Dichoptic difference thresholds for chromatic stimuli

► New measure of binocular function. ► Detection of binocular differences in color. ► Thresholds predicted by apparent difference in hue. We have investigated the properties of binocular color vision using a new measure: the Dichoptic Color Difference Threshold (DCDT). The DCDT is the smallest detectable difference in color between two dichoptically superimposed stimuli. DCDTs differ from conventional measures of binocular rivalry in that they are performance- not appearance-based. The dependency of DCDTs on (a) color direction and (b) color contrast was measured. The colors (chromaticities) of the stimuli were defined according to a scaled version of the MacLeod–Boynton color space, and the luminance and color contrasts of the stimulus pairs were equated using a matching procedure. DCDTs were measured using a forced-choice procedure in which subjects had to chose which of two stimuli had a between-eye-difference in color. DCDTs ranged from 9° to 22° of color angle depending on color direction. DCDTs were lower than binocular rivalry thresholds but higher than thresholds for discriminating the color pairs when placed side-by-side. There were no minima at either the cardinal color or unique hues directions, suggesting that DCDTs are not mediated by these mechanisms. DCDTs were however positively correlated with the measured perceived color difference between the color pairs when placed side-by-side.