Connections of the parahippocampal cortex in the cat. III. Cortical and thalamic efferents

Abstract To study the distribution of the cortical and thalamic efferent projections from the parahippocampal cortex in the cat, a series of injections of anterogradely transported radioactively labeled amino acids were placed in different parts of the entorhinal and perirhinal cortices. Subsequently, some of the identified cortical and thalamic target areas were injected with retrograde tracers such as wheat germagglutinin conjugated with horseradish peroxidase (WGA‐HRP) or with a fluorescent tracer‐fast blue or nuclear yellow‐in order to disclose the laminar origin of the parahippocampal efferent projections. The results indicate that the parahippocampal cortex gives rise to widespread projections to the association cortex, and, to a lesser extent, sends fibers to the limbic cortex and the primary sensory cortex. These projections arise mainly from the deep layers of the parahippocampal cortex and terminate predominantly in superficial layers of the cortex, with a preference for layer I. Within the cortical projections a medial‐to‐lateral topography could be observed such that the entorhinal cortex projects predominantly to the allocortical and periallocortical limbic areas, including parts of the subicular complex, the ventral retrosplenial and the infralimbic cortices, and olfactory related areas‐i.e., the olfactory bulb, the anterior olfactory nucleus, the prepiriform cortex, and the ventral tenia tecta. The more lateral parts of the parahippocampal cortex, which surround the posterior rhinal sulcus, project in addition to extensive parts of the paralimbic association cortex that include the proisocortical cingular, prelimbic, orbitofrontal, and agranular and granular insular cortices. The most lateral portion of the parahippocampal cortex, the perirhinal cortex, furthermore issues projections to widespread neocortical areas on the lateral and medial aspects of the hemisphere that constitute part of the parasensory association cortex. Weak‐to‐moderate projections are found to the cortex of the middle suprasylvian and anterior ectosylvian sulci, as well as the cruciate and splenial sulci, all of which have been reported to constitute sensory convergence areas. The most marked projections from the perirhinal cortex reach a zone of neocortex directly lateral to the perirhinal cortex including ventral parts of the posterior sylvian, posterior ectosylvian, posterior suprasylvian, and lateral gyri. These projections appear to be topographically organized such that rostral parts of the perirhinal cortex project more rostrally, and more caudal parts of the perirhinal cortex project to more caudal parts of this cortical zone. The posterior sylvian gyrus may represent a para‐auditory area, whereas the posterior suprasylvian and lateral gyri contain the paravisual areas 20 and 21. The rostrocaudal axis in the perirhinal cortex is thus related to an auditory‐to‐visual shift in the temporal association cortex. A similar relation is noted in the projections from the perirhinal cortex to the caudodorsal part of the thalamus. Rostrally located neurons in the perirhinal cortex project strongly to the auditory‐related medial geniculate nucleus, whereas more caudally located perirhinal neurons project preferentially to the lateral posterior nucleus, which is strongly associated with the visual system. Additional but weaker projections are observed to reach the parafascicular, subparafascicular, and reuniens nuclei. Projections from the entorhinal cortex to the thalamus could not be demonstrated. It is concluded that major projections from the parahippocampal cortex reach widespread cortical areas and parts of the thalamus that are involved in higher‐order processing of sensory information. This conclusion is discussed with respect to the intricate relations that exist between the parahippocampal cortex and the hippocampal formation.