Element pretraining influences the content of appetitive serial compound conditioning in rats

Four experiments with rat subjects in a Pavlovian appetitive conditioning procedure examined the effects of prior pairings of S2 with the unconditioned stimulus (US) on the nature of the associations formed in S1----S2----US serial compound conditioning. Experiments 1 and 2 showed that prior training of S2 prevented the acquisition of stimulus-stimulus (S-S) associations between S1 and stimulus features of S2, but enhanced the acquisition of stimulus-response (S-R) associations between S1 and the emotional conditioned response (CR) evoked by S2. Experiments 3 and 4 showed that the effects of S2 pretraining were not due to S2 training itself, but rather to its endowing S2 with the ability to evoke a strong CR during the early stages of serial compound conditioning. In Experiment 3, suppression of the CR to a pretrained S2 during serial compound conditioning permitted the establishment of S-S associations. In Experiment 4, the induction of a CR in the presence of an untrained S2 during serial compound conditioning prevented the acquisition of S-S associations. Implications of these data for our understanding of compound conditioning are discussed.