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Acquisition and Performance Accounts of the Overexpectation Effect




Kimble, Whitney

Type of Degree





Overexpectation occurs when a stimulus individually conditioned to become an excitor (conditioned stimulus, CS) elicits attenuated responding due to subsequent reinforcement in compound with another excitatory stimulus (Kamin & Gaioni, 1974; Kremer, 1978). In three experiments with albino rats we examined two theoretical accounts of the overexpectation effect. The Rescorla-Wagner (1972) model accounts for overexpectation using an acquisition-focused approach, in which the individual associations between the CSs and the unconditioned stimuli, US, combine and lead to an expectation of a US larger in magnitude than that predicted by each individual CS. Responding is reduced to match the actual US magnitude when the larger magnitude does not occur. An alternative approach is provided by the Comparator Hypothesis, a performance-based model that suggests overexpectation results from a comparison between associations that occurs at the moment of test. Conditioned responding to the test CS (T) is determined by the association between T and the US in comparison to the extent to which the other stimulus (comparator stimulus) predicts the US. Experiment 1 was an attempt to obtain overexpectation in our preparation. Experiment 2 assessed whether preexposure of the companion stimulus can attenuate overexpectation. Experiment 3 assessed whether compound preexposure attenuated the preexposure effect and whether attenuation of overexpectation occurs when one of stimuli that underwent compound preexposure is extinguished.