Editing Color constancy (section)

==Physiological basis==
The physiological basis for color constancy is thought to involve specialized [[neuron]]s in the [[primary visual cortex]] that compute local ratios of cone activity, which is the same calculation that Land's retinex algorithm uses to achieve color constancy. These specialized cells are called ''double-opponent cells'' because they compute both color opponency and spatial opponency. Double-opponent cells were first described by [[Nigel Daw]] in the [[goldfish]] retina.<ref>{{Cite journal |doi=10.1126/science.158.3803.942 |title=Goldfish Retina: Organization for Simultaneous Colour Contrast |first=Nigel W. |last=Daw   |journal=Science |date=17 November 1967 |volume=158 |issue=3803 |pages=942–944 |pmid=6054169|bibcode=1967Sci...158..942D |s2cid=1108881 }}</ref><ref>{{Cite book |title=Neural Mechanisms of Color Vision: Double-Opponent Cells in the Visual Cortex |author=Bevil R. Conway |url=https://books.google.com/books?id=pFodUlHfQmcC&pg=PR7 |publisher=Springer |year=2002 |isbn=978-1-4020-7092-1}}</ref> There was considerable debate about the existence of these cells in the primate visual system; their existence was eventually proven using reverse-correlation [[receptive field]] mapping and special stimuli that selectively activate single cone classes at a time, so-called "cone-isolating" stimuli.<ref>{{cite journal | last1 = Conway | first1 = BR | last2 = Livingstone | first2 = MS | year = 2006 | title = Spatial and Temporal Properties of Cone Signals in Alert Macaque Primary Visual Cortex (V1) | journal = Journal of Neuroscience | volume = 26 | issue = 42| pages = 10826–10846 | doi=10.1523/jneurosci.2091-06.2006| pmid = 17050721 | pmc = 2963176 }} [cover illustration].</ref><ref>{{cite journal | last1 = Conway | first1 = BR | year = 2001 | title = Spatial structure of cone inputs to color cells in alert macaque primary visual cortex (V-1) | journal = Journal of Neuroscience | volume = 21 | issue = 8| pages = 2768–2783 | doi = 10.1523/JNEUROSCI.21-08-02768.2001 | pmid = 11306629 | pmc = 6762533 }} [cover illustration].</ref> Human brain imaging evidence strongly suggests that a critical cortical locus for generating color constancy is located in cortical area V4,<ref>{{Cite journal|last1=Bartels|first1=A.|last2=Zeki|first2=S.|date=2000|title=The architecture of the colour centre in the human visual brain: new results and a review *|url=https://onlinelibrary.wiley.com/doi/abs/10.1046/j.1460-9568.2000.00905.x|journal=European Journal of Neuroscience|language=en|volume=12|issue=1|pages=172–193|doi=10.1046/j.1460-9568.2000.00905.x|pmid=10651872|s2cid=6787155|issn=1460-9568|url-access=subscription}}</ref> damage in which leads to the syndrome of [[cerebral achromatopsia]].

Color constancy works only if the incident illumination contains a range of wavelengths. The different [[cone cell]]s of the [[Human eye|eye]] register different but overlapping ranges of wavelengths of the light reflected by every object in the scene. From this information, the visual system attempts to determine the approximate composition of the illuminating light. This illumination is then ''discounted''<ref>"Discounting the illuminant" is a term introduced by [[Helmholtz]]: {{cite conference |title=Do humans discount the illuminant? |book-title=Proceedings of [[SPIE]] |volume=5666 |conference=Human Vision and Electronic Imaging X |first=John J. |last=McCann |editor1=Bernice E. Rogowitz |editor2=Thrasyvoulos N. Pappas |editor3=Scott J. Daly |date=March 2005 |pages=9–16 |doi=10.1117/12.594383}}</ref> in order to obtain the object's "true color" or [[reflectance]]: the wavelengths of light the object reflects. This reflectance then largely determines the perceived color.

=== Neural mechanism ===
There are two possible mechanisms for color constancy.   The first mechanism is unconscious inference.<ref>{{cite journal | last1 = Judd | first1 = D. B. | year = 1940 | title = Hue saturation and lightness of surface colors with chromatic illumination | journal = Journal of the Optical Society of America | volume = 30 | issue = 1 | pages = 2–32 | doi = 10.1364/JOSA.30.000002 | bibcode = 1940JOSA...30....2J }}</ref> The second view holds this phenomenon to be caused by sensory adaptation.<ref>{{cite journal | last1 = Helson | first1 = H | year = 1943 | title = Some factors and implications of color constancy | journal = Journal of the Optical Society of America | volume = 33 | issue = 10| pages = 555–567 | doi=10.1364/josa.33.000555| bibcode = 1943JOSA...33..555H }}</ref><ref>{{cite book |last=Hering |first=E. |orig-date=1920 |title=Grundzüge der Lehre vom Lichtsinn |location=Berlin |publisher=Springer |translator-last1=Hurvich |translator-first1=L. M.  |translator-last2=Jameson |translator-first2=D. |date=1964 |trans-title=Outlines of a theory of the light sense}}</ref>   Research suggests color constancy to be related changes in [[retina]]l cells as well as cortical areas related to vision.<ref>{{cite journal | last1 = Zeki | first1 = S | year = 1980 | title = The representation of colours in the cerebral cortex | journal = Nature | volume = 284 | issue = 5755| pages = 412–418 | doi=10.1038/284412a0| pmid = 6767195 | bibcode = 1980Natur.284..412Z | s2cid = 4310049 }}</ref><ref>{{cite journal | last1 = Zeki | first1 = S | year = 1983 | title = Colour coding in the cerebral cortex: The reaction of cells in monkey visual cortex to wavelengths and colours | journal = Neuroscience | volume = 9 | issue = 4| pages = 741–765 | doi=10.1016/0306-4522(83)90265-8| pmid = 6621877 | s2cid = 21352625 }}</ref><ref name=":02">{{Cite journal|last=Hood|first=D.C.|s2cid=12490019|date=1998|title=Lower-Level Visual Processing and Models of Light Adaptation|journal=[[Annual Review of Psychology]]|volume=49|pages=503–535|doi=10.1146/annurev.psych.49.1.503|pmid=9496631}}</ref> This phenomenon is most likely attributed to changes in various levels of the visual system.<ref name=":1"/>

==== Cone adaptation ====
Cones, specialized cells within the retina, will adjust relative to light levels within the local environment.<ref name=":02" /> This occurs at the level of individual neurons.<ref name="ReferenceA">{{cite journal | last1 = Lee | first1 = B. B. | last2 = Dacey | first2 = D. M. | last3 = Smith | first3 = V. C. | last4 = Pokorny | first4 = J. | year = 1999 | title = Horizontal cells reveal cone type-specific adaptation in primate retina | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 96 | issue = 25| pages = 14611–14616 | doi=10.1073/pnas.96.25.14611| pmid = 10588753 | pmc = 24484 | bibcode = 1999PNAS...9614611L | doi-access = free }}</ref> However, this adaptation is incomplete.<ref name=":1" /> [[Chromatic adaptation]] is also regulated by processes within the brain.  Research in monkeys suggest that changes in chromatic sensitivity is correlated to activity in [[Parvocellular cell|parvocellular]] [[Lateral geniculate nucleus|lateral geniculate]] neurons.<ref>{{cite journal | last1 = Creutzfeldt | first1 = O. D. | last2 = Crook | first2 = J. M. | last3 = Kastner | first3 = S. | last4 = Li | first4 = C.-Y. | last5 = Pei | first5 = X. | year = 1991 | title = The neurophysiological correlates of colour and brightness contrast in lateral geniculate neurons: 1. Population analysis | journal = Experimental Brain Research | volume = 87 | issue = 1| pages = 3–21 | doi=10.1007/bf00228503| pmid = 1756832 | s2cid = 1363735 }}</ref><ref>{{cite journal | last1 = Creutzfeldt | first1 = O. D. | last2 = Kastner | first2 = S. | last3 = Pei | first3 = X. | last4 = Valberg | first4 = A. | year = 1991 | title = The neurophysiological correlates of colour and brightness contrast in lateral geniculate neurons: II. Adaptation and surround effects | journal = Experimental Brain Research | volume = 87 | issue = 1 | pages = 22–45 | doi=10.1007/bf00228504| pmid = 1756829 | s2cid = 75794 }}</ref> Color constancy may be both attributed to localized changes in individual retinal cells or to higher level neural processes within the brain.<ref name="ReferenceA"/>