Fraser spiral illusion

The visual distortion is produced by combining a regular line pattern (the circles) with misaligned parts (the differently colored strands).

[2] Zöllner's illusion and the café wall illusion 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 checkered background.

It is a unique illusion, where the observer can verify the concentric strands manually.

[3] When the strands are highlighted in a different colour, it becomes obvious to the observer that no spiral is present.