What is a memory system? Horel's critique revisited

J.A. Horel's critique of what he termed "the hippocampal memory hypothesis" turns out, 23 years later, to have been remarkably discerning and prophetic. There is now an overwhelming weight of evidence to confirm his four key proposals: that selective destruction of the hippocampus or fornix does not produce dense global amnesia; that the effects of hippocampal or fornix lesions are not primarily a memory impairment, but an impairment in processing spatial information; that damage to the anterior temporal stem is part of the explanation of dense temporal lobe amnesia; and that the interaction of temporal cortex with prefrontal cortex is essential in memory. This review summarizes the modern evidence that reinforces each of these four proposals. A final section argues that, not only in the case of the hippocampus but also in the case of other temporal and frontal cortical areas that are involved in normal memory, the concept of a "memory system" is harmful.