Associative and nonassociative theories of the UCS preexposure phenomenon: Implications for Pavlovian conditioning.

Abstract

Excitatory Pavlovian conditioning of a discrete conditioned stimulus is attenuated by prior exposure to the unconditioned stimulus. The unconditioned stimulus preexposure phenomenon is observed in a variety of Pavlovian conditioning procedures as diverse as eyelid conditioning, the conditioned emotional response, and conditioned taste aversion learning. This article discusses the variables that affect the unconditioned stimulus preexposure phenomenon and uses this information in evaluating both associative and nonassociative accounts of the phenomenon. At least one associative account, based on context blocking, and at least one nonassociative account, based on central habituation of the emotional response to the unconditioned stimulus, remain viable. The primary goals of research in Pavlovian conditioning are to determine the variables that influence the formation of conditioned responses and then to specify their mechanisms of action. A number of investigators are currently examining how one such variable, the organism's experience with the unconditioned stimulus, affects the course of conditioning. It has been shown in a variety of Pavlovian conditioning paradigms that exposure to the unconditioned stimulus (UCS) prior to the initiation of pairings of the conditioned stimulus (CS) and the UCS attenuates the formation of the excitatory conditioned response (CR). The UCS preexposure effect interests many investigators because of their conviction that a thorough analysis of this phenomenon will further our understanding of the necessary conditions for Pavlovian conditioning.