Eye movement is the voluntary

Voluntary action

A voluntary action is produced by conscious choice of an organism. The organism, would in turn also be aware of the action while it is executed. This is the opposite of an involuntary action...

 or involuntary

Reflex action

A reflex action, also known as a reflex, is an involuntary and nearly instantaneous movement in response to a stimulus. A true reflex is a behavior which is mediated via the reflex arc; this does not apply to casual uses of the term 'reflex'.-See also:...

 movement of the eye

Human eye

The human eye is an organ which reacts to light for several purposes. As a conscious sense organ, the eye allows vision. Rod and cone cells in the retina allow conscious light perception and vision including color differentiation and the perception of depth...

s, helping in acquiring, fixating and tracking

Eye tracking

Eye tracking is the process of measuring either the point of gaze or the motion of an eye relative to the head. An eye tracker is a device for measuring eye positions and eye movement. Eye trackers are used in research on the visual system, in psychology, in cognitive linguistics and in product...

 visual stimuli. It may also compensate for a body movement, such as when moving the head. In addition, rapid eye movement

Rapid eye movement

Rapid eye movement sleep is a normal stage of sleep characterized by the random movement of the eyes. REM sleep is classified into two categories: tonic and phasic. It was identified and defined by Nathaniel Kleitman, Eugene Aserinsky, and Jon Birtwell in the early 1950s...

occurs during REM sleep.

Introduction

Eyes are the visual organs that have the retina

Retina

The vertebrate retina is a light-sensitive tissue lining the inner surface of the eye. The optics of the eye create an image of the visual world on the retina, which serves much the same function as the film in a camera. Light striking the retina initiates a cascade of chemical and electrical...

, a specialized type of brain

Brain

The brain is the center of the nervous system in all vertebrate and most invertebrate animals—only a few primitive invertebrates such as sponges, jellyfish, sea squirts and starfishes do not have one. It is located in the head, usually close to primary sensory apparatus such as vision, hearing,...

 tissue containing photoreceptors. These specialised cells convert light into electrochemical signals through the ganglion cell layer

Ganglion cell layer

The ganglion cell layer is a layer of the retina that consists of retinal ganglion cells.In the macula lutea, the layer forms several strata....

 and travel along the optic nerve

Optic nerve

The optic nerve, also called cranial nerve 2, transmits visual information from the retina to the brain. Derived from the embryonic retinal ganglion cell, a diverticulum located in the diencephalon, the optic nerve doesn't regenerate after transection.-Anatomy:The optic nerve is the second of...

 fibers to the brain.

Primates and many other invertebrates use two types of voluntary eye movement to track objects of interest: smooth pursuit

Pursuit movement

Smooth pursuit eye movements allow the eyes to closely follow a moving object. It is one of two ways that visual animals can voluntarily shift gaze, the other being saccadic eye movements. Pursuit differs from the vestibulo-ocular reflex, which only occurs during movements of the head and serves to...

 and saccade

Saccade

A saccade is a fast movement of an eye, head or other part of an animal's body or device. It can also be a fast shift in frequency of an emitted signal or other quick change. Saccades are quick, simultaneous movements of both eyes in the same direction...

s. These movements appear to be initiated by a small cortical region in the brain's frontal lobe

Frontal lobe

The frontal lobe is an area in the brain of humans and other mammals, located at the front of each cerebral hemisphere and positioned anterior to the parietal lobe and superior and anterior to the temporal lobes...

. 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.

Not only that, but the visual system is a complicated process and has many steps. Basically, the process starts with the receptor layer (i.e. the retina), which processes raw sensory information from the outside world. It then tranfers this information to higher parts of the brain.

Varieties and purpose of movement

There are three main basic types of eye movements:

• Irregular movements at 30-70 per second, or irregular excursions of 20 seconds of arc
• Saccades, which are worth a few minutes of arc, moving at regular intervals about one second
• Slow irregular drifts up to 6 minutes of arc

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

Convergence

-Mathematics:* Convergence , refers to the notion that some functions and sequences approach a limit under certain conditions* Convergence , the notion that a sequence of transformations come to the same conclusion, no matter what order they are performed in.-Natural sciences:*Convergence ,...

 and smooth pursuit.

Types

Eye movements are typically classified as either duction

Duction

A duction is an eye movement involving only one eye. There are generally six possible movements depending upon the eye's axis of rotation:#Abduction refers to the outward movement of an eye.#Adduction refers to the inward movement of an eye...

s, versions

Version (eye)

A version is an eye movement involving both eyes moving synchronously and symmetrically in the same direction.#Dextroversion / right gaze#Laevoversion / left gaze#Sursumversion / elevation / up gaze#Deorsumversion / depression / down gaze...

, or vergence

Vergence

A vergence is the simultaneous movement of both eyes in opposite directions to obtain or maintain single binocular vision..When a creature with binocular vision looks at an object, the eyes must rotate around a vertical axis so that the projection of the image is in the centre of the retina in both...

s. 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
  • Vestibulo-ocular reflex
    Vestibulo-ocular reflex
    The vestibulo-ocular reflex is a reflex eye movement that stabilizes images on the retina during head movement by producing an eye movement in the direction opposite to head movement, thus preserving the image on the center of the visual field. For example, when the head moves to the right, the...
    
    
  • Optokinetic reflex
    Optokinetic reflex
    The optokinetic reflex allows the eye to follow objects in motion when the head remains stationary . The reflex develops at about 6 months of age.-See also:...
    
    
• Gaze shifting mechanisms
  • Saccadic movement
    Saccade
    A saccade is a fast movement of an eye, head or other part of an animal's body or device. It can also be a fast shift in frequency of an emitted signal or other quick change. Saccades are quick, simultaneous movements of both eyes in the same direction...
    
    
  • Smooth pursuit
    Pursuit movement
    Smooth pursuit eye movements allow the eyes to closely follow a moving object. It is one of two ways that visual animals can voluntarily shift gaze, the other being saccadic eye movements. Pursuit differs from the vestibulo-ocular reflex, which only occurs during movements of the head and serves to...
    
    
  • Vergence movement
    Vergence
    A vergence is the simultaneous movement of both eyes in opposite directions to obtain or maintain single binocular vision..When a creature with binocular vision looks at an object, the eyes must rotate around a vertical axis so that the projection of the image is in the centre of the retina in both...
    
    

Yoked movement vs. antagonistic movement

• Hering's law of equal innervation
  Hering's law of equal innervation
  Hering's law of equal innervation is used to explain the conjugacy of saccadic eye movement in stereoptic animals. The law proposes that conjugacy of saccades is due to innate connections in which the eye muscles responsible for each eye's movements are innervated equally...
  
  
• Sherrington's law of reciprocal innervation
  Sherrington's law of reciprocal innervation
  Sherrington's law of reciprocal innervation, also called Sherrington's law II explains how a muscle will relax when its opposite muscle is activated. René Descartes had hypothesized as much in 1626...
  
  

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

Visual acuity

Visual acuity is acuteness or clearness of vision, which is dependent on the sharpness of the retinal focus within the eye and the sensitivity of the interpretative faculty of the brain....

. This area is called the fovea

Fovea

The fovea centralis, also generally known as the fovea , is a part of the eye, located in the center of the macula region of the retina....

, 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

Hertz

The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....

, 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

Retinal correspondence

Retinal correspondence is the inherent relationship between paired retinal visual cells in the two eyes. Images from one object stimulate both cells, which transmit the information to the brain, permitting a single visual impression localized in the same direction in space.-Types:Normal retinal...

 to avoid the perception of double vision

Diplopia

Diplopia, commonly known as double vision, is the simultaneous perception of two images of a single object that may be displaced horizontally, vertically, or diagonally in relation to each other...

. 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

Orbit (anatomy)

In anatomy, the orbit is the cavity or socket of the skull in which the eye and its appendages are situated. "Orbit" can refer to the bony socket, or it can also be used to imply the contents...

 by six extraocular muscles

Extraocular muscles

The extraocular muscles are the six muscles that control the movements of the eye . The actions of the extraocular muscles depend on the position of the eye at the time of muscle contraction.-List of muscles:-Importance:...

.

Extraocular muscles

Each eye has six extraocular muscles (EOM) that bring about the various eye movements:

• Lateral rectus
  Lateral rectus muscle
  The lateral rectus muscle is a muscle in the orbit. It is one of six extraocular muscles that control the movements of the eye and the only muscle innervated by the abducens nerve, cranial nerve VI....
  
  , (supplied by Abducens nerve)
• Medial rectus
  Medial rectus muscle
  The medial rectus muscle is a muscle in the orbit.As with most of the muscles of the orbit, it is innervated by the inferior division of the oculomotor nerve ....
  
  , (supplied by Oculomotor nerve)
• Inferior rectus
  Inferior rectus muscle
  The inferior rectus muscle is a muscle in the orbit.-Actions:It depresses, adducts, and helps extort the eye.The inferior rectus muscle is the only muscle that is capable of depressing the pupil when it is in a fully abducted position....
  
  , (supplied by Oculomotor nerve)
• Superior rectus
  Superior rectus muscle
  The superior rectus muscle is a muscle in the orbit. It is one of the extraocular muscles. It is innervated by the superior division of the oculomotor nerve...
  
  , (supplied by Oculomotor nerve)
• Inferior oblique
  Inferior oblique muscle
  The Obliquus oculi inferior is a thin, narrow muscle placed near the anterior margin of the floor of the orbit.-Action:Its actions are lateral rotation, elevation and abduction of the eye....
  
  , (supplied by Oculomotor nerve) and
• Superior oblique
  Superior oblique muscle
  For the abdominal muscle see: Abdominal external oblique muscleThe superior oblique muscle, or obliquus oculi superior, is a fusiform muscle originating in the upper, medial side of the orbit which abducts, depresses and internally rotates the eye...
  
   (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. Thus, the eye can be considered as undergoing rotations about a single point in the center of the eye.

Neuroanatomy

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 nerve
  The oculomotor nerve is the 3rd of 12 paired cranial nerves. It enters the orbit via the superior orbital fissure and controls most of the eye's movements, including constriction of the pupil and maintaining an open eyelid by innervating the Levator palpebrae superiors muscle. The optic nerve is...
  
  /Oculomotor nucleus
• IV cranial nerve: Trochlear nerve
  Trochlear nerve
  The trochlear nerve is a motor nerve that innervates a single muscle: the superior oblique muscle of the eye....
  
  /Trochlear nucleus
• VI cranial nerve: Abducens nerve/Abducens nucleus
  Abducens nucleus
  The abducens nucleus is the originating nucleus from which the abducens nerve emerges - a cranial nerve nucleus. This nucleus is located beneath the fourth ventricle in the caudal portion of the pons, medial to the sulcus limitans....
  
  

• Brain
  Brain
  The brain is the center of the nervous system in all vertebrate and most invertebrate animals—only a few primitive invertebrates such as sponges, jellyfish, sea squirts and starfishes do not have one. It is located in the head, usually close to primary sensory apparatus such as vision, hearing,...
  
  
  • Cerebral cortex
    Cerebral cortex
    The cerebral cortex is a sheet of neural tissue that is outermost to the cerebrum of the mammalian brain. It plays a key role in memory, attention, perceptual awareness, thought, language, and consciousness. It is constituted of up to six horizontal layers, each of which has a different...
    
    
    • Frontal lobe
      Frontal lobe
      The frontal lobe is an area in the brain of humans and other mammals, located at the front of each cerebral hemisphere and positioned anterior to the parietal lobe and superior and anterior to the temporal lobes...
      
       – frontal eye fields
      Frontal eye fields
      The frontal eye fields is a region located in the premotor cortex, which is part of the frontal cortex of the primate brain.-Function:...
      
       (FEF), medial eye fields
      Medial eye fields
      Medial eye fields are areas in the frontal lobe of the primate brain that play a role in visually guided eye movement. Most Neuroscientists refer to this area as the supplementary eye fields.-See also:*Saccade*Pursuit movement*Supplementary eye fields...
      
       (MEF), supplementary eye fields
      Supplementary eye fields
      Supplementary eye fields are areas on the dorsal-medial surface of frontal lobe of the primate brain that are involved in planning and control of saccadic eye movements. The SEF was first characterized by John Schlag and colleagues as an area where low intensity electrical stimulation can evoke...
      
       (SEF), dorsomedial frontal cortex (DMFC)
    • Parietal lobe
      Parietal lobe
      The parietal lobe is a part of the Brain positioned above the occipital lobe and behind the frontal lobe.The parietal lobe integrates sensory information from different modalities, particularly determining spatial sense and navigation. For example, it comprises somatosensory cortex and the...
      
       – lateral intraparietal area (LIP), middle temporal area (MT), medial superior temporal area
      Medial superior temporal area
      The medial superior temporal area is a part of the cerebral cortex lying in the dorsal stream of the visual area of the primate brain. The MST receives most of its inputs from the medial temporal area, which is involved primarily in the detection of motion. The MST uses the incoming information...
      
       (MST)
    • Occipital lobe
      Occipital lobe
      The occipital lobe is the visual processing center of the mammalian brain containing most of the anatomical region of the visual cortex. The primary visual cortex is Brodmann area 17, commonly called V1...
      
      
      • Visual cortex
        Visual cortex
        The visual cortex of the brain is the part of the cerebral cortex responsible for processing visual information. It is located in the occipital lobe, in the back of the brain....
        
        
  • Cerebellum
    Cerebellum
    The cerebellum is a region of the brain that plays an important role in motor control. It may also be involved in some cognitive functions such as attention and language, and in regulating fear and pleasure responses, but its movement-related functions are the most solidly established...
    
    
• Midbrain
  • Pretectum
    Pretectum
    The pretectum, also known as the pretectal area, is a region of neurons found between the thalamus and midbrain. It receives binocular sensory input from retinal ganglion cells of the eyes, and is the region responsible for maintaining the pupillary light reflex.-Outputs:The pretectum, after...
    
     – Pretectal nuclei
  • Superior colliculus
    Superior colliculus
    The optic tectum or simply tectum is a paired structure that forms a major component of the vertebrate midbrain. In mammals this structure is more commonly called the superior colliculus , but, even in mammals, the adjective tectal is commonly used. The tectum is a layered structure, with a...
    
    
• Brain stem
  Brain stem
  In vertebrate anatomy the brainstem is the posterior part of the brain, adjoining and structurally continuous with the spinal cord. The brain stem provides the main motor and sensory innervation to the face and neck via the cranial nerves...
  
  
  • Superior colliculus
    Superior colliculus
    The optic tectum or simply tectum is a paired structure that forms a major component of the vertebrate midbrain. In mammals this structure is more commonly called the superior colliculus , but, even in mammals, the adjective tectal is commonly used. The tectum is a layered structure, with a...
    
     (SC)
  • Premotor nuclei in the reticular formation
    Reticular formation
    The reticular formation is a part of the brain that is involved in actions such as awaking/sleeping cycle, and filtering incoming stimuli to discriminate irrelevant background stimuli...
    
     (PMN)
  • Paramedian pontine reticular formation
    Paramedian pontine reticular formation
    The paramedian pontine reticular formation, or PPRF, is part of the pontine reticular formation, a brain region without clearly defined borders in the center of the pons. It is involved in the coordination of eye movements, particularly horizontal gaze and saccades.- Input, Output, and Function...
    
    
  • Cranial nerves
    • III: Oculomotor nerve
      Oculomotor nerve
      The oculomotor nerve is the 3rd of 12 paired cranial nerves. It enters the orbit via the superior orbital fissure and controls most of the eye's movements, including constriction of the pupil and maintaining an open eyelid by innervating the Levator palpebrae superiors muscle. The optic nerve is...
      
      /Oculomotor nucleus
      Oculomotor nucleus
      The fibers of the oculomotor nerve arise from a nucleus in the midbrain, which lies in the gray substance of the floor of the cerebral aqueduct and extends in front of the aqueduct for a short distance into the floor of the third ventricle...
      
      
    • IV: Trochlear nerve
      Trochlear nerve
      The trochlear nerve is a motor nerve that innervates a single muscle: the superior oblique muscle of the eye....
      
      /Trochlear nucleus
    • VI: Abducens nerve/Abducens nucleus
      Abducens nucleus
      The abducens nucleus is the originating nucleus from which the abducens nerve emerges - a cranial nerve nucleus. This nucleus is located beneath the fourth ventricle in the caudal portion of the pons, medial to the sulcus limitans....
      
      
  • Vestibular nuclei
    Vestibular nuclei
    The vestibular nuclei are the cranial nuclei for the vestibular nerve.In Terminologia Anatomica they are grouped in both the pons and medulla.-Subnuclei:There are 4 subnuclei; they are situated at the floor of the fourth ventricle....
    
    
  • Medial longitudinal fasciculus
    Medial longitudinal fasciculus
    The medial longitudinal fasciculus is a pair of crossed fiber tracts , one on each side of the brainstem. These bundles of axons are situated near the midline of the brainstem and are composed of both ascending and descending fibers that arise from a number of sources and terminate in different...
    
    
  • Nucleus prepositus hypoglossi
    Nucleus prepositus hypoglossi
    Located in the medulla, the nucleus prepositus hypoglossi or NPH is part of the horizontal gaze holding system. It functions as a neural integrator....
    
    

Symptoms

• Patients with eye movement disorders may report diplopia
  Diplopia
  Diplopia, commonly known as double vision, is the simultaneous perception of two images of a single object that may be displaced horizontally, vertically, or diagonally in relation to each other...
  
  , nystagmus, poor visual acuity
  Visual acuity
  Visual acuity is acuteness or clearness of vision, which is dependent on the sharpness of the retinal focus within the eye and the sensitivity of the interpretative faculty of the brain....
  
   or cosmetic blemish from squint
  Strabismus
  Strabismus is a condition in which the eyes are not properly aligned with each other. It typically involves a lack of coordination between the extraocular muscles, which prevents bringing the gaze of each eye to the same point in space and preventing proper binocular vision, which may adversely...
  
   of the eyes.

Etiology

• Innervational
  • Supranuclear
  • Nuclear
  • Nerve
  • Synapse
• Muscle anomalies
  • Maldevelopment (e.g. Hypertrophy, atrophy/dystrophy)
  • Malinsertion
  • Scarring secondary to alignment surgery
    Strabismus surgery
    Strabismus surgery is surgery on the extraocular muscles to correct the misalignment of the eyes. With approximately 1.2 million procedures each year, extraocular muscle surgery is the third most common eye surgery in the United States.-Types:...
    
    
  • Muscle diseases (e.g. Myasthenia gravis
    Myasthenia gravis
    Myasthenia gravis is an autoimmune neuromuscular disease leading to fluctuating muscle weakness and fatiguability...
    
    )
• Orbital anomalies
  • Tumor (e.g. rhabdomyosarcoma
    Rhabdomyosarcoma
    A rhabdomyosarcoma is a type of cancer, specifically a sarcoma , in which the cancer cells are thought to arise from skeletal muscle progenitors. It can also be found attached to muscle tissue, wrapped around intestines, or in any anatomic location...
    
    )
  • Excess fat behind globe (e.g. thyroid conditions)
  • Bone fracture
  • Check ligament (e.g. Brown's syndrome, or Superior tendon sheath syndrome)

Selected disorders

• Congenital fourth nerve palsy
  Congenital fourth nerve palsy
  Congenital fourth nerve palsy is a condition present at birth characterized by a vertical misalignment of the eyes due to a weakness or paralysis of the superior oblique muscle....
  
  
• Duane syndrome
  Duane syndrome
  Duane syndrome is a rare, congenital eye movement disorder most commonly characterized by the inability of the eye to abduct or move outwards...
  
  
• Internuclear ophthalmoplegia
  Internuclear ophthalmoplegia
  Internuclear ophthalmoplegia is a disorder of conjugate lateral gaze in which the affected eye shows impairment of adduction. When an attempt is made to gaze contralaterally , the affected eye adducts minimally, if at all. The contralateral eye abducts, however with nystagmus...
  
  
• Nystagmus
• Ophthalmoparesis
  Ophthalmoparesis
  Ophthalmoparesis or ophthalmoplegia refers to paralysis of one or more extraocular muscles which are responsible for eye movements. It is a physical finding in certain neurologic illnesses.-Classification:...
  
  
• Opsoclonus
  Opsoclonus
  Opsoclonus refers to uncontrolled eye movement. Opsoclonus consists of rapid, involuntary, multivectorial , unpredictable, conjugate fast eye movements without intersaccadic intervals. It is also referred to as saccadomania or reflexive saccade...
  
  
• Sixth (abducent) nerve palsy

See also

• Convergence micropsia
  Convergence micropsia
  Convergence micropsia is a type of micropsia characterized by the reduction in apparent size of objects viewed when the eyes are more converged than they need to be for the distance of the object from the eyes....
  
  
• Dissociated vertical deviation
  Dissociated Vertical Deviation
  Dissociated Vertical Deviation is an ocular condition which occurs in association with a squint, typically Infantile Esotropia.- Mechanism :...
  
  
• Eye exercises
  Eye exercises
  Orthoptics is a discipline dealing with the diagnosis and treatment of defective eye movement and coordination , binocular vision, and amblyopia by eye care professionals...
  
  
• Eye movement desensitization and reprocessing
  Eye Movement Desensitization and Reprocessing
  Eye Movement Desensitization and Reprocessing is a form of psychotherapy that was developed by Francine Shapiro to resolve the development of trauma-related disorders caused by exposure to distressing events such as rape or military combat...
  
  
• Eye movement in language reading
  Eye movement in language reading
  Eye movement in reading involves visual processing of words. This was first described by the French ophthalmologist Louis Émile Javal in the late 19th century. He reported that eyes do not move continuously along a line of text, but make short rapid movements intermingled with short stops...
  
  
• Eye movement in music reading
  Eye movement in music reading
  Eye movement in music reading is the scanning of a musical score by a musician's eyes. This usually occurs as the music is read during performance, although musicians sometimes scan music silently to study it, and sometimes perform from memory without score. The phenomenon has been studied by...
  
  
• Eye tracking
  Eye tracking
  Eye tracking is the process of measuring either the point of gaze or the motion of an eye relative to the head. An eye tracker is a device for measuring eye positions and eye movement. Eye trackers are used in research on the visual system, in psychology, in cognitive linguistics and in product...
  
  
• Gaze-contingency paradigm
  Gaze-contingency paradigm
  The gaze-contingency paradigm is a general term for techniques allowing to change the display on a computer screen in function of where the viewer is looking.Gaze-contingent techniques are part of the eye movement field of study in psychology...
  
  
• Microsaccade
  Microsaccade
  Microsaccades are a kind of fixational eye movement. They are small, jerk-like, involuntary eye movements, similar to miniature versions of voluntary saccades. They typically occur during prolonged visual fixation , not only in humans, but also in animals with foveal vision . Microsaccade...
  
  
• Ocular tremor
  Ocular tremor
  Ocular microtremor is a constant, physiological, high frequency , low amplitude eye tremor....
  
  
• Orthoptist
• Rapid eye movement sleep
• Strabismus
  Strabismus
  Strabismus is a condition in which the eyes are not properly aligned with each other. It typically involves a lack of coordination between the extraocular muscles, which prevents bringing the gaze of each eye to the same point in space and preventing proper binocular vision, which may adversely...
  
  
• Voluntary Nystagmus

External links

• 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.