PREDICTIVE HUMAN PURSUIT AND ORBITAL GOAL OF MICROSTIMULATED SMOOTH EYE-MOVEMENTS

1. Anticipatory saccades in smooth pursuit move the point of gaze from near the moving target to well ahead of it, interrupting accurate smo oth pursuit. Their effects on the pursuit process were studied in 22 n ormal human subjects. We presented horizontal periodic target trajecto ries of 30 degrees amplitude and 30 degrees/s constant velocity or 0.4 Hz sinusoidal velocity in 40-s trials. Saccades and surrounding smoot h eye movement (SEM) segments were marked and classified by computer. 2. Anticipatory saccades were often followed by slowed SEM that tended to intercept the target at the endpoint of its trajectory. This was s een in the distribution of projections of the initial 60 ms of postsac cadic SEM to the time of the trajectory endpoint. Magnitude of this SE M tended to follow a function of the time and location of thr endpoint of the anticipatory saccade, decreasing as the anticipatory saccades landed closer to the trajectory endpoint. 3. The time and location of the target trajectory endpoint seemed to be the goal for this SEM. We believe this to demonstrate the predictive use of the period and ampli tude of the trajectory in smooth pursuit, apart from the instantaneous velocity match of the target. 4. Gottlieb and coworkers in the fronta l eye field and Ron and Robinson in the cerebellum produced SEMs in th e monkey by microstimulation. At some sites in both structures, direct ion and velocity of the SEMs depended on the initial position of the e ye in that the elicited SEMs appeared to be converging toward a common point, or ''orbital goal,'' and the SEM velocity diminished as the ga ze neared that goal. 5. Both our SEM after anticipatory saccades and m icrostimulated SEM in the monkey slowed as the initial position was br ought closer to the inferred orbital goal. This similarity suggests th at the goal-directed SEM sites in the monkey might be part of a mechan ism for predictive pursuit.