Intertrial unconditioned stimuli preferentially interfere with delay conditioning.

One way to minimize excitation acquired by the conditioned stimulus (CS) is to introduce intertrial presentations of the unconditioned stimulus (US). However, even in the presence of frequent intertrial USs, Experiments 1a and 1b found that rats anticipated the customary arrival time of a food pellet US when it occurred before (embedded)-versus coincident with (delay)-the termination of a white noise CS. Delay conditioning emerged in Experiment 2 in the absence of intertrial USs; hence, the detrimental effects of intertrial USs depended on the CS-US relationship, delay versus embedded, and not the duration of CS-US interval. Experiments 3a, 3b, and 4 found that random USs located in the early portion of the intertrial interval increased the control acquired by contextual stimuli at the expense of temporal stimuli occasioned near CS termination. Our results suggest that delay relationships leave the CS especially vulnerable to the deleterious effects of intertrial USs.