Simultaneous lightness contrast with double increments.

In this paper we demonstrate the existence of simultaneous lightness contrast in displays in which the target patches are both more luminant than their surrounds. These effects are not predicted by theories of lightness that assume that the highest luminance in a scene is perceived as white, and anchors all the other luminances. We show that the strength of double-increment illusions depends crucially on the luminance of both the surrounds and the target patches. Such luminance prerequisites were not met in previous studies, which explains why simultaneous contrast with incremental targets has so far been regarded as extremely weak or nonexistent.