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.
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