SMOOTH-PURSUIT EYE-MOVEMENTS AND OTOLITH-OCULAR RESPONSES ARE DIFFERENTLY IMPAIRED IN CEREBELLAR-ATAXIA

Horizontal and vertical smooth pursuit was compared with otolith-ocula r responses in 11 patients with cerebellar ataxia and 21 normal subjec ts using three-dimensional magnetic search coil eye movement recording s. Otolith-ocular responses were investigated during off-vertical axis rotation. This stimulus induces nystagmus consisting of the exponenti ally decaying canalicular response, and an eye-velocity modulation and offset which arise from the excitation of the otoliths by the gravity vector, which lasts as long as the rotation continues. Otolith-ocular reflexes are intimately interrelated with visual tracking when real t argets are viewed during linear motion. The responses of both the tran slational vestibule-ocular reflex and the pursuit system have been sho wn to be linearly dependent on the inverse of the viewing distance, so that a common central pathway for the two systems has been suggested, probably travelling through the cerebellum. Thus, the aim of the stud y was to evaluate to what extent these reflexes are disturbed in cereb ellar disease. The results confirm the earlier notion that in normal s ubjects pursuit performance is better for horizontal than for vertical tracking, and that it is better for upward than for downward tracking . This pattern is also found in patients. In addition, smooth pursuit performance is clearly degraded in patients, but the modulation of eye -velocity during off-vertical axis rotation is enhanced. Since the amo unt of this enhancement does not correlate with the amount of pursuit impairment, degradation of smooth pursuit and pathological enhancement of otolith-ocular responses seem to be independent effects of cerebel lar degeneration. Thus, the increase in the otolith-ocular response in patients cannot be attributed to adaptational mechanisms trying to ov ercome the smooth pursuit deficiency; it is more likely to represent p athological disinhibition of otolith derived responses. The absence of compensatory eye-velocity offset during off-vertical axis rotation ma y reflect the fact that in patients the otolith signals are not utiliz ed in computations thought to be important for spatial orientation mec hanisms arising from the interaction of vestibular, visual and somatos ensory signals.