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 hippocampus for learning place responses, animals with damage to the afferent 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 their connections to the temporal lobe, can mediate place learning or that some extrahippocampal structure is sufficient. We examined this question using rats with ibotenic acid lesions of the cells of the hippocampus. Rats were pretrained to swim to a visible platform and then given probe trials on which the visible platform was removed. Video and kinematic 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 response. There were, however, abnormalities in their swimming patterns, and despite having acquired one place response, they did not then acquire new place responses when only the hidden platform training procedure was used. These results demonstrate that place learning can be acquired by rats in which the hippocampus proper is removed. Contrasts between 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.