Eye-position dependence of torsional velocity during step-ramp pursuit and transient yaw rotation in humans

The time course of eye-position-dependent torsion during transient horizontal pursuit and yaw rotation was examined in seven normal human subjects. The stimuli consisted of step-ramp target motion (25, 40°/s) and brief chair rotation (∼200°/s² accelerated to 40°/s) at three different vertical positions (center 0°, up or down 15°). Three-dimensional eye movements were recorded with dual search coils. The kinematics of pursuit and the rotational vestibulo-ocular reflex (rVOR) were assessed by determining the tilt-angle slope, a measure of the variation of the axis of eye-velocity with vertical eye position. We found that the tilt-angle slope during pursuit was initially 0.4 ± 0.07 (mean ± 95% confidence interval) and then gradually rose to 0.64 ± 0.04, at about the time that the steady-state eye-velocity was reached. The rVOR began with a nearly head-fixed axis (0.08 ± 0.04), appropriate for full retinal image stabilization, followed by a gradual increase of the tilt-angle slope to 0.31 ± 0.02. Thus, differences between pursuit and the rVOR with respect to Listing's law can be seen from the onset of transient responses, although in both cases eye-position-dependent torsion increases with time. This temporal evolution of the axis of eye-velocity may involve the velocity-storage mechanism.