Discharge characteristics of single units in superior colliculus of the alert rhesus monkey.

ACCUMULATING EVIDENCE suggests that the superior colliculus plays an important role in orientation and eye movement. Ablation studies have shown that destruction of superior colliculi interferes with spatial orientation and eye movement (Z-29), although the evidence regarding the effects of such lesions in primates is conflicting (1, 7, 17, 18; and unpublished observations). Stimulation studies have disclosed that electrical or chemical stimulation of the superior colliculus elicits eye and head movement in several species (4, 1 I, 20, 22). While most single-unit studies centered on the visual information-processing role of the mammalian superior colliculi (12-16, 30, 32, 33; M. Cynader and N. Berman, unpublished observations), a few studies have recently appeared that demonstrate eye-movement related unit activity (22, 31, 35) The aim of the present study was to investigate the discharge characteristics of single units in the superior colliculus of the alert, unanesthe tized monkey. We wished to assess both the visual receptivefield characteristics of units and the possible relation of unit discharges to eye movement. To carry out this plan one eye of each monkey was immobilized prior to the experiment by transection of the 3rd, 4th, and 6th cranial nerves. This made it possible to study in the monkey, whose head was restrained, receptive fields of units of the immobilized eye, and to relate the discharge characteristics of collicular units to the movement of the normal eye during both saccadic and smooth pursuit eye movements. M. Cynader and N. Berman (unpublished data) recently have shown that nearly all units in the monkey