Spatial Filtering Underlies Brightness Phenomena and Color Constancy

In simultaneous brightness contrast (SBC), identical mid-luminance disks appear non-identical when one is placed on a black background and one on a white background. The strength of SBC can be enhanced when a semi-transparent layer is placed over the display (Meyer's effect). In the first study, we try to separate the causes of Meyer's effect by placing a spatially homogenous transparent layer over a standard SBC display, varying the alpha and color of the layer, and measuring SBC strength with matching and ranking procedures. Paradoxically, with black layers, increasing alpha level weakens SBC when measured with a ranking procedure and strengthens SBC when measured with a matching procedure. With white and gray layers, neither procedure produces Meyer's effect. We account for the differences between white and black layers by positing that the visual system separates luminance from contrast via high-pass spatial filtering. In the second study, we examine a variation of SBC and transparency with the checker-shadow illusion, by varying the alpha of a black shadow surrounding a variablegrayscale test checker. Observers selected which checker appeared whiter between each test checker (always equal to less than 255) and a standard white checker (255). A variety of alpha levels create a "whiter than white" effect, where the test checker appears whiter than the standard. We account for this result with spatial filtering, which produces physical values matching observer responses, and suggest that filtering be brought into the anchoring model. Our third set of studies investigates the color-changing dress, an internet phenomenon in which a dress is reported as blue-black by some observers and white-gold by others. One explanation is that observers make different inferences about the lighting followed by a best guess about the reflectance of the dress. This is based on the idea that reflectance alone remains invariant under changes in lighting conditions. Here we demonstrate an alternate type of invariance across illumination conditions: an object that appears to vary in color under changing illumination does not change color in the high spatial frequency region. A first approximation to color constancy can therefore be accomplished by a high-pass filter. The studies presented here demonstrate thatspatial filtering is able to account for many variations in transparency, shadows, and illumination both in illusions and natural scenes, without recourse to cognitive interpretations of visual stimuli.