Effects of judged dimensions on the Giovanelli, gravity lens, and iris illusions.

We investigated whether the pattern of illusion effects differs between length and position judgments in the Giovanelli illusion, the gravity lens illusion, and Yang's iris illusion. The results revealed that length judgments consistently produced assimilation across all three illusions, whereas position judgments produced assimilation in the Giovanelli and gravity lens illusions but contrast in the iris illusion. The common assimilative effects observed in length judgments may reflect similarities in the global configurations of these illusions. In contrast, the different effects observed in position judgments may arise from differences in the perceptual organization of local stimulus elements. These findings suggest that perceptual organization depends not only on stimulus configuration but also on the perceptual dimension being judged and that different reference structures may underlie length and position processing.