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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. 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 volutary control is obliterated.
- Ductions - A duction is an eye movement involving only one eye.
- Versions - Versions are eye movements involving both eyes in which each eye moves in the same direction.
- Vergences - Vergences are eye movements involving both eyes in which each eye moves in opposite directions.
- Fixational eye movement
- Gaze stabilizing mechanisms
- Gaze shifting mechanisms
Yoked movement vs. antagonistic movement Edit
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.
Extraocular muscles Edit
- Main article: Extraocular muscles
Each eye has six extraocular muscles (EOM) that bring about the various eye movements:
- Lateral rectus, (supplied by Abducens nerve)
- Medial rectus, (supplied by Oculomotor nerve)
- Inferior rectus, (supplied by Oculomotor nerve)
- Superior rectus, (supplied by Oculomotor nerve)
- Inferior oblique, (supplied by Oculomotor nerve) and
- Superior oblique (supplied by Trochlear nerve)
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 (Carpenter, 1988). 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:
- III cranial nerve: Oculomotor nerve/Oculomotor nucleus
- IV cranial nerve: Trochlear nerve/Trochlear nucleus
- VI cranial nerve: Abducens nerve/Abducens nucleus
- Cerebral cortex
- Brain stem
- Superior colliculus (SC)
- Premotor nuclei in the reticular formation (PMN)
- Paramedian pontine reticular formation
- Cranial nerves
- Vestibular nuclei
- Medial longitudinal fasciculus
- Nucleus Prepositus Hypoglossi
- Patients with eye movement disorders may report diplopia, nystagmus, poor visual acuity or cosmetic blemish from squint of the eyes.
- Muscle anomalies
- Orbital anomalies
- Tumor (e.g. rhabdomyosarcoma)
- Excess fat behind globe (e.g. thyroid conditions)
- Bone fracture
- Check ligament (e.g. Brown's syndrome, or Superior tendon sheath syndrome)
Selected disorders Edit
- Congenital fourth nerve palsy
- Duane syndrome
- Internuclear ophthalmoplegia
- Sixth (abducent) nerve palsy
See also Edit
- Convergence micropsia
- Eye convergence
- Eye fixation
- Eye Movement Desensitization and Reprocessing
- Eye movement in language reading
- Eye movement in music reading
- Eye tracker
- Fixational eye movement are small, involuntary eye movements that occur during visual fixation
- Gaze-contingency paradigm
- Microsaccade small, jerk-like, involuntary eye movements
- Ocular tremor
- Rapid eye movement (disambiguation)
- REM dreams
- Rapid eye movement sleep
- Saccade, a fast movement of an eye, head, or other part of an animal's body or device
- Visual search
- Voluntary Nystagmus
- ↑ Krauzlis, RJ. The control of voluntary eye movements: new perspectives. The Neuroscientist. 2005 Apr;11(2):124-37. PMID 15746381.
- ↑ Heinen SJ, Liu M. "Single-neuron activity in the dorsomedial frontal cortex during smooth-pursuit eye movements to predictable target motion." Vis Neurosci. 1997 Sep-Oct;14(5):853-65. PMID 9364724
- ↑ Tehovnik EJ, Sommer MA, Chou IH, Slocum WM, Schiller PH. "Eye fields in the frontal lobes of primates." Brain Res Brain Res Rev. 2000 Apr;32(2-3):413-48. PMID 10760550
- ↑ Kanski, JJ. Clinical Ophthalmology: A Systematic Approach. Boston:Butterworth-Heinemann;1989.
- ↑ Awwad, S. "Motility & Binocular Vision". EyeWeb.org.
- ↑ Westheimer, Gerald & McKee, Suzanne P.; "Visual acuity in the presence of retinal-image motion". Journal of the Optical Society of America 1975 65(7), 847-50.
- ↑ Robinson FR, Fuchs AF. "The role of the cerebellum in voluntary eye movements." Annu Rev Neurosci. 2001;24:981-1004. PMID 11520925
- Carpenter, Roger H.S.; Movements of the Eyes (2nd ed.). Pion Ltd, London, 1988. ISBN 0-85086-109-8.
- eMedicine - Extraocular Muscles, Actions
- Oculomotor Control - Nystagmus and Dizziness Department of Otolaryngology - Queen's University at Kingston, Canada
- Fixation Movements of the Eyes
- An eye movement simulator, which shows changes in eye movements for any given muscle or nerve impairment.
|Sensory system - Visual system - edit|
|Eye | Optic nerve | Optic chiasm | Optic tract | Lateral geniculate nucleus | Optic radiation | Visual cortex|
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