Walsh & Hoyt: Fixation System

Although the eyes may appear to an examiner to be still during steady viewing, fixation actually consists of three distinct types of miniature movements that are not detected by visual inspection of the eyes: microsaccades, microtremor, and microdrift. They occur in horizontal, vertical and torsional directions. Microsaccades usually have amplitudes of less than 26 minutes of arc (average about 6 minutes; there are 60 arc-minutes in 1 degree). By ophthalmoscopy, the examiner can detect saccades as small as about 10 minutes of arc. Microsaccades occur at a mean frequency of approximately 120 per minute. Although it has been suggested that microsaccades prevent images from fading on the retina, vision remains clear when they are voluntarily suppressed. Normal individuals can voluntarily reduce the rate of microsaccades. When they are asked to suppress them, saccade rate is reduced to about 30 per minute. Microsaccades stop during finely guided visual motor tasks such as threading a needle or sighting a rifle. Microsaccades have no known visual function, but they may reflect shifting of visual attention away from a fixated object. Some investigators maintain that microsaccades refresh images to prevent the fading that occurs when the eyes are immobile.