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Jeffrey Saunders, Young Lee; Symmetry facilitates 3D shape discrimination across changes in viewpoint. Journal of Vision 2011;11(11):55. doi: 10.1167/11.11.55.
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© ARVO (1962-2015); The Authors (2016-present)
Perceiving 3D shape constancy across changes in viewpoint is a challenging problem. Some researchers have argued that the presence of symmetry could allow viewpoint invariant perception of 3D shape, even from a monocular image (Vetter et al., 1994; Pizlo & Stevenson, 1999). We tested whether symmetry can facilitate shape discrimination for smoothly-curved, solid 3D objects. Symmetric and asymmetric random shapes were generated to have the same angular power spectra, expressed as spherical harmonics. Objects were presented in three viewing conditions: shading-only, stereo-only, and combined shading and stereo. Shaded objects were illuminated by a diffused point light source, and a mirror haploscope was used to present binocular images. Objects were presented sequentially and observers judged whether the two shapes were same or different. The test objects were either the same orientation or differed by a rotation in depth of ±15°, ±30°, or ±60° relative to the standard object. In all conditions, we found that shape discrimination performance decreased with rotation in depth. Overall, shape discrimination for symmetric objects was better than for asymmetric objects. We also observed an interaction between symmetry and viewing condition. For asymmetric objects, binocular viewing provided a significant benefit, while for symmetric objects, performance in monocular and binocular conditions was equivalent. The results suggest that structural constraints like symmetry are important for viewpoint invariant perception, and that stereo information may not be essential for shape perception of structured 3D objects.
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