Assimilation: Asymmetry Between Brightness and Darkness?

A pincushion formed by four arcs on a gray background looks darker when the arcs are black, and lighter when the arcs are white. Yet, a matching-experiment shows that this difference is relative. Whereas the apparently darker pincushion requires a matching luminance that is lower than the background luminance (i.e. assimilation), the apparently lighter pincushion curiously is also matched to a darker-than-background value (i.e. simultaneous contrast). A change-over in direction of a higher luminance occurs only at the lowest contrast. The size of the decrement required for matching the brightness of the pincushions increases with increasing contrast of the inducing stimulus, as well as with viewing distance. Assimilation is found also in the domain of color, however, only when the luminance of the colored inducers is below that of the background. Analogous asymmetries in the perception of darkness and lightness are discussed.