THE EFFECT OF THE VESTIBULAR APPARATUS ON THE MECHANISM OF ELICITATION OF OPTOKINETIC NYSTAGMUS AS STUDIED ON ELECTRONYSTAGMOGRAM

When a man looks around while on a revolving object, nystagmus is elicited. This type of nystagmus consists of two components, i.e. optokinetic nystagmus and labyrinthine nystagmus. Labyrinthine nystagmus is caused by the movement of endolymph in the semicircular canals and can be elicited by rotating the body in the dark room with the eyes opened (perrotatory nystagmus). Optokinetic nystagmus can be elicited by applying visual stimuli with the eyes opened.Optokinetic nystagmus may or may not be affected by additionally induced rotation nystagmus. It is also known that labyrinthine nystagmus is inhibited by gaze. In the present study electronystagmographic study was performed on the effect of concomitant perrotatory nystagmus (as induced by pendular rotation of the body) on the pattern of optokinetic nystagmus and the inhibitory effect of gaze on perrotatory nystagmus.The subjects were 20 healthy adults. In all subjects an inhibitory effect of gaze on perrotatory nystagmus was noted. The effect, however, was complete (total suppression of perrotatory nystagmus) in 39% of the cases and incomplete (the nystagmus still appeared though diminished in frequency and amplitude) in the other 61%.Optokinetic nystragmus was demonstrated in all cases. When a stimulus of pendular rotatory movements was applied to the subjects while optokinetic nystagmus was being elicited, no appreciable changes were observed in 39% of cases. In the remaining 61% of the cases, clearly noticeable changes in the slow component of the optokinetic nystagmus were observed.In the group whose perrotatory nystagmus was inhibited incompletely, the changes of the optokinetic nystagmus by the rotation was more clealy observed than in the group whose perrotatory nystagmus was inhibited completely.This finding suggests that the inhibition of labyrinthine nystagmus by gase should be taken into account when interaction between optokinetic nystagmus and labyrinthine nystagmus is studied.