Episodic memory and the hippocampus: another view.

A popular view of the function of the hippocampus maintains that this structure temporarily encodes the neocortical representation of the experience of an episode. It uses the encoding to recreate repeatedly the neocortical representation. It is said that in time the episodic memory becomes consolidated in the neocortex and can be retrieved independent of the hippocampus. This paper is critical of that view and begins by raising four concerns. These include a question of how the hippocampus could encode the rich complexity of neocortical representations in sufficient detail to recreate them. And it observes that some data indicate episodic memories remain dependent on the hippocampus for life. Another view of hippocampal function is presented which addresses these concerns. Basically, this view hypothesizes that the ability to retrieve episodic memories involves the interplay between two modes of hippocampal function. Processes during the theta mode facilitate the development of context memory in the hippocampus and the registration of unique events in the neocortex, but block the influence of context memory upon the neocortex. By contrast, during the non-theta mode, context memory is projected onto the neocortex, creating a contextual framework. It is proposed that the ability to retrieve memory of episodes depends on the development, through contiguity, of associations between the representation of the episode (created during theta) and a contextual framework (evoked during non-theta). From this perspective, the episode does not need to be encoded in the hippocampus, but remains dependent indefinitely on associations formed between the episodic memory in the neocortex and a context memory maintained in hippocampal structures. However, for the associations that enable retrieval to form, the creation of the representation of an episode during theta must be followed quickly by the evocation of a contextual framework. During an extended period of the theta mode as occurs during REM sleep dreaming, these associations cannot usually be formed, resulting in amnesia for most dreams.