Hannah Hyman¹, Tanner Lumpkin, Ralph Hale; ¹University of North Georgia

The watercolor illusion (WCI) is a visual phenomenon where a thin colored contour adjacent to a second darker contour induces color spreading, causing the enclosed region to appear tinted and more figure-like. Previous research has shown that the WCI can bias figure-ground (F/G) organization, with the WCI region typically seen as figure. This effect has also been observed to vary with luminance. In the present study, we further examined the interaction between WCI and luminance effects on ambiguous F/G organization. Unlike prior studies, we controlled for contrast by equalizing it between the background and possible figure regions, aiming to isolate the effects of luminance and illusion strength on F/G assignment. The WCI is known to be less visible on dark (black) backgrounds than on light (white) backgrounds, prompting us to investigate whether this difference in illusion magnitude affects F/G organization. We conducted three experiments using ambiguous F/G stimuli with central shapes composed of two regions in different luminance combinations: (1) white and white, (2) black and black, and (3) white and black. A mid-gray screen surrounded each stimulus. Each experiment included four conditions: no WCI, WCI on the left, WCI on the right, and WCI on both. A probe appeared in either the left or right region, and participants indicated if the probed region appeared as figure. Our findings reveal interactions between WCI and luminance on F/G assignment. This research is the first of its kind to explore color spreading and luminance in this manner, contributing novel insights into the role of luminance and the WCI on perceptual organization.