Movements of exploration intact in rats with hippocampal lesions

Prompted by the theoretical prediction that damage to the hippocampus should abolish exploratory behavior, the present study examined exploratory movements in control rats and rats with hippocampal lesions produced with the neurotoxin N-methyl d-aspartate (NMDA). In four daily 30-min sessions, control and hippocampal rats were exposed to an open circular table under room lighting. Both control and hippocampal rats spent a majority of time near, and organized trips away from, a portion of the table (home base) near a large cue placed proximal to the table. On Day 1, control and HPC rats made equal numbers of head orientations and a comparable number of trips, featuring equal travel distance and numbers of stops. By Day 4, dwell times near the home base increased and other movements decreased in the control rats but the activity profile of Day 1 persisted in the hippocampal rats. The high degree of similarity in behavior between hippocampal and control rats on Day 1 and the persistence of this behavior in hippocampal rats on Day 4 suggests that the hippocampus is not necessary for the display of normal exploratory movements per se. The absence of habituation of exploration in hippocampal rats is discussed in relation to contemporary theories of hippocampal function.