HUMAN VESTIBULOOCULAR RESPONSES TO RAPID, HELMET-DRIVEN HEAD MOVEMENTS

High-frequency head rotations in the 2-20 Hz range and passive, unpred ictable head acceleration impulses were produced by a new technique, u tilizing a helmet with a torque motor oscillating a mass. Unrestrained head and eye movements were recorded using magnetic sensor coils in a homogeneous magnetic field. In order to analyze the influence of the visual system on the vestibule-ocular reflex (VOR), we took measuremen ts under three experimental conditions: (1) with a stationary visual t arget; (2) in total darkness with the subject imagining the stationary target; and (3) with a head-fixed target. The results in 15 healthy s ubjects were highly consistent. At 2 Hz, VOR gain was near unity; abov e 2 Hz, VOR gain started to decrease, but this trend reversed beyond 8 Hz, where the gain increased continuously up to 1.1-1.3 at 20 Hz. Pha se lag increased with frequency, from a few deg at 2 Hz to about 45 de grees at 20 Hz. Above 2 Hz, VOR gain was not significantly different f or the three experimental conditions. Head acceleration impulses produ ced a VOR with near-unity gain in both directions. We also tested thre e subjects with clinically total bilateral loss of labyrinthine functi ons. These labyrinthine-defective subjects showed, in comparison to th e normal subjects, strikingly lower gains and much longer delays in th e VOR during sinusoidal and step-like head movements. These results su ggest that our new torque-driven helmet technique is effective, safe a nd convenient, enabling the assessment of the VOR at relatively high f requencies where both visual and mental influences are minimized.