The Role of the Second Oblique in the Poggendorff Illusion

It has been observed previously in the classic Poggendorff illusion figure that the end of the oblique line which abuts onto the vertical parallel appears more misaligned than its free end. Various theories suggest that angular distortions are largest at the point of abutment. Two experiments are reported which, however, suggest that, compared to displays in which a single dot is aligned with an oblique line, the introduction of a second, parallel oblique segment does not increase errors more at the abutting tip than at the free end, but, rather, reduces them more at the free end. These results not only cast doubt on the relative importance of angular effects alone in the classic Poggendorff display, but also emphasise the complexity of that figure as a tool in the investigation of visual processing.