CHEMOARCHITECTONIC SUBDIVISIONS OF THE VISUAL PULVINAR IN MONKEYS AND THEIR CONNECTIONAL RELATIONS WITH THE MIDDLE TEMPORAL AND ROSTRAL DORSOLATERAL VISUAL AREAS, MT AND DLR

The organization of the inferior pulvinar complex (PI) in squirrel monkeys was studied with histochemical localization of the calcium binding proteins calbindin-D28k and parvalbumin, and of cytochrome oxidase. With each of these markers, the inferior pulvinar complex can be subdivided into four distinct regions. Calbindin-D28k immunoreactivity is densely distributed in cells and neuropil within PI, except for a distinct centromedially located gap. This calbindin-poor zone, termed the medial division of the inferior pulvinar (PI(M)), corresponds precisely to a region that contains elevated cytochrome oxidase activity and parvalbumin immunostaining. The PI(M) extends slightly above and behind the classically defined limit of the inferior pulvinar, the corticotectal tract. Regions of inferior pulvinar with intense immunostaining for calbindin-D28k were the posterior division of the inferior pulvinar (PI(P), medial to PI(M)) and the central division (PI(C), lateral to PI(M)). A newly recognized lateral region, PI(L), adjoins the lateral geniculate nucleus and stains more lightly for calbindin and parvalbumin immunoreactivity and for cytochrome oxidase. Staining patterns for calbindin, parvalbumin, and cytochrome oxidase in the pulvinar of rhesus monkeys closely resemble those shown in squirrel monkey inferior pulvinar, suggesting that a common organization exists in all primates. In order to examine cortical connection patterns of the histochemically defined compartments in the inferior pulvinar, injections of up to five neuroanatomical tracers (wheat germ agglutinin conjugated to horseradish peroxidase and fluorescent retrograde tracers) were placed in the same cerebral hemisphere. Single injection sites were in the middle temporal area (MT), and several separate injections were placed in a strip corresponding to the rostral subdivision of the dorsolateral area (DLr). Injections that involved only DLr and not MT labeled principally the PI(C), and more sparsely PI(P) and PI(L). DLr connections occupied a ''shell'' region dorsal to PI(M) that extended from PI(C) into the lateral and medial divisions of the pulvinar, PL and PM. Injection sites that included MT or were largely restricted to MT produced dense label in PI(M) and moderate label in PI(C) and PI(L). The retinotopic organization within the inferior pulvinar was inferred from patterns of connections. Connections with cortex related most closely to central vision were found posteriorly in PI(M) and in adjacent portions of PI(C) as it wraps around the caudal pole of PI(M). Cortex related to more peripheral locations in the lower visual field connected with more rostral PI(M) and PI(C). Patterns of label within the portions of PL and PM that were immediately adjacent to PI(M) roughly paralleled those in PI(M) and PI(C). Thus, a distinct chemoarchitectonic subdivision of the inferior pulvinar, PI(M), is the major source of thalamic projections to MT in squirrel monkeys. Interestingly, the relative scarcity of calbindin immunostaining in PI(M) is characteristic of ''primary'' thalamic relay nuclei, such as the ventroposterior and lateral geniculate nuclei. Thus, the PI(M)-MT pathway may share certain physiological characteristics with primary sensory relays, such as those involving intracellular calcium buffering and, perhaps, rapid and secure impulse transmission. (C) 1993 Wiley-Liss, Inc.