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Stephen Grossberg, Massimiliano Dasara, Baingio Pinna; The problem of the perception of holes and figure-ground segregation in the watercolor illusion. Journal of Vision 2005;5(8):54. doi: https://doi.org/10.1167/5.8.54.
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The watercolor illusion is a long-range assimilative spread of color sending out from a thin colored line running parallel and contiguous to a darker chromatic contour and imparting a strong figural effect. Pinna (1987) showed that the watercolor illusion induces a figural effect with univocal depth segregation similar to a rounded volumetric surface, and a complementary background effect, that, under most conditions, appears as an unequivocal hole. The problem perceiving a hole is related to the unilateral belongingness of the boundaries (Rubin, 1921). However, a hole is something in between figure and background. Phenomenally, the boundaries belong only to the figure and not to the background, which, as a consequence, has no shape and is invisible. When a hole is perceived, the boundaries belong to it and at the same time to the complementary region. The boundaries delineate the hole that is visible as an empty space with a shape. This work demonstrates that the watercolor illusion, due to its strong figural effect, enhances the perception of holes under conditions usually perceived as a figure or as a background. This was psychophysically tested (i) by weakening the whole figural organization through imparting motion to only some components of a stimulus, while other components remain stationary, and (ii) by creating new paradoxical cases. The results showed that, despite the common fate principle, under watercolor conditions, moving and stationary elements group, inducing strong figural and hole effects. The uniqueness of the watercolor illusion in inducing the perception of holes is considered to be due to its peculiar boundary properties - juxtaposition of at least two parallel lines creating a luminance asymmetrical gradient along the boundaries - that can be considered as a principle of figure-hole-background segregation. The results are interpreted in terms of the FACADE neural model of biological vision (Grossberg, 1994).
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