Jesse R. Macyczko¹, Lindsay Peterson², Michael A. Webster¹, Erin Goddard²; ¹University of Nevada, Reno, ²University of New South Wales

Color coding in the retina and geniculate is thought to depend on two opponent dimensions encoding the differences in LvsM or SvsLM cone signals. However, color percepts cannot be accounted for by these independent dimensions, suggesting later transformations of chromatic information. A striking example is that hues change more rapidly with chromatic angle around yellowish directions (one quadrant of the LM vs S cone-opponent plane) than bluish, magenta, or greenish quadrants. The basis for this asymmetry and where it arises in visual coding remain uncertain. We investigated the magnitude of these hue differences across different cortical regions using fMRI. Stimuli consisted of two different chromatic angles presented as circular 1 c/deg gratings in a 10-deg tapered aperture, and counterphased at 1 Hz. The component colors were equal in chromatic contrast but varied from 0 to 90 degrees in angular separation, or “hue contrast,” centered on each of the four diagonals in the LvsM and SvsLM plane. We compared three models using representational similarity analysis (RSA) to determine which models best predicted pairwise classification of the stimuli from neural responses: color quadrant, where the quadrants rather than the contrasts predicted responses; cone-opponent contrast, as predicted by the LvsM and SvsLM space; and perceived contrast, where we used behavioral measures of the hue differences to rank contrasts between quadrants. Color quadrant predicted responses similarly across all cortical ROIs. However, perceived rather than cone-opponent contrast predicted responses better in early cortical regions. This suggests that the asymmetry in hue percepts in color coding arise as early as V1.

Acknowledgements: Supported by EY-010834 to MW and the Australian Research Council (ARC) (DP220100747 to EG)