Enhancement of the vestibule-ocular reflex by prior eye movements

We investigated the effect of visually mediated eye movements made before v elocity-step horizontal head rotations in eleven normal human subjects. Whe n subjects viewed a stationary target before and during head rotation, gaze velocity was initially perturbed by similar to 20% of head velocity; gaze velocity subsequently declined to zero within similar to 300 ms of the stim ulus onset. We used a curve-fitting procedure to estimate the dynamic cours e of the gain throughout the compensatory response to head rotation. This a nalysis indicated that the median initial gain of compensatory eye movement s (mainly because of the vestibule-ocular reflex. VOR) was 0.8 and subseque ntly increased to 1.0 after a median interval of 320 ms. When subjects atte mpted to fixate thr remembered location of the target in darkness, the init ial perturbation of gaze was similar to during fixation of a visible target (median initial VOR gain 0.8), however, the period during which the gain i ncreased toward 1.0 was >10 times longer than that during visual fixation. When subjects performed horizontal smooth-pursuit eye movements that ended (i.e., 0 gaze velocity) just before the head rotation, the gaze velocity pe rturbation at the onset of head rotation was absent or small. The initial g ain of the VOR had been significantly increased by the prior pursuit moveme nts for all subjects (P < 0.05; mean increase of 11%). In four subjects, we determined that horizontal saccades and smooth tracking of a head-fixed ta rget (VOR cancellation with eye stationary in the orbit) also increased the initial VOR gain (by a mean of 13%) during subsequent head rotations. Howe ver, after vertical saccades or smooth pursuit. the initial gaze perturbati on caused by a horizontal head rotation was similar to that which occurred after fixation of a stationary target. We conclude that the initial gain of the VOR during a sudden horizontal head rotation is increased by prior hor izontal, but not vertical, visually mediated gaze shifts. We postulate that this "priming" effect of a prior gaze shift on the gain of the VOR occurs at the level of the velocity inputs to the neural integrator subserving hor izontal eye movements, where gaze-shifting commands and vestibular signals converge.