Editing Color vision (section)

===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>