Cortico-hippocampal communication by way of parallel parahippocampal-subicular pathways

Abstract

The hippocampal memory system, consisting of the hippocampal formation and the adjacent parahippocampal region, is known to play an important role in learning and memory processes. In recent years, evidence from a variety of experimental approaches indicates that each of the constituting fields of the hippocampal memory system may serve functionally different, yet complementary roles. Understanding the anatomical organization of cortico‐parahippocampal‐hippocampal connectivity may lead to a further understanding of these potential functional differences. In the present paper we present the two main conclusions of experiments in which we studied the anatomical organization of the hippocampal memory system of the rat in detail, with a focus on the pivotal position of the entorhinal cortex. We first conclude that the simple traditional view of the entorhinal cortex as simply the input and output structure of the hippocampal formation needs to be modified. Second, our data indicate the existence of two parallel pathways through the hippocampal memory system, arising from the perirhinal and postrhinal cortex. These two parallel pathways may be involved in separately processing functionally different types of sensory information. This second proposition will be subsequently evaluated on the basis of series of electrophysiological studies we carried out in rats and some preliminary functional brain imaging studies in humans. Hippocampus 10:398–410, 2000 © 2000 Wiley‐Liss, Inc.

Mapping neuronal activation and the influence of adrenergic signaling during contextual memory retrieval

“…For example, one could compare salient context-specific activation in proximal versus distal regions (along the transverse axis) of CA1 and the subiculum because anatomic studies indicate distinct differences in connectivity within these regions and in their extrahippocampal projections Witter et al 2000). A clearer picture of activation within the parahippocampal cortices could be obtained by analyzing many more areas and their various layers.…”

“…Several parahippocampal cortical regions were studied because of their extensive connections with the hippocampus and their role in mnemonic processes (Eichenbaum 2000;Witter et al 2000;Brown and Aggleton 2001;Egorov et al 2002). No salient context-specific labeling was present in the medial entorhinal cortex; however, the posterior lateral entorhinal cortex exhibited significant salient context-specific labeling that was present only in control mice (Fig.…”

“…The parahippocampal cortices (perirhinal, postrhinal, and entorhinal) are multimodal association areas that process and relay sensory information between the hippocampus and other cortical regions (Insausti et al 1997;Burwell and Amaral 1998;Dolorfo and Amaral 1998;Naber and Witter 1998;Witter et al 2000). Lesion studies of the entorhinal or perirhinal cortex suggest that these regions may contribute to contextual fear conditioning (Corodimas and LeDoux 1995;Phillips and LeDoux 1995;Maren and Fanselow 1997).…”

Towards a functional organization of the medial temporal lobe memory system: Role of the parahippocampal and medial entorhinal cortical areas

“…Areas of the parahippocampal region then connect with the hippocampus. The flow of information through the cerebral cortex, parahippocampal region, and hippocampus can be described as a hierarchy of connectivity in which widespread areas of the cerebral cortex funnel information onto multiple areas within the parahippocampal region, whose outputs converge on the hippocampus (Lavenex and Amaral, 2000;Witter et al, 2000;Kerr, 2007). Importantly, the outputs of hippocampal processing are directed back down the hierarchy to areas of the parahippocampal region, which in turn send their outputs back to the areas of the cerebral cortex from which the inputs originated.…”

The hippocampal debate: are we asking the right questions?

“…If the accumbens plays a role in action selection [68,124,163], and the entorhinal cortex plays a role in memory formation [28,138,214,218,244], then it is possible that the same internal information is redistributed in different ways across the two pathways. Navigation models extending to anatomical structures beyond hippocampus have tended to include models of accumbens as the 'next stage downstream' [19,22,53,177], while memory models have tended to emphasize the entorhinal-hippocampusentorhinal loop [4,24,33].…”