Luminance contrast and JND threshold show conscious and unconscious impact of luminance on illusory color spreading and figure/ground organization

The watercolor illusion (WCI) is a visual phenomenon in which two thin, contrasting-colored contours create an illusory spread of color into an adjacent region where no physical color is present. This effect is closely linked to perceptual heuristics supporting surface completion and figure–ground (F/G) organization, with the WCI typically biasing the region bordered by the lighter contour to be perceived as the figure. Previous research has shown that luminance differences between adjacent regions can influence the WCI’s ability to bias F/G perception (Hale et al., 2025). The present study sought to determine the luminance contrast levels at which luminance begins to override the WCI, including whether effects emerge at luminance differences that appear to fall below, near, or above a just noticeable difference (JND) threshold based on observers’ responses. Participants viewed images containing two regions divided by a vertical wavy contour. Each display appeared in one of three WCI conditions: illusion on the left, illusion on the right, or absent. Across trials, one of six luminance contrast levels was applied to either side of the image, producing all combinations of WCI condition and luminance contrast. In each trial, a probe appeared on the left or right region, and participants indicated which region they perceived as the figure relative to the probe. This allowed for evaluation of how luminance differences, some of which produced performance patterns consistent with subthreshold or near-threshold discriminability, modified F/G perception. Results revealed that luminance contrast significantly modulated both WCI strength and F/G assignment. Performance patterns suggested that even luminance contrasts that were likely below an observers’ JND could influence F/G judgments, while higher contrasts consistently overrode the WCI’s typical figure bias. These findings help clarify the luminance levels at which unconscious and conscious luminance differences differentially impact illusory color spreading and F/G organization.