The anatomy of memory: an interactive overview of the parahippocampal–hippocampal network

Abstract

Converging evidence suggests that each parahippocampal and hippocampal subregion contributes uniquely to the encoding, consolidation and retrieval of declarative memories, but their precise roles remain elusive. Current functional thinking does not fully incorporate the intricately connected networks that link these subregions, owing to their organizational complexity; however, such detailed anatomical knowledge is of pivotal importance for comprehending the unique functional contribution of each subregion. We have therefore developed an interactive diagram with the aim to display all of the currently known anatomical connections of the rat parahippocampal-hippocampal network. In this Review, we integrate the existing anatomical knowledge into a concise description of this network and discuss the functional implications of some relatively underexposed connections.

The rat perirhinal cortex: A review of anatomy, physiology, plasticity, and function

“…Although, there have been a number of reviews that have specifically examined the anatomy (e.g. Burwell, 2001;van Strien et al, 2010) and functional roles of the perirhinal cortex (Dere et al, 2007;Winters et al, 2008;Warburton and Brown, 2010), to date, there has been no review that brings anatomy, physiology, synaptic plasticity and function of the perirhinal cortex together. Here we attempt to bring these different strands together and also examine how the perirhinal cortex fits within the general hippocampal-parahippocampal circuitry with particular emphasis on the electrophysiological properties of these connections.…”

Functional MRI of long-term potentiation: imaging network plasticity

“…The reported synaptically driven functional reorganization was not associated with global increases in excitability because only a subset of the numerous hippocampal outputs [13,59] were activated. Extrahippocampal activations were entirely contingent on LTP induction, as stimulation of the PP with saturating current intensities (up to 1.0 mA) in controls never produced activations in those regions [48,49].…”

“…Whether this mechanism generalizes across brain regions will require further investigation. For the moment, it is interesting to note that visual perceptual learning [59] in humans modifies the synchrony of spontaneous BOLD signals between visual and frontal -parietal networks in a way that predicts the degree of learning [70,71]. Reorganization of hippocampal and PFC networks in humans has already been demonstrated in fMRI studies during memory recall [42], in line with lesion and imaging (2-deoxyglucose) data in rodents [72,73] and consistent with the idea of a hippocampal to neocortical shift in memory consolidation.…”

Updating hippocampal representations: CA2 joins the circuit

“…Based on anatomy, it is very likely that the grid and border cells in EC layer II project directly to DG as well as CA3 via the PP; grid, HD and border cells in layer III EC project directly to CA1 through the temporoammonic pathway (TA); and projections from both layer II and III neurons converge on the pyramidal cells of CA2 [41] (Figure 1). However, deep EC layers also contribute to PP projections [40], and only cellular-level connectomics will establish the extent to which projections from different EC subpopulations converge and diverge at their hippocampal targets.…”

“…Hippocampal connections with the EC provide numerous direct and indirect routes and shortcuts around the trisynaptic circuit (reviewed in [40]). Based on anatomy, it is very likely that the grid and border cells in EC layer II project directly to DG as well as CA3 via the PP; grid, HD and border cells in layer III EC project directly to CA1 through the temporoammonic pathway (TA); and projections from both layer II and III neurons converge on the pyramidal cells of CA2 [41] (Figure 1).…”