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Below is a glossary of common terms related to eye health.

# | A | B | C | D | E | F | G | H | I | K | L | M | N | O | P | R | S | T | V | W

The expression for normal eyesight (or 6/6 in countries where metric measurements are used). This notation is expressed as a fraction. The numerator (1st number) refers to the distance you were from the test chart, which is usually 20 feet (6 meters). The denominator (2nd number) denotes the distance at which a person with normal eyesight could read the line with the smallest letters that you could correctly read. For example, if your visual acuity is 20/100 that means that the line you correctly read at 20 feet could be read by a person with normal vision at 100 feet. The Snellen chart, which consists of letters, numbers, or symbols, is used to test visual acuity (sharpness of eyesight). A refraction test is used to determine the amount of correction needed for a prescription when treating refractive error such as astigmatism, myopia or hyperopia. See "Refraction Test."

(Eye focusing) The eye's ability to adjust its focus by the action of the ciliary muscle, which increases the lens focusing power. When this accommodation skill is working properly, the eye can focus and refocus quickly and effortlessly, which is similar to an automatic focus feature on a camera. The ciliary muscles must contract to adjust for near vision, which causes the eye’s crystalline lens, which is flexible, to be squashed. For distant vision, the ciliary muscle must relax and the eye’s crystalline lens is stretched out.  The ability of the eye to accommodate does decrease with age due to the crystalline lens becoming less flexible causing a condition called presbyopia. (See "Presbyopia")

Accommodative Esotropia
(clinical condition) when an individual is focusing on a near object and his or her eyes are turning inward too much. It is caused by either uncorrected hyperopic refractive error and/or a high accommodative convergence/accommodation (AC/A) ratio. The average age of onset is 2 1/2 years. It is most noticeable when the child is tired or sick. This is treated with plus lenses (glasses or contacts) to help straighten the eyes. In some cases, vision therapy and corrective lenses are prescribed. (Please note that Accommodative Esophoria is a condition similar to accommodative esotropia but lesser in extent.)

Accommodative Infacility
A clinical condition in which the individual has difficulty changing eye focus from distance to near. Symptoms include eyestrain associated with near work (such as reading or using a computer), periodic blurring of distance vision especially following sustained near visual work, tendency to hold near work closer than expected, headaches with near work, and possible double vision. Clinical signs include: patient will have difficulty with both the plus and the minus lens (fails +/- 2.00 D flipper test), low PRA and NRA, and poor recoveries on Bell Retinoscopy. Vision therapy is an effective treatment option.

Accommodative Insufficiency (AI)
This clinical condition is also called non-presbyopic accommodative insufficiency. It is an under focusing, a lack of focusing ability at a near distance. Symptoms include eyestrain, blurred vision, occasional or constant when doing near work (such as reading or using a computer), occasional unusual sensitivity to light, excess tearing, headaches, and general fatigue. Clinical signs include: patient will have difficulty with a minus lens, low amplitude of accommodation, low PRA and higher NRA. Vision therapy is an effective treatment option.

Clearness of eyesight. Depends on the sharpness of images and the sensitivity of nerve elements in the retina. (See "Near Acuity" and "Distance Acuity")

T he eye's ability to still see an image during eye blinks and even after the viewed object is no longer present. The most common example is seeing light after the flash of a camera.

(Clinical condition) reduced visual acuity (poorer than 20/20) which is not correctable by glasses or contacts and is not caused by structural or pathological anomalies. This condition is often called “lazy eye” because it is typically the result of disuse. It is usually marked by blurred vision in one eye and favoring one eye over the other. About two percent of the population is affected.

Types of functional (reversible) amblyopia:

* Refractive - anisometropia (the two eyes have different refractive powers), or other amblyopiogenic refractive errors (hyperopia, myopia, or astigmatism)

* Strabismic- misalignment of the two eyes in which they point in different directions.

* Form deprivation (may also be referred to as  amblyopia ex anopsia)- caused by conditions that prevent light from entering the eye. These may include congenital ptosis (droopy eyelid), corneal opacity, or cataract.

Any optical error such as hyperopia, myopia, or astigmatism. Also called refractive error.

Amplitude of Accommodation (AA)
A measurement of the eye’s ability to focus clearly on objects at near distances. This eye focusing range for a child is usually about 2-3 inches. For a young adult, it is 4-6 inches. The focus range for a 45-year-old adult is about 20 inches. For an 80-year-old adult, it is 60 inches.

The condition in which the two eyes have different refractive powers.

Eyestrain, symptoms include excessive tearing, itching, burning, visual fatigue, and headache. It can be caused from an uncorrected refractive error, accommodation (eye focusing) disorder, binocularity (eye teaming) disorder, or by extended, intense use of the eyes.

Light rays entering the eye do not all meet at the same point (similar to a frayed string), which results in blurred or distorted vision. An abnormally shaped cornea typically causes this condition. Occasionally astigmatism exists in the lens of the eye. This condition is corrected by a cylindrical (toric) eyeglass or contact lens.

The alignment of the len’s cylindrical part; used for correcting astigmatism. This measurement is given in degrees. The values are typically from 90 degrees to 180 degrees.

Bifocal Glasses
Used to correct vision at two distances, composed of two ophthalmic lenses such as a plus lens for near vision and a minus lens for distance vision.

Binocular Fusion Dysfunction
A clinical condition in which the eyes are not working as a team. Vision therapy is an effective treatment option. (See"General Binocular Vision Disorder")

Binocular Vision
The simultaneous use of the two eyes.

The ability to use both eyes as a team and to be able to fuse (unite) two visual images into one, three-dimensional image (See “Convergence” and “Divergence”).

Bi – Ocularity
Using both eyes, but not together as a team.

Blurred Vision
Lack of visual clarity or acuity.    

Botulinum Toxin Type A (Oculinum, Botox®)
An injection of this poison has been used as an alternative to conventional surgery in selected strabismic patients. It causes a temporary paralysis of an extraocular muscle that leads to a change in eye position. This change has been reported to result in long-lasting and permanent alteration in eye alignment. Although one injection is often sufficient to produce positive results, one-third to one-half of patients may require additional injections. This technique has been most successful when used in adults with small-angle misalignments. It is not commonly used in children. This treatment is also used in patients who have blepharospam (an uncontrollable eye lid spasm).

A condition of the crystalline lens, in which the normally clear lens becomes clouded or yellowed, causing blurred or foggy vision. Cataracts may be caused by aging, eye injuries, disease, heredity, or birth defects. Surgery is a treatment option. The affected lens is removed and is replaced with a substitute (implant) lens or with a special type of contact lens. Generally the success rate of cataract surgery is over 90%, if the eye is otherwise healthy.

Ciliary Body
A structure directly behind the iris of the eye and contains the ciliary muscle.

Ciliary Muscle
A band of muscle and fibers that are attached to the lens that controls the shape of the lens and allows the lens to accommodate (change focus).

Color Perception Test
A test that measures the ability to identify and distinguish colors.

Color Vision Deficiency
Also known as Colorblindness. It is the absence of or defect in the perception of colors. Color vision is based on perception of red, green, and blue. If there is a defect in the perception of one of these colors, a color will be perceived as if it were composed only of the other two colors. Based on the color or colors for which there is defective perception, a person may suffer from red, green, or blue blindness. Color blindness in which all colors are perceived as gray is termed monochromasia. For people with the common, inherited, types of color deficiency there is no cure.

Computer Vision Syndrome (CVS)
The complex of eye and vision problems related to near work that are experienced during or related to computer use. Its symptoms include eyestrain, dry or burning eyes, blurred vision, headaches, double vision, distorted color vision, and neck and backaches. The condition is caused by various internal and external factors. Treatment options may include prescription glasses and/or vision therapy.

A receptor cell which is sensitive to light and is located in the retina of the eye. It is responsible for color vision.

An inflammation of the conjunctiva, the transparent layer covering the inner eyelid and the white portion (sclera) of the eyeball. Conjunctivitis can be caused by a virus, bacteria, or fungus (infectious conjunctivitis, or "pink eye", may be contagious); by allergies to pollen, fabrics, animals, or cosmetics (allergic conjunctivitis); or by air pollution or noxious fumes such as swimming pool chorine (chemical conjunctivitis). Symptoms include red or watery eyes, blurred vision, inflamed inner eyelids, scratchiness in the eyes, or (with infectious conjunctivitis) a puss like or watery discharge and matted eyelids. Conjunctivitis is usually treated with antibiotic eye drops and/or ointment.

The ability to use both eyes as a team and to be able to turn the eyes inward to maintain single vision up close.

Convergence Excess (CE)
A clinical condition in which the eyes have a tendency to turn excessively inward when viewing an object at a near distance. Symptoms may include visual fatigue while reading or using a computer, occasional blurred or double vision, and inability to comprehend or concentrate while reading. Clinical signs include: greater esophoria at near than distance, high AC/A ratio, and a high lag of accommodation. Can be improved with vision therapy and/or glasses. (See "Esophoria")

Convergence Insufficiency (CI)
(Clinical condition) the inability of the eyes to turn inward and/or sustain an inward turn. Symptoms include eye strain with reading and using a computer, headaches, loss of comprehension, difficulty concentrating, blurred or double vision, and eye fatigue. Clinical signs include: near point of convergence of greater than 4 inches (10 cm), greater exophoria at near than at distance, and low AC/A ratio. Vision therapy is an effective treatment option.

The transparent, blood-free tissue covering the central front of the eye (over the pupil, iris, and aqueous humor) that initially refracts or bends light rays as light enters the eye. Contact lenses are fitted over the cornea.

Cover Test
A test of eyeball alignment in which each eye is covered with an occluder (eye cover) and then uncovered to observe eye movements.

Depth Perception Test
A test to measure the ability of the vision system to discern the relative distances of various objects. (Also called a “Stereopsis Test”)

A single object is perceived as two rather than one; double vision.

Distance Acuity
The eye's ability to distinguish an object's shape and details at a far distance such as 20 feet (6 meters).

The ability to use both eyes as a team and be able to turn the eyes out toward a far object.

Divergence Excess (DE)
(Clinical condition) the eye's tendency to drift out relative to the direction of a distant object being viewed. Symptoms include: double vision at distance, headaches, eyestrain, nausea, dizziness, and blurred vision. Clinical signs include: exophoria greater at distance than near, high AC/A ratio, and reduced positive fusional vergence at distance. Can be improved with vision therapy.

Divergence Insufficiency (DI)
(Clinical condition) the eye's tendency to turn more inward than necessary when viewing a distant object. Symptoms include: double vision, headaches, eyestrain, nausea, dizziness, and blurred vision. Clinical signs: esophoria greater at distance than near, low AC/A ratio, and reduced negative fusional vergence at distance. Treated with corrective lenses and vision therapy.

Dominant Eye
The eye that "leads" it partner during eye movements. Humans also have dominant hand, foot, eye, and side of the brain (not necessarily all on the same side).

Inability to "sound out" words. Dysphonesia is a subtype of dyslexia. Children with this form of dyslexia have difficulty sequentially analyzing and remembering what and where the sounds are in words. The resulting phonemic processing problems make it difficult to sound out new words, learn phonics, and make them dependent on their sight vocabulary. When they come to an unknown word they will often substitute a word using context clues. For example, "pony" for "horse", even though the substituted word doesn't look or sound anything like the original word. When spelling unknown words it is often difficult  to determine what the original word is. For example, they may write "fmlue" for "familiar" or "lap" for "lamp". They cannot learn phonics because they cannot process where the sounds are. Their short term sequential auditory memory can be poor and result in repeating "8167" as "8671", or remember to go to their room but forgetting to get the item requested.

A specific language-based disorder. The individual has difficulty with letter or word recognition, spelling, reading, writing, and sometimes naming pictures of objects. Dyslexia varies in degree from mild to very sever. It is caused by an inability of the brain's language centers to decode print or phonetically make the connection between the word's written symbols and their appropriate sounds. Dyslexia is not caused by a vision disorder. Children often are of normal or above normal intelligence. Dyslexia cannot be cured and will never be outgrown. Appropriate teaching methods can be taught to help those with dyslexia overcome their weakness.

Eccentric Fixation
The deviating eye does not use the central foveal (center of the retina that produces the sharpest eyesight) area for fixation. Commonly, individuals with amblyopia and some individuals with strabismus will have this visual adaptation. In esotropia, the eccentrically located retinal point used for fixation is usually in the nasal retina. In exotropia, the eccentrically located retinal point used for fixation is usually in the temporal retina. Vision therapy is a treatment option for those with amblyopia and/or strabismus. It is not a treatment option for an individual with a fovea that has been destroyed.

Normal vision, no correction needed.

Esophoria (Eso)
(Clinical condition) a tendency of the eyes to want to turn more inward than necessary when an individual is viewing an object at near or at distance, which may cause the individual to experience eyestrain and other symptoms. Symptoms of basic esophoria include: eyestrain, headaches, blurred or double vision, apparent movement of print, and difficulty concentrating on and comprehending reading material. Clinical signs of basic esophoria include: AC/A ratio is normal, equal esophoria at distance and near, and normal near point of convergence. Sometimes esophoria is caused by a refractive error such as hyperopia (farsightedness), and glasses or contacts can correct the problem alone. However, sometimes vision therapy is needed to to help re-train the eyes to function more appropriately. (See "Convergence Excess" and "Divergence Insufficiency")

Esotropia (ET)
(Clinical condition) a condition in which an eye is turned either constantly or intermittently inward toward the nose. Esotropia is a type of strabismus. It is caused by a reduction in visual acuity, reduced visual function, high refractive error, traumatic brain injury, oculomotor nerve lesion, or eye muscle injury. Treatment options may include one or more of the following: glasses or contacts, bi-focal lenses, prisms, vision therapy, surgery, or Botulinum Toxin Type A (Oculinum, Botox®) injections. In some cases, esotropia is caused by a refractive error such as hyperopia (farsightedness), and glasses or contacts alone may allow the eyes to straighten. Vision therapy is most appropriate when there are small degrees of misalignment. Surgery, to re-position or shorten the eye muscles, may be required for high degrees of misalignment. If surgery is required, a combination of surgery and vision therapy often yields the best results.

Exophoria (Exo)
(Clinical condition) a tendency of the eyes to want to turn more outward than necessary when an individual is viewing an object at near or at distance, which may cause the individual to experience eyestrain and other symptoms. Symptoms of basic exophoria include: eyestrain, headaches, blurred or double vision, apparent movement of print, and difficulty concentrating on and comprehending reading material. Clinical signs of basic exophoria include: normal AC/A ratio, equal exophoria at distance and near, and decreased near point of convergence. Vision therapy is an effective treatment option. (See "Convergence Insufficiency" and "Divergence Excess")

Exotropia (XT)
(Clinical condition) a condition in which an eye is either constantly or intermittently turned outward toward the ear. Exotropia is a type of strabismus. It may also be called divergent strabismus, wandering eye, or wall eye(s). It is caused by a reduction in visual acuity, reduced visual function, high refractive error, traumatic brain injury, oculomotor nerve lesion, or eye muscle injury. Treatment options may include one or more of the following: glasses or contacts, bi-focal lenses, prisms, vision therapy, surgery, or Botulinum Toxin Type A (Oculinum, Botox®) injections. Vision therapy is most appropriate when there are small degrees of misalignment. If surgery is required, a combination of surgery and vision therapy often yields the best results

Extraocular Muscles
The muscles attached to the outside of the eyeball which control eye movement. Each eye has six muscles (lateral rectus, medial rectus, superior oblique, inferior oblique, superior rectus, and inferior rectus) that are coordinated by the brain.

Eye Hand Coordination
The ability of our eyes to guide our hands, also called visual motor integration.  

Eye Trac
(Equipment) an electronic testing and recording system of eye movements as in reading.

Eye Tracking
The ability of the eyes to smoothly and effortlessly follow a moving target.

Facility of Accommodation
A measure of the ease and speed of the eye(s) to change focus.

The ability to recognize distinct shapes from their background, such as objects in a picture, or letters on a chalkboard.

Fine Motor Skills
The ability to coordinate hand and finger movements.

The ability to direct and maintain steady visual attention on a target. Fixations are a form of pursuits.

Fixation Disparity (FD)
Over-convergence or under-convergence, or vertical misalignment of the eyes under binocular (both eyes) viewing conditions small enough in magnitude so that fusion is present.

Also known as spots, are usually clouded or semi-opaque specks or particles within the eye that are seen in the field of vision. The eyes are filled with fluid which maintains the shape of the eye, supplies it with nutrition and aids in the focusing of light. Often, particles of protein or other natural materials are left floating or suspended in this fluid when the eye is formed before birth. If the particles are large or close together, they cast shadows which make them visible. This is particularly true when nearsightedness occurs or becomes more severe. In most cases this is normal but floaters can also be caused by certain injuries, eye disease or deterioration of eye fluid or its surrounding structures.

Form Constancy
The ability to recognize two objects that have the same shape but different size or position. This ability is needed to tell the difference between "b" and "d", "p" and "q", "m" and "w".

Center of the retina that can produce the sharpest eyesight and contains the most cones.

Fusional Vergence
A convergence response which serves to maintain (fusion) the union of images from each eye into a single image. The eyes will turn with a slow smooth tonic movement or a fast jumping movement called phasic.

Fusional Vergence Dysfunction
See "General Binocular Vision Disorder".

The union of images from each eye into a single image.  There are three degrees of fusion. 1st degree fusion is the superimposition of two dissimilar targets. 2nd degree fusion is flat fusion with a two-dimensional target. 3rd degree fusion is depth perception (stereopsis) with a three-dimensional target.

Fusion Test
Determines the eyes ability to unite the images from each eye into a single image.

General Binocular Vision Disorder
Inability to efficiently utilize and/or sustain binocular vision. Symptoms include eyestrain, headaches, decreased comprehension, inability to concentrate while reading, excessive tearing, and blurred vision. A patient will have difficulty with both base-in and base-out prisms. Vision therapy is an effective treatment option.

Tendency of the eyes to deviate from their normal position for visual alignment. This condition may be observed when one eye is covered.

The eyes are abnormally turned.

A condition in which one eye has a tendency to point higher than the other eye, causing eyestrain. Sometimes improved by prisms in glasses.

Inverse Occlusion
Covering the amblyopic eye. (See "Direct Occlusion" and "Occlusion")

The colored part of the eye located between the lens and cornea; it regulates the entrance of light.)

The sensation of bodily position, presence, or movement resulting chiefly from stimulation of sensory nerve ending in muscles, tendons, and joints.

Lag of Accommodation
A measure of the eye's ability to focus accurately on a given target. The dioptric difference between the eye's focusing response and the stimulus to focus. 

Latent Hyperopia
Hyperopia (farsightedness) is compensated by accommodation and the tonicity (tension) of the ciliary muscle; identified by cycloplegic refraction. In mild cases of hyperopia (farsightedness), the eyes are able to compensate without corrective lenses; otherwise a plus lens (glasses or contacts) is prescribed. Vision therapy is not prescribed. (See "Hyperopia")

Learning Disability (LD)
A disorder that affects people's ability to either interpret what they see and hear or to link information from different parts of the brain. Learning disabilities can be divided into five broad categories: speech and language disorders, reading disorder, arithmetic disorder, writing disorder, and attention disorders. The term learning disability does not include children who have learning problems that are primarily the result of visual, hearing, or motor disorders.

Also called Verometer, is a device used to measure the refractive power of eyeglasses and contact lenses.

The most sensitive part of the retina that is about the size of a pinhead and is where our most detailed vision occurs.

Macular Degeneration
A deterioration of the central portion of the retina known as the macula. 

A voluntary or intentional reduction in visual acuity or other examination data.

Also called microtropia, monofixation syndrome, and small angle strabismus. A small angle deviation (inward or outward, commonly inward) that is less than five degrees with some amount of stereopsis (depth perception) and anomalous retinal correspondence (ARC). Possible mild amblyopia, eccentric fixation, and/or anisometropia may also be present. It frequently results from the treatment of a larger-angle deviation (esotropia or exotropia) by optical correction, vision therapy, pharmacological agents, and/or extraocular muscle surgery. Treatment for microstrabimus consists mostly of correcting significant refractive errors and any coexisting amblyopia. The use of vision therapy and prisms to establish bifoveal fusion has been successful in selected cases.

Minus (-) Lens
Concave lens, stimulates focusing and diverges light. The lens is thinner in the center than the edges. It is used in glasses or contact lenses for people who are nearsighted (myopia).

Monocular Vision
Only one eye having useful vision.

Nearsightedness, an individual will have difficulty seeing clearly at distance. Light entering the eye focuses in front of the retina when the eye is at rest and is corrected with a minus lens. A condition known as high myopia occurs when myopia is greater than 6 diopters. Typically, vision therapy is not prescribed for myopia.

Near Acuity
The eye's ability to distinguish an object's shape and details at a near distance such as 16 inches (40 cm).

Near Point of Convergence (NPC)
The closest point at which the two eyes can maintain a single united image.

Near Point of Convergence Test
Measures the patient’s ability to point the eyes at an approaching object and to keep them fixed on the object as it reaches the patient’s nose.  Normal range is 0 to 4 inches away from the nose.

Negative Relative Accommodation (NRA)
A measure of the maximum ability to relax accommodation while maintaining clear, single binocular vision.

Normal Retinal Correspondence (NRC)
The foveas of the two eyes are corresponding neural points in the visual cortex and binocular vision can occur.

Rhythmic oscillations or tremors of the eyes which occur independent of the normal eye movements. Generally nystagmus is not curable, but it is manageable. Treatments include prescription glasses or contact lenses, prisms, and vision therapy.

To block out light. An eye can be completely or partially blocked. This procedure is used to promote the use of one eye or both eyes. This therapy procedure may be used for people with amblyopia, strabismus, or closed head trauma. It may also be used in a vision therapy program for someone with amblyopia, eye focusing (accommodation) disorder, or poor eye tracking (oculomotor) skill.  An eye patch, black contact, or another device may be used to block out light from an eye. (See "Direct Occlusion" and "Inverse Occlusion")

Ocular Motility
Pertaining to binocular alignment and eye muscle movement. (See "Binocularity", "Strabismus")

Ocular Motor (OM)
General eye movement ability, which include pursuits (to visually track and/or follow moving objects) and saccades (to direct and coordinate eye movement as the eye quickly and voluntarily shift from one target to another).

Ocular Motor Dysfunction
Poor eye movement skills. Vision therapy is an effective treatment option. (See “Pursuits Dysfunction” and “Saccades Dysfunction”)  

Oculomotor Skills
The ability to quickly and accurately move our eyes. These are sensory motor skills that allow us to move our eyes so we can fixate on objects (fixation), move our eyes smoothly from point to point as in reading (saccades), and to track a moving object (pursuits). (See "Fixation", “Pursuits” and “Saccades”)    

Oculus Dexter (OD)
Right eye.

Oculus Sinister (OS)
Left eye.

Oculus Uterque (OU)
Both eyes.

A physician (doctor of medicine (M.D.) or doctor of osteopathy (D.O.) who specializes in the comprehensive care of the eyes and visual system in the prevention of eye disease and injury. The ophthalmologist has completed four or more years of college premedical education, four or more years of medical school, one year of internship, and three or more years of specialized medical and surgical training and experience in eye care. The ophthalmologist is a physician who is qualified by lengthy medical education, training and experience to diagnose, treat and manage all eye and visual system problems, and is licensed by a state regulatory board to practice medicine and surgery. The ophthalmologist is the medically trained specialist who can deliver total eye care: primary, secondary and tertiary care services (i.e., vision services, contact lenses, eye examinations, medical eye care and surgical eye care), and diagnose general diseases of the body. An ophthalmologist is not trained to provide vision therapy. 

A device used to illuminate the inside of the eye and enlarge the image for examining the retina, optic nerve entrance, arteries, and veins.

Optic Nerve
Is a bundle of nerve fiber that connects each eye to the brain and transmits images from the retina to the brain. 

Is a professional in the field of designing, finishing, fitting and dispensing of eyeglasses and contact lenses, based on an eye doctor's prescription. The optician may also dispense colored and specialty lenses for particular needs as well as low-vision aids and artificial eyes.

Optometric Vision Therapy (VT)
As defined by the American Optometric Association: Optometric vision therapy is a treatment plan used to correct or improve specific dysfunctions of the vision system. It includes, but is not limited to, the treatment of strabismus (turned eye), other dysfunctions of binocularity (eye teaming), amblyopia (lazy eye), accommodation (eye focusing), ocular motor function (general eye movement ability), and visual-perception-motor abilities.

Optometric vision therapy is based upon a medically necessary plan of treatment which is designed to improve specific vision dysfunctions determined by standardized diagnostic criteria. Treatment plans encompass lenses, prisms, occlusion (eye patching), and other appropriate materials, modalities, and equipment. (Vision therapy can also be called visual or vision training, orthoptics, eye training, or eye exercises.)  

Please note that the definition above describes Optometric Vision Therapy, which has been clinically shown to improve certain eye disorders, which are described above. However Optometric Vision Therapy is NOT the same as the Bates Method, vision therapy using Bates, integrated vision therapy, or natural eye exercises. These holistic programs use some form of eye exercises associated with relaxation techniques, which claim to improve nearsightedness, farsightedness, astigmatism, decreasing vision with age, and other disorders. There is virtually no statistical studies/results indicating the success of these methods.

A health care professional who is state licensed to provide primary eye care service.  These services include comprehensive eye health and vision examinations; diagnosis and treatment of eye disease and vision disorders; the detection of general health problems; the prescribing of glasses, contact lenses, low vision rehabilitation, vision therapy, and medications; the performing of certain surgical procedures; and the counseling of patients regarding their surgical alternatives and vision needs as related to their occupations, avocations and lifestyle. The optometrist has completed pre-professional undergraduate education in a college or university and four years of professional education at a college of optometry, leading to the doctor of optometry (O.D.) degree. Some optometrists complete a residency.

Organic Amblyopia
Gradual or sudden loss of central vision (partial loss) affecting visual acuity with no treatment options.

Types of organic (irreversible) amblyopia:

* Nutritional amblyopia - Vision loss caused by low levels of vitamin B 12 due to poor nutrition and poor absorption assoicated with drinking alcohol.

* Tobacco-alcohol amblyopia - Clinical evidence exists that a nutritional deficiency is the underlying cause of this vision loss; however, many still believe that the toxic effects of alcohol and/or tobacco are contributing factors.

* Toxic amblyopia - Caused by exposure to toxins such as ethambutol, methyl alcohol (moonshine), ethylene glycol (antifreeze), cyanide, lead and carbon monoxide.

    • Orthophoria (ortho)
      The absence of either esophoria or exophoria. The eyes do not have a tendency to want to turn more inward than necessary or want to turn more outward than necessary when pointed on an object.

    • Orthoptics
      The science of correcting defects in binocular vision. The technique of eye exercises to correct strabismus (esotropia or exotropia), convergence insufficiency (exophoria), or convergence excess (esophoria), amblyopia, and ocular motility disorders. Orthoptics was pioneered by French ophthalmologist Javal in the mid to late 1800’s. Today ophthalmologists use specialty-trained healthcare professionals called orthoptists to evaluate patients and treat them with orthoptics. In America, the non-surgical technique of orthoptics is less commonly used by ophthalmologists compared to other countries. Orthoptics is a limited form of optometric vision therapy.

    • Paresis
      A paralysis that when occurring in ocular muscles causes double vision when looking in some directions. 

    • Perceptual Skills
      Includes the identification, discrimination, spatial awareness, and visual-sensory integration. These are visual cognitive skills used to processes visual information to the brain to be organized and interpreted. (See “Visual Perceptual Disorder”)

    • PTS Computerized Perceptual Home Vision Therapy System
      A home-based computerized perceptual therapy program, which was designed to enhance visual information processing. The therapy procedures address simultaneous processing, sequential processing and/or speed of information processing. This computer program contains 6 activities that are specifically for the following problems: a weakness with visual information processing skills such as figure-ground, form constancy, spatial relations, visual closure, visual discrimination, visual memory, and visualization skills, slow speed of information processing, and acquired brain injury with perceptual-cognitive deficits. This program is available only from a licensed eye care practitioner.

    • Perimetry
      The measurement of a visual field function (the total area that can be seen while looking straight ahead) using targets of different sizes and brightness (light levels). The visual field is measured in degrees. In a normal eye the peripheral field of vision is about 180 degrees. An instrument called a perimeter is used for mapping all areas of a person's eyesight, including peripheral (side) vision. Visual field testing can help detect certain patterns of visual loss, indicating specific types of eye diseases or vision conditions. It is the single best test for diagnosing glaucoma.

    • Peripheral Vision
      The ability to see or be aware of what is surrounding us, our side vision. (See "Visual Field".)

    • Phasic
      Fast, jump movement. (See "Fusional Vergence")

    • Photophobia
      Unusual sensitivity to light. 

    • Physiological Diplopia
      A normal diplopia (double vision) that occurs when an individual is not pointing his/her eyes on a certain object. 

    • Plano Lens
      A lens that has no prescription. No variance between the curvature of the front and back lens surfaces. It is a flat lens.

    • Plus (+) Lens
      Convex lens (thicker in the middle) relaxes focusing and converges light. It is typically used in glasses or contact lenses for people who are farsighted (hyperopic). Although it may also be prescribed for other visual conditions as well.  

    • Polaroid Lens
      A lens used in sunglasses and sometimes 3D glasses which consists of two glass or plastic surfaces with a plastic lamination between the two surfaces, and designed to reduce reflected glare. In optometric vision therapy, these lens are used with 3D pictures such as vectograms and stereograms, which are also polarized.

    • Positive Relative Accommodation (PRA)
      A measure of the maximum ability to stimulate accommodation while maintaining clear, single binocular vision.

    • Presbyopia
      Sometimes called the fourth refractive error, is not truly a refractive error. It is the natural process of the eye losing the ability to accommodate or change the shape of the natural crystalline lens inside the eye to see comfortably at near. This vision defect occurs with the advancement of age; the onset usually occurs between the ages of 40 to 45. Unlike the rest of the body, which stops growing by the age of twenty, the lens of the eye continues to grow throughout life. As the lens ages and grows, it becomes harder in consistency, loses its elasticity, and therefore is resistant to changes in shape. The result is a gradual reduction in accommodation (near eye focus), and a more dependence on reading glasses. A plus lens or multi-focal lens (such as a bifocal lens) is prescribed in the form of glasses or contact lenses. Vision therapy is not a treatment option. 

    • Prism
      A wedge-shaped lens which is thicker on one edge than the other. This plastic or glass lens bends light (opposite direction from its thicker end). Prisms can be used to measure an eye misalignment and/or treat a binocular dysfunction (eye teaming problem). A prism is sometimes added to glasses to help improve eyesight due to an eye misalignment or visual field loss. (See "Base-Down Prism", “Base-In Prism”, “Base-Out Prism”, "Base-Up Prism", "Yoked Prism")

    • Prismatic Effect By Lens
      When light goes through a wedge shaped lens which is called a prism, it bends. Light is also bent when it does not go through the center of a lens. This is an undesirable effect that can occur in glasses. It commonly occurs when the pupillary distance (PD) is not measured or made correctly.

    • Proximal Vergence
      A convergence response attributed to the awareness of, or the impression of nearness of an object of regard. (See "Vergence")

    • Pseudomyopia
      The condition Accommodative Excess/Spasm causes an individual to experience blurry distance vision after prolonged near work such as reading or using a computer. The individual may appear to be nearsighted (myopia). Treatment options may include prescription lenses and/or vision therapy.

    • Ptosis
      Droopy upper eyelid, causing the eye to remain partially closed. 

    • Pupil
      The opening at the center of the iris of the eye. It contracts (dilates) in the dark and when the eye is focused on a distant object.

    • Pupillary Distance (PD)
      The distances between the pupils of the eyes, in millimeters -- a necessary measurement for proper lens prescription. 

    • Pupillary Reflex
      The automatic contraction or enlargement of the pupil when confronted with the presence or absence of light, accommodation, or emotional change. 

A device used to measure the distance between the pupils of the eyes, in millimeters, which is a necessary measurement for proper lens prescription. It also measures the diameter of the pupil.

Pursuit Dysfunction
A condition in which the individual’s ability to follow a moving target is inadequate. Vision therapy is an effective treatment option

Pursuit Test
Measures the eyes ability to follow a moving target.  

The eye’s ability to smoothly follow a moving target. 

Requires the use of good visual skills, which are distance and near acuity, accommodation skills, binocularity skills (convergence), oculomotor skills (saccadic), peripheral vision, figure-ground, form constancy, spatial relations, visual closure, visual discrimination, visual memory, and visualization.

Refraction Test
Determines the eye's refractive error and the best corrective lenses to be prescribed. There are several methods of performing refraction: Retinoscopy, Automated Refractor, and Subjective Refraction.  

Refractive Error
Defects in vision caused by the eye’s inability to bend, or refract light and focus it clearly on the retina. Astigmatism, hyperopia, and myopia are common conditions of refractive error, also called ametropia.

Refractive Power
A lens' ability to bend parallel light rays into focus, as measured by power diopters. In general, the greater the curvature of a lens and the greater the difference between center thickness and edge thickness, the higher the index of refraction and the greater its refractive power. Refractive power can also refer the strength of a person's contact lenses or glasses.  

Refractive Media
The parts of the eye that light travels through before being focused on the retina includes the cornea, crystalline lens, aqueous, and vitreous.

Relative Amblyopia
Functional amblyopia can co-exit with a pathology abnormality. Treatment is possible.

The innermost layer of the eye, a neurological tissue, which receives light rays focused on it by the lens. This tissue contains receptor cells (rods and cones) that send electrical impulses to the brain via the optic nerve when the light rays are present.

This technique determines the eye's refractive error and the best corrective lenses to be prescribed. An instrument called a retinoscope which consists of a light, lens, mirror, and handle, is used to shine light into a patient’s eye. There are two types of retinoscope: streak and spot retinoscope. When light is shone into patient’s eye, the light is reflected back (“reflex”). If the reflection is in the same direction (“with movement”) of the retinoscope then the refractive error is hyperopia (farsightedness) and a plus lens is prescribed. If the reflection is in the opposite direction (“against movement”) of the retinoscope then the refractive error is myopia (nearsightedness) and a minus lens is prescribed. The strength of the prescription is determined when the pupil is suddenly filled with light (“neutralized”) with the appropriate lens powers (strength).

A receptor cell which is sensitive to light and is located in the retina of the eye. It is responsible for night vision (non-color vision in low level light).

The eye's ability to direct and coordinate movement as it quickly and voluntarily shift from one target to another.

Saccades Dysfunction
A condition in which the individual’s ability to scan along a printed page and move his eyes from point to point is inadequate. Symptoms include frequent loss of place while reading, skip or transpose words, and have difficulty comprehending because of an inaccurate eye movement. Vision therapy is an effective treatment option.

Saccadic Test
Measures the eyes ability to move quickly and precisely from point to point. 

The white protective covering of the eye.

Slit Lamp (Biomicroscope)
T his instrument can examine ocular tissue from the front of the cornea to the back of the lens. A narrow "slit" beam of very bright light produced by a lamp. This beam is focused on to the eye which is then viewed under magnification with a microscope. A joystick control is employed to enable instrument to be moved left-right and up-down. A chin rest, head rest and fixation target is also required. Some slit lamps have a tilting mechanism to enable the lamp to be directed from different angles.

Spatial Relation
The ability to judge the relative position of one object to another and the internal awareness of the two sides of the body. These skills allow the individual to develop the concepts of right, left, front, back, up, and down. This ability is needed in reading and math. (See "Directionality/Laterality")

An ophthalmic lens with no cylindrical power or addition. It has the same power in all parts of the lens. 

To be unable to direct both eyes simultaneously toward a point. Also known as strabismus (turned eye). For more information, see "Strabismus". 

The ability to perceive a three dimensional depth which requires adequate fusion (union) of the images from each eye.  

Stereopsis Test
Measures depth perception that is dependent on the accuracy of eye teaming.   

(Clinical condition) turned eye (s), the eyes are misaligned. It is caused by a reduction in visual acuity, reduced visual function, high refractive error, traumatic brain injury, oculomotor nerve lesion, or eye muscle injury. In strabismus, the eyes send conflicting images to the brain, and the brain cannot combine these images as it would in normal vision. The brain compensates by ignoring one image in favor of the other, causing a loss of depth perception. Strabismus in more common in children, and affects four percent of all children (although it may also appear later in life).

It is characterized by using the following categories:

* Unilateral strabismus: Strabismus in which only one eye deviates.

* Alternating strabismus: Strabismus in which the deviating eye can change.

* Intermittent strabismus: Strabismus which is not present all the time.

* Constant strabismus: Strabismus which is present at all times.

* Periodic strabismus: Strabismus which occurs at one testing distance but not at another.

Subconjunctival Hemorrhage
A blood spot on the eye. It occurs when a small blood vessel under the conjunctiva (the transparent coating that covers the inner eyelid and the white of the eye) breaks and bleeds. A common condition caused spontaneously from coughing, heavy lifting, or vomiting. In some cases, it may develop following eye surgery or trauma. It tends to be more common among those with diabetes, hypertension, and taking blood thinners (including aspirin). A subconjunctival hemorrhage is essentially harmless. The blood naturally absorbs within one to three weeks and no treatment is required. If a mild irritation is present, artificial tear drops can be used. You can speed up the healing process by applying cool compresses for the first two days and then warm compresses in the following days.

Subjective Refraction
The procedure in which the patient is asked to report on which lens combination provides the clearest vision. While this is the method of choice for determining prescription in those able to understand the task and respond to the examiner, it is less reliable in children.

Suppression of Binocular Vision
When the brain ignores the image that is seen by one eye. It is the result of weak eye teaming skills (binocularity).

Suppression Test
Determines if there is any tendency for the visual processing center of the brain to ignore or suppress visual data from one eye.

Pertaining to the sense of touch. 

Slow, smooth tension. (See "Fusional Vergence")

Tonic Vergence
Convergence due to the basic tonicity (tension) of the extraocular muscles, which are responsible, in part, for the distance phoria. Deficient tonic vergence would result in exophoria and excessive tonic vergence results in esophoria. (See "Vergence")  

An instrument that measures the pressure within the eye, which is known as intraocular pressure (IOP).

Tunnel Vision
A constriction of the visual field that is commonly caused by chronic glaucoma, retinal degeneration, a tumor, or a brain disorder that interferes with the fibers that connect the optic nerve to the brain. (Please note that a visual stress,  emotional stress, or emotional trauma can also cause a constriction of the visual field.) (See "Streff Syndrome".)

Athree-dimensional picture that is used to strengthen the binocularity system. Available in fixed and variable styles to provide base-in and/or base-out training. 3D glasses are used to view the picture.

To turn the eyes horizontally (convergence- inward or divergence- outward). Accommodative vergence, fusional vergence, proximal vergence, and tonic vergence are needed to maintain single vision. 

Vergence Facility
A measure of the ease and speed of the eyes to change from a converging to diverging position.

A disordered state in which the individual is dizzy or feels that the surrounding environment is whirling.

Visagraph Eye - Movement Recording System
Records and measures eye movements while an individual reads. The system also measures reading efficiency. Specially created goggles and a computer program are used.

The ability to take in information through our eyes and process the information so that it has meaning. 

Vision Therapy (VT)
See "Optometric Vision Therapy"

Vision Therapy Technician
One who works under the supervision of an optometrist in evaluating clients and in planning and implementing vision therapy programs. (See “COVTT”)  

Vision Therapist
Optometrist or an optometric vision therapy technician who develops and administers vision therapy programs. (Typically this term is referring to a vision therapy technician rather than an optometrist.)
Please note that there are some individuals that call themselves vision therapists, but they are not optometrists or vision therapy technicians.

Visual Acuity
Sharpness or clearness of eyesight. (See "Near Acuity" and "Distance Acuity", “20/20”)

Visual Analysis
Refers to figure-ground, form constancy, spatial relation, visual closure, visual discrimination, visual memory, and visualization.

Visual Closure
The ability to identify or recognize a symbol or object when the entire object is not visible.

Visual Discrimination
The ability to discriminate between visible likeness and differences in size, shape, pattern, form, position, and color. Such as the ability to distinguish between similar words like "ran" and "run".

Visual Field
The total area that can be seen while looking straight ahead. (See "Tunnel Vision".) (Note: Perimetry is the method of testing an eye's field of vision. For more information, please see "Perimetry".)

Visual Form Dysfunction
Difficulty with figure-ground, form constancy, visual closure, and visual discrimination. Symptoms include confusion with similar objects, words, or colors. Vision therapy is a treatment option. 

Visual Memory
The ability to recall and use visual information from the past.  (See "Visual Sequential Memory")

Visual Memory Dysfunction
Difficulty with retention, recall, or recognition of things seen. Symptoms can include poor spelling and poor recall of visual information. Vision therapy is a treatment option. 

Visual-Motor Dysfunction
The inability to process and reproduce visual images by writing or drawing. Symptoms can include poor pencil grip/writing, poor organization on written page, poor copying/spacing, and excessive erasing. Vision therapy is a treatment option.

Visual-Motor Integration (VMI)
After visual data is gathered, it is processed and combined in the brain with information from movement (eye hand coordination). 

Visual-Motor Skills
The ability of our eyes to guide our hands (eye hand coordination, visual-motor integration).

Visual Pathway
Route of the nerve impulses from the retina along the optic nerve, and optic nerve radiations to the brain's sensory cortex that is located at the base of the skull.

Visual Perceptual Disorders
Information processing dysfunctions of the visual system. These dysfunctions can be a directionality/laterality disorder, visual form dysfunction, visual memory dysfunction, and visual-motor dysfunction. Vision therapy is a treatment option. (Also see “Perceptual Skills”)    

Visual Perceptual Skills
The ability to organize and interpret information that is seen and give it meaning. These information-processing skills include figure-ground, form constancy, spatial relations, visual closure, visual discrimination, visual memory, and visualization.

Visual-Sensory Integration
After visual data is gathered, it is processed and combined in the brain with information from hearing (auditory-visual integration), balance (gross-motor/bilateral integration), posture, and movement (visual-motor integration).    

Visual Sequential Memory
Ability to recall a sequence of numbers, letters or objects in the order they were originally given.

Visual Skills
Are accommodation (eye focusing), binocularity (eye teaming), and oculomotor skills (eye movement), which are neuro-muscular abilities that are controlled by muscles inside and outside of the eye and are networked with the brain.

The ability to crate and manipulate mental pictures of an object or concept on the basis of past visual experience and memory. Essential in reading and playing sports.

Wandering Eye(s)
See "Exotropia".

Wheatstone Stereoscope
An instrument designed to present separate images to each eye. Each eye can see independently. Two plane mirrors are joined at one edge at a 90-degree angle and two target holders, one opposite one mirror and the other, mounted on a screw base which, when turned, synchronously moves the targets toward or away from each other. The Amblyoscope, Troposcope, Synoptophore, and the Bernell Mirror Stereoscope are examples of this design.