Opponent process

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The opponent process is a color theory that states that the human visual system interprets information about color by processing signals from photoreceptor cells in an antagonistic manner. The opponent-process theory suggests that there are three opponent channels, each comprising an opposing color pair: red versus green, blue versus yellow, and black versus white (luminance). [1] The theory was first proposed in 1892 by the German physiologist Ewald Hering.

Contents

Color theory

Complementary colors

When staring at a bright color for a while (e.g. red), then looking away at a white field, an afterimage is perceived, such that the original color will evoke its complementary color (green, 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. The strongest color contrast a color can have is its complementary color. Complementary colors may also be called "opposite colors" and are understandably the basis of the colors used in the opponent process theory.

Unique hues

Opponent color pairs based on the NCS experiment, including black, white and the four unique hues Opponent colors.svg
Opponent color pairs based on the NCS experiment, including black, white and the four unique hues

The colors that define the extremes for each opponent channel are called unique hues, as opposed to composite (mixed) hues. 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. [2] These definitions have been experimentally refined and are represented today by average hue angles of 353° (carmine-red), 128° (cobalt green), 228° (cobalt blue), 58° (yellow). [3]

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. [4] While there is considerable inter-subject variability when defining unique hues experimentally, [3] an individual's unique hues are very consistent, to within a few nanometers. [5]

Physiological basis

Relation to LMS color space

Diagram of the opponent process Diagram of the opponent process.png
Diagram of the opponent process

Though the trichromatic and opponent processes theories were initially thought to be at odds, it was later shown that the mechanisms responsible for the opponent process receive signals from the three types of cones predicted by the trichromatic theory and process them at a more complex level. [6]

Most humans have three different cone cells in their retinas that facilitate trichromatic color vision. 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. To calculate the opponent process tristimulus values from the LMS color space, the cone excitations must be compared:[ citation needed ]

Neurological basis

Log-log plot of spatial contrast sensitivity functions for luminance and chromatic contrast Opponent process contrast sensitivity functions.svg
Log-log plot of spatial contrast sensitivity functions for luminance and chromatic contrast

The neurological conversion of color from LMS color space to the opponent process is believed to mostly take place in the lateral geniculate nucleus (LGN) of the thalamus, though may also take place in the Retina bipolar cells. Retinal ganglion cells carry the information from the retina to the LGN, which contains three major classes of layers: [7]

Advantage

Transmitting information in opponent channel color space is advantageous over transmitting it in LMS color space ("raw" signals from each cone type). There is some overlap in the wavelengths of light to which the three types of cones (L for long-wave, M for medium-wave, and S for short-wave light) respond, so it is more efficient for the visual system (from a perspective of dynamic range) to record differences between the responses of cones, rather than each type of cone's individual response.[ citation needed ][ dubious – discuss ]

Color blindness

Color blindness can be classified by the cone cell that is affected (protan, deutan, tritan) or by the opponent channel that is affected (red–green or blue–yellow). In either case, the channel can be either inactive (in the case of dichromacy) or have a lower dynamic range (in the case of anomalous trichromacy). For example, individuals with deuteranopia see little difference between the red and green unique hues.

History

Johann Wolfgang von Goethe first studied the physiological effect of opposed colors in his Theory of Colours in 1810. [8] 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." [9] [10]

Ewald Hering proposed opponent color theory in 1892. [2] 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, the eye does not perceive it as such.

Hering's new theory ran counter to the prevailing Young–Helmholtz theory (trichromatic theory), first proposed by Thomas Young in 1802 and developed by Hermann von Helmholtz in 1850. The two theories seemed irreconcilable until 1925 when Erwin Schrödinger was able to reconcile the two theories and show that they can be complementary. [11]

Validation

In 1957, Leo Hurvich and Dorothea Jameson provided psychophysical 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. [12] [13]

In 1959, Gunnar Svaetichin and MacNichol [14] recorded from the retina of fish and reported of three distinct types of cells:

Svaetichin and MacNichol called the chromaticity cells Yellow-Blue and Red-Green opponent color cells.

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 (LGN) through the 1950s and 1960s by De Valois et al., [15] Wiesel and Hubel, [16] and others. [17] [18] [19] [20]

Following 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 retina and LGN. [21] [22] [23] [24] A variety of terms are used in the literature to describe these cells, including chromatically opposed or -opponent, spectrally opposed or -opponent, opponent colour, colour opponent, opponent response, and simply, opponent cell.

In other fields

Others have applied the idea of opposing stimulations beyond visual systems, described in the article on opponent-process theory . In 1967, Rod Grigg extended the concept to reflect a wide range of opponent processes in biological systems. [25] In 1970, Solomon and Corbit expanded Hurvich and Jameson's general neurological opponent process model to explain emotion, drug addiction, and work motivation. [26] [27]

Applications

The opponent color theory can be applied to computer vision and implemented as the Gaussian color model [28] and the natural-vision-processing model . [29] [30] [31]

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 recordings from single cell 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 [32] analyzed opponent-colors theory and found the unique hues did not match the spectrally opposed responses. De Valois himself [33] 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 [34] 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. [35] and Wuerger et al. [36] 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., [37] 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 after-image 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 after image 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 after-image 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 after images. 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 [38] 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 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 produce the illusion of a cyan square. [39]

A 2023 opinion essay of Conway, Malik-Moraleda, and Gibson [40] claimed to "review the psychological and physiological evidence for Opponent-Colors Theory" and bluntly stated "the theory is wrong". [40]

See also

Related Research Articles

<span class="mw-page-title-main">Color</span> Visual perception of the light spectrum

Color or colour is the visual perception based on the electromagnetic spectrum. Though color is not an inherent property of matter, color perception is related to an object's light absorption, reflection, emission spectra, and interference. For most humans, colors are perceived in the visible light spectrum with three types of cone cells (trichromacy). Other animals may have a different number of cone cell types or have eyes sensitive to different wavelengths, such as bees that can distinguish ultraviolet, and thus have a different color sensitivity range. Animal perception of color originates from different light wavelength or spectral sensitivity in cone cell types, which is then processed by the brain.

<span class="mw-page-title-main">Color blindness</span> Decreased ability to see color or color differences

Color blindness or color vision deficiency (CVD) is the decreased ability to see color or differences in color. The severity of color blindness ranges from mostly unnoticeable to full absence of color perception. Color blindness is usually an inherited problem or variation in the functionality of one or more of the three classes of cone cells in the retina, which mediate color vision. The most common form is caused by a genetic condition called congenital red–green color blindness, which affects up to 1 in 12 males (8%) and 1 in 200 females (0.5%). The condition is more prevalent in males, because the opsin genes responsible are located on the X chromosome. Rarer genetic conditions causing color blindness include congenital blue–yellow color blindness, blue cone monochromacy, and achromatopsia. Color blindness can also result from physical or chemical damage to the eye, the optic nerve, parts of the brain, or from medication toxicity. Color vision also naturally degrades in old age.

<span class="mw-page-title-main">Primary color</span> Fundamental color in color mixing

A set of primary colors or primary colours consists of colorants or colored lights that can be mixed in varying amounts to produce a gamut of colors. This is the essential method used to create the perception of a broad range of colors in, e.g., electronic displays, color printing, and paintings. Perceptions associated with a given combination of primary colors can be predicted by an appropriate mixing model that reflects the physics of how light interacts with physical media, and ultimately the retina. The most common color mixing models are the additive primary colors and the subtractive primary colors . Primary colors can also be conceptual, either as additive mathematical elements of a color space or as irreducible phenomenological categories in domains such as psychology and philosophy. Color space primaries are precisely defined and empirically rooted in psychophysical colorimetry experiments which are foundational for understanding color vision. Primaries of some color spaces are complete but necessarily imaginary. Phenomenological accounts of primary colors, such as the psychological primaries, have been used as the conceptual basis for practical color applications even though they are not a quantitative description in and of themselves.

<span class="mw-page-title-main">Natural Color System</span> Proprietary perceptual color model

The Natural Color System (NCS) is a proprietary perceptual color model. It is based on the color opponency hypothesis of color vision, first proposed by German physiologist Ewald Hering. The current version of the NCS was developed by the Swedish Colour Centre Foundation, from 1964 onwards. The research team consisted of Anders Hård, Lars Sivik and Gunnar Tonnquist, who in 1997 received the AIC Judd award for their work. The system is based entirely on the phenomenology of human perception and not on color mixing. It is illustrated by a color atlas, marketed by NCS Colour AB in Stockholm.

<span class="mw-page-title-main">Color constancy</span> How humans perceive color

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<span class="mw-page-title-main">Color vision</span> Ability to perceive differences in light frequency

Color vision, a feature of visual perception, is an ability to perceive differences between light composed of different frequencies independently of light intensity.

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<span class="mw-page-title-main">Tetrachromacy</span> Type of color vision with four types of cone cells

Tetrachromacy is the condition of possessing four independent channels for conveying color information, or possessing four types of cone cell in the eye. Organisms with tetrachromacy are called tetrachromats.

<span class="mw-page-title-main">Afterimage</span> Image that continues to appear in the eyes after a period of exposure to the original image

An afterimage is an image that continues to appear in the eyes after a period of exposure to the original image. An afterimage may be a normal phenomenon or may be pathological (palinopsia). Illusory palinopsia may be a pathological exaggeration of physiological afterimages. Afterimages occur because photochemical activity in the retina continues even when the eyes are no longer experiencing the original stimulus.

<span class="mw-page-title-main">Trichromacy</span> Possessing of three independent channels for conveying color information

Trichromacy or trichromatism is the possession of three independent channels for conveying color information, derived from the three different types of cone cells in the eye. Organisms with trichromacy are called trichromats.

<span class="mw-page-title-main">Cone cell</span> Photoreceptor cells responsible for color vision made to function in bright light

Cone cells or cones are photoreceptor cells in the retinas of vertebrates' eyes. They respond differently to light of different wavelengths, and the combination of their responses is responsible for color vision. Cones function best in relatively bright light, called the photopic region, as opposed to rod cells, which work better in dim light, or the scotopic region. Cone cells are densely packed in the fovea centralis, a 0.3 mm diameter rod-free area with very thin, densely packed cones which quickly reduce in number towards the periphery of the retina. Conversely, they are absent from the optic disc, contributing to the blind spot. There are about six to seven million cones in a human eye, with the highest concentration being towards the macula.

Dichromacy is the state of having two types of functioning photoreceptors, called cone cells, in the eyes. Organisms with dichromacy are called dichromats. Dichromats require only two primary colors to be able to represent their visible gamut. By comparison, trichromats need three primary colors, and tetrachromats need four. Likewise, every color in a dichromat's gamut can be evoked with monochromatic light. By comparison, every color in a trichromat's gamut can be evoked with a combination of monochromatic light and white light.

<span class="mw-page-title-main">Monochromacy</span> Type of color vision

Monochromacy is the ability of organisms to perceive only light intensity without respect to spectral composition. Organisms with monochromacy lack color vision and can only see in shades of grey ranging from black to white. Organisms with monochromacy are called monochromats. Many mammals, such as cetaceans, the owl monkey and the Australian sea lion are monochromats. In humans, monochromacy is one among several other symptoms of severe inherited or acquired diseases, including achromatopsia or blue cone monochromacy, together affecting about 1 in 30,000 people.

<span class="mw-page-title-main">Spectral color</span> Color evoked by a single wavelength of light in the visible spectrum

A spectral color is a color that is evoked by monochromatic light, i.e. either a spectral line with a single wavelength or frequency of light in the visible spectrum, or a relatively narrow spectral band. Every wave of visible light is perceived as a spectral color; when viewed as a continuous spectrum, these colors are seen as the familiar rainbow. Non-spectral colors are evoked by a combination of spectral colors.

In color science, a color model is an abstract mathematical model describing the way colors can be represented as tuples of numbers, typically as three or four values or color components. When this model is associated with a precise description of how the components are to be interpreted, taking account of visual perception, the resulting set of colors is called "color space."

<span class="mw-page-title-main">Young–Helmholtz theory</span> Postulated existence of three photoreceptor types in the eye

The Young–Helmholtz theory, also known as the trichromatic theory, is a theory of trichromatic color vision – the manner in which the visual system gives rise to the phenomenological experience of color. In 1802, Young postulated the existence of three types of photoreceptors in the eye, with different but overlapping response to different wavelengths of visible light.

Visual perception is the ability to interpret the surrounding environment through photopic vision, color vision, scotopic vision, and mesopic vision, using light in the visible spectrum reflected by objects in the environment. This is different from visual acuity, which refers to how clearly a person sees. A person can have problems with visual perceptual processing even if they have 20/20 vision.

<span class="mw-page-title-main">Impossible color</span> Color that cannot be perceived under ordinary viewing conditions

Impossible colors are colors that do not appear in ordinary visual functioning. Different color theories suggest different hypothetical colors that humans are incapable of perceiving for one reason or another, and fictional colors are routinely created in popular culture. While some such colors have no basis in reality, phenomena such as cone cell fatigue enable colors to be perceived in certain circumstances that would not be otherwise.

<span class="mw-page-title-main">Unique hues</span> Pure blue, green, yellow or red hues that cannot be described as a mixture of other hues

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. The neural correlate of the unique hues are approximated by the extremes of the opponent channels in opponent process theory. 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.

<span class="mw-page-title-main">Russell L. De Valois</span>

Russell L. De Valois was an American scientist recognized for his pioneering research on spatial and color vision.

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