Mechanisms underlying fixation-dependent purple perception beyond the S-cone-free zone

Determine the physiological and perceptual mechanisms, beyond the S-cone–free zone of the human fovea, that account for the fixation- and distance-dependent color illusion where purple stimuli on a blueish background are perceived as purple only at fixation and bluish in the periphery, and explain why the effective area of purple perception expands with viewing distance despite the S-cone–free zone being approximately 0.4° in diameter.

Background

The paper introduces a reproducible optical illusion in which purple elements presented on a blueish background appear purple only at the point of fixation, while identical elements in the visual periphery appear bluish. As viewing distance increases, more elements are perceived as purple, eventually approaching uniform purple perception across the pattern.

While the author posits that classic color contrast, the non-spectral nature of purple (requiring combined L- and S-cone signals), and the biological distribution of cones (including the absence of S-cones in the very center of the fovea) likely contribute, the observed spatial extent of the effect with distance does not align with the known size of the S-cone–free zone (~0.4°). This discrepancy suggests additional mechanisms are involved, which remain to be identified and characterized.