Pinna's Intertwining Illusion

Discovered by Baingio Pinna · University of Sassari, Italy

Look at the figure above. The rings appear to spiral inward and intertwine with one another — yet every path is a perfect concentric circle. None of them cross.

The illusion arises from two interacting properties. Within each ring, squares alternate between dark and light. Between adjacent rings, the tilt direction of the squares reverses — what leans clockwise in one ring leans counter-clockwise in the next. This combination confuses the visual system's edge-detection machinery.

Early visual processing relies on centre-surround cell assemblies that sharpen edges by adding dark fringes to bright edges and bright fringes to dark ones. The spacing and scale of the squares align these enhancement fringes in a way that mimics the visual signature of a spiral — a wave-like interference pattern that overrides the normal circular path signal. The brain infers a global spiral from purely local tilt cues.

The strength of the illusion is tunable. Tilt angles near 45° and 75° tend to produce the most compelling effect. Contrast between the dark and light squares matters too — reduce it and the spiral weakens. Square size relative to ring spacing controls how tightly the fringes align.

Read more at opticalillusion.net — Pinna's Intertwining Illusion

I first encountered this illusion in the wonderful, classic The Ultimate Book of Optical Illusions by Al Seckel.

This tuner was made by Mark Safire · MIT Licensed