STIMULUS-PROCESSING AND STIMULUS SELECTION IN RATS WITH HIPPOCAMPAL-LESIONS

Rats with dorsal and ventral hippocampal lesions and sham-operated controls were either (a) trained to discriminate between two compound stimuli which contained a common element or (b) exposed to the same compounds under a condition in which each element was correlated with reinforcement 50% of the time. When each element was tested in isolation following training to condition (a), the partially reinforced cue, the element common to each compound, was overshadowed by elements more highly correlated with reinforcement among sham animals. However, no overshadowing was found with hippocampal rats: The common element was as effective in controlling behavior as elements more highly correlated with reinforcement. By contrast, hippocampal and sham rats assigned to condition (b) behaved similarly to the common element when it shared the same correlation to reinforcement as other elements. The data suggest that the attention-like deficit of hippocampal subjects may be due to an inability to encode stimuli properly. They appear deficient in forming appropriate internal representations of contingent events. © 1979 Academic Press, Inc.