The Leaning Tower Illusion

2007 First prize
Frederick Kingdom, Ali Yoonessi and Elena Gheorghiu

McGill University, Canada

Here is a novel illusion that is as striking as it is simple. The two images of the Leaning Tower of Pisa are identical, yet one has the impression that the tower on the right leans more, as if photographed from a different angle. The reason for this is because the visual system treats the two images as if part of a singlescene. 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.

Leaning tower illusion Frederick A. A. Kingdom, Ali Yoonessi, Elena Gheorghiu Scholarpedia 2007. 2(12):5392.

The Leaning Tower illusion: a new illusion of perspective Frederick A. A. Kingdom, Ali Yoonessi, Elena Gheorghiu Perception. 2007. 36(3):475-477

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Where Has All the Motion Gone?

2007 Third prize
Arthur Shapiro & Emily Knight

Bucknell University, USA
This movie requires Flash Player 9

Click on the big button to toggle between a blurred version of the display and an unblurred version. When the display is blurred, the motion is dramatic; when the display is not blurred, there is only minimal motion. The effect can also be seen with a defocused lens. Blur eliminates high-spatial frequencies. It does not add information to the image. Why, therefore, does the removal of high-spatial frequencies add motion to the display? The buttons and levers allow control over the many of the parameters in the display.

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Bouncing Brains

Thorsten Hansen, Kai Hamburger, & Karl R. Gegenfurtner

University of Giessen, Germany
This movie requires Flash Player 9

Please relax and look at the colorful brains: aren’t they rotating and bouncing?! They are, but only in your head.

What’s going on? Some regions in the brains are darker, some lighter than the background. The perceived location of the separation between light and dark regions changes as the background is modulated, causing each brain to jiggle and bounce. In some regions these illusory motions of neighboring brains are coherent and are grouped together to give rise to an even stronger illusion.

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’Weaves’ and the Hermann Grid

Kai Hamburger & Arthur Shapiro

University of Giessen, Germany, and Bucknell University, USA
This movie requires Flash Player 9

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Drifting Background Illusion

Masaharu Kato

Uppsala University, Sweden
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In the illusion , a small pink object can be seen moving back and forth in front of three types of background. When observers track the pink target moving back and forth in front of a background consisting of dynamic noise, they report the percept of a drifting background. The direction of this drift is opposite to that of the small pink object, even though the background in fact does not move into a particular direction. This illusory motion arises for a static gray Gaussian background, but is actually much stronger when the background is dynamic random noise. However, the illusion does not occur when the background consists of static random noise.

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Swimmers, Eels and Other Gradient-Gradient Illusions

Emily Knight & Arthur Shapiro

Bucknell University, USA
This movie requires Flash Player 9

The red button adds/removes half of the background grating. The swimmers bob up and down when they are in front of the grating but not when they are in front of a uniform background

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Steel Magnolias and Breeze in the Trees Illusions

Michael Pickard

Sunderland University, UK

Breeze in the Trees

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Kaleidoscopic Motion and Velocity Illusions

Peter van der Helm

Radboud University Nijmegen, The Netherlands

Animation

You will see a rotating wheel that pulsates each time it aligns with two stationary shapes. You may also see that, at the same time, the inner stationary shape wiggles. The pulsations seem to be caused by color assimilation, and the other effects by ambiguous figure-ground segregation.

Read more about the illusion and possible explanations at Peter van der Helm’s website

For another interactive version of this Illusion, see Michael Bach’s “Optical Illusions & Visual Phenomena” website.

Kaleidoscopic motion and velocity illusions Peter A. van der Helm Vision Research, 2007. 47:460–465

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It’s a Circle, Honest!

David Whitaker

University of Bradford, UK

The illusion in the figure on the left consists of two sinusoidal gratings at 45° and 135° which combine to form a plaid. The contrast of this plaid is windowed by a perfect circle. Despite this, the percept is far from circular – rather, it appears octagonal with distinct sides. The percept is generated by attraction and repulsion of the circular envelope in the orientation domain by the sinusoidal carrier gratings. It relies upon the sharp transition between Fraser illusion (attraction) and Zöllner illusion (repulsion) at the knee-points of the octagon.
Whilst the illusion is scale-invariant in that it does not change with viewing distance, if the scale of the carrier grating is lowered (Figure on the right) relative to the circle, the percept changes from an octagon to a diamond. This is well-predicted by the variation in the strength of the Fraser and Zöllner illusions as the relative spatial scale of carrier and envelope is varied (Skillen et al. (2002) Vision Research 42, 2447-2455).

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