Summary

It is well established that the hippocampal region is critical to declarative and relational memory. Therefore, it is reasonable to expect that damage to the hippocampal region would cause impairment on any test that requires some feature of declarative/relational memory, and that on any other test, hippocampal damage is expected to have no effect. However, there have been several reports of paradoxical facilitation of learning and memory following hippocampal damage. Here, several examples in the study of animal learning and memory are discussed. In some experiments, hippocampal region damage results in the facilitation of learning simple stimulus–reward–response associations. In other experiments, the ‘flexibility’ of memory, exhibited in reversal learning and in learning multiple partially contradictory choice problems, is also facilitated following hippocampal damage. In yet other experiments, recognition based on the familiarity of stimulus combinations is improved following hippocampal damage. Each of these cases of paradoxical facilitation of learning and memory informs us about the distinctions between hippocampal-dependent memory processing and memory processes supported by other brain areas or systems. Furthermore, these findings show that competition between these systems can result in slower learning when the hippocampus is engaged, compared to when its contribution is removed.

Introduction

In the field of human memory research, our understanding of how the brain supports memory began with neuropsychological studies of patients with pervasive, ‘global’ amnesia.