CS-US interval as a determinant of the form of Pavlovian appetitive conditioned responses.

The effects of variation in temporal and predictive relations between the onset of visual and auditory conditioned stimuli (CSs) and delivery of a food unconditioned stimulus (US) on the form of conditioned responses (CRs) were examined in four experiments with rat subjects. In Experiment 1, substantially different CRs were evoked by CSs differing in duration and CS-US interval in delay conditioning. Using a second-order conditioning technique to assess the associative strength of CSs of different delay intervals, Experiment 2 showed that shorter intervals generated more strength than longer intervals, but the differences in CR form observed in Experiment 1 were not dependent on differences in associative strength. In Experiment 3, CSs of equal duration but different CS-US intervals in trace conditioning evoked CRs similar in form to those observed with delay procedures in Experiment 1. In Experiment 4, the onset of a CS of intermediate CS-US interval was made less predictive of US delivery by separate nonreinforced short duration presentations of that stimulus. Behavior evoked by the intermediate interval CS was more like that evoked by CSs of longer CS-US intervals. These data are discussed in terms of an orienting response hypothesis proposed earlier to account for differences in the nature of CRs evoked by CSs differing in physical characteristics such as modality.