Surface color from boundaries: a new ‘watercolor’ illusion

Abstract

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).

Spatial Dependence of Color Assimilation by the Watercolor Effect

“…Others have argued against a substantial contribution of aberration (Fach and Sharpe 1986;Moulden et al 1993). This difference is reflected in studies suggesting that assimilation is present (Pinna et al 2001;Monnier and Shevell 2003) or absent (Broerse et al 1999) depending on whether or not stimuli were observed through an achromatizing lens. Our data show that chromatic aberration cannot account for the WCE in the wider stimuli that have been used in previously published work.…”

“…Pinna et al (2001) pointed out that the induced color in the WCE continues to be seen up to about 45 deg. Our results show that with large stimuli the effect is present with an achromatizing lens.…”

Using afterimages for orientation and color to explore mechanisms of visual filling-in

“…Illusions such as neon color spreading and the water-color illusion (da Pos & Bressan, 2003;Pinna, Brelstaff, & Spillmann, 2001) have been taken as direct evidence of filling-in. Filling-in processes have also been invoked to explain a variety of other percepts, including brightness perception (Grossberg & Todorović, 1988;Todorović, 1987), properties of McCollough afterimages (Broerse, Vladusich, & O'Shea, 1999;Grossberg, Hwang, & Mingolla, 2002), properties of color complement afterimages (Shimojo, Kamitani, & Nishida, 2001), and some aspects of 3-D perception (Grossberg, 1997).…”

Illusory colors promote interocular grouping during binocular rivalry

“…One trick, for example, creates illusory colors by rapidly flickering specially configured achromatic contours (see, e.g., Benham, 1894), and another creates illusory colors within the uncolored interior of a figure defined by chromatic boundaries (see, e.g., Pinna, Brelstaff, & Spillmann, 2001). While fascinating in their own right, illusory colors also offer an effective means for dissociating physical stimulation from perceptual experience.…”