Visual influence on head shaking using the vestibular autorotation test

In this study we investigated the vestibular system by recording eye movements in response to voluntary high-frequency head-only movements using the Vestibular Autorotation Test (VAT; Western System Research Inc., Los Angeles, California). Our objective was to evaluate if the VAT could be implemented as one of the screening tests for vestibular integrity in aircrews and potential pilots. We attempted to record horizontal and vertical eye movements using electrooculography and head velocity with calibrated rotational velocity sensors. The gain and phase of the input and output signals were computed by discrete Fourier analysis. Seated subjects were instructed to fixate on a real or imaginary target while making smooth head oscillations about the spinal axis in time to an audible cue from 0.5 to 6.0 Hz/during an 18-second test period. Test trials included two conditions in the light with subjects fixated on a real target (C1) or on an imaginary target on a blank screen (C2); three conditions in the dark in which subjects fixated on an imaginary target (C3), fixated on a remembered LED target in the dark after it was extinguished (C4), or fixated on a real target (C5). All the dark trials were performed after dark adaptation for 30 minutes. We were not able to obtain consistent vertical VOR response (when the subjects oscillated their head about the interaural axis) using the VAT. For horizontal eye movements from 2.0 to just over 4.7 Hz, when subjects fixated on an imaginary target, there was an unexpected and significant increase in the gain of the eye movement velocities in the dark as compared to the gain obtained in the light conditions. In the dark trials, the gain was significantly higher when the subjects fixated on an imaginary target as opposed to a real target. There was no difference in phase among all conditions. This test could potentially serve for preliminary screening for the integrity of the vestibular system as it is noninvasive and of short duration. However, caution must be exercised in controlling various variables. Extensive normative data are needed to properly assess this test as a screening tool for aircrews.