Trichromat-like Categorical Color Naming of Dichromats

Abstract

It has been shown that some dichromats can categorize colors using color names in quite similar manner to trichromats. The previous studies suggested some possible mechanisms to account for this trichromat-like categorical color naming ability, such as rod contribution, anomalous cone pigments, a nonlinear parallel chromatic channel or luminance cue. However none of them has not been able to fully explain the categorical color naming of dichromats. In this paper we tried to find out what observing conditions of color stimuli mostly affect the color categorization of dichromats to explore this naming mechanism. Dichromats and trichromats participated in the categorical color naming experiments, in which they named 424 OSA uniform color samples twice using only the Berlin and Kay’s eleven basic color terms under various chromatic illuminants. They also observed OSA color samples, simulated on a CRT monitor, in various observing conditions, i.e., a small visual angle, a short duration, equal luminance and blurred edge. A trichromat could use the same color names for the same stimuli in almost all observing conditions whereas a dichromat showed some confusion of color names along the green-red direction under some illuminants and for some CRT stimuli. This confusion was greater with a small visual angle and with equal luminance. Individual differences were much greater for dichromats than for trichromats. Our results suggest that dichromats might not have the same color category mechanism, but have different mechanisms that determine their degree of deficiency. These mechanisms must yield barely visible differences, not enough to be used for color discrimination, but enough for color categorization.