PubMed · 2225273
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C Corbe. [Contrast sensitivity].. https://pubmed.ncbi.nlm.nih.gov/2225273/
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Our ability to see the world in depth is a major accomplishment of the brain. Previous models of how positionally disparate cues to the two eyes are binocularly matched limit possible matches by invoking uniqueness and continuity constraints. These approaches cannot explain data wherein uniqueness fails and changes in contrast alter depth percepts, or where surface discontinuities cause surfaces to be seen in depth, although they are registered by only one eye (da Vinci stereopsis). A new stereopsis model explains these depth percepts by proposing how cortical complex cells binocularly filter their inputs and how monocular and binocular complex cells compete to determine the winning depth signals.
Models of the dipper effect seen in contrast discrimination experiments predict that small amounts of noise should facilitate detection of a subthreshold sinusoidal grating. Although facilitation of chromatic sine waves has been measured with chromatic or luminance noise, a facilitory effect of luminance sinusoidal gratings has not been measured, most likely because the stimulus characteristics were not tuned for revealing facilitation. The present study measures contrast detection thresholds (CDTs) of sinusoidal gratings in two-dimensional, static, band-limited white noise and low-pass and high-pass filtered noise using a two-interval forced-choice paradigm. The results show facilitation in near threshold white noise of middle frequency sinusoidal gratings, and facilitation in filtered noise of sinusoidal gratings whose frequency is far outside the pass band of the noise. Based on these results, a model of contrast detection thresholds is modified such that the facilitation is attributed to reduced observer uncertainty caused by small amounts of noise.
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.