The tuning of human photopigments may minimize red—green chromatic signals in natural conditions

Humans and other Old World primates (Catarrhini) share very similar L (long-wavelength, 'red') and M (medium-wavelength, 'green') cone photopigment spectral sensitivities, with peaks at around 563 nm and 535 nm, respectively. Changes of single amino acid residues at critical sites in photopigment opsins can alter this peak tuning. Moreover, the photopigment alleles and spectral sensitivities of human populations are polymorphic, so there is potential for adaptive change or genetic drift. The manifest lack of variability suggests that the tuning of the L and M photopigments has adaptive significance, but the reason for this conservatism is unclear. To assess how natural spectral reflectances may have influenced pigment tuning, we have measured the chromatic (i.e. difference) signals available in natural scenes, and estimated how these signals would vary if spectral sensitivities of the pigments moved to longer or to shorter wavelengths. The size of the chromatic signal is, predictably, dependent principally on the spectral separation of the photopigments, but in addition we find that for a fixed separation there is a marked dependence on the specific peak tuning of the photopigments. Indeed, the naturally occurring L and M cone peaks may be set at a pair of points on the spectrum that on average minimizes the 'L-M' (i.e. red-green) chromatic signal. This somewhat paradoxical observation supports the view that red-green vision has evolved for a specific task, such as finding fruit, whilst minimizing interference by the chromatic signal in luminance vision to which both L and M cones contribute.