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

The organization of the inferior pulvinar complex (PI) in squirrel mon keys was studied with histochemical localization of the calcium bindin g proteins calbindin-D28k and parvalbumin, and of cytochrome oxidase. With each of these markers, the inferior pulvinar complex can be subdi vided into four distinct regions. Calbindin-D28k immunoreactivity is d ensely distributed in cells and neuropil within PI, except for a disti nct centromedially located gap. This calbindin-poor zone, termed the m edial division of the inferior pulvinar (PI(M)), corresponds precisely to a region that contains elevated cytochrome oxidase activity and pa rvalbumin immunostaining. The PI(M) extends slightly above and behind the classically defined limit of the inferior pulvinar, the corticotec tal tract. Regions of inferior pulvinar with intense immunostaining fo r calbindin-D28k were the posterior division of the inferior pulvinar (PI(P), medial to PI(M)) and the central division (PI(C), lateral to P I(M)). A newly recognized lateral region, PI(L), adjoins the lateral g eniculate nucleus and stains more lightly for calbindin and parvalbumi n immunoreactivity and for cytochrome oxidase. Staining patterns for c albindin, parvalbumin, and cytochrome oxidase in the pulvinar of rhesu s monkeys closely resemble those shown in squirrel monkey inferior pul vinar, suggesting that a common organization exists in all primates. I n order to examine cortical connection patterns of the histochemically defined compartments in the inferior pulvinar, injections of up to fi ve neuroanatomical tracers (wheat germ agglutinin conjugated to horser adish peroxidase and fluorescent retrograde tracers) were placed in th e same cerebral hemisphere. Single injection sites were in the middle temporal area (MT), and several separate injections were placed in a s trip corresponding to the rostral subdivision of the dorsolateral area (DLr). Injections that involved only DLr and not MT labeled principal ly the PI(C), and more sparsely PI(P) and PI(L). DLr connections occup ied a ''shell'' region dorsal to PI(M) that extended from PI(C) into t he 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 centra l vision were found posteriorly in PI(M) and in adjacent portions of P I(C) as it wraps around the caudal pole of PI(M). Cortex related to mo re peripheral locations in the lower visual field connected with more rostral PI(M) and PI(C). Patterns of label within the portions of PL a nd PM that were immediately adjacent to PI(M) roughly paralleled those in PI(M) and PI(C). Thus, a distinct chemoarchitectonic subdivision o f the inferior pulvinar, PI(M), is the major source of thalamic projec tions 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 genicul ate 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.