How could we have missed it? Hundreds, perhaps thousands, of visual scientists, psychologists, neuroscientists, visual artists, architects, engineers and biologists all missed it–until three years ago. 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. 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 Contestants submit novel visual illusions (that is, unpublished or published no earlier than the previous year).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 that was created by renowned Italian sculptor Guido Moretti).

In the leaning tower illusion, the tower on the right appears to be leaning more than the tower on the left. Yet these two photographs of the Leaning Tower of Pisa are duplicates.

The illusion reveals the way in which the human visual system uses perspective to help construct our perception of 3-D objects. We say “construct” because the visual system has no direct access to 3-D information about the world. Our perception of depth results from neural calculations based on a set of rules.

These rules include the following: perspective (parallel lines appear to converge in the distance); stereopsis (our left and right eyes receive horizontally displaced images of the same object, resulting in the perception of depth); occlusion (objects near us occlude objects farther away); chiaroscuro (the contrast of an object as a function of the position of the light source); and sfumato (the feeling of depth that one gets from the interplay of in- and out-of-focus elements in an image, as well as from the level of transparency of the atmosphere itself). Because the towers pictured in these paired images do not converge as they recede, the brain mistakenly perceives them as nonparallel and diverging.

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The leaning tower illusion shows that the brain uses the convergence angle of two reclining objects as they recede into the distance to calculate the relative angle between them. When two parallel towers appear in the same photograph, such as the Petronas Twin Towers in Kuala Lumpur, we perceive them as parallel because they appear to converge in the distance as they recede.

Further analysis of similar images reveals subtleties in the way our visual system processes the perception of depth and perspective. For instance, the leaning tower illusion also works with paired images of train tracks, violating the classical rules of perspective. It is hard to believe, but these are actually identical images of parallel train tracks. Although the angles are the same in both images, the brain perceives them as being quite different.

The leaning tower illusion does not occur when viewing two leaning Japanese manga girls, even though the two cartoon images are tilted. The reason is that the cartoon girls do not appear to recede in depth, so our brain does not expect that they would converge in the distance. This phenomenon demonstrates that the brain applies its depth-perception tool kit only in specific situations.

The leaning tower illusion is such a fundamental feature of our visual system that it works even if one draws a 3-D solid object as it recedes into the distance. The parallel lines give the illusion of diverging in the distance. That is, the box appears wider at the back than it does at the front, when it fact the back and front are precisely the same width on the retina.

Just as the painter creates the illusion of depth on a flat canvas, our brain creates the illusion of depth based on information arriving from our essentially 2-D retinas. Visual illusions show us that depth, color, brightness and shape are not absolute terms but are subjective, relative experiences actively created by complicated brain circuits. This is true not only of visual experiences but of any sensation. Whether we experience the feeling of “redness,” the appearance of “squareness,” or emotions such as love and hate, these are the results of the electrical activity of neurons in our brain.

In the movie The Matrix, Morpheus asks Neo: “What is real? How do you define real? If you're talking about what you can feel, what you can smell, what you can taste and see, then real is simply electrical signals interpreted by your brain.” What the movie doesn't tell us is that even when Neo awakens from the fake world of the “Matrix” into the “real world,” his brain will continue to construct his subjective experience, as all our brains do, and this experience may or may not match reality. So in a way, we all live in the illusory “matrix” created by our brain.

Thanks to the brain's rules of perspective, artists can fool the brain into perceiving two-dimensional drawings as three-dimensional. Artist Kurt Wenner's 3-D pavement paintings–such as Muses in Lucerne, Switzerland–are anamorphic illusions that create an impression of three dimensions when seen from one particular viewpoint (above). From the “wrong” side, however, you can see the distortions that Wenner uses to create the 3-D effect (right). The word “anamorphic” comes from the Greek meaning “formed again.”