Eye movement is the voluntary or involuntary movement of the eyes, helping in acquiring, fixating and tracking visual stimuli. It may also compensate for a body movement, such as when moving the head. In addition, rapid eye movement occurs during REM sleep.
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Eyes are the visual organs that have the retina, a specialized type of brain tissue containing photoreceptors. These specialised cells convert light into electrochemical signals through the ganglion cell layer and travel along the optic nerve fibers to the brain.
Primates and many other invertebrates use three types of voluntary eye movement to track objects of interest: smooth pursuit , vergence shifts [1] and saccades.[2] These movements appear to be initiated by a small cortical region in the brain's frontal lobe.[3][4] This is corroborated by removal of the frontal lobe. In this case, the reflexes (such as reflex shifting the eyes to a moving light) are intact, though the voluntary control is obliterated.[5]
There are three main basic types of eye movements:
These movements are needed to provide a new set of rods and cones, because, if it was fixed on one set, the signal would be cancelled due to adaptation of the stimulus after some time. Saccades are faster than convergence and smooth pursuit.[5]
Eye movements are typically classified as either ductions, versions, or vergences.[6][7] A duction is an eye movement involving only one eye; a version is an eye movement involving both eyes in which each eye moves in the same direction; a vergence is an eye movement involving both eyes in which each eye moves in opposite directions.
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.[8] 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 many vertebrate 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 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 are not rigidly attached to anything, but are held in the orbit by six extraocular muscles.
Each eye has six extraocular muscles (EOM) that bring about the various eye movements:
credit: Patrick J. Lynch
When the muscles exert differential tensions (contractions in synergistic muscles and relaxation of antagonist muscles), a torque is exerted on the globe that causes it to turn. This is an almost pure rotation, with only about one millimeter of translation.[9] Thus, the eye can be considered as undergoing rotations about a single point in the center of the eye.
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:
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Wehner, R. (2005). Sensory physiology: Brainless eyes. Nature: International Weekly Journal of Science , 157-159.