Additivity of opponent chromatic valence

The additivity of chromatic valences that were produced by two different wavelengths λ₁ and λ₂ was investigated for four chromatic sensations, red, green, yellow, and blue, by the summation index method. Linear additivity was found for the green and the blue chromatic valence giving the summation index of 0.30. The linear additivity was also found for the red sensation if both stimuli were in the same region of long wavelengths or of short wavelengths. A slight additivity failure of the enhancement type was, however, found for λ₁ and λ₂ pairs when λ₁ was chosen in the short wavelength region while λ₂ in the long wavelength region, respectively. The additivity failure of the same type was found for the yellow chromatic valence for two subjects when λ₁ was 607 nm and λ₂ was 570, 550, and 553 nm, the last combination giving a summation index of as much as 0.64. The results may imply the existence of two different cones whose responses do not add linearly to yield the yellow sensation.