The brightness that results from stimulating a particular test-region of the retina may be depressed or enhanced by simultaneous stimulation of other "inducing"-regions. The test-region brightness may be affected by contiguous inducing-regions (local contrast effects), and by non-contiguous inducing regions (long-range effects sometimes called "assimilation"). We describe a computational model for early vision that can predict the results of brightness-matching procedures commonly used to measure these phenomena. According to this model, brightness depression reflects primarily lateral inhibition that underlies local contrast effects; whereas brightness enhancement results from processes similar, in spirit, to those described in Helson's adaptation-level theory.
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