Parsing Pulses: Testing the Limits of Temporal Phase Perception in Human Vision

Introduction: Humans can detect luminance flicker exceeding 60 Hz, but the threshold for perceiving the flicker’s phase is much lower (~7-10 Hz). As a precursor to future experiments investigating this temporal bottleneck and the broader dynamics of visual perception, we replicated Aghdaee and Cavanagh (2007) using stimuli devoid of spatial and temporal transients. Methods: Twelve subjects judged whether two monochromatic Gaussians, oscillating sinusoidally between black and white, were in-phase or 180° out-of-phase. A 1440 Hz PROPixx projector (VPixx Technologies) displayed stimulus pairs at 4° eccentricity, spaced 1.8° or 5° apart, either: 1) left and right of the vertical meridian in the opposite-hemifield condition, or 2) above and below the horizontal meridian within the same hemifield. Using the method of constant stimuli, we measured phase detection thresholds across 11 oscillation frequencies (1–31 Hz), conducting 25 repetitions for each of the randomly interleaved conditions. To prevent visual offset artifacts, stimuli oscillated continuously until subjects responded. Thresholds were determined by fitting a cumulative normal function with a lower asymptote parameter. Results: Inter-stimulus spacing distance revealed a significant main effect, indicating subjects discriminated phase at higher frequencies for closely-spaced stimuli (11.13 Hz) than for farther stimuli (8.15 Hz). The main effect of hemifield was not significant, and no significant interaction with distance was observed. Notably, the asymptote parameter differed significantly from zero in the near condition, with subjects retaining a small (~60%) but significant ability to determine phase at even the highest frequencies tested. Conclusion: The advantage of near stimuli suggests the involvement of a low-level primary sensory mechanism, such as local motion detection circuits. In contrast, comparing two far-spaced stimuli requires higher-level (non-local) and slower mechanisms which possess timing consistent with conscious awareness. Future work should consider mechanisms such as discrete perception and onset artifacts similar to the Fröhlich Illusion.