This is the second article in a new Mind Matters series on the neuroscience behind visual illusions. 

How could we have missed it? Hundreds, perhaps thousands, of visual scientists, psychologists, neuroscientists, visual artists, architects, engineers and biologists all missed it—until now. The “it” in question is the Leaning Tower Illusion, discovered by Frederick Kingdom, Ali Yoonessi, and Elena Gheorghiu of McGill University. In this illusion, two identical side-by-side images of the same tilted and receding object appear to be leaning at two different angles [see slideshow]. This incredible effect was first noticed just last year in images of the famed Leaning Tower of Pisa, but it also works with paired images of other tilted objects.

The Leaning Tower Illusion is one of the simplest visual tricks one can produce, but also one of the most profound to our understanding of depth perception. This fact is why vision scientists are shaking their heads in disbelief that they did not notice the illusion earlier. Kingdom and colleagues first announced the illusion at the 2007 Best Visual Illusion of the Year contest, where it won the First Prize. The annual contest, which we organized and which is hosted by the Neural Correlate Society, celebrates the ingenuity and creativity of the world’s premier visual illusion creators, both artists and scientists. Contestants submit novel visual illusions (that is, unpublished, or published no earlier than the previous year). An international panel of impartial judges conducts the initial review, and narrows the dozens of submissions down to the Top Ten best entries. The Top Ten creators then compete in Naples, Florida, during a gala celebration, in which the audience chooses the Top Three winners. First, Second and Third prizes take home the coveted “Guido” (a 3-D illusion sculpture created by the renowned Italian sculptor, Guido Moretti).

View 3-D Illusions Slide Show

Mind Matters is edited by Jonah Lehrer the science writer behind the blog The Frontal Cortex  and the book Proust Was a Neuroscientist.