Effects of fixation and optokinetic stimulation on vestibulo-ocular reflex suppression.

Suppression of the vestibulo-ocular reflex was assessed in normal subjects and patients with neurological disorders to determine the relative effects on suppression of a single fixation target and an optokinetic field. Subjects were rotated sinusoidally in yaw at varying frequencies of up to 0.5 Hz whilst seated in a Barany chair. A comparison was made between eye movements in darkness, those produced during fixation on a central target mounted to the chair, and eye movements during fixation on the target plus an "earth-fixed" or "chair-fixed" visual background. Presentation of a background produced only minimal effects on the suppression of the vestibulo-ocular reflex in normal subjects. In patients with impairment of fixation suppression, suppression of the vestibulo-ocular reflex was not improved after presentation of either form of optokinetic field. The results demonstrate that central fixation is the predominant requirement for suppression of the vestibulo-ocular reflex. This correlates closely with the ability to pursue. Although the optokinetic reflex generates following eye movements similar to pursuit, it cannot be used to mediate suppression of the vestibulo-ocular reflex in the absence of an intact pursuit system. The findings strengthen the view that the optokinetic reflex evolved to act in synergy with the vestibulo-ocular reflex in generating compensatory eye movements.