The world’s best visual illusion of 2007

The Neural Correlate Society holds such an annual contest. According to them:

The contest is a celebration of the ingenuity and creativity of the world’s premier visual illusion research community.

For 2007, the first prize went to Frederick Kingdom, Ali Yoonesi and Elena Gheorghiu of McGill University, Canada with their entry entitled “The Leaning Tower Illusion”:

Perhaps the fact that it’s very striking, yet very simple is the main factor that contributed to its triumph. We have 2 pictures The Leaning Tower of Pisa, put side by side. They are actually the same picture, yet your eyes will tell you that the tower on the right leans somewhat more, as though the picture on the right was photographed from a different angle.

The explanation, as given by the contest website is:

The reason for this is because the visual system treats the two images as if part of a single scene. Normally, if two adjacent towers rise at the same angle, their image outlines converge as they recede from view due to perspective, and this is taken into account by the visual system. So when confronted with two towers whose corresponding outlines are parallel, the visual system assumes they must be diverging as they rise from view, and this is what we see. The illusion is not restricted to towers photographed from below, but works well with other scenes, such as railway tracks receding into the distance. What this illusion reveals is less to do with perspective, but how the visual system tends to treat two side-by-side images as if part of the same scene. However hard we try to think of the two photographs of the Leaning Tower as
separate, albeit identical images of the same object, our visual system regards them as the ‘Twin Towers of Pisa’, whose perspective can only be interpreted in terms of one tower leaning more than the other.

Source
Neural Correlate Society’s best visual illusion contest

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