Eye movements

Eye movements are the voluntary or involuntary movements of the eye.

Neuroanatomy

 * Brain
 * Cerebral cortex
 * Frontal lobe - frontal eye fields (FEF), medial eye fields (MEF), supplementary eye fields (SEF), dorsomedial frontal cortex (DMFC)
 * Parietal lobe - lateral intraparietal area (LIP), middle temporal area (MT), medial superior temporal area (MST)
 * Occipital lobe
 * Visual cortex
 * Cerebellum


 * Midbrain
 * Pretectum - Pretectal nuclei
 * Superior colliculus


 * Brain stem
 * Superior colliculus (SC)
 * Premotor nuclei in the reticular formation (PMN)
 * Paramedian pontine reticular formation
 * Cranial nerves
 * III: Oculomotor nerve/Oculomotor nucleus
 * IV: Trochlear nerve/Trochlear nucleus
 * VI: Abducens nerve/Abducens nucleus
 * Vestibular nuclei
 * Medial longitudinal fasciculus

Ocular anatomy: the extraocular muscles
Each eye has six muscles that control its movements: 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 :
 * 1) Ductions - A duction is an eye movement involving only one eye.
 * 2) Versions - Versions are eye movements involving both eyes in which each eye moves in the same direction.
 * 3) Vergences - Vergences are eye movements involving both eyes in which each eye moves in opposite directions.

Eye movement systems

 * Fixational eye movements
 * Gaze stabilizing mechanisms
 * Vestibulo-ocular reflex
 * Optokinetic reflex


 * Gaze shifting mechanisms
 * Saccadic movements
 * Pursuit movements
 * Vergence movements

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 movements vs. antagonistic movements

 * Hering's law of equal innervation
 * Sherrington's law of reciprocal innervation

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.

Having two eyes is an added complication, because the brain must point both of them accurately enough that the object of regard falls on corresponding points of the two retinas; otherwise, we would see double. Before dealing with this problem, we shall discuss the movements of one eye alone, and restrict our discussion to 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 no exception, but they have special advantages not shared by skeletal muscles and joints, and so are considerably different. First, the eye is not rigidly attached to anything, but is held in the orbit by six extraocular muscles.

Symptoms

 * Patients with eye movement disorders may report diplopia or poor visual acuity in the affected eye.

Etiology

 * Innervational
 * Supranuclear
 * Nuclear
 * Nerve
 * Synapse


 * Muscle anomalies
 * Maldevelopment
 * Malinsertion
 * Scarring secondary to alignment surgery
 * Diseased muscle


 * Orbita anomalies
 * Tumor
 * Excess fat behind globe (e.g. thyroid conditions)
 * Bone fracture
 * Check ligament (e.g. Brown's syndrome, or Superior tendon sheath syndrome)

Disorders

 * Congenital fourth nerve palsy
 * Duane syndrome
 * Internuclear ophthalmoplegia
 * Nystagmus
 * Ophthalmoparesis
 * Opsoclonus
 * Sixth (abducent) nerve palsy
 * Strabismus