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Peter J. Bex, Isabelle Mareschal, Steven C. Dakin; Contrast gain control in natural images. Journal of Vision 2005;5(8):597. doi: 10.1167/5.8.597.
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© ARVO (1962-2015); The Authors (2016-present)
Objective: Contrast sensitivity for grating stimuli is approximately invariant of the relative phase of components at other spatial scales (Graham & Nachmias, 1971, Vision Research, p251). In natural images however, the relative phases across scales define the distribution of structure and contrast. We examined how this property determines contrast gain control.
Methods: Contrast increments were applied exclusively to one octave spatial frequency bands within natural images. The relative alignment of the target octave was left intact or was de-correlated from the rest of the image by rotation, mirror reversal or spatial displacement, leaving the amplitude spectrum unchanged. RMS contrast increment thresholds were measured across the full contrast range as a function of spatial frequency.
Results: TvC functions were characteristically ‘dipper’ shaped and were used to infer sigmoidal contrast response functions. Contrast increment sensitivity was higher to decorrelated images by up to 4 dB and implied divisive gain control with no change in sensitivity. The magnitude of enhanced contrast sensitivity increased with the spatial displacement of the target band up to about 2 wavelengths.
Conclusions: Contrast gain control operates across spatial scales in broad band natural images, but only over a relatively limited spatial area. This property attenuates contrast responses in regions of high local contrast energy and leaves a greater dynamic range at other points in the image.
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