Projections of the superior colliculus to subdivisions of the inferior pulvinar in New World and Old World monkeys.

Patterns of terminals labeled after WGA-HRP injections in the superior colliculus (SC) in squirrel monkeys and macaque monkeys, and after DiI application in marmosets, were related to the architecture of the pulvinar and dorsal lateral geniculate nucleus (LGN). In all studied species, the SC projects densely to two architectonic subdivisions of the inferior pulvinar, the posterior inferior pulvinar nucleus (PIp) and central medial inferior pulvinar nucleus (PIcM). These projection zones expressed substance P. Thus, sections processed for substance P reveal SC termination zones in the inferior pulvinar. The medial subdivision of the inferior pulvinar, PIm, which is known to project to visual area MT, does not receive a significant collicular input. Injections in MT of a squirrel monkey revealed no overlap between SC terminals and neurons projecting to area MT. Thus, PIm is not the significant relay station of visual input from the SC to MT. The SC also sends an input to the LGN, however, this projection is sparser than the input directed to pulvinar.