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Andrea Li, Qasim Zaidi; Unmasking of orientation flows in 3-D shape perception. Journal of Vision 2008;8(6):449. doi: 10.1167/8.6.449.
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© ARVO (1962-2015); The Authors (2016-present)
When a textured surface is slanted in depth, components not aligned with the slant increase in spatial frequency, and orientation flows along the slant become more visible. We examine whether this is due to a decrease in visibility of the components that increase in frequency or to a decrease in cross-orientation suppression (COS) of the orientation flows whose frequency is essentially constant. Planar surfaces textured with a 2.0 cpd horizontal grating were presented either in fronto-parallel orientation, or slanted right or left by 50 deg at a viewing distance of 1.0 m. Contrast thresholds for these gratings were measured in isolation and with the addition of full-contrast vertical gratings of either the same frequency, or 0.33 or 3.0 times the frequency. The presence of the vertical grating increased thresholds for detecting the flows, consistent with COS. However, the vertical grating of the same frequency raised thresholds 1.3 to 2.5 times more than did the gratings of three times higher or lower frequencies. Similar results were obtained for 2.0 cpd horizontal-vertical plaid surfaces slanted at 50 deg. Thus the visibility of the orientation flows that convey 3-D slant is unmasked when slanting the surface increases the frequencies of the other texture components, and this enhancement is independent of any direct effect of the decrease in visibility of these components. The results provide evidence for a COS mechanism that is frequency-selective and broadband in orientation, that suppresses responses particularly for those textures that contain similar frequencies across orientations, and the release from this masking due to surface slant gives enhanced responses for image segments where the texture provides cues to 3-D shape. The frequency selectivity of this mechanism may distinguish it from COS mechanisms attributed to contrast saturation in the LGN (Li et al. 2006; Priebe et al. 2006).
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