Overshadowing and associability change: examining the contribution of differential stimulus exposure.

In two appetitive conditioning experiments with rats, we investigated the mechanisms responsible for demonstrations of the superior associability of overshadowed conditioned stimuli (CSs) relative to control CSs. In Experiment 1, we investigated whether previous demonstrations were a consequence of differences in the relationship between the CSs and the unconditioned stimulus (US) or of differences in the conditions of exposure to the CSs. Rats received trials with X, Y, and an AB compound, but no delivery of the US (X-, Y-, AB-). A subsequent AY-, AX+, BY + test discrimination revealed that the AY/BY component of the discrimination was solved more readily than the AY/AX component--suggesting the contribution of an exposure effect. In Experiment 2, we better equated the conditions of exposure between A and Y by using AB+, XY+, X- training in Stage 1. In Stage 2, instrumental responses were rewarded during an AY compound. A final test revealed that Y took better control of instrumental responding than did A. The results of these experiments are discussed in terms of classical and contemporary theories of learning and attention.