Classical conditioning in animals.

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Ann. Rev. Psychol. 1978. 29:587-612 Copyright © 1978 by Annual Reviews Inc. All rights reserved

CLASSICAL CONDITIONING

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IN ANIMALSl A. Dickinson and N J. Mackintosh2 Laboratory of Experimental Psychology, University of Sussex, Brighton, BNl 9QG, England

The experimental study of learning by psychologists grew out of classical associa tionist theory. The value of this heritage is now often questioned by cognitive psychologists, psycholinguists, and others, who have doubted whether the ways in which children learn to talk or adults to solve problems are illuminated by being forced into this associationist framework. We may sympathize with these doubts, but they only make it more rather than less important to study those phenomena, such as conditioning in animals, which can plausibly be regarded as simple cases of associative learning. We cannot know how adequate associative principles are to the explanation of complex behavior until we have developed a satisfactory theory of associative learning. Recent developments in the study of conditioning suggest that such a theory will be much more complex than has hitherto been expected. In a conditioning experiment, the experimenter arranges a relationship between certain events in the subject's environment, and, we assume, the subject comes to associate those (or related) events together. Conditioning is, therefore, as simple a form of apparently associative learning as we are likely to find. It remains a mat ter of some doubt whether traditional theories of association are adequate to ex plain it. In the standard procedure for excitatory classical conditioning, a CS is paired with a reinforcer or a US and the experimenter records an increase in the probability or amplitude of a discrete CR. Given a behavioristic perspective, the natural inter pretation of an experiment such as Pavlov's

(129)

was to say that the dog learned

to salivate at the sound of the bell. But there are other conditioning procedures where no such discrete, peripheral response is, or perhaps can be, recorded. In 'The following abbreviations are used: CS, conditioned stimulus; CR, conditioned response; US, unconditioned stimulus; UR, unconditioned response; CS, omission of CS; ITI, intertrial interval. 2The preparation of this chapter was supported by a grant from the United Kingdom Science Research Council. We are grateful to S. Revusky for his comments on an earlier draft. 587

0066-4308/78/0201-0587$01.00


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DICKINSON & MACKINTOSH

conditioned suppression, for example, the course of conditioning is measured as a decline in the rate of lever pressing or licking. In taste-aversion conditioning, the experimenter records a decline in the consumption of a substance associated with illness. The apparently natural interpretation of Pavlov's experiment is, in fact, highly restrictive and applicable only with difficulty to many conditioning procedures. We shall assume that subjects may associate a variety of events in a conditioning experiment, and that the change in behavior recorded by the experimenter is simply a convenient index of the formation of this association (149). This view implies a sharp distinction between learning and performance, and we shall have little to say on the important question of how an associatiol1l between, let us suppose, representa tions of a CS and US actually produces the change in behavior recorded by the experimenter. It does, however, suggest that the associative process studied in conditioning experiments is one of greater scope and generality than is sometimes implied. This process is responsible for organisms learning about relationships between events, enabling them to build up an associative representation of the causal structure of their environment ( 193). CONTROLLING CONTINGENCIES

Operationally we define a classical conditioning experiment as one in which the experimenter arranges some relationship between a stimulus and a reinforcer (a stimulus-reinforcer contingency) regardless of the subject's own behavior. The im plied contrast is with an instrumental conditioning experiment, in which the occur rence of the reinforcer depends upon the performance of a particular response by the subject (a response-reinforcer contingency). The purely operational nature of this distinction continues to lead many to doubt whether it can reflect any funda mental difference between the processes of classical and instrumental conditioning (61). Such doubts are strengthened by the realization that, since the CR will occur in close temporal contiguity with the US, any operationally defined classical experi ment may contain an implicit response-reinforcer contingency in addition to the explicit stimulus-reinforcer contingency (131). There remains, nevertheless, an important theoretical question. Granted that an operationally defined classical experiment contains implicit instrumental contingen cies, we can still ask whether the change in behavior that we record is a consequence of the explicit classical stimulus-reinforcer contingency or of the implicit instrumen tal response-reinforcer contingency. Only in the former case should we say that classical conditioning had occurred. How can the question be answered? A partial solution, first suggested by Sheffield (179), is to add an explicit instrumental contin gency whose effects (perhaps because they should act in an opposite direction) will be readily discernible from those of the classical contingency. If, for example, Pavlov's dogs salivated because they learned that salivation improves the taste of dry food (an instrumental, response-reinforcer relationship), the implication is that they could equally well have learned not to salivate if this had been the only way of obtaining food. An omission contingency would be added to the classical contin-


CLASSICAL CONDITIONING IN ANIMALS

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gency, food being delivered if and only if the dogs refrained from salivating when the bell was sounded. Sheffield found that one of two dogs continued to salivate for several hundred trials when salivation cost him food in this way, and subsequent work (65) has confirmed that an omission schedule will maintain a much higher rate of salivation than will an extinction schedule, albeit a lower rate of responding than the original classical schedule. Several studies have shown that pigeons will continue to peck a key light previously associated with food although such pecks now cause the omission of food (100, 222). Rats may continue to make contact with a lever originally paired with the delivery of food, even when such contact causes the omission of food (186), although in other experiments an omission schedule has immediately suppressed such responses (95). More striking evidence of the suffi ciency of a stimulus-reinforcer contingency for the establishment of conditioning is that animals trained on an omission schedule from the outset will show reliable acquisition of a CR [key pecking by pigeons (172, 221); jaw movement by rabbits (54); licking by rats (128)]. Appropriate comparisons with animals receiving the same overall frequency of reinforcement in the absence of any response contingency (172) suggest that an omission schedule may have some suppressive effect on the probability of respond ing. At most, of course, this would imply only that the response in question could be modified by its consequences, not that the instrumental contingencies supposedly implicit in an ordinary classical schedule are in fact responsible for the acquisition of the response. Moreover, it is always possible that the addition of an omission contingency will actually .TIodify the stimulus-reinforcer contingency in crucial ways (118). If the illumination of a pigeon's response key signals the delivery of food, but only provided that the pigeon does not peck the illuminated key, then the sight of the key just prior to a peck becomes a reliable signal for the absence of food. The argument seems particularly apt in the case where the omission contingency is applied not just to key pecks but to approach toward the key (100, 216). It can obviously explain why key pecking is eliminated when approach to within an inch or two causes a change in key color as well as the omission of food (100). Even without such an explicit negative stimulus, if a distant view of the key signals the availability of food while a closer view signals its omission, to the extent that these two sets of stimuli are discriminably different, the stimulus-reinforcer contingencies inherent in the situation are sufficient to explain why the omission contingency should suppress responding. What is surely remarkable is that even when approach to the key does cause the omission of food, key pecking can still be reliably estab lished (62). If an omission contingency is added to a classical contingency between a CS and aversive US, the performance of the CR emibles the subject to avoid the US. The comparison between classical and avoidance schedules was already being made 40 years ago with a view to determining whether CRs were modified by their conse quences (167). It now seems clear that dogs may learn to flex their leg less reliably on a classical schedule than i, differences in the form and latency of the classical and avoidance CRs, as well as other results pointing to the relative imperviousness of a well established classical


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flexion CR to an instrumental contingency (183), strongly imply that both classical and instrumental contingencies are capable of producing flexion CRs. Experiments on the nictitating membrane CR of rabbits (28, 53), on the other hand, suggest that this response is affected almost exclusively by classical contingencies. The question whether a response is modified by its consequences is clearly amena ble to experimental analysis, even if interpretation of the data is often difficult. In some cases, stimulus-reinforcer contingencies alone appear sufficient to modify be havior; other responses are apparently more readily modified by their consequences. In the absence of explicit experimental analysis, we cannot always be certain whether a given response is affected by classical or instrumental contingencies. We can, however, be certain that the distinction jis not merely an operational one. VARIETIES OF CONDITIONING

As we have suggested, one reason for stressing the importance of classical condition ing is the realization that his not only local reflexes such as salivation or leg flexion that are modified by stimulus-reinforcer contingencies. These response systems are representative of but one category of CRs, what Konorski (82) called consummatory CRs. Other consummatory CRs are jaw movement (54) or licking (128) to water as the US, and directed pecking by pigeons to food or water USs (73), as well as defensive eRs such as blinking or nictitating membrane closure to an aversive US (28, 53). Konorski proposed a second category of CR, preparatory CRs, characterized by the fact that they were conditionable to ess of longer duration (such as contextual, apparatus cues) and did not necessarily produce discrete, reflex responses. The concept of a preparatory CR is similar to that proposed by two-factor theorists who have talked of the conditioning of central incentive or motivational states (31, 59, 152). The best examples are the conditioned suppression of appetitively motivate:d instrumental responding, and the conditioned ,acceleration of aversively motivated instrumental responding, by a stimulus associated with an aversive US (I5, 93). Two-factor theory assumes that fear conditiom:d to such a es somehow modulates the motivational state maintaining responding. It is possible, however, that changes in the baseline instrumental response result from some interaction with overt CRs conditioned to the es (199); for example, a es associated with shock by no means always produces an acceleration of avoidance re:sponding (8, 173). There are, never theless, data which render any interpretation in terms of peripheral response compe tition extremely implausible for all cases (37, 126). The search continues for appetitive analogues of the conditioned acceleration of avoidance responding. Will the presentation of a CS associated with food affect appetitively motivated instrumental responding? If the baseline response is lever pressing by rats or monkeys or panel pressing by dogs, the normal outcome is that an appetitive es suppresses rather than enhances rate of responding (4, 80, 184). It is hard to see why this should not be regarded as a motivational effect even if not that predicted by some versions of two-factor theory (152). The only reliable in stance of such a CS enhancing responding is when pigeons are trained to peck a



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response key, and the CS is the illumination of that key (96, 170). This is clearly a case of peripheral response interaction, since such a CS will directly elicit pecking as a CR, and these classical pecks will summate with the baseline pecks; when the CS is a change in the color of a different key, it suppresses rather than enhances the rate of baseline pecking, because the pigeon now directs pecks at the second key when the CS is turned on (170). An apparently quite distinct possibility is that hunger or thirst themselves could be conditioned to a CS associated with an increase in such drives (29). A CS paired with an artificially induced abrupt increase in thirst may indeed elicit an increase in drinking (177), a finding suggesting that earlier failures were due to the slow change in drive produced by normal deprivation procedures. But since equally abrupt changes in thirst induced by the injection of natural precursors of thirst do not support conditioning (43), it is possible that successful conditioning is a conse quence of the aversive state generated by nonphysiological injections being amelio rated by the instrumental act of drinking. A recent attempt to condition hunger (114) reported at best very small effects, and these only when the CS, a novel flavor, was one more naturally associated with internal regulatory states than the sort of CS routinely used in previous studies (e.g. a black goal box).

Conditioning of Directed Movements Two phenomena that have received intensive study during the past decade, auto shaping and taste-aversion conditioning, have, however, done most to reshape our current views about the domain of classical conditioning. The discovery that pigeons would peck an illuminated response key in the absense of an explicit response reinforcer contingency (24), and that such key pecking was controlled by stimulus reinforcer contingencies (13, 46), demanded substantial rethinking about the status of this supposedly operant response (64, 118, 171). But it is perhaps of even greater long-term significance that general approach and withdrawal behavior may be con ditioned by purely classical contingencies (25, 63, 210). Pigeons will not only peck at a localized visual stimulus paired with the delivery of food; they will also ap proach such a stimulus even though it takes them away from the food magazine itself (64). Although it is obviously adaptive to blink one's eye or flex one's leg in anticipation of a puff of air to the eye or a shock to the foot, one might still question the general significance of a process that operated only on such special occasions. But since it must be to an animal's general advantage to approach (and investigate and manipulate) stimuli associated with appetitive reinforcers and to withdraw from stimuli or places associated with danger (113, 174), it is now clear that classical conditioning plays an unexpectedly important role in modifying an animal's behav ior. From the experimental psychologist's point of view, moreover, it is not only key pecking whose operant status is called into question: we must accept that when a rat is trained to run down an alley for food in the goal box, to solve a discrimination by approaching the positive and avoiding the negative stimulus, or to run away from the compartment of a shuttle box in which shock is delivered, the changes in behavior we observe may be largely a consequence of the classical contingencies inherent in these situations rather than of the explicit instrumental contingencies (14).

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Taste-Aversion Conditioning Taste-aversion conditioning, a phenomenon long neglected in spite of clear signs of its potential significance (47, 48, 165), but now rapidly assuming the status of a bandwagon [a recent bibliography (158) lists 403 articles], is presumably one exam ple of the role of conditioning in the control of general regulatory functions. A rat made sick by such treatments as X-irradiation (49, 154), injection of lithium chloride (49, 121) or apomorphine (161), or by prolonged rotation (21, 55) will subsequently show a marked aversion to any recently ingested, relatively novel substance. The fact that aversions are readily formed to substances which the subject has tast�:d without ingesting and in the absence of any consummatory response makes it clear that the classical stimulus-reinforcer contingency is sufficient to produce condition ing (19, 38, 40). Two characteristics of taste-aversion conditioning have excited substantial con troversy: conditioning apparently occurs in spilte of an interval between CS and US of several hours (154, 182); and conditioning occurs selectively to certain classes of stimuli (tastes and odors) rather than to others such as the visual characteristics of food or water or the place in which it is eaten or drunk (41, 49, 50). This second characteristic, it should be noted, is not all-or-none, since rats will show an aversion to a place or food container associated with sickness (12, 117, 157), while birds (220), guinea pigs (20), and monkeys (74) show aversions to the visual characteris tics of water associated with sickness. Nor is there reason to believe that this characteristic is exclusive to taste-aversion conditioning. There are now other exam ples of certain classes of stimuli being apparently more readily associated with certain classes of reinforcer than with others (44, 194). It is even possible that rats associate tastes rather than exteroceptive stimuli with later sickness in part because of the intrinsic properties of gustatory stimuli, such as their slow onset and longev ity, rather than any particular associability of tastes and sickness. Although tastes are less readily associated with immediate shock than are exteroceptive cues, a delay of 3.5 min between CS and US results in stronger conditioning to the taste than to the exteroceptive stimulus (86). Nevertheless, it is hard to believe that this is a sufficient explanation of conditioning between tastes and sickness over an interval of several hours. In spite of these possibly u�haracteristics, there are numerous parallels between taste aversion conditionmg and other more conventional paradigms. The strength of the aversion to a taste CS depends upon the intensity and other charac teristics of the CS (11, 17, 123) and on the intensity of the US (2, 55, 154). Although conditioning occurs over reliably longer intervals than those effective in other para digms, it still varies inversely with the length of the interval separating CS and US (2, 55, 154, 182). An aversion produced by pairing a particular substance with sickness does not generalize completely to substances with different tastes (39, 123, 161). These last two findings can only mean that the aversion is a consequence of the association between CS and US. In fact, throughout this review we shall see that most of the more interesting phenomena obsc!rved in conventional conditioning paradigms have their counterparts in taste-aven.ion conditioning. This suggests that


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a certain caution is in order before accepting the claim that taste-aversion condition ing represents a unique adaptive specialization of conditioning (162), let alone the more extravagant assumption that there are no general laws of learning (94, 176).


CATEGORIES OF CONDITIONING

Conditioned stimuli may be categorized along two orthogonal dimensions: the relationship between CS and US, and the affective value of the US. Variations along these dimensions produce CSs with apparently opposing behavioral effects.

CS-US Relationship EXCITATORY CSs An excitatory CS is one that becomes capable of eliciting the conditioned response of interest as a result of some associative process. The tradi tional procedure for establishing an excitatory CS is to arrange temporal pairing of the CS and some effective US with the onset of the CS preceding that of the US (103). It is now clear, however, that temporal conjunction is not a sufficient condition for excitatory conditioning, and contemporary views emphasize the role of concepts such as "predictability" (77), "validity" (200), and "contingency" (51, 138). The insufficiency of simple temporal contiguity is demonstrated by experiments showing that when the number of contiguous pairings is kept constant, the level of excitatory conditioning systematically decreases as the number of USs presented at random times during the inter-CS intervals increases (45, 139). Such a result would not be expected if excitatory conditioning depended solely upon close temporal pairings of the CS and US. If the USs are distributed randomly during the inter-CS interval, an increase in their frequency should result in a larger number of USs being presented in close temporal proximity to the CS. The dependence of excitatory conditioning upon the rate of US presentation in the absence of the CS is often interpreted as showing that a necessary requirement for excitatory conditioning is the presence of a positive contingency or correlation between the CS and US. A positive contingency holds when the probability that the US will occur in close temporal conjunction with the CS [P(US/CS)] is greater than the probability that the US will occur temporally remote from the CS [P(US/CS)]. One implication of such a view is that no conditioning should occur if P(US/CS) P(US/CS), irrespective of the number of pairings involved in such a schedule. Indeed, it has been suggested that this schedule represents the appropriate control procedure for nonassociative effects (135, 138), and many studies have found no conditioning in animals exposed to this so-called "truly random control" condition (46, 137, 214). These studies have typically used relatively low frequencies of CS and US presentation, however, thereby minimizing the likelihood of chance pairings. When the overall probability of CS and US presentation is specifically manipulated, the level of conditioning increases with presentation rate (87). This is not necessarily at variance with the contingency view of conditioning, since even at the higher presentation rates the level of conditioning is not related to the total number of CS-US pairings (9). Furthermore, it is clear that animals cannot necessarily estimate the overall correlation or contingency from exposure to a limited sample of the truly =


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random schedule. If P(US/CS) > P(US/CS) during initial exposure, excitatory condi tioning would be expected even on the contingency view. In accordance with this prediction, an initial positive contingency may be necessary for excitatory condition ing (9), and degrading this initial contingency by systematically omitting or delaying chance CS-US pairings until the animals have: experienced unpaired presentations of the US will reduce the level of conditioning (3). Although the contingency vie:w is compatible with conditioning during limited exposure to a random schedule, sw:h conditioning should be abolished by prolonged exposure. Again the prediction is confirmed (79, 145). Moreover, the events responsible for this decline in condition ing with prolonged exposure are just those that bring the animal in contact with the full contingency, namely the conjoint experience of unpaired CSs and USs (79). The fact that extended exposure to a random schedule fails to endow a CS with excitatory properties in spite of innumerable CS-US pairings does not mean that animals learn nothing about the CS-US rela.tionship. Several experiments have found that prior exposure to a random schedule may retard subsequent excitatory conditioning (7, 46, 102, 180). Although such retardation probably does not reflect the presence of inhibitory properties [(87); see also below], it is unclear to what extent it represents learning about the contingency between the CS and US rather than the addition of CS and US pre-exposure effects. Even if animals learn some thing during exposure to a random schedule, such learning leaves the CS neutral in its capacity to elicit the target conditioned response. INHIBITORY CSs Rescorla (142) and Hearst (60) have suggested that a CS should pass two tests in order to be regarded as having a true inhibitory effect. The first is the summation or combined-cue test. This involves demonstrating that when the putative inhibitor is combined with either a conditioned or unconditioned excitor for the particular target response, the resulting compound maintains a lower re sponse strength than either the excitor alone or a compound of the excitor and some control stimulus. This test captures the central idea underlying the concept of a behavioral inhibitor, namely that it controls a response tendency opposite to that of an excitor. The second test, the so-called retardation or resistance-to-reinforce ment procedure, involves showing that the supposed inhibitor requires a greater number of reinforcements than some appropriate control stimulus to develop excita tory properties. By these criteria, a number of schedules can· produce an inhibitory CS. In a conditioned inhibition procedure: the CS is either compounded with, or presented immediately before or after, a nonreinforced excitor. Inhibitory CSs established by this procedure have been found to be capable of exerting an inhibitory action on conditioned motivational states (15 I, 214), discrete skeletal responses (112) and taste aversions (10, 189). Differential conditioning, in which nonreinforced CSs are intermixed with reinforced presentations of an excitor, also endows the CS with the capacity to inhibit conditioned motivational sltates (151, 214), autoshaping (217), and taste aversion (10). It appears that for differential conditioning to be successful as an inhibitory procedure, the CS should not initially evoke any CR by generaliza tion from the excitor (84, 195). Finally, inhibiitory properties have been reported following backward conditionirig, in which the CS is associated with the termination



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of the US, both for conditioned motivational states (32, 120) and for discrete skeletal responses (180). There is some evidence that the development of an inhibitory CS by this procedure requires extended training (66, 181). The very diversity of these procedures makes it difficult to specify the critical features for generating conditioned inhibition. We saw that a necessary condition for excitatory conditioning was that P(US/CS) > P(US/CS). It would be gratifying if a necessary condition for the generation of inhibition as an opposing response tendency was the opposite contingency, namely P(US/CS) > P(US/CS). This con tingency does in fact hold in all the inhibitory procedures considered, and more direct evidence of its importance comes from studies showing that both aversive (151, 214) and appetitive inhibitors (25, 46, 63, 212, 226) can be produced by an explicitly unpaired procedure in which USs are simply programmed to occur during the inter-CS interval. Indeed, when P(US/CS) 0, the inhibitory properties of the CS systematically increase with P(US/CS), in spite of the increased likelihood that a US will then occur in chance temporal conjunction with the es (141). Just as in excitatory conditioning, however, the temporal relationship between CS and US also appears to play a role in inhibitory conditioning. For instance, inhibi tory conditioning in both the differential (215) and backward procedures (120) increases as the minimum time from a CS presentation to the next US presentation is lengthened. This has led to the idea that a CS which predicts a period free from USs acquires inhibitory properties. However, an alternative interpretation is that a long US-free interval ensures that no US occurs during traces of the es, thereby enhancing the effective negative contingency. A further role for temporal contiguity is suggested by studies of backward conditioning which have shown that presenting the CS in close temporal association with the prior US enhances inhibitory condi tioning even when the overall contingency and the interval to the next US are controlled (110, 134). In summary, it appears that excitatory and inhibitory conditioning depend both on the temporal relationship and on the contingency between es and US. The explanation of this dependency, however, may require a relatively sophisticated theory of conditioning. (See section on Mechanisms of Association below.) =

Motivational Significance of the US Appetitive conditioning results from using a US which should be capable of acting as a reinforcer to increase the probability of an instrumental response. Aversive conditioning, by contrast, employs a US whose omission should increase the proba bility of an instrumental response. The reason for distinguishing between these classes of conditioning is that conditioned appetitive and aversive excitors appear to exert inhibitory effects on each other in a manner that parallels the relationship between conditioned excitors and inhibitors (37). It is well known that aversive ess will inhibit appetitively motivated behavior in a summation test (173); similarly, pairing a CS with water will retard subsequent aversive conditioning to that CS (166). In a symmetrical fashion, an appetitive CS will suppress aversively motivated behavior (26, 56), although the only evidence that appetitive training retards subsequent aversive conditioning rests with Konorski & Szwejkowska's classic study (85). However, further evidence for this inhibitory


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interaction comes from a counterconditioning experiment showing that making an aversive US, a shock, a CS for an appetitive US, food, decreased the aversive properties of the shock (130). The relationship between conditioned inhibitors and between excitors and inhibi tors of contrasted affective value has not really been explored in terms of the summation and retardation tests. There is evidence, however, that conditioned excitors and inhibitors of opposite affective polalrity have similar capacities as instru mental reinforcers (30, 71, 119, 140, 213). ELEMENTS OF ASSOCIATION

Classical excitatory conditioning often results in the development of a CR which bears strong similarities to the UR. This suggests that the contingency between the CS and the elicitation of the UR produces a direct associative link between an internal representation of the CS and the UR-generating mechanism (an S-R associ ation). There are, however, two problems with this account. First, several studies have shown that associative learning can occur in the absence of any obvious similarity between the emergent CR and the UR elicited during training. Secondly, there is reason to believe that some internal representation of the US itself is associated with the CS, since postconditioning changes in the potency of the US

affect the CR.

Conditioning in the Absence of the Appropriate UR One simple way of showing that the presence of a contingency between the CS and a UR related to the observed CR is not necessary for conditioning is to block the appropriate UR during conditioning. A whole series of studies has shown that blocking peripheral URs with pharmacological agents during aversive conditioning does not prevent the occurrence of the CR following removal of the blocking agent (103). Similar results occur in appetitive conditioning (225), and the same conclu sion is implied by the finding that blocking physical but not visual access to the food magazine in experiments on autoshaping in pigeons does not prevent associative learning (25, 226). A simple S-R interpretation is also unable to explain how conditioning could occur in the absence of any obvious relationship between the form of the CR and UR. For example, pairing key illumination with direct water injection into the mandibles of pigeons can result in behavior din:cted at the key which was not seen in response to the water-injection US (223). Similarly, key pecking by chicks may be conditioned with the illumination of a heat source as the US even though the UR does not contain any pecking-like components (208, 211). Finally, in experiments on sensory preoonditioning, associative learning occurs in the absence of any obvious overt UR during l:raining. Joint presentation of two neutral stimuli, CS, and CS2, followed by the association of CSz with an effective US results in the capacity of CS1 to elicit a CR related to the US. This presents difficulties for an S-R thcory,because the target CS, CS" is never directly associated with the occurrence of the UR. Contemporary experiments have shown good sen-


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sory preconditioning in both conditioned suppression ( 1 60) and taste-aversion learn ing (91 ).


Postconditioning Changes in the Significance of the US A series of studies by Rescorla and his colleagues has shown that operations that apparently change the potency or value of the US following conditioning can alter the magnitude of the CR observed on test. Devaluation of the US either by habitua tion following aversive conditioning ( 1 46) or by associating a food-US with an aversive event following appetitive conditioning (69) reduced the magnitude of the CR subsequently elicited by the CS. In contrast, postconditioning exposure to a stronger US than that employed in conditioning was found both to enhance a simple aversive CR ( 1 47) and to reinstate a previously extinguished response ( 1 50). Again, these studies provide obvious difficulties for a simple S-R notion and suggest that some representation of the US itself is involved in the associative structure mediating the CR (an S-S association). This may not, however, be the universal rule. There is evidence that postconditioning exposure to the US can sometimes leave the original CR relatively intact (58, 70, 1 59), and Rescorla ( 1 60) has argued that this is the normal result after second-order conditioning. Second order conditioning is established by first pairing CSt with an effective US, and then pairing another neutral stimulus, CS2, with CSt in the absence of the US. Reliable second-order conditioning has been demonstrated in conditioned suppression ( 1 60), taste-aversion conditioning ( 1 8), and in general activity conditioning in rats (69) and autoshaping in pigeons (92, 1 36) with food as the original US. In some of these experiments, modifications of the value of CSt or of the original US after second order conditioning has little effect on responding to CS2 (69, 146, 1 47, 160), although in others a reliable effect has been demonstrated (96, 1 36). Rescorla has interpreted his own data to mean that the association mediating second-order conditioning is best characterized as one between an internal representation of CS2 and the response elicited by CSt on the higher-order conditioning trials. Even in this case, however, the response associated with CS2 cannot be the peripheral CR elicited by CSt; there are reliable demonstrations of second-order conditioning in which the actual CRs controlled by CSt and CS2 have a very different form (68, 96). Any conception of the first-order response that becomes associated with CS2 must be central in nature and refer to something like the general affective state aroused by CSt. Once we allow that the responses associated with the CS are central events, the distinction between the S-S and S-R positions becomes blurred. Given the obscurity of this distinction, it is probably best to characterize classical conditioning as the formation of an association between some internal representation of the CS and a representation of information about events related to, as well as properties of, the US. This characterization implicitly recognizes that the nature of the US-related information varies with different conditioning procedures and parameters. The fact that conditioning with one US can modulate conditioning with a different US of either similar (5, 1 50) or contrasted affective value (35) implies that in certain cases some of the information activated by the CS refers only to the general affective value

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of the US. On the other hand, the observation that in some experiments on condi tioning of consummatory CRs, the CR typically reflects properties specific to a given US, such as its locus of application and sensory qualities, suggests that the CS may be associated with a representation containing detailed information about the actual US employed (127). Annu. Rev. Psychol. 1978.29:587-612. Downloaded from www.annualreviews.org Access provided by University of Texas Southwestern Medical Center on 01/24/15. For personal use only.

CS as a Determinant of CR Form The general similarity of the form of CR and UR observed in classical conditioning is often explained by the principle of "stimulus substitution." However, the diver gence in form noted above and the variation in susceptibility of the CR to postcondi tioning changes in the value of the US suggest that a strict principle of stimulus substitution is untenable. Several authors have suggested alternative characteriza tions of a classical CS as, for example, a "learned releaser" (223), an "objc::ct substitute" (64), or an "incentive stimulus" (89). Stimulus-substitution theory, how ever, can deal with some divergences between CR and UR if we accept that the CS can substitute only for the actual information about the US that is encoded in some internal representation of that event, and that this information varies with the conditioning procedure. Even with this modification, however, stimulus-substitution theory will not be able to account for all variations in the form of the CR, since it is clear that some of this variation is attributable to the nature of the CS. The nature of the CR elicited in rats by a CS associated with food depends on whether the CS is visual or auditory (68), a block of wood or another rat (196). If the illumination of a pigeon key is paired with an appetitive US, the form of at least some components of the CR will usually reflect that of the UR (73). However, presenting the same US paired with an auditory stimulus or in the context of general nonlocalized background stimuli does not lead to responding obviously resembling the UR (13, 169, 185). In many of these cases, however, although different CSs control different CRs when paired with the same US, there is good reason to believe that very similar associations are established regardless of the nature of the CS (16, 68, 92). Although contextual or background cues may be associated with the US without eliciting overt CRs, they may play an essential supporting role in the production of specific CRs to a discrete stimulus. In a striking example of this, the association of contextual cues with a water US in rabbits has been shown to underly the appear ance of a jaw movement response (the normal UR to water) to a novel tone nev.!r itself associated with the US (178). Two modifications, therefore, are required to stimulus-substitution theory: the information encoded in the representation of the US must be assumed to vary with the conditioning procedure, and the elicitation of certain CRs must depend upon properties of the CS.

The Associative Structure of Inhibitory Conditioning The mechanisms governing inhibitory conditioning appear to parallel those for excitatory conditioning in that a number of phenomena found for one category are also found in the other (148, 187). These parallels suggest that we should look for similar associative structures underlying the two categories of conditioning.



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In the previous section, we argued for the view that excitatory conditioning is mediated by the formation of an excitatory association between internal representa tions of events that are in fact related in the environment. A parallel account of inhibitory conditioning is that an association is formed between the internal repre sentations of the CS and of events related to the omission of the US. As such omission produces no specific source of exteroceptive stimulation, these events must be the animal's own responses to the omission. Frustration theory has long recog nized that omitting an appetitive US in the context of a relevant excitor produces a set of responses, such as withdrawal, that are antagonistic to those typically controlled by appetitive excitors (1, 175). Both rats and pigeons will withdraw and escape from CSs negatively correlated with an appetitive US (30, 63, 72, 210). In an analogous fashion, Denny (33) has argued that the omission of an aversive US leads to the conditioning of responses or states antagonistic to those maintained by an aversive excitor. Once again, however, these inhibitory responses cannot be interpreted solely in terms of their peripheral manifestations, and it is clear that the excitatory-inhibitory interactions seen in summation and retardation tests can represent more than com petition between peripheral responses. The evidence for this contention comes from two main sources. First, in the absence of a conditioned or unconditioned excitor, the potential properties of an inhibitor are neither manifest (6) nor changed by repeated exposure of the inhibitory CS (227). Secondly, inhibitory CSs have proper ties that go beyond simple attenuation of the CR. For instance, if a neutral stimulus is compounded with an inhibitory CS and the compound is then paired with a US, the level of excitatory conditioning to the neutral CS is enhanced (16, 144, 189). These properties are most easily reconciled with the idea that an inhibitory CS acts directly on the central mechanisms mediating the excitatory response. There are two views of the underlying associative structure involved in such an interaction, both originally developed by Konorski (81-83). According to the first, a direct inhibitory association is formed between an internal representations of the CS and of the US. Activating this association will then oppose the arousal of the US representation (81). The alternative view is that the associative connection is between the CS and some other central mechanism whose arousal leads to the inhibition of the US representation (82). The first assumes that the intrinsic nature of the associations brought about by conditioning are radically different in the excitatory and inhibitory procedures, while the second does not. MECHANISMS OF ASSOCIATION

Selective Association in Conditioning The traditional view of conditioning implies that subjects automatically associate appropriately paired events. Successful conditioning depends only on arranging adequate temporal contiguity between a CS of adequate sensory properties and a US of adequate reinforcing properties. We have already seen from experiments on contingencies that although some degree of temporal contiguity between CS and US may be necessary, it is not sufficient to ensure conditioning. But the traditional view will not be salvaged by adding contingency to contiguity as a necessary requirement.

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On the contrary, the role of contingencies in conditioning will probably be under stood only in the context of a more radical n:vision of traditional theory. The need for that revision is simply explained: a degree of contiguity and contingency between CS and US that is sufficient to ensure excellent conditioning when that CS is conditioned in isolation may result in little or no conditioning if other stimuli predict the occurrence of the US. The simplest case of so-called "overshadowing" was first observed by Pavlov (129). Reliable conditioning would occur to a weak CS pre sented on its own, but not if it was presented only in conjunction with a more salient CS from the same or another modality (106, 155). Differences in temporal relation ship (23, 219) or correlation with reinforcement (101, 200, 203, 209) also produce overshadowing. Thus a stimulus imperfectly correlated with reinforcement will be adequately conditioned only if no better predictor of reinforcement is available; little or no conditioning will occur to it if it is presented only in conjunction with other stimuli more highly correlated with reinforcement. Finally, in experiments on block ing (77, 112, 155), prior conditioning to one element of a compound CS may prevent conditioning to the other element. Conditioning, it seems, occurs selectively in favor of better predictors of reinforcement at the expense of worse predictors; different CSs may be said to compete for association with a given US. The general importance of selective association in conditioning can be seen by noting that a CS is never in fact presented in isolation, but always in a given context or experimental situation. If the elements of a compound CS may be said to compete for association with the US, then a single CS may have to compete with these contextual cues. Thus conditioning to contextual cues may have hitherto unsus pected effects on conditioning to the experimenter's nominal CS. This is particularly likely to happen in the random control procedure, since here the CS is less well correlated with the US than are the contextual cues. If a zero correlation between CS and US produces little or no conditioning to the CS in spite of occasional chance pairings of the two, this may not be because contingencies are the fundamental controlling variables in conditioning, but rather because the con textual cues themselves acquire associative stn!ngth and come to overshadow the CS (145, 153). There is good evidence that conditioning to contextual cues does occur during the course of exposure to the random control procedure (88, 122, 124), and that the contingencies likely either to enhance or to degrade such conditioning produce complementary changes in conditioning to the CS (42, 79). Even if the CS is perfectly correlated with the US, variations in the effective relationship between US and contextual cues may be expected to affect the course of conditioning. One way of varying this relationship would be via the intertrial interval (ITI): the shorter the ITI the higher the probability of reinforcement for a given amount of exposure to the contextual cues. Thus the finding that rate of conditioning is inversely related to ITI (52, 116, 132, 190) may reflect the fact that better conditioning to contextual cues at nonoptimal ITIs (113, 224) produces more overshadowing of the nominal CS. Overshadowing by contextual cues may also mediate some of the effects on conditioning of the temporal relationship between CS and US (155). As the interval between the onset of CS and US departs from some optimal value (which may vary



601

for different CRs), the level of conditioning declines (103). At the same time, consistent with an overshadowing analysis, conditioning to contextual cues in creases (125). It is also often assumed that excitatory conditioning depends upon forward CS-US pairings, in which the onset of the CS precedes that of the US, and, as we have seen, backward pairings in which US precedes CS can often produce inhibitory conditioning. There are exceptions to this, however, and several recent studies have reported reliable excitatory conditioning from backward pairings (66, 67,78,109). One reason why backward pairings do not always produce excitatory conditioning may be that the US is associated with contextual cues, which therefore overshadow the backward CS-US association (206). Finally, prior conditioning to contextual cues may significantly block condition ing to a CS subsequently introduced into that context. The deleterious effect of prior exposure to the US on subsequent conditioning (21, 70, 76, 115) may be partly a consequence of such blocking by contextual cues. Such an interpretation is sup ported by several lines of evidence: the effect of exposure to the US on subsequent conditioning can be attenuated by giving exposure and conditioning trials in differ ent contexts (76, 197, 198); by extinguishing the original contextual conditioning (22,198); by signaling the US during pre-exposure by a stimulus that is not present during subsequent conditioning (27,197); and by preventing good conditioning to contextual cues (163).

Theoretical Analysis of Selective Association Selective association in conditioning may be partly responsible for a variety of phenomena often considered in isolation. Its major importance remains, however, in the revisions it requires to traditional theories of association. How are we to explain that conditioning depends not only on the relation between a given CS and US, but also on the relation between that US and other events? Two broad answers have been suggested: failures of conditioning caused by the presence of other predic tors of the US may be attributed either to the ineffectiveness of the US or to the ineffectiveness of the CS. INEFFECTIVENESS OF US The former notion has been expressed in different ways by Kamin (77), Rescorla & Wagner (153, 204), and Revusky (ISS). Their shared assumption is that a US will reinforce conditioning to a CS only to the extent that its occurrence is not already predicted by some other event. According to Kamin, a US already predicted is not surprising, and some element of surprise is necessary to instigate the rehearsal process responsible for establishing new associations. According to Revusky, a US can enter into only a limited number of associations: once associated with one event it cannot as readily be associated with others. The only formal theory, that of Rescorla and Wagner, may be interpreted in a variety of ways. Their central proposition is that the reinforcing effectiveness of a US on a given trial is proportional to the discrepancy between the asymptote of condition ing supportable by that US and the extent to which the US is predicted by all the stimuli present on that trial. Wagner (202) has since proposed a more general theory which captures this discrepancy principle by assuming that a US will be relatively


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ineffective to the extent that it is already represented in short-term memory at the moment of reinforcement. This may occur either because a representation of the US is retrieved from long-term memory by an already conditioned CS (133), or because the US itself has recently been presented (191). Experimental analysis of Kamin's blocking effect (77) provides considerable sup port for these theories. In the standard blocking experiment, little or no conditioning accrues to the added element II of a compound CS, AB, since prior conditioning ensures that the occurrence of the US will be fully predicted by A. Suppose, how ever, that instead of reinforcing the AB compound, it is presented without reinforce ment. The Rescorla-Wagner model predicts, and the prediction is confirmed, that inhibitory conditioning will accrue to B in direct proportion to the extent of prior excitatory conditioning to A alone (201). Conversely, if prior training had estab lished A, not as a signal for the US but as a signal for its absence (i.e. as a conditioned inhibitor), reinforcement of the AB compound will result in supernormal excitatory conditioning to B (16, 144, 189). The symmetry between its explanations of excita tory and inhibitory conditioning is one of the greatest virtues of the Rescorla Wagner model, even if there is evidence that this symmetry is not as complete as the original model required (6, 227). Several other experiments confirm that blocking is a consequence of AB signaling the same reinforcer as that signaled by A alone during prior training. Kamin (77) reported that blocking was attenuated when !.hock intensity was increased from A alone to AB compound trials; either the surprise engendered by the change in US, or the additional reinforcing properties of the stronger US, was sufficient to ensure some conditioning to the added element II. II: as Rescorla and Wagner imply, the increase in reinforcement is responsible for the attenuation of blocking, not all changes to the US should be equally effective:. Blocking may indeed remain intact when the US is changed to a longer but less intense shock (145); but it is attenuated by the addition of an unexpected second shock shortly after each compound trial (77) and, more seriously for the model, by the postponement or complete omission of an expected second shock (36). This latter finding points rather to Kamin's notion that the normal conditioning process may be restored by the addition of any surpris ing event to the otherwise unsurprising US. Other data, however, imply that the occum:nce of a surprising event shortly after a conditioning trial may interfere with the course of conditioning (205). One inter pretation of this finding is that the events of a. conditioning trial require processing in a limited capacity short-term store for some time after each trial; the occurrence of a surprising event during this time interval will prevent adequate processing of the events of the trial. There is suggestive evidence that an unexpected US will be processed for a longer time in short-term memory than an expected one (192), but it is clearly too soon to say with any confidence what are the various effects of surprising changes in reinforcement. The alternative explanation of selective association is that failures of conditioning are due to the ineffectiveness not of the US but of the blocked or overshadowed CS. Subjects fail to attend to the added element ill a

INEFFECTIVENESS OF CS



603

blocking experiment, and hence fail to condition to it. According to one version of such a theory, stimuli must compete for access to a limited capacity system; in the blocking experiment, the pretrained element preempts attention and thereby pre vents subjects attending to the added element (188). The problem is to see why, for example, training subjects to attend to A as a signal for food should not prevent them attending to B when AB signals shock; such pretraining, however, enhances rather than blocks conditioning to B (35). This version of selective attention theory cannot readily handle the central feature of blocking-that it depends on the pre dictability of the US. If the US is already predicted by A on AB trials, then B is redundant. If blocking, and selective association in general, is to be attributed to an attentional process, then the redundancy of a stimulus must be the critical variable affecting the attention paid to it. A more plausible theory of attention, therefore, might assume that attention is determined by the relative predictive value of a stimulus (104), and explain blocking by saying that subjects learn to ignore the added element B because it predicts no change in reinforcement from that predicted by A alone. Such a theory is able to explain some features of blocking that other theories are less equipped to handle, for example, that little or no blocking occurs on the first compound trial (105), that a surprising change in reinforcement attenuates blocking not on the trial on which the surprise occurs, but only on the subsequent trial (107), and that a blocked es is less readily associated with a subsequent change in reinforcement than is a novel es (108). This last observation suggests a parallel between blocking and the phenomenon of latent inhibition: exposure to the es alone before the start of conditioning interferes with the subsequent course of both excitatory and inhibitory conditioning (97, 143). The phenomenon is also observed in experiments on taste-aversion condi tioning (156), where it has often been interpreted in terms of "learned safety" (75). A familiar taste, however, is not the same as one which the animal has learned is a safe one (10, 189). In general, it is clear that a familiar es is less readily associated with any subsequent change in reinforcement than is a novel es. This decline in the associability of a stimulus with reinforcement is the sort of effect implied by Mackintosh's theory of attention (104), and it is possible that such a theory, designed to explain the phenomena of selective association in terms of changes in attention to redundant stimuli (i.e. stimuli that signal no change in the prevailing rate of reinforcement), will also explain latent inhibition. It follows from such a theory that if a es signals some other event during pre-exposure, latent inhibition will be attenuated (34, 99). An alternative approach is to note that the procedure for establishing latent inhibition is exactly the same as that for producing habituation (57): the only difference between the two types of experiment is that in one the experimenter records a change in the subject's current response to the es, while in the other he records a change in the subsequent rate of conditioning to the es. It is not perhaps surprising, therefore, that variables such as the number and distribution of trials and the intensity of the es have similar effects on habituation and latent inhibition (90, 168); and that both effects are susceptible to dishabituation by a novel stimulus (90,


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98, 164, 2 1 8). One attempt to integrate habituation, latent inhibition, and certain features of selective association is the theory advanced by Wagner (202), who proposes that all three may reflect the decline in effectiveness of a CS as repea.ted presentations cause its occurrence to be anticipated by the subject. The basic mecha nism for this effect is the same as that incorporated in the Rescorla-Wagner model: an expected event, be it CS or US, is less efft:ctive than an unexpected one. To say that an habituated CS is expected is simply to say that by a conditioning process it will be associated with the contextual cues in which it was presented. Thus a change in those contextual Clles will disrupt both habituation (202) and latent inhibition (98). There is a sharp contrast between the mechanism proposed by Wagner for this decline in the effectiveness of a CS and that suggested by Mackin tosh. According to Wagner, a CS loses effectiveness as its occurrence is predicted by other events, but according to Mackintosh, the decline occurs when the CS itself fails to predict any significant change. As yet, however, there are no data that would allow one to choose between these alternative conceptions. Literature Cited I . Amsel, A. 1958. The role of frustrative non reward in noncontinuous reward situations. Psychol. Bull. 5 5 : 1 02-19 2. Andrews, E. A., Braveman, N. S. 1 975. The combined effects of dosage level and interstimulus interval on the forma tion of one-trial poison-based aversions in rats. Anim. Learn. Behav. 3:287-89 3. Ayres, J. J. B., Benedict, J. 0. , Witcher, E. S. 1975. Systematic manipulation of individual events in a truly random con trol in rats. J. Compo PhysioL Psychol. 88:97-103 4. Azrin, N. H., Hake, D. F. 1969. Posi tive conditioned suppression: Condi tioned suppression using positive rein forcers as the unconditioned stimuli. 1. Exp. Anal. Behav. 1 2 : 1 67-73 5. Bakal, C. W., Johnson, R. D., Rescorla, R. A. 1 974. The effect of change in US quality on the blocking effect. Pavlovian J. Biol Sci. 9:97-103 6. Baker, A. O. 1 974. Conditioned inhibi tion is not the symmetrical opposite of conditioned excitation: A test of the Re scoria-Wagner model. Learn. Motiv. 5:369-79 7. Baker, A. O. 1 976. Learned irrelevance and learned helplessness: Rats learn that stimuli, reinforcers and responses are uncorrelated. 1. Exp. Psychol.: Anim. Behav. Proc. 2 : 1 30-4 1 8. Barbaree, H. E., Weisman, R. O. 1 975. On the failure of transfer of control from separately conducted Pavlovian conditioning to free-operant avoidance

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