Lightness contrast and failures of constancy: a common explanation.

Abstract

Observers were asked to select, from a grid of 16 achromatic Munsell chips presented on a white background in bright illumination, a sample to match a light gray chip simultaneously presented on the same white background but in a shadowed region adjacent to the brightly illuminated region. The border dividing the two fields of illumination was made to appear as either a reflectance edge or an illumination edge by either concealing or revealing the larger context. Each condition thus constituted an experiment on either contrast or constancy, allowing these two phenomena to be compared under comparable conditions. The results indicate that constancy effects are far greater than contrast effects, casting doubt on conventional reductions of the two phenomena to a single explanatory mechanism. Closer analysis of the data indicates that it may be contrast effects andfailures of constancy that share a common explanation. Such an explanation, in terms of edge-processing algorithms, is offered and is supported with additional experiments as well as a brief review of previous contrast and constancy findings.