Analysis of US-Preexposure Effects in Appetitive Conditioning

In two experiments, rats received preexposure to one type of food followed by autoshaping in which presentation of one lever was associated with the preexposed food, and presentation of another lever with a novel type of food pellet. In both it was found that acquisition of the leverpress response occurred more readily on the lever associated with the novel food. This example of the US (unconditioned stimulus) preexposure effect is not to be explained in terms of the development of competing responses during preexposure. Explanations in terms of blocking by contextual cues and of habituation to the US are considered.