Dorsal hippocampal function in unreinforced spatial learning

This study examined learning about the spatial environment by rats during a single 10 min period of exploration on an eight‐arm radial maze. Because no specific behaviors were learned during this procedure, the existence of learned spatial information was inferred from its retarding effect on subsequent conditioned cue preference (CCP) learning on the same maze. Previous experiments have shown that this form of spatial learning, measured in this way, requires an intact fimbria‐fornix and functional N ‐methyl‐ D ‐aspartate receptors. However, in the present experiments, large neurotoxic lesions of the dorsal hippocampus that impaired win‐shift learning failed to eliminate the retarding effect of exploration on CCP learning. This result was obtained in three independent replications. These findings fail to confirm the hypothesis that the hippocampus is involved in spatial learning when that learning occurs in the absence of reinforcers and does not produce any specific learned behaviors. Previous work showed that this form of “pure” spatial learning requires an intact fimbria‐fornix for acquisition but not for expression; the present findings suggest that the hippocampus is not required for either of these processes. The fimbria‐fornix may interact with other temporal lobe structures in mediating this form of learning. The function of the hippocampus may be limited in some way to situations that involve reinforcers and/or situations in which specific behaviors are learned. Hippocampus 10:226–235, 2000 © 2000 Wiley‐Liss, Inc.