Jaeseon Song¹ (), Andrew Lisech², Josiane Mukahirwa³, Keith Schneider⁴; ¹University of Delaware

Introduction: Temporal phase perception, which involves comparing modulations between temporal stimuli, is significantly slower than peripheral stimulus detection, suggesting a central temporal bottleneck. While phase perception thresholds for luminance flicker (7–10 Hz) have been studied, those for chromatic flicker remain largely unexplored. The critical flicker fusion frequency for chromatic flicker (~30 Hz) is about half that of luminance flicker (~60 Hz), reflecting slower mechanisms in chromatic processing. It remains unclear whether this slower peripheral processing plays a greater role in central phase comparisons or if a general central temporal bottleneck, independent of peripheral stimulus characteristics, has a greater impact. To investigate, we measured phase discrimination thresholds for chromatic modulations across a range of temporal frequencies (TFs). Methods: Phase discrimination thresholds were measured using two chromatic circles rotating through isoluminant hue space (0–360° in HSV color space). Isoluminence was determined for R, G, and B channels using flicker photometry, and the isoluminant HSV values were calculated. Thresholds were measured across seven TFs (1–20 Hz) under two distance conditions (3° and 12°). Stimuli were displayed using a 480 Hz VPixx PROPixx projector at 4° eccentricity, with the circles positioned to the left and right of the screen center. Participants adjusted the test chromatic flicker’s TF and phase to match a reference flicker. Results: Phase error magnitude increased with TF under both distance conditions, indicating greater difficulty in phase discrimination at higher frequencies, particularly at 3–5 Hz and above. Phase error was generally lower in the near condition compared to the far condition. Conclusion: Our findings show that phase perception for chromatic modulations operates at a temporal scale slower than phase comparisons in luminance modulations, challenging the idea of a constant central temporal bottleneck. Instead, this suggests that the temporal limitations of peripheral stimuli play an important role for central phase comparisons.