Central processing of human ocular torsion analyzed by galvanic vestibularstimulation

We examined the dynamics of human ocular torsion (OT) responses to sinusoid al galvanic vestibular stimulation (GVS) (0.005-1.67 Hz). The tonic OT show ed a lowpass characteristic with a rime constant of 1.74 s and a gain of 0. 93 deg/mA. In two subjects, nystagmus dominated the observable OT pattern a t frequencies <0.1 Hz. The nystagmus slow phases showed an exponential traj ectory with a rime constant of 1.49 s. The dynamics of both tonic OT and to rsional nystagmus in our study were similar to the dynamics of OT induced b y rotation and linear acceleration found in the literature. We propose a mo del for the central processing of torsional eye movements that is based on a common neural integration of semicircular canal (SC) and utricular inputs as well as nystagmus bursts. The sensitivity of all vestibular afferents t o GVS was derived to be 0.76 spikes/s/mA. SC effects on OT are at least 3.5 times higher than utricular effects. NeuroReport 11:1559-1563 (C) 2000 Lip pincott Williams & Wilkins.