Variable otolith contribution to the galvanically induced vestibulo-ocular reflex

The torsional eye movements elicited by sinusoidal galvanic vestibular stimulation (GVS) (0.012-3.13 Hz) were examined in healthy humans. GVS consistently induced sinusoidal modulation of the torsional slow phase velocity (SPV), which was linearly related to stimulus intensity. At low frequencies (< 0.1 Hz) nystagmic responses could be discriminated from an underlying 'tonic' modulation of eye position, which was prominent in some, but negligible in other subjects, and was not correlated with the SPV modulation. The actual SPV modulation consistently exceeded the (hypothetical) velocity modulation derived from the tonic positional components, albeit variably by almost 20-fold across subjects. This indicates that the contribution of possibly otolith-related response components to the galvanic vestibulo-ocular reflex may vary considerably in normal individuals.