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 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. When the strands are highlighted in a different colour, it becomes obvious to the observer that no spiral is present.
- Fraser J (1908) A New Visual Illusion of Direction. British Journal of Psychology 2:307–320
- Cucker, Felipe (2013). Manifold Mirrors: The Crossing Paths of the Arts and Mathematics. Cambridge University Press. pp. 163–166. ISBN 978-0-521-72876-8.
- Stern, Tom (October 1, 2013). Philosophy and Theatre: An Introduction. Routledge. p. 57. ISBN 9781134575985.
- See Bach, Michael. "Fraser Illusion". Visual Phenomena & Optical Illusions.