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 hi normal humans. Methods. Horizontal and vertical smooth pursuit were measured in different eye-in-orbit po sitions in normal subjects, using a magnetic search coil technique wit h sinusoidal and step-ramp stimuli. Pursuit also was tested in differe nt horizontal head-on-trunk positions. Results. Pursuit gain to sinuso idal targets averaged approximately 15% less with the eyes centered 30 degrees horizontally or vertically from the primary position than wit h the eyes near the orbital midline. In contrast, initial pursuit resp onses to step-ramp stimuli were similar regardless of eye position. Fo r sinusoidal and step-ramp responses in eccentric eye positions, no si gnificant differences were found between pursuit movements directed to ward the orbital midposition and pursuit movements directed away from it. Changes in head position had no effect on smooth pursuit. Conclusi ons. Sinusoidal smooth pursuit function decreases modestly for horizon tal and vertical motion in eccentric eye positions. This effect is not caused by reductions in gain for centrifugal movements compared to ce ntripetal movements, implying that the pursuit nonlinearities expected to arise from orbital mechanics are largely eliminated by central pro cessing. Eye position-related differences in retinal or eye motion fee dback or in predictive input may explain the influence of eye position on smooth pursuit maintenance. Changes in target position with respec t to a trunk-centered frame of reference did not produce the orbital e ccentricity effects that were documented because sinus-soidal pursuit gain did not vary with head rotation.