A new method for the study of color vision is introduced where two test flashes are used in an increment-threshold experiment. Summation index is defined as the logarithm of the factor by which threshold is reduced when the two light flashes are presented together in the ratio of their individual threshold radiances. When the wavelength of one test flash is fixed at 630 mμ and the wavelength of the other flash is varied, a summation-index curve can be drawn as a function of the wavelength of the second flash. This curve is assumed to reveal the interrelations between the underlying mechanisms for color discrimination which are stimulated by the first test flash and the second test flash, respectively. Two different durations of the test flash are employed and two different summation-index curves are obtained. It is found that under certain conditions the summation index becomes smaller than that predicted by probability summation, which suggests an inhibition between two mechanisms. With the aid of the experimental spectral sensitivity curves obtained under the same adapting conditions as above, the summation-index curves are interpreted with the help of conceptions of opponent-colors theory.
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