OSCILLOPSIA SUPPRESSION AND FOVEATION-PERIOD VARIATION IN CONGENITAL,LATENT, AND ACQUIRED NYSTAGMUS

We studied the relative importance of clear and stable epochs of visio n and extraretinal signals of eye movements in suppressing illusory mo tion of the world (oscillopsia) in eight subjects with two types of in fantile nystagmus, congenital nystagmus (CN) and latent/manifest laten t nystagmus (LMLN), and two acquired forms of nystagmus (AN), pendular and jerk. Three subjects with CN and no oscillopsia did not always ex hibit well-developed foveation periods, unless their CN was therapeuti cally damped. Two subjects with CN and AN had transient oscillopsia th at coincided in time and plane with the lack of well-developed foveati on. Two subjects with AN and oscillopsia had well-developed foveation (one after gabapentin). One subject with LMLN and vertical AN experien ced oscillopsia solely in the plane of the AN, despite the presence of good foveation in both planes. Our findings argue against the role of foveation periods in suppression of oscillopsia. In CN, lack of well- developed foveation does not result in oscillopsia, suggesting that ef ference copy of the CN may be responsible for the stability of vision. In CN with AN or AN alone, oscillopsia may occur irrespective of well -developed foveation, if the AN is not monitored by efference copy. Th e existence of well-developed foveation in some AN subjects supports t he assertion that they reflect normal fixation reflexes, rather than d evelopmental adaptation to CN. In LMLN with vertical AN, efference cop y of the LMLN prevents horizontal oscillopsia whereas the absence of e fference copy of the AN results in vertical oscillopsia.