2 TYPES OF FOVEATION STRATEGY IN LATENT NYSTAGMUS - FIXATION, VISUAL-ACUITY AND STABILITY

The authors studied the foveation dynamics of two individuals with lat ent/manifest latent nystagmus (LMLN) to test the hypothesis that oscil lopsia suppression and good visual acuity require periods of accurate target foveation at low slip velocities. Congenital nystagmus (CN) wav eforms contain post-saccadic foveation periods; the LMLN waveform does not and yet allows for both oscillopsia suppression and good acuity. During fixation with both eyes open, there were intervals when the eye s were still and correctly aligned; at other times, there was esotropi a and nystagmus with slow-phase velocities less than +/-4 deg/sec and each fast phase pointed the fovea of the fixating eye at the target. H owever, cover of either eye produced LN and a different strategy was e mployed: the fast phases carried the fixating eye past the target and the fovea subsequently reacquired it during the slowest parts of the s low phases. The authors confirmed this in both subjects, whose high ac uities were made possible by foveation occurring during the low-veloci ty portions of their slow phases. A nystagmus foveation function (NFF) , originally developed for CN, was calculated for both LN and MLN inte rvals of fixation and it was found to track visual acuity less accurat ely for individuals with high acuity. Individuals with LMLN exhibit tw o different foveation strategies: during low-amplitude LMLN, the targe t is foveated immediately after the fast phases; and during high-ampli tude LMLN, target foveation occurs towards the end of the slow phases. Therefore, the saccadic system can be used to create retinal error ra ther than eliminate it if this strategy is beneficial. Individuals wit h LMLN foveated targets with the same eye-position and -velocity accur acy as those with CN and the NFF provides a rough estimate of acuity i n both. Current calibration methods for both infrared and search-coil techniques need to be altered for subjects with LMLN.