Types of Eye Movement
The eye is a complex organ, composed of numerous rods, cones, fluids and light-sensitive components. Each eye has six extraocular muscles that control its movements, rotations and reactions. Movements can be both voluntary and involuntary and occur when an individual is conscious or unconscious. Each of these movements is classified into a sub-category that explains its respective characteristics.
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Saccades
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Saccades are abrupt movements of the eye that change the point of fixation. Simply put, they can be thought of as looking from point A to point B. Saccades can be voluntary or involuntary. An example of voluntary movement would be when an individual chooses to look at one side of a room and then the other. An example of involuntary movement would be the unconscious motions of the eye that occur during sleep (known as rapid eye movement or REM).
Smooth Pursuit
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Smooth pursuit incorporates a gradual movement of the eye, typically while focusing on a singular moving subject. For example, if you place your hand in front of your face and slowly move it to one side while following it with your eyes, a smooth pursuit movement occurs. Interestingly, it is very difficult to make a voluntary smooth pursuit movement in the absence of a moving subject (such as your hand). Without a subject, the eye is more prone to rapid movements that would be considered saccades.
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Vergence
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Vergence movements align the fovea (central portion of the eye that affords acute vision) with different targets, one closer and one further away. These movements are much like the focusing of a camera. They can adjust the eyes to blur or clarify objects at different distances. Vergence movements are also used to describe the process or turning the eyes in separate directions (known as "crossing" the eyes).
Vesibulo-Ocular
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Vestibulo-ocular movements allow the eyes to remain fixated on a subject while the head moves. This is a reflex response that keeps objects in the periphery of an individual's vision from moving beyond the field of view as the head rotates.
Fixation Maintenance
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These movements are actually characterized as saccades. However, they occur only as involuntary movements. When the eyes are fixated on a singular subject, they rapidly move from side to side. These movements are very minute and often unnoticeable from the observer's point of view.
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