Kara Wang¹, Cameron Zhang¹, Jason Chow², Philip Guan², Alex Williams³, David H. Brainard¹, Fangfang Hong¹; ¹Department of Psychology, University of Pennsylvania, ²Reality Lab Research, Meta, ³New York University, Center of Neural Science

Color discrimination thresholds—the smallest perceptible differences between colors—have been extensively explored in trichromats, but remain less well-characterized in dichromats. Measuring these thresholds provides foundational data to constrain mechanistic models of color vision and can inform the design of color-rendering algorithms that make visual information more accessible to dichromatic populations. We recruited two deuteranope participants and measured their color discrimination thresholds in an LS cone-isolating plane. Participants performed a three-alternative forced-choice oddity task in which three matte, blobby stimuli were presented for 1 s: two shared the same surface color, one differed, and participants reported the odd one out. Reference and comparison stimuli were selected using a non-parametric adaptive procedure targeting 66.7% correct performance. Each participant completed 7,500 trials, along with additional fallback trials inserted when the adaptive procedure timed out. We fit a semi-parametric Wishart Process Psychophysical Model (WPPM) to the data for each participant. Once fit, the model enables prediction of discrimination performance for any pair of reference and comparison stimuli, effectively characterizing the entire psychometric field in the LS cone-isolating plane. We then compared threshold contours with those obtained from a larger trichromat dataset collected over the full 3D RGB cube, from which thresholds in the LS plane were read out by slicing the fitted psychometric field. The comparison revealed three key regularities across participants: (1) thresholds along the S-cone contrast were comparable between deuteranopes and trichromats, (2) thresholds along the L-cone contrast were substantially larger in deuteranopes, and (3) the major and minor axes of threshold ellipses in deuteranopes align roughly with S+L and S-L, respectively. Together, these results suggest that color discrimination in deuteranopes is governed by an additive mechanism and an opponent mechanism constructed from the available L and S cone signals, with greater sensitivity along the S-L opponent direction than along the additive direction.