SHADING AND STEREO IN EARLY PERCEPTION OF SHAPE AND REFLECTANCE

Recent experiments involving shaded 2-D stimuli have shown that early- vision mechanisms are capable of interpreting 3-D shape from shading. Tn particular, target discrimination tasks suggest that a target 'pops out' when background distracters, but not the target, can be interpre ted as convex and lit from above or top-left. Since the problem of ext racting 3-D shape from shading is intrinsically ill-defined, early vis ion may need to make these twin assumptions of convexity and top-left lighting in order to constrain the problem. Would these assumptions be recognized as unnecessary and consequently discarded when 3-D shape c ould be unambiguously defined by some other cue, like stereo disparity ? A 2AFC stimulus onset asynchrony paradigm with masking was used in t arget discrimination experiments. The performance of five naive subjec ts on tasks where only shading cues were present was compared with tha t on tasks involving shading as well as stereo cues that define shape unambiguously. The results show that although stereo disparity informa tion is incorporated by early-vision 3-D mechanisms, it is not used to overturn the default assumptions of lighting and shape. Stereo inform ation is interpreted within the framework of top-left lighting, and a consistent preference for convexity is seen over concavity.