Central processing of human ocular torsion analyzed by galvanic vestibular stimulation

We examined the dynamics of human ocular torsion (OT) responses to sinusoidal galvanic vestibular stimulation (GVS) (0.005-1.67 Hz). The tonic OT showed a lowpass characteristic with a time constant of 1.74 s and a gain of 0.93 deg/mA. In two subjects, nystagmus dominated the observable OT pattern at frequencies <0.1 Hz. The nystagmus slow phases showed an exponential trajectory with a time constant of 1.49 s. The dynamics of both tonic OT and torsional nystagmus in our study were similar to the dynamics of OT induced by rotation and linear acceleration found in the literature. We propose a model 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 to GVS was derived to be 0.76 spikes/s/mA. SC effects on OT are at least 3.5 times higher than utricular effects.