GAIN AND DELAY OF HUMAN VESTIBULOOCULAR REFLEXES TO OSCILLATION AND STEPS OF THE HEAD BY A REACTIVE TORQUE HELMET

Vestibule-ocular reflexes (VOR) were evaluated with a reactive torque helmet that imposed high-frequency oscillation (2-20 Hz) or step displ acements of the head in the horizontal plane. The present paper descri bes the experimental and analytical techniques and the results for nor mal subjects, which will serve as a baseline for the evaluation of ves tibular pathology. For comparison, manually controlled head steps were also applied, as described in the literature. Eye and head movements were recorded with magnetic search coils. Non-vestibular effects were avoided by the use of high stimulus frequencies and the analysis of sh ort time-windows (<100 msec) after steps. Helmet-imposed steps caused a virtually uniform head acceleration (average magnitude 770 degrees/s ec) in the first 90 msec. This resulted in a linear relation between e ye and head velocities; the gain and delay of the VOR could be calcula ted independently from the slope and offset of this relation. Such est imates appear more reliable than those obtained with conventional tech niques. Normal subjects had a VOR gain of about 0.9 and a delay of abo ut 5 msec. The results of sinusoidal head oscillation were in agreemen t with the results for steps. The responses to manually generated head steps agreed in general with those to helmet-induced steps, but becau se of the non-uniform acceleration they allowed a less exact analysis of function.