Neocortical and basal telencephalic origins of the anterior commissure of the cat.

The neocortical and basal telencephalic origins of the anterior commissure of the cat have not been described in earlier studies of the great cerebral commissures. In this anatomical study, all cerebral commissures, except the anterior commissure, of twelve cats were first transected. Subsequent unilateral injections of large quantities of horseradish peroxidase throughout the right hemisphere revealed the entire origins of the three branches of the anterior commissure in the left hemisphere. Since the anterior commissure was the only interhemispheric fibre system remaining, only the cells, fibres and anterogradely-labelled terminals of the anterior commissure were labelled by horseradish peroxidase in the uninjected hemisphere. Approximately three-quarters of the neurons of the anterior commissure are in the neocortex, mostly in layers V and VI. These neocortical cells occupy an extensive field stretching from gyrus proreus to the posterior ectosylvian gyrus and from the rhinal sulcus to the suprasylvian sulcus. Other fibres of the anterior commissure, however, were found to have their cell bodies in regions not considered part of the neocortex, and these included the anterior olfactory nucleus, the pyriform cortex, olfactory tubercles, nucleus of the lateral olfactory tract, part of the amygdaloid nuclei, the periamygdaloid nucleus and the lateral entorhinal area. Finally, it was found that the fibres of the anterior commissure do not have an exclusive neocortical territory from which cells of other commissural fibres are excluded. Rather, there appears to be a substantial overlap between the field of origin of fibres of the anterior commissure and those of the largest cerebral commissure, the corpus callosum. The disposition of this field may help to explain why visual information fails to transfer between the hemispheres in cats whose corpus callosum has been cut, in contrast to the success of such transfer in primates.