Surface color from boundaries: a new 'watercolor' illusion

A colored line flanking a darker border will appear to assimilate its color onto the enclosed white area over distances of up to 45 deg (the Watercolor Effect). This coloration is uniform and complete within 100 ms. We found that thin (6 arcmin), winding inducing lines with different contrasts to the ground are generally more effective than thick, straight, and equiluminant lines. Blue and red lines induce the strongest effects, but watercolor spreading may also be seen with green and yellow. On a white background, color spreading is stronger than on chromatic, gray or black backgrounds. Little or no color is perceived when a narrow white zone (gap) is inserted in between the two inducing lines. However, chains of colored dots instead of continuous lines suffice to produce spreading. Edge-induced color is also observed when the two colored lines are presented dichoptically, suggesting a cortical origin. The Watercolor Effect described here may serve to enhance figure-ground segregation by imparting surface color onto the enclosed area, and to promote grouping between distant stimulus elements. As a grouping factor, watercolor coloration wins over proximity. Assimilative color spreading may arise in two steps: First, weakening of the contour by lateral inhibition between differentially activated edge cells (local diffusion); and second, unbarriered flow of color onto the enclosed area (global diffusion).