Does the vestibulo-ocular reflex (VOR) merit redefinition?

The VOR has historically been described by parameters of the slow phase of nystagmus while fast phases have been systematically ignored. However, ample neurophysiologic and theoretical evidence suggests that fast phases are an integral part of the VOR and are not to be discarded in evaluation of vestibular function. In a series of experiments, normal subjects and compensated patients with total or partial loss of unilateral peripheral vestibular function were tested. They were exposed to sinusoidal oscillations (20 degrees, 60 degrees, and 120 degrees/second at 1/6 Hz and 25 degrees/second at 1 Hz) with no actual or imaginary visual targets (doing mental arithmetic) and the usual slow-phase parameters (gain, phase, etc.) were computed. The gaze (total eye position, slow and fast phase of nystagmus vs. head + head position in space) was also plotted vs. head position in space. It was found that in patients, fast phases of nystagmus are unequally (right vs. left) produced, extending the linear range of VOR function. It was also found that in normal subjects and patients, gaze essentially tracks the head in the absence of visual targets. It is concluded that fast phases of nystagmus contribute to gaze control during head rotations in darkness and that the definition and the testing of the VOR have to be modified to accommodate these observations.