Human smooth pursuit: effects of stimulus extent and of spatial and temporal constraints of the pursuit trajectory.

We compared the quality of monocular smooth pursuit obtained with either a single point, a full-field stripe pattern or their combination as the target for unidirectional stimulus motion at velocities between 9 and 90 deg/sec. A point target moving in a fixed trajectory maximally constrains target selection as well as pursuit trajectory, whereas a full-field multicontoured pattern leaves the subject maximal freedom in these respects. To unconfound effects of pattern extent from those of spatial and temporal constraints, we presented point targets under conditions in which the subject was free to choose the location, extent and temporal structure of his pursuit trajectory ("free range"). Pursuit velocity gains were lowest for the point target moving in a fixed trajectory. Gain improved when the subject was free to pursue the same target moving at the same velocity in his own preferred range and rhythm. A further improvement was reached by showing the stripe pattern in addition to, and moving in conjunction with the spot. A final increase in gain occurred when the spot was removed, and the subject was allowed to pursue any feature of the uniformly moving, multicontoured pattern. No asymmetries were found between monocular pursuit with the right or the left eye, pursuit of rightward and leftward motion or between nasal- and temporalward motion. Effects of the type of target on the structure of the nystagmoid pursuit eye movements were slight or absent.