Lightness contrast and failures of constancy: a common explanation.

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 reflec-tance 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 and failures 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. Lightness constancy and simultaneous lightness contrast (SLC) have traditionally occupied central roles in the field of lightness perception. Theories have sought not only to relate these two phenomena within the same theoretical structure, but to explain them with the same principle (Freeman, 1967). Constancy and contrast illustrate, in different ways, that a given luminance will vary in lightness (perceived surface reflectance) when it is presented in different contexts. This lack of correlation between lu-minance and lightness has always seemed significant because of the assumed fundamental role of luminance in Historically, theories of lightness perception have differed, not on the importance of luminance information, but on the role of the context in altering the basic response to luminance. Helmholtz (18681 1962), for example, suggested that the luminance of a surface is meaningless until it is placed (by means of an unconscious inference) within a framework of perceived illumination, and that contrast effects involve the inappropriate use of such frameworks. Helson (1964) operationalized Helmholtz's framework as the weighted average of all luminances in the visual field. The average replaced Helmholtz's cog-nitive judgment and the weighting handled contrast effects. Wallach (1948) put the luminance of a surface into a ratio with the luminance of the surrounding region as a variable to account for contrast as well as constancy effects. The direct linkage of contrast and constancy effects reaches its sharpest …