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Kyle McDermott, Saloni Sharma, Michael Webster; Adaptation and contrast constancy in natural images. Journal of Vision 2007;7(9):267. doi: https://doi.org/10.1167/7.9.267.
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© ARVO (1962-2015); The Authors (2016-present)
Many perceptual judgments have a well defined norm that can be biased in predictable ways by adaptation, so that the adapting stimulus appears more neutral and thus induces a negative aftereffect in the original neutral stimulus. We examined how adaptation affects the norm for judgments of contrast in natural images, in order to examine how perceived contrast is calibrated by experience and how norms are established for intensive dimensions like contrast that do not have a qualitatively distinct perceptual null (e.g. as in color or motion). Stimuli were grayscale images of natural scenes with contrast titrated over a wide range. The level in successive images was varied in a forced-choice staircase to find the subjectively correct image, with settings repeated before or after adaptation to the same images at high (150%) or low (50%) contrasts. Subjects could reliably set the images to appear natural and thus had a well defined norm. Surprisingly, adaptation to the 50% contrast images shifted this norm to lower contrasts relative to the pre-adapt settings. That is, the original images appeared to have a higher contrast after adapting to their low-contrast versions than to a zero-contrast field, even though adaptation to any contrast in simple gratings reduces rather than increases perceived contrast (Georgeson, 1985). To directly test for changes in contrast sensitivity in the images, settings were repeated with an asymmetric matching task in which an unadapted image was adjusted to equal the perceived contrast in the adapted image. This showed only a weak effect for the 50% adapt level, implying that the renormalization may primarly reflect short-term shifts in criterion rather than sensitivity. These shifts may be important for understanding how visual appearance is recalibrated for changes in viewing conditions or the observer (cataracts) that alter contrast in the retinal image.
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