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 Watercolo r Effect), This coloration is uniform and complete within 100 ms. We found that thin (6 arcmin). winding inducing lines with different contrasts to th e ground are generally more effective than thick, straight. and equiluminan t lines. Blue and red lines induce the strongest effects, but watercolor sp reading may also be seen with green and yellow. On a white background, colo r spreading is stronger than on chromatic. gray or black backgrounds. Littl e or no color is perceived when a narrow white zone (gap) is inserted in be tween the two inducing lines. However, chains of colored dots instead of co ntinuous lines suffice to produce spreading. Edge-induced color is also obs erved 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 sprea ding may arise in two steps: First, weakening of the contour by lateral inh ibition between differentially activated edge cells (local diffusion); and second, unbarriered flow of color onto the enclosed area (global diffusion) . (C) 2001 Elsevier Science Ltd. All rights reserved.