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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Previous anterograde studies in the macaque monkey have shown that, in addition to the projection to striate cortex (V1), the dorsal lateral geniculate nucleus (DLG) has a sparse, horizontally segregated projection to layers IV and V of prestriate cortex (V4). However, the distribution and degree of axon collateralization of DLG cells which give rise to these projections are unknown. This study was designed to answer these questions. The DLG (along with the pulvinar and other subcortical regions) was examined for the presence of single- or double-labeled cells after injections of two different (fluorescent or HRP) retrograde tracers into corresponding retinotopic points in visual cortical areas V1 and V4. In the DLG, it was found that cells projecting to V4, which reside in or near the tectorecipient interlaminar zones of the DLG, do not project to V1 and thus represent a separate population of cells. The organization of the macaque geniculo-prestriate projection thus seems quite different from that of carnivores. Both single- and double-labeled cells were found in other subcortical areas, e.g., single-labeled cells were found in the claustrum, hypothalamus and lateral pulvinar, and a double-labeled cell population was found in the inferior pulvinar.

The pattern of acetylcholinesterase (AChE) reactivity was studied in the pulvinar and dorsal lateral geniculate nucleus (DLG) of the adult macaque monkey. Discrete islands of AChE reactivity were found that correlated precisely in location with the pattern of projections from the superior colliculus and pretectum. Specifically, AChE overlies terminal fields of superior colliculus projections in the DLG, in four foci in the medial and lateral pulvinar, and in several foci in the inferior pulvinar. All of these tectal projection areas have very high AChE reactivity such that they are easily distinguished. In addition, the pretectum projects to a specific focus in the lateral pulvinar that also has a very dense AChE histochemical reaction. A number of these AChE foci could be further distinguished from other areas in the pulvinar by myeloarchitectonic characteristics. Some of the foci in the lateral and inferior pulvinars could also be distinguished by unique cytoarchitectonic features (as seen with both Nissl and Golgi stains). In an attempt to determine the possible origin of a cholinergic input to the pulvinar, horseradish peroxidase (HRP) injections and choline acetyltransferase immunohistochemistry were also done. The results of this experiment indicate that the AChE reactivity seen in the midbrain projection zones to the thalamus may be due to the precise overlap of terminal projections from the brainstem cholinergic cell groups, Ch5, Ch6, and Ch8. These results, taken together, imply that there are several anatomically and histochemically distinct zones related to extrageniculate pathways located within classically defined thalamic boundaries.