The Parahippocampal Region: Corticocortical Connectivity

Abstract

The parahippocampal region, as defined in this review, comprises the cortical regions that surround the rodent hippocampus including the perirhinal, postrhinal, and entorhinal cortices. The comparable regions in the primate brain are the perirhinal, parahippocampal, and entorhinal cortices. The perirhinal and postrhinal/parahippocampal cortices provide the major polysensory input to the hippocampus through their entorhinal connections and are the recipients of differing combinations of sensory information. The differences in the perirhinal and postrhinal cortical afferentation have important functional implications, in part, because these two regions project with different terminal patterns to the entorhinal cortex. The perirhinal cortex projects preferentially to the lateral entorhinal area (LEA), and the postrhinal cortex projects preferentially to the medial entorhinal area (MEA) and the caudal portion of LEA. Although the perirhinal and postrhinal cortices provide the major cortical input to the entorhinal cortex, the entorhinal cortex itself receives some direct cortical input. An examination of the cortical afferentation of the entorhinal cortex reveals an interesting principle of connectivity among these regions; the composition of the direct neocortical input to the LEA is more similar to that of the perirhinal cortex, and the composition of the direct neocortical input to the MEA is more similar to that of the postrhinal cortex. Thus, polymodal associational input to the LEA and the MEA exhibits some segregation and is organized in parallel. The organization of intrinsic connections for each of the parahippocampal regions also contributes to the segregation of information into parallel pathways.

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

“…From this and previous work (Burwell, 2000;Witter et al, 2000;Craig and Commins, 2007), clear predictions can be made in terms of the remaining projections and future work should focus on determining the electrophysiologically excitatory and inhibitory capabilities of the other projections in the hippocampal-parahippocampal network. We suggest, for example, that the projections from the proximal subiculum and lateral entorhinal to the perirhinal cortex will both be electrophysiologically excitatory in nature.…”

“…There are a number of anatomical differences (Burwell et al, 1995;Burwell and Amaral, 1998a,b;Pitkä nen et al, 2000;Burwell, 2000Burwell, , 2001) and functional differences (Norman and Eacott, 2005) existing between the two areas and it has been suggested that the rat postrhinal cortex is homologous to the monkey parahippocampal cortex (Burwell et al, 1995;Burwell, 2000). Therefore, we follow Burwell's classification (2000) and distinguish between the perirhinal and postrhinal cortices as two distinct anatomical regions in the rat brain.…”

“…The cortices that make up the parahippocampal region all form reciprocal connections with each other. As shown above, the perirhinal cortex forms reciprocal connections with the entorhinal (Wyss, 1981;Kosel et al, 1982;Kö hler, 1988;Insausti et al, 1997;Naber et al, 1997;Burwell and Amaral, 1998a) and postrhinal cortices (Burwell and Amaral, 1998b;Burwell, 2000). The entorhinal and postrhinal cortices also project to and from each other to complete the cortical aspect of the hippocampalparahippocampal network (Burwell and Amaral, 1998a,b).…”

“…While the perirhinal and postrhinal cortices of the rat are respectively analogous to the perirhinal and parahippocampal cortices of the primate (Burwell et al, 1995;Burwell, 2000), this is not to say that all the functions ascribed to these regions can be translated between these species. The brains of primates are more adapted to visual information compared to rodents and therefore more elaborate visual processing may occur in the primate parahippocampal region compared to rats.…”

“…The intrinsic projections of the perirhinal cortex follow a dorsal-ventral gradient within area 36 and between areas 35 and 36 (Burwell, 2000). Area 36 projects laterally into area 35 with the majority of connections originating in the ventral region of area 36 in layers II, V and VI and terminating in layers I and V of area 35 (Burwell and Amaral, 1998a).…”