Comparison of sensitivity to color changes in natural and phase-scrambled scenes

Traditionally, thresholds for detecting photometric changes have been measured by using stimuli such as disks or gratings and accounted for in terms of relatively low-level mechanisms in the visual pathway. Therefore one might not expect the higher-order structures that characterize natural scenes to influence thresholds for detecting uniform photometric changes. We compared thresholds for detecting uniform photometric changes for natural and phase-scrambled versions of images of natural scenes. The chromaticity and luminance of every pixel was represented as a vector in a modified version of the MacLeod-Boynton color space and was translated, rotated, or compressed within that color space. Thresholds for all types of transformation were significantly lower in the raw compared with phase-scrambled scenes, and we attribute this to the influence of higher-order structure.