A saccade is an eye movement when both eyes move quickly in tandem. This might be accompanied by movement of the head, neck or other parts of the body. It is a process of the brain that allows the viewer to perceive the world around him or her in small, focused areas. The speed and path of a saccade can be used to detect certain neurological problems.
There are four distinct types of saccadic movements. Visually guided saccades show the eye moving toward a newly introduced image. Memory-guided saccades will show the eye moving toward a location that is remembered but not necessarily present. Predictive saccades are when the eyes predict the movements of an object in the visual field and follow it. Finally, anti-saccades are when the eye moves away from an object sometimes in expectation of movement that does not occur.
Different methods and machines can be used to test whether the saccades in a human are functioning as they are intended to. These tests measure the speed of eye movement both vertically and horizontally. Certain conditions can be diagnosed if the speed is too fast or too slow in either direction, including pulsion and overshoot dysmetria.
Saccadic movement is primarily controlled by the frontal eye fields, which are areas of the brain located near the top of the head. A second area known as the median eye fields help control the physical tracking motions of the eyes, especially during a saccade. When combined, the eyes are able to move and fixate quickly on areas of the surrounding environment allowing a detailed, three-dimensional mental image of the area to be established.
The human eye, during a saccade, is the fastest moving part of the body, and because of this, it has developed a mechanism known as visual saccadic suppression. This function of the eyes will block an image that the eye perceives as blurry from reaching the brain. This avoids having meaningless information transmitted to the brain when it would be impossible to process it.
There are several disorders that can affect the saccadic motion of the eyes in humans. One of the more common is known as nystagmus and is characterized by slow tracking followed by quick saccades afterward. Undershoot and overshoot dysmetria is a disorder in which the eyes either over-compensate or under-compensate when attempting to fixate on a point. A glissade is a type of saccade disorder in which the eye does not stop at a fixed point but rather moves slowly past the point.