The Spinning Disks Illusion

Johannes Zanker

The Royal Holloway, University of London, UK

When sets of disks with tangential greylevel gradients are arranged in concentric circles (see image above, most observers perceive these disks moving around the centre, similar to Kitaoka’s ‘snake illusion’. This motion illusion is enhanced for large-scale and bright images and depends to a large extent to dynamic changes in the stimulus such as elicited by involuntary eye movements or blinks – fixating the centre of the pattern does abolish the illusion, whereas scanning the picture the motion sensation. A reliably effective version of this illusion, which does not require eye movements (i.e. persists when observers fixate the target in the centre of the image), can be generated by modulating the background luminance of the array of disks (see attached animated gif file ‘spin_disks.gif’). This stimulus offers the opportunity of studying this motion illusion – the percept of spinning disks in the absence of any physical displacement – in a highly controlled manner in psychophysical and physiological experiments, because it is not depending on involuntary eye movements or eye blinks. Work in preparation (Zanker 2005) will demonstrate how this illusion can be explained in terms of a two-dimensional motion detector network (2DMD, cf. Zanker & Walker, Naturwissenschaften 91, 149 – 156, 2004).

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The Rotating-Tilted-Lines Illusion

Simone Gori & Kai Hamburger

Brain Research Unit University of Freiburg, Germany

When one approaches the stimulus pattern, the radial lines appear to rotate in a counterclockwise direction, whereas when one recedes from it, they appear to rotate clockwise. In the complex version of the pattern, the illusory rotation is stronger and there may be some residual counter rotation in the surround.

Read more about the illusion and possible explanations

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