Eye movement (sensory)
Eye movement (ocular motility) 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.
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 vertebrates use three types of voluntary eye movement to track objects of interest: smooth pursuit, vergence shifts  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 voluntary control is obliterated.
Varieties and purpose of movement
There are three main basic types of eye movements:
- Vergence Movements or convergence are the movements of both eyes to make sure that the image of the object being looked at falls on the corresponding spot on both retinas.
This type of movement helps in the depth perception of objects 
- Saccades are the rapid movement of eyes that is used while scanning a visual scene. In our subjective impression, the eyes do not move smoothly across the printed page during reading. Instead, our eyes make short and rapid movements called saccades. During each saccade the eyes move as fast as they can and the speed cannot be consciously controlled in between the stops. The movements are worth a few minutes of arc, moving at regular intervals about three to four per second. One of the main uses for these saccadic eye movements is to be able to scan a greater area with the high resolution fovea of the eye.
- Pursuit Movements or Smooth pursuit are the movements that the eyes make while tracking an object's movement, so that its moving image can remain maintained on fovea.
Additionally, the eyes are never completely at rest. They make fast random jittering movements even when we are fixated on one point. The reason for these random movements are the photoreceptors and the ganglion cells. It appears that a constant visual stimulus can make the photoreceptors or the ganglion cells to become unresponsive; on the other hand a changing stimuli will not. Therefore, these random eye movements constantly change the stimuli that fall on the photoreceptors and the ganglion cells making the image more clear.
Saccades are faster than vergence and pursuit eye movements.
Eye movements are typically classified as either ductions, versions, or vergences. 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.
- Fixational eye movement
- Gaze-stabilizing mechanisms
- Gaze shifting mechanisms
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 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 most vertebrates (humans, mammals, reptiles, birds), 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:
- 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)
Eye movement of lateral rectus muscle, superior view
Eye movement of medial rectus muscle, superior view
Eye movement of inferior rectus muscle, superior view
Eye movement of superior rectus muscle, superior view
Eye movement of superior oblique muscle, superior view
Eye movement of inferior oblique muscle, superior view
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. Thus, the eye can be considered as undergoing rotations around 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)
- Congenital fourth nerve palsy
- Duane syndrome
- Internuclear ophthalmoplegia
- Sixth (abducent) nerve palsy
- Convergence micropsia
- Dissociated vertical deviation
- Eye exercises
- Eye movement desensitization and reprocessing
- Eye movement in language reading
- Eye movement in music reading
- Eye tracking
- Gaze-contingency paradigm
- Ocular tremor
- Rapid eye movement sleep
- Progressive supranuclear palsy
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- eMedicine – Extraocular Muscles, Actions
- Oculomotor Control – Nystagmus and Dizziness Department of Otolaryngology – Queen's University at Kingston, Canada
- Fixation Movements of the Eyes
- Software system for simulating eye motility disorders and their surgical correction
- An eye movement simulator, which shows changes in eye movements for any given muscle or nerve impairment.
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