FOVEATION DYNAMICS IN CONGENITAL NYSTAGMUS .3. VESTIBULOOCULAR REFLEX

It has been shown that, during fixation of a stationary target with a fixed head, an individual with congenital nystagmus (CN) can repeatedl y (beat-to-beat) foveate (within 13 minarc) and maintain low retinal s lip velocities (less than 4-degrees/sec). With the head in motion, ves tibuloocular reflex (VOR) data showed eye velocities during these fove ation periods that approximation head veloicty. Despite some claims th at the VOR of CN subjects was deficient or absent, individuals with CN hardly ever complain of oscillopsia or exhibit any of the symptoms th at would accompany such deficits in the VOR, whether during simple wal king and running or while skiing down a mogul field. We developed and describe several different and unrelated methods to accurately assess the function of the VOR in an individual with typical idiopathic CN. W e investigated the dynamics of CN foveation periods during head rotati on to test the hypothesis that eye velocities would match head velocit ies during these periods. At about 1 Hz, horizontal VOR instantaneous (beat-to-beat) gains were 0.96 in the light and 0.94 in the dark while imagining a stationary target. Vertical VOR gains were 1.00 and 0.99 for these two conditions at the same frequency; the CN was horizontal. Also, during the VOR there is a CN neutral-zone shift comparable to t hat found during smooth pursuit. Our methods demonstrated that gaze ve locity was held constant during foveation periods and we conclude that the VOR in this subject is functioning normally in the presence of th e CN oscillation. Based on our findings in this and previous studies, we hypothesize that CN may be due to a peripheral instability.