What does the hippocampus really do?

Much of the evidence used to implicate the hippocampus in learning and memory has been obtained from clinical cases and/or experimental studies with animals where the damage is extensive and includes more than just the hippocampus. When the damage is limited to the cells that comprise the hippocampus (CA1-CA3 pyramidal cells, hilar and granule cells in the dentate gyrus) the effect on behavior in the rat is more limited than what is usually reported. Selective, axon-sparing ibotenic acid lesions of the hippocampus were used in the experiments that are reviewed to study the effects of removing the hippocampus on: (1) the acquisition of spatial and non-spatial information; (2) complex, non-spatial representational learning; and (3) acquisition and utilization of contextual information. The results indicated that rats with the hippocampus removed were impaired on those tasks that require the utilization of spatial and contextual information but performed like controls in learning about and handling (even complex) non-spatial information. Future research utilizing selective lesions of the hippocampus and sensitive behavioral testing techniques should help clarify the extent to which the impairments in the acquisition of spatial information and the ability to utilize contextual, background cues can be reduced to a single, underlying learning process.