What is the Opponent Process Theory of Color Vision?

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Color vision is based on the type of incident light on the cone cells of the retina. While the trichromatic theory explains the way light is perceived by the cone cells in the eye, and how the signals are sent to the brain; the interpretation of these colors by the brain is explained by the opponent process theory of color vision, which is interpreted and elucidated in this article.
Color vision is the capability of an organism to detect and distinguish objects based on the type of wavelengths of light that are reflected, emitted, or transmitted by each object. The perception of colors by the eye and its translation into meaningful information by the brain is a subjective process. These two organs work in accord to help the organism sense color. The mechanism of this process can be explained by two complementary theories - the trichromatic theory and the opponent process theory.
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The trichromatic theory, also called the Young–Helmholtz theory, explains how the retina of the eye detects color via the help of three types of cones - short (S), medium (M), and large (L). It claims that there are three primary colors-red, blue, and green-that are perceived by the different types of cone cell. The S cones detect blue, M cones detect green, and L cones detect red color. All other colors are perceived due to the variation in the different wavelengths incident on these cones.

This theory is supported by the color matching experiments, which involves an observer to look at a certain color and try to match it by mixing the correct wavelengths of light. But this theory failed to explain why a colorblind or color deficient person is unable to discern between two of the primary colors (for example, red and green). Since logically if a cone was defective, the person would be unable to detect one particular color and all its related shades, instead, in actuality the person fails to distinguish between two distinct primary colors and perceives them as the same. Also, the mechanism underlying the phenomenon of negative afterimages could not be elucidated by this theory.

Opponent Process Theory of Color Vision

► The first recorded instance of this theory was in the "Theory of Colors" by Johann Wolfgang von Goethe in the year 1810. He presented a color wheel with all the colors arranged symmetrically, and explained - "the colors 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".

► 82 years later, Ewald Hering proposed his opponent color theory, where he claimed that four primary colors existed (red, green, blue, and yellow) and all other colors could be described by the combinations of these four colors. He also theorized that these set of four colors existed as parts of opposite pairs and were also perceived as such, i.e., activation of one color of the pair resulted in the inhibition of the other. For example, one perceives either red or green, but never reddish green or greenish red, and although the combination of red and green yields yellow, it is not perceived as such by the eyes.

► This theory attempted to resolve and account for the mechanisms involved in receiving and processing the information obtained from the cone cells. The biological basis for this theory involves not only the cone cells of the retina but also the bipolar and ganglion cells. It posits that the information that is generated by the cone cells on the incidence of light, is received by the bipolar cells, which then process it and pass it on to the ganglion cells. Two types of ganglion cells are involved in this process: magnocellular and parvocellular. The parvocellular cells handle majority of the incoming information and are further categorized into two types - one that differentiates data of L and M cones (red-green difference), and another that separates the data obtained from S cones and a mix of L and M cones (blue-yellow differences). Intensity of the light is also determined by these cells. Hence, these cells help decipher the difference between three pairs - red/green, blue/yellow, and white/black.

► In 1957, the efforts of Leo Hurvich and Dorothea Jameson yielded qualitative data in support of Hering's color theory. They named the method used to generate this data, "hue cancellation". It involved selecting an initial color (bluish green) and then trying to determine how much of the opponent color of one of the constituents of the initial color (e.g. yellow) must be added to it, in order to cancel it out leaving only the other constituent color behind (e.g. green).
Evidential Support

Initially, the opponent process theory was believed to refute the trichromatic theory of color, but due to the experimental work of many scientists and the development of the hue cancellation method, this theory was accepted as an advanced and developed form of trichromatic theory. The opponent theory is also supported by the fact that it is able to successfully explain two-color vision related phenomena that were left unexplained by the other theory. These phenomena include color-blindness and complementary color afterimages.

Colorblindness
★ In this case, the inability to decipher between colors is due to the malfunction of any one type of the cone cells in the retina. Typically, a red/green or blue/yellow color blindness is observed, i.e. if a cone cells perceiving a given pair of opponent colors is defunct, both the colors are not perceived by the eye, or both are perceived as the same color. For example, red/green color-blind people see red and green both as a dark grayish color. This supports the opponent process theory, since the hampering of the perception of one color automatically hampers that of its opponent pair,i.e., one does not encounter any individual that is solely color blind for red but can see green, and vice versa.

Complementary Color Afterimages
★ Afterimages are produced as a result of the overstimulation of one type of photoreceptors, leading to a temporary loss of sensitivity of it. For example, if one stares or looks at a blue image for a minute or two, the ganglion cells activates the blue photoreceptors and inhibit the yellow receptors. Then, if one stops looking at the image and looks at a blank white paper, he/she is able to see an imprint of a yellow colored version of the image on to the paper. This occurs due to the fatigue of the blue receptors. In order to overcome, this, the ganglion cells try to balance it by inhibiting the blue and activating the yellow (opponent color) photoreceptors, thereby producing a yellow imprint of the image. In layman's terms, after staring at a particular color, its opposite color is observed. The same is true for the red/green pair. Hence, this provides support for the validity of the opponent process theory of color vision, as only the opposite color is seen later, instead of just any random color.
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The discovery of electrophysiological responses that resulted in opponent processing of color along with the application of this theory, by Richard Solomon in 1970, to explain emotion, drug addiction, and motivation revolutionized the understanding of this theory. Subsequently this theory was applied to computers mechanical perception of color in the form of "Gaussian color model" and the "natural vision processing model".