Symmetry facilitates shape constancy for smoothly curved 3D objects.

We tested whether the presence of symmetry improves shape discrimination across changes in viewpoint and lighting for smoothly curved 3D objects. We constructed symmetric and asymmetric versions of random 3D shapes by manipulating their spherical harmonic representations. Matched objects had the same power spectra and appear highly similar except for the presence of symmetry. Observers discriminated sequentially presented pairs of either symmetric or asymmetric objects. Objects were presented in conditions that provided different 3D cues: shading only, stereo only, and combined shading and stereo. To control for 2D cues, standard and test objects had matched boundary contours and were rendered with different light sources. Test objects were also rotated in depth by variable amounts (0° to 60°). Across all viewpoint and 3D cue conditions, we found that shape discrimination for symmetric objects was better than for asymmetric objects. The symmetry benefit was not limited to monocular viewing or to conditions with large rotations in depth. In a second experiment, we blocked trials by viewpoint rotation to eliminate uncertainty in object orientation. This improved performance for asymmetric objects relative to symmetric objects, suggesting that symmetry contributes by providing a cue to object orientation. However, a symmetry advantage was still observed in all shape cue conditions, so this was not the sole source of benefit. Our results demonstrate that symmetry improves shape constancy for smooth 3D objects and suggest that one role of symmetry is to provide a reference orientation for an object.