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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 cones in an antagonistic manner. The three types of cones 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 latter 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. Although, according to the trichromatic theory, 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 (Kandel et al., 2000). The three types of cones, S, M, and L, respond best to short-, medium- and long-wavelength light respectively. Information from the cones is passed to 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, and parvocellular or small-cell layers (Kandel et al., 2000). Parvocellular cells, or P cells, are the major cells to handle information about color. They 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 (Kandel et al., 2000). These two subtypes are responsible for processing red-green and blue-yellow differences respectively. Cells in the parvocellular pathway transmit information not only about color, but also about intensity of light (how much of it there is) because of their receptive fields.
History The opponent color theory was first proposed by Ewald Hering in 1872 (Hering, 1964). 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. (Note that although yellow is a mixture of red and green in the RGB color theory, the eye does not perceive it as such.) In 1957 Leo Hurvich and Dorothea Jameson provided quantitative data for Hering's color opponency theory (Hurvich et al., 1957). The opponent color theory can be applied to computer vision and implemented as the Gaussian color model (Geusebroek et al., 2001). Reddish green and yellowish blue 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 proving that, under special viewing conditions involving the use of an eye tracker, it is apparently possible to override the opponency mechanisms and, for a moment, get some people to perceive novel colors: "''s''ome observers indicated that although they were aware that what they were viewing was a color (that is, the field was not achromatic), they were unable to name or describe the color. One of these observers was an artist with a large color vocabulary. Other observers of the novel hues described the first stimulus as a reddish-green." * See also | ||||||||
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