The effects of contrast on the achromatic watercolor illusion

Abstract

The watercolor illusion involves color spreading generated by patterns formed by two touching thin lines with different colors. Usually the color spreading is more salient on the side of the line that has lower contrast against the background. According to Pinna & Grossberg (J Opt Soc Am A; 2005 22(10):2207–21) the watercolor effect results from a combination of weakening of boundary signals from local competition between line representations and filling-in of illusory color across weakened boundary signals. Most previous studies of the watercolor effect used chromatic stimuli and reported color saturation matches to non-illusory stimuli. It is difficult to study the effects of contrast in a chromatic setting, so in this study, we develop a series of achromatic stimuli that have a similar configuration to classic watercolor stimuli and generate measurable achromatic watercolor effects. Further, we use a precise matching paradigm to quantitatively explore the achromatic watercolor illusion, whereby the lightness of a watercolor surface region is compared to that of a non-illusory region bounded by a single dark line. The measure of strength of the illusion is the amount of luminance change in the non-illusory stimulus that is needed to generate a lightness match to a watercolor stimulus. The achromatic watercolor effect seems to be stable with only small variability as the contrast of the lines changes greatly. Our results thus help constrain the search for possible neural mechanism underlying the phenomenon.