Discrimination of Reflectance Changes under Simultaneous Illumination Changes

Relational colour constancy refers to the perception of constant relations among surface colours under illumination changes. We tested whether the ability of human observers to discriminate illumination from reflectance changes in a scene involves high-level attentional mechanisms. Observers were shown pairs of computer-generated Mondrian patterns with the number of patches varying between 2 and 49. On each trial a Mondrian pattern was presented for 1 s and replaced by a second pattern. In half of the trials in each experimental condition, the chromaticity of one patch was changed. Observers were required to discriminate whether a reflectance change occurred. Three conditions were run. In condition A, the illumination of the second Mondrian underwent a chromaticity change. In condition B, the chromaticity of the second Mondrian illumination remained constant, whereas its overall level was either increased or decreased. In condition C (control), the illumination was exactly the same in both Mondrians. Functions relating reaction time to number of patches in the display were compared. Data suggest that different mechanisms are involved in discriminating reflectance changes associated with different types of illuminant changes.