THE JOURNAL zyxwvu OF COMPARATIVE NEUROLOGY 220:168-190 (1983) Afferent Connections of the Perirhinal Cortex zyx in the Rat TERRENCE W. DEACON, HOWARD EICHENBAUM, PAUL ROSENBERG, AND KENNETH W. ECKMANN Department of Biological Anthropology, Harvard University, Cambridge, Massachusetts 02138 (T.W.D., P.R.) and Department of Biology, Wellesley College, Wellesley zyxwvut , Massachusetts 02 181 (H.E ., K. W.E .) ABSTRACT Connections of the perirhinal cortex in the rat brain were studied using anterograde (3H-proline/leucine) and retrograde (horseradish peroxidase) tracers. The perirhinal cortex receives major projections from medial precen- tral, anterior cingulate, prelimbic, ventral lateral orbital, ventral and poste- rior agranular insular, temporal, superior and granular parietal, lateral occipital, agranular retrosplenial, and ectorhinal cortices, and from the pre- subiculum, subiculum, and diagonal band of Broca. Rostral neocortical areas project predominantly to rostral perirhinal regions while more caudal neocortical and subicular areas project predominantly to caudal perirhinal regions. Terminal fields are further segregated within perirhinal cortex to either the dorsal or ventral banks of the rhinal sulcus. All afferents from frontal areas terminate predominantly in the deep layers of its ventral bank; affer- ents from temporal, parietal, and lateral occipital areas terminate predomi- nantly in the deep and superficial layers along its dorsal bank: and afferents from ectorhinal cortex terminate in a column within its dorsal bank. Cortical cells which project to perirhinal areas are found predominantly in layer I1 and the superficial part of layer 111. However, ventrolateral orbi- tal, parietal, and lateral occipital cortex projections originate predominantly from layer V. Perirhinal areas also receive afferents from the nucleus reuniens of the thalamus, lateral nucleus of the amygdala, claustrum, supramammillary nuclei, and the dorsal raphe nuclei. Key words: cerebral cortex, rhinal sulcus, limbic system, axon tracers The rhinal fissure is a constant feature in the mam- malian brain, even in those whose cerebrum is otherwise lisencephalic. This unique cortical landmark forms the topological boundary between the phylogenetically and cytoarchitectonically distinct isocortex and allocortex. Considerable anatomic evidence supports the notion that the cortex within the rhinal sulcus also serves as an impor- tant functional relay between neocortical systems and the hippocampal formation. In primates the so-called "association areas," including prefrontal, parietal, and temporal areas, as well as cingu- late cortex are the source of neocortical input into the hip- pocampal complex (Van Hoesen and Pandya, '71; Van Hoesen et al,, '72, zyxwvutsrq '75, '79; Seltzer and Pandya, '74, '76; Van Hoesen and Pandya, '75a,b; Rosene and Van Hoesen, '77; Seltzer and Van Hoesen, '79; Turner et al., '80). However,in general, these cortical areas do not synapse directly in the hippocampus itself but terminate in one or more of the ad- j acent parahippocampal areas: the subicular, entorhinal, and perirhinal cortices. These areas in turn project into the CA fields and dentate gyrus of the hippocampus. By comparison, we know relatively little about the neo- cortical pathways to the hippocampus in species whose cortex is less differentiated (e.g., the rat). In particular, connections of the perirhinal cortex in rodents have been noted only incidentally in a number of studies. Summariz- ing these incidental findings: Neocortical projections to the perirhinal cortex have been traced from the supracallo- Accepted May 25, 1983. zyxwv c ' 1983 NAN R. TJSS. INC.