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The color opponent process is a color theory that states that the human visual system interprets information about color by processing signals from cones and rods in an antagonistic manner. The three types of cones (L for long, M for medium and S for short) have some overlap in the wavelengths of light to which they respond, so it is more efficient for the visual system to record differences between the responses of cones, rather than each type of cone's individual response. The opponent color theory suggests that there are three opponent channels: red versus green, blue versus yellow, and black versus white (the last type is achromatic and detects light-dark variation, or luminance). Responses to one color of an opponent channel are antagonistic to those to the other color. That is, opposite opponent colors are never perceived together – there is no "greenish red" or "yellowish blue".
While the trichromatic theory defines the way the retina of the eye allows the visual system to detect color with three types of cones, the opponent process theory accounts for mechanisms that receive and process information from cones. Though the trichromatic and opponent processes theories were initially thought to be at odds, it later came to be understood that the mechanisms responsible for the opponent process receive signals from the three types of cones and process them at a more complex level.
Besides the cones, which detect light entering the eye, the biological basis of the opponent theory involves two other types of cells: bipolar cells, and ganglion cells. Information from the cones is passed to the bipolar cells in the retina, which may be the cells in the opponent process that transform the information from cones. The information is then passed to ganglion cells, of which there are two major classes: magnocellular, or large-cell layers, and parvocellular, or small-cell layers. Parvocellular cells, or P cells, handle the majority of information about color, and fall into two groups: one that processes information about differences between firing of L and M cones, and one that processes differences between S cones and a combined signal from both L and M cones. The first subtype of cells are responsible for processing red–green differences, and the second process blue–yellow differences. P cells also transmit information about intensity of light (how much of it there is) due to their receptive fields.
Johann Wolfgang von Goethe first studied the physiological effect of opposed colors in his Theory of Colours in 1810. 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."
Ewald Hering proposed opponent color theory in 1892. 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; although yellow is a mixture of red and green in the RGB color theory, the eye does not perceive it as such.
In 1957, Hurvich and Dorothea Jameson provided quantitative data for Hering's color opponency 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 eliminate any hint of that component from the starting color (Wolfe, Kluender, & Levi, 2009).
Griggs expanded the concept to reflect a wide range of opponent processes for biological systems in this book Biological Relativity (c) 1967.
If we stare at a red square for forty seconds, and immediately look at a white sheet of paper we'll often perceive a green square on the blank sheet. This complementary color afterimage is more easily explained by the opponent theory than the trichromatic; in the opponent-process theory, fatigue of pathways promoting red produce the illusion of a green square.
Under normal circumstances, there is no hue one could describe as a mixture of opponent hues; that is, as a hue looking "redgreen" or "yellowblue". However, in 1983 Crane and Piantanida carried out an experiment under special viewing conditions in which red and green stripes (or blue and yellow stripes) were placed adjacent to each other and the image held in the same position relative to the viewer's eyes (using an eye tracker to compensate for minor muscle movements). Under such conditions, the borders between the stripes seemed to disappear and the colors flowed into each other, making it apparently possible to override the opponency mechanisms and, for a moment, get some people to perceive novel colors. :
However, some subjects in the Crane and Piantanida study merely reported seeing hallucinatory textures, such as blue specks on a yellow backdrop. A possible explanation is that the study did not control for variations in the perceived luminance of the colors from subject to subject (two colors are equiluminant for an observer when rapidly alternating between the colors produces the least impression of flickering). To investigate this, Vincent Billock, Gerald Gleason and Brian Tsou set up a similar experiment which controlled for luminance. They had the following observation:
This led them to propose a 'soft-wired model of cortical color opponency', in which populations of neurons compete to fire and in which the 'losing' neurons go completely silent. In this model, eliminating competition by, for instance, inhibiting connections between neural populations can allow mutually exclusive neurons to fire together.