Opponent process
The opponent process is a hypothesis of color vision that states that the human visual system interprets information about color by processing signals from the three types of photoreceptor cells in an antagonistic manner. The three types of cones are called L, M, and S. The names stand for "Long wavelength sensitive,” "middle wavelength sensitive," and "short wavelength sensitive." The opponent-process theory implicates three opponent channels: L versus M, S versus, and a luminance channel. These cone-opponent mechanisms were at one time thought to be the neural substrate for a psychological theory called Hering's Opponent Colors Theory, which calls for three psychologically important opponent color processes: red versus green, blue versus yellow, and black versus white. The Opponent Colors Theory is named for the German physiologist Ewald Hering who proposed the idea in the late 19th century. However, it has been argued that Hering’s Opponent Colors Theory lacks adequate phenomenological and empirical support, and may not be a necessary feature of normal human color experience. Correspondingly, considerable physiological and behavioral evidence proves that the physiological cone opponent mechanisms do not constitute the neurobiological basis for Hering's Opponent Colors Theory.
Color theory
Complementary colors
When staring at a bright color for a while, then looking away at a white field, an afterimage is perceived, such that the original color will evoke its complementary color. When complementary colors are combined or mixed, they "cancel each other out" and become neutral. That is, complementary colors are never perceived as a mixture; there is no "greenish red" or "yellowish blue", despite claims to the contrary. The strongest color contrast that a color can have is its complementary color. Complementary colors may also be called "opposite colors" and they were originally considered the primary evidence in support of Hering's Opponent Colors Theory. There are two fatal problems with this evidence. First, the complement of red is not green, as called for by Hering's theory; it is bluish-green. And second, there exists a complementary color for every color, so there is nothing special about the set of complementary pairs picked out by Hering's theory.Unique hues
The colors that define the extremes for each opponent channel are called unique hues, as opposed to composite hues. Ewald Hering first defined the unique hues as red, green, blue, and yellow, and based them on the concept that these colors could not be simultaneously perceived. For example, a color cannot appear both red and green. These definitions have been experimentally refined and are represented today by average hue angles of 353°, 128°, 228°, 58°.The unique hues are a defining feature of many psychological color spaces, but there is substantial evidence showing that the unique hues are not hard wired in the nervous system, contrary to the stipulations of Hering's Opponent Colors Theory. Unique hues can differ between individuals and are often used in psychophysical research to measure variations in color perception due to color-vision deficiencies or color adaptation. While there is considerable inter-subject variability when defining unique hues experimentally, an individual's unique hues are very consistent, to within a few nanometers.
Physiological basis
Relation to LMS color space
The trichromatic theory is in conflict with Hering's Opponent Colors Theory, although it is compatible with a physiological opponent process that compares the outputs of the different classes of cone types. The poles of these cone opponent mechanisms do not correspond to the unique hues of Hering's Opponent Colors Theory and unlike the unique hues, have no privilege in color perception.Most humans have three different cone cells in their retinas that facilitate trichromatic color vision. Colors are determined by the proportional excitation of these three cone types, i.e. their quantum catch. The levels of excitation of each cone type are the parameters that define LMS color space. To calculate the opponent process tristimulus values from the LMS color space, the cone excitations must be compared:
- The luminous opponent channel is a weighted sum of all three cone cells.
- The red–green opponent channel is equal to the difference of the L- and M-cones.
- The blue–yellow opponent channel is equal to the difference of the S-cone and the average/weighted sum of the L- and M-cones.
Cone opponent mechanisms are encoded in the retina
The output of different types of cones are compared by cells in the retina including retina bipolar cells and bistratified retinal ganglion cells. The output of bipolar cells is relayed to the visual cortex by the retinal ganglion cells by way of a thalamic relay station called the lateral geniculate nucleus of the thalamus. Much of the scientific knowledge of retinal ganglion cell physiology was obtained by neural recordings of cells in the LGN.The cone-opponent mechanisms in the retina and LGN represent a fundamental physiological opponent process but do not represent the unique hues. For example, the colors that best elicit responses of the bistratified S--opponent neurons are best described as purplish and lime-green, not "blue" and "yellow". The neurons are sometimes referred to as "blue–yellow" neurons, but this is a historical artifact dating to the time when it was thought that Hering's Opponent Colors Theory was hardwired by the retina and the mismatch between the colors to which they are optimally tuned and Hering's Opponent Colors was overlooked. Cone opponent mechanisms exist in the retinas of many mammals, including monkeys, mice, and cats. In primates, the LGN contains three major classes of layers:
- Magnocellular layers responsible largely for the luminance channel
- Parvocellular layers responsible largely for red–green opponency
- Koniocellular layers responsible largely for blue–yellow opponency, poor spatial resolution, long latency
Advantage
Transmitting information in opponent-channel color space could be advantageous over transmitting it in LMS color space. There is some overlap in the wavelengths of light to which the three types of cones 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.Hurvich and Jameson argued that the use of opponent-channel color space would increase color contrast, making the information easier to process by later stages of vision.
Color blindness
can be classified by the cone cell that is affected or by the opponent channel that is affected. In either case, the channel can either be inactive or have a lower dynamic range. For example, individuals with deuteranopia see little difference between the red and green unique hues.History
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: Even though yellow is a mixture of red and green in the RGB color theory, humans do not perceive it as such.
Hering's new theory ran counter to the prevailing Young–Helmholtz theory, first proposed by Thomas Young in 1802 and developed by Hermann von Helmholtz in 1850. The two theories seemed irreconcilable until 1925 when Erwin Schrödinger was able to reconcile the two theories and show that they can be complementary.
Psychophysical investigations
In 1957, Leo Hurvich and Dorothea Jameson claimed to provide a psychophysical validation for Hering's theory. Their method was called hue cancellation. Hue cancellation experiments start with a color and attempt to determine how much of the opponent color of one of the starting color's components must be added to reach the neutral point. The problem with the method of Hurvich and Jameson is that it defined the unique hues as the colors used in the cancellation; it did not test whether these colors are unique. So, participants were only ever asked to assess the proportion of the four colors in mixtures; they were never asked whether these four colors are the only possible set of primaries as would be required for a scientifically valid test of Hering's Opponent Colors Theory. Bosten and colleagues showed in 2014 that other colors can be used as primaries.In 1959, Gunnar Svaetichin and MacNichol recorded from the retinae of fish and reported three distinct types of cells:
- One cell responded with hyperpolarization to all light stimuli regardless of wavelength and was termed a luminosity cell.
- Another cell responded with hyperpolarization at short wavelengths and with depolarization at mid-to-long wavelengths. This was termed a chromaticity cell.
- A third cellalso a chromaticity cellresponded with hyperpolarization at fairly short wavelengths, peaking about 490 nm, and with depolarization at wavelengths longer than about 610 nm.
Similar chromatically or spectrally opposed cells, often incorporating spatial opponency, were found in the vertebrate retina and lateral geniculate nucleus through the 1950s and 1960s by De Valois et al., Wiesel and Hubel, and others.
Following Gunnar Svaetichin's lead, the cells were widely called opponent color cells: red–green and yellow–blue. Over the next three decades, spectrally opposed cells continued to be reported in primate retinae and LGN. A variety of terms are used in the literature to describe these cells, including chromatically opposed or chromatically opponent, spectrally opposed or spectrally opponent, opponent colour, colour opponent, opponent response, and simply, opponent.