Effect of eyelid closure and vocalization upon the vestibulo-ocular reflex during rotational testing

During rotational testing of the vestibulo-ocular reflex (VOR), eyelid closure is known to reduce both VOR gain and nystagmus frequency. We tested the effect of vocalization, such as counting aloud, on the VOR in ten healthy normal subjects. Velocity-step and sinusoidal stimuli were applied in complete darkness under three test conditions: 1) eyes open, performing mental arithmetic; 2) eyes closed, performing mental arithmetic; and 3) eyes closed and vocalizing. With velocity-step stimuli, eyelid closure significantly reduced VOR gain (peak eye velocity/head velocity) and nystagmus duration, but vocalization reversed these effects and resulted in the highest gain values. Similar results were obtained for VOR gain during sinusoidal stimulation. This study demonstrates that when eyelid closure is necessary or unavoidable during VOR testing, continuous vocalization ensures responses that are comparable to or better than those with eyes open in darkness.