Visual acuity

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

Visual acuity is a measure of the spatial resolution of the visual processing system. VA is tested by requiring the person whose vision is being tested to identify characters (like letters and numbers) on a chart from a set distance. Chart characters are represented as black symbols against a white background (for maximum contrast). The distance between the person's eyes and the testing chart is set at a sufficient distance to approximate infinity in the way the lens attempts to focus. Twenty feet, or six metres, is essentially infinity from an optical perspective.

(The difference in optical power required to focus at 20 ft versus infinity is only 0.164 diopters.)

In an eye exam, lenses of varying powers are used to precisely correct for refractive errors. A pinhole is also used to largely correct for refractive errors. English alphabet letters are typically used (as in the classic Snellen chart) as most people will recognise them but other symbols (such as a letter E facing in different directions) are also used.

In the expression, 20/20 vision, the numerator refers to the distance in feet between the subject and the chart. The denominator indicates the size of the letters, expressed in millimeters. The denominator does not measure the height of the letters. Rather, the denominator measures the distance of the separation of the lines that make up those letters.

The letters in the lowest line in the Snellen chart are composed of lines that are separated by a visual angle of one arc minute. This corresponds to lines that are spaced only 1.75 mm apart. Each succeeding higher line on the Snellen chart contains letters whose lines are separated by correspondingly larger visual angles.

A person who can correctly identify letters on the lowest line is able to discern individual lines that are separated by a visual angle of one arc minute.

The metric equivalent of 20/20 vision is 6/6 vision. At 20 feet or 6 metres, a human eye with nominal performance is able to separate lines that are one arc minute apart (equivalent to lines that are spaced 1.75 mm apart). A vision of 20/20 is considered nominal performance for human distance vision. A vision of 20/40 is considered half as good as nominal performance. A vision of 20/10 is considered twice as good as nominal performance.

The 20/x number does not directly relate to the eyeglass prescription required to correct vision, because it does not specify the nature of the problem corrected by the lens, only the resulting performance. Instead an eye exam seeks to find the prescription that will provide the best corrected visual performance achievable. This may be greater or lesser than 6/6 for many reasons. In other words, 20/20 vision or 6/6 vision does not necessarily correspond to the best possible visual acuity a subject may achieve, but once this standard is attained the subject is considered to have achieved "normal" visual acuity.

Physiology of visual acuity
20-20 is normal vision. 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 they 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 in and a distance of 20 ft.

Thus, visual acuity, or resolving power, is the property of cones. To resolve detail, the eye's optical system has to project a focused image on the fovea, a region inside the macula having the highest density of cone photoreceptor cells (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 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 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, 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 of light by the pupil (see diffraction limit). 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 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, to assess the health of the retina before subjecting them to surgery.

The visual cortex is the part of the cerebral cortex in the posterior part of the brain responsible for processing visual stimuli, called the occipital lobe. The central 10° of field (approximately the extension of the macula) 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, severe eye turn or strabismus, 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 and depth perception, or stereopsis. The period of time over which an animal is highly sensitive to such visual deprivation is referred to as the critical period.

The eye is connected to the visual cortex by the optic nerve coming out of the back of the eye. The two optic nerves come together behind the eyes at the optic chiasm, 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. This ultimately forms the physiological basis of binocular vision. The tracts project to a relay station in the midbrain called the lateral geniculate nucleus, part of the thalamus, 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.

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.

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, expressed as the (decadic) 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 ft 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 ft away the same as a person with normal eyesight would see it from 40 ft 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), although recent test subjects have exceeded 20/8 vision. Some birds of prey, such as hawks, are believed to have an acuity of around 20/2; 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 to 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."

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 that 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.

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 ft.
 * N (near) for the evaluation done at 15.7 in.
 * 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 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.

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. 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, it is a  hyperacuity (scientific term). 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 uniformly 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.