Characteristics of foveating and defoveating fast phases in latent nystagmus

PURPOSE: Under certain conditions, the fast phases of latent/manifest latent nystagmus (LMLN) can defoveate the target of interest instead of foveating it, as was thought to be their only function. LMLN fast phases in the waveforms from four subjects were studied with the goals of better understanding their characteristics and determining what triggers both foveating and defoveating fast phases. METHODS: Eye movement records were made using both the scleral search coil and infrared methods. Relationships of fast-phase sizes with slow-phase positions and velocities before and after fast phases were analyzed, as were relationships of saccade size with peak velocity and duration. RESULTS. The data showed that LMLN with defoveating fast phases occurred in the presence of higher slow-phase velocities. Also, larger saccade sizes corresponded to larger presaccadic and postsaccadic slow-phase velocities. The peak velocities and durations of LMLN fast phases were in the same ranges as normal saccades. CONCLUSIONS: Defoveating fast phases with decreasing-velocity slow phases may be the result of the addition of saccadic pulses to linear slow phases. Mechanisms are suggested to explain the switch from foveating to defoveating fast phases in LMLN.