PREDICTIVE SMOOTH-PURSUIT OF COMPLEX 2-DIMENSIONAL TRAJECTORIES IN MONKEY - COMPONENT INTERACTIONS

Smooth pursuit eye movements were studied in monkeys tracking target s pots that moved two-dimensionally. Complex target trajectories were cr eated by applying either two or three sinusoids to horizontal and vert ical axes in various combinations. The chance of observing predictable performance was increased by repeated training on each trajectory. Da ta analyses were based upon repeated presentations of each trajectory within sessions and on successive days. We wished to determine how acc urately monkeys could pursue targets moving along these trajectories a nd to observe interactions among frequency components. At intermediate frequencies, tracking performance was smooth and consistent during re peated presentations with saccadic corrections that were well integrat ed with smooth pursuit. The mean gain for eight different sum-of-sines trajectories was 0.83 and the mean magnitude (absolute value) of the phase error was 6 degrees. In light of the long delays that have been associated with the processing of visual information, these values ind icate that the monkeys were pursuing predictively. Five factors influe nced predictive pursuit performance: (1) there was a decline in perfor mance with increasing frequency; (2) horizontal pursuit was better tha n vertical pursuit; (3) high-frequency components were tracked with hi gher gains and phase lags, while lower-frequency components were track ed with lower gains and phase leads; (4) the gain of sinusoidal pursui t was always reduced when a second sinusoid was applied to the same ax is or, to a lessor extent, when a second sinusoid of higher frequency was applied to the orthogonal axis; (5) the phase of sinusoidal pursui t shifted from a phase lag to a phase lead when combined with a second sinusoid of higher frequency, but was not affected by the addition of a lower-frequency sinusoid. Findings 1 and 2 confirm, in monkeys, res ults reported for humans, and 3 extends to monkeys and to two-dimensio nal pursuit results based upon human subjects. All of these findings d emonstrate that complex predictive tracking is controlled by a nonline ar and nonhomogeneous system that uses predictive strategies in concer t with feedback control to generate good pursuit.