Eye movements, visual acuity and spatial constancy.

How are eye movements related to acuity or to the lack of spatial constancy, oscillopsia (OSOP)? How do subjects with congenital nystagmus (CN) suppress OSOP? Can we apply their strategies to cases of acquired nystagmus? In normals, the maintenance of target foveation with low retinal slip is thought to be necessary for good visual acuity. Retinal slip velocities below 1.67-4 deg/sec have been given as upper bounds for good vision. Subjects with CN do not usually have OSOP and can have good (even normal) visual acuity. CN subjects can maintain target position (SD = 0.21 deg) and low retinal slip velocities (SD = 1.97 deg/sec). Previously, we identified two possible mechanisms for the suppression of OSOP in subjects with CN : 1) efference copy of the CN waveform to negate the effects of the oscillation or 2) the stable vision available during foveation periods. A 48 year-old man with hereditary CN lost consciousness; when he came to, he had troublesome, intermittent OSOP. Recordings revealed a jerk left with extended foveation (JLef) waveform when OSOP was absent (SD of eye position was 0.24 deg and SD of retinal slip velocity was 1.87 deg/sec). However, with the onset of OSOP, his CN waveform abruptly changed to jerk right (JR) and was biased several degrees to the right of the target. Phase-plane analysis revealed that, during the periods of OSOP, the JR waveform did not enter the foveation window defined by the limits, 0 +/- 0.5 deg and 0 +/- 4.0 deg/sec.(ABSTRACT TRUNCATED AT 250 WORDS)