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 vestibule-ocular reflex (VOR) which are defined by ocular motor, perceptual, and postural man ifestations. While the ocular motor syndrome was often studied investi gations on the perceptual consequences for spatial orientation and mot ion perception are rare. Subjective visual straight ahead (SVA) and pe rception 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 deviation s (median -7.8 degrees) in upbeat nystagmus. SVA was deviated toward t he slow phase of the vertical nystagmus in the pitch plane and associa ted with increased fore-aft body sway. Perception of object motion was more severely impaired for vertical (particularly for motion in the d irection of slow nystagmus phases) than for horizontal directions in b oth downbeat and upbeat nystagmus. Impairment of motion perception in the vertical pitch plane of the VOR is beneficial to the extent that i t alleviates disturbing oscillopsia due to the involuntary retinal sli p. 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 . (C) 1998 Published by Elsevier Science Ireland Ltd.