Cortico-cortical connections of the limbic cortex of the rat.

By means of the retrograde transport of fluorescent tracers (Fast Blue, True Blue, Fluorogold and Diamidino Yellow), the cortico-cortical connections of prelimbic, insular, anterior and posterior cingulate (retrosplenial) areas have been studied. Our results demonstrate that there are, in the cortex of the adult rat, a few cells which have branched axons with connections in the ipsilateral hemisphere (associational neurons) or in the contralateral hemisphere (callosal neurons). The callosal neurons could be separated into two categories: "double callosal" neurons which project both axon collaterals to two cortical areas of the contralateral hemisphere, and "associational-callosal" neurons which send axon collaterals to both hemispheres.