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Avoidance learning, Pavlovian conditioning, and the development of phobias
Blo/ogicQf ~~yc~~fu~ North-Holland
167
27 (1988) 167-183
AVOIDANCE LEARNING, PAVLOVIAN CONDITIONING, AND THE DEVELOPMENT OF PHOBIAS * David A.T. SIDDLE School
and Nigel W. BOND
**
of3ehff~~o~~l Sciences, Macquarie Uniuersiry, Sydmy, NSW 2109, Australia
This paper examines the role of Pavlovian conditioning in the acquisition, maintenance and elimination of human phobias, Because many conceptualizations of human fears and phobias are based on data from studies of avoidance learning in animals, we first review theories of avoidance. Our conclusion is that none of the extant theories provides an adequate account of avoidance learning, and we propose a model of avoidance that involves Pavlovian, but not instrumental learning. We then analyse critically arguments that Pavlovian conditioning plays only a small role in the aetiology of fears. Finally, the paper examines the implications of a conditioning model of avoidance for the study of human fears and phobias.
1. Introduction Theories about the acquisition, maintenance, and treatment of human phobias have been influenced profoundly by empirical work on the effects of aversive stimulation in animals. Thus, Wolpe’s (1958) initial development of systematic desensitization and its theoretical explanation in terms of reciprocal in~bition rested upon his empirical work on fear reduction in cats and on Hull’s (1943, 1952) theories of animal (and human) learning. Similarly, the deveiopment of flooding or implosive therapy by Stampfl and Levis (1967) was influenced by findings reported by Masserman (1964) and by Solomon, Kamin, and Wynne (1953) concerning the extinction of avoidance responding in cats and dogs respectively. It is also related to work on extinction of avoidance responding in rats through response prevention (Baum, 1966, 1970). At a theoretical level, experimental models of the development of fear-related behaviour have relied on conceptualizations of both avoidance learning and Pavlovian conditioning. Perhaps the best example of this approach can be seen in the application of the two-factor theory of avoidance (Mowrer, 1947) to the development of phobia. However, it can be argued that there is now a * Preparation of this manuscript was supported by a grant from the Scheme. Thanks are due to Jeanette Packer for her comments on ** Request for reprints should be addressed to: David Siddle Behavioural Sciences, Macyuarie University, Sydney, NSW 2109,
0301-0511/88/$3.50
@ 1988, Elsevier Science Publishers
Australian Research Grants an earlier version. or Nigel Bond, School of Australia.
B.V. (North-HoBand)
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mismatch between the theories developed to account for the various facets of human phobias and the animal learning theories on which they are supposedly based. This paper takes as its starting point an examination of current theories of avoidance learning in animals. We conclude that these theories are inadequate and we then resurrect a theory of avoidance based upon Pavlovian conditioning. An approach of this sort allows us to employ formal theories of associative learning, for example, the Rescorla-Wagner (1972) theory, which have been developed to account for the empirical phenomena of conditioning, to make predictions about avoidance behaviour, and concomitantly, human phobias. However, any model of psychopathology which ascribes a key role to Pavlovian conditioning must be able to deal with the type of criticisms that have been mounted by Rachman (1977). Thus, the final section of the paper addresses specifically the issues raised by Rachman (1977) in his assertion that principles of conditioning cannot provide a comprehensive account of the development of fear.
2. Theories of avoidance learning 2.1. Two-factor
theory
theory” has sometimes As Rachman (1977) has noted, the term “two-factor been used in connection with the issue of whether there are one or two fundamental learning processes. By two-factor theory, we are referring to a theory which proposes that both Pavlovian conditioning and instrumental learning are important in avoidance learning and behaviour. Defined in this way, two-factor theory has undoubtedly been the most influential account of avoidance learning, especially in terms of its application to clinical phenomena (Mowrer, 1947; Rescorla & Solomon, 1967). In accounting for avoidance behaviour, two-factor theory argues that a previously neutral stimulus (a conditioned stimulus, CS), acquires associative strength through adventitious pairing with an aversive event (an unconditional stimulus, US). Fear elicited by the CS then motivates avoidance behaviour which is reinforced through a reduction of CS-induced fear, that is, avoidance behaviour is negatively reinforced. The theory is parsimonious and attractive in that it links together the two forms of associative learning, Pavlovian conditioning and instrumental learning. However, it has fallen into disfavour for a number of theoretical and empirical reasons. Bolles (1970) has pointed out that it is inconceivable that any animal can avoid predation on the basis of two-factor theory. Predators simply do not indicate their approach with a signal which begins 5 s before they arrive and which ceases just as they do arrive. Further, if the animal avoids the aversive US, then Pavlovian extinction of the fear CR should take
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place; yet, animals can make hundreds of avoidance responses without the response appearing to weaken (Mackintosh, 1974). Empirically, the theory demands that animals should always be fearful of the CS; otherwise there can be no fear reduction to reinforce the avoidance response. In fact, a series of studies has demonstrated that after extended training, independent measures of fear of the CS indicate that the animal does not fear the CS, even though efficient avoidance continues to follow CS presentation (Mineka, 1979). Thus, despite its simplicity and elegance, two-factor theory has failed to account for avoidance behaviour in animals. 2.2. Species-specific
defence reactions
2.2.1. Bolles (1970) has detailed a theory of avoidance which is based first upon Species-Specific Defense Reactions (SSDRs) and second, on punishment. According to Bolles, an avoidance response will be acquired rapidly if it constitutes one of the animal’s innate defense responses. If the avoidance response is not based on an innate defense response, it will be acquired slowly, if at all. Bolles has argued that if an animal is placed into an avoidance learning situation, and all behaviours that are not based on a defense reaction (i.e., non-avoidance behaviours) are followed by an aversive event, the punishment contingency leads to cessation of the non-avoidance behaviours. The major theoretical problem with the SSDR account is that it resorts to an analysis of avoidance based upon punishment, a concept that itself requires explanation. Empirically, it is also clear that animals can occasionally acquire avoidance behaviours that are not part of their innate repertoire (Crawford & Masterson, 1982). 2.2.2. Masterson and Crawford (1982) have articulated a theory of avoidance behaviour based upon the concept of a defense motivation system. They have suggested that the motivational state produced by noxious stimuli, or by cues previously paired with noxious stimuli, has two major effects. First, its activation makes relevant response patterns more likely to occur. Second, the motivational system responds to “ideal” consummatory stimuli, that is to those stimuli normally associated with the completion of a consummatory response. In the rat, there are essentially four relevant responses, fleeing, freezing, fighting, and burying. The response which is emitted is determined by the presence of a specific set of environmental cues, for example, the presence of a conspecific or an intruding predator is needed to elicit, the response of fighting. However, flight is preeminent and to the extent that the required avoidance response leads to consummatory stimuli for flight, reinforcement and thus learning will occur.
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Although Masterson and Crawford (1982) noted that the rat possesses four basic defensive responses, their discussion of reinforcement revolves around the necessity for movement from a dangerous to a safe place. Thus, the analysis overlooks the acquisition of passive avoidance, that is, the witholding of a response, enabling the animal to remain in a safe place. Empirically, the theory requires that the avoidance response be functionally effective, that is, reinforced. Such a position is vulnerable to findings (Bond, 1984; Kamin, 1956; Katzev & Mills, 1974) that rats acquire anticipatory running responses rapidly when exposed to a condition in which they are presented with a brief inescapable shock regardless of whether they run or not. The Pavlovian conditioning procedure provides no obvious reinforcement for the animal and yet it is effective in eliciting anticipatory responding to the CS (warning signal). 2.3. Cognitive theory 2.3.1. Seligman and Johnston (1973) have developed a theory of avoidance which is based on Irwin’s (1971) notion of “preferences” and “act-outcome” expectancies. In brief, the theory posits (1) that the animal prefers no shock to shock, (2) expects that if he or she responds within a given time, no shock rather than shock will occur, and (3) expects that if he or she does not respond within a given time, shock rather than no-shock will occur. Expectancies are strengthened when they are confirmed and weakened when they are disconfirmed. Unfortunately, the theory involves a circular argument in that there is no independent way of determining the animals’ expectancies. Expectancy is said to explain the behaviour, and the behaviour is used to index expectancy. Empirically, the theory holds that if the US is not avoided, there will be little acquisition of anticipatory responding; obviously, an animal cannot develop an expectancy that if it responds, no shock will occur. As noted earlier, however, Kamin (1956) Katzev and Mills (1974) and Bond (1984) observed moderate to high levels of responding under conditions in which the US was presented regardless of the animals behaviour. 2.3.2. Bolles (1978) has also formulated an expectancy model which employs some tenets from his earlier SSDR theory. It involves the learning of stimulus-stimulus and response-stimulus expectancies. The two expectancies are then put together to form a “psychological syllogism” so that the animal’s learning can be translated into behaviour. In a two-way avoidance situation, for example, the animal learns that the side that it is on is “dangerous” and that the other side is “safe”. As the learning of the discrimination proceeds, so the invariant running response becomes more probable. The theory suffers
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from much the same problems as the earlier SSDR theory in that it cannot account for avoidance responses which are not part of the animals innate repertoire. Empirically, it is disconfirmed by the occurrence of anticipatory running in a classical conditioning paradigm.
3. A Pavlovian conditioning account of avoidance behaviour Pavlovian conditioning has played a relatively insignificant role in accounting for avoidance behaviour, particularly following the explication of two-factor theory by Mowrer (1947). Early attempts at an analysis based solely on Pavlovian conditioning were discarded because anticipatory responses could be established that were different from the response required to escape the shock (Hull, 1929) and because the analyses could not account for free-operant avoidance (Mackintosh, 1974). The first issue is clearly not a problem. Thus, if a rat is placed in a distinctive chamber and receives a shock, the UR elicited by the shock is running, but the CR elicited by the contextual cues is freezing (Fanselow, 1980). Clearly, one should not assume that anticipatory responses will mimic escape responses. Second, although an account of avoidance based upon Pavlovian conditioning cannot handle free-operant avoidance, it should be noted that such avoidance is rather poorly acquired. It may well be that free operant avoidance is an example of what Seligman (1970) has termed “contraprepared” learning, and it can be argued that it is unlikely that any theory of avoidance will be able to account easily for contraprepared associations. Hollis (1982) has examined a large body of literature supporting the view that Pavlovian conditioning enables animals to prepare themselves to deal with the US in an efficient manner. For example, rats which have food availability signalled each day, eat more food and gain more weight than a group which has the food and signal presented independently (Zamble, 1973). Further, Farris (1967) demonstrated that male Japanese quail engaged in courtship and copulation much sooner if the imminent appearance of a female was signalled by a buzzer, in comparison with a group which was presented with the female and the buzzer independently. Finally, Hollis (1984) observed that territorial male blue gouramis were more likely to win a contest against another territorial male if the contest was signalled by a stimulus that had been previously paired with the presentation of another male. These three examples indicate that the function of Pavlovian conditioning is to enable the animal to deal more efficiently with presentation of the US. More importantly, however, they also demonstrate that the CR is not a poor copy of the UR, but involves a complex of responses in addition to those that are selected for measurement.
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In addition to arguing that Pavlovian contingencies are both necessary and sufficient for avoidance learning, we can question whether avoidance learning involves instrumental reinforcement. Evidence from a number of sources suggests that avoidance behaviour is heavily influenced by hereditary factors. First, Bolles (1970) has made the point that although an animal can learn rapidly to jump out of a box to avoid shock, the same animal may take hundreds of trials to learn to press a lever to avoid shock. Belles (1970) proposed that to the extent that the required avoidance response was one of the animals natural defensive behaviours (SSDRs), it would learn the response rapidly. Arbitrary responses are acquired slowly, if at all. Second, a number of experimenters have been able to select for high- and low-avoidance performance in rats (Bammer, 1982; Bignami, 1965; Brush, Froehlich, & Sakellaris, 1979). Third, an examination of the literature dealing with strain differences in avoidance performance, has led Fuller and Thompson (1978) to conclude that the differences in performance derive from genetically related differences in initial responses to the CS or US. These three lines of evidence converge in such a way as to suggest that avoidance behaviour has little to do with learning of an instrumental nature. The conclusion that instrumental learning is not involved in avoidance behaviour is at variance with the assumption made by most theorists that the removal of an avoidance contingency produces a large deterioration in avoidance performance. In fact, an examination of the evidence suggests that the deterioration is less than one might expect. Although Bolles, Stokes. and Younger (1966) observed a relatively profound decrement in avoidance responding following removal of the avoidance contingency, Kamin (1956) observed only a moderate decrement. Further, Katzev and Mills (1974) and Bond (1984) found very little difference in anticipatory responding when comparing an avoidance procedure in which an escape response terminated both the CS and the shock, and a Pavlovian conditioning procedure in which neither avoidance nor escape was possible. The latter condition involved a brief, 0.5-s shock which was presented following the termination of the buzzer CS. The animal could not escape or avoid the shock, which was delivered if the animal remained where it was, or was presented as soon as the animal crossed from one side of the shuttlebox to the other. Despite this lack of functional effectiveness, animals developed high levels of anticipatory responding (Bond, 1984; Katzev & Mills, 1974). Further, the latency to cross from one side to the other declined over trials (Bond, 1984). A pseudoconditioning control group which received unpaired presentations of the buzzer and the shock, displayed very little anticipatory responding, indicating that the behaviour of the Pavlovian conditioning group was due to the pairing of the buzzer and the shock (Bond, 1984). The data outlined above indicate that substantial anticipatory responding can develop under a Pavlovian conditioning contingency. This finding poses a
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considerable problem for most extant theories of avoidance. However, if a Pavlovian conditioning model is to account for avoidance responding, there remains the problem of why successful avoidance responding, which necessarily means that the CS is not followed by the US, does not lead to Pavlovian extinction. Masterson and Crawford (1982) have outlined one possible reason for this phenomenon, namely that defense system activation will be highly volatile. It will be aroused whenever the animal may be hurt, not just when it is certain to be hurt. They note that the defense system must be volatile, because the failure to be ready for danger is potentially life-threatening. Empirically, we can ask whether avoidance responding is really any different from other Pavlovian paradigms in terms of resistance to extinction. Mackintosh (1974) has outlined evidence indicating that avoidance responses are not always highly resistant to extinction. Moreover, there are many demonstrations that responses acquired following uppetitiue Pavlovian conditioning are also hig/zhly resistant to extinction. The classic examples can be found in studies involving omission (Sheffield, 1965) negative automaintenance in autoshaping (Williams & Williams, 1969) and the “long-box” paradigm developed by Jenkins (1977). All of these examples demonstrate that CRs based upon appetitive Pavlovian conditioning are highly resistant to extinction. Clearly, the “problem” of resistance to extinction is not restricted to aversive Pavlovian conditioning. It seems that where biologically significant events are concerned, that is, Pavlovian USs, it is more important for the animal to be ready for their occurrence, even if they do not occur, than vice-versa. If a Pavlovian conditioning model of avoidance learning is to be utilized in the study of the development of human phobias, we must address the argument that conditioning plays only a minor role in the development of human fear. Before addressing this issue, however, we must first trace briefly some important changes which have occurred in theoretical accounts of Pavlovian conditioning during the past 25 years.
4. Theoretical developments
in Pavlovian conditioning
Detailed discussions of important changes which have taken place in the study of conditioning have been provided by several authors (e.g., Furedy & Riley, 1987; Mackintosh, 1983) and only a brief overview will be provided here. It is difficult to overemphasize the influence of Hull (Hull, 1943) in shaping approaches to conditioning. As Mackintosh (1983) has noted, however, Hull’s analysis owed more to Thorndike (1911) and his “law of effect” than to Pavlov’s (1927) theorizing. Whereas for Pavlov, the process underlying conditioning was one of stimulus substitution (i.e., the CS coming to substitute for the US), Hull’s analysis emphasized the strengthening of receptor-effector
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connections through drive reduction. Hull’s approach represented S-R behaviourism par excellence. That is, conditioning was regarded as the acquisition of new reflexes and was said to involve an S-R (CS-CR) association. In contrast to the Hullian position, Tolman (1932) and his colleagues adopted a position which asserted that conditioning involves the learning of relationships between events occurring in the environment. In this sense, conditioning is said to involve the acquisition of new knowledge. It follows from a Tolmanian approach that any single measure of conditioning is only one of several possible indicators of the central learning process. In contrast to the Hullian position that there is a close relationship between learning and performance, Tolman stressed the distinction between learning and performance, and argued that what is learned is not always evident through immediate changes in behaviour. This has been referred to as the problem of behavioural silence (Dickinson, 1980). There is no doubt that the Hullian approach was dominant from 1930 to the 1960s. Important theoretical developments since 1960, however, owe much more to Tolman’s S-S approach than to Hull’s S-R approach. Essentially, extant theories of conditioning represent an information-processing approach in which for the most part, associations are said to be learned between stimuli. In other words, Pavlovian conditioning is considered to allow organisms to track causal relationships in their environments. Current animal-based theories emphasize the unexpectedness of the US (Rescorla & Wagner, 1972) the extent to which the CS and the US are primed or prerepresented in a short-term memory store (Wagner, 1978), the relative predictive accuracy of all cues present on a CS-US pairing trial (Mackintosh, 1974; Pearce & Hall, 1980), or the type of processing (automatic or controlled) that is devoted to the CS (Pearce & Hall, 1980). A key assumption of all theories is that the CS and US must be processed conjointly in a limited capacity processing mechanism in order for learning to occur. Recent developments in accounts of human conditioning have followed a similar route. Thus, ohman (1979, 1983) has stressed that the CS and the US must be processed conjointly in a short-term memory system, and that for this to occur, both stimuli must capture the subject’s attention, That is, both must elicit orienting. Dawson and Schell(1985,1987) have argued that conscious, controlled processing of the CS and the US is necessary for conditioning to occur. Of course, not all theorists claim that Pavlovian conditioning involves S-S learning in all situations (see Furedy, 1988, this issue; Furedy & Riley, 1987). Rescorla (1980) has provided some evidence that second-order conditioning may involve S-R associations, and ohman and his colleagues (e.g., ohman, Dimberg, & Esteves, in press) have argued that CSs which form part of “prepared” associations may be learned about and may control behaviour in situations in which controlled processing is prevented. Nevertheless, the S-S approach in one form or another has proved to be useful in providing a
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framework for explanations of phenomena such as latent inhibition, blocking, overshadowing, and inhibitory conditioning (Mackintosh, 1983). Having sketched some of the important developments in theories of Pavlovian conditioning during the past 25 years, we can now examine the assertion that Pavlovian conditioning plays only a minor role in the development of human phobias.
5. Pavlovian conditioning and the development of human phobias In an influential review, Rachman (1977) concluded that although it is clear that laboratory animals can acquire fear through conditioning, principles of conditioning cannot provide a comprehensive account of the development of human fear. Rachman’s conclusions were based on (1) the failure to acquire fear in fear-evoking situations, (2) the difficulty of producing conditioned fear reactions in humans, (3) the reliance of conditioning theory on the premise of equipotentiality, (4) the distribution of fears in normal and neurotic populations, (5) the fact that phobics recount histories that are inconsistent with conditioning theory, and (6) fears can be acquired vircariously. Before discussing these issues, we need to examine the type of model of Pavlovian conditioning adopted by Rachman (1977). According to Rachman, “neutral stimuli which are associated with a fear or pain-producing state of affairs, develop fear qualities” (p. 376). Moreover, the strength of the conditioned fear is said to be determined by “the number of repetitions of the association between the pain/fear experience and the stimuli, and also by the intensiy of the fear or pain experienced in the presence of the stimuli” (p. 376). Although Rachman (1977) did not discuss the kind of model of conditioning on which he based his critique, his statements seem to imply an S-R approach. With this in mind, we can now examine each of Rachman’s (1977) arguments in turn. 5. I. Failures to acquire fear According to Rachman’s (1977) review, repeated air-raids during the Second World War did not produce an increase in psychiatric disorders and few prolonged phobic reactions. These findings, according to Rachman, militate against a conditioning account of the acquisition of fear. However, S-S theories assume that an association would have been learned between specific stimuli and the complex of events which constitutes an air-raid. Thus, fear which resulted from air-raids might be expected to occur only in the presence of CSs which reliably predicted air-raids. As far as we can see, an S-S conditioning model does not predict diffuse consequences such as an increase
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in psychiatric disorders, but does predict the occurrence of fear only in the presence of the CSs which have been reliably followed by air-raids. At a more general level, it should be noted that most theories of conditioning emphasize the importance of not only temporal contiguity between the CS and the US, but also CS-US contingency. That is, an important determinant of conditioning is said to be the probability of US occurrence in the presence of the CS (p(US/CS)) relative to the probability of US occurrence in the absence of the CS (p(US/CS)). When p(US/CS) = p(US/CS), no conditioning occurs (Rescorla, 1967). Thus, repeated CS-US pairings do not necessarily produce conditioning when p(US/cs) is high. 5.2. Conditioning
human fears
According to Rachman (1977) there is little evidence of laboratory fear conditioning in humans. Although we do not disagree with this conclusion, we do not view it as an indictment of a conditioning model of the acquisition of fear. Most theories of conditioning hold that the asymptotic level of learning is determined by the intensity of the US (usually denoted by X), and it is clear that the majority of human studies use somewhat less than intense USs. The most common description of a shock US in human work is “uncomfortable, but not painful”, and US intensity is almost always set by subjects themselves. Moreover, to argue that human fear conditioning has rarely been demonstrated in the laboratory is not to say that it cannot be demonstrated. Indeed, Rachman himself cited work by Sanderson, Laverty, and Campbell (1963) which demonstrated human fear conditioning with a potent US, injections of scoline. Our contention is simply that in most human conditioning studies, the US is not sufficiently intense to produce fear. That is not to say, of course, that learning does not occur with nonnoxious USs or that different processes are involved. 5.3. The equipotentiality
premise
Rachman (1977) has argued that the fact that not all stimuli are equally conditionable poses difficulties for a conditioning model of fear acquisition. This argument can be analysed on different levels. On one level, it can be noted that all theories of conditioning include a CS learning rate parameter (usually denoted by a) which determines the rate at which learning occurs. That is, rate of learning is determined by the salience (Rescorla & Wagner, 1972) or associability of the CS (Pearce & Hall, 1980). In addition, when a number of potential CSs occur together and are followed by a US, the most salient CS will overshadow the less salient CSs. That is, the less salient CS acquires less associative strength when presented in compound with a more salient CS than it does when presented alone. Thus, when compound CSs are
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followed by a US, it cannot be assumed that all CSs will acquire associative strength. At another level, differences in ease of conditioning can be examined in terms of differences in associations. For example, Seligman (1970) has proposed that associations can be ordered along a dimension of preparedness from prepared, through unprepared, to contraprepared. General process theories of conditioning do not incorporate the notion that associations themselves are differentially responsive to CS-US contingencies; however, it is by no means clear that they should do so. Both human (e.g., Qhman, Dimberg, & Ost, 1985) and animal data (e.g., Garcia & Koelling, 1966) on selective associations can often be interpreted without recourse to a concept of preparedness (Maltzman & Boyd, 1984; McNally, 1987; Schwartz, 1974). Moreover, Logue (1979), on the basis of a comprehensive review of the taste aversion literature, concluded that different principles are not required to account for taste aversion and “ traditional” learning. 5.4. Distribution
of fears
As Rachman (1977) has noted, the distribution of fears in a general population and the distribution of phobias is biased. Thus, fear of snakes is represented much more strongly than is fear of dental treatment or fear of injections (Agras, Sylvester, & Oliveau, 1969). From Rachman’s perspective, these findings pose difficulties for a conditioning account of fear acquisition. There are two ways of handling this criticism. First, we have already noted that theories of conditioning propose that more salient stimuli will gain associative strength more rapidly than will less salient stimuli, and if CSs of high and low salience occur together, the former will overshadow the latter. In addition, the phenomenon of latent inhibition may be able to account for the fact that fear of some events is prevalent whereas fear of others is not. Latent inhibition refers to a retardation of conditioning as a consequence of preexposure to the CS in the absence of a US and has been demonstrated in both human and nonhuman animals (Siddle & Remington, 1987). To use the examples provided by Rachman, it seems more likely that lambs are more likely to be encountered in the absence of an aversive consequence than are snakes. Another approach is to argue that the association between snakes and an aversive outcome is selective in the sense that it is prepared (e.g., ohman et al., 1985). Essentially, ijhman (1988, this issue) has argued that because reptiles were dangerous to prototypical mammals, a propensity to associate reptiles with aversive outcomes has become part of an open genetic program. It is important to note, however, that even if it is assumed that phobias represent prepared learning (Seligman, 1971) the underlying mechanism of acquisition is conditioning.
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5.5. Patients’ reports of fear onset The fact that patients often cannot recall a conditioning-like episode in marking the onset of a phobia has been taken by Rachman (1977) to indicate that conditioning is not a satisfactory account of the genesis of fears. However, more recent evidence indicates that with patient groups, a large proportion (46681%) of animal phobics, social phobics, claustrophobics, blood phobics, dental phobics, and agoraphobics ascribe the onset of their phobias to a conditioning experience (Ost & Hugdahl, 1981, 1983, 1985). In the case of subclinical fears, the frequency of conditioning experiences at the onset of fears is lower (Fazio, 1972; Rimm, Janda, Lancaster, Nahl, & Dittmar, 1977)., It is interesting to note, however, that Garb and Stunkard (1974) have reported that 87% of a normal sample recalled that a conditioning episode marked the start of a food aversion. Thus, although the recall data suggest that conditioning cannot account for all phobias and subclinical fears, it nevertheless figures in a substantial proportion of them. 5.6. Vicarious transmission
of fear
In his discussion of the vicarious transmission of fears, Rachman (1977) has drawn a distinction between vicarious conditioning and the social transmission of fear through information. There is little doubt that learning can occur through both of these mechanisms, but we are not convinced that the phenomena pose problems for a conditioning account of fear acquisition. For example, Hygge and ohman (1978) have demonstrated that electrodermal conditioning can occur vicariously to facial expressions of emotion. With regard to the information variables, much of the human conditioning literature during the past 20 years has been concerned with the effects of instructions on the acquisition of conditioned responses (Dawson & Schell, 1985). It is quite clear that conditioning effects can be obtained through instruction alone. Thus, rather than viewing vicarious acquisition of fear as a phenomenon which provides difficulties for a conditioning account of fear acquisition, we view it as a phenomenon to be integrated within a conditioning model. In summary, our contention is that the criticisms which Rachman (1977) made of a conditioning model of the acquisition of fear are misplaced. Our argument is that the model of conditioning implicit in Rachman’s critique represents an S-R approach in which temporal contiguity is both necessary and sufficient for learning to occur. We argue further that an information processing model of conditioning in which the emphasis is on the development of S-S associations and on CS/US contingency provides considerably more explanatory power and avoids the criticisms made by Rachman (1977). AS Reiss (1980) has noted, “. . even before behavior therapists had discovered problems in applying contiguity theory to human behavior, animal learning
D.A. T. Siddle and N. W.
theorists 386).
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6. Implications of a conditioning model of avoidance 6.1. A Pavlovian learning
account
of avoidance
learning
and theories
of associative
A major advantage of the present approach is that it can utilize formal models of Pavlovian conditioning to formulate predictions which allow us to distinguish between competing theoretical accounts. For example, a preparedness approach to the role of learning in the acquisition and maintenance of human phobias (6hman et al., 1985; Seligman, 1971) predicts that human conditioning to fear-relevant stimuli will be more resistant to extinction than will conditioning to fear-irrelevant stimuli. In contrast, Maltzman and Boyd (1984) have outlined an account in which fear-relevant stimuli are viewed simply as being more salient. Salience of a CS is a parameter in the RescorlaWagner (1972) model of associative learning, and other things being equal, the model predicts that CRs to more salient stimuli will extinguish faster than will CRs to less salient stimuli. The bulk of the evidence suggests conditioning with fear-relevant stimuli is more resistant to extinction, as predicted by “preparedness theory” (McNally, 1987). 6.2. Dissociation
between behavioural,
autonomic,
and subjective measures of fear
As already noted, most models of avoidance learning have considered fear-reduction as the reinforcer for avoidance behaviour. These models have been embarrassed by the frequent finding that animals display efficient avoidance behaviour without displaying fear to the putative CS (Mineka, 1979). The same problem occurs when fear-reduction theories of avoidance are used to account for the acquisition and maintenance of human fears. In contrast, the present model does not conceptualize fear-reduction as a reinforcer of avoidance behaviour. We suggest that the learning of an association between a CS and an aversive US will produce conditioning of both fear and the appropriate response, for example, running. However, the latter does not depend on the former and the two will not necessarily covary in a tight manner. Given that our account is based on the premise of biological functionality, we have to answer the question of why fear occurs at all in avoidance situations. An answer can be found in the perceptual-defensive-recuperative model proposed by Bolles and Fanselow (1980). Briefly, Bolles and Fanselow have suggested that conditioned fear is associated with the release of endogenous opiates which serve to ameliorate pain. The reduction of pain serves to
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prevent the occurrence of recuperative behaviour which might interfere with the animal’s ability to deal efficiently with the US. The reasoning suggests that an animal will display fear in the early stages of acquisition. However, when it has begun to deal efficiently with the US, fear is expected to subside. A mechanism for the di~nution of fear can be found in Schull’s (1979) conditioned opponent-process model. Clearly, our approach can handle the bothersome fact that once animals start to avoid efficiently, they no longer display fear. If, however, the animal’s current response is prevented, fear will again be elicited in order to enable adjustment to the new contingency (Baum, 1970). Our account serves to indicate why behavioural, autonomic, and subjective measures of fear do not necessarily covary. &en that the components of fear perform different functions and that the relative importance of these functions may change over time, we do not expect a close correspondence between them; this is indeed the case (Lang, 1968). Another prediction which follows from our analysis concerns the effects of phobic stimulation in phobic patients. As already noted, it can be argued that conditioned fear results in the release of endogenous opiates (Bolles & Fanselow, 1980). If some human phobias are acquired through conditioning, it is possible to argue that, because fear is associated with the release of endogenous opiates, human phobics will exhibit analgesia in the presence of the relevant phobic stimulus. Furthermore, the analgesia will dissipate following a course of successful treatment.
7. Conclusion In summa~, current conceptualizations of classical conditioning indicate that it is a dynamic process which enables animals to deal effectively with causal relationships in the environment and thus with biologically important events. Conceptualizations of this sort enable us to account for the acquisition and maintenance of avoidance behaviour in animals and for phobias in humans. The theoretical integration of animal and human work provides a rich base in which current models of Pavlovian learning enable us to make predictions about the outcome and effects of particular interventions.
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Baum, M. (1966). Rapid extinction of an avoidance response following a period of response prevention in the avoidance apparatus. Psychological Reporis, 18, 59-64. Baum, M. (1970). Extinction of avoidance responding through response prevention (flooding). Psychological Bulletin, 74, 276-284. Bignami, G. (1965). Selection for high rates and low rates of avoidance conditioning in the rat. Animal Behavior, 13, 221-227. Belles, R.C. (1970). Species-specific defense reactions and avoidance learning. Psychological Review, 77, 32-48. Bolles, R.C. (1978). The role of stimulus learning in defensive behavior. In S.H. Hulse, H. Fowler, & W.K. Honig (Eds.), Cognitive processes in animal behavior (pp. 89-107). Hillsdale, NJ: Erlbaum. Belles, R.C., & Fanselow, M.S. (1980). A perceptual-defensive-recuperative model of fear and pain. Behavioural and Brain Sciences, 3, 291-301. Belles, R.C., Stokes, L.W., & Younger, M.S. (1966). Does CS termination reinforce avoidance behavior? Journal of Comparative and Physiological Psychology, 62, 201-207. Bond, N.W. (1984). Avoidance, classical, and pseudoconditioning as a function of species-specific defense reactions in high- and low-avoider rat strains. Animal Learning and Behavior, 12, 323-331. Brush, F.R. Froehlich, J.C., & Sakellaris, P.C. (1979). Genetic selection for avoidance behavior in the rat. Behavior Genetics, 9, 309-316. Crawford, M., & Masterson, F.A. (1982). Species-specific defense reactions and avoidance learning. Pavlovian Journal of Biological Science, 17, 204-214. Dawson, M.E., & Schell, A.M. (1985). Information processing and human autonomic classical conditioning. In P.K. Ackles, J.R. Jennings, & M.G.H. Coles (Eds.), Advances in Psychophysiology, vol. 1 (pp. 89-165). Greenwich, Connecticut: JAI Press. Dawson, M.E., & Schell, A.M. (1987). Human autonomic and skeletal classical conditioning; The role of conscious cognitive factors. In G. Davey (Ed.), Cognitive processes and Pavlovian conditioning in humans (pp. 27-55). Chichester: Wiley. Dickinson, A. (1980). Contemporary animal learning theov. Cambridge: Cambridge University Press. Fanselow, M.S. (1980). Conditional and unconditional components of post-shock freezing. Pavlovian Journal of Biological Science, 15, 177-182. Farris, H.E. (1967). Classical conditioning of courting behavior in the Japanese quail, Coturmx coturnix japonica. Journal of Experimental Analysrs of Behavior, IO, 213-217. Fazio, A.F. (1972). Implosive therapy with semiclinical phobias. Journal of Abnormal Psychology, 80, 183-188. Fuller, J.L., & Thompson, W.R. (1978). Foundations of behavior genetics. St. Louis: C.V. Mosby & co. Furedy, J.J. (1988). Arguments for and proposed tests of a revised S-R contiguity-reinforcement theory of human Pavlovian autonomic conditioning: Some contra-cognitive claims. Biological Psychology, 27, 137-151, this issue. Furedy, J.J., & Riley, D.M. (1987). Human Pavlovian autonomic conditioning and the cognitive paradigm. In G. Davey (Ed.), Cognitive processes and Pavlovian conditioning in humans (pp. l-25). Chichester: Wiley. Garb, J.J., & Stunkard, A.J. (1974). Taste aversion in man. American Journal of Psychiatty, 131, 1204-1207. Garcia, J., & Koelling, R.A. (1966). Relation of cue to consequence in avoidance learning. Psychonomic Science, 4, 123-124. Hollis, K.L. (1982). Pavlovian conditioning of signal-centred action patterns and autonomic behavior: A biological analysis of function. Advances in the study of behavror, 12, l-64. Hollis, K.L. (1984). The Biological function of Pavlovian conditioning: The best defence is a good offence. Journal of Experimental Psychology: Animal Behavior Process, 10, 413-425.
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Hull, CL. (1929). A functional inte~retation of the conditioned reflex. Psyc~?o~ogica~Review, 36, 498-511. Hull, CL. (1943). Principles of behavior. New York: Appleton-Century-Crofts. Hull, CL. (1952). A behavior system. New Haven: Yale University Press. Hygge, S., & Ohman, A. (1978). Modeling processes in the acquisition of fears: Vicarious electrodermal conditioning to fear-relevant stimuli. Journal of Persona&y and Social Psvehology, 36, 271-279. Irwin, F.W. (1971). Intentiona! behnvior and motivation: A cognitive theory. New York: J.B. Lippincott. Jenkins, H.M. (1977). Sensitivity of different response systems to stimulus-reinforcer and response-reinforcer relations. In H. Davis & H.M.B. Hurwitz (Eds.), Operant Pavlovian interact~ons (pp. 47-66). Hillsdale, NJ: Erlbaum. Kamin, L.J. (1956). The effects of t~~ination of the CS and avoidance of the US on avoidance learning. Journal of Comparative and Physioiogicul Psycholoa, 49, 420-424. Katzev, R.D., & Mills, S.K. (1974). Strain differences in avoidance conditioning as a function of the classical CS-US contingency. Journal of Comparative and Physiological Psychology, 87, 661-671. Lang, P.J. (1968). Fear reduction and fear behavior: Probtems in treating a construct. In J.M. Shilier (Ed.), Research in psychorherapy, Vol. III (pp. 90-102). Washington, DC: American Psychological Association. Logue, A.W. (1979). Taste aversion and the generality of the laws of learning. Psycho/ogical Bulletin, 86, 276-296. Mackintosh, N.J. (1974). The psycholo@ of animal leurning. New York: Academic Press. Mackintosh, N.J. (1983). Conditioning and associative learning. Oxford: Clarendon Press. Maltzman, I., & Boyd, G. (1984). Stimulus significance and bilateral SCRs to potentially phobic pictures. Journal of Abnormal Psychology, 93, 41-46. Masserman, J.H. (1964). Behavior and neurosis: An experimental and psychoanalytic approach to psychobiologic principles. New York: Hafner. Masterson, F.A., & Crawford, M. (1982). The defense motivation system: A theory of avoidance behavior. The Behavioral and Brum Sciences, 5, 661-696. McNally, R.J. (1987). Preparedness and phobias: A review. Psychologicnl Bulletin, IOI, 283-303. Mineka, S. (1979). The role of fear in theories of avoidance learning, flooding, and extinction. Psychological Bulletin, 86, 985-1010. Mowrer, O.H. (1947). On the dual nature of learning: A reinterpretation of “conditioning” and “problem solving”. Harvard Educational Reuiew, 17, 102-148. ohman, A. (1979). Fear relevance, autono~c conditioning, and phobias: A laboratory model. In P.O. Sjoden, S. Bates, & W.S. Dockens (Eds.), Trends in behavior therapy (pp. 107-133). New York: Academic Press. iihman, A. (1983). The orienting response during Pavlovian conditioning. In D. Siddle (Ed.), Orienting and habituation: Perspectives in human research (pp. 315-369). Chichester: Wiley. iihman, A. (1988). Nonconscious control of autonomic responses: A role for Pavlovian conditioning? Biological Psycho~o~, 37, 113-135, this issue. 6hman, A., Dimberg, U., & Esteves. F. (in press). Preattentive activation of aversive emotions. In T. Archer, & L-G. Nilsson (Eds.), Aversion, avoidance and anxiety: Perspectives on aversively motivated behavior. Hillsdale, NJ: Erlbaum. ijhman, A., Dimberg, U., & Ost, L-G. (1985). Animal and social phobias. Biological constraints on learned fear responses. In S. Reiss & R.R. Bootzin (Eds.), Theoretical issues in behaviour therapy (pp. 123-175). New York: Academic Press. Ost, L-G., & Hugdahl, K. (1981). Acquisition of phobias and anxiety response patterns in clinical patients. Behaviour Research and Therapy, 19, 439-447. Ost, L-G, & Hugdahl, K. (1983). Acquisition of agoraphobia, mode of onset and anxiety response patterns. Behavior Research and Therapy, ZI, 623-631.
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AVOIDANCE LEARNING, PAVLOVIAN CONDITIONING, AND THE DEVELOPMENT OF PHOBIAS * David A.T. SIDDLE School
and Nigel W. BOND
**
of3ehff~~o~~l Sciences, Macquarie Uniuersiry, Sydmy, NSW 2109, Australia
This paper examines the role of Pavlovian conditioning in the acquisition, maintenance and elimination of human phobias, Because many conceptualizations of human fears and phobias are based on data from studies of avoidance learning in animals, we first review theories of avoidance. Our conclusion is that none of the extant theories provides an adequate account of avoidance learning, and we propose a model of avoidance that involves Pavlovian, but not instrumental learning. We then analyse critically arguments that Pavlovian conditioning plays only a small role in the aetiology of fears. Finally, the paper examines the implications of a conditioning model of avoidance for the study of human fears and phobias.
1. Introduction Theories about the acquisition, maintenance, and treatment of human phobias have been influenced profoundly by empirical work on the effects of aversive stimulation in animals. Thus, Wolpe’s (1958) initial development of systematic desensitization and its theoretical explanation in terms of reciprocal in~bition rested upon his empirical work on fear reduction in cats and on Hull’s (1943, 1952) theories of animal (and human) learning. Similarly, the deveiopment of flooding or implosive therapy by Stampfl and Levis (1967) was influenced by findings reported by Masserman (1964) and by Solomon, Kamin, and Wynne (1953) concerning the extinction of avoidance responding in cats and dogs respectively. It is also related to work on extinction of avoidance responding in rats through response prevention (Baum, 1966, 1970). At a theoretical level, experimental models of the development of fear-related behaviour have relied on conceptualizations of both avoidance learning and Pavlovian conditioning. Perhaps the best example of this approach can be seen in the application of the two-factor theory of avoidance (Mowrer, 1947) to the development of phobia. However, it can be argued that there is now a * Preparation of this manuscript was supported by a grant from the Scheme. Thanks are due to Jeanette Packer for her comments on ** Request for reprints should be addressed to: David Siddle Behavioural Sciences, Macyuarie University, Sydney, NSW 2109,
0301-0511/88/$3.50
@ 1988, Elsevier Science Publishers
Australian Research Grants an earlier version. or Nigel Bond, School of Australia.
B.V. (North-HoBand)
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mismatch between the theories developed to account for the various facets of human phobias and the animal learning theories on which they are supposedly based. This paper takes as its starting point an examination of current theories of avoidance learning in animals. We conclude that these theories are inadequate and we then resurrect a theory of avoidance based upon Pavlovian conditioning. An approach of this sort allows us to employ formal theories of associative learning, for example, the Rescorla-Wagner (1972) theory, which have been developed to account for the empirical phenomena of conditioning, to make predictions about avoidance behaviour, and concomitantly, human phobias. However, any model of psychopathology which ascribes a key role to Pavlovian conditioning must be able to deal with the type of criticisms that have been mounted by Rachman (1977). Thus, the final section of the paper addresses specifically the issues raised by Rachman (1977) in his assertion that principles of conditioning cannot provide a comprehensive account of the development of fear.
2. Theories of avoidance learning 2.1. Two-factor
theory
theory” has sometimes As Rachman (1977) has noted, the term “two-factor been used in connection with the issue of whether there are one or two fundamental learning processes. By two-factor theory, we are referring to a theory which proposes that both Pavlovian conditioning and instrumental learning are important in avoidance learning and behaviour. Defined in this way, two-factor theory has undoubtedly been the most influential account of avoidance learning, especially in terms of its application to clinical phenomena (Mowrer, 1947; Rescorla & Solomon, 1967). In accounting for avoidance behaviour, two-factor theory argues that a previously neutral stimulus (a conditioned stimulus, CS), acquires associative strength through adventitious pairing with an aversive event (an unconditional stimulus, US). Fear elicited by the CS then motivates avoidance behaviour which is reinforced through a reduction of CS-induced fear, that is, avoidance behaviour is negatively reinforced. The theory is parsimonious and attractive in that it links together the two forms of associative learning, Pavlovian conditioning and instrumental learning. However, it has fallen into disfavour for a number of theoretical and empirical reasons. Bolles (1970) has pointed out that it is inconceivable that any animal can avoid predation on the basis of two-factor theory. Predators simply do not indicate their approach with a signal which begins 5 s before they arrive and which ceases just as they do arrive. Further, if the animal avoids the aversive US, then Pavlovian extinction of the fear CR should take
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place; yet, animals can make hundreds of avoidance responses without the response appearing to weaken (Mackintosh, 1974). Empirically, the theory demands that animals should always be fearful of the CS; otherwise there can be no fear reduction to reinforce the avoidance response. In fact, a series of studies has demonstrated that after extended training, independent measures of fear of the CS indicate that the animal does not fear the CS, even though efficient avoidance continues to follow CS presentation (Mineka, 1979). Thus, despite its simplicity and elegance, two-factor theory has failed to account for avoidance behaviour in animals. 2.2. Species-specific
defence reactions
2.2.1. Bolles (1970) has detailed a theory of avoidance which is based first upon Species-Specific Defense Reactions (SSDRs) and second, on punishment. According to Bolles, an avoidance response will be acquired rapidly if it constitutes one of the animal’s innate defense responses. If the avoidance response is not based on an innate defense response, it will be acquired slowly, if at all. Bolles has argued that if an animal is placed into an avoidance learning situation, and all behaviours that are not based on a defense reaction (i.e., non-avoidance behaviours) are followed by an aversive event, the punishment contingency leads to cessation of the non-avoidance behaviours. The major theoretical problem with the SSDR account is that it resorts to an analysis of avoidance based upon punishment, a concept that itself requires explanation. Empirically, it is also clear that animals can occasionally acquire avoidance behaviours that are not part of their innate repertoire (Crawford & Masterson, 1982). 2.2.2. Masterson and Crawford (1982) have articulated a theory of avoidance behaviour based upon the concept of a defense motivation system. They have suggested that the motivational state produced by noxious stimuli, or by cues previously paired with noxious stimuli, has two major effects. First, its activation makes relevant response patterns more likely to occur. Second, the motivational system responds to “ideal” consummatory stimuli, that is to those stimuli normally associated with the completion of a consummatory response. In the rat, there are essentially four relevant responses, fleeing, freezing, fighting, and burying. The response which is emitted is determined by the presence of a specific set of environmental cues, for example, the presence of a conspecific or an intruding predator is needed to elicit, the response of fighting. However, flight is preeminent and to the extent that the required avoidance response leads to consummatory stimuli for flight, reinforcement and thus learning will occur.
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and phobm
Although Masterson and Crawford (1982) noted that the rat possesses four basic defensive responses, their discussion of reinforcement revolves around the necessity for movement from a dangerous to a safe place. Thus, the analysis overlooks the acquisition of passive avoidance, that is, the witholding of a response, enabling the animal to remain in a safe place. Empirically, the theory requires that the avoidance response be functionally effective, that is, reinforced. Such a position is vulnerable to findings (Bond, 1984; Kamin, 1956; Katzev & Mills, 1974) that rats acquire anticipatory running responses rapidly when exposed to a condition in which they are presented with a brief inescapable shock regardless of whether they run or not. The Pavlovian conditioning procedure provides no obvious reinforcement for the animal and yet it is effective in eliciting anticipatory responding to the CS (warning signal). 2.3. Cognitive theory 2.3.1. Seligman and Johnston (1973) have developed a theory of avoidance which is based on Irwin’s (1971) notion of “preferences” and “act-outcome” expectancies. In brief, the theory posits (1) that the animal prefers no shock to shock, (2) expects that if he or she responds within a given time, no shock rather than shock will occur, and (3) expects that if he or she does not respond within a given time, shock rather than no-shock will occur. Expectancies are strengthened when they are confirmed and weakened when they are disconfirmed. Unfortunately, the theory involves a circular argument in that there is no independent way of determining the animals’ expectancies. Expectancy is said to explain the behaviour, and the behaviour is used to index expectancy. Empirically, the theory holds that if the US is not avoided, there will be little acquisition of anticipatory responding; obviously, an animal cannot develop an expectancy that if it responds, no shock will occur. As noted earlier, however, Kamin (1956) Katzev and Mills (1974) and Bond (1984) observed moderate to high levels of responding under conditions in which the US was presented regardless of the animals behaviour. 2.3.2. Bolles (1978) has also formulated an expectancy model which employs some tenets from his earlier SSDR theory. It involves the learning of stimulus-stimulus and response-stimulus expectancies. The two expectancies are then put together to form a “psychological syllogism” so that the animal’s learning can be translated into behaviour. In a two-way avoidance situation, for example, the animal learns that the side that it is on is “dangerous” and that the other side is “safe”. As the learning of the discrimination proceeds, so the invariant running response becomes more probable. The theory suffers
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from much the same problems as the earlier SSDR theory in that it cannot account for avoidance responses which are not part of the animals innate repertoire. Empirically, it is disconfirmed by the occurrence of anticipatory running in a classical conditioning paradigm.
3. A Pavlovian conditioning account of avoidance behaviour Pavlovian conditioning has played a relatively insignificant role in accounting for avoidance behaviour, particularly following the explication of two-factor theory by Mowrer (1947). Early attempts at an analysis based solely on Pavlovian conditioning were discarded because anticipatory responses could be established that were different from the response required to escape the shock (Hull, 1929) and because the analyses could not account for free-operant avoidance (Mackintosh, 1974). The first issue is clearly not a problem. Thus, if a rat is placed in a distinctive chamber and receives a shock, the UR elicited by the shock is running, but the CR elicited by the contextual cues is freezing (Fanselow, 1980). Clearly, one should not assume that anticipatory responses will mimic escape responses. Second, although an account of avoidance based upon Pavlovian conditioning cannot handle free-operant avoidance, it should be noted that such avoidance is rather poorly acquired. It may well be that free operant avoidance is an example of what Seligman (1970) has termed “contraprepared” learning, and it can be argued that it is unlikely that any theory of avoidance will be able to account easily for contraprepared associations. Hollis (1982) has examined a large body of literature supporting the view that Pavlovian conditioning enables animals to prepare themselves to deal with the US in an efficient manner. For example, rats which have food availability signalled each day, eat more food and gain more weight than a group which has the food and signal presented independently (Zamble, 1973). Further, Farris (1967) demonstrated that male Japanese quail engaged in courtship and copulation much sooner if the imminent appearance of a female was signalled by a buzzer, in comparison with a group which was presented with the female and the buzzer independently. Finally, Hollis (1984) observed that territorial male blue gouramis were more likely to win a contest against another territorial male if the contest was signalled by a stimulus that had been previously paired with the presentation of another male. These three examples indicate that the function of Pavlovian conditioning is to enable the animal to deal more efficiently with presentation of the US. More importantly, however, they also demonstrate that the CR is not a poor copy of the UR, but involves a complex of responses in addition to those that are selected for measurement.
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In addition to arguing that Pavlovian contingencies are both necessary and sufficient for avoidance learning, we can question whether avoidance learning involves instrumental reinforcement. Evidence from a number of sources suggests that avoidance behaviour is heavily influenced by hereditary factors. First, Bolles (1970) has made the point that although an animal can learn rapidly to jump out of a box to avoid shock, the same animal may take hundreds of trials to learn to press a lever to avoid shock. Belles (1970) proposed that to the extent that the required avoidance response was one of the animals natural defensive behaviours (SSDRs), it would learn the response rapidly. Arbitrary responses are acquired slowly, if at all. Second, a number of experimenters have been able to select for high- and low-avoidance performance in rats (Bammer, 1982; Bignami, 1965; Brush, Froehlich, & Sakellaris, 1979). Third, an examination of the literature dealing with strain differences in avoidance performance, has led Fuller and Thompson (1978) to conclude that the differences in performance derive from genetically related differences in initial responses to the CS or US. These three lines of evidence converge in such a way as to suggest that avoidance behaviour has little to do with learning of an instrumental nature. The conclusion that instrumental learning is not involved in avoidance behaviour is at variance with the assumption made by most theorists that the removal of an avoidance contingency produces a large deterioration in avoidance performance. In fact, an examination of the evidence suggests that the deterioration is less than one might expect. Although Bolles, Stokes. and Younger (1966) observed a relatively profound decrement in avoidance responding following removal of the avoidance contingency, Kamin (1956) observed only a moderate decrement. Further, Katzev and Mills (1974) and Bond (1984) found very little difference in anticipatory responding when comparing an avoidance procedure in which an escape response terminated both the CS and the shock, and a Pavlovian conditioning procedure in which neither avoidance nor escape was possible. The latter condition involved a brief, 0.5-s shock which was presented following the termination of the buzzer CS. The animal could not escape or avoid the shock, which was delivered if the animal remained where it was, or was presented as soon as the animal crossed from one side of the shuttlebox to the other. Despite this lack of functional effectiveness, animals developed high levels of anticipatory responding (Bond, 1984; Katzev & Mills, 1974). Further, the latency to cross from one side to the other declined over trials (Bond, 1984). A pseudoconditioning control group which received unpaired presentations of the buzzer and the shock, displayed very little anticipatory responding, indicating that the behaviour of the Pavlovian conditioning group was due to the pairing of the buzzer and the shock (Bond, 1984). The data outlined above indicate that substantial anticipatory responding can develop under a Pavlovian conditioning contingency. This finding poses a
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173
considerable problem for most extant theories of avoidance. However, if a Pavlovian conditioning model is to account for avoidance responding, there remains the problem of why successful avoidance responding, which necessarily means that the CS is not followed by the US, does not lead to Pavlovian extinction. Masterson and Crawford (1982) have outlined one possible reason for this phenomenon, namely that defense system activation will be highly volatile. It will be aroused whenever the animal may be hurt, not just when it is certain to be hurt. They note that the defense system must be volatile, because the failure to be ready for danger is potentially life-threatening. Empirically, we can ask whether avoidance responding is really any different from other Pavlovian paradigms in terms of resistance to extinction. Mackintosh (1974) has outlined evidence indicating that avoidance responses are not always highly resistant to extinction. Moreover, there are many demonstrations that responses acquired following uppetitiue Pavlovian conditioning are also hig/zhly resistant to extinction. The classic examples can be found in studies involving omission (Sheffield, 1965) negative automaintenance in autoshaping (Williams & Williams, 1969) and the “long-box” paradigm developed by Jenkins (1977). All of these examples demonstrate that CRs based upon appetitive Pavlovian conditioning are highly resistant to extinction. Clearly, the “problem” of resistance to extinction is not restricted to aversive Pavlovian conditioning. It seems that where biologically significant events are concerned, that is, Pavlovian USs, it is more important for the animal to be ready for their occurrence, even if they do not occur, than vice-versa. If a Pavlovian conditioning model of avoidance learning is to be utilized in the study of the development of human phobias, we must address the argument that conditioning plays only a minor role in the development of human fear. Before addressing this issue, however, we must first trace briefly some important changes which have occurred in theoretical accounts of Pavlovian conditioning during the past 25 years.
4. Theoretical developments
in Pavlovian conditioning
Detailed discussions of important changes which have taken place in the study of conditioning have been provided by several authors (e.g., Furedy & Riley, 1987; Mackintosh, 1983) and only a brief overview will be provided here. It is difficult to overemphasize the influence of Hull (Hull, 1943) in shaping approaches to conditioning. As Mackintosh (1983) has noted, however, Hull’s analysis owed more to Thorndike (1911) and his “law of effect” than to Pavlov’s (1927) theorizing. Whereas for Pavlov, the process underlying conditioning was one of stimulus substitution (i.e., the CS coming to substitute for the US), Hull’s analysis emphasized the strengthening of receptor-effector
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connections through drive reduction. Hull’s approach represented S-R behaviourism par excellence. That is, conditioning was regarded as the acquisition of new reflexes and was said to involve an S-R (CS-CR) association. In contrast to the Hullian position, Tolman (1932) and his colleagues adopted a position which asserted that conditioning involves the learning of relationships between events occurring in the environment. In this sense, conditioning is said to involve the acquisition of new knowledge. It follows from a Tolmanian approach that any single measure of conditioning is only one of several possible indicators of the central learning process. In contrast to the Hullian position that there is a close relationship between learning and performance, Tolman stressed the distinction between learning and performance, and argued that what is learned is not always evident through immediate changes in behaviour. This has been referred to as the problem of behavioural silence (Dickinson, 1980). There is no doubt that the Hullian approach was dominant from 1930 to the 1960s. Important theoretical developments since 1960, however, owe much more to Tolman’s S-S approach than to Hull’s S-R approach. Essentially, extant theories of conditioning represent an information-processing approach in which for the most part, associations are said to be learned between stimuli. In other words, Pavlovian conditioning is considered to allow organisms to track causal relationships in their environments. Current animal-based theories emphasize the unexpectedness of the US (Rescorla & Wagner, 1972) the extent to which the CS and the US are primed or prerepresented in a short-term memory store (Wagner, 1978), the relative predictive accuracy of all cues present on a CS-US pairing trial (Mackintosh, 1974; Pearce & Hall, 1980), or the type of processing (automatic or controlled) that is devoted to the CS (Pearce & Hall, 1980). A key assumption of all theories is that the CS and US must be processed conjointly in a limited capacity processing mechanism in order for learning to occur. Recent developments in accounts of human conditioning have followed a similar route. Thus, ohman (1979, 1983) has stressed that the CS and the US must be processed conjointly in a short-term memory system, and that for this to occur, both stimuli must capture the subject’s attention, That is, both must elicit orienting. Dawson and Schell(1985,1987) have argued that conscious, controlled processing of the CS and the US is necessary for conditioning to occur. Of course, not all theorists claim that Pavlovian conditioning involves S-S learning in all situations (see Furedy, 1988, this issue; Furedy & Riley, 1987). Rescorla (1980) has provided some evidence that second-order conditioning may involve S-R associations, and ohman and his colleagues (e.g., ohman, Dimberg, & Esteves, in press) have argued that CSs which form part of “prepared” associations may be learned about and may control behaviour in situations in which controlled processing is prevented. Nevertheless, the S-S approach in one form or another has proved to be useful in providing a
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framework for explanations of phenomena such as latent inhibition, blocking, overshadowing, and inhibitory conditioning (Mackintosh, 1983). Having sketched some of the important developments in theories of Pavlovian conditioning during the past 25 years, we can now examine the assertion that Pavlovian conditioning plays only a minor role in the development of human phobias.
5. Pavlovian conditioning and the development of human phobias In an influential review, Rachman (1977) concluded that although it is clear that laboratory animals can acquire fear through conditioning, principles of conditioning cannot provide a comprehensive account of the development of human fear. Rachman’s conclusions were based on (1) the failure to acquire fear in fear-evoking situations, (2) the difficulty of producing conditioned fear reactions in humans, (3) the reliance of conditioning theory on the premise of equipotentiality, (4) the distribution of fears in normal and neurotic populations, (5) the fact that phobics recount histories that are inconsistent with conditioning theory, and (6) fears can be acquired vircariously. Before discussing these issues, we need to examine the type of model of Pavlovian conditioning adopted by Rachman (1977). According to Rachman, “neutral stimuli which are associated with a fear or pain-producing state of affairs, develop fear qualities” (p. 376). Moreover, the strength of the conditioned fear is said to be determined by “the number of repetitions of the association between the pain/fear experience and the stimuli, and also by the intensiy of the fear or pain experienced in the presence of the stimuli” (p. 376). Although Rachman (1977) did not discuss the kind of model of conditioning on which he based his critique, his statements seem to imply an S-R approach. With this in mind, we can now examine each of Rachman’s (1977) arguments in turn. 5. I. Failures to acquire fear According to Rachman’s (1977) review, repeated air-raids during the Second World War did not produce an increase in psychiatric disorders and few prolonged phobic reactions. These findings, according to Rachman, militate against a conditioning account of the acquisition of fear. However, S-S theories assume that an association would have been learned between specific stimuli and the complex of events which constitutes an air-raid. Thus, fear which resulted from air-raids might be expected to occur only in the presence of CSs which reliably predicted air-raids. As far as we can see, an S-S conditioning model does not predict diffuse consequences such as an increase
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in psychiatric disorders, but does predict the occurrence of fear only in the presence of the CSs which have been reliably followed by air-raids. At a more general level, it should be noted that most theories of conditioning emphasize the importance of not only temporal contiguity between the CS and the US, but also CS-US contingency. That is, an important determinant of conditioning is said to be the probability of US occurrence in the presence of the CS (p(US/CS)) relative to the probability of US occurrence in the absence of the CS (p(US/CS)). When p(US/CS) = p(US/CS), no conditioning occurs (Rescorla, 1967). Thus, repeated CS-US pairings do not necessarily produce conditioning when p(US/cs) is high. 5.2. Conditioning
human fears
According to Rachman (1977) there is little evidence of laboratory fear conditioning in humans. Although we do not disagree with this conclusion, we do not view it as an indictment of a conditioning model of the acquisition of fear. Most theories of conditioning hold that the asymptotic level of learning is determined by the intensity of the US (usually denoted by X), and it is clear that the majority of human studies use somewhat less than intense USs. The most common description of a shock US in human work is “uncomfortable, but not painful”, and US intensity is almost always set by subjects themselves. Moreover, to argue that human fear conditioning has rarely been demonstrated in the laboratory is not to say that it cannot be demonstrated. Indeed, Rachman himself cited work by Sanderson, Laverty, and Campbell (1963) which demonstrated human fear conditioning with a potent US, injections of scoline. Our contention is simply that in most human conditioning studies, the US is not sufficiently intense to produce fear. That is not to say, of course, that learning does not occur with nonnoxious USs or that different processes are involved. 5.3. The equipotentiality
premise
Rachman (1977) has argued that the fact that not all stimuli are equally conditionable poses difficulties for a conditioning model of fear acquisition. This argument can be analysed on different levels. On one level, it can be noted that all theories of conditioning include a CS learning rate parameter (usually denoted by a) which determines the rate at which learning occurs. That is, rate of learning is determined by the salience (Rescorla & Wagner, 1972) or associability of the CS (Pearce & Hall, 1980). In addition, when a number of potential CSs occur together and are followed by a US, the most salient CS will overshadow the less salient CSs. That is, the less salient CS acquires less associative strength when presented in compound with a more salient CS than it does when presented alone. Thus, when compound CSs are
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followed by a US, it cannot be assumed that all CSs will acquire associative strength. At another level, differences in ease of conditioning can be examined in terms of differences in associations. For example, Seligman (1970) has proposed that associations can be ordered along a dimension of preparedness from prepared, through unprepared, to contraprepared. General process theories of conditioning do not incorporate the notion that associations themselves are differentially responsive to CS-US contingencies; however, it is by no means clear that they should do so. Both human (e.g., Qhman, Dimberg, & Ost, 1985) and animal data (e.g., Garcia & Koelling, 1966) on selective associations can often be interpreted without recourse to a concept of preparedness (Maltzman & Boyd, 1984; McNally, 1987; Schwartz, 1974). Moreover, Logue (1979), on the basis of a comprehensive review of the taste aversion literature, concluded that different principles are not required to account for taste aversion and “ traditional” learning. 5.4. Distribution
of fears
As Rachman (1977) has noted, the distribution of fears in a general population and the distribution of phobias is biased. Thus, fear of snakes is represented much more strongly than is fear of dental treatment or fear of injections (Agras, Sylvester, & Oliveau, 1969). From Rachman’s perspective, these findings pose difficulties for a conditioning account of fear acquisition. There are two ways of handling this criticism. First, we have already noted that theories of conditioning propose that more salient stimuli will gain associative strength more rapidly than will less salient stimuli, and if CSs of high and low salience occur together, the former will overshadow the latter. In addition, the phenomenon of latent inhibition may be able to account for the fact that fear of some events is prevalent whereas fear of others is not. Latent inhibition refers to a retardation of conditioning as a consequence of preexposure to the CS in the absence of a US and has been demonstrated in both human and nonhuman animals (Siddle & Remington, 1987). To use the examples provided by Rachman, it seems more likely that lambs are more likely to be encountered in the absence of an aversive consequence than are snakes. Another approach is to argue that the association between snakes and an aversive outcome is selective in the sense that it is prepared (e.g., ohman et al., 1985). Essentially, ijhman (1988, this issue) has argued that because reptiles were dangerous to prototypical mammals, a propensity to associate reptiles with aversive outcomes has become part of an open genetic program. It is important to note, however, that even if it is assumed that phobias represent prepared learning (Seligman, 1971) the underlying mechanism of acquisition is conditioning.
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5.5. Patients’ reports of fear onset The fact that patients often cannot recall a conditioning-like episode in marking the onset of a phobia has been taken by Rachman (1977) to indicate that conditioning is not a satisfactory account of the genesis of fears. However, more recent evidence indicates that with patient groups, a large proportion (46681%) of animal phobics, social phobics, claustrophobics, blood phobics, dental phobics, and agoraphobics ascribe the onset of their phobias to a conditioning experience (Ost & Hugdahl, 1981, 1983, 1985). In the case of subclinical fears, the frequency of conditioning experiences at the onset of fears is lower (Fazio, 1972; Rimm, Janda, Lancaster, Nahl, & Dittmar, 1977)., It is interesting to note, however, that Garb and Stunkard (1974) have reported that 87% of a normal sample recalled that a conditioning episode marked the start of a food aversion. Thus, although the recall data suggest that conditioning cannot account for all phobias and subclinical fears, it nevertheless figures in a substantial proportion of them. 5.6. Vicarious transmission
of fear
In his discussion of the vicarious transmission of fears, Rachman (1977) has drawn a distinction between vicarious conditioning and the social transmission of fear through information. There is little doubt that learning can occur through both of these mechanisms, but we are not convinced that the phenomena pose problems for a conditioning account of fear acquisition. For example, Hygge and ohman (1978) have demonstrated that electrodermal conditioning can occur vicariously to facial expressions of emotion. With regard to the information variables, much of the human conditioning literature during the past 20 years has been concerned with the effects of instructions on the acquisition of conditioned responses (Dawson & Schell, 1985). It is quite clear that conditioning effects can be obtained through instruction alone. Thus, rather than viewing vicarious acquisition of fear as a phenomenon which provides difficulties for a conditioning account of fear acquisition, we view it as a phenomenon to be integrated within a conditioning model. In summary, our contention is that the criticisms which Rachman (1977) made of a conditioning model of the acquisition of fear are misplaced. Our argument is that the model of conditioning implicit in Rachman’s critique represents an S-R approach in which temporal contiguity is both necessary and sufficient for learning to occur. We argue further that an information processing model of conditioning in which the emphasis is on the development of S-S associations and on CS/US contingency provides considerably more explanatory power and avoids the criticisms made by Rachman (1977). AS Reiss (1980) has noted, “. . even before behavior therapists had discovered problems in applying contiguity theory to human behavior, animal learning
D.A. T. Siddle and N. W.
theorists 386).
discovered
problems
Bond / Pavlovianconditioningandphobias
in applying
the model
to animal
behavior”
119 (p.
6. Implications of a conditioning model of avoidance 6.1. A Pavlovian learning
account
of avoidance
learning
and theories
of associative
A major advantage of the present approach is that it can utilize formal models of Pavlovian conditioning to formulate predictions which allow us to distinguish between competing theoretical accounts. For example, a preparedness approach to the role of learning in the acquisition and maintenance of human phobias (6hman et al., 1985; Seligman, 1971) predicts that human conditioning to fear-relevant stimuli will be more resistant to extinction than will conditioning to fear-irrelevant stimuli. In contrast, Maltzman and Boyd (1984) have outlined an account in which fear-relevant stimuli are viewed simply as being more salient. Salience of a CS is a parameter in the RescorlaWagner (1972) model of associative learning, and other things being equal, the model predicts that CRs to more salient stimuli will extinguish faster than will CRs to less salient stimuli. The bulk of the evidence suggests conditioning with fear-relevant stimuli is more resistant to extinction, as predicted by “preparedness theory” (McNally, 1987). 6.2. Dissociation
between behavioural,
autonomic,
and subjective measures of fear
As already noted, most models of avoidance learning have considered fear-reduction as the reinforcer for avoidance behaviour. These models have been embarrassed by the frequent finding that animals display efficient avoidance behaviour without displaying fear to the putative CS (Mineka, 1979). The same problem occurs when fear-reduction theories of avoidance are used to account for the acquisition and maintenance of human fears. In contrast, the present model does not conceptualize fear-reduction as a reinforcer of avoidance behaviour. We suggest that the learning of an association between a CS and an aversive US will produce conditioning of both fear and the appropriate response, for example, running. However, the latter does not depend on the former and the two will not necessarily covary in a tight manner. Given that our account is based on the premise of biological functionality, we have to answer the question of why fear occurs at all in avoidance situations. An answer can be found in the perceptual-defensive-recuperative model proposed by Bolles and Fanselow (1980). Briefly, Bolles and Fanselow have suggested that conditioned fear is associated with the release of endogenous opiates which serve to ameliorate pain. The reduction of pain serves to
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conditmning and phobias
prevent the occurrence of recuperative behaviour which might interfere with the animal’s ability to deal efficiently with the US. The reasoning suggests that an animal will display fear in the early stages of acquisition. However, when it has begun to deal efficiently with the US, fear is expected to subside. A mechanism for the di~nution of fear can be found in Schull’s (1979) conditioned opponent-process model. Clearly, our approach can handle the bothersome fact that once animals start to avoid efficiently, they no longer display fear. If, however, the animal’s current response is prevented, fear will again be elicited in order to enable adjustment to the new contingency (Baum, 1970). Our account serves to indicate why behavioural, autonomic, and subjective measures of fear do not necessarily covary. &en that the components of fear perform different functions and that the relative importance of these functions may change over time, we do not expect a close correspondence between them; this is indeed the case (Lang, 1968). Another prediction which follows from our analysis concerns the effects of phobic stimulation in phobic patients. As already noted, it can be argued that conditioned fear results in the release of endogenous opiates (Bolles & Fanselow, 1980). If some human phobias are acquired through conditioning, it is possible to argue that, because fear is associated with the release of endogenous opiates, human phobics will exhibit analgesia in the presence of the relevant phobic stimulus. Furthermore, the analgesia will dissipate following a course of successful treatment.
7. Conclusion In summa~, current conceptualizations of classical conditioning indicate that it is a dynamic process which enables animals to deal effectively with causal relationships in the environment and thus with biologically important events. Conceptualizations of this sort enable us to account for the acquisition and maintenance of avoidance behaviour in animals and for phobias in humans. The theoretical integration of animal and human work provides a rich base in which current models of Pavlovian learning enable us to make predictions about the outcome and effects of particular interventions.
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