Fraser spiral illusion
Encyclopedia
The Fraser spiral illusion is an optical illusion
Optical illusion
An optical illusion is characterized by visually perceived images that differ from objective reality. The information gathered by the eye is processed in the brain to give a perception that does not tally with a physical measurement of the stimulus source...

 that was first described by the British psychologist James Fraser
James Fraser
James Fraser may refer to:*James Baillie Fraser , Scottish author*James A. Fraser , gold miner and Canadian politician*James E. Fraser James Fraser may refer to:*James Baillie Fraser (1783–1856), Scottish author*James A. Fraser (1843–1937), gold miner and Canadian politician*James E. Fraser...

 in 1908.

The illusion is also known as the false spiral, or by its original name, the twisted cord illusion. The overlapping black arc segments appear to form a spiral; however, the arcs are a series of concentric circles.

The visual distortion is produced by combining a regular line pattern (the circles) with misaligned parts (the differently colored strands). Zöllner's illusion and the café wall illusion
Café wall illusion
The café wall illusion is a geometrical-optical illusion in which the parallel straight dividing lines between staggered rows with alternating black and white "bricks" appear to be sloped....

are based on a similar principle, like many other visual effects, in which a sequence of tilted elements causes the eye to perceive phantom twists and deviations.

The illusion is augmented by the spiral components in the chequered background.

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