Habituation of startle response, lick suppression, and exploratory behavior in rats with hippocampal lesions.

Rats with bilateral hippocampal lesions and controls with neocortical lesions were compared on the habituation of lick suppression and startle response. The animals with hippocampal lesions showed no consistent differences from controls on any measure within these two response systems. Importantly, the rats with hippocampal lesions showed significant retention of habituation over periods of 24 hr and 21 days. Experimental and control differences were not revealed when stimulation was presented on a 1-sec interstimulus interval. None of these results varied with the extent of the hippocampal lesions, which ranged from relatively small lesions restricted to the dorsal hippocampus to large lesions that damaged the hippocampus in its dorsal, posterior, and ventral aspects. In contrast to the startle response and lick suppression results, hippocampal lesions significantly disrupted Y-maze exploratory behavior, and the disruption was directly related to the extent of hippocampal damage. The data suggest that the hippocampus is not involved in any important way in the control of either short-term or long-term habituation of elicited, reflex-like behaviors but is importantly involved in the control of emitted, exploratory behaviors.