SUPPRESSION OF VESTIBULAR-NYSTAGMUS BY FORCED CONVERGENCE IN NORMAL HUMAN-SUBJECTS

Background: It has been suggested that the principal mechanism of nyst agmus suppression in the nystagmus blockage syndrome is either adducti on of the eye or convergence, We examined this issue using the nystagm us of the vestibule-ocular reflex (VOR) as a model. Methods: A motoriz ed, computer-controlled rotary chair was used to produce VOR in darkne ss, using either sinusoidal or velocity step stimulation. Left eye pos ition was monitored and horizontal slow-phase eye velocity was calcula ted. Subjects were cued to converge or perform other gaze tasks. Resul ts: Convergence suppressed nystagmus. With sinusoidal stimulation, nys tagmus was nearly extinguished in extreme lateroversion, probably due to mechanical tethering of the eye. However, VOR gain suppression of 4 7% during convergence was observed even when the monitored eye was clo se to primary position. With velocity step stimulation, nystagmus was nearly extinguished at moderate angles of adduction. Conclusions: Conv ergence is sufficient to suppress nystagmus, without vision and withou t regard to whether the eye is adducted.