The role of local framework luminance range in the simultaneous lightness contrast illusion with double increments

In double increment Simultaneous Lightness Contrast two equiluminant squares rest on darker backgrounds that differ in luminance. Research on such displays has produced conflicting results as to whether an illusion is observed. Anchoring theory of lightness predicts no illusion with double increment displays. Here we test the hypothesis that an illusion can be predicted if the framework containing the target and the darkest background carries more weight in lightness computations. We tested two displays, one with a general white and one with a general black background. An illusion was obtained only in the first display. These results suggest that an illusion can occur with double increment displays when the global value of the targets differs from their local value, as these conditions allow different local framework weighting to affect the targets’ final lightness. We propose that this parameter be added to the Anchoring Theory