Single-unit recording and stimulation in superior colliculus of the alert rhesus monkey.

RECENT SINGLE-UNIT recording studies of the alert rhesus monkey superior colliculus have disclosed that the deeper layers of this structure contain cells which discharge in association with eye movement (15-17, 23, 24). Such units fire selectively prior to saccades of a specific direction and size, irrespective of the position of the eye in orbit. Stimulation studies of the superior colliculus have so far produced conflicting results. Some reports suggest that in the alert cat stimulation brings the eye to a certain position in orbit irrespective of eye position prior to stimulation (1, 9, 19). Thus, the size and direction of elicited saccades is reported to vary as a function of initial eye position. By contrast, in the alert monkey it has recently been reported that stimulation produces conjugate saccades of a specific size and direction; these parameters are the same no matter where the eye is in the orbit prior to stimulation (12, 15, 16). The stimulation map of Robinson (12) shows a reasonable correspondence with the receptive-field map of the superior colliculus reported by Cynader and Berman (5). The aim of this study was to clarify the relationship between recording and stimulation data by using methods which allow a direct comparison. We used alert rhesus monkeys which had one eye immobilized for the mapping of visual receptive fields. The other eye was normal, thus permitting the study of eye movement. Stimulation and recording were carried out using the same low-resistance microelectrode; for each site sampled, records were obtained for both single-unit activity and for electrical stimulation.