Ralph R. Miller

The Comparator Hypothesis: A Response Rule for The Expression of Associations

Publisher Summary This chapter describes the potential explanatory power of a specific response rule and its implications for models of acquisition. This response rule is called the “comparator hypothesis.” It was originally inspired by Rescorlas contingency theory. Rescorla noted that if the number and frequency of conditioned stimulus–unconditioned stimulus (CS–US) pairings are held constant, unsignaled presentations of the US during training attenuate conditioned responding. This observation complemented the long recognized fact that the delivery of nonreinforced presentations of the CS during training also attenuates conditioned responding. The symmetry of the two findings prompted Rescorla to propose that during training, subjects inferred both the probability of the US in the presence of the CS and the probability of the US in the absence of the CS and they then established a CS–US association based upon a comparison of these quantities. The comparator hypothesis is a qualitative response rule, which, in principle, can complement any model of acquisition.

Recovery of an overshadowed association achieved by extinction of the overshadowing stimulus

Abstract Three experiments are reported in which conditioned lick suppression by water-deprived rats was used as an index of associative strength. In Experiment 1, overshadowing of a light by a tone was observed when the light-tone compound stimulus was paired with foot shock. After initial compound pairings, the tone-shock association was extinguished in one group of subjects. Subsequently, these animals demonstrated significantly higher levels of suppression to the light relative to a control group in which the tone had not been extinguished. Experiment 2 replicated this effect while failing to find evidence to support the possibilities that extinction presentations of the overshadowing tone act as retrieval cues for the light-shock association, or that, via second-order conditioning, the light-shock association is actually formed during extinction of the tone. Experiment 3 determined that the recovery from overshadowing observed in Experiments 1 and 2 was specific to the extinction of the overshadowing stimulus rather than the extinction of any excitatory cue. Collectively, these results suggest that the debilitated response to an overshadowed stimulus does not represent an acquisition failure, but rather the failure of an acquired association to be manifest in behavior.

Conducting exposure treatment in multiple contexts can prevent relapse

The acquisition of anxiety disorders (e.g., phobias) is often thought to be mediated by classical conditioning processes (e.g., Wolpe, 1958, Psychotherapy by reciprocal inhibition Wolpe and Rowan, 1989, Behaviour Research and Therapy, 27, 583-585). Thus, the success of exposure therapy is possibly a consequence of extinction, and factors affecting extinction in Pavlovian conditioning are potentially relevant to clinicians who administer exposure therapy. The present experiments investigated the effects of conducting extinction in multiple contexts using rats as subjects in a conditioned suppression paradigm. In Experiment 1, subjects received conditioned stimulus (CS) and unconditioned stimulus (US) pairings in one context followed by extinction of that CS in one or three other contexts. When tested in an associatively neutral context (i.e., different from those of conditioning or extinction), rats that had received extinction in three contexts exhibited less responding to the CS than rats that had received extinction in one context. In Experiment 2, CS-US training occurred in either one or three contexts, followed by extinction of that CS in three other contexts. Testing in a neutral context revealed that rats conditioned in multiple contexts showed greater responding to the CS than rats trained in a single context. The results are discussed in the framework of memory retrieval, and the clinical implications are explored.

Sometimes-Competing Retrieval (SOCR): A Formalization of the Comparator Hypothesis.

Cue competition is one of the most studied phenomena in associative learning. However, a theoretical disagreement has long stood over whether it reflects a learning or performance deficit. The comparator hypothesis, a model of expression of Pavlovian associations, posits that learning is not subject to competition but that performance reflects a complex interaction of encoded associative strengths. That is, subjects respond to a cue to the degree that it signals a change in the likelihood or magnitude of reinforcement relative to that in the cues absence. Initially, this performance-focused view was supported by studies showing that posttraining revaluation of a competing cue often influences responding to the target cue. However, recently developed learning-focused accounts of retrospective revaluation have revitalized the debate concerning cue competition. Further complicating the picture are phenomena of cue facilitation, which have been addressed less frequently than cue competition by formal models of conditioning of either class. The authors present a formalization and extension of the comparator hypothesis, which results in sharpened differentiation between it and the new learning-focused models.

Information and expression of simultaneous and backward associations: Implications for contiguity theory ☆

Abstract Five conditioned lick-suppression experiments with water-deprived rats examined the possibility that simultaneous and backward associations are learned, but are not expressed as anticipatory responses in common indexes of associative strength. Experiments 1–4 used a sensory preconditioning procedure in which clicks preceded the onset of a tone. Subsequently, the tone was paired with footshock in either a forward, simultaneous, or backward arrangement. In no case did the tone trained in the simultaneous or backward manner elicit a conditioned response. However, Experiments 1, 2, and 3 determined that the clicks, which predicted the tone, evoked equally strong conditioned responses regardless of whether the tone was paired with the shock in a forward, simultaneous, or backward manner. Experiment 4 found that responding to the clicks was degraded following postconditioning extinction of the tone, regardless of whether the tone had been paired with the shock in a forward or simultaneous manner. Experiment 5 determined that if the click and tone were paired simultaneously, the click failed a test for excitation following tone-shock simultaneous pairings but passed a test for excitation following tone-shock forward pairings. Collectively, these findings suggest that predictive information (i.e., a forward relationship between stimuli) is not necessary for the acquisition of an association, but may promote the expression of the association in an anticipatory response system. Moreover, these results suggest that associations are not simple linkages, but contain information regarding the temporal relationship of the associates.

Biological significance in forward and backward blocking: resolution of a discrepancy between animal conditioning and human causal judgment.

Similarities between Pavlovian conditioning in nonhumans and causal judgment by humans suggest that similar processes operate in these situations. Notably absent among the similarities is backward blocking (i.e., retrospective devaluation of a signal due to increased valuation of another signal that was present during training), which has been observed in causal judgment by humans but not in Pavlovian responding by animals. The authors used rats to determine if this difference arises from the target cue being biologically significant in the Pavlovian case but not in causal judgment. They used a sensory preconditioning procedure in Experiments 1 and 2, in which the target cue retained low biological significance during the treatment, and obtained backward blocking. The authors found in Experiment 3 that forward blocking also requires the target cue to be of low biological significance. Thus, low biological significance is a necessary condition for a stimulus to be vulnerable to blocking.

Massive extinction treatment attenuates the renewal effect

Two experiments with rats as subjects investigated whether massive extinction can attenuate the renewal effect. Experiment 1 investigated whether moderate or massive extinction could prevent the return of conditioned responding following Pavlovian conditioning in Context A, extinction in Context B, and subsequent testing in Context C (i.e., ABC renewal). Experiment 2 examined whether massive extinction could prevent renewal following training in Context A, extinction in Context B, and testing in Context A (i.e., ABA renewal). Both experiments observed attenuated renewal following massive, but not moderate extinction. Results are discussed in terms of contemporary theories of extinction.

Outcome Additivity and Outcome Maximality Influence Cue Competition in Human Causal Learning

Recent research suggests that outcome additivity pretraining modulates blocking in human causal learning. However, the existing evidence confounds outcome additivity and outcome maximality. Here the authors present evidence for the influence of presenting information about outcome maximality (Experiment 1) and outcome additivity (Experiment 2) on subsequent forward blocking. The results of Experiment 3 confirm that, with outcome maximality controlled, outcome additivity affects backward blocking but not release from overshadowing. Finally, the results of Experiment 4 demonstrate that information about outcome additivity has a similar effect on forward blocking if presented after the blocking training instead of before. The results are compatible with the idea that blocking results from inferential processes at the time of testing and not from a failure to acquire associative strength during training.

Reasoning Rats: Forward Blocking in Pavlovian Animal Conditioning Is Sensitive to Constraints of Causal Inference

Forward blocking is one of the best-documented phenomena in Pavlovian animal conditioning. According to contemporary associative learning theories, forward blocking arises directly from the hardwired basic learning rules that govern the acquisition or expression of associations. Contrary to this view, here the authors demonstrate that blocking in rats is flexible and sensitive to constraints of causal inference, such as violation of additivity and ceiling considerations. This suggests that complex cognitive processes akin to causal inferential reasoning are involved in a well-established Pavlovian animal conditioning phenomenon commonly attributed to the operation of basic associative processes.

Associations to Contextual Stimuli as a Determinant of Long-Term Habituation

A series of experiments was performed to determine whether long-term habituation of the acoustic startle response in rats is mediated by conditioned associations between contextual cues and the test stimulus. Experiment 1 established parameters yielding demonstrable long-term habituation of the startle response. Experiment 2 attempted to overshadow the hypothesized associations to contextual cues by providing a more reliable predictor of the acoustic stimulus. Experiment 3 investigated the effect of changes in contextual cues on long-term habituation. Experiment 4 provided treatments designed to extinguish the hypothesized associations between the context and the habituated stimulus. Experiment 5 sought latent inhibition of the hypothesized association between the contextual cues and the acoustic stimulus. The results of these experiments uniformly failed to support an associative model of long-term habituation of the startle response, but they are consistent with a nonassociative model emphasizing habituation to the entire experimental situation rather than exclusively to the iterated stimulus.