Context-Specificity of Target Versus Feature Inhibition

Copyright 1994 by the American Psychological Association. Inc. 0097-7403/941$3.00

Journal of Experimental Psychology: Animal Behavior Processes 1994. Vol. 20. No. 1.51-65

Context-Specificity of Target Versus Feature Inhibition in a Feature-Negative Discrimination Mark E. Bouton and James B. Nelson

Four experiments with rats examined the effects of a context switch on inhibition that was acquired during a feature-negative discrimination. A target conditioned stimulus was paired with food when it was presented alone but occurred without food when it was combined with a feature stimulus. A context switch following training did not disrupt inhibition conditioned to the feature. However, responding to the target was more difficult to inhibit when it was tested in a different context. It is suggested that both the target and the feature acquired inhibition and that the target's inhibition was especially sensitive to the context. The feature may inhibit responding to the target (a) by directly suppressing the representation of the food and (b) by activating the target's own inhibitory association with food, which is at least partly context-specific. Implications for theories of inhibition and negative occasion-setting are discussed.

view, the context-specificity of extinction could come about if inhibition is relatively specific to the context in which it is learned. The present experiments were therefore designed to test whether conditioned inhibition is reduced with a change of context. To produce inhibition, we used a feature-negative discrimination procedure. In this sort of procedure, a target CS, such as a tone, is paired with a US when it is presented alone (T +) but is presented without a US when it is combined with a "feature" stimulus, such as a light (LT-). The featurenegative procedure is widely known to produce inhibition to the feature (L). Indeed, it is sometimes taken as the basic means of producing conditioned inhibition (e.g., Rescorla & Wagner, 1972). If conditioned inhibition is specific to its context, then a context switch after feature-negative training might cause a loss of inhibition to the feature CS. Recent theories of conditioning, however, suggest that the target stimulus might also acquire some inhibition in the feature-negative discrimination. As is expected when a CS is nonreinforced in extinction, the target itself might acquire an inhibitory association when it is nonreinforced in compound with the feature in this paradigm (e.g., Pearce & Hall, 1980; Wagner, 1981). Thus, both feature and target may acquire some inhibition when the compound is nonreinforced in the feature-negative paradigm. We designed the present experiments to investigate the effects of switching the context after feature-negative discrimination training. We used an appetitive conditiOning preparation in which we monitored the number of times the rat entered the food cup or magazine (magazine entries) during the CS (e.g., Hall & Channell, 1985; Hall & Honey, 1990; Kaye & Mackintosh, 1990; Wilson & Pearce, 1990). The results suggest that inhibition is indeed attenuated when the context is switched after featurenegative training. However, in the present experiments, the context switch reduced inhibition to the target CS but not inhibition to the feature. The results have implications for the issue of what is learned in feature-negative discrimination learning.

The results of a number of experiments suggest that extinction performance is relatively specific to the context in which it is learned. For example, if conditioned stimulus (CS)-unconditioned stimulus (US) pairings are given in one context and then CS-alone training (extinction) is given in another context, a return of the CS to the original conditioning context can renew extinguished performance to the CS (e.g., Bouton & Bolles, 1979; Bouton & King, 1983; Bouton & Peck, 1989). Although extinction performance is specific to its context, conditioning performance is much less so. When the context is switched after conditioning, there is often relatively little diminution of the conditioned response (CR) (e.g., Bouton & King, 1983; Bouton & Peck, 1989; Bouton & Swartzentruber, 1986; Hall & Honey, 1989; Kaye & Mackintosh, 1990; Kaye, Preston, Szabo, Druiff, & Mackintosh, 1987). It may be reasonable to suppose that the CS acquires a second association during extinction and that this association is especially sensitive to the context (Bouton, 1993). One possibility is that the animal learns a new inhibitory association with the CS (e.g., CS-no US) in extinction. Such learning has been proposed by several conditioning models (e.g., Konorski, 1967; Pearce, 1987; Pearce & Hall, 1980; Wagner, 1981; see also Pavlov, 1927) that assume that nonreinforcement of the CS in extinction will cause acquisition of a second inhibitory association instead of the unlearning of the original CS-US association (e.g., Rescorla & Wagner, 1972). According to these models, at the end of extinction, the CS has both excitatory and inhibitory values. On this

Mark E. Bouton and James B. Nelson, Department of Psychology, University of Vermont. This research was supported by Grants BNS 89-08535 and mN 92-09454 from the National Science Foundation. Correspondence concerning this article should be addressed to Mark E. Bouton, Department of Psychology, University of Vermont, Burlington, Vermont 05405.

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MARK E. BOUTON AND JAMES B. NELSON

Experiment 1 The purpose of the first experiment was to assess some of the properties acquired by the feature in the feature-negative procedure to be used in Experiments 2-4. There were two groups. Group FN (feature-negative training) received a feature-negative procedure in which a tone was the target (T+) and a houselight-off stimulus was the negative feature (LT-). In this and all subsequent experiments, the elements of the compound (L and T) were presented simultaneously. In each session, an intermittent white noise was also reinforced (N + ). L was ultimately tested in compound with both N and T at the end of training. If L had acquired true inhibition, we expected it to inhibit responding to both T and N. Notice that the transfer target (N) was never nonreinforced before testing; if L inhibited responding to N, its inhibitory effect could not depend on any inhibition conditioned to the target. A control group received the same T + and N + trials during training, but L alone trials (L-) replaced LT-. This is a somewhat conservative control for inhibition to L in Group FN, because L- training could produce some inhibition to L. However, consistent with several conditioning models (e.g., Rescorla & Wagner, 1972; Pearce & Hall, 1980; Wagner, 1981), more inhibition often accrues to a CS when it is trained as a feature in the feature-negative arrangement (e.g., Rescorla & Holland, 1977).

Method Subjects The subjects were 16 male Wistar rats, bred at the University of Vermont, that were approximately 110 days old at the start of the experiment. They were individually housed in standard stainless steel cages in a room maintained on a 18:6 hr Jight:dark cycle; the experiment was conducted on consecutive days during the light part of the cycle. The rats were food-deprived to 80% of their original body weight throughout the experiment.

Apparatus There were two counterbalanced sets of four conditioning boxes; these provided two "contexts" in Experiments 2-4 but were not used in that capacity here. Each box was housed in a sound attenuation chamber and was constructed of clear acrylic plastic. The boxes were 23 X 13 X 11 cm. The front and right side walls were transparent, and the exterior of the back wall and left side were covered with black paper. Illumination was provided by two 7.5-W white incandescent bulbs mounted on the ceiling of the sound attenuation chambers, 25 cm above the box floors. A small 1.2-W keylight (3 cm in diameter) was positioned behind the wall in the upper right comer of the boxes, 12 cm above the floor and 3.5 cm from the right wall. The rats were placed in the boxes through the ceiling. On the right wall of each box a stainless steel recessed food cup was positioned 3 cm above the floor and 1.5 cm from the back wall. The food cup was accessed through a 6-cm square opening, and an infrared photocell was mounted 3 mm behind the opening, 2.5 cm from the bottom. In all boxes, inactive operant levers were located on the back wall near the right comer 5 em above the floor. Background noise in all boxes was a constant 65 dB (re 20 /-LN/m2 [AD.

In one set of four boxes, the floor consisted of 3-mm bars mounted parallel to the side wall, staggered so that the oddnumbered bars were mounted 6 mm above the even-numbered bars. Odd-numbered bars were 1.6 cm apart (as were the even-numbered bars). The black side and back walls were lined with l-cm horizontal white stripes spaced 1 cm apart. A dish containing 10 ml of a 4% McCormick coconut extract solution (McCormick, Hunt Valley, Maryland) was positioned near the food cup but was outside the box on the floor of the sound attenuation chamber. The floors of the second set of four boxes consisted of3-mm bars spaced 1.8 cm apart mounted diagonally with respect to the chamber walls. A similarly positioned dish of 2% McCormick anise extract solution provided the distinctive scent. In summary, the two sets of boxes differed in visual (stripes vs. no stripes), tactile (staggered narrowly spaced bars vs. wide level bars), olfactory (anise vs. coconut scent), and spatial (they were housed in different rooms) respects. Four CSs were available. Two auditory CSs were presented through a speaker mounted on the ceiling of the sound attenuation chambers, 25 cm above the floor of each box (the speakers for each box were identical). One CS was an 80-dB 3000-Hz tone (T), the other was an intermittent (6 pulses per second) 65-dB white noise (N). One of two visual cues (L) was provided by the offset of the house lights, which caused complete darkness. The other visual CS was the illumination of the keylight (K). The US was always two 45-mg food pellets (traditional formula, P. J. Noyes, Lancaster, New Hampshire) delivered 0.2 s apart.

Procedure The experiment was run on consecutive days. Each rat was run consistently in the same box (counterbalanced). Magazine training. The rats were initially placed in the boxes for two 30-min exposure sessions on each of 2 days. On the next day, they received two sessions of hand shaping in which they were trained to approach and eat from the food cup at the sound of the feeder's activation. The first and second sessions were approximately 30 and 15 min long, respectively. Each rat received a total of about 40 pellets. Conditioning. The animals were then randomly assigned to two groups (n = 8). On each of the next 2 days, both groups received one 75-min session in which T and N were each paired with the US on six trials. CSs were always 30 s in duration; US onset was contiguous with CS offset. Trials were presented in a double alternating fashion (T+ T+ N+ N+ ... ) with the intertrial interval (ITI) averaging 345 s. Inhibition training was then carried out over the next seven 75min sessions. For both groups, each session contained 4 T + and 4 N + trials distributed evenly through the session. There were also 12 nonreinforced trials involving L. For Group FN, these were compound trials in which L and T were presented simultaneously (LT-). For the control group, L was presented alone (L-). Trial sequences were changed daily. On any day, T + , N +, and nonreinforced trials occurred in the same trial positions for both groups. The ITI was variable with a mean of 195 s. Test. On the day following the conclusion of inhibition training, both groups received a series of test trials designed to assess L's effect on responding to T and N. Half the rats in each group received three repetitions of the sequence LT-, T+, N+, LN-, L-, and the other half received the sequence LN-, N+, T+, LT-, L-. (The L-trials were included to assess possible second order conditioning to L.) The test session lasted 56 min, and the average ITI was 195 s. Data collection and analysis. The computer recorded the number of magazine entries each rat made during the 30-s CS and during

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CONTEXT AND INHIBmON the 3(}-s period immediately before the CS (the pre-CS period). Responding during the CS was evaluated by analyses of variance (ANOVAs), and planned comparisons were made using the methods discussed by Howell (1987, pp. 431-443). Responding during the pre-CS periods was analyzed with distribution-free tests because a substantial number of scores of zero often made the distributions nonnormal. Throughout, the rejection criterion was set at p < .05.

Results Conditioning and inhibition training proceeded uneventfully. During testing, Group FN showed signs of conditioned inhibition to L, whereas the control group did not. Conditioning. Magazine entries during the conditioning sessions are summarized in Figure 1. The groups were compared on responding to the reinforced and nonreinforced stimuli in separate ANOVAs. For the reinforced CSs, a Group X CS (T or N) X Session analysis on the two initial conditioning days that preceded actual inhibitory conditioning revealed a main effect of session, F(1, 14) = 7.98; some excitation was acquired before inhibition training began. All effects and interactions involving group and CS were nonreliable, Fs < 1. For the 7 days that involved inhibition trials, a Group X CS (T or N) X Session ANOVA revealed no main effects, Fs(l, 14) < 3.0, but there were significant Group X Session andCS X Session interactions, Fs(6, 84) > 2.71. The Group X Session interaction was the result of a reliable increase in responding over sessions in Group FN, F(6, 84) = 3.89, with no such effect occurring in the control group, F < 1. We do not have an explanation of this difference. The CS X Session interaction took the form of an increase in responding to N over training, F(6, 84) = 5.90, and responding to T did not increase reliably above that observed during the two initial conditioning days, F(6, 84) = 1.75.

A Group X Session ANOVA on the nonreinforced stimuli (LT- in Group FN and L- in the control group) confirmed a group effect, F(l, 14) = 126.17, a session effect, F(6, 84) = 6.14, and a Group X Session interaction, F(6, 84) = 3.01. The interaction confirms that Group FN's initial responding to LT declined overtraining, F(6, 84) = 8.70, and responding to L in the control group was always low and did not change significantly over sessions, F < 1. These data are consistent with the possibility that initial responding to T became inhibited by L over sessions in Group FN. Pre-CS responding during the phase averaged 1.32 for Group FN and 0.84 for the control group. The difference was not reliable, U(8, 8) = 15. Test. Responding during testing is summarized in Figure 2, which shows the groups' mean responding during the first presentations of the tone and noise targets alone and in compound with L. (First trials are reported and analyzed because of our emphasis on the first test trial in subsequent experiments; analyses of session means supported identical conclusions.) The effects of L on responding to T and N were evaluated in separate Group X CS-type (target vs. compound) ANOVAs. The ANOVA comparing trials with the tone target revealed a CS-type effect that fell short of significance, F(I, 14) = 3.56, p = .08, and a significant Group X CS-type interaction, F(I, 14) = 6.05. The group effect was not reliable, F < 1. Simple effect comparisons confrrmed that there was reliably less responding to the LT compound than T in Group FN, F(l, 14) = 9.45, but not in the control group, F .097. The animals came to discriminate reinforced and nonreinforced trials over training, the two discriminations (T+, LT- and T+, KT-) were learned about equally rapidly, and performance did not differ between the groups. Response rates during the pre-CS periods were 1.59 for the group eventually tested in A and 1.65 for the group eventually tested in B, U(8, 8) = 34. Neither group had a difference between pre-CS responding on reinforced and nonreinforced trials, Ts(8) > 10. Test. Figure 4 summarizes performance to LT and T on their first test presentations for the groups tested in Context A and Context B. (We focused on the first test with each stimulus because previous work with extinction predicted an effect primarily on the first trial.) L inhibited performance to T in both contexts. A Group (context) X CS type (LT vs. T) ANOVA revealed a strong effect of CS type, F(l, 14) = 399.03; overall, there was less responding to LT than T. Neither the group effect nor the Group X CS type interaction approached reliability, Fs(1, 14) < 1. Planned comparisons indicated no difference in responding to the compound in the two contexts, Fs < 1, and no difference in responding to the tone alone, F(1, 26) = 1.32. L's ability to inhibit T was not reduced when it was switched to Context B. The animals tested in A and B had pre-CS rates of 2.37 and 3.63, respectively, on the compound trial, U(8, 8) = 31, and 0.88 and 2.62 on the T-alone trial. The latter difference was significant, U(8, 8) = 12, but as responding to T in the two contexts was entirely consistent with the performance we had observed

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throughout the experiment, there seems to be little need to reinterpret CS responding. The groups did not differ in responding to either T or LT during testing.

Discussion Whether L was tested in its usual context (A) or in a new context (B), it strongly inhibited performance to T. The results provide no evidence that inhibition to the feature was attenuated by a context switch in this procedure. With the present method, inhibition to the feature CS transferred well between the contexts.

reinforcement. However, in this experiment the animals received L + ,LT- training in Context A and K + ,KT- training in Context B. T was now a feature common to both contexts, whereas Land K were targets uniquely connected with Contexts A and B, respectively. During testing, the rats received LT and L in either Context A, the home context, or in Context B. As in Experiment 2, our focus was the effects of the context switch on the houselight-off CS (L), because complete darkness should be perceived as the same stimulus in both contexts. But in the present experiment L played the role of the target, rather than the feature, in the discrimination that was learned in Context A. If inhibition to the target is lost with a context switch, we should observe an increase in responding to LT in Context B.

Experiment 3 We began the present experiments with the idea that inhibition learned to a CS during extinction might be specific to the context in which it is learned. Although we began by asking whether feature inhibition is context-specific (Experiment 2), an extinguished CS may have more in common with the target CS, rather than the feature CS, in a feature-negative discrimination. Unlike a purely inhibitory feature, targets and extinguished CSs share a mixed history of reinforcement and nonreinforcement; both may have mixed excitation and inhibition (e.g., Pearce & Hall, 1980; Wagner, 1981). The third experiment was therefore designed to ask whether inhibition to the target, rather than to the feature, is attenuated when the context is changed following feature-negative training. The design, illustrated in Table 1, was the mirror image of Experiment 2. During initial conditioning, Contexts A and B were again home to feature-negative discriminations, and were therefore associated with both reinforcement and non-

Method Subjects and Apparatus The subjects were 16 male Wistar rats from the same stock as in the preceding experiments; they were approximately 120 days old at the start of the experiment. Housing, maintenance conditions, and apparatus were the same as in the previous experiments.

Procedure The experiment was run on consecutive days following the twosession-a-day procedure used in Experiment 2. Magazine training. Box exposure and magazine training were conducted following the procedure used in Experiment 2. Conditioning. There were then 2 days in which L and K were paired with the US in Contexts A and B, respectively. The two sessions given on the first day each began with two nonreinforced

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exposures to the CSs to be used in that context (L and T in Context A and K and T in Context B). The remainder of each session then contained 12 trials in which the appropriate excitor (L in A and K in B) was reinforced. On the second day, all rats received another 12 L + trials in A and 12 K + trials in B. Trial spacing was the same as in Experiment 2. Inhibitory conditioning was then conducted over the next 9 days. On each day there was a session of L+, TL- in A and a session of K+, TK- in B. The position of reinforced and nonreinforced trials were changed daily and followed the schedule used in Experiment 2. The sequence of exposure to Contexts A and B was also organized and balanced following the method used in Experiment 2. Test. Rats were divided into two groups (n = 8) matched on performance to Land TL over training. Once again, box and A-B sequence given during conditioning were balanced over groups. On the day following the end of inhibitory conditioning, both groups received tests of Land TL. One group was tested in Context A; the other group was tested in Context B. There were four repetitions of the trial sequence TL-, TL-, L +, L +. Trial spacing was the same as in Experiment 2.

Results The conditioning phase proceeded uneventfully. Most important, during testing, target inhibition was lost: The rats responded more to TL in a new context (Context B) than in its original context (Context A). Conditioning. Responding to the excitors and inhibitory compounds during conditioning is shown in Figure 5. The data from Days 3-11 (those that involved both excitor and compound trials) were subjected to a Group X CS type (excitor vs. inhibitory compound) X Context (containing either

Lor K) X Session ANOVA. The analysis revealed a main effect of session, F(8, 112) = 1.96, CS type, F(1, 14) = 13.87, and a CS type X Session interaction, F(8, 112) = 4.04. There was more responding to the excitors than to the compounds, and this difference increased over training: Simple effects analyzing the interaction revealed both an increase in responding to the excitors, F(8, 112) = 4.24, and a decrease in responding to the compounds, F(8, 112) = 2.06, over days. No effects involving context (Le., L vs. K) or group approached significance, Fs < 1.26, ps > .20. As in Experiment 2, Land K had roughly equivalent effects. The groups eventually tested in A and B had mean pre-CS rates of 1.56 and 1.71, respectively. These did not differ, U(8, 8) = 28, and there was no difference in pre-CS responding on reinforced and nonreinforced trials in either group, Ts(8) > 12. Test. Figure 6 shows responding to TL and L on their first presentations for the groups tested in Contexts A and B. A Group X CS type ANOVA revealed a significant Group X CS type interaction, F(1, 14) = 13.51; the difference between responding to the excitor alone and to the compound clearly depended on the context. The CS type main effect was also reliable, F(I, 14) = 12.22, although the main effect of group was not, F(1, 14) = 1.85. Simple effects revealed no difference in responding to L in Contexts A and B, F(1, 20) < 1. However, there was more responding to TL in Context B than in Context A, F(1, 20) = 8.07. When the context was changed, there was a substantial increase in responding to the TL compound. The groups tested in A and B had mean pre-CS rates of 1.25 and 2.25 on the test of TL and 2.88 and 1.13 on the trial with Lalone. Pre-CS rates did not differ on either trial, Us(8, 8) > 16.

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Discussion The results of this experiment suggest that performance to the TL compound increased when it was switched from Context A to Context B. As the feature (T) was not new to that context, the effect was evidently due to some effect of the switch on the target (L). One possibility is that inhibition to the target was reduced when the context was switched following feature-negative training. The reduction was evident primarily when the target was presented in compound. Excitation to L was not reduced demonstrably with a context switch in this experiment. This result is consistent with previous research in this laboratory and in others (e.g., Bouton & King, 1983; Bouton & Peck, 1989; Hall & Honey, 1989; Kaye et aI., 1987; Kaye & Mackintosh, 1990) in suggesting that excitation is not readily reduced by a context switch. It should be noted, however, that previous reports with the magazine-entry preparation have actually produced conflicting evidence on this point (Hall & Honey, 1989; Kaye & Mackintosh, 1990).

Experiment 4 The results of Experiments 2 and 3 begin to suggest that a context switch reduces inhibition to the target (Experiment 3) more than inhibition to the feature (Experiment 2). The purpose of Experiment 4 was to compare the effects of a context switch on the target and the feature within a single experiment. The design is illustrated in Table 1. We used a withinsubjects method. Each rat received pairings of T and N with food in both Contexts A and B. In addition, each excitor was nonreinforced in compound with either L or K in one of the two contexts. In Context A, Twas nonreinforced in com-

pound with L; in Context B, N was nonreinforced with K. The subsequent test was designed to pit the effect of a context switch on target inhibition against the effect on feature inhibition. Each target was tested in the context in which it had been inhibited (the home context) and in the context in which it had not been inhibited (the away context). When tested away, the target was tested with a feature that was in its conditioning (i.e., home) context. When tested in its home context, the target was compounded with a feature that was outside its conditioning context (i.e., away). If target inhibition is more sensitive than feature inhibition to the effects of a context switch, then we would expect more responding to a compound when the target was tested away, even though it was compounded with a feature that was at home. In this experiment, the compound tests were transfer tests in which targets and features were tested in new combinations.

Method Subjects and Apparatus The subjects were 16 male Wistar rats, from the same stock as before, that were approximately 90 days old at the start of the experiment. Housing, maintenance conditions, and the apparatus were the same as in the preceding experiments. In this experiment, white semitransparent paper covered the key light CS.

Procedure The experiment was run on consecutive days with two sessions (one in each context) on each day. Magazine training. All rats initially received 15 min of exposure to both contexts with the food magazines baited with food pellets. On the next day, there were two 30-min sessions (one in each context) in which the rats were trained to retrieve pellets from

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the magazine at the sound of the feeder. Approximately 20 food pellets were delivered in each context. Conditioning. On the next day, all CSs were preexposed in both contexts. The rats were run in Context A and then Context B. The sessions were identical and were 46 min long; each session contained two repetitions of the sequence K-, T-, L-, N-, with an average ITI of 3.5 min. The next 2 days involved conditioning of both excitors in both contexts. In each of the two daily 75-min sessions, there were six T+ and six N + trials (average ITI = 345 s) given in a double alternating fashion. On Day 1, all rats were run in Contexts A and B in the sequence AB, and on Day 2, all rats received BA. Over the next 18 days, the rats received inhibitory conditioning treatments in the two contexts. Each of the two-daily 75-min sessions contained 4 T+ trials, 4 N+ trials, and 12 trials in which a compound stimulus was nonreinforced. In Context A, the nonreinforced compound was LT, and in Context B, the compound was KN. On odd days of the phase, the rats were run in the context sequence AB; on even days they received BA. The trial sequence was changed daily; the average ITI was 195 s. Test. On the final 2 days of the experiment, all animals were tested with T, N, LN, and KT in both contexts. Half of the subjects (n = 8) received testing in Context A on Day I and Context B on Day 2; the other half received the reverse sequence. Each rat received one session each day. In each session, the rats received four repetitions of one of four stimulus sequences: KT, T, LN, N; LN, N, KT, T; T, KT, N, LN; or N, LN, T, KT. The excitors were reinforced, whereas the compounds were nonreinforced. Half the rats received an inhibitory compound first, and the other half received an excitor first. Assignment to stimulus sequence was balanced with respect to context sequence. Each test session was 60 min long with an average ITI of 195 s.

Results Conditioning proceeded uneventfully; the animals learned to respond to the excitors and refrain from responding to the

compounds. During testing, there was more responding to the compounds that contained targets away from their inhibited contexts, even though the inhibitory feature was at home. Conditioning. The results of the conditioning phase are presented in Figure 7. For simplicity, the figure collapses over CS identity. Some data were lost from four sessions (5, 10, 16, and 19), and these sessions were excluded from the figure and the statistical analysis. Responding to the excitors and inhibitory compounds was analyzed in separate ANOVAs. Responding to the excitors during the inhibitory conditioning sessions shown in the figure was put through a CS (T vs. N) X Session type (in which a given excitor was or was not nonreinforced) X Day ANOVA. The analysis revealed a main effect of day, F(13, 195) = 7.61. There was no CS type effect, F < 1. The CS X Day interaction was reliable, F(13, 195) = 2.62, although simple effect tests analyzing the interaction revealed significant increases in responding to both T and N over days, Fs(13, 195) > 12.99. No other main effects or interactions were significant except for the session-type main effect, F(I, 15) = 39.08; overall, there was less responding to the targets when they were presented in sessions in which they were also being nonreinforced. In principle, this effect could have been cued by context, because each excitor was nonreinforced in only one context. However, when we isolated responding on only the first trials of the sessions, we found no difference between responding to the targets in the nonreinforced context (mean = 10.03) and reinforced-only context (mean = 9.27), F(1, 15) < 1. Thus, previous nonreinforced trials within the session, rather than an anticipation cued by context, appeared to cause the weaker average responding in the sessions in which the targets were nonreinforced.

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DAY Figure 7. Mean responding to the conditioned stimuli (eSs) during the conditioning phase of Experiment 4. Target Away = context in which the target was always reinforced; Target Home = context in which the target was nonreinforced in compound with a feature.

Responding to the feature-target compounds was subjected to a CS type (KN vs. LT) X Day ANOVA. The analysis revealed a reliable effect of day, F(13, 195) = 33.73. The CS type effect was also reliable, F(I, 15) = 13.86; there was more responding to KN than to LT. The interaction was not significant, F < 1. The animals averaged 1.92 responses during the pre-CS periods of the conditioning phase. There were no differences in pre-CS rates to the excitors when they were in sessions containing nonreinforced trials or not, and there was no difference in responding prior to the two types of inhibitory compound trials, Ts(l5) > 36. Test. Responding to the targets and feature-target compounds during testing is shown in Figure 8. For simplicity, the figure collapses over test order, which had no systematic effect on the results, and over the identities of the excitors and inhibitory compounds. The data were analyzed with a withinsubjects ANOVA that included context (target at home or away), trial-type (target alone or in compound), target-type (T and KT vs. N and LN), and trials as the factors. There was significantly less responding to the compounds than to the targets alone, trial-type main effect, F(I, 15) = 88.21. However, as suggested by the figure, the difference between excitor and compound depended on the context, as indicated by a Context X Trial-type interaction, F(I, 15) = 11.35. Simple effect tests indicated that there was more responding to the compound when the target was away than when it was at home, F(1, 15) = 37.98; responding to the targets alone did not differ between the contexts, F(l, 15) = 1.95. (A separate but identical analysis of the first-trial data confirmed a difference between the compounds, F( 1, 15) = 4.67, but no difference between the excitors tested alone, F < 1.) These data make it clear that target inhibition was attenuated with a change of context in this procedure. There were also significant effects of target type and

a Target-type X Trial-type interaction, F(l, 15) > 23.87, which were consistent with the conditioning phase in suggesting weaker inhibition by K than by L. No other interactions involving target-type approached reliability. The overall analysis also confirmed reliable effects of Trial, F(3, 45) = 3.16, and a Trial-type X Trial interaction, F(3, 45) = 9.40; over testing, target-alone responding increased, F(3, 45) = 7.42, and compound responding decreased, F(3, 45) = 6.07. The reliable increase in responding to the targets suggests that they were not at a response ceiling at the beginning of testing. No other main effects or interactions approached significance. Pre-CS responding averaged 2.16 during testing. There were no differences in responding in the two contexts on either compound or target-alone trials, Ts(15) > 20.

Discussion During testing, there was more responding to the compounds when the targets were presented in the context in which they had never been inhibited; inhibition was lost when the target was tested away. This pattern was evident even though it meant that inhibition was stronger in the contexts in which the features had never been presented before. Somewhat paradoxically, there was greater inhibition when the features were tested in a new context. These results strengthen the conclusion suggested by Experiments 2 and 3: In the present preparation, the target is more likely than the feature to lose inhibition when the context is switched following feature-negative training. Two details of Experiment 4 are worth noting. First, unlike Experiment 3, in the present design visual cues provided the features and auditory cues provided the targets. The loss of target inhibition found here suggests that the com-

CONTEXT AND INHIBITION

patible result in Experiment 3 was not due to its unique use of an auditory feature and a visual target. Second, unlike Experiments 2 and 3, the two visual CSs (L and K) were balanced in the present design. We found a consistent pattern of results with these cues; during testing, there was more responding to LN in Context A than Context B (see Table 1) and more responding to KT in Context B than Context A. Such results help eliminate any concern raised by the fact that L and K were not balanced in the previous experiments. The present findings are general across several different stimulus compounds. Finally, it should be noted that the inhibition tests used in Experiment 4 were transfer tests in which the elements were combined in novel combinations. Evidently, the loss of target inhibition with a context switch is general to tests with both familiar (Experiment 3) and novel (Experiment 4) featuretarget combinations.

General Discussion The present experiments examined the effects of a context switch on the performance controlled by both the feature and the target after feature-negative discrimination training. The main result can be summarized simply: Performance to the target stimulus was more difficult to inhibit when the target was tested in a context in which it had never been inhibited before (Experiments 3 and 4). The simplest explanation may be to assume that the target acquires some type of inhibition when it is nonreinforced during discrimination training (e.g., Pearce & Hall, 1980; Wagner, 1981) and that this inhibition is lost at least partly when the context is changed. It is important to note that the loss of target inhibition was not ap-

parent when the target was simply presented (tested) alone. When the context was switched, we obtained a loss of target inhibition that was primarily restricted to responding to the feature-target compounds. In contrast to inhibition to the target, inhibition to the feature appeared to transfer readily between our contexts. In Experiment 2, a negative feature presented in a new context was effective at inhibiting responding to its usual target there. In Experiment 4, a negative feature presented in a new context was effective at inhibiting a transfer target there. In both cases, the feature inhibited a target that had previously been inhibited in the new context. We do not argue that feature inhibition cannot be reduced by a context switch with other conditioning or testing procedures. But with the methods used in the present experiments, a loss of inhibition to the feature was surprisingly difficult to detect. The results of Experiment 4, in addition to Experiments 2 and 3, suggest that any effect of a context switch on inhibition to the feature must be weaker than its effect on inhibition to the target. The primary result, then, is that the context switch appeared to reduce the target's "inhibitability." One possibility is that the animal learned that the target was sometimes inhibited and that this property was lost with a context switch. The "inhibitability" idea is similar to Holland's (1989b, 1992) view that animals store a CS in a "conditional memory system" when it has been the target in an occasion-setting discrimination. When a target is stored in this system, it is recognized as having a conditional value. However, it is not clear that a conditional memory system would allow a target's inhibitability to be specific to its context; why should membership in the system be revoked with a context change? We propose, instead, that the present results may follow more

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62

MARK E. BOUTON AND JAMES B. NELSON

Figure 9. Model of extinction. Tone (T) conditioned stimulus has two types of associations with the unconditioned stimulus (US): an excitatory association (arrow) acquired during conditioning and an inhibitory association (blocked line) acquired during extinction. Inhibitory association is gated, and requires simultaneous input from context (A) and tone for activation.

directly from how inhibition may be represented in a general memory system. The present results are compatible with previous work on the context-specificity of extinction (e.g., Bouton, 1991), and we can begin an account of them by first considering extinction. Figure 9 presents one possible representation of what is learned when a target tone CS has been paired with a US and then extinguished. When the tone is initially paired with a US, a CS-US association is learned. Conditioned performance would depend on the CS's ability to activate the representation of the US (e.g., Rescorla, 1973; Wagner, 1981). When the tone is then nonreinforced in extinction, a second, new inhibitory association is learned (e.g., Pearce & Hall, 1980; Wagner, 1981). Activation (or retrieval) of the inhibitory association would interfere with performance otherwise evoked by the excitatory association (e.g., Bouton, 1993). In contrast to the excitatory association, however, activation of the inhibitory association is highly dependent on the presence of the context present during extinction (A). In Figure 9, input from both T and A converges on an intermediate "control element" (e.g., Estes, 1976) that in tum activates the inhibitory association. The control element is assumed to function as an AND gate; complete activation of the inhibitory link thus depends on the joint presence of the CS (tone) and the extinction context (A). This model predicts that extinction performance to T will be specific to the extinction context and is consistent with a memory-retrieval analysis of extinction (Bouton, 1991, 1993). An extinguished CS is "inhibitable" in the sense that it has acquired an inhibitory link, as well as an excitatory link, with the representation of the US. The same associative structure may be embedded in what is learned during feature-negative training. Figure 10

presents a possible representation of the content of featurenegative learning. As before, the target (T) enters into both excitatory and inhibitory associations with the US, and the inhibitory association is also linked, by the control element, to its context (A). The new inhibitory feature (L), however, is encoded so that it may inhibit responding to T through activation of one of either of two links. In the first link, L may suppress activation of the US representation directly (e.g., Rescorla & Holland, 1977). This link is suggested by the results of Experiment 1, where an inhibitory feature inhibited performance to a new CS that presumably had no inhibitory association because it had never been nonreinforced. Several theoretical approaches to conditioning have accepted this as the mechanism of action for conditioned inhibitory stimuli (e.g., Pearce & Hall, 1980; Rescorla & Holland, 1977; Wagner, 1981). The feature's second link is the new mechanism suggested by the results of Experiments 3 and 4. Here the feature inhibits responding to the target by activating the target's own inhibitory association with the US. This link operates through the control element, which still functions as an AND gate. After feature-negative training, target inhibition therefore depends on the joint presence of the feature, the target, and the context in which the target's inhibition was learned (A). Notice that the feature's effect through Link 1 does not depend on context; this would allow the feature's action to transfer partly between contexts (Experiment 4). However, the action of the inhibitor through the target's inhibitory association will depend on the target being in a context in which its own inhibition has been learned (Experiment 3). This is the important result obtained in the present experiments.

Figure 10. Model of a feature-negative discrimination. Tone target (T) conditioned stimulus has both excitatory and inhibitory associations with the unconditioned stimulus (US). Negative feature (L) inhibits responding to T through two mechanisms: (1) a direct inhibitory association with the US and (2) an excitatory link with T's own inhibitory association. As in Figure 9, T's inhibitory association is gated, in this case requiring input from three stimuli for activation: the context (A), T, and L.

CONTEXT AND INHIBITION

The feature's effect through Link 2 may be related to the hypothetical actions of negative occasion setters (e.g., Holland, 1985). Such stimuli are sometimes thought to act by suppressing the target's own excitatory association with the US (e.g., Holland, 1985; see also Rescorla, 1991). One implication of the present experiments is that negative occasion setters may instead activate the target's inhibitory association. Another implication is that both of the links shown in Figure 10 may be present following a single feature-negative discrimination procedure. It seems possible, however, that some procedures could encourage the encoding of one type of linkage over the other. For instance, serial presentation of feature and target (e.g., Holland, 1984), or a relatively weak or nonsalient feature (Holland, 1989a), may encourage the feature's effect on the target's inhibitory association (Link 2). However, the present data suggest that both mechanisms of action may be present to some degree in a single featurenegative discrimination. One problem with the scheme represented in Figure 10 is that it does not account for all the facts surrounding a negative feature's ability to transfer and control responding to new targets. To the extent that a negative feature can suppress the US representation directly (Link 1), then inhibition would transfer to any excitor associated with the same US (e.g., Rescorla & Holland, 1977). But transfer of the inhibition mediated by Link 2 would clearly be restricted. As Figure 10 suggests, the second link could not affect responding to an excitor that has no inhibitory association with the US. The fact that responding to targets that have never been nonreinforced are not affected by a negative occasion setters (Holland & Lamarre, 1984; Lamarre & Holland, 1987; Rescorla, 1985) may be consistent with this possibility. However, a negative occasion setter's ability to suppress performance does transfer to excitors that have been in a negative occasion setting relationship (Holland & Lamarre, 1984; Lamarre & Holland, 1987). The scheme represented in Figure 10 does not explain why transfer should ever occur with Link 2. The problem is significant, because transfer of Link 2 inhibition appears to have occurred in Experiment 4, where a feature inhibited a separately trained target when testing occurred in the context in which the target had been inhibited. The facts of transfer thus demand that a target's inhibitory association be activated by separately trained feature stimuli. One possible extension of the scheme presented in Figure 10 is illustrated in Figure 11. The figure models Experiment 4: Two negative features (L and K) have been trained in featurenegative discriminations with separate targets (T and N, respectively) in separate contexts (A and B, respectively). As in Figure 10, the features operate by two mechanisms. In one, they suppress the US representation directly, and in the other they activate the target's own context-specific inhibitory association with the US. The only new idea is that the features now first activate an intermediate unit (X) that in tum activates the target's inhibitory link. If the separate features converge on an intermediate unit, complete transfer of a feature's inhibition to a new target would be possible. Further, the model predicts that transfer will be most complete if the transfer target is encoded, as shown, with an inhibitory association resulting from feature-negative training (Holland &

63

Figure 11. Model of two feature-negative discriminations that allows the features to inhibit responding to new targets in the targets' home contexts (Experiment 4). A tone target (T) and light feature (L) have been trained in Context A (A); a noise target (N) and keylight feature (K) have been trained in Context B (B). (Solid and dashed lines merely separate the two discriminations.) The features (L and K) can inhibit responding to T and N through two mechanisms: (a) direct inhibitory associations with the unconditioned stimulus (US) and (b) excitatory links with an intermediate unit (X) that in tum excites the targets' inhibitory associations with the unconditioned stimulus (US). Each target's inhibitory association with the US is gated and requires input from the target, the intermediate unit, and the context for activation.

Lamarre, 1984; Lamarre & Holland, 1987). The intermediate unit may actually embody what Holland first set out to capture in his conditional memory system: The inhibitory associations of various targets are combined together by a unit that effectively labels them as inhibitable. Like Holland's system, the present one suggests that transfer will be most complete when the target has been inhibited in its own discrimination. However, unlike Holland's system, the present scheme further requires that the transfer target be tested in a context in which it has been inhibited before. That was the important new result of Experiment 4. Pearce (1987) proposed a generalization model that provides a fundamentally different account of many phenomena of feature-negative discrimination learning. In this model, the animal learns excitation or inhibition to the aggregation of cues present on any conditioning trial; performance to any new combination of CSs then depends on the amount of excitation and inhibition that generalizes from other conditioned stimuli. Contextual stimuli have no special status in the theory. Despite its success with some aspects of featurenegative discrimination learning, the Pearce model does not predict the present findings. In fact, our simulations suggest that it often predicts the opposite of the results we obtained. Consider Experiment 2. In that design (Table 1), T +, LTtraining in Context A would cause the learning of strong excitation to the compound AT and inhibition to the compound ALT. The treatment in Context B (T +, KT-) would yield excitation to BT and inhibition to BKT. Test performance to LT in Contexts A and B (ALT and BLT, respectively) would then depend on generalization from all of the

64

MARK E. BOUTON AND JAMES B. NELSON

conditioned stimuli. The degree of generalization is determined by the similarity between the conditioned and test stimuli. Because similarity in the model depends on the proportion of elements common to the conditioned and test stimuli, excitation learned to BT should generalize heavily to performance to BLT-lOO% of BT is in BLT. Generalization of inhibition from BKT is weaker, because only 67% of BKT is in BLT. The prediction for ALT is different. Excitation would again generalize heavily from AT to ALT, but here it is completely offset by inhibition conditioned directly to ALT. The model thus predicts greater responding to BLT than ALT during testing, a result that was not obtained in Experiment 2.1 Now consider the Pearce (1987) model's account of Experiment 3. In that experiment, L +, TL- training in A would cause strong excitation to AL and inhibition to ATL, whereas K +, TK- training in B should cause strong excitation to BK and inhibition to BTK. Once again, the test compounds were ATL and BTL. Excitation from AL should generalize heavily to ATL, but this would be offset by inhibition conditioned directly to ATL. In contrast, neither of the excitatory compounds (AL and BK) would generalize heavily enough to BTL (50% of each is in BTL) to offset the inhibition generalizing there from ATL and BTK (67% of each is in BTL). Consequently, Pearce's model does not predict the strong excitatory responding to BTL that was observed in Experiment 3. In fact, the model predicts stronger responding to BTL in Experiment 2 than in Experiment 3, the opposite of what we actually obtained. Simulations of Experiment 4 also produced predictions that reversed the actual outcomes: The model predicted strong excitation to the targets in the contexts in which they were also inhibited, and this excitation was expected to generalize most heavily to (and cause strongest responding to) the compounds containing targets in their inhibitable contexts. In actuality, however, those compounds elicited the weakest responding. The Pearce model does not predict the results of Experiments 2-4. To our knowledge, the present data are not anticipated by any existing interpretation of feature-negative discriminations. We know of no theory that predicts that target inhibitability, but not feature inhibition, will be specific to its context. The present experiments have produced new evidence that both the feature and the target may acquire inhibitory associations with the US in the feature-negative paradigm. Although the feature's ability to inhibit targets appears to transfer across contexts, the target's inhibitability does not. The results suggest that an inhibitory feature may sometimes inhibit performance by activating the target's own context-specific inhibitory association with the US.

1 We describe the percentage of elements in the training stimulus that were common to the trained and tested stimuli. The Pearce model also considers the percentage of elements in the test stimulus that were shared; these were incorporated in our simulations, but are left out of the discussion for simplicity.

References Bouton, M. E. (1991). Context and retrieval in extinction and in other examples of interference in simple associative learning. In L. Dachowski & c. F. Flaherty (Eds.), Current topics in animal learning: Brain, emotion, and cognition (pp. 25-53). Hillsdale, NJ: Eribaum. Bouton, M. E. (1993). Context, time, and memory retrieval in the interference paradigms of Pavlovian learning. Psychological Bulletin, 114, 80-99. Bouton, M. E., & Bolles, R. C. (1979). Contextual control of the extinction of conditioned fear. Learning and Motivation, 10, 445466. Bouton, M. E., & King, D. A. (1983). Contextual control of the extinction of conditioned fear: Tests for the associative value of the context. Journal of Experimental Psychology: Animal Behavior Processes, 9, 248-265. Bouton, M. E., & Peck, C. A. (1989). Context effects on conditioning, extinction, and reinstatement in an appetitive conditioning preparation. Animal Learning & Behavior, 17, 188-198. Bouton, M. E., & Swartzentruber, D. (1986). Analysis of the associative and occasion-setting properties of contexts participating in a Pavlovian discrimination. Journal of Experimental Psychology: Animal Behavior Processes, 12, 333-350. Estes, W. K. (1976). Structural aspects of associative models of memory. In C. N. Cofer (Ed.), The structure of human memory (pp. 31-53). New York: Freeman. Hall, G., & Channell, S. (1985). Differential effects of contextual change on latent inhibition and on the habituation of an orienting response. Journal of Experimental Psychology: Animal Behavior Processes, 11, 470-481. Hall, G., & Honey, R. C. (1989). Contextual effects in conditioning, latent inhibition, and habituation: Associative and retrieval functions of contextual cues. Journal of Experimental Psychology: Animal Behavior Processes, 15, 232-241. Hall, G., & Honey, R. C. (1990). Context-specific conditioning in the conditioned-emotional-response procedure. Journal of Experimental Psychology: Animal Behavior Processes, 16, 271278. Holland, P. C. (1984). Differential effects of reinforcement of an inhibitory feature after serial and simultaneous feature negative discrimination training. Journal of Experimental Psychology: Animal Behavior Processes, 10, 461--475. Holland, P. C. (1985). The nature of conditioned inhibition in serial and simultaneous feature negative discriminations. In R. R. Miller & N. E. Spear (Eds.), Information processing in animals: Conditioned inhibition (pp. 267-297). Hillsdale, NJ: Erlbaum. Holland, P. C. (I 989a). Occasion setting with simultaneous compounds in rats. Journal of Experimental Psychology: Animal Behavior Processes, 15, 183-193. Holland, P. C. (l989b). 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. (1992). Occasion setting in Pavlovian conditioning. In D. L. Medin (Ed.), The psychology of learning and motivation (Vol. 28, pp. 69-125). New York: Academic Press. Holland, P. c., & Lamarre, J. (1984). Transfer of inhibition after serial and simultaneous feature negative discrimination training. Learning and Motivation, 15, 219-243. Howell, D. C. (1987). Statistical methods for psychology. Boston: Duxbury Press.

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animals: Conditioned inhibition (pp. 299-326). Hillsdale, NJ: Erlbaum. Rescorla, R. A. (1991). Associative relations in instrumental learning: The eighteenth Bartlett memorial lecture. Quarterly Journal of Experimental Psychology, 43B, 1-23. Rescorla, R. A., & Holland, P. C. (1977). Associations in Pavlovian conditioned inhibition. Learning and Motivation, 8, 429-447. Rescorla, R. A., & Wagner, A. R. (1972). A theory of Pavlovian conditioning: Variations in the effectiveness of reinforcement and nonreinforcement. In A. H. Black & W. F. Prokasy (Eds.), Classical conditioning II: Current research and theory (pp. 64-99). New York: Appleton-Century-Crofts. Wagner, A. R. (1981). SOP: A model of automatic memory processing in animal behavior. In N. E. Spear & R. R. Miller (Eds.), Information processing in animals: Memory mechanisms (pp. 5-47). Hillsdale, NJ: Erlbaum. Wilson, P. N., & Pearce, J. M. (1990). Selective transfer of responding in conditional discriminations. Quarterly Journal of Experimental Psychology, 42B, 41-58.

Kaye, H., & Mackintosh, N. J. (1990). A change of context can enhance perfonnance of an aversive but not of an appetitive conditioned response. Quarterly Journal of Experimental Psychology, 42B, 113-134. Kaye, H., Preston, G., Szabo, L., Druiff, H., & Mackintosh, N. J. (1987). Context specificity of conditioning and latent inhibition: Evidence for a dissociation of latent inhibition and associative interference. Quarterly Journal of Experimental Psychology, 39B, 127-145. Konorski, J. (1967). Integrative activity of the brain. Chicago: University of Chicago Press. Lamarre, J., & Holland, P. C. (1987). Transfer of inhibition after serial feature negative discrimination training. Learning and Motivation, 18, 319-342. Pavlov, I. P. (1927). Conditioned reflexes. London: Oxford University Press. Pearce, J. M. (1987). A model for stimulus generalization in Pavlovian conditioning. Psychological Review, 94, 61-73. Pearce, J. M., & Hall, G. (1980). A model for Pavlovian learning: Variations in the effectiveness of conditioned but not of unconditioned stimuli. Psychological Review, 87, 532-552. Rescorla, R. A. (1973). Effect of US habituation following conditioning. Journal of Comparative and Physiological Psychology, 82, 137-143. Rescorla, R. A. (1985). Conditioned inhibition and facilitation. In R. R. Miller & N. E. Spear (Eds.), Information processing in

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