Rapid processing of cast and attached shadows.

We used a visual-search method to investigate the role of shadows in the rapid discrimination of scene properties. Targets and distractors were light or dark 2-D crescents of identical shape and size, on a mid-grey background. From the dark stimuli, illusory 3-D shapes can be created by blurring one arc of the crescent. If the inner arc is blurred, the stimulus is perceived as a curved surface with attached shadow. If the outer arc is blurred, the stimulus is perceived as a flat surface casting a shadow. In a series of five experiments, we used this simple stimulus to map out the shadow properties that the human visual system can rapidly detect and discriminate. To subtract out 2-D image factors, we compared search performance for dark-shadow stimuli with performance for light-shadow stimuli which generally do not elicit strong 3-D percepts. We found that the human visual system is capable of rapid discrimination based upon a number of different shadow properties, including the type of the shadow (cast or attached), the direction of the shadow, and the displacement of the shadow. While it is clear that shadows are not simply discounted in rapid search, it is unclear at this stage whether rapid discrimination is acting upon shadows per se or upon representations of 3-D object shape and position elicited by perceived shadows.