Editing Color vision (section)

==Physiology of color perception==

[[File:Cone-fundamentals-with-srgb-spectrum.svg|thumb|upright=1.25|Normalized response spectra of human cones to monochromatic spectral stimuli, with wavelength given in nanometers]]
[[File:Spectrum locus 12.png|thumb|upright=1.25|The same figures as above represented here as a single curve in three (normalized cone response) dimensions]]Perception of color begins with specialized retinal cells known as [[cone cell]]s. Cone cells contain different forms of opsin – a pigment protein – that have different [[spectral sensitivity|spectral sensitivities]]. Humans contain three types, resulting in [[trichromatic color vision]].

Each individual cone contains pigments composed of [[opsin]] apoprotein covalently linked to a light-absorbing [[prosthetic group]]: either [[Retinal|11-''cis''-hydroretinal]] or, more rarely, 11-''cis''-dehydroretinal.<ref>{{cite journal | vauthors = Nathans J, Thomas D, Hogness DS | title = Molecular genetics of human color vision: the genes encoding blue, green, and red pigments | journal = Science | volume = 232 | issue = 4747 | pages = 193–202 | date = April 1986 | pmid = 2937147 | doi = 10.1126/science.2937147 | bibcode = 1986Sci...232..193N | s2cid = 34321827 | jstor = 169687 }}</ref>

The cones are conventionally labeled according to the ordering of the wavelengths of the peaks of their [[spectral sensitivity|spectral sensitivities]]: short (S), medium (M), and long (L) cone types. These three types do not correspond well to particular colors as we know them. Rather, the perception of color is achieved by a complex process that starts with the differential output of these cells in the retina and which is finalized in the [[visual cortex]] and associative areas of the brain.

For example, while the L cones have been referred to simply as [[red#Seeing red|red]] receptors, [[microspectrophotometry]] has shown that their peak sensitivity is in the greenish-yellow region of the spectrum. Similarly, the S cones and M cones do not directly correspond to [[blue#Optics|blue]] and [[green#Color vision and colorimetry|green]], although they are often described as such. The [[RGB color model]], therefore, is a convenient means for representing color but is not directly based on the types of cones in the human eye.

The peak response of human cone cells varies, even among individuals with so-called normal color vision;<ref>{{cite journal | vauthors = Neitz J, Jacobs GH | title = Polymorphism of the long-wavelength cone in normal human colour vision | journal = Nature | volume = 323 | issue = 6089 | pages = 623–5 | year = 1986 | pmid = 3773989 | doi = 10.1038/323623a0 | s2cid = 4316301 | bibcode = 1986Natur.323..623N }}</ref>
in some non-human species this polymorphic variation is even greater, and it may well be adaptive.{{technical inline |date=May 2018 | Cited source doesn't mention adaptive color responses or sensitivity in the publically–available summary.  I can guess what “adaptive” describes in this context, but I could be incorrect, and therefore cannot clarify:  it could be photoreceptors which shift their sensitivity, or it could be visual processing which sees some colors more or less strongly. }}<ref>{{cite journal | vauthors = Jacobs GH | title = Primate photopigments and primate color vision | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 93 | issue = 2 | pages = 577–81 | date = January 1996 | pmid = 8570598 | pmc = 40094 | doi = 10.1073/pnas.93.2.577 | bibcode = 1996PNAS...93..577J | doi-access = free }}</ref>

===Theories===
[[File:Diagram of the opponent process.png|thumb|upright=1.85|Opponent process theory]]
Two complementary theories of color vision are the [[trichromatic theory]] and the [[opponent process]] theory. The trichromatic theory, or [[Young–Helmholtz theory]], proposed in the 19th century by [[Thomas Young (scientist)|Thomas Young]] and [[Hermann von Helmholtz]], posits three types of cones preferentially sensitive to blue, green, and red, respectively. Others have suggested that the trichromatic theory is not specifically a theory of color vision but a theory of receptors for all vision, including color but not specific or limited to it.<ref name=":0">{{Cite journal |last=Zeki |first=Semir |date=2022-10-09 |title=The Paton prize lecture 2021: A colourful experience leading to a reassessment of colour vision and its theories |url=http://dx.doi.org/10.1113/ep089760 |journal=Experimental Physiology |volume=107 |issue=11 |pages=1189–1208 |doi=10.1113/ep089760 |pmid=36114718 |s2cid=252335063 |issn=0958-0670}}</ref> Equally, it has been suggested that the relationship between the phenomenal opponency described by [[Ewald Hering]] and the physiological opponent processes are not straightforward (see below), making of physiological opponency a mechanism that is relevant to the whole of vision, and not just to color vision alone.<ref name=":0" /> Hering proposed the opponent process theory in 1872.<ref>{{cite journal|title=Zur Lehre vom Lichtsinne |journal=Sitzungsberichte der Mathematisch–Naturwissenschaftliche Classe der Kaiserlichen Akademie der Wissenschaften |issue=III Abtheilung |volume=LXVI. Band | vauthors = Hering E |author-link=Ewald Hering |year=1872 | url=https://books.google.com/books?id=u5MCAAAAYAAJ&q=1872+hering+ewald+Zur+Lehre+vom+Lichtsinne.+Sitzungsberichte+der+kaiserlichen+Akademie+der+Wissenschaften.+Mathematisch%E2%80%93naturwissenschaftliche+Classe,&pg=PA5 |publisher=K.-K. Hof- und Staatsdruckerei in Commission bei C. Gerold's Sohn}}</ref> It states that the visual system interprets color in an antagonistic way: red vs. green, blue vs. yellow, black vs. white.  Both theories are generally accepted as valid, describing different stages in visual physiology, visualized in the adjacent diagram.<ref name="Ali_1985" />{{rp|168}}

Green–magenta and blue–yellow are scales with mutually exclusive boundaries. In the same way that there cannot exist a "slightly negative" positive number, a single eye cannot perceive a bluish-yellow or a reddish-green. Although these two theories are both currently widely accepted theories, past and more recent work has led to [[Opponent process#Criticism and the complementary color cells|criticism of the opponent process theory]], stemming from a number of what are presented as discrepancies in the standard opponent process theory. For example, the phenomenon of an after-image of complementary color can be induced by fatiguing the cells responsible for color perception, by staring at a vibrant color for a length of time, and then looking at a white surface. This phenomenon of complementary colors shows that cyan, rather than green, is the complement of red, and that magenta, rather than red, is the complement of green. It therefore also shows that the reddish-green color supposed to be impossible by opponent process theory is actually the color yellow. Although this phenomenon is more readily explained by the trichromatic theory, explanations for the discrepancy may include alterations to the opponent process theory, such as redefining the opponent colors as red vs. cyan, to reflect this effect. Despite such criticisms, both theories remain in use.

A newer theory proposed by [[Edwin H. Land]], the [[Color constancy#Retinex theory |Retinex Theory]], is based on a demonstration of [[color constancy]], which shows that the color of any surface that is part of a complex ''natural scene'' is to a large degree independent of the wavelength composition of the light reflected from it.  Also the after-image produced by looking at a given part of a complex scene is also independent of the wavelength composition of the light reflected from it alone. Thus, while the color of the after-image produced by looking at a green surface that is reflecting more "green" (middle-wave) than "red" (long-wave) light is magenta, so is the after–image of the same surface when it reflects more "red" than "green" light (when it is still perceived as green). This would seem to rule out an explanation of color opponency based on retinal cone adaptation.<ref>{{cite journal | vauthors = Zeki S, Cheadle S, Pepper J, Mylonas D | title = The Constancy of Colored After-Images | language = English | journal = Frontiers in Human Neuroscience | volume = 11 | pages = 229 | date = 2017 | pmid = 28539878 | pmc = 5423953 | doi = 10.3389/fnhum.2017.00229 | doi-access = free }} [[File:CC-BY icon.svg|50px]]  Text was copied from this source, which is available under a [https://creativecommons.org/licenses/by/4.0/  Creative Commons Attribution 4.0 International License].</ref>

According to Land's Retinex theory, color in a ''natural scene'' depends upon the three sets of cone cells ("red," "green," and "blue") separately perceiving each surface's relative lightness in the scene and, together with the [[visual cortex]], assigning color based on comparing the lightness values perceived by each set of cone cells.<ref>{{Cite journal|url=https://www.jstor.org/stable/24953876|title=The Retinex Theory of Color Vision|last=Land|first=Edwin|date=December 1977|journal=Scientific American|pmid=929159|doi=10.1038/scientificamerican1277-108|volume=237|issue=6|pages=108–28|jstor=24953876 |bibcode=1977SciAm.237f.108L}}</ref>

===Cone cells in the human eye===
[[File:Distribution of Cones and Rods on Human Retina.png|thumb|Cones are present at a low density throughout most of the retina, with a sharp peak in the center of the fovea. Conversely, rods are present at high density throughout most of the retina, with a sharp decline in the fovea.]]
A range of wavelengths of light stimulates each of these receptor types to varying degrees. The brain combines the information from each type of receptor to give rise to different perceptions of different wavelengths of light.
{| class="wikitable"
!Cone type || Name || Range || Peak wavelength<ref name= "Wyszecki_1982">{{cite book | vauthors = Wyszecki G, Stiles WS | year = 1982 | title = Color Science: Concepts and Methods, Quantitative Data and Formulae | edition = 2nd | publisher = Wiley Series in Pure and Applied Optics | location = New York | isbn = 978-0-471-02106-3 }}</ref><ref>{{cite book | vauthors = Hunt RW | year = 2004 | title = The Reproduction of Colour | edition = 6th | pages = [https://archive.org/details/reproductionofco0000hunt/page/11 11–2] | publisher = Wiley–IS&T Series in Imaging Science and Technology | location = Chichester UK | isbn = 978-0-470-02425-6 | url = https://archive.org/details/reproductionofco0000hunt/page/11 }}</ref>
|-
|S || β  || 400–500 [[Nanometre|nm]] || 420–440&nbsp;nm
|-
|M || γ  || 450–630&nbsp;nm || 534–555&nbsp;nm
|-
|L || ρ  || 500–700&nbsp;nm || 564–580&nbsp;nm
|}

Cones and rods are not evenly distributed in the human eye. Cones have a high density at the [[Fovea centralis|fovea]] and a low density in the rest of the retina.<ref>{{cite journal| vauthors = Purves D, Augustine GJ, Fitzpatrick D, Katz LC, LaMantia AS, McNamara JO, Williams SM |date=2001|title=Anatomical Distribution of Rods and Cones|url=https://www.ncbi.nlm.nih.gov/books/NBK10848/|journal=Neuroscience. 2nd Edition|language=en}}</ref> Thus color information is mostly taken in at the fovea. Humans have poor color perception in their peripheral vision, and much of the color we see in our periphery may be filled in by what our brains expect to be there on the basis of context and memories. However, our accuracy of color perception in the periphery increases with the size of stimulus.<ref name="Johnson_1986">{{cite journal | vauthors = Johnson MA | title = Color vision in the peripheral retina | journal = American Journal of Optometry and Physiological Optics | volume = 63 | issue = 2 | pages = 97–103 | date = February 1986 | pmid = 3953765 | doi = 10.1097/00006324-198602000-00003 }}</ref>

The opsins (photopigments) present in the L and M cones are encoded on the X [[chromosome]]; defective encoding of these leads to the two most common forms of [[color blindness]]. The ''[[OPN1LW]]'' gene, which encodes the opsin present in the L cones, is highly [[polymorphism (biology)|polymorphic]]; one study found 85 variants in a sample of 236 men.<ref>{{cite journal | vauthors = Verrelli BC, Tishkoff SA | title = Signatures of selection and gene conversion associated with human color vision variation | journal = American Journal of Human Genetics | volume = 75 | issue = 3 | pages = 363–75 | date = September 2004 | pmid = 15252758 | pmc = 1182016 | doi = 10.1086/423287 }}</ref> A small percentage of women may have an extra type of color receptor because they have different alleles for the gene for the L opsin on each X chromosome. [[X chromosome inactivation]] means that while only one opsin is expressed in each cone cell, both types may occur overall, and some women may therefore show a degree of [[tetrachromat]]ic color vision.<ref>{{cite web | vauthors = Roth M | date = 2006 | url = http://www.post-gazette.com/pg/06256/721190-114.stm | title = Some women may see 100 million colors, thanks to their genes | archive-url = https://web.archive.org/web/20061108171635/http://www.post-gazette.com/pg/06256/721190-114.stm | archive-date=2006-11-08 | work = Post-Gazette.com }}</ref> Variations in ''[[OPN1MW]]'', which encodes the opsin expressed in M cones, appear to be rare, and the observed variants have no effect on [[spectral sensitivity]].

===Color in the primate brain===
[[File:Ventral-dorsal streams.svg|thumb|left|Visual pathways in the human brain. The [[ventral stream]] (purple) is important in color recognition. The [[dorsal stream]] (green) is also shown. They originate from a common source in the [[visual cortex]].]]

Color processing begins at a very early level in the visual system (even within the retina) through initial color opponent mechanisms. Both Helmholtz's trichromatic theory and Hering's opponent-process theory are therefore correct, but trichromacy arises at the level of the receptors, and opponent processes arise at the level of [[retinal ganglion cell]]s and beyond. In Hering's theory, opponent mechanisms refer to the opposing color effect of red–green, blue–yellow, and light-dark. However, in the visual system, it is the activity of the different receptor types that are opposed. Some midget retinal ganglion cells oppose L and M cone activity, which corresponds loosely to red–green opponency, but actually runs along an axis from blue-green to magenta. Small bistratified retinal ganglion cells oppose input from the S cones to input from the L and M cones. This is often thought to correspond to blue–yellow opponency but actually runs along a color axis from yellow-green to violet.

Visual information is then sent to the brain from retinal ganglion cells via the [[optic nerve]] to the [[optic chiasma]]: a point where the two optic nerves meet and information from the temporal (contralateral) visual field crosses to the other side of the brain. After the optic chiasma, the visual tracts are referred to as the [[optic tract]]s, which enter the [[thalamus]] to synapse at the [[lateral geniculate nucleus]] (LGN).

The lateral geniculate nucleus is divided into laminae (zones), of which there are three types: the M-laminae, consisting primarily of M-cells, the P-laminae, consisting primarily of P-cells, and the koniocellular laminae.  M- and P-cells receive relatively balanced input from both L- and M-cones throughout most of the retina, although this seems to not be the case at the fovea, with midget cells synapsing in the P-laminae. The koniocellular laminae receives axons from the small bistratified ganglion cells.<ref name = "Rodieck_1988">{{cite book | vauthors = Rodieck RW | title = The First Steps in Seeing | publisher = Sinauer Associates, Inc. | location = Sunderland, Massachusetts, USA | date = 1998 | isbn = 978-0-87893-757-8 }}</ref><ref name = "Hendry_1970">{{cite journal | vauthors = Hendry SH, Reid RC | title = The koniocellular pathway in primate vision | journal = Annual Review of Neuroscience | volume = 23 | pages = 127–53 | date = 1970-01-01 | pmid = 10845061 | doi = 10.1146/annurev.neuro.23.1.127 }}</ref>

After [[synapse|synapsing]] at the LGN, the visual tract continues on back to the primary [[visual cortex]] (V1) located at the back of the brain within the [[occipital lobe]]. Within V1 there is a distinct band (striation). This is also referred to as "striate cortex", with other cortical visual regions referred to collectively as "extrastriate cortex". It is at this stage that color processing becomes much more complicated.

In V1 the simple three-color segregation begins to break down. Many cells in V1 respond to some parts of the spectrum better than others, but this "color tuning" is often different depending on the adaptation state of the visual system. A given cell that might respond best to long-wavelength light if the light is relatively bright might then become responsive to all wavelengths if the stimulus is relatively dim. Because the color tuning of these cells is not stable, some believe that a different, relatively small, population of neurons in V1 is responsible for color vision. These specialized "color cells" often have receptive fields that can compute local cone ratios. Such "double-opponent" cells were initially described in the goldfish retina by Nigel Daw;<ref>{{cite journal | vauthors = Daw NW | title = Goldfish retina: organization for simultaneous color contrast | journal = Science | volume = 158 | issue = 3803 | pages = 942–4 | date = November 1967 | pmid = 6054169 | doi = 10.1126/science.158.3803.942 | s2cid = 1108881 | bibcode = 1967Sci...158..942D }}</ref><ref>{{cite book| title = Neural Mechanisms of Color Vision: Double-Opponent Cells in the Visual Cortex | vauthors = Conway BR | url = https://books.google.com/books?id=pFodUlHfQmcC&q=goldfish+retina+by+Nigel-Daw&pg=PR7 | publisher = Springer | year = 2002 | isbn = 978-1-4020-7092-1 }}</ref> their existence in primates was suggested by [[David H. Hubel]] and [[Torsten Wiesel]], first demonstrated by C.R. Michael<ref>{{Cite journal|last=Michael|first=C. R.|date=1978-05-01|title=Color vision mechanisms in monkey striate cortex: dual-opponent cells with concentric receptive fields|url=https://journals.physiology.org/doi/abs/10.1152/jn.1978.41.3.572|journal=Journal of Neurophysiology|volume=41|issue=3|pages=572–588|doi=10.1152/jn.1978.41.3.572|pmid=96222|issn=0022-3077}}</ref> and subsequently confirmed by [[Bevil Conway]].<ref>{{cite journal | vauthors = Conway BR | title = Spatial structure of cone inputs to color cells in alert macaque primary visual cortex (V-1) | journal = The Journal of Neuroscience | volume = 21 | issue = 8 | pages = 2768–83 | date = April 2001 | pmid = 11306629 | pmc = 6762533 | doi = 10.1523/JNEUROSCI.21-08-02768.2001 }}</ref> As Margaret Livingstone and David Hubel showed, double opponent cells are clustered within localized regions of V1 called [[Blob (visual system)|blobs]], and are thought to come in two flavors, red–green and blue-yellow.<ref>{{cite book| title = Neurons, and Networks: An Introduction to Behavioral Neuroscience | vauthors = Dowling JE | publisher = Harvard University Press | year = 2001 | isbn = 978-0-674-00462-7 | url = https://books.google.com/books?id=adeUwgfwdKwC&q=Margaret+Livingstone+David+Hubel+double+opponent+blobs&pg=PA376 }}</ref> Red–green cells compare the relative amounts of red–green in one part of a scene with the amount of red–green in an adjacent part of the scene, responding best to local color contrast (red next to green). Modeling studies have shown that double-opponent cells are ideal candidates for the neural machinery of [[color constancy]] explained by [[Edwin H. Land]] in his [[retinex]] theory.<ref>{{cite book | veditors = McCann M | date = 1993 | title = [[Edwin H. Land]]'s Essays | location = Springfield, Va. | publisher = Society for Imaging Science and Technology }}</ref>
[[File:16777216colors.png|thumb|When viewed in full size, this image contains about 16 million pixels, each corresponding to a different color in the full set of RGB colors. The [[human eye]] can distinguish about 10 million different colors.<ref name="Judd_1975">{{cite book | vauthors = Judd DB, Wyszecki G |title=Color in Business, Science and Industry|publisher=[[Wiley-Interscience]]|series=Wiley Series in Pure and Applied Optics|edition=third|location=New York|year= 1975|page=388|isbn=978-0-471-45212-6}}</ref>]]

From the V1 blobs, color information is sent to cells in the second visual area, V2. The cells in V2 that are most strongly color tuned are clustered in the "thin stripes" that, like the blobs in V1, stain for the enzyme cytochrome oxidase (separating the thin stripes are interstripes and thick stripes, which seem to be concerned with other visual information like motion and high-resolution form).  Neurons in V2 then synapse onto cells in the extended V4. This area includes not only  V4, but two other areas in the posterior inferior temporal cortex, anterior to area V3, the dorsal posterior inferior temporal cortex, and posterior TEO.<ref name="Conway_2007">{{cite journal | vauthors = Conway BR, Moeller S, Tsao DY | title = Specialized color modules in macaque extrastriate cortex | journal = Neuron | volume = 56 | issue = 3 | pages = 560–73 | date = November 2007 | pmid = 17988638 | doi = 10.1016/j.neuron.2007.10.008 | pmc = 8162777 | s2cid = 11724926 | url = https://authors.library.caltech.edu/100800/ }}</ref><ref name="Conway_2009">{{cite journal | vauthors = Conway BR, Tsao DY | title = Color-tuned neurons are spatially clustered according to color preference within alert macaque posterior inferior temporal cortex | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 106 | issue = 42 | pages = 18034–9 | date = October 2009 | pmid = 19805195 | pmc = 2764907 | doi = 10.1073/pnas.0810943106 | bibcode = 2009PNAS..10618034C | doi-access = free }}</ref> Area V4 was initially suggested by [[Semir Zeki]] to be exclusively dedicated to color,<ref>{{cite journal | vauthors = Zeki SM | title = Colour coding in rhesus monkey prestriate cortex | journal = Brain Research | volume = 53 | issue = 2 | pages = 422–7 | date = April 1973 | pmid = 4196224 | doi = 10.1016/0006-8993(73)90227-8 }}</ref> and he later showed that V4 can be subdivided into subregions with very high concentrations of color cells separated from each other by zones with lower concentration of such cells though even the latter cells respond better to some wavelengths than to others,<ref name="Zeki_1983">{{cite journal | vauthors = Zeki S | title = The distribution of wavelength and orientation selective cells in different areas of monkey visual cortex | journal = Proceedings of the Royal Society of London. Series B, Biological Sciences | volume = 217 | issue = 1209 | pages = 449–70 | date = March 1983 | pmid = 6134287 | doi = 10.1098/rspb.1983.0020 | bibcode = 1983RSPSB.217..449Z | s2cid = 39700958 }}</ref> a finding  confirmed by subsequent studies.<ref name="Conway_2007" /><ref>{{cite journal | vauthors = Bushnell BN, Harding PJ, Kosai Y, Bair W, Pasupathy A | title = Equiluminance cells in visual cortical area v4 | journal = The Journal of Neuroscience | volume = 31 | issue = 35 | pages = 12398–412 | date = August 2011 | pmid = 21880901 | pmc = 3171995 | doi = 10.1523/JNEUROSCI.1890-11.2011 }}</ref><ref>{{cite journal | vauthors = Tanigawa H, Lu HD, Roe AW | title = Functional organization for color and orientation in macaque V4 | journal = Nature Neuroscience | volume = 13 | issue = 12 | pages = 1542–8 | date = December 2010 | pmid = 21076422 | pmc = 3005205 | doi = 10.1038/nn.2676 }}</ref>  The presence in V4 of orientation-selective cells led to the view that V4 is involved in processing both color and form associated with color<ref name="Zeki_2005">{{cite journal | vauthors = Zeki S | title = The Ferrier Lecture 1995 behind the seen: the functional specialization of the brain in space and time | journal = Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences | volume = 360 | issue = 1458 | pages = 1145–83 | date = June 2005 | pmid = 16147515 | pmc = 1609195 | doi = 10.1098/rstb.2005.1666 }}</ref> but it is worth noting that the orientation selective cells within V4 are more broadly tuned than their counterparts in V1, V2 and V3.<ref name="Zeki_1983" /> Color processing in the extended V4 occurs in millimeter-sized color modules called [[glob (visual system)|globs]].<ref name="Conway_2007"/><ref name="Conway_2009"/> This is the part of the brain in which color is first processed into the full range of [[hue]]s found in [[color space]].<ref>{{Cite journal|last=Zeki|first=S.|date=1980|title=The representation of colours in the cerebral cortex|url=https://www.nature.com/articles/284412a0|journal=Nature|language=en|volume=284|issue=5755|pages=412–418|doi=10.1038/284412a0|pmid=6767195|bibcode=1980Natur.284..412Z |s2cid=4310049|issn=1476-4687}}</ref><ref name="Conway_2007"/><ref name="Conway_2009"/>

Anatomical studies have shown that neurons in extended V4 provide input to the inferior [[temporal lobe]]. "IT" cortex is thought to integrate color information with shape and form, although it has been difficult to define the appropriate criteria for this claim. Despite this murkiness, it has been useful to characterize this pathway (V1 > V2 > V4 > IT) as the [[ventral stream]] or the "what pathway", distinguished from the [[dorsal stream]] ("where pathway") that is thought to analyze motion, among other features.