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Tandra Ghose, Stephen Palmer; Gradient cut alignment: A cue to ground in figure-ground and depth perception. Journal of Vision 2007;7(9):903. doi: 10.1167/7.9.903.
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In a previous study of figure-ground effects due to the presence of shading gradients at extremal edges (Palmer & Ghose, VSS 2006), we found that when edges were parallel to the equiluminance contours of the shading gradient (i.e., at extremal edges), perception was very powerfully biased toward seeing the gradient-side as closer and figural. ; We also found that when the edges were misaligned with the gradients, perception seemed to be biased toward seeing the gradient-side as farther and ground. ; The goal of the present research was to systematically study this cue to ground arising from gradient cuts: edges that are not parallel to the equiluminance contours of a shading gradient. ; We varied the nature of the shared edge between bipartite displays, one side of which contained a simple sinewave-like shading gradient (typically seen as a series of cylinders lying side-by-side) and the other side of which was a homogeneous gray (seen as a solid flat surface). ; Although the angle of a straight edge relative to the equiluminance contours of the gradient had surprisingly little effect, more complex edge shapes containing turning points (e.g., triangular and sinusoidal edges) produced large and systematic figural/depth effects that depended on the alignment between the turning points and the shading gradient. ; In particular, the gradient side was perceived as closer when deep concavities in the gradient side of the edge were aligned with luminance minima in the gradient, but the flat side was perceived as closer when they were aligned with luminance maxima. ; Further experiments with asymmetrical triangular-wave edges demonstrated that alignment with luminance minima is crucial: Y-junctions with the luminance minima bias perception toward the gradient side being seen as figural, and arrow-junctions bias perception toward the gradient side being seen as ground, with T-junctions having an intermediate effect.
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