Fig. 16 Fusion maldevelopment nystagmus (FMN). With both eyes open there is either low-amplitude nystagmus (when only one eye is fixating) or, rarely, no nystagmus (when both eyes are fixating). Closure of either eye results in jerk nystagmus with fast phases toward the viewing (unoccluded) eye. When both eyes are open, the nystagmus fast phases are toward the fixating eye. Slow phases may be either linear (usually when both eyes are open) or decreasing-velocity exponentials (usually upon occlusion of one eye), unlike those of IN. Note that the fast phases may be foveating (for low-amplitude FMN with linear slow phases) or defoveating (for the higher amplitude FMN with decreasing velocity slow phases).