In most natural visual scenes we can easily distinguish patterns of illumination resulting from shadows, highlights and shading, from patterns of object lightness and colour. How ? Shadows have certain unique physical characteristics, and we have begun to study our visual sensitivity to them using a novel psychophysical paradigm. Human test subjects were required to detect and discriminate small rectangular targets superimposed on larger, complex, 'Mondrian-like' backgrounds. The targets simulated one or more of the figural/luminance/chromatic conditions of shadows and material surfaces. We found that a combination of three factors found in natural shadows, namely X-junctions, continuity of colour across the shadow border, and consistent polarity of luminance contrast across the shadow border, maximized shadow detectability. Our simulated shadows were slightly more easily detected, but no more efficiently discriminated from material targets, when on chromatic compared to achromatic backgrounds.
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