Direction-specific impairment of motion perception and spatial orientation in downbeat and upbeat nystagmus in humans.

Downbeat and upbeat nystagmus can be classified as central vestibular syndromes in the vertical (pitch) plane of the vestibulo-ocular reflex (VOR) which are defined by ocular motor, perceptual, and postural manifestations. While the ocular motor syndrome was often studied investigations on the perceptual consequences for spatial orientation and motion perception are rare. Subjective visual straight ahead (SVA) and perception of object motion were measured in 11 patients with downbeat (n=6) and upbeat (n=5) nystagmus. Upward deviations of SVA (median +5.2 degrees) were found in downbeat nystagmus, and downward deviations (median -7.8 degrees) in upbeat nystagmus. SVA was deviated toward the slow phase of the vertical nystagmus in the pitch plane and associated with increased fore-aft body sway. Perception of object motion was more severely impaired for vertical (particularly for motion in the direction of slow nystagmus phases) than for horizontal directions in both downbeat and upbeat nystagmus. Impairment of motion perception in the vertical pitch plane of the VOR is beneficial to the extent that it alleviates disturbing oscillopsia due to the involuntary retinal slip. Thus, our findings confirm the hypothesis that downbeat and upbeat nystagmus reflect a central tone imbalance of the VOR in the vertical pitch plane with ocular motor, postural, and perceptual manifestations.