EVALUATION OF THE HUMAN VESTIBULO-OCULAR REFLEX AT HIGH-FREQUENCIES WITH A HELMET, DRIVEN BY REACTIVE TORQUE

A new technique was developed to test the VOR in humans. A tightly but comfortably fitting helmet was provided with an electronically contro lled torque motor that rotated a mass around a vertical axis. Accelera tion of the motor caused reactive torque of the helmet in the opposite direction, which was transmitted to the (Freely movable) head. Sinuso idal head oscillations in the frequency range 2-20 Hz (peak-to-peak am plitudes about 10 degrees and 0.1 degrees, respectively) were easily a chieved, as well as step displacements of the head with accelerations on the order of 1000 degrees/s(2). Limitation of the maximum torque an d lack of any rigid attachment of the head to a fixed structure made t he technique safe and comfortable. Eye and head rotations were recorde d, independently of head translations, with eye and head search coils in a homogeneous a.c. magnetic field. In normal subjects, gain was nea r unity at 2 Hz; above 8 Hz, gain increased progressively toward 1.1-1 .3 at 20 Hz. Phase lag increased from a few degrees at 2 Hz to about 4 5 degrees at 20 Hz. Above 2 Hz, these results were unaffected by visua l conditions; lower gains and increased phase lags were found in subje cts with bilateral or unilateral vestibular loss. For step displacemen ts, gain (measured in the first 100 ms) was near unity for normals, ne ar zero after bilateral vestibular loss and strongly asymmetrical afte r unilateral vestibular loss. Thus, the technique seems highly effecti ve in testing vestibular function with minimal contamination by other systems.