Abstract
In this study, we measured the relative contributions of luminance and S-cone signals to color discrimination thresholds and supra-threshold color differences in color deficient observers. In a trial of the color discrimination experiment, four luminance or S-cone Gaussian blobs were presented on the gray background, and one of them had a different color from the others. The observers were asked to indicate which blob had a different color in four-alternative forced choice manner. The discrimination thresholds were measured with the PSI staircase procedure. In a trial of the color difference experiments, four colored spots were presented: two spots at the left of the central fixation point, and the other two at the right. The observers were asked to judge which of the left or right spot pair appeared to have larger color differences. The perceptual color differences between colors on the luminance and S-cone axes were measured with the maximum likelihood difference scaling method. In the results, the ratio of discrimination thresholds on the S-cone axis to those on the luminance axis were comparable across the color vision types of the observers. In contrast, the perceptual color differences were extremely smaller on the S-cone axis than on the luminance axis in several deuteranomaly observers, while the color differences in the other types of observers were comparable between the luminance and S-cone axes. These results suggest that, in some color deficient observers, S-cone signals were considerably undervalued in the perception of supra-threshold color differences, but not in color discrimination.