Connections of the parahippocampal cortex. I. Cortical afferents

In the present study in the cat the parahippocampal cortex denotes the caudoventral part of the limbic lobe and is composed of the entorhinal and perirhinal cortices. The cytoarchitecture of these areas and their borders with adjacent cortical areas are briefly discussed. The organization of the cortical afferents of the parahippocampal cortex was studied with the aid of retrograde and anterograde tracing techniques. In order to identify the source of cortical afferents, injections of retrograde tracers such as wheat germ agglutinin conjugated with horseradish peroxidase (WGA‐HRP), or the fluorescent substances fast blue or nuclear yellow, were placed in different parts of the parahippocampal cortex. In an attempt to further disclose the topographical and laminar organization of the afferent pathways, injections of tritiated amino acids were placed in cortical areas that were found to project to the parahippocampal cortex. The results of these experiments indicate that fibers from olfactory‐related areas, the hippocampus, and other parts of the limbic cortex project only to the entorhinal cortex. The afferents from olfactory structures terminate predominantly superficially, whereas hippocampal and limbic cortical afferents are directed mainly to layers deep to the lamina dissecans. Paralimbic areas, including the anterior cingulate and the prelimbic cortices on the medial aspect, and the orbitofrontal and granular and agranular insular cortices on the lateral aspect of the hemisphere, project to the entorhinal cortex and medial parts of area 35 of the perirhinal cortex. These mostly mesocortical afferents terminate in both the superficial and deep layers of the entorhinal and perirhinal cortices. Parasensory association areas, which form part of the neocortex, do not project farther medially in the parahippocampal cortex than the perirhinal areas 35 and 36. These afferents mainly stem from a rather wide rim of neocortex that lies directly adjacent to area 36 and extends from the posterior sylvian gyrus via the posterior ectosylvian gyrus into the posterior suprasylvian gyrus. There is a rostrocaudal topographical arrangement in these projections such that rostral cortical areas distribute more rostrally and caudal parts project to more caudal parts of the perirhinal cortex. The cortex of the posterior suprasylvian gyrus contains the paravisual areas 20 and 21. The posterior sylvian gyrus most probably represents a para‐auditory association area, whereas the most ventral part of the posterior ectosylvian gyrus may constitute a convergence area for visual and auditory inputs. The projections from this ventral zone on the lateral aspect of the hemisphere terminate in a columnarlike pattern in area 36. Additional but considerably lighter projections stem from areas located more dorsally on the lateral convexity of the hemisphere. These areas, which are more closely associated with the primary sensory fields, encompass the posterior ectosylvian (Ep) and AII auditory areas, the somatosensory area SII, and the cortex in the ventral and dorsal banks of the anterior ectosylvian sulcus, which contains auditory, somatosensory, as well as visual association areas. The fibers from these areas project to the deep layers of area 35. It is concluded that through several corticocortical steps information from all sensory modalities may reach the parahippocampal cortex in which further convergence occurs. Since the present observations indicate that medial and lateral parts of the parahippocampal cortex receive different inputs, which in turn are transmitted, respectively, to the temporal and septal poles of the hippocampus (Witter and Groenewegen: J. Comp. Neurol. 224: 371–385, '84), it is suggested that these parts of the hippocampus serve different functional roles.