Vestibulo-ocular reflex (VOR) biases in normal subjects and patients with compensated vestibular loos

The properties of the vestibulo-ocular reflex (VOR) were examined during sinusoidal passive head rotation in the dark at 1/6 Hz, in 9 normal subjects and 14 unilateral vestibular patients. Rotation speeds ranged from 90 to 180°/s. The bias (offset of slow-phase velocity from zero) and gain in the VOR were estimated by using a polynomial (cubic) fit between head and slow-phase eye velocity, thereby allowing for possible non-linearities in the reflex. The gain in the VOR in this context refers to the linear components of the fit, and so predicts sensitivity only at low head velocities. The aim of the study was to verify previous theroretical predictions that VOR bias could vary with the rotation parameters, that this bias could be used to detect the side of a vestibular lesion even at low frequency rotation, and make non-linearities more obivious. Confirming these predictions, the VOR bias in a given test is never equal to any spontaneous nystagmus, even if present before rotation. The range of values for the gain in the VOR (as defined above) in normals and compensated unilateral vestibular patients overlap, so that they cannot be statistically separated into two response sets. The main conclusions for patients are : i) the VOR bias always moves toward the lesioned or weaker side, as the peak velocity of the rotation test is increased, even at 1/6 Hz ; ii) the relationship between eye velocity and head velocity becomes more non-linear as the rotation speed is increased, justifying the use of polynomial VOR descriptions ; iii) the non-linearity appears mainly on the lesioned side as a soft saturation, while the response remains approximately linear on the healthy side. These results have implications in the clinical interpretation of vestibular tests, and in the validity of population averages to detect deficient vestibular funciton after compensation.