Iris Wright¹, Tiffany Little¹, Ralph Hale¹; ¹University of North Georgia

The watercolor illusion (WCI) is a visual phenomenon in which two thin, contrasting borders create the appearance of color spreading into an adjacent region where no physical color is present. Previous research has shown that strong luminance differences between regions can compete with the WCI and disrupt its influence on F/G perception (Hale et al., 2025). The present study investigated whether the WCI would continue to bias F/G perception when luminance gradients extended beyond the contours of the stimulus, or whether luminance would dominate as the stronger cue under these conditions. Participants viewed six reversible figures, each containing left and right regions with luminance gradients traveling upward, downward, leftward, or rightward. Each stimulus appeared with the WCI on the left, right or absent, and included a probe on the left or right region. Participants indicated which region they perceived as the figure. This design allowed us to determine whether extended luminance gradients would affect the WCI’s ability to influence F/G organization. Contrary to earlier findings, luminance gradients did not significantly influence F/G perception in this study. Instead, the WCI continued to bias F/G judgments as though no luminance contrast was present. We speculate that the gradients may have been perceptually discounted as illumination cues: although they produced physical left-right luminance differences (bottom-up), observers may have unconsciously inferred that the gradients indicated lighting cues rather than surface properties (top-down), effectively treating the two regions as equivalent. This would allow the WCI to exert its typical influence on F/G organization. Stimuli without the WCI were perceived as reversible regardless of gradient direction, whereas stimuli containing the WCI consistently biased perception toward the side containing the illusion. These findings challenge assumptions about the interaction between luminance and the WCI and contribute to a clearer understanding of how perceptual heuristics shape F/G organization.