Unique Hues

One can see so many different colors in the environment. And yet there are only four colors that occupy a special place in color perception. They are called unique hues and they originate from the opponent colors theory proposed by Ewald Hering in 1878 [1, 2]. The theory postulates three opponent processes: two chromatic processes of red-green and blue-yellow and one achromatic process of white-black. Unique hues are perceived when one of the two chromatic processes is polarized in one direction and the other is at equilibrium. For instance, one perceives unique red when the red-green process is polarized toward red and the blue-yellow process is at equilibrium. Phenomenologically, one can describe any color he or she sees by a mixture of various ratios of two unique hues. However, one does not perceive two opposing unique hues at the same and at the same location. Namely, one does not see greenish red or bluish yellow. The opponent colors theory was at odd with the trichromatic theory that was initially proposed by Thomas Young in 1802 and developed by Hermann von Helmholtz in 1850. The trichromatic theory states that there are three types of receptors that are responsible for conveying color signals. These two theories of color vision were the focus of controversy among scientists until they were integrated into the two-stage model in the 1960s [3]. The trichromatic theory holds at the receptor level in the retina where there are three types of cones. These are called short-, medium-, or long-wavelengthsensitive (S, M, or L) cone receptors, and the names are derived from different spectral region of the peak in their spectral sensitivities. Then the opponent colors theory is at work in the second stage such as retinal ganglion cells and the lateral geniculate nucleus (LGN). Even though this idea of integration of the two major theories appears attractive, recent research has shown that spectrally opponent neurons found in these sites do not really represent red-green and yellow-blue processes of the opponent colors theory. Rather, spectrally opponent neurons in the LGN represent neural signals created by the L-cone inputs antagonized by the M-cone inputs (L-M axis) and those created by the S-cone inputs opposed by a combination of Land M-cone signals (S-(L+M)