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Additional recommended knowledge
Eye movement systems 
Eyes are the visual organs that have the retina, a specialized type of brain tissue containing photoreceptors and interneurons. These specialised cells convert light into electrochemical signals that travel along the optic nerve fibers to the brain.
Primates and many other invertebrates use two types of voluntary eye movement to track objects of interest: smooth pursuit and saccades. These movements appear to be initiated by a small cortical region in the brain's frontal lobe.
Physiology of eye movement
Yoked movement vs. antagonistic movement
The visual system in the brain is too slow to process that information if the images are slipping across the retina at more than a few degrees per second. Thus, to be able to see while we are moving, the brain must compensate for the motion of the head by turning the eyes. Another specialisation of visual system in frontal-eyed animals is the development of a small area of the retina with a very high visual acuity. This area is called the fovea, and covers about 2 degrees of visual angle in people. To get a clear view of the world, the brain must turn the eyes so that the image of the object of regard falls on the fovea. Eye movements are thus very important for visual perception, and any failure to make them correctly can lead to serious visual disabilities. To see a quick demonstration of this fact, try the following experiment: hold your hand up, about one foot (30 cm) in front of your nose. Keep your head still, and shake your hand from side to side, slowly at first, and then faster and faster. At first you will be able to see your fingers quite clearly. But as the frequency of shaking passes about 1 Hz, the fingers will become a blur. Now, keep your hand still, and shake your head (up and down or left and right). No matter how fast you shake your head, the image of your fingers remains clear. This demonstrates that the brain can move the eyes opposite to head motion much better than it can follow, or pursue, a hand movement. When your pursuit system fails to keep up with the moving hand, images slip on the retina and you see a blurred hand.
The brain must point both eyes accurately enough that the object of regard falls on corresponding points of the two retinas in order to avoid the perception of double vision. In primates (monkeys, apes, and humans), the movements of different body parts are controlled by striated muscles acting around joints. The movements of the eye are slightly different in that the eyes not rigidly attached to anything, but are held in the orbit by six extraocular muscles.
Anatomy of eye movement
Ocular anatomy: the extraocular muscles
Each eye has six extraocular muscles (EOM) that bring about the various eye movements:
The brain exerts ultimate control over both voluntary and involuntary eye movements. Three cranial nerves carry signals from the brain to control the extraocular muscles. They are:
Disorders of eye movement
Psychology of eye movement
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Eye_movement". A list of authors is available in Wikipedia.|