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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.

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In order to study the cooperation of peripheral motor subsystems, the degree of curvature of human saccades along cardinal (right, up, left, down) and oblique directions was computed from an extensive set of experimental data. Our curvature measure allows comparison of fast and slow saccade trajectories elicited in different experimental conditions, independent of the speed of execution. Although we found clear and consistent subject-specific differences, the most common pattern in oblique visually-guided (i.e., fast) saccades reflected early dominance of the horizontal velocity signal as expressed in saccade trajectories curving away from the horizontal axis. Plots of curvature against direction yielded consistent idiosyncratic patterns with periodical increases and decreases in saccade curvature which were largely independent of saccade amplitude. At the cardinal axes, mean saccade curvature was generally less, but rarely entirely absent, and fitted smoothly into the curvature pattern of neighbouring quadrants. Memory-guided saccades, which have been shown earlier to be considerably slower than visually-guided saccades and to be more variable in their dynamic properties, showed a strikingly similar dependence of curvature on saccade direction, although some small but consistent differences were noticed. This finding suggests that saccade curvature is determined by mechanisms residing in the final common pathway for both saccade types. The curvature data were compared with quantitative predictions from three different models for the generation of oblique saccades. By quantifying the curvature of human saccades and thereby revealing the shortcomings of these models, the present paper documents new constraints with which future models of the saccadic system must comply and allows certain suggestions on how these might be developed.