# Opponent process
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{{short description|Theory regarding color vision in humans}}
{{for|the universal psychological and neurological model|opponent-process theory}}
{{Missing information|details of color perception|date=June 2022}}
The '''opponent process''' is a hypothesis of color vision that states that the human [visual system](/source/visual_system) interprets information about [color](/source/color) by processing signals from the three types of [photoreceptor cell](/source/photoreceptor_cell)s in an antagonistic manner. The three types of cones are called L, M, and S. The names stand for "Long wavelength sensitive,” "middle wavelength sensitive," and "short wavelength sensitive." The opponent-process theory implicates three '''opponent channels''': L versus M, S versus (L+M), and a luminance channel (+ versus -). These cone-opponent mechanisms were at one time thought to be the neural substrate for a psychological theory called Hering's Opponent Colors Theory, which calls for three psychologically important opponent color processes: [red](/source/red) versus [green](/source/green), [blue](/source/blue) versus [yellow](/source/yellow), and [black](/source/black) versus [white](/source/white) ([luminance](/source/luminance)).[{{cite book | title = A Text-book of physiology | author = Michael Foster | publisher = Lea Bros. & Co | year = 1891 | page = [https://archive.org/details/bub_gb_Swn8ztLFTdkC/page/n905 921] | url = https://archive.org/details/bub_gb_Swn8ztLFTdkC }}] The Opponent Colors Theory is named for the German physiologist [Ewald Hering](/source/Ewald_Hering) who proposed the idea in the late 19th century. However, it has been argued that Hering’s Opponent Colors Theory lacks adequate phenomenological and empirical support, and may not be a necessary feature of normal human color experience.[{{Cite journal |last=Arstila |first=Valtteri |date=May 2018 |title=What makes unique hues unique? |url=http://link.springer.com/10.1007/s11229-017-1313-3 |journal=Synthese |language=en |volume=195 |issue=5 |pages=1849–1872 |doi=10.1007/s11229-017-1313-3 |issn=0039-7857|url-access=subscription }}] Correspondingly, considerable physiological and behavioral evidence proves that the physiological cone opponent mechanisms do not constitute the neurobiological basis for Hering's Opponent Colors Theory.[{{cite journal | journal = [Cell](/source/Cell_(journal)) | 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 }}]
==Color theory==
===Complementary colors===
{{main|Complementary colors}}
When staring at a bright color for a while (e.g. red), then looking away at a white field, an [afterimage](/source/afterimage) is perceived, such that the original color will evoke its [complementary color](/source/complementary_colors) (cyan, in the case of red input). When complementary colors are combined or mixed, they "cancel each other out" and become neutral (white or gray). That is, complementary colors are never perceived as a mixture; there is no "greenish red" or "yellowish blue", despite [claims to the contrary](/source/Impossible_color). The strongest color contrast that a color can have is its complementary color. Complementary colors may also be called "opposite colors" and they were originally considered the primary evidence in support of Hering's Opponent Colors Theory. There are two fatal problems with this evidence. First, the complement of red is not green, as called for by Hering's theory; it is bluish-green. And second, there exists a complementary color for every color, so there is nothing special about the set of complementary pairs picked out by Hering's theory.
===Unique hues===
[[File:Opponent colors.svg|right|thumb|240px|Opponent color pairs based on the [NCS](/source/Natural_Color_System) experiment, including black, white and the four [unique hues](/source/unique_hues)]]
{{main|Unique hues}}
The colors that define the extremes for each opponent channel are called [unique hues](/source/unique_hues), as opposed to composite (mixed) hues. [Ewald Hering](/source/Ewald_Hering) first defined the unique hues as red, green, blue, and yellow, and based them on the concept that these colors could not be simultaneously perceived. For example, a color cannot appear both red and green.[ These definitions have been experimentally refined and are represented today by average [hue](/source/hue) angles of 353° (carmine red), 128° (cobalt green), 228° (cobalt blue), 58° (yellow).][
The unique hues are a defining feature of many psychological color spaces, but there is substantial evidence showing that the unique hues are not hard wired in the nervous system, contrary to the stipulations of Hering's Opponent Colors Theory. Unique hues can differ between individuals and are often used in psychophysical research to measure variations in color perception due to color-vision deficiencies or color adaptation.][{{cite journal |last1=Tregillus |first1=Katherine |title=Long-term adaptation to color. |journal=Current Opinion in Behavioral Sciences |date=2019 |volume=30 |pages=116–121 |doi=10.1016/j.cobeha.2019.07.005 |s2cid=201042565 |doi-access=free }}] While there is considerable inter-subject variability when defining unique hues experimentally,[{{cite journal| pmc=1404500 | pmid=15002843 | doi=10.2466/pms.2003.97.3f.1038 | volume=97 | title=Focal colors and unique hues | journal=Perceptual and Motor Skills | pages=1038–1042 | last1 = Miyahara | first1 = E. | year=2003 | issue=3_suppl }}] an individual's unique hues are very consistent, to within a few nanometers of wavelength.[{{cite journal |last1=Mollon |first1=J. D. |title=On the nature of unique hues |journal=John Dalton's Colour Vision Legacy |date=1997 |pages=381–392 |url=https://www.researchgate.net/publication/265079435}}]
==Physiological basis==
===Relation to LMS color space===
right|thumb|360px|Diagram of the opponent process {{citation needed|date=June 2022}}
The [trichromatic theory](/source/Young%E2%80%93Helmholtz_theory) is in conflict with Hering's Opponent Colors Theory, although it is compatible with a physiological opponent process that compares the outputs of the different classes of cone types. The poles of these cone opponent mechanisms do not correspond to the unique hues of Hering's Opponent Colors Theory and unlike the unique hues, have no privilege in color perception.
Most humans have three different [cone cell](/source/cone_cell)s in their retinas that facilitate [trichromatic color vision](/source/trichromacy). Colors are determined by the proportional excitation of these three cone types, i.e. their ''quantum catch''. The levels of excitation of each cone type are the parameters that define [LMS color space](/source/LMS_color_space). To calculate the opponent process [tristimulus values](/source/CIE_1931_color_space) from the LMS color space, the cone excitations must be compared:[{{cite journal |last1=Ghodrati |first1=Masoud |last2=Khaligh-Razavi |first2=Seyed-Mahdi |last3=Lehky |first3=Sidney R. |title=Towards building a more complex view of the lateral geniculate nucleus: Recent advances in understanding its role |journal=Progress in Neurobiology |date=September 2017 |volume=156 |pages=214–255 |doi=10.1016/j.pneurobio.2017.06.002|pmid=28634086 |hdl=1721.1/120922 |hdl-access=free }}]
* The luminous (achromatic) opponent channel is a weighted sum of all three cone cells (plus the [rod cell](/source/rod_cell)s in some conditions).
* The red–green opponent channel is equal to the difference of the L- and M-cones.
* The blue–yellow opponent channel is equal to the difference of the S-cone and the average/weighted sum of the L- and M-cones.
Most mammals have no L cone (the primate L cone arose from a [gene duplication](/source/gene_duplication) of the M cone opsin gene). These mammals still show two kinds of opponent channels in their retinal ganglion cells: the achromatic channel and the blue-yellow opponency channel.[{{cite journal |last1=Schwartz |first1=Gregory W. |title=Color vision: More than meets the eye |journal=Current Biology |date=August 2021 |volume=31 |issue=15 |pages=R948–R950 |doi=10.1016/j.cub.2021.06.044|pmid=34375596 |bibcode=2021CBio...31.R948S |doi-access=free}}]
===Cone opponent mechanisms are encoded in the retina===
{{see also|Lateral geniculate nucleus#Color processing}}
thumb|Spatial contrast sensitivity functions for luminance and chromatic contrast.
The output of different types of cones are compared by cells in the retina including [retina bipolar cell](/source/retina_bipolar_cell)s (which compare signals from L and M cones) and bistratified retinal ganglion cells (which compare S cone signals with L and M cone signals). The output of bipolar cells is relayed to the [visual cortex](/source/visual_cortex) by the [retinal ganglion cell](/source/retinal_ganglion_cell)s (RGCs) by way of a thalamic relay station called the [lateral geniculate nucleus](/source/lateral_geniculate_nucleus) (LGN) of the [thalamus](/source/thalamus). Much of the scientific knowledge of retinal ganglion cell physiology was obtained by neural recordings of cells in the LGN.
The cone-opponent mechanisms in the retina and LGN represent a fundamental physiological opponent process but do not represent the unique hues (or Hering's Opponent Colors Theory). For example, the colors that best elicit responses of the bistratified S-(L+M)-opponent neurons are best described as purplish (or lavender) and lime-green, not "blue" and "yellow". The neurons are sometimes referred to as "blue–yellow" neurons, but this is a historical artifact dating to the time when it was thought that Hering's Opponent Colors Theory was hardwired by the retina and the mismatch between the colors to which they are optimally tuned and Hering's Opponent Colors was overlooked. Cone opponent mechanisms exist in the retinas of many mammals, including monkeys, mice, and cats.[
===Color blindness===
{{main|Color blindness}}
[Color blindness](/source/Color_blindness) can be classified by the [cone cell](/source/cone_cell) that is affected (protan, deutan, tritan) or by the opponent channel that is affected ([red–green](/source/Color_blindness) or [blue–yellow](/source/Color_blindness)). In either case, the channel can either be inactive (in the case of [dichromacy](/source/Color_blindness)) or have a lower dynamic range (in the case of [anomalous trichromacy](/source/Color_blindness)). For example, individuals with [deuteranopia](/source/deuteranopia) see little difference between the red and green [unique hues](/source/unique_hues).
==History==
[Johann Wolfgang von Goethe](/source/Johann_Wolfgang_von_Goethe) first studied the physiological effect of opposed colors in his ''[Theory of Colours](/source/Theory_of_Colours)'' in 1810.][{{cite web | url = http://webexhibits.org/colorart/ch.html | title = Goethe's Color Theory | work = Vision science and the emergence of modern art | url-status = live | archive-url = https://web.archive.org/web/20080916130507/http://www.webexhibits.org/colorart/ch.html | archive-date = 2008-09-16 }}] Goethe arranged his color wheel symmetrically "for the colours diametrically opposed to each other in this diagram are those which reciprocally evoke each other in the eye. Thus, yellow demands purple; orange, blue; red, green; and vice versa: Thus again all intermediate gradations reciprocally evoke each other."[
{{cite book | vauthors = Goethe J | title = Theory of Colours, paragraph #50 | year = 1810 }}][
{{cite journal | journal = The Art-Union | volume = 2 | issue = 18 | title = Goethe on Colours | url = https://books.google.com/books?id=H7TlAAAAMAAJ&q=%22reciprocally+evoke+each+other+in+the+eye%22&pg=RA1-PA121 | date = July 15, 1840 | page = 107 | url-status = live | archive-url = https://web.archive.org/web/20171221235903/https://books.google.com/books?id=H7TlAAAAMAAJ&pg=RA1-PA121&dq=%22reciprocally+evoke+each+other+in+the+eye%22#v=onepage&q=%22reciprocally%20evoke%20each%20other%20in%20the%20eye%22&f=false | archive-date = December 21, 2017 }}]
[Ewald Hering](/source/Ewald_Hering) proposed opponent color theory in 1892.[Hering E, 1964. ''Outlines of a Theory of the Light Sense''. Cambridge, Mass: Harvard University Press.] He thought that the colors red, yellow, green, and blue are special in that any other color can be described as a mix of them, and that they exist in opposite pairs. That is, either red or green is perceived and never greenish-red: Even though yellow is a mixture of red and green in the RGB color theory, humans do not perceive it as such.
Hering's new theory ran counter to the prevailing [Young–Helmholtz theory](/source/Young%E2%80%93Helmholtz_theory) (''trichromatic theory''), first proposed by [Thomas Young](/source/Thomas_Young_(scientist)) in 1802 and developed by [Hermann von Helmholtz](/source/Hermann_von_Helmholtz) in 1850. The two theories seemed irreconcilable until 1925 when [Erwin Schrödinger](/source/Erwin_Schr%C3%B6dinger) was able to reconcile the two theories and show that they can be complementary.[{{cite journal |last1=Niall |first1=Keith K. |title=On the trichromatic and opponent-process theories: An article by E. Schrödinger |journal=Spatial Vision |date=1988 |volume=3 |issue=2 |pages=79–95 |doi=10.1163/156856888x00050|pmid=3153667 }}]
===Psychophysical investigations===
In 1957, [Leo Hurvich](/source/Leo_Hurvich) and [Dorothea Jameson](/source/Dorothea_Jameson) claimed to provide a [psychophysical](/source/Psychophysics) validation for Hering's theory. Their method was called ''hue cancellation''. Hue cancellation experiments start with a color (e.g. yellow) and attempt to determine how much of the opponent color (e.g. blue) of one of the starting color's components must be added to reach the neutral point.[{{cite journal | vauthors = Hurvich LM, Jameson D | title = An opponent-process theory of color vision | journal = Psychological Review | volume = 64, Part 1 | issue = 6 | pages = 384–404 | date = November 1957 | pmid = 13505974 | doi = 10.1037/h0041403 | s2cid = 27613265 }}][{{cite book |author1-link= Jeremy M. Wolfe | vauthors = Wolfe JM, Kluender KR, Levi DM | date = 2009 | title = Sensation & Perception | publisher = Sinauer Associates, Inc. | edition = third | isbn = 978-1-60535-875-8 |location=New York }}] The problem with the method of Hurvich and Jameson is that it defined the unique hues as the colors used in the cancellation; it did not test whether these colors are unique. So, participants were only ever asked to assess the proportion of the four colors (red, green, blue, yellow) in mixtures; they were never asked whether these four colors are the only possible set of primaries as would be required for a scientifically valid test of Hering's Opponent Colors Theory. Bosten and colleagues showed in 2014 that other colors can be used as primaries.
In 1959, [Gunnar Svaetichin](/source/Gunnar_Svaetichin) and MacNichol[{{cite journal | vauthors = Svaetichin G, Macnichol EF | title = Retinal mechanisms for chromatic and achromatic vision | journal = Annals of the New York Academy of Sciences | volume = 74 | issue = 2 | pages = 385–404 | date = November 1959 | pmid = 13627867 | doi = 10.1111/j.1749-6632.1958.tb39560.x | bibcode = 1959NYASA..74..385S | s2cid = 27130943 }}] recorded from the retinae of fish and reported three distinct types of cells:
* One cell responded with [hyperpolarization](/source/hyperpolarization_(biology)) to all light stimuli regardless of wavelength and was termed a ''luminosity cell''.
* Another cell responded with hyperpolarization at short wavelengths and with [depolarization](/source/depolarization) at mid-to-long wavelengths. This was termed a ''chromaticity cell''.
* A third cell{{dash}}also a chromaticity cell{{dash}}responded with hyperpolarization at fairly short wavelengths, peaking about 490 nm, and with depolarization at wavelengths longer than about 610 nm.
Svaetichin and MacNichol called the chromaticity cells ''yellow–blue'' and ''red–green opponent color cells'', following the assumption of the day that Hering's Opponent Colors Theory was hardwired in the brain.
Similar chromatically or spectrally opposed cells, often incorporating spatial opponency (e.g. red "on" center and green "off" surround), were found in the vertebrate retina and [lateral geniculate nucleus](/source/lateral_geniculate_nucleus) (LGN) through the 1950s and 1960s by De Valois et al.,[{{cite journal | vauthors = De Valois RL, Smith CJ, Kitai ST, Karoly AJ | title = Response of single cells in monkey lateral geniculate nucleus to monochromatic light | journal = Science | volume = 127 | issue = 3292 | pages = 238–9 | date = January 1958 | pmid = 13495504 | doi = 10.1126/science.127.3292.238 | bibcode = 1958Sci...127..238D }}] Wiesel and Hubel,[{{cite journal | vauthors = Wiesel TN, Hubel DH | title = Spatial and chromatic interactions in the lateral geniculate body of the rhesus monkey | journal = Journal of Neurophysiology | volume = 29 | issue = 6 | pages = 1115–56 | date = November 1966 | pmid = 4961644 | doi = 10.1152/jn.1966.29.6.1115 }}] and others.[{{cite journal | vauthors = Wagner HG, Macnichol EF, Wolbarsht ML | title = Opponent Color Responses in Retinal Ganglion Cells | journal = Science | volume = 131 | issue = 3409 | pages = 1314 | date = April 1960 | pmid = 17784397 | doi = 10.1126/science.131.3409.1314 | s2cid = 46122073 | bibcode = 1960Sci...131.1314W }}][{{cite journal | vauthors = Naka KI, Rushton WA | title = S-potentials from colour units in the retina of fish (Cyprinidae) | journal = The Journal of Physiology | volume = 185 | issue = 3 | pages = 536–55 | date = August 1966 | pmid = 5918058 | pmc = 1395833 | doi = 10.1113/jphysiol.1966.sp008001 | bibcode = 1966JPhsg.185..536N }}][{{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 }}][{{cite journal | vauthors = Byzov AL, Trifonov JA | title = The response to electric stimulation of horizontal cells in the carp retina | journal = Vision Research | volume = 8 | issue = 7 | pages = 817–22 | date = July 1968 | pmid = 5664016 | doi = 10.1016/0042-6989(68)90132-6 }}]
Following [Gunnar Svaetichin](/source/Gunnar_Svaetichin)'s lead, the cells were widely called ''opponent color cells'': ''red–green'' and ''yellow–blue''. Over the next three decades, spectrally opposed cells continued to be reported in primate retinae and LGN.[{{cite journal | vauthors = Gouras P, Zrenner E | title = Color coding in primate retina | journal = Vision Research | volume = 21 | issue = 11 | pages = 1591–8 | date = January 1981 | pmid = 7336591 | doi = 10.1016/0042-6989(81)90039-0 | s2cid = 46225236 }}][{{cite journal | vauthors = Derrington AM, Krauskopf J, Lennie P | title = Chromatic mechanisms in lateral geniculate nucleus of macaque | journal = The Journal of Physiology | volume = 357 | issue = 1 | pages = 241–65 | date = December 1984 | pmid = 6512691 | pmc = 1193257 | doi = 10.1113/jphysiol.1984.sp015499 }}][{{cite journal | vauthors = Reid RC, Shapley RM | title = Spatial structure of cone inputs to receptive fields in primate lateral geniculate nucleus | journal = Nature | volume = 356 | issue = 6371 | pages = 716–8 | date = April 1992 | pmid = 1570016 | doi = 10.1038/356716a0 | s2cid = 22357719 | bibcode = 1992Natur.356..716R }}][{{cite journal | vauthors = Lankheet MJ, Lennie P, Krauskopf J | title = Distinctive characteristics of subclasses of red–green P-cells in LGN of macaque | journal = Visual Neuroscience | volume = 15 | issue = 1 | pages = 37–46 | date = January 1998 | pmid = 9456503 | doi = 10.1017/s0952523898151027 | citeseerx = 10.1.1.553.5684 | s2cid = 1558413 }}] A variety of terms are used in the literature to describe these cells, including ''chromatically opposed'' or ''chromatically opponent'', ''spectrally opposed'' or ''spectrally opponent'', ''opponent colour'', ''colour opponent'', ''opponent response'', and simply, ''opponent''.
===In other fields===
{{main|Opponent-process theory}}
Others have applied the idea of opposing stimulations beyond visual systems, described in the article on ''[opponent-process theory](/source/opponent-process_theory)''. In 1967, [Rod Grigg](/source/Rod_Grigg) extended the concept to reflect a wide range of opponent processes in biological systems.[{{cite book | vauthors = Grigg ER | date = 1967 | title = Biologic Relativity | publisher = Chicago: Amaranth Books }}] In 1970, [Solomon](/source/Richard_Solomon_(psychologist)) and Corbit expanded Hurvich and Jameson's general neurological opponent process model to explain emotion, drug addiction, and work motivation.[{{cite journal | vauthors = Solomon RL, Corbit JD | title = An opponent-process theory of motivation. II. Cigarette addiction | journal = Journal of Abnormal Psychology | volume = 81 | issue = 2 | pages = 158–71 | date = April 1973 | pmid = 4697797 | doi = 10.1037/h0034534 }}][{{cite journal | vauthors = Solomon RL, Corbit JD | title = An opponent-process theory of motivation. I. Temporal dynamics of affect | journal = Psychological Review | volume = 81 | issue = 2 | pages = 119–45 | date = March 1974 | pmid = 4817611 | doi = 10.1037/h0036128 | citeseerx = 10.1.1.468.2548 }}]
==Applications==
The opponent color theory can be applied to [computer vision](/source/computer_vision) and implemented as the ''[Gaussian color model](/source/Gaussian_color_model)''[{{cite journal| vauthors = Geusebroek JM, van den Boomgaard R, Smeulders AW, Geerts H |title=Color invariance|journal=IEEE Transactions on Pattern Analysis and Machine Intelligence|volume=23|issue=12|date=December 2001|pages=1338–1350| doi=10.1109/34.977559 |bibcode=2001ITPAM..23.1338G }}] and the ''[natural-vision-processing model](/source/natural-vision-processing_model)''.[{{cite book | vauthors = Barghout L | date = 2014 | chapter = Visual taxometric approach to image segmentation using fuzzy-spatial taxon cut yields contextually relevant regions | title = Information Processing and Management of Uncertainty in Knowledge-Based Systems | publisher = Springer International Publishing }}][{{cite patent | inventor = Barghout L, Lee L | pubdate = 25 March 2004 | title = Perceptual information processing system | country = US | number = 2004059754 }}][{{cite book | vauthors = Barghout L | title = Vision: Global Perceptual Context Changes Local Contrast Processing, Updated to include computer vision techniques. | publisher = Scholars' Press | date = 21 February 2014 }}]
==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](/source/Impossible_color), or color combinations humans should not be able to see under the opponent-processing theory. However, many criticize that this result may just be illusory 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-cell recordings](/source/Single-unit_recording) 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[{{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}}] analyzed opponent-colors theory and found the unique hues did not match the spectrally opposed responses. De Valois himself[{{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 }}] 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[{{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}}] 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.[{{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 }}] and [Wuerger](/source/Sophie_Wuerger) et al.[{{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 }}] 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.,[{{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 | article-number = 229 | date = 2017 | pmid = 28539878 | pmc = 5423953 | doi = 10.3389/fnhum.2017.00229 | doi-access = free }}] 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](/source/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[{{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}}] 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 color](/source/complementary_colors) afterimage is easily explained by the trichromatic color theory ([Young–Helmholtz theory](/source/Young%E2%80%93Helmholtz_theory)); in the opponent-process theory, fatigue of pathways promoting red produces the illusion of a cyan square.[{{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](/source/Worth_Publishers) | year = 2009 | edition = 2 | oclc = 213815202 | isbn = 978-1-4292-0082-0 | quote = color information is processed at the post-[receptor cell](/source/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 }}]
In 2018, Arstila