SPATIAL-DISTRIBUTION AND DISCHARGE CHARACTERISTICS OF SUPERIOR COLLICULUS NEURONS ANTIDROMICALLY ACTIVATED FROM THE OMNIPAUSE REGION IN MONKEY

One proposed role of the superior colliculus (SC) in oculomotor contro l is to suppress or excite the activity of brain stem omnipause neuron s (OPNs) to initiate or terminate saccades, respectively. Although con nections from the SC to the OPNs have been demonstrated, the spatial d istribution and discharge characteristics of the projecting neurons fr om the SC remain unknown. We mapped the spatial distribution of the de eper-layer neurons of the SC by stimulating the region of the OPNs to identify antidromic projections and found that the density of direct p rojections from the SC to the OPNs was greatest in the most rostral re gion and decreased gradually for more caudal sites. On the basis of sa ccade-related discharge characteristics, the antidromically driven neu rons were predominantly fixation and buildup neurons. The spatially di stributed SC projections to the OPNs and the discharge characteristics of the SC neurons suggest that the direct projections from SC to OPNs are excitatory. Finally, we propose how excitation and disfacilitatio n from SC activity can contribute to modulation of OPN response and co ntrol saccades.