We find that additivity of trichromatic color matches holds, even in parafoveal vision, for most observers if rods are prevented from seeing the stimuli. Some heterozygous women are exceptions. The failure of the additivity law for these women implies that their eyes contain more than three types of cone with different spectral sensitivities.
© 1981 Optical Society of America
Allen L. Nagy, Donald I. A. MacLeod, Nicholas E. Heyneman, and Alvin Eisner, "Four cone pigments in women heterozygous for color deficiency," J. Opt. Soc. Am. 71, 719-722 (1981)