CONTRAST OVERCONSTANCY

In foveal vision there is a tendency toward contrast constancy above threshold. Contrast sensitivity varies greatly across different spatial and temporal frequencies and at different luminances, but perception of contrast above threshold varies much less (in logarithmic terms). In parafoveal vision (1-2-degrees eccentricity) the contrast of high-spatial-frequency gratings appeared higher than a 3-cycle per degree (c/deg) comparison grating, even though their threshold sensitivity was lower. I call this overconstancy. For two experienced observers, the effect was greatest at high spatial frequencies (12-18 c/deg) and at contrasts close to threshold. A simple model of contrast coding was able to fit all the data accurately: R' = [(C - T)/ (C(norm) - T)]m, where R' is the final response, C is contrast, T is threshold contrast, and C(norm) and m are parameters. C(norm) represents the contrast at which different response functions converge to a common value (R' = 1), while m is the slope of the upper branch of a given function on logarithmic axes. This model, incorporating threshold subtraction, nonlinear compression, and response normalization, was derived from research on contrast matching, magnitude estimation, and discrimination. On this model overconstancy results when C < C(norm), and the contrast response for the test condition is more compressive (lower m) than in the comparison condition. In the parafovea m decreased linearly with increasing spatial frequency for both subjects, but C(norm) was approximately constant at 40-50%.