Color appearance of spatial patterns compared by direct estimation and conjoint measurement

Perceptual scales of color saturation obtained by direct estimation (DE) and maximum likelihood conjoint measurement (MLCM) were compared for red checkerboard patterns and uniform red squares. For the DE task, observers were asked to rate the saturation level as a percentage, indicating the chromatic sensation for each pattern and contrast. For the MLCM procedure, observers judged on each trial which of two stimuli that varied in chromatic contrast and/or spatial pattern evoked the most salient color. In separate experiments, patterns varying only in luminance contrast were also tested. The MLCM data confirmed previous results reported with DE indicating that the slope of the checkerboard scale with cone contrast levels is steeper than that for the uniform square. Similar results were obtained with patterns modulated only in luminance. DE methods were relatively more variable within an observer, reflecting observer uncertainty, while MLCM scales showed greater relative variability across observers, perhaps reflecting individual differences in the appearance of the stimuli. MLCM provides a reliable scaling method, based only on ordinal judgments between pairs of stimuli and that provides less opportunity for subject-specific biases and strategies to intervene in perceptual judgements.