Saccade

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 one part of the retina, the macula, has a high concentration of light-receptive nerve endings. 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, nature has contrived to produce a system that makes efficient use of body resources. If we saw an entire scene in high resolution, the diameter of the optic nerve would need to be larger than the diameter of they eyeball itself. Subsequent processing of such a high-resolution image would require a brain size many times larger than its current size.

Saccades can add spatial and frequency resolution to many other kinds of sensors. For example, some animals use saccades as part of echolocation.

Dynamics of the saccadic eye motion give insight into the complexity of the mechanism that controls the saccadic motion of the eye. Again, let's have the human eye serve as an example. The saccade is the fastest movement of external part of human body. The peak angular speed of the eye during a saccade reaches several hundred degrees per second. Saccades last from about 0.02 seconds, up to about 0.8 seconds.

The duration of a saccade depends on the amplitude of a saccade. Amplitude of a saccade is 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 more lineary dependent on the amplitude.

To explain some implications of saccadic eye motion, it's worth modeling the eye as a photo camera. A saccadic eye movement is quite fast compared to any motion on the scene that's being observed. Thus, the image on the retina during a saccade is blurred. It's the same phenomenon as if you were moving a camera with an open shutter -- the photograph would get blurred.

See also saccadic masking.