Faa. Kingdom et P. Whittle, CONTRAST DISCRIMINATION AT HIGH CONTRASTS REVEALS THE INFLUENCE OF LOCAL LIGHT ADAPTATION ON CONTRAST PROCESSING, Vision research, 36(6), 1996, pp. 817-829
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.