Convergence of intra‐ and interhemispheric cortical afferents: Lack of collateralization and evidence for a subrhinal cell group projecting heterotopically

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Abstract
The distribution of corticocortical projecting neurons in the rat's brain was investigated with fluorescent dyes and the retrograde transport of horseradish peroxidase. Although the fluorescent techniques especially revealed the existence of a considerable number of neurons interconnecting the limbic areas (sub‐ and perirhinal cortex, prefrontal, cingulate, and retrosplenial cortex) both intra‐ and interhemispherically, only a negligibly small number of neurons with collateralized axons could be detected. In the rat's anterior dorsolateral cortex an area is described whose neurons are organized in a columnlike fashion and project intra‐ and interhemispherically to the limbic areas examined. The density of efferent connections differed between areas, with an especially high density found in a small region of the subrhinal cortex.Injections of horseradish peroxidase into different regions of the cingulate and retrosplenial cortex confirmed the existence of widespread heterotopic interhemispheric connections originating from this defined subrhinal area, though the number of retrogradely labeled cells remained consistently smaller than that obtained following the injection of fluorescent dyes. Among the regions studied with horseradish peroxidase injections, those into the retrosplenial cortex showed the highest density of labeled cells within this subrhinal area.A more detailed examination of the subrhinal region containing the densely labeled neurons (projecting to the contralateral hemisphere) made use of Nissl stains and revealed a morphologically separable area which was characterized by medium‐sized, dark‐staining neurons whose long axons were oriented mediolaterally. The region includes portions of the insular‐perirhinal, entorhinal, and piriform cortex.It is suggested that the corticocortical projections are basically noncollateralized in the rat. However, there apparently is a dense interhemispheric interconnectivity between the limbic areas. Functional evidence for the defined subrhinal area suggests a prominent role of its neurons in cognitive information processing.The present evidence for considerable interhemispheric cortical projections may provide a new impetus for studying the intercommunication of the two sides of the brain with both anatomical and behavioral methods.