A Perspective on 3-D Visual Illusions

What the leaning tower and related illusions reveal about how your brain constructs 3-D images

By Stephen L. Macknik & Susana Martinez-Conde

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How could we have missed it? Hundreds, perhaps thousands, of visual scientists, psychologists, neuroscientists, visual artists, architects, engineers and biologists all missed it—until six years ago. The “it” in question is the leaning tower illusion, discovered by Frederick Kingdom, Ali Yoonessi and Elena Gheorghiu, all then at 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. This incredible effect was first noticed in images of the famed Leaning Tower of Pisa, but it also works with paired images of other receding objects.

The leaning tower illusion is one of the simplest visual tricks one can produce, but it is also one of the most profound in relation 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 his colleagues announced the illusion at the 2007 Best Illusion of the Year Contest, where it won first prize.

The annual contest, which we organize and which is hosted by the Neural Correlate Society, celebrates the ingenuity and creativity of the world's premier creators of visual illusions, 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 Contestants submit novel visual illusions (that is, unpublished or published no earlier than the previous year).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 10 best entries. The top 10 creators then compete in Naples, Fla., during a gala celebration, in which the audience chooses the top three winners. First, second and third prizes take home the coveted “Guido” (a three-dimensional illusion sculpture created by renowned Italian sculptor Guido Moretti).

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