Trichromacy
Since the retina contains four different types of receptor it might be thought that the neural pathways would carry four different signals to the brain, and more precisely to the primary visual cortex which is at the back and rear portion of the brain. It is generally believed, however, that colour information is coded by the retinal and post-retinal neural structures as just three types of signals that are often called ''channels". The idea of ''channels" in the brain is central to the way in which the operation of the brain can be viewed as an information- or signal-processing task. A channel is a conceptual processing route and thus for the visual system we can say that the information from the cones is processed in three separate channels. Remembering that colour perception is only one function of the visual system, there are other channels that are responsible for providing other information about the outside world that enables the perception of form, motion, and distance for example. The existence of channels for the processing of colour information helps explain the two contradictory theories of colour vision that were prevalent during the 19th century: the trichromatic theory and the opponent-colours theory.
The trichromatic theory was postulated by Young and later by Helmholtz and was based upon colour matching experiments carried out by Maxwell. Maxwell's experiments demonstrated that most colours can be matched by superimposing three separate light sources known as primaries; a process known as additive mixing. Although any light sources could be used as primaries it will be seen later that the use of monochromatic sources of radiation enables the widest gamut of colours to be obtained by additive mixing. The Young-Helmholtz theory of colour vision was built around the assumption of there being three classes of receptors although direct proof for this was not obtained until 1964 when microspectrophotopic recordings of single cone cells were obtained. The roots of trichromacy are firmly understood to be in the receptoral stage of colour vision. It is important to realize that a yellow stimulus produced by the additive mixture of appropriate red and green lights does not simply match monochromatic yellow light but is indistinguishable from it. Thus, the trichromatic nature of vision is essential for the operation of many colour reproducing processes such as television, photography, and three-colour printing.