Afterimage watercolors: an exploration of contour-based afterimage filling-in

We investigated filling-in of colored afterimages and compared them with filling-in of "real" colors in the watercolor illusion. We used shapes comprising two thin adjacent undulating outlines of which the inner or the outer outline was chromatic, while the other was achromatic. The outlines could be presented simultaneously, inducing the original watercolor effect, or in an alternating fashion, inducing colored afterimages of the chromatic outlines. In Experiment 1, using only alternating outlines, these afterimages triggered filling-in, revealing an "afterimage watercolor" effect. Depending on whether the inner or the outer outline was chromatic, filling-in of a complementary or a similarly colored afterimage was perceived. In Experiment 2, simultaneous and alternating presentations were compared. Additionally, gray and black achromatic contours were tested, having an increased luminance contrast with the background for the black contours. Compared to "real" color filling-in, afterimage filling-in was more easily affected by different luminance settings. More in particular, afterimage filling-in was diminished when high-contrast contours were used. In the discussion we use additional demonstrations in which we further explore the "watercolor afterimage." All in all, comparisons between both types of illusions show similarities and differences with regard to color filling-in. Caution, however, is warranted in attributing these effects to different underlying processing differences.