Abstract
Previous studies have shown that humans can discriminate spectral changes in illumination and that this sensitivity depends both on the chromatic direction of the illumination change and on the ensemble of surfaces in the scene. These studies, however, always used stimulus scenes with a fixed surface-reflectance layout. Here we compared illumination discrimination for scenes in which the surface reflectance layout remains fixed (fixed-surfaces condition) to those in which surface reflectances were shuffled randomly across scenes, but with the mean scene reflectance held approximately constant (shuffled-surfaces condition). Illumination discrimination thresholds in the fixed-surfaces condition were commensurate with previous reports. Thresholds in the shuffled-surfaces condition, however, were considerably elevated. Nonetheless, performance in the shuffled-surfaces condition exceeded that attainable through random guessing. Analysis of eye fixations revealed that in the fixed-surfaces condition, low illumination discrimination thresholds (across observers) were predicted by low overall fixation spread and high consistency of fixation location and fixated surface reflectances across trial intervals. Performance in the shuffled-surfaces condition was not systematically related to any of the eye-fixation characteristics we examined for that condition, but was correlated with performance in the fixed-surfaces condition.
Highlights
The visual system receives information about the environment when illumination from the light sources reflects off of objects and reaches the eye
The difference between the test illuminations is much easier to see in the fixed-surfaces condition than in the shuffled-surfaces condition. This is consistent with the results we present below: Illumination discrimination thresholds were higher in the shuffled-surfaces condition than in the fixed-surfaces condition
We previously established that when the surfaces in the scene remain fixed, illumination discrimination thresholds fall well within the range of illumination variations we used (Radonjicet al., 2016)
Summary
The visual system receives information about the environment when illumination from the light sources reflects off of objects and reaches the eye. Variations in illumination—both over time and across space—are ubiquitous in natural scenes (Nascimento, Amano, & Foster, 2016; Spitschan, Aguirre, Brainard, & Sweeney, 2016) and can dramatically modulate the light reflected from objects and, the information available to vision about their physical properties, such as surface reflectance. For this reason, the visual system adjusts its processing of the retinal image across changes in illumination, to maintain relatively stable perception of object colors (Brainard & Radonjic, 2014).
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