Effects of eye and head position on horizontal and vertical smooth pursuit.

PURPOSE To identify and explain the effects of eye and head position on smooth pursuit eye movements in normal humans. METHODS Horizontal and vertical smooth pursuit were measured in different eye-in-orbit positions in normal subjects, using a magnetic search coil technique with sinusoidal and step-ramp stimuli. Pursuit also was tested in different horizontal head-on-trunk positions. RESULTS Pursuit gain to sinusoidal targets averaged approximately 15% less with the eyes centered 30 degrees horizontally or vertically from the primary position than with the eyes near the orbital midline. In contrast, initial pursuit responses to step-ramp stimuli were similar regardless of eye position. For sinusoidal and step-ramp responses in eccentric eye positions, no significant differences were found between pursuit movements directed toward the orbital midposition and pursuit movements directed away from it. Changes in head position had no effect on smooth pursuit. CONCLUSIONS Sinusoidal smooth pursuit function decreases modestly for horizontal and vertical motion in eccentric eye positions. This effect is not caused by reductions in gain for centrifugal movements compared to centripetal movements, implying that the pursuit nonlinearities expected to arise from orbital mechanics are largely eliminated by central processing. Eye position-related differences in retinal or eye motion feedback or in predictive input may explain the influence of eye position on smooth pursuit maintenance. Changes in target position with respect to a trunk-centered frame of reference did not produce the orbital eccentricity effects that were documented because sinusoidal pursuit gain did not vary with head rotation.