Associative changes with a random CS-US relationship.

Three experiments examined conditioned magazine approach in rats when a positive unconditioned stimulus (US) bore a random relation to a conditioned stimulus (CS). Experiment 1 found that over the course of conditioning the CS initially elevated responding relative to the baseline but then lost the power to do so. Transfer tests revealed that a CS-US association developed early and persisted despite the decline in magazine responding. Experiment 2 confirmed the persistence of CS-US associations and found them to be more substantial when a different US occurred during the CS than in its absence. In Experiment 3, when the situation was exposed to US-alone presentations prior to introducing the CS, there was little evidence that a subsequent random relation between the CS and US produced an association between them. These results agree with those of blocking and overshadowing experiments using discrete CSs and support an interpretation of the random procedure in terms of competition between the background and CS for conditioning.