Modulation of mean eye position during vestibular induced eye movements of the rhesus monkey

Vestibular induced nystagmus in the dark was measured during sinusoidal rotational stimulation in three male monkeys using electrooculography. A periodic modulation of mean eye position was observed. This periodic modulation was quantified by least squares fitting through the nystagmus a sinusoid whose frequency was the same as that of the stimulus. The magnitude and the phase of the best fitting sinusoid were used to determine gain and phase delay at various combinations of stimulus amplitudes and frequency. Results indicate that the mean eye position in the alert animals is predominantly modulated in the direction of the fast component of nystagmus. The magnitude of this modulation of mean eye position is independent of the amplitude of the stimulus. The phase relationship of this modulation is dependent on the frequency of stimulation. In drowsy animals, the modulation of mean eye position is predominantly in the direction of the slow component of nystagmus. We conclude that the modulation of mean eye position during vestibular induced nystagmus operates as an automatic gain control system which is sensitive to the animal's level of alertness. The quantitative data in this report are useful in refining current models of vestibular nystagmus.