a US feature and pseudofeature both modulated performance to a separately trained cue. Feature–target interval tests. Figure 10 shows response latencies during the feature–target interval tests in Groups USa and USpd. The identity of the feature (grape or orange), target (tone or noise), and test session (first or second with each test interval) did not affect responding, so the data for all statistical tests were collapsed across those factors. The original feature-negative discriminations were maintained in both groups throughout testing: the R1 response latencies on T1/ trials were significantly lower than those on US1 r T10 trials with the original 30-s training interval in each session [T’s(7 or 8) Å 0]. In addition, in Group USa, the US2-alone cue inhibited R2 responding to T2 on US2 r T2 trials with 30-s intervals in each test session [T’s(8) Å 0]. In contrast, in Group USpd, the pseudofeature US2 failed to reliably slow R2 responding to T2 [T’s(7) ú 5, with the exception of performance during tests that included 45-s interval test trials, T(7) Å 4]. Response latencies on test trials were systematically related to the feature– target interval. In Group USa, the R1 response latencies on US1 r T1 test trials with 15-, 30-, 45-, and 60-s test intervals were significantly greater than those on T1/ trials [T’s(8) õ 5]; at the same time, the R1 latencies on test trials with 45-, 60-, 120-, and 240-s test intervals were significantly lower than the response latencies on US1 r T1 (30 s) trials [T’s(8) õ 3]. Similarly, the R2 response latencies on US2 r T2 test trials with 15-, 30-, 45-, and 60s test intervals were significantly greater than those on T2/ trials [T’s(8) õ 5]; at the same time, the R2 latencies on US2 r T2 test trials with 60-, 120-, and 240-s test intervals were significantly lower than those on US2 r T2 (30 s) trials [T’s(8) Å 0]. In Group USpd, the R1 response latencies on US1 r T1 test trials with 15-, 30-, 45-, and 60-s test intervals were significantly greater than those on T1/ trials [T ’s(8) õ 5]; at the same time, the R1 latencies on test trials with 60-, 120-, and 240-s test intervals were significantly lower than the response latencies on US1 r T1 (30 s) trials [T ’s(8) õ 2], but the R1 latencies on test trials with a 15-s interval were reliably longer than those

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FIG. 10. Response latencies during the feature-target interval tests in Experiment 2. In training, all subjects received one operant serial feature-negative discrimination with a US feature. Group USa (top) also received nonreinforced US2-alone trials and reinforced trials with another target T2/ trials). Group USpd (bottom) received an additional pseudodiscrimination (T2/ and US2-T2/ trials).

on US1 r T1 (30 s) trials, [T(8) Å 2.5]. In contrast, performance on trials with the pseudofeature (US2) was not as clearly affected by the US2 – T2 interval: the R2 response latencies were reliably longer on test trials than on T2/ trials only with the 15-s test interval [T(8) Å 0]; with that interval the test trial R2 response latencies were also reliably longer than those on US2 r T2 (30 s) trials [T(8) Å 4].

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FIG. 11. Frequency of food cup behavior during the 120-s interval test sessions in Experiment 2. The line parallel to the abscissa indicates the mean response levels that occurred during all pretrial intervals (coincidentally, the pretrial response rate was identical in the two groups). Percentage total behavior was computed by dividing the number of observations on which a food cup response was scored by the total number of observations made during that interval.

Therefore, the feature–target interval tests showed that, in Group USa, a US-alone control cue resembled a US feature in modulating performance to other cues during feature–target interval testing. The same was not generally the case with a US pseudofeature in Group USpd. However, in Group USpd, response latencies on the pseudotarget cue (T2/) trials were significantly higher than response latencies on the target cue (T1/) trials in all feature– target interval test sessions [T’s(7 or 8) õ 4], perhaps precluding the observation of large differences between the response latencies on F2 r T2 test trials and T2/ trials. Figure 11 shows food cup entry data scored from the video tapes. As in Experiment 1, delivery of the flavored sucrose solutions produced high levels of food cup behavior followed by suppression below baseline levels. In the sixth 5-s interval after delivery of both US1 and US2, this suppression was reliable in both groups [T’s(8) õ 4], and in the seventh 5-s interval, the suppression was reliable for US1 in both groups but was reliable for US2 only in Group USa [T’s(8) õ 4]. Discussion Learning and performance of serial feature negative discriminations with US features was similar to that observed in Experiment 1: acquisition was very rapid, transfer was extensive, and the inhibitory effects persisted over long feature–target test intervals. However, these response-inhibition properties of the US feature apparently did not depend on arrangement of the serial

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feature–negative contingency: in Group USa, the effects of a serial feature (US1) and a US feature that was presented alone in training (US2) were indistinguishable in both transfer and interval tests. These observations are consistent with Goddard’s (1995) claims that simple spaced presentation of USs can endow them with response-inhibiting powers. The effects of the pseudofeature (US2) in Group USpd are more difficult to interpret. Although the pseudofeature failed to inhibit responding to its original target (T2) in training, transfer testing, and in most of the interval testing sessions, it readily inhibited responding to other cues. Several aspects of the results suggest that the performance of rats in Group USpd was strongly affected by generalization between the two US flavors. First, acquisition of the true serial feature-negative discrimination was slowed in Group USpd relative to Group USa. This outcome would be expected if there was substantial generalization between US1 and US2, and the pseudodiscrimination procedure was slower to establish inhibition to US2 than the US-alone procedure, or it established excitatory tendencies to US2. Second, in the initial transfer test, rats in Group USpd responded at low levels during both US1 r T1 and US2 r T1 compounds, but at high levels during both US1 r T2 and US2 r T2 trials, as if at the time of target presentation the rats were unable to determine whether they had just received US1 or US2. Third, both the feature and the pseudofeature inhibited responding during the separately trained target cue (panel light and/or clicker). Thus, the effective feature may have comprised stimulus aspects common to the grape and orange flavors used as the feature and pseudofeature cues, e.g., sweet taste. If this interpretation is correct, then the equivalent effects of US1 and US2 in Group USa in this experiment, and the occurrence of substantial transfer in Group US–US in Experiment 1, are hardly surprising. Of course, because responding on US2 r T2 trials in Group USpd (Experiment 2) was not suppressed, this interpretation demands that these rats also explicitly learned to respond to T2 in the presence of the common feature at the same time they were acquiring inhibitory tendencies to that feature. Thus, the rats in Group USpd effectively received a T1/, F r T10, T2/, F r T2/ discrimination, in which T2 sets the occasion for responding to the common feature, F. Unfortunately, these experiments provide no independent evidence by which this interpretation might be evaluated. GENERAL DISCUSSION

Recently, Goddard and Holland (1996) explicitly compared operant serial feature-positive discrimination performance with visual (CS) and sucrose (US) features. Their results showed that discrimination performance proceeded at roughly the same rate in all conditions and that, in many respects, responding with US features resembled that with CS features. For example, a US feature’s occasion-setting ability transferred to a target cue that had been trained with another US feature. Similarly, extinction of a US feature

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did not importantly reduce original occasion-setting performance or transfer performance to a target cue that had been trained with another US feature. However, occasion-setting transfer was poor when a US feature was paired with a target cue that had been originally trained with a CS feature; similarly, occasion-setting transfer was also poor when a CS feature was paired with a target cue that had been originally trained with a US feature. Furthermore, those transfer effects were not the result of simple generalization between the features: CSs and USs trained as pseudofeatures did not enhance responding in transfer tests. In the present study, the apparatus, the feature and target cues, and the original training and testing parameters were kept as similar as possible to those used by Goddard and Holland (1996) with the exception that an operant serial feature-negative, rather than feature-positive, discrimination was studied. Despite the many similarities between the procedures of the two studies, operant serial feature-negative discrimination training led to results considerably different from those reported in operant serial feature-positive discrimination training. First, and most noticeable, discrimination performance was dramatically more rapid with a US, in comparison to a CS, feature. Second, whereas a CS feature did not reliably inhibit performance to other targets or separately trained cues, a US feature inhibited responding to both kinds of targets. Finally, performances during compounds of various types of target cues and either the feature or the pseudofeature were similar. The first two of these differences may reflect a powerful tendency for temporally spaced USs to acquire inhibitory power (described previously). The third difference suggests that whereas intermixed serial feature-positive and pseudodiscrimination trials produce only small amounts of generalization between the feature and the pseudofeature, intermixing of serial feature-negative and pseudodiscrimination trials produces substantial generalization between the two US cues. It is not evident why serial feature-positive procedures would encourage more specific encoding of US features than serial featurenegative procedures. The results shown in feature – target interval testing also merit comment. In general, Experiments 1 and 2 showed that operant serial feature-negative discrimination performance with both US and CS features improved as the feature – target interval was shortened and worsened as the feature – target interval was lengthened. However, in a recent study, Holland et al. (1997) showed that in an operant serial feature-positive discrimination with CS features, discrimination performance was best with the original feature – target intervals and worsened as the intervals were either shortened or lengthened. This finding suggests that in serial feature-positive learning, temporal information about the original feature – target relation is strongly coded. However, in serial feature-negative learning, performance may depend more on simple inhibition to the feature. This feature inhibition may be strongest immediately after the feature and then gradu-

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ally decay. The results from the magazine entry data corroborate this suggestion; subjects showed very few magazine entries in the interval shortly after the delivery of a US feature but magazine entries gradually recovered later in the interval. Such a function has also been reported by Goddard (1995) in an appetitive conditioning preparation with rats. REFERENCES Bottjer, S. W., & Hearst, E. (1979). Food delivery as a conditional stimulus: Feature learning and memory in pigeons. Journal of the Experimental Analysis of Behavior, 31, 189–207. Farwell, B. J., & Ayres, J. J. B. (1979). Stimulus-reinforcer and response–reinforcer relations in the control of conditioned appetitive head poking (‘‘goal tracking’’) in rats. Learning and Motivation, 10, 295–312. Goddard, M. J. (1995). Acquisition of US–no US associations. Learning and Motivation, 26, 264–277. Goddard, M. J., & Holland, P. C. (1996). Type of feature affects transfer in operant serial featurepositive discriminations. Animal Learning and Behavior, 24, 266–276. 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., & Coldwell, S. E. (1993). Transfer of inhibitory stimulus control in operant feature-negative discriminations. Learning and Motivation, 24, 345–375. Holland, P. C., & Forbes, D. T. (1982). Control of conditional discrimination performance by CS-evoked event representations. Animal Learning and Behavior, 10, 249–256. Holland, P. C., Hamlin, P. A., & Parsons, J. P. (1997). Temporal specificity in serial feature positive discrimination learning. Journal of Experimental Psychology: Animal Behavior Processes, 23, 95–109. Holland, P. C., & Lamarre, J. (1984). Transfer of inhibition after serial and simultaneous feature negative discrimination training. Learning and Motivation, 15, 219–243. Honey, R. C., & Hall, G. (1989). The acquired equivalence and distinctiveness of cues. Journal of Experimental Psychology: Animal Behavior Processes, 15, 338–346. Lamarre, J., & Holland, P. C. (1987). Transfer of inhibition after serial feature-negative discrimination training. Learning and Motivation, 18, 319–342. Reberg, D., & Memmott, J. (1979). Shock as a signal for shock or no-shock: A feature-negative effect in conditioned suppression. Journal of the Experimental Analysis of Behavior, 32, 387–397. Rescorla, R. A. (1985). Conditioned inhibition and facilitation. In R. R. Miller & N. E. Spear (Eds.), Information processing in animals: Conditioned inhibition (pp. 299–326). Hillsdale, NJ: Erlbaum. Schmajuk, N. A., Lamoureux, J., & Holland, P. C. (1994, November). Occasion setting and stimulus configuration: A neural network approach. Paper presented at the annual meeting of the Psychonomic Society, St. Louis. Swartzentruber, D. (1995). Modulatory mechanisms in Pavlovian conditioning. Animal Learning and Behavior, 23, 123–143. Received January 17, 1997 Revised April 9, 1997

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