Saccade

DEFINITION

A rapid, simultaneous movement of both eyes from one still position to another. Derived from a French word for twitch, saccade is a typical occurence in visual perception ? the eyes fixing on one point after another in the visual field. saccadeHumans and other animals do not look at a scene in a steady way. Instead, the eyes move around, locating interesting parts of the scene and building up a mental 'map' corresponding to the scene. One reason for saccades of the human eye is that the central part of the retina, the fovea, plays a critical role in resolving objects. By moving the eye so that small parts of a scene can be sensed to obtain greater closure, the body's optical resources can be used more efficiently.The dynamics of saccadic eye motion give insight into the complexity of the mechanism that controls the motion of the eye. The saccade is the fastest movement of an external part of the human body. The peak angular speed of the eye during a saccade reaches up to 1000 degrees per second. Saccades last from about 20 to 200 milliseconds.The duration of a saccade depends on its amplitude. The amplitude of a saccade is the angular distance that the eye needs to travel during the movement. For amplitudes up to about 60 degrees, the duration of a saccade linearly depends on the amplitude (so called "saccadic main sequence"). In that range, the peak velocity of a saccade linearly depends on the amplitude. In saccades larger than 60 degrees, the peak velocity remains constant at the maximum velocity attainable by the eye. Thus, the duration of these large saccades is no longer linearly dependent on the amplitude.In addition to the kind of saccades described above, the human eye is in a constant state of vibration, oscillating back and forth at a rate of about 60 per second. These microsaccades are such small movements that they are entirely imperceptible under normal circumstances. They serve to refresh the image being cast onto the rods and cones at the back of the eye. Without microsaccades, staring fixedly at something causes visual distortions after a few seconds since rods and cones respond best to changes in color and luminosity. (See afterimage.)The word appears to have been coined in about 1878 by French ophthalmologist Louis ?mile Javal (1839-1907), who used a mirror on one side of a page to observe that in reading, the eyes are involved in a succession of discontinuous individual movements. Others noted that saccades occur in all visual activities.Saccadic is the adjectival form, and saccadically the adverbial form.(pr. sə-KAHD)Also see sight.

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