Abstract
It is still unclear whether there is a special role for hue in color discrimination. Strikingly, the answer seems to vary for different regions of color space. We and others showed that in the “orange” quadrant of DKL space (positive LM, negative S-LM), hue thresholds are significantly lower than in the “violet” quadrant (positive LM, positive S-LM) and also significantly lower than chroma thresholds. Here, we explored whether this disparity might arise due to differences in perceived saturation. We first measured points of isosaturation for the “orange” and “violet” quadrants. Observers completed a 2AFC task viewing two 4.5° disks on an isoluminant gray background. For each trial they indicated which disk appeared more saturated; trials were controlled using an adaptive staircase. We found systematic differences between perceived saturation in the two quadrants. At equal radial distance (chromatic contrast), “violet” stimuli appeared more saturated than “orange” ones. We then measured hue and chroma JNDs in the two quadrants at various chromatic contrasts in DKL space. Observers viewed four disks in a square configuration and chose the odd one out, which differed in chroma or hue. The magnitude of the difference was controlled using the QUEST adaptive staircase. In the “orange” quadrant, hue thresholds were consistently lower than chroma thresholds, irrespective of chromatic contrast. In the “violet” quadrant, the ratio of hue to chroma thresholds was close to 1 for low chromatic contrasts and decreased for higher contrasts. However, “orange” quadrant ratios were always less than “violet” quadrant ratios, indicating that differences in perceived saturation cannot account for the relationship between chroma and hue thresholds in different regions of color space.