Impact of luminance gradients on illusory color spreading and figure/ground organization

The watercolor illusion (WCI) is a visual phenomenon in which two thin, contrasting borders create the appearance of color filling an area where no color exists. This illusion occurs because the visual system uses perceptual heuristics, to achieve surface completion and figure/ground (F/G) organization. F/G organization refers to how the visual system distinguishes an object (the “figure”) from its background (the “ground”), while surface completion describes how the brain fills in missing information to perceive a complete and unified object. Previous research on the WCI has shown that strong differences in luminance can compete with this illusion, disrupting its influence on perception (Hale et al., 2025). The present study investigated whether the WCI would continue to influence F/G perception when luminance gradients extended beyond the contours of the stimulus, or whether luminance would dominate as the stronger cue. Participants were shown six reversible figures that each contained a left and right side with luminance gradients traveling upward, downward, leftward, or rightward. Each stimulus appeared with or without the WCI and included a probe on either the left or right side. Participants were asked to indicate which side they perceived as the figure. Contrary to earlier findings, luminance did not significantly affect F/G organization in this study. Instead, the WCI continued to bias F/G perception as though no luminance contrast were present. Stimuli without the WCI were perceived as reversible regardless of gradient direction, whereas stimuli with the WCI consistently biased perception toward the side containing the illusion. These findings challenge assumptions about the interaction between luminance and the WCI and provide new insight into how perceptual heuristics shape F/G organization. This research contributes to a broader understanding of how the visual system simplifies complex scenes to create our perception of the world.