Backward conditoning as an inhibitory procedure

Four independent groups of subjects received classical conditioning acquisition following exposure to the CS, the US, both in a noncontingent manner, or both in a backward-paired manner (the US always immediately preceding the CS). A fifth group received no preconditioning experience with the conditioning stimuli. In both the conditioned suppression situation with rats (Experiment 1) and the eyelid conditioning situation with rabbits (Experiment 2), preconditioning experience with the conditioning stimuli retarded acquisition. This retardation was most pronounced in subjects which had preexposure to the CS and the US in a backward-paired manner, suggesting that the backward contingency produced inhibitory tendencies in excess of those expected simply on the basis of any nonassociative effects of adaptation to the CS and US.