Saccade
From Freepedia
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. Humans do not look at a scene in a steady way. Instead, the 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 cone cells. The rest of the retina is mainly made up of monochrome nerve endings called rod cells, which are especially good for motion detection. Thus, the macula makes up the high-resolution central part the 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 many times larger than its current size.
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 20–800 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 a particular saccade. For amplitudes of 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, the duration of these large saccades is no longer linearly dependent on the amplitude.
During the saccade, no information is passed through the optic nerve to the brain. Transmission ceases as soon as the eyeball starts to move, and starts again as soon as the eyeball stops. This is similar to the situation with a motion picture camera: exposure is made only when the film is stopped behind the lens. A shutter closes just before film starts to move, and opens again when the film is stationary on the next frame. A good way to see this effect for yourself is to stand 18 inches away from a mirror and look at your left eye, then look at your right eye, then look back at the left, and so on. You will not see any movement of your eyes, nor any evidence that the optic nerve has momentarily ceased transmitting. A companion standing next to you, however, will see your eyes shifting rapidly side to side. Due to saccadic masking, the eye/brain system not only hides an observer's eye movements from himself, it hides evidence that anything has been hidden.
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 tiny movements, roughly 1/180th of a degree in excursion, and completely imperceptible under normal circumstances. They serve to refresh the image being cast onto the rod cells and cone cells at the back of the eye. If there were no microsaccades, staring fixedly at something would cause the vision to cease after a few seconds since rods and cones only respond to a change in luminance.
