Characterization of the vestibulo-ocular reflex evoked by high-velocity movements.

Abstract

The horizontal angular vestibulo-ocular reflex (VOR) plays an important role in stabilizing images on the retina throughout head rotations. Current evidence suggests that the VOR behaves linearly at low velocities and nonlinearly at high velocities. The aim of the research was to evaluate and characterize the normal behavior of the reflex evoked by high-velocity head rotations. Case control study. Manually applied head-thrust movements with peak velocities in the range of 100degrees to 500degrees/s and peak accelerations up to 7,000degrees/s were performed on normal volunteers. These head thrusts were comparable with those described in detail by Halmagi and coworkers. Eye and head movements were recorded using the magnetic search coil method. The gain of the VOR is linear at low velocities and saturates at head velocities greater than 350degrees/s. The values for the normal gain of the reflex were approximated by means of the area between two nonlinear functions. The directional difference parameter, exploring the symmetry of the reflex, indicated that the VOR in normal subjects is symmetric. The gain of the VOR in individuals with intact vestibular function is nonlinear at high angular head velocities. We propose a quantitative means using two nonlinear functions to characterize the normal range of values for the gain of the VOR in individuals with normal vestibular function. A directional difference parameter used in conjunction with the normal range of gains can detect small differences in the symmetry of the VOR and, consequently, reveal unilateral vestibular loss.