Three-dimensional eye-movement responses to off-vertical axis rotations inhumans

We recorded three-dimensional eye movements elicited by velocity steps abou t axes that were tilted with respect to the earth-vertical. Subjects were a ccelerated in 1 s from zero to 100 degrees/s, and the axis of rotation was tilted by 15 degrees, 30 degrees, 60 degrees, or 90 degrees. This stimulus induced a constant horizontal velocity component that was directed opposite to the direction of rotation, as well as a modulation of the horizontal, v ertical and torsional components with the frequency of the rotation. The ma ximum steady-state response in the horizontal constant-velocity component w as much smaller than in other species (about 6 degrees/s), reaching a maxim um at a tilt angle of about 60 degrees. While the amplitude of the horizont al modulation component increased up to a tilt angle of 90 degrees (8.4 deg rees/s), the vertical and torsional modulation amplitudes saturated around 60 degrees (ca. 2.5 degrees/s). At small tilt angles, the horizontal modula tion component showed a small phase lag with respect to the chair position, which turned into a small phase lead at large tilt angles. The torsional c omponent showed a phase lead that increased with increasing tilt angle. The vertical and torsional velocity modulation at large tilt angles was not pr edicted by a recent model of otolith-canal interaction by Merfeld. Agreemen t between model and experimental data could be achieved, however, by introd ucing a constant force along the body's z-axis to compensate for the gravit ational pull on the otoliths in the head-upright position. This approach ha d been suggested previously to explain the direction of the perceived subje ctive vertical during roll under different a-levels, and produced in our mo del the observed vertical and torsional modulation components at large tilt angles.