Comprehensive characterization of human color discrimination thresholds
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Discrimination thresholds reveal the limits of human perception; scientists have studied them since the time of Fechner in the 1800s. Forced-choice psychophysical methods combined with the method of constant stimuli or parametric adaptive trial-placement procedures are well-suited for measuring one-dimensional psychometric functions. However, extending these methods to characterize psychometric fields in higher-dimensional stimulus spaces, such as three-dimensional color space, poses a significant challenge. Here, we introduce a novel Wishart Process Psychophysical Model (WPPM) that leverages the smooth variation of threshold across stimulus space. We demonstrate the use of the WPPM in conjunction with a non-parametric adaptive trial-placement procedure by characterizing the full psychophysical field for color discrimination in the isoluminant plane. Each participant (N = 8) completed between 6,000 and 6,466 three-alternative forced-choice (3AFC) oddity color discrimination trials. The WPPM was fit to these trials. Importantly, once fit, the WPPM allows readout of discrimination performance between any pair of stimuli, providing a comprehensive characterization of the psychometric field. In addition, the WPPM readouts were validated for each participant by comparison with 25 probe psychometric functions. These were measured with an additional 6,000 trials per participant that were held out from the WPPM fit. The dataset offers a foundational resource for developing perceptual color metrics and for benchmarking mechanistic models of color processing. This approach is broadly generalizable to other perceptual domains beyond color.
Color discrimination thresholds— the smallest detectable color changes—provide a benchmark for models of color vision, guide theories of color’s perceptual representation, and inform the design of display technologies. Despite their importance, comprehensive characterization of color discrimination thresholds has been considered intractable due to the extensive time required from participants. In this study, we leverage advances in semi-parametric modeling and a non-parametric adaptive trial-placement procedure to fully characterize color discrimination in the isoluminant plane. Our results enable readouts of discrimination performance for any pair of stimuli across color space and provide a novel foundational dataset. Moreover, our framework generalizes beyond color, offering a powerful approach for characterizing perception in any domain where performance varies smoothly across the underlying stimulus space.
