Unique hues are unmixed colors without a tint of any other colors. For example, unique yellow is a pure yellow that is not tinged by red or green. There are four unique hues: red, green, blue, and yellow.
One can see so many different colors in the environment. And yet there are only four colors that occupy a special place in color perception. They are called unique hues and they originate from the opponent colors theory proposed by Ewald Hering in 1878 [1, 2]. The theory postulates three opponent processes: two chromatic processes of red-green and blue-yellow and one achromatic process of white-black. Unique hues are perceived when one of the two chromatic processes is polarized in one direction and the other is at equilibrium. For instance, one perceives unique red when the red-green process is polarized toward red and the blue-yellow process is at equilibrium. Phenomenologically, one can describe any color he or she sees by a mixture of various ratios of two unique hues. However, one does not perceive two opposing unique hues at the same and at the same location. Namely, one does not see greenish red or bluish yellow.
The opponent colors theory was at odd with the trichromatic theory that was initially proposed by Thomas Young in 1802 and developed by Hermann von Helmholtz in 1850. The trichromatic theory states that there are three types of receptors that are responsible for conveying color signals. These two theories of color vision were the focus of controversy among scientists until they were integrated into the two-stage model in the 1960s . The trichromatic theory holds at the receptor level in the retina where there are three types of cones. These are called short-, medium-, or long-wavelength-sensitive (S, M, or L) cone receptors, and the names are derived from different spectral region of the peak in their spectral sensitivities. Then the opponent colors theory is at work in the second stage such as retinal ganglion cells and the lateral geniculate nucleus (LGN). Even though this idea of integration of the two major theories appears attractive, recent research has shown that spectrally opponent neurons found in these sites do not really represent red-green and yellow-blue processes of the opponent colors theory. Rather, spectrally opponent neurons in the LGN represent neural signals created by the L-cone inputs antagonized by the M-cone inputs (L-M axis) and those created by the S-cone inputs opposed by a combination of L- and M-cone signals (S-(L+M) axis) . Psychophysical data from humans indicate that unique red closely lies along the L+direction on the L-M axis but that all the other unique hues are clustered along the intermediate directions between the L-M and S-(L+M) axes . In order to address this discrepancy, recent models of color vision usually include a third stage where the further processing of the second-stage mechanisms creates neural signals that correspond to color perception suggested by Hering’s opponent colors theory. However, such neural signals in any particular site in the brain have yet to be discovered, leaving unique hues a mystery in color science.
Traditionally, unique hues have been measured with spectral lights that are created by a monochromator to show a light of only one wavelength. More recently, broadband stimuli such as those printed on paper and those presented on a computer screen have been also used. When the broadband stimuli are used, it is common to calculate their dominant wavelengths to characterize such stimuli. Because the long-wavelength end of the visible spectrum (400–700 nm) appears red with a slight tint of yellow for many observers, there have not been many measurements of unique red with spectral lights. Even though the exact unique hue loci vary between different studies, approximate wavelength ranges for other unique hues are 458–495 nm for unique blue, 490–555 nm for unique green, and 544–594 nm for unique yellow .
Unique hue loci vary greatly among people with normal color vision. For instance, the wavelength for unique green can vary up to 80 nm, which is more than a quarter of the entire visible spectrum . However, the sources of such individual differences remain unknown. Possible physiological sources of individual differences include prereceptoral light filtering by the lens and macular pigment, spectral sensitivity and optical density of cones, relative number of different types of cones, and relative sensitivity of the two chromatic mechanisms at the second stage. Among these possibilities, variations in prereceptoral filtering and optical density of cones are not large enough to account for observed individual differences in unique hues . Cone spectral sensitivity differences estimated by Rayleigh color match do not correlate with unique green variations . The L/M cone ratio can vary by more than a 30-fold range among individuals, and it is far too large to account for the range of observed unique hue differences . Further, no relationship is found between relative sensitivity of the L-M and S-(L+M) mechanisms and unique hue settings .
If all these physiological factors are eliminated as potential sources of individual differences, what else can there be to account for variations in unique hues? Scientists have turned to suggest factors that shape color perception are in the environment, not inside observers. For example, Joel Pokorny and Vivianne C. Smith in 1977 suggested that unique yellow may correspond to the average illuminant in the observer’s environment . This position will make a clear prediction that variations in unique hues among observers would be less when they judge surface colors composed of broadband stimuli than when they judge spectral colors that are monochromatic stimuli . Indeed, meta-analysis of ten different studies from nearly 600 observers confirms this prediction except for unique green .
Physiological factors are rejected as sources of individual differences of unique hues among people with normal color vision as described above. However, cone spectral sensitivity seems to play a role when color vision defective observers are considered. Anomalous trichromats have three types of cones like color normal observers, but the spectral sensitivity of one type of cones is shifted. Among them, deuteranomalous observers have M-cones whose spectral sensitivity is shifted toward a longer wavelength, making the difference between M-cone and L-cone spectral sensitivity smaller. This anomaly leads to inefficient red-green color vision and predicts longer wavelength for their unique yellow compared to color normal observers . Multiple studies show that deuteranomalous observers set their unique yellow at substantially longer wavelengths compared to normal observers , confirming the prediction.
Entwined with individual variations, potential differences in unique hues involve cultural differences. Michael A. Webster and his colleagues measured unique hues from a total of 349 observers in India and the USA in 2002 using color palettes printed on paper . The groups from India included optometry school students, urban workers, and two groups of rural farmers. The observers from the USA were college students. English was used for the students and the urban workers, whereas the separate native languages were used for the two groups of rural residents. The results showed that there are large differences within groups, again confirming individual differences of unique hues. However, there were also smaller but consistent differences between groups. These differences may reflect many variations in observers’ lives such as particular languages spoken, cultural norms and color term usage standards, the environment that causes long-term adaptation, and the kinds of colors encountered frequently on a daily basis. For example, particular shades of color may be more desirable in certain culture, leading to long-term adaptation of the visual system as well as certain usage of color terms. Thus, environmental and cultural factors may be often difficult to separate. Interestingly, unique hue choices made by one of the rural resident groups tended to be more similar to those made by the US students . Overall, these findings seem to indicate that unique hue loci are determined by a combination of various environmental and cultural differences.
Unique Hues and Focal Colors
Separately from the line of research on unique hues in color science, Brent Berlin and Paul Kay published a very influential study on color terms in 1969 in the context of linguistics . In order to test their hypothesis of universality of color terms across languages, they collected color naming data from native speakers of 20 languages. Berlin and Kay presented a palette of Munsell color chips to human observers and asked them to choose a best example of the color term and to draw a boundary of the chips that can be called by the color term. The results showed that the best example chips were clustered in small regions of the palette. Focal colors are the best examples of the color terms as this was the wording used in Berlin and Kay’s survey. Though their origins are completely different, unique hues and focal colors for categories red, green, blue, and yellow have been shown to closely correspond with each other empirically .
Unique hues are four primary colors in color perception. Unique hue loci vary dramatically among individuals with normal color vision. However, the sources of such differences are not clearly identified yet. Physiological factors have been excluded as the sources. The emerging picture is that unique hues are shaped by a combination of environmental factors that influence an observer’s long-term adaptation and cultural factors that include the language and the customs of color term usage. Focal colors can be considered as a synonym of unique hues red, green, blue, and yellow.
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