Unique hues

{{short description|Pure blue, green, yellow or red hues that cannot be described as a mixture of other hues}}

Image:Opponent color circle 1917.png (1917)]]

Unique hue is a term used in perceptual psychology of color vision and generally applied to the purest hues of blue, green, yellow and red. The proponents of the opponent process theory believe that these hues cannot be described as a mixture of other hues, and are therefore pure, whereas all other hues are composite.{{cite journal |last=Allen |first=Keith |year=2010 |title=Locating the Unique Hues. |journal=Rivista di estetica |volume=43 |issue=43 |pages=13–28 |doi=10.4000/estetica.1786 |doi-access=free}} The neural correlate of the unique hues are approximated by the extremes of the opponent channels in opponent process theory.{{cite journal |last1=Valberg |first1=Arne |title=Unique hues: an old problem for a new generation. |journal=Vision Research |date=2001 |volume=41 |issue=13 |pages=1645–1657 |doi=10.1016/S0042-6989(01)00041-4 |doi-access=free |pmid=11348647 }} In this context, unique hues are sometimes described as "psychological primaries" as they can be considered analogous to the primary colors of trichromatic color theory.{{cite book |last1=Wright |first1=Wayne |chapter=Psychologically Pure Colors |title=Encyclopedia of Color Science and Technology |date=2013 |pages=1–4 |doi=10.1007/978-3-642-27851-8_78-8|isbn=978-3-642-27851-8 }}

Opponent process theory

{{main|Opponent process}}

File:Opponent colors.svg, a model based on the opponent process theory of color vision.]]

The concept of certain hues as 'unique' came with the introduction of opponent process theory, which Ewald Hering introduced in 1878.{{cite book |last1=Hering |first1=Ewald |title=Zur Lehre von Lichtsinne |date=1878}}{{cite book |last1=Hering |first1=Ewald |title=Outlines of a Theory of the Light Sense |year=1964 |publisher=Harvard }} Hering first proposed the idea that red, green, blue, and yellow were unique hues ("Urfarben"), based on the concept that these colors could not be simultaneously perceived. These hues represented the extremes of two perpendicular axes of color: a red-green axis and a blue-yellow axis. While this theory with 4 unique hues was initially considered contradictory to the Young-Helmholtz trichromatic theory's three primary colors, the two theories were reconciled theoretically by Erwin Schrödinger{{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 }} and the later discovery of color-opponent cells in the retina and lateral geniculate nucleus (LGN) related the two theories physiologically.{{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 }}

Physiology

File:Diagram of the opponent process.png

File:Gray722.png

A physiological pathway from the cones in the retina to a neural correlate for the psychological unique hues has been elusive.{{cite journal |last1=Wuerger |first1=Sophie M. |last2=Atkinson |first2=Philip |last3=Cropper |first3=Simon |title=The cone inputs to the unique-hue mechanisms |journal=Vision Research |date=1 November 2005 |volume=45 |issue=25 |pages=3210–3223 |doi=10.1016/j.visres.2005.06.016|pmid=16087209 |s2cid=5778387 }} Mollon and Jordan state: “...the nature of the unique hues remains mysterious and we do not know whether they tell us anything about the neural organisation of the visual system.” The first transformation of light to a neuronal signal (visual phototransduction) yields 3 channels, each proportional to the quantal catch of one cone type (L-, M- and S-), estimated by the LMS color space. The second transformation occurs in the color-opponent cells and produces the opponent process channels: L+M (luminance), L-M (red-green), and S-(L+M) (blue-yellow), the latter of which form the cardinal axes.

Hering and researchers until the mid 20th century expected that the cardinal axes would correspond to the unique hues, i.e. the unique hues would exist when one opponent channel is maximally stimulated and the other opponent channel is in equilibrium.{{cite journal |last1=Hurvich |first1=Leo |title=Some quantitative aspects of an opponent-colors theory. II. Brightness, saturation, and hue in normal and dichromatic vision. |journal=JOSA |date=1955 |volume=45 |issue=8 |pages=602–616 |doi=10.1364/JOSA.45.000602 |pmid=13243163 }} However, subsequent psychophysical tests demonstrated that while unique red lies on the extreme of the L-M axis, the other unique hues do not lie on the extremes of either opponent channel (L-M and S-(L+M) axes).{{cite journal |last1=Krauskopf |first1=John |last2=Williams |first2=David R. |last3=Heeley |first3=David W. |title=Cardinal directions of color space |journal=Vision Research |date=January 1982 |volume=22 |issue=9 |pages=1123–1131 |doi=10.1016/0042-6989(82)90077-3|pmid=7147723 |s2cid=10480870 }} Therefore, the cardinal axes are not a direct correlate of our experience of unique hues and a further (third) transformation must be applied to identify correlates, i.e. each unique hue is a synthesis of the opponent process channels. One theory suggests a conversion at a point later than the LGN, and that this produces non-linear combinations resulting in our experience of color being non-linear to the cardinal axes.{{cite journal |last1=De Valois |title=A multi-stage color model. |journal=Vision Research |date=1993 |volume=33 |issue=8 |pages=1053–1065 |doi=10.1016/0042-6989(93)90240-W |doi-access=free |pmid=8506645 }} However, while opponent-cells have been found in the LGN that respond to cone combinations other than those of the cardinal axes, such as M-S,{{cite journal |last1=Valberg |first1=Arne |title=Neurones with strong inhibitory S-cone inputs in the macaque lateral geniculate nucleus. |journal=Vision Research |date=1986 |volume=26 |issue=7 |pages=1061–1064 |doi=10.1016/0042-6989(86)90040-4 |doi-access=free |pmid=3798743 }} there is no physiological understanding of this third transformation. An opposing theory therefore suggests that hues are learned based on variations in the visual environment; that unique hues represent an adaptation away from the cardinal axes and unique hues cannot be explained by relative numbers of excited L- and M-cones or their sensitivities.{{cite journal |last1=Mollon |first1=J.D. |title=Color vision |journal=Annual Review of Psychology |year=1982 |volume=33 |pages=41–85 |doi=10.1146/annurev.ps.33.020182.000353 |pmid=6977310 }}

There is mixed evidence as to whether unique hues are perceptually privileged compared to other colors. Some research suggests that there is no greater sensitivity for unique hues compared to other colors,{{Cite journal |last1=Krauskopf |first1=John |last2=Karl |first2=Gegenfurtner |date=1992 |title=Color discrimination and adaptation |journal=Vision Research |volume=32 |issue=11 |pages=2165–2175 |doi=10.1016/0042-6989(92)90077-v |doi-access=free |pmid=1304093 |issn=0042-6989}} but other evidence suggests there is greater sensitivity for yellows and blues, which may be due to them coinciding with the daylight locus.{{Cite journal |last1=Danilova |first1=M. V. |last2=Mollon |first2=J. D. |date=2014-02-12 |title=Symmetries and asymmetries in chromatic discrimination |journal=Journal of the Optical Society of America A |volume=31 |issue=4 |pages=A247-53 |doi=10.1364/josaa.31.00a247 |pmid=24695178 |bibcode=2014JOSAA..31A.247D |issn=1084-7529}} There is no direct evidence that larger populations of neurons are dedicated to unique hues compared to other colors, but some EEG research suggests that the latency of some EEG components may be shorter for unique hues compared to non-unique hues,{{Cite journal |last1=Forder |first1=Lewis |last2=Bosten |first2=Jenny |last3=He |first3=Xun |last4=Franklin |first4=Anna |date=2017-02-10 |title=A neural signature of the unique hues |journal=Scientific Reports |volume=7 |issue=1 |page=42364 |doi=10.1038/srep42364 |pmid=28186142 |issn=2045-2322|pmc=5301231 |bibcode=2017NatSR...742364F }} and that colors can be decoded with a higher accuracy from EEG signals when they are unique hues.{{cite bioRxiv |last1=Chauhan |first1=Tushar |last2=Jakovljev |first2=Ivana |last3=Thompson |first3=Lindsay N. |last4=Wuerger |first4=Sophie M. |last5=Martinovic |first5=Jasna |date=2021-06-17 |title=Decoding of EEG signals reveals non-uniformities in the neural geometry of colour |biorxiv=10.1101/2021.06.17.448044}}

Measurement

Unique hues are typically quantified as wavelength of monochromatic light, Munsell color,{{cite journal |last=Kuehni |first=Rolf G. |year=2004 |title=Variability in unique hue selection: A surprising phenomenon |journal=Color Research & Application |volume=29 |number=2 |pages=158–162 |url=http://www.imbs.uci.edu/~kjameson/KuehniCRA.pdf |doi=10.1002/col.10237}} or hue degree derived from a RGB color space.{{cite journal |last1=De Valois |first1=Russell L |last2=De Valois |first2=Karen K |last3=Switkes |first3=Eugene |last4=Mahon |first4=Luke |title=Hue Scaling of Isoluminant and Cone-specific Lights |journal=Vision Research |date=April 1997 |volume=37 |issue=7 |pages=885–897 |doi=10.1016/s0042-6989(96)00234-9|doi-access=free |pmid=9156186 }}{{cite journal |last1=Webster |first1=Michael A. |last2=Miyahara |first2=Eriko |last3=Malkoc |first3=Gokhan |last4=Raker |first4=Vincent E. |title=Variations in normal color vision II Unique hues |journal=Journal of the Optical Society of America A |date=1 September 2000 |volume=17 |issue=9 |pages=1545 |doi=10.1364/josaa.17.001545|bibcode=2000JOSAA..17.1545W }} The subject is asked to determine the hue that is not contaminated by neighboring unique hues, either by the method of adjustment, where the subject freely adjusts the color until they reach the unique hue, or two-alternative forced choice (2AFC) staircases. In the latter, the subject iteratively chooses which of two spectral color options is more pure. The unchosen color is replaced with a color on the opposite side of the chosen color. When the same color is chosen twice in a row, this constitutes a reversal, and the step size decreases. After a certain number of reversals, the wavelength/hue of the unique hue is determined.

Variability

The unique hues have been experimentally determined to represent average hue angles of 353° (carmine-red), 128° (cobalt green), 228° (cobalt blue), 58° (yellow). However, the values have large inter-subject{{cite journal| doi=10.2466/pms.2003.97.3f.1038 | volume=97 | title=Focal colors and unique hues | journal=Perceptual and Motor Skills | pages=1038–42 | last1 = Miyahara | first1 = E| year=2003 | issue=3_suppl | pmid=15002843 | pmc=1404500 }} and slight intra-subject variability, depending on the state of adaptation of the visual system.{{cite book |last1=Mollon |first1=J. D. |last2=Jordan |first2=Gabriele |year=1997 |chapter=On the Nature of Unique Hues |title=John Dalton's Colour Vision Legacy |editor1-last=Dickinson |editor1-first=C. |editor2-last=Murray |editor2-first=I. |editor3-last=Carden |editor3-first=D. |publisher=Taylor and Francis |pages=381–392 |chapter-url=https://www.researchgate.net/publication/265079435}} For example, the wavelength attributed to unique green varies by up to 70 nm between subjects. The variance greatly exceeds the variance that would be expected from differing L:M cone ratios or spectral sensitivities, but the source of this variance has not been identified.

Unique hues are a useful tool in measuring intra-subject variability in color perception.{{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 |doi-access=free}} Neitz et al (2002) show that unique yellow shifts towards longer wavelengths following multi-day adaptation to red environments, and is also shifted for deuteranomalous colorblind observers.{{cite journal |last1=Neitz |first1=Jay |title=Color perception is mediated by a plastic neural mechanism that is adjustable in adults. |journal=Neuron |year=2002 |volume=35 |issue=4 |pages=783–792 |doi=10.1016/S0896-6273(02)00818-8 |doi-access=free }} The researchers interpret these results as suggesting a long-term normalisation mechanism which can change the weighting of cone inputs to compensate for global changes in illumination, allowing color vision to remain optimal in a changing chromatic environment. Unique hues have also been shown to change over the course of the year as a result of adaptation to differences in the color spectrum of the environment in summer compared to winter,{{cite journal |last1=Welbourne |first1=Lauren |title=Human colour perception changes between seasons. |journal=Current Biology |year=2015 |volume=25 |issue=15 |pages=R646-7 |doi=10.1016/j.cub.2015.06.030 |doi-access=free|pmid=26241135 }} and have been shown to change after surgery to remove cataracts.{{cite journal |last1=Delahunt |first1=Peter |title=Long-term renormalization of chromatic mechanisms following cataract surgery. |journal=Visual Neuroscience |date=2004 |volume=21 |issue=3 |pages=301–307 |doi=10.1017/S0952523804213025 |pmid=15518204 |pmc=2633455 }}

Unique hues have played an important role in understanding linguistic relativity or the idea that language has a significant influence on thought. The way in which language and culture affects color naming is debated and not yet fully understood. The Universalist side of the debate argues that unique color terms are biologically tied to the human visual system and the visual environment and are the same regardless of language and culture. The Relativist side argues that language contextualizes thought and therefore perception, the idea being that having a different environment and culture causes the perception of the individual to be different.{{cite journal|doi=10.1002/col.21793 | volume=39 | title=Unique hues and their stimuli-state of the art | year=2013 | journal=Color Research & Application | pages=279–287 | last1 = Kuehni | first1 = Rolf G.| issue=3 }}

=In color vision deficiency=

Unique hues have different meaning in subjects with color vision deficiency. Unique yellow was determined to skew to higher wavelengths for anomalous trichromats (deuteranomaly), approaching 700 nm for strong deutans. Dichromats, who possess a single chromatic opponent channel, thereby have unique hues at the extremes of their visible spectrum, where each cone is excited independently, which renders unique hues an ineffective tool for quantifying dichromatic color vision. However, it is common to use similar techniques for defining the wavelength corresponding to "unique white" (achromatic point) of dichromats as means for quantifying their color vision.{{cite journal |last1=Massof |first1=Robert W. |last2=Bailey |first2=James E. |title=Achromatic points in protanopes and deuteranopes |journal=Vision Research |date=1976 |volume=16 |issue=1 |pages=53–58 |doi=10.1016/0042-6989(76)90076-6|pmid=1083102 |s2cid=9909092 }} While imbalance in the L:M cone ratio is linked to mild red-green color vision deficiency, there is no dependence of unique yellow on the L:M ratio. Likewise, there is no change to unique yellow for carriers of dichromacy.

See also

References

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Category:Color

Category:Visual perception