Editing Visual acuity (section)

== Physiology ==
Daylight vision (i.e. [[photopic vision]]) is subserved by [[cone cell|cone]] receptor cells which have high spatial density (in the [[Fovea centralis|central fovea]]) and allow high acuity of 6/6 or better. In [[scotopic vision|low light]] (i.e., [[scotopic vision]]), [[cone cell|cones]] do not have sufficient sensitivity and vision is subserved by [[rod cell|rods]]. Spatial resolution is then much lower. This is due to spatial summation of [[rod cell|rods]], i.e. a number of rods merge into a [[Bipolar neuron|bipolar cell]], in turn connecting to a [[ganglion cell]], and the resulting [[Receptive field|unit]] for resolution is large, and acuity small. There are no rods in the very center of the [[visual field]] (the [[foveola]]), and highest performance in low light is achieved in [[Peripheral vision|near peripheral vision]].<ref name="Strasburger-2011" />

The maximum [[angular resolution]] of the human eye is 28 arc seconds or 0.47 arc minutes;<ref>{{Cite web |last=Deering |first=Michael F. |name-list-style=vanc |title=The Limits of Human Vision |url=http://www.swift.ac.uk/about/files/vision.pdf}}.</ref> this gives an angular resolution of 0.008 degrees, and at a distance of 1&nbsp;km corresponds to 136&nbsp;mm. This is equal to 0.94 arc minutes per line pair (one white and one black line), or 0.016 degrees. For a pixel pair (one white and one black pixel) this gives a pixel density of 128 pixels per degree (PPD).

6/6 vision is defined as the ability to resolve two points of light separated by a visual angle of one minute of arc, corresponding to 60 PPD, or about 290–350 pixels per inch for a display on a device held 250 to 300&nbsp;mm from the eye.<ref>{{Cite web |title=Visual Acuity of the Human Eye |url=http://www.ndt-ed.org/EducationResources/CommunityCollege/PenetrantTest/Introduction/visualacuity.htm |url-status=dead |archive-url=https://web.archive.org/web/20120906014717/http://www.ndt-ed.org/EducationResources/CommunityCollege/PenetrantTest/Introduction/visualacuity.htm |archive-date=6 September 2012 |access-date=7 May 2006 |website=NDT Resource Center}}</ref>

Thus, visual acuity, or resolving power (in daylight, central vision), is the property of cones.<ref>{{Cite book |last1=Ali |first1=Mohamed Ather |title=Vision in Vertebrates |last2=Klyne |first2=M.A. |publisher=Plenum Press |year=1985 |isbn=978-0-306-42065-8 |location=New York |page=28 |name-list-style=vanc}}</ref>
To resolve detail, the eye's optical system has to project a focused image on the [[Fovea centralis|fovea]], a region inside the [[macula]] having the highest density of [[cone cell|cone]] [[photoreceptor cell]]s (the only kind of photoreceptors existing in the fovea's very center of 300 μm diameter), 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.

[[File:AcuityHumanEye.svg|270px|thumb |left| The diagram shows the relative acuity<ref>acuity as reciprocal of degrees visual angle, divided by the foveal value</ref> of the human eye on the horizontal meridian.
<ref>Original figure in {{Cite journal |last=Østerberg |first=G. |year=1935 |title=Topography of the layer of rods and cones in the human retina |journal=Acta Ophthalmologica |volume=13 |issue=Suppl. 6 |pages=11–103 |doi=10.1111/j.1755-3768.1935.tb04723.x |s2cid=220560741}}. Østerberg's figure is reproduced in Strasburger et al. (2011), Fig. 4</ref><ref name="Strasburger-2011" /><ref>{{Cite book |last=Hunziker |first=Hans-Werner |title=Im Auge des Lesers: foveale und periphere Wahrnehmung – vom Buchstabieren zur Lesefreude |date=2006 |publisher=Transmedia Stäubli Verlag |isbn=978-3-7266-0068-6 |location=Zürich |language=de |trans-title=The eye of the reader: foveal and peripheral perception – from letter recognition to the joy of reading |name-list-style=vanc}}</ref> {{Dubious | "Peripheral acuity too low" |date=January 2020}} The [[Blind spot (vision)|blind spot]] is at about 15.5° in the outside direction (e.g. in the left visual field for the left eye).<ref>{{Cite journal |last=Rohrschneider |first=K. |year=2004 |title=Determination of the location of the fovea on the fundus |journal=Investigative Ophthalmology & Visual Science |volume=45 |issue=9 |pages=3257–3258 |doi=10.1167/iovs.03-1157 |pmid=15326148}}</ref>]]

The grain of a photographic mosaic has just as limited resolving power as the "grain" of the [[retinal mosaic]]. 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. 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 [[Retina amacrine cell|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.<ref name="Britannica-2008" />

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. In many vertebrates, such as cats, where high visual acuity is not a priority, there is a reflecting [[tapetum lucidum|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&nbsp;mm), which occurs in low-light conditions. When the pupil is small (1–2&nbsp;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&nbsp;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 6/2.6 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 6/2.6 can be demonstrated using a [[Interferometry|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 [[cataract]]s, 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 a human or 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 of time, such as a [[cataract]], severe eye turn or [[strabismus]], [[anisometropia]] (unequal refractive error between the two eyes), or covering or patching the eye during medical treatment, will usually result in a severe and permanent decrease in visual acuity and pattern recognition in the affected eye if not treated early in life, a condition known as [[amblyopia]]. 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 cells connected to both eyes in [[Visual cortex|cortical area V1]], resulting in a loss of [[stereopsis]], i.e. [[depth perception]] by [[binocular vision]] (colloquially: "3D vision"). 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 radiation]].

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 [[cataract]]s and scarred [[cornea]]s, 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 [[tumor]]s and [[multiple sclerosis]], and diseases affecting the visual cortex such as tumors and strokes.

Though the resolving power depends on the size and packing density of the photoreceptors, the neural system must interpret the receptors' information. As determined from single-cell experiments on the cat and primate, different ganglion cells in the retina are tuned to different [[spatial frequency|spatial frequencies]], so some ganglion cells at each location have better acuity than others.  Ultimately, however, it appears that the size of a patch of cortical tissue in [[visual cortex|visual area V1]] that processes a given location in the visual field (a concept known as [[cortical magnification]]) is equally important in determining visual acuity. In particular, that size is largest in the fovea's center, and decreases with increasing distance from there.<ref name="Strasburger-2011" />

=== 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 uncorrected 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.<ref name="Britannica-2008" />