Influence of context on spatial expanse of color spreading in the watercolor illusion.

The watercolor illusion (WCI) occurs when a physically non-colored region surrounded by contrasting contour and fringe appears filled in with a hue similar to the fringe. The present experiments explored how local and global stimulus factors influence the spatial expanse of WCI color spreading. Experiment 1 utilized two- and three-dimensional-appearing stimuli with the WCI in only one part of each stimulus. Some conditions fully enclosed the color-spreading region with fringe on all sides. Others removed fringe from one side, opening up the color-spreading region to another part of the stimulus. Regardless of perceived dimensionality or enclosure, color did not spread beyond the fringed color-spreading region as confirmed by illusion magnitude ratings and handwritten shading. Experiment 2 consisted of transparent "wireframe" versions of the opaque-appearing stimuli used in Experiment 1. This altered the local context by adding physical contours inside the fringed color-spreading region. As in Experiment 1, color did not spread beyond physically open regions. Furthermore, illusory color filled a space bound by a combination of physical and illusory contours depending on the fringe end-cuts and other perceptual organization cues within the stimulus. Our main focus in these experiments was to determine where color spreads in a variety of contexts. Perceptual organization factors other than perceived depth seem more likely to impact the spatial expanse of WCI color spreading. These are some of the first experiments to explore the impact of changes to local and global context on the spatial expanse of the WCI.