Fine oculomotor control is required for good visual performance. To provide maximum visual acuity, it has been shown that retinal image motion must be kept below about 2.5°/s (Westheimer & McKee 1975), and that the image of the target of regard must be within approximately 0.5° of the fovea (Jacobs 1979). This is a strict set of parameters for the oculomotor system, and an eye movement problem or sustained instability such as nystagmus may therefore affect visual function. 

Nystagmus can be classified into congenital forms that are present at birth or which appear soon after, and acquired types that generally appear later in life. Nystagmus can be thought of as a failure of mechanisms which hold gaze steady, and where possible, is classified according to the underlying defect.