Editing Color vision (section)

===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]].