The visual system is able to infer three-dimensional (3D) shape from the surface shading-gradient of objects. Using Global Dot Motion (GDM) stimuli, we investigated the influence of shape from shading on the perception of coherent local and global motion. In Experiment 1, we report that the visual system is unable to detect the local motion of dots that undergo a change in 3D shape (convex to concave shape) from frame to frame. For this condition, GDM detection thresholds were approximately four times higher than when dots do not change shape. However, when shaded dots were perceptually two-dimensional (as with bipartite and horizontally shaded dots) GDM the visual system was able to detect the global motion regardless of a change in shading direction. Finally in Experiment 3, we demonstrated that the addition of noise dots interferes with the detection of global motion only when they have same 3D shape as signal dots. GDM detection thresholds were unaffected if additional noise dots were of the opposite 3D shape. The findings of the present study demonstrate that 3D shape from shading information impacts of GDM detection, particularly, that this depth form-cue is used as a basis for independent motion analysis at both local and global levels of processing.
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