Chromatic sensitivity: A new insight into color discrimination

Color discrimination varies between individuals even in the absence of a congenital color vision defect. Attempts to explain this variation have shown no link to macular pigment optical density, L:M cone ratios, non-verbal IQ, or female carriers of color vision deficiencies. This study examined individuals' relative luminance and color-opponent systems sensitivity to their Farnsworth-Munsell 100-Hue (FM-100) color discrimination ability. Thirty normal color vision subjects performed FM-100 and increment threshold testing for white, blue, green and red on a 150 cd/m2 white background. White increment thresholds were compared to colored increment thresholds to produce a luminance to color-opponent system sensitivity ratio (L/C ratio). The sum of these ratios produced a total L/C ratio for each subject. Total error scores (TES) on FM-100 testing were compared to a total L/C ratio to determine a linear regression correlation. Our key finding indicates that variation in normal human color discrimination shows a reliable correlation to individual differences in relative luminance and color-opponent systems sensitivities (r= -0.44, p = 0.016). As total L/C ratio increased, subject total error score decreased. In keeping with the Helmholtz-Kohlrausch Effect, colored increment L/C values were high for blue and red followed by green, thus demonstrating this effect at threshold. We presume this difference in systems sensitivity improves color discrimination by enhancing color saturation.