Veiled Factors in the Poggendorff Illusion

The Poggendorff illusion has been interpreted as a result of the physical interaction between a line element and an inducing element. We investigated the perceptual properties of the Poggendorff illusion for different depth relations between the two elements with binocular viewing in cases with and without physical interaction between them. The results show that this illusion occurred only when the depth of the line element was perceived as farther than or equal to that of the inducing element even without physical interaction between the two elements in which the gap between the line element and the inducing element was kept at 1.4 deg visual angle and the illusion was difficult to recognize in the individual view; the magnitude of the illusion under vertical conditions did not change with depth difference in the case without physical interaction, while it changed in the case with physical interaction. Then, we inferred that some veiled factors, which have not been considered before, exist in this illusion. Using the configuration without the physical interaction under horizontal conditions, we demonstrated that the perceived depth relation between two elements is one of the crucial factors for the Poggendorff illusion.