Poggendorff Illusion

The Poggendorff illusion is a geometrical-optical illusion involving the misperception of the position of one segment of a straight transverse line that has been interrupted by the contour of an intervening shape, such as a rectangle drawn across it. It is named after the editor Johann Christian Poggendorff, who discovered the effect in 1860 while examining diagrams submitted by Johann Karl Friedrich Zollner; Zollner had been investigating an entirely different illusion, but his diagrams happened to reveal this separate diagonal-line-misalignment effect as well. The illusion has become a widely used tool in vision science and psychology for investigating perceptual systems, neurological function and cognitive development; its strength depends on the properties of the obscuring shape and its borders, and research shows that acute angles in the figure tend to look expanded to viewers, while the illusion weakens or disappears entirely when the lines are perfectly horizontal or vertical.