Hippocampal-neocortical interaction: A hierarchy of associativity

The structures forming the medial temporal lobe appear to be necessary for the establishment of long-term declarative memory. In particular, they may be involved in the "consolidation" of information in higher-order associati onal cortices, perhaps through feedback projections. This review highlights the fact that the medial temporal lobe is organized as a hierarchy of asso ciational networks. Indeed, associational connections within the perirhinal , parahippocampal, and entorhinal cortices enables a significant amount of integration of unimodal and polymodal inputs, so that only highly integrate d information reaches the remainder of the hippocampal formation. The feedb ack efferent projections from the perirhinal and parahippocampal cortices t o the neocortex largely reciprocate the afferent projections from the neoco rtex to these areas. There are, however, noticeable differences in the degr ee of reciprocity of connections between the perirhinal and parahippocampal cortices and certain areas of the neocortex, in particular in the frontal and temporal lobes. These observations are particularly important for model s of hippocampal-neocortical interaction and long-term storage of informati on in the neocortex. Furthermore, recent functional studies suggest that th e perirhinal and parahippocampal cortices are more than interfaces for comm unication between the neocortex and the hippocampal formation. These struct ures participate actively in memory processes, but the precise role they pl ay in the service of memory or other cognitive functions is currently uncle ar. (C) 2000 Wiley-Liss, Inc.