One-trial overshadowing: evidence of distributive processing.

Evidence of "overshadowing" following a single training trial was obtained in each of five experiments measuring stimulus-evoked suppression of licking in rats. In three conditioned emotional response experiments, less conditioned suppression was observed to either a light or a tone when the stimuli had been paired with shock in compound that when either had been paired with shock in isolation. This difference occurred when the stimuli were diffuse and produced differential orienting behaviors on the training trial (Experiment 1). But, it was as demonstrable when the stimuli were designed to be localizable from the same source and produce apparently compatible orienting behaviors (Experiments 2A and 3A). The difference was also as substantial when the stimulus compound was composed of successive, nonoverlapping stimulus elements as when composed of simultaneous elements (Experiment 3A). In two companion habituation studies (Experiments 2B and 3B) using the same stimulus arrangements but no pairing with shock, there was consistently less habituation of the unconditioned suppression to the visual and tonal stimuli when exposed in compound rather than in isolation. One-trial "overshadowing" is at odds with those theories that approach overshadowing only as a multiple-trial phenomenon. The apparent generality of the effect provides impetus for a theoretical account that supposes some manner of distributive processing.