Editing Opponent process (section)

==Criticism and the complementary color cells==
Much controversy exists over whether opponent-processing theory is the best way to explain color vision. A few experiments have been conducted involving image stabilization (where one experiences border loss) that produced results that suggest participants have seen "impossible" colors, or color combinations humans should not be able to see under the opponent-processing theory. However, many criticize that this result may just be illusionary experiences. Critics and researchers have instead started to turn to explain color vision through references to retinal mechanisms, rather than opponent processing, which happens in the brain's visual cortex.

As [[Single-unit recording|single-cell recordings]] accumulated, it became clear to many physiologists and psychophysicists that opponent colors did not satisfactorily account for single-cell spectrally opposed responses. For instance, Jameson and D’Andrade<ref>{{Citation| vauthors = Jameson K, D'Andrade RG |chapter=It's not really red, green, yellow, blue: An inquiry into perceptual color space |pages=295–319|publisher=Cambridge University Press|isbn=9780511519819|doi=10.1017/cbo9780511519819.014|title=Color Categories in Thought and Language|year=1997}}</ref> analyzed opponent-colors theory and found the unique hues did not match the spectrally opposed responses. De Valois himself<ref>{{cite journal | vauthors = De Valois RL, De Valois KK | title = A multi-stage color model | journal = Vision Research | volume = 33 | issue = 8 | pages = 1053–65 | date = May 1993 | pmid = 8506645 | doi = 10.1016/0042-6989(93)90240-w | s2cid = 53187961 }}</ref> summed it up: "Although we, like others, were most impressed with finding opponent cells, in accord with Hering's suggestions, when the Zeitgeist at the time was strongly opposed to the notion, the earliest recordings revealed a discrepancy between the Hering–Hurvich–Jameson opponent perceptual channels and the response characteristics of opponent cells in the macaque lateral geniculate nucleus." Valberg<ref>{{Cite journal| vauthors = Valberg A |date=September 2001|title=Corrigendum to "Unique hues: an old problem for a new generation"|journal=Vision Research|volume=41|issue=21|pages=2811|doi=10.1016/s0042-6989(01)00243-7|s2cid=1541112|issn=0042-6989|doi-access=free}}</ref> recalls that "it became common among neurophysiologists to use colour terms when referring to opponent cells as in the notations ''red-ON cells'', ''green-OFF cells'' ... In the debate ... some psychophysicists were happy to see what they believed to be opponency confirmed at an objective, physiological level. Consequently, little hesitation was shown in relating the unique and polar color pairs directly to cone opponency. Despite evidence to the contrary ... textbooks have, up to this day, repeated the misconception of relating unique hue perception directly to peripheral cone opponent processes. The analogy with Hering's hypothesis has been carried even further so as to imply that each color in the opponent pair of unique colors could be identified with either excitation or inhibition of one and the same type of opponent cell." Webster et al.<ref>{{cite journal | vauthors = Webster MA, Miyahara E, Malkoc G, Raker VE | title = Variations in normal color vision. II. Unique hues | journal = Journal of the Optical Society of America A | volume = 17 | issue = 9 | pages = 1545–55 | date = September 2000 | pmid = 10975364 | doi = 10.1364/josaa.17.001545 | bibcode = 2000JOSAA..17.1545W }}</ref> and [[Sophie Wuerger|Wuerger]] et al.<ref>{{cite journal | vauthors = Wuerger SM, Atkinson P, Cropper S | title = The cone inputs to the unique-hue mechanisms | journal = Vision Research | volume = 45 | issue = 25–26 | pages = 3210–23 | date = November 2005 | pmid = 16087209 | doi = 10.1016/j.visres.2005.06.016 | s2cid = 5778387 | doi-access = free }}</ref> have conclusively re-affirmed that single-cell spectrally opposed responses do not align with unique-hue opponent colors.

More recent experiments show that the relationship between the responses of single "color-opponent" cells and perceptual color opponency is even more complex than supposed. Experiments by Zeki et al.,<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 }}</ref> using the Land Color Mondrian, have shown that when normal observers view, for example, a green surface which is part of a multi-colored scene and which reflects more green than red light it looks green and its afterimage is magenta. But when the same green surface reflects more red than green light, it still looks green (because of the operation of color constancy mechanisms) and its afterimage is still perceived as magenta. This is true also of other colors and may be summarized by saying that, just as surfaces retain their color categories in spite of wide-ranging fluctuations in the wavelength-energy composition of the light reflected from them, the color of the afterimage produced by viewing surfaces also retains its color category and is therefore also independent of the wavelength-energy composition of the light reflected from the patch being viewed.  There is, in other words, a constancy to the colors of afterimages. This serves to emphasize further the need to search more deeply into the relationship between the responses of single opponent cells and perceptual color opponency on the one hand and the need for a better understanding of whether physiological opponent processes generate perceptual opponent colors or whether the latter are generated after colors are generated.

In 2013, Pridmore<ref>{{Cite journal| vauthors = Pridmore RW |date=2012-10-16|title=Single cell spectrally opposed responses: opponent colours or complementary colours?|journal=Journal of Optics|volume=42|issue=1|pages=8–18|doi=10.1007/s12596-012-0090-0|s2cid=122835809|issn=0972-8821}}</ref> argued that most red–green cells reported in the literature in fact code the red–cyan colors. Thus, the cells are coding complementary colors instead of opponent colors. Pridmore reported also of green–magenta cells in the retina and V1. He thus argued that the red–green and blue–yellow cells should be instead called  ''green–magenta'', ''red–cyan'' and ''blue–yellow'' complementary cells. An example of the complementary process can be experienced by staring at a red (or green) square for forty seconds, and then immediately looking at a white sheet of paper. The observer then perceives a cyan (or magenta) square on the blank sheet. This [[complementary colors|complementary color]] afterimage is more easily explained by the trichromatic color theory ([[Young–Helmholtz theory]]) than the traditional RYB color theory; in the opponent-process theory, fatigue of pathways promoting red produces the illusion of a cyan square.<ref>{{cite book | vauthors = Griggs RA | page = [https://archive.org/details/psychologyconcis0000grig_u2k4/page/92 92] | chapter = Sensation and perception | title = Psychology: A Concise Introduction | publisher = [[Worth Publishers]] | year = 2009 | edition = 2 | oclc = 213815202 | isbn = 978-1-4292-0082-0 | quote = color information is processed at the post-[[receptor cell]] level (by bipolar, ganglion, thalamic, and cortical cells) according to the opponent-process theory. | chapter-url = https://archive.org/details/psychologyconcis0000grig_u2k4/page/92 }}</ref>

A 2023 opinion essay of [[Bevil Conway|Conway]], Malik-Moraleda, and Gibson<ref name="conway-cell"/>  claimed to "review the psychological and physiological evidence for Opponent-Colors Theory" and bluntly stated "the theory is wrong".<ref name="conway-cell">{{cite journal | journal = [[Cell (journal)|Cell]] | title = Color appearance and the end of Hering's Opponent-Colors Theory | doi = 10.1016/j.tics.2023.06.003 | volume=27 | issue=9 | date = June 30, 2023 | last1 = Conway | first1 = Bevil R. | author-link1 = Bevil Conway | last2 = Malik-Moraleda | first2 = Saima | last3 = Gibson | first3 = Edward | pages = 791–804 | pmid = 37394292 | pmc = 10527909 }}</ref>