We studied and compared the ipsilateral efferents of the superficial and deep layers of the superior colliculus of the rhesus monkey. Using a stereotaxic 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 smaller 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 ('67). Following lesions of the superficial layers of the superior colliculus, definite degenerated axonal endings were found in the dorsal and ventral lateral geniculate nuclei, inferior pulvinar, centrointermediate nucleus, magnocellular dorsomedial nucleus, anterior pretectal nucleus and pretectal region. 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, suprageniculate nucleus, limitans nucleus, lateral posterior nucleus, medial and oral pulvinar, nucleus of the accessory optic tract, zona incerta, subdivisions of the ventral lateral and ventral posterior lateral nuclei, ventral posterior inferior nucleus, denosocellular and multiform dorsomedial nuclei, all intralaminar nuclei, inferior colliculus, 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 portion of the superior colliculus projected to the rostral portions of thalamic nuclei and the rostral portion of the superior colliculus projected to the caudal portions of thalamic nuclei. All the degeneration patterns seen after lesions of the superficial and deep layers were accounted for by large lesions which involved all layers of the superior colliculus. It is concluded that the superficial and deep layers of the rehesus monkey superior colliculus have different ascending projections. The finding, are related to the organization of visual and multimodal thalamocortical systems in primates and other mammals.

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The organization of five projections from the retinorecipient and nonretinorecipient layers of the superior colliculus (SC) and nuclei of the pretectum to the lateral pulvinar, medial pulvinar, and dorsal lateral geniculate nucleus (DLG) of the thalamus in the macaque monkey were established by using both anterograde autoradiographic and retrograde horseradish peroxidase tracing techniques. Some projections from these midbrain regions to the inferior pulvinar are also described as are projections to the thalamus from the tectorecipient parabigeminal nucleus. First, the retinorecipient and nonretinorecipient pretectal nuclei and SC layers were determined by Nissl stains, myelin stains, and the distribution of label resulting from intraocular injections of tritiated amino acids. The possible projections of these pretectal nuclei, layers of the SC (as well as the parabigeminal nucleus), to the pulvinar and DLG were then determined by both horseradish peroxidase and autoradiographic tracing methods. The retinal projections to the SC extend throughout layers 11-1, 11-2, 11-3, and dorsal I11 with obvious patches in layers 11-1 and 11-2 as described before by others. There were a number of pretectal nuclei which also had connections with the retina. In the nomenclature adopted, these include the sublentiform nucleus, the nucleus of the optic tract (not to be confused with the thalamic limitans nucleus), the posterior (or principal) pretectal nucleus, and the laminated olivary nucleus (with a head and tail). The nucleus of the posterior commissure and the nucleus of the pretectal area do not receive retinal projections.The retinorecipient midbrain regions which project to the pulvinar are as follows. Aside from a complicated multifocal projection to the inferior pulvinar, the superficial retinorecipient SC layers project to three terminal foci or zones in one or more of the lateral pulvinar subdivisions alpha, beta, and gamma (Rezak and Benevento, '77). Focus 1 lies mainly along the dorsoventral lateral border of subdivisions of PL-gamma and PL-beta, extending ventrally to PL-alpha, and arises from SC layers 11-3 and 111. Focus 2 occupies the lateral 20-50% of PL-alpha and arises from SC layers 11-2.11-3, and 111. There is a topographical arrangement to the SC projections to foci 1 and 2 which may reflect a representation of more than two visual quadrants in the lateral pulvinar.Focus 3, which consists of several clusters, is located medially in PLgamma and PL-beta and overlaps their mutual border with the medial pulvinar. Focus 3 is complicated as it is made up of a mixed input from the retinorecipient (superficial) and nonretinorecipient (deep) SC layers. The portion of focus 3 located along the medial edge of the lateral pulvinar arises mainly from SC layers II-2,11-3, and 111, while the portion of the focus located along the adjacent lateral edge of the medial pulvinar arises mainly from SC layers Accepted March 10, 1983. 0 1983 ALAN R. LISS, INC. L.A. BENEVENTO AND G.P. STANDAGEIV, V, and VI. These projections to th...

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