Comparison of saccades evoked by visual stimulation and collicular electrical stimulation in the alert monkey.

In the alert monkey we have compared the properties of saccades elicited by a visual stimulus (V-saccades) with those generated by electrical stimulation in the superior colliculus (E-saccades). We found that whereas there exists a graded relation between E-saccade amplitude and current strength, E-saccade direction is remarkably independent of electrical stimulation parameters. At sufficiently high current strengths (about 20 microA), E-saccades are consistently directed toward the center of the movement field of nearby cells, except when stimulation is performed at sites near the collicular borders. Further interesting differences between the amplitude and direction behaviour were observed when the variability in E-saccade vectors, obtained with fixed stimulation parameters, was analyzed. In all cases, E-saccade amplitude scatter exceeds direction scatter, suggesting the possibility of a polar coordinate organization for the coding of saccade metrics. These data are compared with V-saccade scatter data, recently obtained in the human (Van Opstal and Van Gisbergen 1989c). Finally, an analysis of saccade dynamics shows that E-saccades can reach V-saccadic velocities at higher current strengths. However, at near-threshold current strengths, where E-saccade amplitude decreases, we found at most stimulation sites (22/37) that E-saccades are consistently slower than V-saccades of the same amplitude. Possible mechanisms underlying the collicular role in saccade generation are discussed.