PEAK VELOCITIES OF VISUALLY AND NONVISUALLY GUIDED SACCADES IN SMOOTH-PURSUIT AND SACCADIC TASKS

Smooth pursuit typically includes corrective catch-up saccades, but ma y also include such intrusive saccades away from the target as anticip atory or large overshooting saccades. We sought to differentiate catch -up from anticipatory and overshooting saccades by their peak velociti es, to see whether the higher velocities of visually rather than nonvi sually guided saccades in saccadic tasks may be found also in saccades in pursuit. In experiment 1, 12 subjects showed catch-up, anticipator y, and overshooting saccades to comprise 70.4% of all saccades in purs uit of periodic, 30 degrees/s constant-velocity targets. Catch-up sacc ades were faster than the others. Saccadic tasks were run as well, on 19 subjects, including the 12 whose pursuit data were analyzed, with t arget-onset, target-remaining (saccade to the remaining target when th e other three extinguish), and antisaccade tasks. For 17 of the 19 sub jects, antisaccade velocities were lower than for either target-onset or target-remaining tasks. Velocities for the target-remaining task we re near those for target onset, indicating that target presence, not i ts onset, defines visually guided saccades. Error and reaction-time da ta suggest greater cognitive difficulty for target remaining than for target onset, so that the cognitive difficulty of typical nonvisually guided saccade tasks is not sufficient to produce their lowered veloci ty. To produce reliably, in each subject, catch-up and anticipatory sa ccades with comparable amplitude distributions, nine new subjects were asked in experiment 2 to make intentional catch-up and anticipatory s accades in pursuit, and were presented with embedded target jumps to e licit catch-up saccades, all with periodic target trajectories of 15 d egrees/s and 30 degrees/s. Velocities of intentional anticipatory sacc ades were lower than velocities of intentional catch-up saccades, whil e velocities of intentional and embedded catch-up saccades were simila r. Target-onset and remembered-target saccadic tasks were run, showing the expected higher velocity for the target-onset task in each subjec t. Both experiments demonstrate higher peak velocities for catch-up sa ccades than for anticipatory saccades, suggesting that cortical struct ures preferentially involved in nonvisually guided saccades may initia te the anticipatory and overshooting saccades in pursuit.