High-frequency vestibuloocular reflex as a diagnostic tool.

During locomotion, the head is subject to rotational perturbations with fundamental frequencies in the range 0.5-5.0 Hz, and significant harmonics up to 20 Hz. Patients who have lost labyrinthine function complain of oscillopsia and visual impairment during locomotion. Measurements of head movements during walking and running in place in such patients indicate that head stability is similar to that in normal subjects. Therefore, head stability is mainly guaranteed by mechanical, not neurogenic, factors. On the other hand, the visual symptoms of such patients can be ascribed to instability of gaze. Thus, it seems that other mechanisms such as visual following, the cervicoocular reflex, or anticipatory eye movements cannot compensate for loss of the VOR during locomotion (though they may do so for lower-frequency or active head rotations). The indispensable role of the VOR during locomotion is probably a reflection of its short latency (16 mseconds or less in the horizontal and vertical planes), which guarantees short phase lags during high-frequency head rotations. Our results indicate that laboratory testing of patients with vestibular symptoms should employ stimuli that correspond to those occurring during locomotion.