The poggendorff illusion: Apparent displacement of the oblique lines

Abstract

Four main experiments and one subsidiary experiment were designed to establish whether linear or angular displacement of the oblique line contributes to the Poggendorff misalignment illusion. Exp. 1 and 2 and a subsidiary experiment showed that apparent extent in the obtuse angle with the real or hypothetical median line is greater than in the acute, suggesting that the point of intersection of the oblique might undergo apparent shift towards the acute angle and the complete line apparent rotation. However, the data from Exp. 3 and 4, in which the illusion with collinear and orthogonally disposed obliques was established, indicated that such perceptual shifts are unlikely. The results were interpreted as showing that while apparent extent in the obtuse angle is greater than that in the acute the point of intersection of the oblique line and the line itself are perceptually stable.