Three-dimensional kinematics of ocular drift in humans with cerebellar atrophy

One of the signs of the cerebellar ocular motor syndrome is the inability t o maintain horizontal and vertical fixation. Typically, in the presence of cerebellar atrophy, the eyes show horizontal gaze-evoked and vertical downb eat nystagmus. We investigated whether or not the cerebellar ocular motor s yndrome also includes a torsional drift and, specifically, if it is indepen dent from the drift in the horizontal-vertical plane. The existence of such a torsional drift would suggest that the cerebellum is critically involved in maintaining the eyes in Listing's plane. Eighteen patients with cerebel lar atrophy (diagnosis confirmed by magnetic resonance imaging) were tested and compared with a group of normal subjects. Three-dimensional eye moveme nts (horizontal, vertical, and torsional) during attempted fixations of tar gets at different horizontal and vertical eccentricities were recorded by d ual search coils in a three-field magnetic frame. The overall ocular drift was composed of an upward drift that increased during lateral gaze, a horiz ontal centripetal drift that appeared during lateral gaze, and a torsional drift that depended on horizontal eye position. The vertical drift consiste d of two subcomponents: a vertical gaze-evoked drift and a constant vertica l velocity bias. The increase of upward drift velocity with eccentric horiz ontal gaze was caused by an increase of the vertical velocity bias; this co mponent did not comply with Listing's law. The horizontal-eye-position-depe ndent torsional drift was intorsional in abduction and extorsional in adduc tion, which led to an additional violation of Listing's law. The existence of torsional drift that is eye-position-dependent suggests that the cerebel lum is critically involved in the implementation of Listing's law, perhaps by mapping a tonic torsional signal that depends on the direction of the li ne of sight. The magnitude of this signal might reflect the difference in t orsional eye position between the torsional resting position determined by the mechanics of the eye plant and the torsional position required by Listi ng's law.