Inter-ocular differences of the horizontal vestibulo-ocular reflex during impulsive testing.

Impulsive testing, which accurately measures the gain of the vestibulo-ocular reflex (VOR) is mostly recorded from only one eye. We measured the horizontal VOR of both eyes in response to graded velocity head impulses in 14 normal subjects. VOR acceleration gains were similar for each eye in both directions at low accelerations, but as head acceleration increased, gains of the adducting eye exceeded gains of the abducting eye. These inter-ocular VOR gain differences were due to longer latencies but higher accelerations of the adducting eye compared to the abducting eye. Consequently, directional VOR gain asymmetry occurred at high accelerations if the same eye was always analysed--with an average inter-ocular difference of 15.3%. If only the abducting eye was analysed, VOR gains were symmetrical in both directions. For accurate measurements of the VOR at high accelerations, the inter-ocular VOR gain differences require binocular recording.