Unique hues

[5][6] Hering first proposed the idea that red, green, blue, and yellow were unique hues ("Urfarben"), based on the concept that these colors could not be simultaneously perceived.

[8] A physiological pathway from the cones in the retina to a neural correlate for the psychological unique hues has been elusive.

[9] Mollon and Jordan state: “...the nature of the unique hues remains mysterious and we do not know whether they tell us anything about the neural organisation of the visual system.”[10] The first transformation of light to a neuronal signal (visual phototransduction) yields 3 channels, each proportional to the quantal catch of one cone type (L-, M- and S-), estimated by the LMS color space.

The second transformation occurs in the color-opponent cells and produces the opponent process channels: L+M (luminance), L-M (red-green), and S-(L+M) (blue-yellow), the latter of which form the cardinal axes.

[13] However, while opponent-cells have been found in the LGN that respond to cone combinations other than those of the cardinal axes, such as M-S,[14] there is no physiological understanding of this third transformation.

[22] However, the values have large inter-subject[22] and slight intra-subject variability, depending on the state of adaptation of the visual system.

[26] Unique hues have played an important role in understanding linguistic relativity or the idea that language has a significant influence on thought.

Unique yellow was determined to skew to higher wavelengths for anomalous trichromats (deuteranomaly), approaching 700 nm for strong deutans.

However, it is common to use similar techniques for defining the wavelength corresponding to "unique white" (achromatic point) of dichromats as means for quantifying their color vision.