Absence of overshadowing in rats with hippocampal lesions

Abstract

Rats with hippocampal, cortical, and sham lesions were trained in two simultaneous discriminations: one between two odors, and the other between two visual stimuli. One odor signaled shock and another odor “no shock,” both with probability of 1. A visual stimulus signaled shock with probability of 1, but the other visual stimulus signaled “no shock” only two-thirds of the time. In this situation, the odor dimension overshadowed the visual dimension in sham animals but not in the hippocampal-lesioned animals. No assessment can be made of the behavior of animals with only neocortical damage, since they did not learn the visual stimulus-shock association even when the visual stimulus was given alone. These results support a description of some of the sequelae of the hippocampal lesions in terms of attentional deficits.