Evidence for patchwork approximation of shape primitives.

Investigators have proposed that qualitative shapes are the primitive information of spatial vision: They preserve an approximately one-to-one mapping between surfaces, images, and perception. Given their importance, we examined how the visual system recovers these primitives from sparse disparity fields that do not provide sufficient information for their recovery. We hypothesized that the visual system interpolates sparse disparities with planes, resulting in a patchwork approximation of the implicitly defined shapes. We presented observers with stereo displays simulating planar or smooth curved surfaces having different curvatures. The observers' task was to detect whether dots deviated from these surfaces or to discriminate planar from curved or planar from scrambled surfaces. Consistent with our hypothesis, increasing curvature had detrimental effects on observers' performance (Experiments 1-3). Importantly, this patchwork approximation leads to the recovery of the proposed shape primitives, since observers were more accurate at discriminating planar-from-curved than planar-from-scrambled surfaces with matched disparity range (Experiment 4).