Yangyi Shi¹, Rhea T. Eskew, Jr.¹; ¹Northeastern University

Human visual perception exhibits distinct asymmetries in the processing of increments and decrements. Psychophysical studies show that perceptual scales of achromatic and S-cone increments follow a saturating curve, while their decrements both follow a reversed, inverted S-shaped curve; discrimination thresholds on increment pedestals are higher than on decrement pedestals (Shi & Eskew, 2024, 2025, Gabree, Shepard & Eskew, 2018), and field spectral sensitivities are different for S increment and decrement tests (McLellan & Eskew, 2000). The present study investigates whether these psychophysical polarity differences could be related to natural image statistics. Hyperspectral images of natural and urban scenes (Cauwerts & Jost, 2019; Nascimento et al., 2016) were analyzed to characterize the distribution of cone response contrasts. Spectral data were converted to L, M, and S cone excitations, and then filtered with Difference of Gaussians (DoG) using different spatial scales for each type of cone to simulate cone response contrast. Post-receptoral channel responses were then calculated using weighted sums of cone contrasts. The analysis revealed a consistent statistical dominance of decrements over increments across L, M, and S cones, at multiple spatial scales. In post-receptoral channels, the Red-Green channel showed a prevalence of "greenish" over "reddish" responses, while the Yellow-Blue channel showed a prevalence of "yellowish" over "bluish" responses, with few exceptions. Both Luminance (L+M) and Achromatic (L+M+S) channels exhibited a strong bias toward decremental responses. These results align qualitatively with previous psychophysical asymmetries. The observed decrement dominance may be attributed to the physics of material reflectance: objects typically exhibit low reflectances due to light-absorbing pigments (Richards, 1982). Furthermore, consistent with the efficient coding hypothesis, the high sensitivity observed at low contrasts for both polarities in psychophysics can be explained by the prevalence of low-contrast content in the natural environment.

Acknowledgements: NSF BCS-2239356