Connectional and neurochemical subdivisions of the pulvinar in Cebus monkeys

Based on cytoarchitectonic criteria, the primate pulvinar nucleus has been subdivided into medial (PM), lateral (PL), and inferior (PI) regions. Howev er, these subdivisions show no correlation with those established by electr ophysiological, immunocytochemical, or neuroanatomical tracer studies. In t his work, we studied the connections of the pulvinar nucleus of Cebus monke y with visual areas V1, V2, V4, MT, and PO by means of retrograde fluoresce nt tracers injected into these areas. Based on the projection zones to cort ical visual areas, the visual portion of the pulvinar. of Cebus monkey was subdivided into three subregions: P1, P2, and P3, similar to those describe d in the macaque (Ungerleider ct al., 1984). In Cebus, P1 includes the cent rolateral portion of traditionally defined PI and adjacent portion of FL. P ? is located in the dorsal portion of PL and P3 includes the medial portion of FI and extends dorsally into adjacent PL and PM. In addition, eve studi ed the histology of the pulvinar using multiple criteria, such as cytoarchi tecture and myeloarchitecture; histochemistry for cytochrome oxidase, NADPH -diaphorase, and acetylcholinesterase; and immunocytochemistry for two calc ium-binding proteins, calbindin and parvalbumin, and for a neurofilament re cognized by the SMI-32 antibody. Some of these stains, mainly calbindin, sh owed additional subdivisions of the Cebus pulvinar, beyond the traditional PI, FL, and PM. Based on this immunohistochemical staining, the border of P I is moved dorsally above the brachium of the superior colliculus and PI ca n be subdivided in five regions (PIP, PIM, PIC, PIL, and PILS). Regions P1, P2, and P3 defined based on efferent connections with cortical visual area s are not architectonically/neurochemically homogeneous. Rather. they appea r to consist of further chemoarchitectonic subdivisions. These distinct his tochemical regions might be related to different functional modules of visu al processing within one connectional area.