Different effects of unilateral and bilateral hippocampal lesions in rats on the performance of radial maze and odor-paired associate tasks.

The hippocampus plays an important role in the declarative or explicit memory in humans and is necessary for allocentric spatial learning and olfactory memory in animals. In primates and rodents, the bilateral hemispheres of the brain (especially the forebrain) symmetrically and asymmetrically contribute to diverse cognitive manipulations. In this study, we investigated the role of the hippocampus in spatial memory and in odor-paired associate memory by unilaterally or bilaterally lesioning this region in rats. The bilateral removal, but not the unilateral removal, of the hippocampus impaired both the acquisition of spatial working memory in the radial maze task and the retrieval of maze performance tested 1 month after the acquisition trials. In contrast, neither bilateral nor unilateral removal impaired the odor-paired associate learning. These findings suggest that the hippocampus is critical to the spatial memory, and that a unilateral hippocampus is sufficient for executing a spatial task. The present results also indicate that the hippocampus plays a minor role in odor-dominated associate learning and that some kinds of memories in rats may be processed independently by the left or right hippocampus.