The WPPM is equivariant under affine transformations of color space (Appendix 1.4), allowing threshold contours derived in our model space to be transformed into other colorimetric representations. In his seminal work, the ellipses do not represent discrimination thresholds per se, but rather the standard deviation of color matches for each reference stimulus. Across all panels, black contours represent thresholds from prior studies, whereas colored contours represent the 66.7% discrimination thresholds estimated in our study. In particular, thresholds were smallest near the adapting point, increased with distance from it, and the ellipses generally pointed toward the adapting point. Their experiment used a fixed adapting point and a 4AFC oddity task, with individual thresholds estimated using a three-down-one-up staircase procedure.