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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