Predictive human pursuit and "orbital goal" of microstimulated smooth eye movements

P. van Gelder, S. Lebedev and W. H. Tsui Nathan S. Kline Institute for Psychiatric Research, Orangeburg, New York 10962, USA. 1. Anticipatory saccades in smooth pursuit move the point of gaze from near the moving target to well ahead of it, interrupting accurate smooth pursuit. Their effects on the pursuit process were studied in 22 normal human subjects. We presented horizontal periodic target trajectories of 30 degrees amplitude and 30 degrees/s constant velocity or 0.4 Hz sinusoidal velocity in 40-s trials. Saccades and surrounding smooth 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 seen in the distribution of projections of the initial 60 ms of postsaccadic SEM to the time of the trajectory endpoint. Magnitude of this SEM tended to follow a function of the time and location of the 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 amplitude of the trajectory in smooth pursuit, apart from the instantaneous velocity match of the target. 4. Gottlieb and coworkers in the frontal eye field and Ron and Robinson in the cerebellum produced SEMs in the monkey by microstimulation. At some sites in both structures, direction and velocity of the SEMs depended on the initial position of the eye in that the elicited SEMs appeared to be converging toward a common point, or "orbital goal", and the SEM velocity diminished as the gaze neared that goal.2+ Both our SEM after anticipatory saccades and microstimulated SEM in the monkey slowed as the initial position was brought closer to the inferred orbital goal. This similarity suggests that the goal-directed SEM sites in the monkey might be part of a mechanism for predictive pursuit.