Local context and the comparator hypothesis

ROBERT C. BARNET 0 1 2 3 NICHOLAS J. GRAHAME 0 1 2 3 RALPH R. MILLER 0 1 2 3 0 The data reported here formed part of a thesis submitted by the first author to the State University of New York at Binghamton in partial fulfillment of the requirements for the master of arts degree. R.C.B. wishes to acknowledge the support and advice of his committee mem bers , Ralph Miller, Stanley Scobie, and Norman Spear, during all stages of this investigation. Support for this research was provided by National Institute of Mental Health Grant 33881 and the SUNY-Binghamton Center for Cognitive and Psycholinguistic Sciences. R.C.B. was sup ported by a Natural Sciences and Engineering Research Council of Canada postgraduate scholarship. Thanks are due to Hua Yin for com ments on an earlier version of the manuscript and to Douglas Dufore and Gerard Newcomer for assistance in data collection. Requests for reprints should be addressed to R. R. Miller , SUNY-Binghamton, Bing hamton, NY 13902-6000 1 State University of New York , Binghamton, New York 2 BARNET, GRAHAME, AND MILLER 3 Michael Domjan Department of Psychology University of Texas Austin , TX 78712 "Comparator" accounts of associative conditioning (e.g., Gibbon & Balsam, 1981; Miller & Matzel, 1988) suggest that performance to a Pavlovian CS is determined by a comparison of the US expectancy of the CS with the US expectancy of general background cues. Recent research indicates that variation in the excitatory value of cues in the local temporal context of a CS may have a profound impact on conditioned responding to the CS (e.g., Kaplan & Hearst, 1982), implicating US expectancy based on local, rather than overall, background cues as the critical comparator term for a CS. In two experiments, an excitatory training context attenuated responding to a target CS. In Experiment 1, the context was made excitatory by interspersing unsignaled USs with target CS-US trials. In this case, posttraining extinction of the conditioning context restored responding to the target CS. In Experiment 2, the target CS's local context was made excitatory by the placement of excitatory "cover" stimuli in the immediate temporal proximity of each target CS- US trial. In this experiment, posttraining extinction of the proximal cover stimuli, not extinction of the conditioning context alone, restored responding to the target CS. An observation from both experiments was that signaling the otherwise unsignaled USs did not appear to influence the associative value of the conditioning context. The results are discussed in relation to a local context version of the comparator hypothesis and serve to emphasize the importance oflocal context cues in the modulation of acquired behavior. Taken together with other recent reports (e.g., Cooper, Aronson, Balsam, & Gibbon, 1990; Schachtman & Reilly, 1987), the present observations encourage contemporary comparator theories to reevaluate which aspects of the conditioning situation comprise the CS's comparator term. - In a typical Pavlovian conditioning experiment, a con ditioned stimulus (CS), such as a light or tone, is paired with an unconditioned stimulus (US), such as food or foot shock. The consequence of these pairings is commonly assumed to be the establishment of a CS-US association, and learning is said to occur when the CS elicits a condi tioned response. However, acquisition of conditioned re sponding to a target CS is often impaired if unsignaled USs are interspersed among the CS-US pairings during the conditioning session. This response deficit, induced by unsignaled USs, has been demonstrated in a variety of species, including rats (e.g., Ayres, Benedict, &. Witcher, 1975; Rescorla, 1968, 1972, 1984), pigeons (e.g., Durlach, 1983, 1986, 1989; Gamzu & Williams, 1973; Jenkins, Barnes, & Barrera, 1981), and humans (e.g., Champion, 1961). Thus, at the behavioral level, there is good agreement that interspersed unsignaled USs interfere with conditioned responding to the target CS. At the theoretical level, there is much less agreement about the consequences of these unsignaled USs. Durlach (1983) and Rescorla (1984), for example, have argued that the detrimental effect of the unsignaled USs reflects a deficit in learning about the target CS that occurs be cause the training context blocks target CS acquisition. According to this blocking-by-context view, the unsig naled USs enhance the associative strength of the static contextual cues. Then, during target CS-US pairings, the excitatory context blocks the target CS from accruing as sociative strength (e.g., Tornie, 1981). This can beviewed as a form of selective learning in which cues present when a US is delivered (e.g., CS and contextual cues) compete for associative strength and thus behavioral control (i.e., Rescorla, 1972; Rescorla & Wagner, 1972). An alternative view of the effects of unsignaled USs is provided by Gibbon and Balsam's (1981; Gibbon, 1977) scalar expectancy theory (SET). Although these investi gators agree that the effect of unsignaled USs is to en hance the associative strength of the context, they diverge from the selective learning view by assuming that the as sociative outcome of target CS-US pairings proceeds in dependently from the associative value of the context. In this view, conditioned responding is determined by a com parison of the expectancy of the US during the entire con ditioning situation ("C" or cycle time, which is the aver age delay between USs) with the expectancy of the US during the CS ("T" or trial time, which is the average delay between CS onset and the US). Specifically, con ditioned responding is predicted when the CIT ratio ex ceeds a threshold value of approximately 2.0. According to SET, unsignaled USs decrease the cycle time, thereby lowering the CIT ratio, and consequently attenuate re sponding to the CS. A conceptually similar alternative to the selective learn ing view is provided by Miller and Schachtman's (1985) comparator hypothesis (see Miller & Matzel, 1988, p. 80 for a discussion of the differences between SET and the comparator hypothesis). This view concurs with SET in suggesting that conditioned responding is not determined by the absolute associative strength of the target CS, but is determined instead by a comparison of the associative value of the target CS to the associative value of other cues present during target CS training (i.e., comparator stimuli). According to the comparator hypothesis, defi cient excitatory responding to the target CS will be ob served when the associative value of the comparator stim ulus (e.g., context) is large relative to the associative value ofthe target CS. Accordingly, unsignaled USs would be expected to promote context-US associations that would be large relative to the target CS-US association and, con sequently, deficient excitatory responding to the target CS would be predicted. The positions taken by Gibbon and Balsam (1981) and Miller and Schachtman ( (...truncated)