The role of shape-from-shading information in the perception of local and global form in Glass patterns.

Three-dimensional (3D) shape can be inferred from the surface shading gradient of objects. Using Glass patterns, we investigated the importance of shape-from-shading information to the perception of global form. We examined whether different 3D shapes inferred from shading affect the extraction of local dipole orientations (local analysis) and the integration of dipoles in the perception of Glass structure (global analysis). In Experiment 1, we showed that incongruence in shading between partner dots prevents the recovery of the dipole orientation: partner dots with different 3D shapes are not paired to recover the dipole orientation. However, when incongruent "bipartite" partner dots (that have the same contrast polarity as shaded dots, but are two-dimensional) were used, the visual system was able to extract the local dipole orientation and detect the global pattern (Experiment 2). In Experiment 3, we showed that additional noise dipoles affect the detection of Glass structure regardless of the 3D shape difference between signal and additional noise dipoles. This demonstrates that the visual system combines different oriented 3D dipoles to detect Glass structure. Our findings show that shape-from-shading information impacts on the ability to detect form structure but in different ways at local and global stages of processing.