CONTRAST DISCRIMINATION AT HIGH CONTRASTS REVEALS THE INFLUENCE OF LOCAL LIGHT ADAPTATION ON CONTRAST PROCESSING

Previous measurements of contrast discrimination threshold, Delta C, a s a function of pedestal contrast, C, for sine-wave gratings have show n a power law relationship between Delta C and C at suprathreshold lev els of C. However, these studies have rarely used contrasts greater th an 50%. Whittle (1986), using incremental and decremental patches, fou nd that Delta C increased with C only up to about 50%, At higher contr asts it decreased, Since a periodic stimulus can be considered to be c omposed of increments and decrements, we thought we might find such an inverse U-shaped function for gratings if we used contrasts up to 100 %, We tested this for both sine-wave and square-wave stimuli at spatia l frequencies from 0.0625 to 8.0 c/deg, We found that for frequencies up to 0.5 c/deg, Delta C in nearly all cases 'dipped down' after about C = 50% contrast, At 4.0 and 8.0 c/deg, however, no dip-down occurred , Additional experiments showed that the dip-down was unlikely to be d ue to cortical long-term adaptation and most likely an effect of local ized light adaptation to the dark bars, We argue that the absence of d ip-down at high spatial frequencies was mainly due to the attenuation of contrast by the optics of the eye. As for the results of Whittle (1 986), a Weber's Law in W = (L(max) - L(min))/L(min) describes the inve rse U-shaped contrast discrimination function well, Two other contrast expressions also linearize the data on log-log plots, We show how som e familiar notions about the physiological operation of localized ligh t adaptation can easily account for the form of the contrast discrimin ation function, Finally we estimate the number of discriminable steps in contrast from detection threshold to maximum contrast for the vario us spatial frequencies tested.