Head slippage during broad-frequency rotational chair testing.

Broad-frequency rotational chair testing is employed in clinical and research settings to evaluate the response of the vestibulo-ocular reflex (VOR) over a range of frequencies. Accurate computation of the gain and phase of the VOR is dependent upon the assumption that the subject's head is rigidly coupled to the rotating chair over the range of frequencies employed. We tested this assumption by examining head slippage in 20 normal subjects using a standard rotational chair over the frequency range 0.025 to 2 Hz. Measurements were made with the subjects' head optimally restrained according to our standard clinical protocol and with the head minimally restrained. Head slippage was expressed as gain and phase of the head with respect to the chair. Computation of these parameters was made by comparing the signal received from a biteblock-mounted rate sensor rigidly coupled to the skull with that of the chair's tachometer. We found highly significant slippage of the head with respect to the chair occurred at 0.5, 1 and 2 Hz, even with the head optimally restrained, leading to increased gain and a phase lag. Gain and phase were highly variable for both conditions at 2 Hz indicating inadequate head fixation using our methods. Below 0.5 Hz, minimal head slippage occurred whether the head was restrained or simply rested against a contoured headrest. Consideration of these results may lead to changing some practices currently employed in broad-frequency rotational chair testing at frequencies of 0.5 Hz and above.