The Leaning Tower Illusion - A conflict between two- and three-dimensional parallelism

Abstract

The Leaning Tower Illusion is a visual illusion in which two identical images, originally of the leaning tower of Pisa, appear to lean away from each other when juxtaposed. The illusion is explained as perspective cues distorting our sense of 2-dimensional direction.1 We used human subjects to test this explanation and to explore the way in which lines create a sense of perspective. We predict that the strength of the illusion scales linearly with the horizontal distance between the image, and is independent of their vertical length. The results turn out to be inconclusive because of large standard deviations in individual measurements. We do show how different compositions of lines, or even a single line, are seen in perspective and give rise to the notion of depth.