Identifying cortical inputs to the rat hippocampus that subserve allocentric spatial processes: a simple problem with a complex answer.

A consideration of the cortical projections to the hippocampus provides a number of candidate regions that might provide distal sensory information needed for allocentric processing. Prominent among the input regions are the entorhinal cortex, the perirhinal cortex, the postrhinal cortex, and the retrosplenial cortex. A review of these sites reveals the surprising fact that in spite of their anatomical connections, removal of the perirhinal and postrhinal cortices has little or no effect on spatial tasks and hence does not functionally disconnect the hippocampus. Extensive retrosplenial lesions have only mild effects, and even lesions of the entorhinal cortex only partially mimic the effects of hippocampal lesions upon tests of spatial memory. In contrast, studies using c-fos imaging support the involvement of the entorhinal, postrhinal, and retrosplenial cortices, but not the perirhinal cortex. It is argued that there exist multiple aspects of spatial memory, and this is reflected in the multiple routes by which cortical information can reach the hippocampus. One consequence is that lesions in a single site often have surprisingly mild effects on standard spatial tests.