Cortical field of origin of the anterior commissure of the rhesus monkey.

The full extent of the cortical field of origin of the anterior commissure of the rhesus monkey was mapped by horseradishperoxidase (HRP) histochemistry. Two adult monkeys were first subjected to complete callosal commissurotomies and permitted to fully recover 6 months prior to a second operation involving the massive unilateral injection of HRP into the entire left temporal lobe. Because the anterior commissure is the only direct fiber system ramifying to the contralateral cerebral cortical hemisphere following callosotomy. the only cells labeled with HRP in the uninjected hemisphere are those giving rise to the anterior commissure. Only layer III cortical pyramidal cells were labeled by HRP. The outer boundaries of the field of origin of the anterior commissure extend from the temporal pole to the occipitotemporal border and from the inferior half of the insular cortex to the parahippocampal gyrus. The field of origin delineated in the present investigation is much more extensive than the terminal projection field of the anterior commissure delineated by prior investigators utilizing silver degeneration methods.