Walsh & Hoyt: Fixation and Gaze-Holding Ability

In an awake individual, the eyes are never absolutely still. Fixation is interrupted by three distinctive types of miniature eye movements: (a) microsaccades, with an average amplitude of about 6 minutes of arc and a mean frequency of about 2 per second; (b) continuous microdrift at rates of less than 20 minutes of arc/second; and (c) microtremor, consisting of high frequency (4060 Hz) oscillations of 530 seconds of arc. Square wave jerksspontaneous, horizontal saccades of about 0.5 , followed about 200 msec later by a corrective saccade and occurring at a rate of less than 9 per minutecan also be observed during fixation in normal individuals. Herishanu and Sharpe suggested that in normal individuals, square wave jerks are sporadically enlarged microsaccades. St. Cyr and Fender postulated that the spontaneous eye movements that occur during fixation correct minor fixation errors, whereas Ditchburn believed that such movements may prevent images from fading on the retina. Kowler and Steinman, however, stated that these small saccades serve no useful purpose, since vision remains clear when they are voluntarily suppressed.