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.

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