The ascending projections of the superior colliculus in the rhesus monkey (Macaca mulatta)
We studied and compared the ipsilateral afferents of the superficial and deep layers of the superior colliculus of the rhesus monkey. Using a stereptaxic method, microelectrodes were inserted through the contralateral hemisphere in order to make electrolytic lesions of the superior colliculus. Large lesions involved all layers of the superior colliculus, while smaler lesions involved either the superficial or the deep layers of the superior colliculus. Following various survival times, the brains were prepared with the Fink-Heimer technique (1967). Following lesions of the superficial layers of the superior colliculus, definite degenerated axonal endings were found in the dorsal and ventral lateral genuculate nuclei, inferior pulvinar, centrontermediate nucleus, magnocellular dorsomedial nucleus, anterior pretectal nucleus and pretectal refion. Sparse degenerated axonal endings were found in the limitans nucleus, lateral posterior nucleus and some intralaminar nuclei following lesions of the superficial layers in the rostral portion of the superior colliculus. Following lesions of the deep layers of the superior colliculus, degenerated axonal endings were found in the central gray, magnocellular medial geniculate nucleus, suprageniculatre nucleus, limations nucleus, lateral posterior nulceus, medial and oral pulvinarm nucleus of athe accessory optic tract, zona incerta, subdivisions of the ventral lateral and ventral posterior lateral nuclei, ventral posterior inferior nulceus, densocellular and mulriform dorsomedial nuclei, all intralaminar nucleim inferior coliculus, parabigeminal nucleus, olivary nucleus, reunions nucleus, Forel's Field H and an undefined midbrain nucleus. In general the projections were topographically organized in that the caudal portions of thalamic nuclei. All the degeneration patterns seen after lesions of the superficial and dep layers were accoundted for by large lesions which involved all layers of the superior colliculus. It is conclude that the superficial and deep layers of the rhesus monkey superior colliculus have different ascending projections. The findings are related to the organization of visual and multimodal thalamocortical system in primates and other mammals.