No abstract

Intermixed exposure to two similar stimuli, for example, AX and BX, improves subsequent discrimination between them compared to blocked exposure (the intermixed/blocked effect). Salience modulation models, developed mainly from research with nonhuman animals and exposure to widely spaced similar stimuli, explain this effect in terms of increased salience of the unique elements, A and B. Conversely, the results from experiments initially conducted with humans and exposure to close spaced similar stimuli have led to the suggestion that it is the development of well-unitized representations of unique elements that leads to better discrimination, leaving the unique elements with less effective salience. The experiments carried out here aim to replicate the intermixed/blocked effect in rats using an exposure procedure with rapid succession between stimuli and to assess the effective salience of unique elements. In Experiment 1, an aversion to a new flavor, Y, was conditioned and then an external inhibition test with AY was given. In Experiment 2, an aversion to A was conditioned and its extinction was measured on unreinforced trials. In Experiment 3, an aversion to AY was conditioned and the associated aversion to Y was measured. We found after rapid intermixed preexposure a reduction in generalization from the aversive Y element to the compound AY (Experiment 1) as well as a reduction in A's salience (Experiments 2 and 3) compared to the effects of blocked preexposure. The results are discussed in terms of the various mechanisms underlying perceptual learning, which appear to depend on the details of the task.