Second-order conditioning of Pavlovian conditioned inhibition

Second-order conditioning of Pavlovian conditioned inhibition

LEARNING AND MOTIVATION 7, 16l- 172 (1976) Second-Order Conditioning of Pavlovian Conditioned Inhibition ROBERT A. RESCORLA Yale University Two...

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LEARNING

AND

MOTIVATION

7,

16l- 172 (1976)

Second-Order Conditioning of Pavlovian Conditioned Inhibition ROBERT A. RESCORLA Yale

University

Two experiments are reported which investigated the conditioning of inhibition to a neutral stimulus as a result of its repeated pairing with a previously conditioned inhibitor. Both experiments employed a conditioned suppression technique with rat subjects. Experiment 1 detected the second-order inhibition through the retarded acquisition of concurrently administered excitatory conditioning. Experiment 2 found similar retardation in the acquisition of excitatory fear conditioning following previous second-order conditioning of inhibition to the stimulus. Implications are discussed for theories of the nature of inhibition and for second-order conditioning as an assessment technique.

The experiments reported here had two intentions. The first was to investigate an important feature of conditioned inhibition, its ability to function in the absence of concurrent excitation. The second was to explore the usefulness of second-order conditioning as an analytic tool in exposing that feature of inhibition. There has now accumulated substantial evidence that certain Pavlovian relations between a conditioned stimulus (CS) and an unconditioned stimulus (US) can result in that CS developing inhibitory power (e.g., Boakes & Halliday, 1972; Pavlov, 1927; Rescorla, 1969). Perhaps the most potent of such relations was originally described by Pavlov (1927) as the conditioned inhibition paradigm. In that procedure, one stimulus, A, is reinforced when presented singly but not reinforced when presented in conjunction with another stimulus, X. In such an A+/AXparadigm, X becomes capable of inhibiting the response normally observed to the singly presented A. This outcome has frequently been interpreted as evidence that X controls an inhibitory process parallel to the excitatory process controlled by A. Despite such results, there has remained one major source of skepticism about conditioned inhibition: In order to detect its presence, special procedures must be used. Simply presenting an inhibitory X singly This research was supported by National Science Foundation Grants GB-28703X and GB-38691X. Conversations with Donald Heth, Peter Holland, and Charles Zimmer-Hart greatly improved this research. Thanks are due to Sherri Bruno for technical assistance. Requests for reprints should be sent to Robert A. Rescorla, Department of Psychology, 2 Hillhouse Avenue, New Haven, Connecticut 06510. I61 Copyright 0 1976 by Academic Pre\\. Inc. All rIghI\ of reproducflon in any form revned

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generates little change in the organism’s behavior and, hence, little evidence for the presence of inhibition. To demonstrate that X is other than a neutral stimulus, one must arrange for it to occur in the context of some excitatory tendency, by using either a summation or a retardation acquisition test (cf. Rescorla, 1969). One may view this as a purely technical problem: Perhaps the separately presented inhibitor has an effect upon the organism, but that effect goes undetected by the investigator unless he arranges something for it to inhibit. In this view, separately presented inhibitors affect the organism just as do separately presented excitors, but the former effect demands additional conditions for its exhibition. An alternative interpretation is that inhibitors are not only undetected but actually nonfunctional when presented alone. Unless excitation is present, an organism may treat an inhibitor as indistinguishable from a neutral stimulus. In this view, unlike excitation, inhibition would require the presence of an antagonistic tendency even to be operative. An additional observation which has encouraged this view is that repeated separate presentation of an inhibitor in the absence of its reinforcing condition has little decremental effect upon its inhibitory power (ZimmerHart & Rescorla, 1974). Separately, presented inhibitors appear not to extinguish; in this regard, they behave like neutral stimuli. The distinction between these interpretations bears on the historically recurrent question of the comparability of excitation and inhibition (see Konorski, 1967). If conditioned inhibition is assumed to be simply the equivalent of conditioned excitation, but with an opposite sign, then one might anticipate that a separately presented inhibitor would produce some observable response. This expectation would also arise from a competing response view of inhibition. Consequently, such interpretations must account for the lack of effect produced by separately presented inhibitors in terms of our failure to arrange conditions for its detection. On the other hand, a view of inhibiton as a separate process with properties different from those of excitation would permit separately presented inhibitors to be inconsequential. This might accord with the threshold notion of inhibition suggested by Konorski (1948), in which inhibitors act solely to raise the threshold for the action of excitors but produce no other consequence of their own. In this view, the failure to see a response is not simply one of measurement: the inhibitor simply does not produce a response opposite to that generated by excitors. One approach to separating these views would be to search out other techniques for the detection of inhibition, techniques which do not require the concurrent presence of excitation. If such excitation-free techniques could reveal the effects of a separately presented inhibitor, the first interpretation would receive some support. One such technique, second-order conditioning, has been suggested by Rescorla and Holland

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(1975). They argue that the ability of a stimulus to serve as a reinforcer in second-order conditioning may be taken as one measure of its conditioned strength. They cite cases in which the conditioned excitatory strength of a stimulus is especially well revealed by its ability to condition excitation to a neutral stimulus. It is, then, of interest to ask whether the conditioned inhibitory strength of a stimulus could be similarly revealed. Even though a separately presented conditioned inhibitor produces no detectable change in performance, its effectiveness might be measured by its ability to second-order condition inhibition to antecedent stimuli. The experiments reported here address this possibility within the context of fear conditioning, as measured by conditioned suppression procedures. Experiment 1 is an initial investigation designed to be especially sensitive in detecting second-order conditioning of inhibition. Experiment 2 replicates those results in a somewhat less complex design. EXPERIMENT

1

This experiment elaborated upon the outline of a standard secondorder conditioning experiment. One group of experimental animals first received pretraining designed to make one stimulus a learned reinforcer; they then were subjected to pairings of a neutral stimulus with that learned reinforcer. Control animals were treated similarly, except that one or the other of those two stages was modified. For one control group, the original stimulus was not made a reinforcer, while for the other group the stimulus and reinforcer were presented during stage two but in an unpaired fashion. These three groups have been suggested elsewhere (Rizley & Rescorla, 1972) as the minimum set for demonstrating second-order conditioning. The present experiment deviated from typical second-order experiments in three respects. First, the pretraining was designed not to make the “reinforcer” excitatory but rather inhibitory. Second, to reveal the supposed second-order conditioning, the stimulus was not simply presented; rather an attempt was made to convert it into an excitor, after the manner of retardation tests (Rescorla, 1969). In the present case, the stimulus was paired with a first-order excitor and the test measured retardation of the development of second-order excitatory conditioning. Third, the retardation test was not conducted following the completion of inhibitory conditioning but rather was interwoven with it. Thus all animals received second-order conditioning of excitation on some trials; however, on other trials they received differing treatments, designed to differentially establish second-order conditioning of inhibition to that same stimulus. This tactic was employed because of the uncertainty surround-

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ing the optimum number of pairings for generating order conditioning of inhibition.

any possible second-

Method Subjects and apparatus. The subjects were 32 male Sprague-Dawley rats about 100 days old at the start of the experiment. They were maintained throughout the experiment at 80% of their normal body weight. The experimental chambers consisted of eight identical Skinner boxes, 22.9 x 20.3 x 20.3 cm. Each chamber had a recessed food magazine in the center of the end wall and a bar to the left of the magazine. The floor of the chamber was composed of 0.48 cm stainless steel rods, spaced 1.9 cm apart. This grid could be electrified through a relay-sequence scrambler from a high-voltage high-resistance shock source. The two end walls of the chambers were aluminum; the side walls and top were clear acrylic plastic. Each Skinner box was enclosed in a sound- and light-resistant shell. Mounted on the rear wall of this shell were a 61/2-W bulb and two speakers. The speakers permitted presentation of a 1,800HZ square wave tone and an 78-db white noise. Experimental events were controlled and recorded automatically by relay equipment located in an adjoining room. Procedure. In the first session, all rats were magazine trained automatically with food pellets (P. J. Noyes, 45 mg) delivered on a variable interval (VI) 1-min schedule. In addition, each bar press yielded a food pellet. This session continued until the animal had emitted about 50 bar presses. Starting with the second experimental day, all sessions were 2 hr long and all involved food reinforcement on a VI schedule. For the first 20 min ofDay 2 the schedule was VI 1 min; thereafter, it was VI 2 min. Simple VI training continued for five sessions. During the next two sessions, the animals received pretest exposure to the stimuli which would later be conditioned. On each day, each animal received four 30-set CS presentations, delivered independently of its bar pressing and without programmed consequence. Two presentations were a 2 set flashing on of the houselight, one an 1800-Hz tone and one a white noise. The next day began first-order conditioning, designed to establish the excitatory and inhibitory conditioned stimuli later to be used in second-order conditioning. This procedure, together with the subsequent phases of the experiment, is outlined in Table 1. On the first two days of first-order conditioning, each animal received two 30-set presentations of the tone, each terminating in a 0.5-n& OS-set footshock. These 2 days established substantial fear of the tone while minimizing conditioning of the background cues. Days 3 through 13 of this phase continued reinforced presentations of the tone, one on each day. Each animal also received three

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DESIGN OF EXPERIMENT 1 Group I-P 1-u N-P I-C

First-order conditioning T+, T+, T+, T+,

TNTNNTN-

Second-order conditioning L-+N, L/N, L+N, L+TN,

L+T L+T L+T L+O

nonreinforced trials per day. For three groups (I-P, I-U, and I-C) those trials consisted of a 30-set presentation of the tone and white noise in compound. As indicated by the first letter in each group label, that treatment was intended to endow the noise with inhibitory power. For group N-P, the three nonreinforced trials presented noise alone, a treatment designed to leave the noise relatively neutral. It should be noted that this group provides a conservative comparison, since there is some evidence that this discrimination treatment can endow the noise with some inhibitory power (Rescorla, 1969). However, there is both empirical and theoretical reason to expect that such a discriminated noise will be more neutral than is the noise given a T+/TN - treatment (Rescorla & LoLordo, 1965; Marchant & Moore, 1974; Weisman & Litner, 1969). This should be especially so when care has been taken to reduce the conditioning of excitation to background cues (Wagner & Rescorla, 1972). Following completion of this first-order training, a single pretest session was given with the light CS, to guarantee its continued neutrality. During this session, each animal received four 30-set presentations of the flashing light while engaging in bar pressing. On the next day, second-order conditioning began. On the first second-order day, Groups I-P and N-P received four paired presentations in which the 30-set light was followed immediately by the 30-set noise. Animals in Group I-U received unpaired presentations of four lights tind four noises. This day was intended to allow second-order conditioning of inhibition to begin prior to intermixing excitatory conditioning. On each of the next 5 days of second-order conditioning, these treatments continued except that only two trials of each type were given. Intermingled with those trials, all animals received two pairings of the light with the excitatory tone. On Days 3 and 5 they additionally received a single “refresher” trial on which the tone alone was followed by shock. Thus all animals received excitatory second-order conditioning of the light. The groups differed in their treatment on the other light trials which were either paired with an inhibitor (I-P), paired with a neutral stimulus (N-P), or unpaired with an inhibitor (I-U).

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During second-order conditioning, the treatment of Group I-C (inhibitory-compound) was identical to that of Group 1-P except for the following changes. Instead of two light-noise and two light -tone pairings, animals in Group I-C received two trials on which the light was followed by the tone-noise compound and two on which it was followed by nothing. Thus, both groups received the same number of light, tone, and noise presentations and the same number of pairings of light with tone and noise. They differed in whether the tone and noise were paired with the light together or separately. The reason for this group was to evaluate the degree to which the noise has a different inhibitory effect if presented in conjunction with the tone than when presented alone. The measure of conditioning used throughout was the amount of bar pressing produced during a CS presentation. In order to attenuate the effects of individual differences in overall rate of responding, the results are plotted in terms of a suppression ratio. This ratio has the form A/(A + B), where A is the rate of responding during the CS and B is the rate of responding in a comparable period prior to CS onset. Thus, a ratio of 0 indicates no responding during the CS (good excitatory conditioning), whereas one of 0.5 indicates similar rates of responding during the CS and pre-CS periods (little excitatory conditioning). Results

and Discussion

Figure 1 shows the course of first-order discrimination learning in the three groups receiving T+/TNand the one group receiving T+/Ntraining. Although discrimination proceded rapidly, the former discrimina-

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FIG. 1. First-order discrimination in Experiment 1. Groups I-P, I-U, and I-C received tone (T) reinforced and tone-noise (TN) nonreinforced. Group N- P received tone reinforced and noise (N) nonreinforced.

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tion was more difficult. That outcome is consonant with the expectation that the noise must acquire greater inhibition during TN- presentation in order to counteract the highly excitatory tone. Of more interest are the results of the light presentation during second-order conditioning, displayed in Fig. 2. Over days, the initially neutral light developed net excitatory strength as a result of being paired with the excitatory tone. But the rate and level of suppression to the light differed according to its treatment on other trials. Of the three groups comprising a standard second-order design, Group I-P developed the least suppression. Over the final two second-order conditioning days, it was reliably less suppressed than either Group I-U (U = 7, p < .Ol or Group N-P (U = 15, p < .05). Thus, intermingled pairings of the light with the inhibitory noise retarded the excitatory acquisition produced by light-tone pairings. This suggests that the separately presented inhibitory noise was active enough to condition its inhibition to the light in Group I-P. Group I-C, which received the light followed by the tone-noise compound on a 50% schedule, acquired its terminal level of suppression rapidly, but that level was reliably above those of the three other groups ( Us < 11, ps < .Ol). The rapidity of conditioning in Group I-C probably reflects the fact that on Day 1 of second-order conditioning it was the only group to receive pairings of the light with the tone. The higher asymptote of Group I-C compared with that of Group I-P suggests that the inhibitory noise was more effective in attenuating conditioning when its presentation occurred in conjunction with the excitatory tone. One interpretation is that the presence of the tone augmented the action of the inhibitor. It is also worth noting that the superior conditioning of Group I-U to Group I-C is

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FIG. 2. Second-order conditioning to the light resulting from light-tone pairings in Experiment 1. Group designations indicate the nature of intermixed treatment of the light.

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consistent with the view that second-order conditioning depends upon the magnitude of the response evoked by the first-order stimulus (Rescorla, 1973). Throughout second-order conditioning, presentations of neither the inhibitory (I-P and I-U) nor the relatively neutral (N-P) noise produced any systematic effects upon response rate. In both cases, the suppression ratios hovered around 0.5, confirming the observation that inhibitors and neutral stimuli are not differentiable on the basis of simple performance. This result also suggests that the present procedures were successful in minimizing any general excitatory conditioning of background cues. Had such conditioning been present, presentation of the inhibitory noise would have generated increases in response rate (see Hammond, 1966). That conclusion is also supported by the maintenance of stable base rates of responding throughout the experiment. The results of this experiment provide preliminary evidence that a separately presented inhibitor functions differently from a neutral stimulus. However, the design may have been unnecessarily complex; in particular, there may be no necessity to intermingle conditioning of excitation during the establishment of second-order conditioned inhibition. In fact, that intermingling permits interpretations other than one in terms of the effectiveness of a singly presented inhibitor. In Group I-P, the light received second-order excitatory conditioning prior to many of its pairings with the inhibitor. Therefore, it is possible that this excitation was critical to the activation of the inhibitor. That is, the previously conditioned excitation may leave a postlight excitatory aftereffect which enables the inhibitor to be active when presented in that period. Because of that aftereffect, the inhibitor may not have been presented in the absence of excitation. Experiment 2 attempts to avoid this alternative by separating the inhibitory and excitatory conditioning into two phases. EXPERIMENT

2

This experiment constitutes a second demonstration of second-order conditioning of inhibition. It differs from Experiment 1 primarily in completing the conditioning of inhibition prior to the institution of the retardation test. Method Subjects and apparatus. The subjects were 32 male Sprague-Dawley rats, about 90 days old at the start of the experiment. They were maintained throughout at 80% of their normal body weight. The apparatus was that of Experiment 1. Procedure. After initial bar press training identical to that of Experiment 1, all animals received pretest exposures of the three stimuli later to be conditioned. On each of 2 days, four 30-set CSs (two flashing lights, one

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1800-Hz tone and one 78-db white noise) were presented. These and all other sessions were 2 hr long and were administered with a VI 2-min schedule of food reinforced bar pressing in effect. On the next day, first-order conditioning was begun. On the first 2 first-order days, all animals received two 30-set tones, each terminating in a 0.5 mA, 0.5 set footshock. For the next 9 days, two groups of eight animals each (I-P and I-P) received two tone-reinforced and six tone-noise compounds ending without shock on each day. The remaining two groups (N-P and N-U) received the same treatment with six noise alone trials replacing the tone-noise trials. As before, these treatments were intended to make the noise inhibitory in the first two groups and leave it relatively neutral in the others, while giving all groups an excitatory tone. The next day involved a single pretest session in which all animals received four 30-set presentations of the flashing houselight to ensure its neutrality. On the next 5 days, an attempt was made to carry out second-order conditioning of inhibition. Each day, Groups I-P and N-P received four trials on which a 30-set light was followed by a 30-set noise. Groups N-U and I-U received four lights and four noises, explicitly unpaired. On the next day, refresher trials were given to ensure similar levels of fear conditioning to the tone. All animals received four 30-set presentations of the tone, each ending in footshock. Finally, the next 6 days constituted second-order conditioning of excitation to the light. On each day, all animals received four presentations of the light, a random half of which terminated in the presentation of the 30-set tone. The 50% schedule was used to generate a moderate rate of conditioning to facilitate observation of between group differences. On Days 3 and 5, a single tone-shock trial was additionally delivered to maintain first-order conditioning of the tone. The consequence of this sequence of treatments is that all groups received second-order conditioning of excitation to the light, but they differed in the prior treatment of that light. Group I-P had the light previously paired with an inhibitor, whereas GroupN-P had it paired with a neutral stimulus. Groups I-U andN-U received the light unpaired with either an inhibitor or neutral stimulus. Results and Discussion

First-order discrimination training proceded similarly to that of Experiment 1. The data of most interest are the rates of second-order conditioning of excitation, displayed in Fig. 3. Despite the use of partial reinforcement, all groups showed some evidence of excitatory secondorder conditioning. However, Group I-P, whose light had previously been paired with the inhibitory noise, showed substantially less suppression than did the three comparison groups (Us < 11, ps < .02).

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FIG. 3. Second-order Experiment

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conditioning to the light resulting from light-tone pairings designations indicate the nature of prior training to the light.

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This confirms the finding-of Experiment 1 that simple presentation of an inhibitor had enough effect to transfer inhibition to a preceding neutral stimulus. Throughout second-order excitatory conditioning, the tone consistently produced complete suppression in all groups. This suggests that the prior second-order inhibitory conditioning may not act through attenuation of the response to the first-order excitor. This would be consistent with parallel results from first-order inhibitors which indicate that they do not act by attenuating the response to the US (see Rescorla & Wagner, 1972). But it may also suggest that this inhibitory conditioning does not summate with first-order excitation. Of course, these considerations must be tempered because complete suppression may obscure evidence of summation. GENERAL DISCUSSION

Taken together, these experiments provide evidence for the phenomenon of second-order conditioned inhibition. That phenomenon suggests two important things about conditioned inhibitors. First, they have a substantial impact upon the organism even when presented alone. They do not seem to require that excitation be present to activate them. Second, they share with conditioned excitors the ability to serve as reinforcers, transferring their effects to antecedent stimuli. These observations encourage a view of conditioned inhibition as generally parallel to conditioned excitation. The success of some theories of conditioning (e.g., Konorski, 1948, 1967; Rescorla & Wagner, 1972) in providing a common framework for describing the acquisition of inhibition and excitation further argues for

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their commonality. However, one discordant result is the failure of Zimmer-Hart and Rescorla (1974) to find extinction of inhibition like the extinction of excitation which results from repeated separate presentation. Although they interpreted that failure in terms of the ineffectiveness of separately presented inhibitors, the present results make that interpretation seem less likely. Perhaps second-order conditioning is a more sensitive index of the operation of inhibition than is the degree of change induced by its nonreinforced presentation. In another context, Rescorla (1975) has argued for the plausibility of a threshold value which associative strengths must exceed in the course of undergoing associative changes. One possibility is that such a threshold does not equally apply to the ability of a stimulus to reinforce antecedent events. It should be noted that the present experiments have all employed only one test of inhibition, its ability to retard the acquisition of excitation. Moreover, the particular retardation test used here was unusual in its use of second-order excitatory conditioning. These procedures were employed in order to maximize the sensitivity to the presence of second-order conditioned inhibition. But it is not clear to what extent these procedures are required for the detection of this inhibition. For instance, some accounts of second-order conditioning would suggest that it is sufficiently distinct from first-order conditioning as to anticipate difficulty in transfer of inhibition from first- to second-order stimuli (Rescorla, 1973); but the present experiments provide no evidence on that point. We have argued elsewhere for the use of multiple assessment techniques for inhibition, in particular for the use of summation tests. Although that argument remains valid, it should be noted that the present experiments were designed to blunt an important criticism of retardation tests, their susceptability to latent inhibition. The present demonstration compares acquisition rates withgroups which were subjected tocomparableamounts of the preexposure responsible for latent inhibition. Moreover, there is available some evidence that second-order conditioning of inhibition can be detected in summation procedures. Pavlov (1927) reports an experiment by Volbarth in which an extinguished stimulus supposedly conditions its inhibition to an antecedent neutral stimulus, as later assessed by summation. Unfortunately, that experiment is difficult to interpret both because other evidence suggests that extinguished stimuli are not inhibitory (Rescorla, 1969) and because currently standard control procedures were omitted. A more substantial report is given by Lindberg (1933), who also used a salivary preparation, but who included useful controls. In addition to providing some evidence on the nature of inhibition, the present experiments also illustrate the general potential of second-order conditioning as an assessment device. Typical conditioning experiments measure the success of first-order conditioning by examining the ability of the CS to provoke a response. But in some cases, one suspects that this

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measure may be insensitive to the modifications in the organism produced by conditioning manipulations (e.g., Annau & Kamin, 1961). As we have suggested elsewhere (Rescorla & Holland, 1973, consequences of first-order conditioning which are not observed in the response to the first-order CS may nevertheless be observed in its ability to establish conditioning to second-order stimuli. In some cases, then, second-order conditioning may provide us with an important tool for understanding first-order conditioning processes. REFERENCES Annau, Z., & Kamin, L. J. The conditioned emotional response as a function of intensity of the US. Journal of Comparative and Physiological Psychology, 1961, 54, 56-58. Boakes, R. A., & Halliday. S. (Eds.) inhibition and learning. New York: Academic Press, 1972. Hammond, L. J. Increased responding to CS in differential CER. Psychonomic Science, 1966, 5, 337-338. Konorski, J. Conditioned reflexes and neuron organization. Cambridge: Cambridge University Press, 1948. Konorski, J. Integrative activity of the brain. Chicago: University of Chicago Press, 1%7. Lindberg, A. A. The formation of negative conditioned reflexes by coincidence in time with the process of differential inhibition. Journal of GeneralPsychology, 1933, 392-419. Marchant, H. G., III, & Moore, J. W. Below-zero conditioned inhibition of the rabbit’s nictitating membrane response. Journal of Experimental Psycho/ogy, 1974, 102, 350-352. Pavlov, I. P. Conditioned rejlexes. Oxford: Oxford University Press, 1927. Rescorla, R. A. Pavlovian conditioned inhibition. Psychological Bulletin, 1969, 72, 77-94. Rescorla, R. A. Second-order conditioning: Implications for theories of learning. In F. J. McGuigan &D. B. Lumsden (Eds.), Contemporary approaches to conditioning and learning. Washington: V. H. Winston, 1973. Rescorla, R. A. Some comments on a model of conditioning. Paper presented at a conference on theories of learning, University of Texas at Arlington, February 1975. Rescorla, R.A., & Holland, P. C. Some behavioral approaches to the study of learning. In E. Bennet and M. Rosenzweig, (Eds.), Neural mechanisms qf learning and memory. Cambridge, Mass.: MIT Press, 1975. Rescorla, R. A., & LoLordo, V. M. Inhibition of avoidance behavior. Journal qf Comparative and Physiological Psychology, 1965. 59, 406-412. Rescorla, R. A., & Wagner, A. R. A theory of Pavlovian conditioning: Variations in the effectiveness of reinforcement and nonreinforcement. In A. Black & W. F. Prokasy (Eds.), C’lassicul conditioning II. New York: Appleton-Century-Crofts, 1972. Rizley, R. C., & Rescorla, R. A. Associations in second-order conditioning and sensory preconditioning. Journal of Comparati,je and Physiological Psychology. 1972, 81, I-11. Wagner, A. R., & Rescorla, R. A. Inhibition in Pavlovian conditioning: Application of a theory. In R. A. Boakes & S. Halliday, (Eds.). inhibition and [earning. New York: Academic Press, 1972. Weisman, R. G., & Litner, J. S. The course of Pavlovian excitation and inhibition offear in rats. Journal qfcomparative and Physiological Psychology, 1969, 69, 667-672. Zimmer-Hart. C. L.. & Rescorla. R. A. Extinction of Pavlovian conditioned inhibition. Journal

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837-845.