The effect of luminance gradients on lightness constancy

The term lightness constancy describes the ability of the visual system to perceive surface colour correctly despite changes in the level of illumination. It has been shown that smooth luminance changes are strong cues of changes in the level of illumination. In the present work, we simulated a classical-lightness-constancy-type display on a CRT monitor (Bruno, 1994 Vision Research 17 2205 - 2214) where a smooth luminance gradient was presented at the border dividing the lower from the higher luminances side of the display. Observers had to perform a matching task in a within-subject design: 3 luminance ratios (30:1, 10:1, 2:1) × 4 conditions, ie gradient throughout the whole display dividing border ('congruent' condition), gradient throughout the inner background dividing border only (first 'incongruent' condition), gradient throughout the outer background dividing border only (second 'incongruent' condition), and no gradient (control condition). For all the tested ratios and compared to the control condition, we found that congruent condition holds better lightness constancy, while the two incongruent conditions show only little failures of constancy. Congruent condition increases lightness constancy. However, since the loss of constancy in the incongruent conditions is modest, it seems that the visual system considers smooth luminance gradients as an important source of information on illumination changes, even when edge cues are partially incongruent.