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 o f foveating it, as was thought to be their only function. LMLN fast ph ases in the waveforms from four subjects were studied with the goals o f better understanding their characteristics and determining what trig gers both foveating and defoveating fast phases. METHODS. Eye movement records were made using both the scleral search coil and infrared met hods. Relationships of fast-phase sizes with slow-phase positions and velocities before and after fast phases were analyzed, as were relatio nships of saccade size with peak velocity and duration. RESULTS. The d ata showed that LMLN with defoveating fast phases occurred in the pres ence of higher slow-phase velocities. Also, larger saccade sizes corre sponded 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 s accadic pulses to linear slow phases. Mechanisms are suggested to expl ain the switch from foveating to defoveating fast phases in LMLN.