Abstract
The watercolor illusion (WCI), a color spreading illusion induced by contrasting outer and inner borders, results in a perception of a pale illusory diffusion of a hue similar to the lighter border. This illusion is a strong figural cue similar to, and often stronger than, other Gestalt cues for figure-ground organization. Our previous research has demonstrated the WCI can bias reversible figure-ground stimuli, like Rubin’s faces-vase. This research used a white background as the color spreading surface. A follow-up study found poor color spreading on contrasting gray and white backgrounds. Past research has demonstrated that the WCI is reduced in illusion magnitude on all but white backgrounds including shades of gray, other colors, and black backgrounds. In our present study, we matched contrast within our color spreading at a non-white luminance in our ambiguous figure-ground regions in order to demonstrate the ability of the WCI to still bias figure-ground assignment on non-white backgrounds. Images consisted of a centrally located light gray square on a dark gray background, with each square divided into two parts by a vertical wavy contour. Each of these images had no WCI, WCI left, and WCI right conditions. Participants reported whether the left or right region appeared to be the figure. Results showed a large effect of the WCI with significant pairwise comparisons between all three WCI conditions. This study supports previous work by Folds, Hale, and others suggesting the WCI acts as a strong figural cue and is able to bias reversible stimuli. Additionally, this study is the first of its kind to show ambiguous figure-ground stimuli can be biased on a non-white background using a color spreading illusion like the WCI. This expands our understanding of the interaction between luminance contrast, color spreading mechanisms, and perceptual organization.