Memory formation by refinement of neural representations: the inhibition hypothesis.

There is no reasonable doubt that the hippocampus plays an important role in memory processing. A virtually uncountable number of studies in animals and humans have revealed changes in neural activity in this structure during memory formation [Squire LR. Memory and the hippocampus: a synthesis from findings with rats, monkeys, and humans. Psychol Rev 1992;99:195-231; Squire LR, Stark CE, Clark RE. The medial temporal lobe. Annu Rev Neurosci 2004;27:279-306], and hippocampal damage reliably leads to impairments in a large number of memory tests. However, while several correlates of successful memory formation have been found in the hippocampus, it is still an open question why specific neural processes support encoding of a particular item. An answer to this question would help to resolve current debates about which memory functions are actually supported by the hippocampus, and why activity in the neural networks of the hippocampus is involved in, or even necessary for, some memory processes but not for others. In this review, we first summarize findings on the electrophysiological activity within the hippocampus during different memory processes. We try to differentiate whether the hippocampus is merely involved in these processes, or whether the hippocampus appears to be necessary for them. Based on a distinction between a more general "encoding state" and the more specific process of "content-specific memory formation", we review data on neural representations within hippocampus and neocortex. We suggest that during memory formation, the hippocampus renders neural representations more sparse by providing an inhibitory signal to the neocortex.