Normal optic nystagmus may be defined as that conjugate response of the eyes to a succession of moving stimuli, which is composed of an alternation of slow and quick phases in opposite directions, when each slow phase consists of the pursuit or fixation of a moving object; each quick phase represents the fixation of a new object of pursuit within the moving field. The slow or pursuit phases consequently consist of eye movements regularly in the direction of the moving visual field. The quick phases of refixation are usually in the opposite direction, determined by some peripherally seen object which at the instant engages the attention or automatically becomes the new object of regard.
This phenomenon has been studied for the most part by direct observation of the response of the eyes to a moving visual field of relative simplicity in the form of alternate vertical black and white stripes
FOX JC, DODGE R. OPTIC NYSTAGMUSII. VARIATIONS IN NYSTAGMOGRAPHIC RECORDS OF EYE MOVEMENT. Arch NeurPsych. 1929;22(1):55-74. doi:10.1001/archneurpsyc.1929.02220010058006