Abstract
Previous studies have shown that MT neurons continue to respond selectively to the direction of moving visual stimuli following the elimination of their primary source of input, V1. A proposed substrate for this preserved function is through the superior colliculus (SC) relay of retinal inputs to visual cortex via the pulvinar nucleus. The inferior portion of the pulvinar (PI) is subdivided into several nuclei that provide distinct projections to extrastriate visual cortex. The medial subdivision, PIm, provides the main projections to MT and its “satellite” areas, while a second, lateral subdivision, PIL, projects almost exclusively to MT. However, it remains to be shown whether MT projection zones in the pulvinar receive direct SC input. Previous studies using anterograde tracer injections in the SC and retrograde tracer injections in MT showed no evidence of overlap in PIm. Here we use rabies virus as a transynaptic tracer to directly examine disynaptic connections from SC to MT in macaque monkeys. Three days (required time for disynaptic transport) following injections in MT, retrogradely labeled cells were found in superficial layers of SC. These results show that MT and SC are disynaptically connected. Whether these colliculocortical connections are relayed through, as yet, undetected SC inputs to PIm or PIL remains to be determined.