Color fusion and flicker fusion frequencies using tritanopic pairs

We determined color fusion and flicker fusion frequencies for tritanopic color pairs, so that only short wavelength sensitive cones were modulated. The minimally distinct border (MDB) technique was used to determine tritanopic pairs for each observer. If S-cones do not contribute to the luminance channel, color fusion and flicker fusion frequencies should be equal, because the achromatic flicker perceived beyond color fusion frequency is supposed to be elicited just by the luminance channel. The results show a significant difference between color fusion and flicker fusion frequencies for all pairs and for all subjects, which suggests a contribution of S-cones to the luminance channel. However, the results also showed that the residual achromatic flicker can not be eliminated by adjusting relative luminance of two colors flickering in counter-phase. The relative luminance stayed the same as the MDB setting when residual flicker was minimized. This means that minimum flicker was obtained when L- and M-cones are temporally silenced, and suggests little or no contribution of S-cones to luminance channel. These conflicting results might be explained by phase shift between responses of S-cones and other longer-wavelength-sensitive cones.