Horizontal ocular vergence and the three-dimensional response to whole-body roll motion

We evaluated the human binocular response to roll motion in the dark and du ring visual fixation with horizontal convergence. Six normal human subjects were exposed to manually driven, whole-body rotation about an earth-vertic al, naso-occipital axis, under two conditions: (I) oscillation at 0.4 Hz (p eak velocity 69+/-3.8 degrees /s) in the dark, and whilst fixating an axial light-emitting diode at 48 cm ('near') and at 206 cm ('far'); (II) constan t velocity rotation (56.5+/-3.1 degrees /s) for 40 s, clockwise and counter -clockwise, in the dark, and sudden stops. Eye and head movements were moni tored using scleral search coils. In head-fixed, angular velocity coordinat es roll motion always evoked conjugate ocular torsion, with small conjugate horizontal and disconjugate vertical components. The resultant binocular e ye responses were rotations about convergent axes. During oscillation with target fixation the convergence of the rotation axes was larger than that p redicted by target geometry, producing disconjugate oscillations of vertica l gaze about the target ('skewing'). Fast-phase eye movements were primaril y resetting rotations about the same convergent rotation axes as the slow p hases, but the small vertical velocity components had oscillatory, asymmetr ical profiles. In response to velocity steps the slow-phase eye velocity de cayed exponentially with time constants of 4.5+/-1.5 s for the torsional co mponent and 5.8+/-1.9 s for the 'vertical vergence' component (right eye-le ft eye recordings). We conclude that in normal human subjects dynamic verti cal canal stimulation with horizontal gaze convergence evokes rotation of t he eyes about convergent axes and a small skewing of the eyes.