A robust, normal ocular motor system model with latent/manifest latent nystagmus and dual-mode fast phases.

Latent/manifest latent nystagmus (LMLN) is a specific type of infantile nystag­ mus that occurs subsequent to strabismus in some patients.1.2 The amplitude of LMLN usually follows Alexander's law, increasing as the fixating eye moves into abduction and decreasing in adduction. 1 The slow phases of LMLN may be either linear or of decreasing velocity in the same patient. 3 Depending on the slow-phase velocity, LMLN fast phases may cause the target image to fall either within (foveat­ in g) or outside (defoveating) the foveal area.3 Higher slow-phase velocities precipi­ tate defoveating fast phases.4 Also, as presaccadic slow-phase velocities grow, fast­ phase amplitudes follow. 'fie present a dual-mode, ocular motor system model capable of producing nor­ mal saccades and both foveating and defoveating fast phases in LMLN. Additional­ ly, the model contains a mechanism by which linear slow phases undergo the transition to decreasing velocity slow phases. The model includes programmable Alexander's law behavior and fixation conditions (binocular, rightand left-eye viewing), allowing simulation of the idiosyncratic characteristics of a broad spec­ trum of individuals with LMLN.5