Luminance contrast affects motion coherency in plaid patterns by acting as a depth-from-occlusion cue

Moving plaid patterns composed of component gratings that differ in luminance contrast tend not to cohere perceptually. Plaid patterns configured to mimic one occlusive grating overlying another also fail to cohere. We hypothesized that plaids constructed of components with different luminance contrasts fail to cohere because these components are interpreted as occlusive surfaces lying in different depth planes. It is known that when depth-from-occlusion and depth-from-binocular disparity cues support the same depth-ordering, both segregation in depth and motion non-coherency are more likely to be perceived than when these two cues conflict. We exploited this interaction and tested our hypothesis by introducing horizontal binocular disparity between two superimposed component gratings of different luminance contrasts. We found that both depth segregation and motion non-coherency were much more likely when the high-contrast grating was stereoscopically in front of the low-contrast grating. From these results we infer that luminance contrast acts as a depth-cue in plaid patterns, with higher contrast gratings appearing to lie in front of lower contrast gratings. Perceptual motion coherency parallels these depth-ordering judgments. We conclude that luminance contrast affects motion coherency by acting as a depth-from-occlusion cue.