The architecture and some of the interconnections of the rat's amygdala and lateral periallocortex.

The connections between the cerebral cortex and amygdala were studied in the rat by means of silver degeneration techniques. To help define the sites of origin and termination of cortico-amygdaloid connections, the architecture of the cortex and the amygdala was studied in sections from normal brains stained for cells, fibers, acetylcholinesterase activity, and heavy metals (Timm staining). The amygdalopetal cortex on the dorsal and lateral surfaces of the rat brain is limited to a narrow strip of periallocortex that forms the dorsal wall and lip of the rhinal sulcus. Histochemical stains indicate that this cortex comprises several stages of cortical differentiation that are intermediate between the ventrally adjacent allocortices and the dorsally adjacent neocortices. The lateral periallocortex consists of two major divisions, the agranular insula (area 13) anteriorly, and a temporal agranular cortex (areas 35 and 36) posteriorly. The principal amygdaloid target for this cortex is the lateral nucleus. Anterior area 13 and posterior area 35 project to the anterior and posterior halves, respectively, of the medial division of this nucleus, while posterior area 13 and anterior area 35 projects to the lateral division of this nucleus. All divisions of periallocortex also send projections to a part of the putamen that surrounds the lateral half of the central nucleus. All of area 13 also sends efferents to the anterior part of the basal nucleus, while the anterior half of area 13 sends an additional projection to the central nucleus. Comparison of these data with those obtained in the cat and monkey suggests that a constant feature of eutherian brains is the existence of a subset of efferents from each of the four neocortical sensory systems that is routed so as to provide subcortical limbic structures with modality-specific information. The initial sequence in this sensorilimbic system consists of one or more modality-specific corticocortical relays that originate in the primary sensory cortices and terminate in one of four topographically adjacent, modality-specific areas of the insular and temporal cortices. These insular and temporal areas then each establish modality-specific connections within the amygdaloid complex. The final set of relays presumably comprises the connection that each of these amygdaloid areas makes with the autonomic and endocrine nuclei of the brain.