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The opponent process is a colour theory that states that the human visual system interprets information about colour 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 colour 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 colour of an opponent channel are antagonistic to those to the other colour.
Although, according to the trichromatic theory, the retina of the eye allows the visual system to detect colour 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 colour, but also about intensity of light (how much of it there is) because of their receptive fields.
History
The opponent colour theory was first proposed by Ewald Hering in 1872 (Hering, 1964). He thought that the colours red, yellow, green, and blue are special in that any other colour 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 colour opponency theory (Hurvich et al., 1957).
The opponent colour theory can be applied to computer vision and implemented as the Gaussian colour 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:
See also
References
- Geusebroek J. M., van den Boomgaard R., Smeulders A. W. M., Geerts H., Color invariance, IEEE Trans. Pattern Anal. Machine Intell., 2001; 23(12):1338-1350.
- Hering E., Outlines of a Theory of the Light Sense, Cambridge, Mass., Harvard Univ. Press, 1964.
- Hurvich LM, Jameson D, An opponent-process theory of color vision, Psychol Rev 1957; 64:384-404.
- Kandel E.R., Schwartz, J.H., Jessell, T.M., Principles of Neural Science, 4th ed., pp.577-580. McGraw-Hill, New York (2000).
"Opponent process"