Oscillopsia suppression and foveation-period variation in congenital, latent, and acquired nystagmus

We studied the relative importance of clear and stable epochs of vision and extraretinal signals of eye movements in suppressing illusory motion of the world (oscillopsia) in eight subjects with two types of infantile nystagmus, congenital nystagmus (CN) and latent/manifest latent nystagmus (LMLN), and two acquired forms of nystagmus (AN), pendular and jerk. Three subjects with CN and no oscillopsia did not always exhibit well-developed foveation periods, unless their CN was therapeutically damped. Two subjects with CN and AN had transient oscillopsia that coincided in time and plane with the lack of well-developed foveation. Two subjects with AN and oscillopsia had well-developed foveation (one after gabapentin). One subject with LMLN and vertical AN experienced 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 efference 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. The existence of well-developed foveation in some AN subjects supports the assertion that they reflect normal fixation reflexes, rather than developmental adaptation to CN. In LMLN with vertical AN, efference copy of the LMLN prevents horizontal oscillopsia whereas the absence of efference copy of the AN results in vertical oscillopsia.