While unique hues do correspond to null points on the red-green/blue-yellow axes, the red-green/blue-yellow axes do not correspond to the L-M and S-(L+M) cardinal axes. However, later research suggests that the L-M and S-(L+M) axes do not correspond to the perceptual equivalent of red-green or yellow-blue. This introduced the concept of ‘neutralisation’, where an equal stimulation of red and green light would cancel out to a neutral point on the red-green axis (which could correspond to yellow, white, or blue). Later research suggested that unique hues could be defined as the ‘equilibrium state’ of the red-green or blue-yellow cardinal axes. Hering suggested that these mutually exclusive hues would correspond to the ‘building up’ (assimilation) or ‘breaking down’ (dissimilation) of a visual substance corresponding to the red-green or blue-yellow color axis. Similarly, the yellow-blue (S-(L+M)) axis should produce unique hues when it is maximally stimulated at either pole. The L-M axis, corresponding to the red-green axis proposed by Hering, should produce unique hues when maximally stimulated at either pole.
The recombination of color signals into L+M, L-M, and S-(L+M) channels (often referred to as the cardinal axes, or dkl color space) makes clear predictions for the physiological correlate of unique hues. The axes proposed for these recombinations are commonly taken to be L+M, S-(L+M), and L-M. It became widely accepted that the three cone types were recombined into three cone contrast pathways, two encoding color, and one encoding luminance, thereby reducing the redundancy of correlated cone signals. While this theory was initially considered contradictory to Young and Helmholtz’s trichromatic theory, the discovery of color-opponent cells in the retina and lateral geniculate nucleus (LGN) reconciled the two theories. These colors are perceptually impossible and suggest an opponent relationship between red and green, and blue and yellow. This theory is based strongly on the existence of perceptually impossible colors or color hue mixtures that have no meaning such as redgreen or yellowblue. His theory suggests that color vision is based on two opposing axes of color: a red-green axis and a blue-yellow axis. Ewald Hering first proposed the idea that red, green, blue, and yellow were unique in 1892.
The concept of certain hues as 'unique' came with the advent of Opponent process theory. They are often used in psychophysical research to measure variations in color perception due to color abnormalities or color adaptation. Unique hues can differ between people, and depend on the state of adaptation of the visual system. There is a great deal of variability when defining unique hues experimentally, however, a single observer can usually set their experience of a unique hue extremely consistently, to within a few nanometers. For example, a color cannot appear both red and green the color would cancel out to yellow.
Ewald Hering first defined the unique hues as red, green, yellow, and blue, and based them on the concept that these colors could not be simultaneously perceived. A unique hue is defined as a color which an observer perceives as a pure, without any admixture of the other colors. Unique hue is a term used in certain theories of color vision, which implies that human perception distinguishes between "unique" (psychologically primary) and composite (mixed) hues. A concept of four unique hues of psychologist Charles Hubbard Judd (1917)
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