When humans are accelerated along the body vertical, the right and left eyes show oppositely directed torsional modulation (cyclovergence). The origin of this paradoxical response is unknown.Westudied cyclovergence during linear sinusoidal vertical motion in healthy humans. A small head-fixed visual target minimized horizontal and vertical motion of the eyes and therefore isolated the torsional component. For stimuli between 1 and 2 Hz (near the natural range of head motion), the phase of cyclovergence with respect to inertial acceleration was 8.7±2.4° (mean±95% CI) and the sensitivity (in degrees per second per g) showed a small but statistically significant increase with frequency. These characteristics contrast with those of cycloversion (conjugate torsion) during horizontal (interaural) inertial stimuli at similar frequencies. From these and previous results, we propose that cyclovergence during vertical translation has two sources, one, like cycloversion, from the low-frequency component of linear acceleration, and another, which we term dynamic cyclovergence, with high-pass characteristics. Furthermore, we suggest that this cyclovergence response in humans is a vestige of the response of lateral-eyed animals to vertical linear acceleration of the head.
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