Leaning Tower Illusion

Abstract

First paragraph: Two identical side by side images of the Leaning Tower of Pisa, photographed from below, appear to rise at differ­ent angles (Kingdom, Yoonessi, & Gheorghiu, 2007a,b; see Fig. II.21- 1). Although the name of the illusion is a play on the name of the famous building, the illusion works for any image of a receding object. The illusion was discovered by accident in 2007 during a psychophysics (behavioral) experiment conducted by two of the authors. Test observ­ers were shown color- manipulated pairs of images of the same scene and noticed that the pair of Pisa towers ap­peared different in form not just color. So striking was the effect that it was assumed that the two images had been taken from different photographs. The illusion won an in­ternational visual illusion competition in the same year (Kingdom, Yoonessi, & Gheorghiu, 2007c) and has subse­quently been featured in numerous books, journals, talks, newspaper articles, calendars, scientific websites, and blogs. Part of the attraction of the leaning tower illusion is its simplicity— one only has to place two identical copies of a photograph of any receding object (e.g., buildings photo­graphed from below, railway lines etc.) side by side to obtain the illusion.