Luminance discrimination, color contrast, and multiple mechanisms.

The purpose of this study was to investigate the effect of color contrast on luminance discrimination. Observers were required to indicate the more intense of two stimuli presented briefly in a surround. In some conditions the two stimuli were the same chromaticity as the surround, while in other conditions the stimuli differed in chromaticity from the surround. The luminance of the fixed test stimulus was varied in different conditions over a range from below the surround level to above the surround level. Difference thresholds were proportional to the luminance difference between test and surround over much of the range. However, difference thresholds were higher at low luminance contrasts when the chromatic contrast between the stimuli and the surround was high. Results also indicate that the effects of chromatic contrast may be mediated by local contrast mechanisms, but that the relationship between threshold and luminance contrast is not mediated entirely by these local contrast mechanisms.