The effect of illumination cues on color constancy in simultaneous identification of illumination and reflectance changes.

To provide a stable percept of the surface color of objects, the visual system needs to account for variation in illumination chromaticity. This ability is called color constancy. The details of how the visual system disambiguates effects of illumination and reflectance on the light reaching the eye are still unclear. Here we asked how independent illumination and reflectance judgments are of each other, whether color constancy depends on explicitly identifying the illumination chromaticity, and what kinds of contextual cues support this identification. We studied the simultaneous identification of illumination and reflectance changes with realistically rendered, abstract three-dimensional scenes. Observers were tasked to identify both of these changes between sequentially presented stimuli. The stimuli included a central object whose reflectance could vary and a background that only varied due to changes in illumination chromaticity. We manipulated the visual cues available in the background: local contrast and specular highlights. We found that identification of illumination and reflectance changes was not independent: While reflectance changes were rarely misidentified as illumination changes, illumination changes clearly biased reflectance judgments. However, correct identification of reflectance changes was also not fully dependent on correctly identifying the illumination change: Only when there was no illumination change in the stimulus did it lead to better color constancy, that is, correctly identifying the reflectance change. Discriminability of illumination changes did not vary based on available visual cues of local contrast or specular highlights. Yet discriminability of reflectance changes was improved with local contrast and, to a lesser extent, with specular highlights, in the stimulus. We conclude that a failure of color constancy does not depend on a failure to identify illumination changes, but additional visual cues still improve color constancy through better disambiguation of illumination and reflectance changes.