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Saccade

From Academic Kids

A saccade is a fast movement of an eye, head, or other part of an animal's body or of a device. It can also be a fast shift in frequency of an emitted signal, or other such fast change.

Its purpose can be illustrated by the human eye. We do not look at a scene in a steady way. Instead, our eyes jerk around, locating interesting parts of the scene and building up an intelligent mental 'map' corresponding to the scene. In the human eye, one reason for saccades is that only the central part of the retina, the macula, has a high concentration of color sensitive nerve endings (called cones). The rest of the retina is mainly made up of monochrome nerve endings (called rods) which are especially good for motion detection. Thus, the macula makes up the high-resolution central part of human retina.

By moving the eye so that small parts of a scene can be sensed with greater resolution, body resources can be used efficiently. If we saw an entire scene in high resolution, the diameter of the optic nerve would need to be larger than the diameter of the eyeball itself. Subsequent processing of such a high-resolution image would require a brain size many times larger than its current size.

Dynamics of the saccadic eye motion give insight into the complexity of the mechanism that controls the saccadic 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 0.02 seconds, up to about 0.8 seconds.

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 a particular saccade. For amplitudes up to about 60 degrees, the duration of a saccade linearly depends on the amplitude. In that range, the peak velocity of a saccade also linearly depends on the amplitude. In saccades larger than about 60 degrees the peak velocity remains constant at the maximum velocity attainable by the eye. Thus, their duration is no longer linearly dependent on the amplitude.

To explain some implications of saccadic eye motion, one may compare the eye to a photo camera. A saccadic eye movement is quite fast compared to any motion on the scene being observed. Thus, the image on the retina during a saccade is blurred. It is the same phenomenon as if a camera were being moved with the shutter open: the photograph would get blurred.

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