The inhibitory effect of backward conditioning as a function of the number of backward pairings

Independent groups of Ss received classical conditioning acquisition following 0, 5, 10, 25, or 50 preexposures to the conditioning stimuli in a backward-paired manner (the US preceding the CS). In both the conditioned suppression situation with rats and the eyelid conditioning situation with rabbits, backward preexposure retarded acquisition (in agreement with earlier findings). Furthermore, increasing backward-paired experience with the conditioning stimuli was associated with increasingly deleterious effects on acquisition performance. These results are contrary to suggestions that backward-conditioning-induced retardation of subsequent acquisition results only from extensive backward preexposures, with fewer such preexposures producing excitatory effects.