It’s a Circle, Honest!

David Whitaker

University of Bradford, UK

(© 2007 David Whitaker)

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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The Freezing Rotation Illusion

2006 First prize
Max Dürsteler

Universitätsspital Zürich, Switzerland

(© 2006 Max Dürsteler)

An object (e.g. airplane) is turning on a surround (greenhouse), which is swaying back and forth. Observe the rotation of the object. Is it turning smoothly all the time? Or does it “freeze” from time to time? Convince yourself by covering the swaying surround that the object is really turning continuously. If the object is swaying back and forth and the surround is turning continuously we do not perceive a slow-down of the surround. Assuming a stable surround, our visual system probably uses the surround as a reference to measure motion of the included objects.

Read more about the illusion and possible explanations

See an interactive version of the The Freezing Rotation Illusion at Michael Bach’s “Optical Illusions & Visual Phenomena” website

The Freezing Rotation IllusionMax R. DürstelerNature Precedings 2007. 371.1

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The Infinite Regress Illusion

2006 Second prize
Peter Tse
Dartmouth College, USA

 

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(© 2006 Peter Tse)

Fixate the black fixation point on the far left side of the image. Note that the figure appears to move steadily away from the fixation point, even though it is in fact only moving up and down.


See another version of the illusion

The infinite regress illusion reveals faulty integration of local and global motion signals Peter U. Tse & Po-Jang Hsieh Vision Research. 2006. 46:3881-5

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The Bar-Cross-Ellipse Illusion

2006 Third prize
Gideon Caplovitz & Peter Tse

Dartmouth College, USA

(© 2006 Gideon Caplovitz & Peter Tse)

Here we present a new multistable stimulus generated by continuously rotating an ellipse behind four fixed occluders. Observers can perceive one of four percepts: (1) a continuously morphing cross, (2) two independent perpendicular bars oscillating in depth, (3) a rigidly rotating ellipse observed behind the occluders, or (4) a fixed cross observed through a continuously rotating, elliptical aperture.

The bar – cross – ellipse illusion: Alternating percepts of rigid and nonrigid motion based on contour ownership and trackable feature assignment Gideon P. Caplovitz & Peter U. Tse Perception. 2006. 35:993-7

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Gradient-Offset Induced Motion

Po-Jang Hsieh

Dartmouth College, USA

(© 2006 Po-Jang Hsieh)

When a gradient stimulus, whose luminance contrast ranges gradually from white on one side to black on the other, is made to disappear all at once so that only the uniform white background remains visible, illusory motion is perceived. This motion lasts ~700ms, as if the stimulus moves from the low to the high luminance contrast side. This gradient-offset induced motion does not occur for equiluminant color-defined gradient offsets, suggesting that it relies mainly on the magnocellular pathway. We hypothesize that this illusion is caused by the difference of decay rates within the gradient afterimage.

See different versions of the illusion

Illusory motion induced by the offset of stationary luminance-defined gradients Po-Jang Hsieh, Gideon P. Caplovitz & Peter U. Tse Vision Research. 2006. 46:970-8

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Coffer Illusion

Anthony Norcia

Smith-Kettlewell Eye Research Institute, USA

(© 2006 Anthony Norcia)

First time viewers of this display invariably do not see the 16 circles segmented from the background. Rather, they see a series of rectangles that they frequently describe as “door panels”. The illusion pits segmentation cues against what appears to be a very strong prior to interpret the image as a series of 3-D structures “coffers” with closed boundaries. (A coffer is a decorative sunken panel.) It appears that the prior involves both closure and shape-from shading assumptions. The Coffer Illusion is a variation on Gianni Sarcone’s “Op Art illusion”.

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The Occlusion Velocity Illusion

Evan Palmer & Phillip Kellman

Harvard Medical School & UCLA, USA
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(© 2006 Evan Palmer & Phillip Kellman)

The Occlusion Velocity Illusion shown in the video demonstrates that when one part of a moving object goes out of view before another part, the two pieces appear misaligned, even though they are not (top row). This illusion can be counteracted by misaligning one portion of the rod in the direction opposite to the perceived misalignment (middle row). If observers are instructed to attend to the rod’s shape only within the blue box, they are still subject to the illusion (bottom row). This final observation indicates that the illusion is obligatory and not under volitional control.

Read more about the illusion and possible explanations

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Clones and Donors Have Opposite Inclinations (in Vision)

Oronzo Parlangeli & Sergio Roncato

Università di Padova, Italy
Clones and Donors Have Opposite Inclinations (in vision)

(© 2006 Oronzo Parlangeli & Sergio Roncato)

Fig. 1 and Fig. 2 show the same inducing pattern.
However, if the blue lines in fig 1 are seen as donors, we can easily appreciate that when they give birth to two clones as in fig 2, these clones grow apart and they show a diametrically opposed inclination: convex lines become concave and concave lines becomes convex.
A clone for each donor is sufficient to produce the effect.

Read more about the illusion and possible explanations

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Dynamic Luminance-Gradient Effect

Alan Stubbs

University of Maine, USA
Dynamic Luminance-Gradient Effect

(© 2006 Alan Stubbs)

For the primary effect, one should sit at a comfortable distance and then move forward toward the center of the figure. An interesting change in apparent brightness and to some degree form will result—what may be called a “here comes the sun” effect. By moving back and forth, this apparent change will repeat.

See different versions of the illusion

Read more about the illusion and possible explanations

A new set of illusionsthe Dynamic Luminance-Gradient Illusion and the Breathing Light Illusion Simone Gori & D. Alan Stubbs Perception. 2006. 35:1573-7

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