Complex interactions between lightness, chroma, and hue in color ensemble perception

Human color perception is trichromatic, and color spaces that quantify the range of perceived colors are commonly mapped in three coordinate dimensions. The human visual system can abstract a single color representation from spatial color variation, but such representations do not always conform to a colorimetric average. Here, we investigate how spatial variation in lightness, chroma, and hue, as well as their mutual relationships, affect perceptual color averages. To obtain baselines, we estimated simple discrimination thresholds for lightness, chroma, and hue. Next, we used simple, spatial color ensembles with uncorrelated and correlated variation in lightness, chroma, and hue to investigate their interactions. First, in a comparison experiment, polychromatic stimuli were compared with uniformly colored stimuli. Second, in an adjustment experiment, uniformly colored stimuli were matched to polychromatic stimuli. Biases in the perceptual averages depended on the hue range of color distributions, dimensions of color variation, as well as their intercorrelation. We found a consistent bias towards higher-than-average chroma in line with previous reports. In contrast, we found little evidence for a bias towards higher-than-average lightness previously reported for real objects. Average percepts of hue were sometimes biased towards hues correlated with higher chroma and/or lightness, and these effects varied between stimulus colors and experiments. This inconsistency in hue biases suggests that they are not a byproduct of selective subsampling or weighting of the stimulus elements with higher chroma or lightness. Even with simple, abstract stimuli, ensemble percepts of color are complicated by interactions between different dimensions of color variation.