Recent studies in humans have indicated that the neurons located in one portion of the hippocampus proper, called the CA3 region, are critical for the formation of new memories — declarative or episodic types of memories (not procedural). This means that in order to "remember" some new fact or event, the new information must be registered within the hippocampal formation. This information is "processed" through some complex circuitry in these structures and is retained for a brief period of seconds. In order for it to be remembered for longer periods, some partially understood process occurs so that the transient memory trace is transferred to other parts of the brain, and this is now stored in working memory or as a long-term memory. The process of memory storage consolidation may require a period of hours, if not days.

In the study of the function of the hippocampus in animals, there is considerable evidence that the hippoc-ampal formation is involved in constructing a "spatial map." According to this literature, this part of the brain is needed to orient in a complex environment (such as a maze). It is not quite clear whether this is a memory function or whether this spatial representation depends upon the connections of the hippocampal formation and parahippocampal gyrus with other parts of the brain.

Clinical Aspect

The clinical implications of the functional involvement of the hippocampal formation in memory will be further elaborated with Figure 73.

It is now possible to view the hippocampal area in detail on MRI and to assess the volume of tissue. Bilateral damage here apparently correlates with the loss of memory function in humans with Alzheimer's dementia, particularly for the formation of memories for new events or for new information (further discussed with Figure 73).

Additional Detail

A vestigial part of the hippocampal formation is still found above the corpus callosum, as shown in this illustration — not labeled.

Hippocampal Formation

FIGURE 72A: Hippocampus 1 — Hippocampal Formation

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