EVIDENCE FOR EXTRAHIPPOCAMPAL INVOLVEMENT IN-PLACE LEARNING AND HIPPOCAMPAL INVOLVEMENT IN PATH INTEGRATION

Although there is a good deal of evidence that animals require the hip pocampus for learning place responses, animals with damage to the affe rent and efferent fibers coursing through the fimbria-fornix have been shown to acquire a place response. This finding suggests either that the cells of the hippocampus proper (CA1-4 and dentate gyrus), via the ir connections to the temporal lobe, can mediate place learning or tha t some extrahippocampal structure is sufficient. We examined this ques tion using rats with ibotenic acid lesions of the cells of the hippoca mpus. Rats were pretrained to swim to a visible platform and then give n probe trials on which the visible platform was removed. Video and ki nematic analyses showed that the hippocampal rats expected to find the platform at its previous location because they swam directly to that location and paused and turned at that location after the platform was removed. Additional tests confirmed that they had learned a place res ponse. There were, however, abnormalities in their swimming patterns, and despite having acquired one place response, they did not then acqu ire new place responses when only the hidden platform training procedu re was used. These results demonstrate that place learning can be acqu ired by rats in which the hippocampus proper is removed. Contrasts bet ween conditions in which hippocampal rats acquire a place response and conditions in which they fail suggests that the hippocampus may serve as an on line system for monitoring movement and integrating movement paths. (C) 1996 Wiley-Liss, Inc.