saccade

saccade, fast, intermittent eye movement that redirects gaze. Saccades may involve the eyes alone or, more commonly, the eyes and the head. Their function is to place the fovea, the central region of the retina where vision is most acute, onto the images of parts of the visual scene of interest. Their duration and peak velocity vary systematically with their size. The smallest “microsaccades” move the eye through only a few minutes of arc (one minute of arc equals one-sixtieth of one degree). They last about 20 milliseconds and have maximum velocities of about 10 degrees per second. The largest saccades (excluding the contributions of head movements) can be up to 100 degrees, with a duration of up to 300 milliseconds and a maximum velocity of about 500–700 degrees per second.

During saccades, vision is seriously impaired for two reasons. First, during large saccades, the image is moving so fast that it is blurred and unusable. Second, an active blanking-off process, known as saccadic suppression, occurs, and this blocks vision for the first part of each saccade. Between saccades, the eyes are held stationary in fixations. It is during these periods, which last on average about 190 milliseconds, that the eyes take in visual information. Saccades can be reflexive in nature; for example, when an object appears in one’s peripheral field of view. However, as Russian psychologist Alfred L. Yarbus showed, saccades are often information-seeking in nature, directed to particular objects or regions by the requirements of ongoing behaviour.