Visual acuity (VA) is acuteness or clearness of vision Visual perception is the ability to interpret information and surroundings from visible light reaching the eye. The resulting perception is also known as eyesight, sight, or vision . The various physiological components involved in vision are referred to collectively as the visual system, and are the focus of much research in psychology, cognitive, especially form vision, which is dependent on the sharpness of the retinal focus within the eye Eyes are organs that detect light, and convert it to electro-chemical impulses in neurons. The simplest photoreceptors connect light to movement . In higher organisms complex neural pathways exist that connect the eye, via the optic nerve to the visual cortex and other areas of the brain. Complex optical systems with resolving power have come in and the sensitivity of the interpretative faculty of the brain.^[1]

Visual acuity is a measure of the spatial resolution of the visual processing system and is usually tested in a manner to optimise and standarise the conditions. To this end black symbols on a white background are used (for maximum contrast Contrast is the difference in visual properties that makes an object distinguishable from other objects and the background. In visual perception of the real world, contrast is determined by the difference in the color and brightness of the object and other objects within the same field of view. Because the human visual system is more sensitive to) and a sufficient distance allowed to approximate infinity in the way the lens The lens is a transparent, biconvex structure in the eye that, along with the cornea, helps to refract light to be focused on the retina. The lens, by changing shape, functions to change the focal distance of the eye so that it can focus on objects at various distances, thus allowing a sharp real image of the object of interest to be formed on the attempts to focus. Twenty feet is essentially infinity Infinity is a concept in many fields, most predominantly mathematics and physics, that refers to a quantity without bound or end. People have developed various ideas throughout history about the nature of infinity. The word comes from the Latin infinitas or "unboundedness." from an optical perspective (the difference in optical power required to focus at 20 feet versus infinity is only 0.164 diopters A dioptre, or diopter, is a unit of measurement of the optical power of a lens or curved mirror, which is equal to the reciprocal of the focal length measured in metres . For example, a 3 dioptre lens brings parallel rays of light to focus at 1/3 metre. The same unit is also sometimes used for other reciprocals of distance, particularly radii of). Whilst in an eye exam An eye examination is a battery of tests performed by an ophthalmologist, optometrist, or orthoptist assessing vision and ability to focus on and discern objects, as well as other tests and examinations pertaining to the eyes. All people should have periodic and thorough eye examinations as part of routine primary care, especially since many eye lenses of varying powers are used to precisely correct for refractive errors A refractive error, or refraction error, is an error in the focusing of light by the eye and a frequent reason for reduced visual acuity, using a pinhole will largely correct for refractive errors and allow VA to be tested in other circumstances. Letters are normally used (as in the classic Snellen chart A Snellen chart is an eye chart used by eye care professionals and others to measure visual acuity. Snellen charts are named after the Dutch ophthalmologist Herman Snellen who developed the chart during 1862) as most people will recognise them but other symbols (such as a letter E facing in different directions) can be used instead.

In the term "20/20 vision" the numerator A fraction is a number that can represent part of a whole refers to the distance in feet between the subject and the chart. The denominator A fraction is a number that can represent part of a whole. The earliest fractions were reciprocals of integers: ancient symbols representing one part of two, one part of three, one part of four, and so on. A much later development were the common or "vulgar" fractions which are still used today (½, ⅝, ¾, etc.) and which consist of a is the distance at which the lines that make up those letters would be separated by a visual angle The visual angle is the angle a viewed object subtends at the eye, usually stated in degrees of arc. It also is called the object's angular size of 1 arc minute The standard symbol for marking the arcminute is the prime (U+2032), though a single quote (') (U+0027) is commonly used where only ASCII characters are permitted. One arcminute is thus written 1′. It is also abbreviated as arcmin or amin or, less commonly, the prime with a circumflex over it (), which for the lowest line that is read by an eye with no refractive error (or the errors corrected) is usually 20 feet. The metric equivalent is 6/6 vision where the distance is 6 meters. This means that at 20 feet or 6 meters, a typical human eye,able to separate 1 arc minute, can resolve lines with a spacing of about 1.75mm The metre , symbol m, is the base unit of length in the International System of Units (SI). Originally intended to be one ten-millionth of the distance from the Earth's equator to the North Pole, its definition has been periodically refined to reflect growing knowledge of metrology. Since 1983, it is defined as the distance travelled by light in a. 20/20 vision can be considered nominal The distinction between real value and nominal value occurs in many fields. From a philosophical viewpoint, nominal value represents an accepted condition which is a goal or an approximation as opposed to the real value, which always is actually present. Often a "nominal" value is a de facto standard rather than a typical or average performance for human distance vision; 20/40 vision can be considered half that acuity for distance vision and 20/10 vision would be twice normal acuity. The 20/x number does not directly relate to the eyeglass prescription An eyeglass prescription is a written order by an optometrist or ophthalmologist. It specifies the refractive power to which each lens should be made in order to correct blurred vision due to refractive errors, including myopia, hyperopia, astigmatism, and presbyopia. It is typically determined using a phoropter asking the patient which lens is required to correct vision, because it does not specify the nature of the problem with the lens, only the resulting performance. Instead an eye exam An eye examination is a battery of tests performed by an ophthalmologist, optometrist, or orthoptist assessing vision and ability to focus on and discern objects, as well as other tests and examinations pertaining to the eyes. All people should have periodic and thorough eye examinations as part of routine primary care, especially since many eye seeks to find the prescription that will provide at least 20/20 vision.^{[citation needed]}

History

Physiology of visual acuity

In low light vision, there is low resolution despite the high sensitivity thereof. This is due to spatial summation of rods, so 100 rods could merge into many bipolars, in turn converging on ganglion cells, and the unit for resolution is very large, thus acuity being small. The farther a pattern of white and black lines is presented to a person, the less he can distinguish the lines, culminating to a distance when the pattern is seen as a uniform gray. The angle subtended by the detail at minimum acuity is the resolving power, and its reciprocal is the visual acuity. For example, a visual acuity of 1 subtends 1 minute on the retina, that of 2 is 1/2 minutes (30 seconds) of arc. Visual acuity is much better in bright light than dim light, the former reaching 2 with a bright center and surrounding, the latter perhaps having visual acuity of 0.04 (25 minutes on eye). In this case, the stimulus is 1.7 inches (4.4 cm) and a distance of 20 ft (6 m). ^[4]

Thus, visual acuity, or resolving power, is the property of cones.^[5] To resolve detail, the eye's optical system has to project a focused image on the 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. The fovea is responsible for sharp central vision , which is necessary in humans for reading, watching television or movies, driving, and any activity where visual detail is of primary importance. The fovea is, a region inside the macula The macula or macula lutea is an oval-shaped highly pigmented yellow spot near the center of the retina of the human eye. It has a diameter of around 5 mm and is often histologically defined as having two or more layers of ganglion cells. Near its center is the fovea, a small pit that contains the largest concentration of cone cells in the eye and having the highest density of cone Cone cells, or cones, are photoreceptor cells in the retina of the eye that function best in relatively bright light. The cone cells gradually become sparser towards the periphery of the retina photoreceptor cells A photoreceptor, or photoreceptor cell, is a specialized type of neuron found in the eye's retina that is capable of phototransduction. The great biological importance of photoreceptors is that as cells they convert light (electromagnetic radiation) into the beginning of a chain of biological processes. More specifically, the photoreceptor absorbs (the only kind of photoreceptors existing on the fovea), thus having the highest resolution and best color vision. Acuity and color vision, despite being mediated by the same cells, are different physiologic functions that do not interrelate except by position. Acuity and color vision can be affected independently.

The grain of a photographic mosaic has just as limited resolving power as the "grain" of the retinal mosaic. In order to see detail, two sets of receptors must be intervened by a middle set. The maximum resolution is that 30 seconds of arc, corresponding to the foveal cone diameter or the angle subtended at the nodal point of the eye. In order to get reception from each cone, as it would be if vision was on a mosaic basis, the "local sign" must be obtained from a single cone via a chain of one bipolar, ganglion, and lateral geniculate cell each. A key factor of obtaining detailed vision, however, is inhibition. This is mediated by neurons such as the amacrine and horizontal cells, which functionally render the spread or convergence of signals inactive. This tendency to one-to-one shuttle of signals is powered by brightening of the center and its surroundings, which triggers the inhibition leading to a one-to-one wiring. This scenario, however, is rare, as cones may connect to both midget and flat (diffuse) bipolars, and amacrine and horizontal cells can merge messages just as easily as inhibit them.

Light Light is electromagnetic radiation of a wavelength that is visible to the human eye . In physics, the term light sometimes refers to electromagnetic radiation of any wavelength, whether visible or not travels from the fixation object to the fovea through an imaginary path called the visual axis. The eye's tissues and structures that are in the visual axis (and also the tissues adjacent to it) affect the quality of the image. These structures are: tear film, cornea, anterior chamber, pupil, lens, vitreous, and finally the retina. The posterior part of the retina, called the retinal pigment epithelium (RPE) is responsible for, among many other things, absorbing light that crosses the retina so it cannot bounce to other parts of the retina. Interestingly, in many vertebrates, such as cats, where high visual acuity is not a priority, there is a reflecting tapetum The tapetum lucidum is a layer of tissue in the eye of many vertebrate animals, that lies immediately behind or sometimes within the retina. It reflects visible light back through the retina, increasing the light available to the photoreceptors. This improves vision in low-light conditions, but can cause the perceived image to be blurry from the layer that gives the photoreceptors a "second chance" to absorb the light, thus improving the ability to see in the dark. This is what causes an animal's eyes to seemingly glow in the dark when a light is shone on them. The RPE also has a vital function of recycling the chemicals used by the rods and cones in photon detection. If the RPE is damaged and does not clean up this "shed" blindness can result.

As in a photographic lens A photographic lens is an optical lens or assembly of lenses used in conjunction with a camera body and mechanism to make images of objects either on photographic film or on other media capable of storing an image chemically or electronically, visual acuity is affected by the size of the pupil. Optical aberrations of the eye that decrease visual acuity are at a maximum when the pupil is largest (about 8 mm), which occurs in low-light conditions. When the pupil is small (1–2 mm), image sharpness may be limited by diffraction Diffraction refers to various phenomena which occur when a wave encounters an obstacle. It is described as the apparent bending of waves around small obstacles and the spreading out of waves past small openings. Similar effects are observed when light waves travel through a medium with a varying refractive index or a sound wave through one with of light by the pupil (see diffraction limit The resolution of an optical imaging system like a microscope or telescope or camera can be limited by multiple factors like imperfections in the lenses or misalignment. There is however a fundamental maximum to the resolution of any optical system which is due to diffraction. An optical system with the ability to produce images with angular). Between these extremes is the pupil diameter that is generally best for visual acuity in normal, healthy eyes; this tends to be around 3 or 4 mm.

If the optics of the eye were otherwise perfect, theoretically acuity would be limited by pupil diffraction which would be a diffraction-limited acuity of 0.4 minutes of arc (minarc) or 20/8 acuity. The smallest cone cells in the fovea have sizes corresponding to 0.4 minarc of the visual field, which also places a lower limit on acuity. The optimal acuity of 0.4 minarc or 20/8 can be demonstrated using a laser interferometer Interferometry is the technique of diagnosing the properties of two or more waves by studying the pattern of interference created by their superposition. The instrument used to interfere the waves together is called an interferometer. Interferometry is an important investigative technique in the fields of astronomy, fiber optics, engineering that bypasses any defects in the eye's optics and projects a pattern of dark and light bands directly on the retina. Laser interferometers are now used routinely in patients with optical problems, such as cataracts A cataract is a clouding that develops in the crystalline lens of the eye or in its envelope, varying in degree from slight to complete opacity and obstructing the passage of light. Early in the development of age-related cataract the power of the lens may be increased, causing near-sightedness , and the gradual yellowing and opacification of the, to assess the health of the retina before subjecting them to surgery.

The visual cortex The term visual cortex refers to the primary visual cortex and extrastriate visual cortical areas such as V2, V3, V4, and V5. The primary visual cortex is anatomically equivalent to Brodmann area 17, or BA17. The extrastriate cortical areas consist of Brodmann area 18 and Brodmann area 19. There is a visual cortex for each hemisphere of the brain is the part of the 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 composition in terms of neurons and connectivity. The human in the posterior part of the brain The brain is the center of the nervous system in all vertebrate, and most invertebrate, animals. Some primitive animals such as jellyfish and starfish have a decentralized nervous system without a brain, while sponges lack any nervous system at all. In vertebrates, the brain is located in the head, protected by the skull and close to the primary responsible for processing visual stimuli, called the 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 . Human V1 is located on the medial side of the occipital lobe within the calcarine sulcus; the full extent of V1 often continues onto the. The central 10° of field (approximately the extension of the macula The macula or macula lutea is an oval-shaped highly pigmented yellow spot near the center of the retina of the human eye. It has a diameter of around 5 mm and is often histologically defined as having two or more layers of ganglion cells. Near its center is the fovea, a small pit that contains the largest concentration of cone cells in the eye and) is represented by at least 60% of the visual cortex. Many of these neurons are believed to be involved directly in visual acuity processing.

Proper development of normal visual acuity depends on an animal having normal visual input when it is very young. Any visual deprivation, that is, anything interfering with such input over a prolonged period, such as a cataract A cataract is a clouding that develops in the crystalline lens of the eye or in its envelope, varying in degree from slight to complete opacity and obstructing the passage of light. Early in the development of age-related cataract the power of the lens may be increased, causing near-sightedness , and the gradual yellowing and opacification of the, severe eye turn or 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 that prevents bringing the gaze of each eye to the same point in space and preventing proper binocular vision, which may adversely affect depth perception. Strabismus can be either a, or covering or patching the eye during medical treatment, will usually result in a severe and permanent decrease in visual acuity in the affected eye if not treated early in life. The decreased acuity is reflected in various abnormalities in cell properties in the visual cortex. These changes include a marked decrease in the number of cells connected to the affected eye as well as few cells connected to both eyes, resulting in a loss of binocular vision Binocular vision is vision in which both eyes are used together. The word binocular comes from two Latin roots, bini for double, and oculus for eye. Having two eyes confers at least four advantages over having one. First, it gives a creature a spare eye in case one is damaged. Second, it gives a wider field of view. For example, a human has a and depth perception Depth perception arises from a variety of depth cues. These are typically classified into binocular cues that require input from both eyes and monocular cues that require the input from just one eye. Binocular cues include stereopsis, yielding depth from binocular vision through exploitation of parallax. Monocular cues include size: distant, or stereopsis Stereopsis is the process in visual perception leading to the sensation of depth from the two slightly different projections of the world onto the retinas of the two eyes. The differences in the two retinal images are called horizontal disparity, retinal disparity, or binocular disparity. The differences arise from the eyes' different positions in. The period of time over which an animal is highly sensitive to such visual deprivation is referred to as the critical period In general, a critical period is a limited time in which an event can occur, usually to result in some kind of transformation. A "critical period" in developmental psychology and developmental biology is a time in the early stages of an organism's life during which it displays a heightened sensitivity to certain environmental stimuli,.

The eye is connected to the visual cortex by the optic nerve The optic nerve is the second of twelve paired cranial nerves but is considered to be part of the central nervous system as it is derived from an outpouching of the diencephalon during embryonic development. Consequently, the fibres are covered with myelin produced by oligodendrocytes rather than the Schwann cells of the peripheral nervous system coming out of the back of the eye. The two optic nerves come together behind the eyes at the optic chiasm The optic chiasm or optic chiasma is the part of the brain where the optic nerves (CN II) partially cross. The optic chiasm is located at the bottom of the brain immediately below the hypothalamus, where about half of the fibers from each eye cross over to the opposite side and join fibers from the other eye representing the corresponding visual field, the combined nerve fibers from both eyes forming the optic tract It is a continuation of the optic nerve and runs from the optic chiasm to the lateral geniculate nucleus. This ultimately forms the physiological basis of binocular vision Binocular vision is vision in which both eyes are used together. The word binocular comes from two Latin roots, bini for double, and oculus for eye. Having two eyes confers at least four advantages over having one. First, it gives a creature a spare eye in case one is damaged. Second, it gives a wider field of view. For example, a human has a. The tracts project to a relay station in the midbrain In biological anatomy, the mesencephalon comprises the tectum (or corpora quadrigemina), tegmentum, the ventricular mesocoelia (or "iter"), and the cerebral peduncles, as well as several nuclei and fasciculi. Caudally the mesencephalon adjoins the pons (metencephalon) and rostrally it adjoins the diencephalon (Thalamus, hypothalamus, et called the lateral geniculate nucleus The lateral geniculate nucleus is the primary processing center for visual information received from the retina of the eye. The LGN is found inside the thalamus of the brain, and is thus part of the central nervous system, which is part of the thalamus The thalamus is a midline paired symmetrical structure within the brains of vertebrates, including humans. It is situated between the cerebral cortex and midbrain, both in terms of location and neurological connections. Its function includes relaying sensation, special sense and motor signals to the cerebral cortex, along with the regulation of, and then to the visual cortex along a collection of nerve fibers called the optic radiations.

Any pathological process in the visual system, even in older humans beyond the critical period, will often cause decreases in visual acuity. Thus measuring visual acuity is a simple test in accessing the health of the eyes, the visual brain, or pathway to the brain. Any relatively sudden decrease in visual acuity is always a cause for concern. Common causes of decreases in visual acuity are cataracts and scarred corneas, which affect the optical path, diseases that affect the retina, such as macular degeneration and diabetes, diseases affecting the optic pathway to the brain such as tumors and multiple sclerosis, and diseases affecting the visual cortex such as tumors and strokes.

Though the resolving power depends on size and packing density of the photoreceptors, the neural system of receptors must interpret this resolving power. As determined from various experiments on the cat, different ganglion cells are tuned to different frequencies of detail, as from a grating, so some ganglion cells have better acuity than others. In humans the results are the same, this time utilizing the same method as well as a device to read electrical changes in the scalp.^[6]

Optical aspects

Besides the neural connections of the receptors, the optical system is an equally key player in retinal resolution. In the ideal eye, the image of a diffraction grating, can subtend 0.5 micrometre on the retina. This is certainly not the case, however, and furthermore the pupil can cause diffraction of the light. Thus, black lines on a grating will be mixed with the intervening white lines to make a gray appearance. Defective optical issues (such as myopia) can render it worse, but suitable lenses can help. Images (such as gratings) can be sharpened by lateral inhibition, i.e., more highly excited cells inhibiting the less excited cells. A similar reaction is in the case of chromatic aberrations, in which the color fringes around black-and-white objects are inhibited similarly.^[7]

Visual acuity expression

Visual acuity is often measured according to the size of letters viewed on a Snellen chart or the size of other symbols, such as Landolt Cs or Tumbling E.

In some countries, acuity is expressed as a vulgar fraction, and in some as a decimal number.

Using the foot as a unit of measurement, (fractional) visual acuity is expressed relative to 20/20. Otherwise, using the metre, visual acuity is expressed relative to 6/6. For all intents and purposes, 6/6 vision is equivalent to 20/20. In the decimal system, the acuity is defined as the reciprocal value of the size of the gap (measured in arc minutes) of the smallest Landolt C that can be reliably identified. A value of 1.0 is equal to 20/20.

LogMAR is another commonly used scale which is expressed as the logarithm of the minimum angle of resolution. LogMAR scale converts the geometric sequence of a traditional chart to a linear scale. It measures visual acuity loss; positive values indicate vision loss, while negative values denote normal or better visual acuity. This scale is rarely used clinically; it is more frequently used in statistical calculations because it provides a more scientific equivalent for the traditional clinical statement of “lines lost” or “lines gained”, which is valid only when all steps between lines are equal, which is not usually the case.

A visual acuity of 20/20 is frequently described as meaning that a person can see detail from 20 feet away the same as a person with normal eyesight would see from 20 feet. If a person has a visual acuity of 20/40, he is said to see detail from 20 feet away the same as a person with normal eyesight would see it from 40 feet away. It is possible to have vision superior to 20/20: the maximum acuity of the human eye without visual aids (such as binoculars) is generally thought to be around 20/10 (6/3) however, recent test subjects have exceeded 20/8 vision.^{[citation needed]} Some birds, such as hawks, are believed to have an acuity of around 20/2;^[8] in this respect, their vision is much better than human eyesight.

When visual acuity is below the largest optotype on the chart, the reading distance is reduced until the patient can read it. Once the patient is able to read the chart, the letter size and test distance are noted. If the patient is unable to read the chart at any distance, he or she is tested as follows:

Many humans have one eye that has superior visual acuity over the other.

Legal Definitions

Various countries have defined statutory limits for poor visual acuity that qualifies as a disability. For example, in Australia, the Social Security Act defines blindness as:

A person meets the criteria for permanent blindness under section 95 of the Social Security Act if the corrected visual acuity is less than 6/60 on the Snellen Scale in both eyes or there is a combination of visual defects resulting in the same degree of permanent visual loss. —Table 13, Schedule 1B, Social Security Act 1991

In the USA, the relevant federal statute defines blindness as follows:

[T]he term "blindness" means central visual acuity of 20/200 or less in the better eye with the use of a correcting lens. An eye which is accompanied by a limitation in the fields of vision such that the widest diameter of the visual field subtends an angle no greater than 20 degrees shall be considered for purposes in this paragraph as having a central visual acuity of 20/200 or less.

^[9]

A person's visual acuity is registered documenting the following: whether the test was for distant or near vision, the eye(s) evaluated and whether corrective lenses (i.e. glasses or contact lenses) were used:

• Distance from the chart
  • D (distant) for the evaluation done at 20 feet (or 6 meters).
  • N (near) for the evaluation done at 15.7 inches (or 40 cm).
• Eye evaluated
  • OD (Latin oculus dexter) for the right eye.
  • OS (Latin oculus sinister) for the left eye.
  • OU (Latin oculi uterque) for both eyes.
• Usage of spectacles during the test
  • cc (Latin cum correctore) with correctors.
  • sc: (Latin sine correctore) without correctors.
• Pinhole occluder
  • The abbreviation PH is followed by the visual acuity as measured with a pinhole occluder, which temporarily corrects for refractive errors such as myopia or astigmatism.

So, distant visual acuity of 20/60 and 20/25 with pinhole in the right eye will be: DscOD 20/60 PH 20/25

Distant visual acuity of count fingers and 20/50 with pinhole in the left eye will be: DscOS CF PH 20/50

Near visual acuity of 20/25 with pinhole remaining at 20/25 in both eyes with spectacles will be: NccOU 20/25 PH 20/25

"Dynamic visual acuity" defines the ability of the eye to visually discern fine detail in a moving object.

Measurement

Visual acuity is typically measured monocularly rather than binocularly with the aid of an optotype chart for distant vision, an optotype chart for near vision, and an occluder to cover the eye not being tested. The examiner may also occlude an eye by sliding a tissue behind the patient's eyeglasses, or instructing the patient to use his or her hand. This latter method is typically avoided in professional settings as it may inadvertently allow the patient to peek through his or her fingers, or press the eye and alter the measurement when that eye is evaluated.

1. Place the chart at 20 feet (or 6 meters) and illuminate to 480 lux at that distance.
2. If the patient uses glasses, then the test is performed using them.
3. Place the occluder in front of the eye that is not being evaluated. The first evaluated eye is the one that is believed to see less or the one the patient says that is seeing less.
4. Start first with the big optotypes and proceed to the smaller ones. The patient has to identify every one on the line being presented and communicate it to the physician.
5. If the measurement is reduced (below 20/20) then the test using a pinhole should be done and register the visual acuity using the pinhole. Both measures should be registered, with and without using pinhole.
6. Change the occluder to the other eye and proceed again from the 4th step.
7. After both eyes have been evaluated in distant visual acuity, proceed to evaluate near visual acuity placing a modified snellen chart for near vision (such as the Rosembaum chart) at 15.7 inches (or 40 centimeters). Then repeat the test from the 2nd step.

In some cases, binocular visual acuity will be measured, because usually binocular visual acuity is slightly better than monocular visual acuity.

Measurement considerations

Visual acuity measurement involves more than being able to see the optotypes. The patient should be cooperative, understand the optotypes, be able to communicate with the physician, and many more factors. If any of these factors is missing, then the measurement will not represent the patient's real visual acuity.

Visual acuity is a subjective test meaning that if the patient is unwilling or unable to cooperate, the test cannot be done. A patient being sleepy, intoxicated, or having any disease that can alter the patient's consciousness or his mental status can make the measured visual acuity worse than it actually is.

Illiterate patients who cannot read letters and/or numbers will be registered as having very low visual acuity if this is not known. Some of the patients will not tell the physician that they don't know the optotypes unless asked directly about it. Brain damage can result in a patient not being able to recognize printed letters, or being unable to spell them.

A motor inability can make a person respond incorrectly to the optotype shown and negatively affect the visual acuity measurement.

Variables such as pupil size, background adaptation luminance, duration of presentation, type of optotype used, interaction effects from adjacent visual contours (or “crowding") can all affect visual acuity measurement.

Visual acuity testing in children

The newborn’s visual acuity is approximately 20/400, developing to 20/20 well after the age of six in most children, according to a study published in 2009.^[10]

The measurement of visual acuity in infants, pre-verbal children and special populations (for instance, handicapped individuals) is not always possible with a letter chart. For these populations, specialised testing is necessary. As a basic examination step, one must check whether visual stimuli can be fixed, centered and followed.

More formal testing using preferential looking techniques use Teller acuity cards (presented by a technician from behind a window in the wall) to check if the child is more visually attentive to a random presentation of vertical or horizontal bars on one side compared with a blank page on the other side — the bars become progressively finer or closer together, and the endpoint is noted when the child in its adult carer's lap equally prefers the two sides.

Another popular technique is electro-physiologic testing using visual evoked potentials (VEP), which can be used to estimate visual acuity in doubtful cases and expected severe vision loss cases like Leber's congenital amaurosis.

VEP testing of acuity is somewhat similar to preferential looking in using a series of black and white stripes or checkerboard patterns (which produce larger responses than stripes). However, behaviorial responses are not required. Instead brain waves created by the presentation of the patterns are recorded. The patterns become finer and finer until the evoked brain wave just disappears, which is considered to be the endpoint measure of visual acuity. In adults and older, verbal children capable of paying attention and following instructions, the endpoint provided by the VEP corresponds very well to the perceptual endpoint determined by asking the subject when they can no longer see the pattern. There is an assumption that this correspondence also applies to much younger children and infants, though this does not necessarily have to be the case. Studies do show the evoked brain waves, as well as derived acuities, are very adult-like by one year of age.

For reasons not totally understood, until a child is several years old, visual acuities from behavioral preferential looking techniques typically lag behind those determined using the VEP, a direct physiological measure of early visual processing in the brain. Possibly it takes longer for more complex behavioral and attentional responses, involving brain areas not directly involved in processing vision, to mature. Thus the visual brain may detect the presence of a finer pattern (reflected in the evoked brain wave), but the "behavioral brain" of a small child may not find it salient enough to pay special attention to.

A simple but less-used technique is checking oculomotor responses with an optokinetic nystagmus drum, where the subject is placed inside the drum and surrounded by rotating black and white stripes. This creates an involuntary flicking or nystagumus of the eyes as they attempt to track the moving stripes. There is a good correspondence between the optikinetic and usual eye-chart acuities in adults. A potentially serious problem with this technique is that the process is reflexive and mediated in the low-level brain stem, not in the visual cortex. Thus someone can have a normal optokinetic response and yet be cortically blind with no conscious visual sensation.

Normal vision

Visual acuity depends upon how accurately light is focused on the retina (mostly the macular region), the integrity of the eye's neural elements, and the interpretative faculty of the brain. ^[11] Normal visual acuity is frequently considered to be what was defined by Snellen as the ability to recognize an optotype when it subtended 5 minutes of arc, that is Snellen's chart 20/20 feet, 6/6 meter, 1.00 decimal or 0.0 logMAR. In humans, the maximum acuity of a healthy, emmetropic eye (and even ametropic eyes with correctors) is approximately 20/16 to 20/12, so it is inaccurate to refer to 20/20 visual acuity as "perfect" vision. 20/20 is the visual acuity needed to discriminate two points separated by 1 arc minute—about 1/16 of an inch at 20 feet. This is because a 20/20 letter, E for example, has three limbs and two spaces in between them, giving 5 different detailed areas. The ability to resolve this therefore requires 1/5 of the letter's total arc, which in this case would be 1 minute. The significance of the 20/20 standard can best be thought of as the lower limit of normal or as a screening cutoff. When used as a screening test subjects that reach this level need no further investigation, even though the average visual acuity of healthy eyes is 20/16 to 20/12.

Some people may suffer from other visual problems, such as color blindness, reduced contrast, or inability to track fast-moving objects and still have normal visual acuity. Thus, normal visual acuity does not mean normal vision. The reason visual acuity is very widely used is that it is a test that corresponds very well with the normal daily activities a person can handle, and evaluate their impairment to do them.

Other measures of visual acuity

Normally visual acuity refers to the ability to resolve two separated points or lines, but there are other measures of the ability of the visual system to discern spatial differences.

Vernier acuity measures the ability to align two line segments. Humans can do this with remarkable accuracy. Under optimal conditions of good illumination, high contrast, and long line segments, the limit to vernier acuity is about 8 arc seconds or 0.13 arc minutes, compared to about 0.6 arc minutes (20/12) for normal visual acuity or the 0.4 arc minute diameter of a foveal cone. Because the limit of vernier acuity is well below that imposed on regular visual acuity by the "retinal grain" or size of the foveal cones, it is thought to be a process of the visual cortex rather than the retina. Supporting this idea, vernier acuity seems to correspond very closely (and may have the same underlying mechanism) enabling one to discern very slight differences in the orientations of two lines, where orientation is known to be processed in the visual cortex.

The smallest detectable visual angle produced by a single fine dark line against a uniformally illuminated background is also much less than foveal cone size or regular visual acuity. In this case, under optimal conditions, the limit is about 0.5 arc seconds, or only about 2% of the diameter of a foveal cone. This produces a contrast of about 1% with the illumination of surrounding cones. The mechanism of detection is the ability to detect such small differences in contrast or illumination, and does not depend on the angular width of the bar, which cannot be discerned. Thus as the line gets finer, it appears to get fainter but not thinner.

Stereoscopic acuity is the ability to detect tiny differences in depth with the two eyes. For more complex targets, stereoacuity is similar to normal monocular visual acuity, or around 0.6-1.0 arc minutes, but for much simpler targets, such as vertical rods, may be as low as only 2 arc seconds. Although stereoacuity normally corresponds very well with monocular acuity, it may be very poor or even absent even with normal monocular acuities. Such individuals typically have abnormal visual development when they are very young, such as an alternating strabismus or eye turn, where both eyes rarely or never point in the same direction and therefore do not function together.

See also

• Dioptre
• Eye examination
• Optical resolution
• Pediatric ophthalmology
• Refractive error
• Strabismus
• Troxler's fading
• Retinal summation

References

1. ^ Cline D; Hofstetter HW; Griffin JR. Dictionary of Visual Science. 4th ed. Butterworth-Heinemann, Boston 1997. ISBN 0-7506-9895-0
2. ^ Herman Snellen (www.whonamedit.com)
3. ^ ^{a b} http://www.ski.org/Colenbrander/Images/Measuring_Vis_Duane01.pdf
4. ^ "eye, human."Encyclopædia Britannica. 2008. Encyclopædia Britannica 2006 Ultimate Reference Suite DVD
5. ^ Ali, Mohamed Ather; Klyne, M.A. (1985). Vision in Vertebrates. New York: Plenum Press. p. 28. ISBN 0-306-42065-1.
6. ^ "eye, human."Encyclopædia Britannica. 2009. Encyclopædia Britannica 2006 Ultimate Reference Suite DVD
7. ^ "eye, human."Encyclopædia Britannica. 2008. Encyclopædia Britannica 2006 Ultimate Reference Suite DVD
8. ^ Kirschbaum, Kari. "Family Accipitridae" (HTML). AnimalDiversity Web. University of Michigan Museum of Zoology. http://animaldiversity.ummz.umich.edu/site/accounts/information/Accipitridae.html. Retrieved 2010-01-30.
9. ^ 42 U.S.C. § 416(i)(1)(B) (Supp. IV 1986).[1] http://www.law.cornell.edu/socsec/rulings/ssr/SSR90-05.html
10. ^ http://www.ncbi.nlm.nih.gov/pubmed/19430325
11. ^ Carlson, N; Kurtz, D.; Heath, D.; Hines, C. Clinical Procedures for Ocular Examination. Appleton & Lange: Norwalk, CT. 1990.

Further reading

• Duane's Clinical Ophthalmology, V.1 C.5, V.1 C.33, V.2 C.2, V.2 C.4, V.5 C.49, V.5 C.51, V.8 C.17, Lippincott Williams & Wilkins, 2004.
• (Russian) Головин С.С. Сивцев Д.А. Таблица для исследования остроты зрения. 3 изд. М., 1927

External links

• How Visual Acuity is Measured
• Visual Acuity of the Human Eye
• Golovin-Sivtsev Table for testing visual acuity - used in the USSR and post-Soviet states in different formats (as pdf or cdr files)
• Visual Acuity Chapter from the Webvision reference, University of Utah
• Blur simulator (for eyeglass prescriptions)

Categories: Ophthalmology | Medical signs

See Also:

• Visual Field: Encyclopedia
• Visual Pathways: Encyclopedia
• Visual Impairment: Encyclopedia
• Visual Perception: Encyclopedia
• Visual Field Test: Encyclopedia

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