The origin of square wave jerks: conditions of fixation and microsaccades.

The effects of various fixation stimuli on square waves of eye movements were studied in 16 normal subjects. Fixation eye movements were recorded by infrared reflection oculography under visual and nonvisual conditions. The target size was varied from 9 to 72 minutes of arc in four steps. The mean amplitude of the square wave was enlarged more than 0.5 degrees with the targets over 18 minutes of arc and in total darkness. The findings indicate that the square wave jerks are enhanced microsaccades and are closely related to the instability of the fixation eye movements.