Neurochemical subdivisions of the inferior pulvinar in macaque monkeys

The architecture of the pulvinar of rhesus monkeys was investigated by acetylcholinesterase (AChE) histochemistry, and by immunocytochemistry for calbindin-D28k and the SMI-32 antibody. The presence of four inferior subdivisions, comparable to those found in architectonic-connectional studies in squirrel monkeys (C.G. Cusick, J.L. Scripter, J.G. Darensbourg, and J.T. Weber, 1993, J. Comp. Neurol. 336:1-30), provided a basis for a proposed revised terminology for visual sectors of the macaque pulvinar. In the present study, the inferior pulvinar (PI) was identified as a neurochemically distinct region that included the traditional cytoarchitectonic nucleus PI and adjacent portions of the lateral and medial pulvinar nuclei, PL and PM. In calbindin-D28k stains, the lateral subdivision of the inferior pulvinar (PIL) had less intense neuropil staining than the adjacent central division, PIC. The PIL was characterized by large, intensely immunopositive neurons seldom found within PIC. PIL occupied the traditional PL and PI and exhibited a narrow shell zone, PIL-S, restricted to PL. The medial division of the inferior pulvinar (PIM) was in a location previously shown to be strongly connected with the middle temporal visual area (MT) in macaques. PIM was found in the medial one-half of the traditional PI and extended into adjacent portions of the traditional PM and PL. PIM was distinguished by less intense neuropil staining for calbindin and many cells stained with the SMI-32 antibody for neurofilament protein. In AChE stains, PIL was moderately dark, PIC appeared lighter, and PIM was characterized by small, intensely stained patches. The small posterior division (PIP) stained darkly for calbindin, lightly for AChE, and was unstained with the SMI-32 antibody. Thus, neurochemical, and perhaps connectional, subdivisions exist within PI, the region of the pulvinar that relays information to striate, "lower order" extrastriate, and inferotemporal visual cortex.