|
Eye movement is the voluntary or involuntary movement of the eye. Neuroanatomy
Ocular anatomy: the extraocular muscles Each eye has six muscles that control its movement: the lateral rectus, the medial rectus, the inferior rectus, the superior rectus, the inferior oblique, and the superior oblique. When the muscles exert different tensions, 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 (Carpenter, 1988). Thus, the eye can be considered as undergoing rotations about a single joint in the center of the eye. Types Eye movements are typically classified as either ductions, versions, or vergences: Eye movement systems [http://www2.gsu.edu/~biopsk/bio8020/Eye-movements.pdf] Primates use two types of voluntary eye movements to track objects of interest: pursuits and saccades. These movements appear to be initiated by a small cortical region in the brain's frontal lobe. Yoked movement vs. antagonistic movement Eyes are the structural organs that contain the retina, a specialized type of brain tissue that contains the photoreceptors and interneurons that convert light into electrochemical signals that travel along the fibers of the optic nerve to the brain. 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 complication for vision 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. Symptoms Etiology
Notes See also | |||||||
|
| ||||||||
 |
|
| |