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Chuan Hou, Anthony M. Norcia; Neural correlates of shape-from-shading. Journal of Vision 2002;2(10):130. doi: 10.1167/2.10.130.
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Shading, combined with boundary contour information provides a powerful monocular cue for three-dimensional shape. Here we used Visual Evoked Potentials to determine the time-course of boundary and shading interactions using a variant of Ramachandran's (1998, Scientific American) windowed grating stimulus. VEPs were elicited by sections of square wave grating undergoing 90 deg spatial phase shifts at 1 Hz. The gratings were windowed with a mid-gray aperture that had triangular edges which were either aligned with the black-white transitions in the grating or mis-aligned. When the boundary and the shading information where aligned, the stimulus appeared to be three dimensional and lit from the side (roof-tops). When the boundary and grating were mis-aligned, the stimulus appeared flat. In the test condition, alternation was between aligned (3-D) and misaligned states (2D) and in a control condition, the same grating motion was made between symmetrically misaligned states (2D–2D). The response to the transition from 3D and 2D was not the same as the transition between 2D and 3D in the test condition, whereas the response in the control condition was the same after each transition. The test response thus contained odd-harmonic components that were not present in the control condition. Test and control responses first differed at around 90 msec. After prolonged viewing, the test condition becomes bi-stable. In one state, the percept alternates between 2D and 3D at 1 Hz and in the other state, there is no percept of 3D. Observers sorted their EEG data depending on which state they saw and the response spectra were then calculated contingent on the percept. The 2D–3D percept led to significantly more odd-harmonic power than the 2D–2D percept, indicating that the odd-harmonics identified in the first experiment were associated with the perception of depth from convergent shape and shading information.
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