Turning Rings

Growing becomes turning

Nothing rotates; the picture only grows and shrinks. As it grows, every square moves straight outwards from the centre, but each one’s dark and light edges are tilted, and the cells that detect motion at such an edge can only tell how it moves across its own direction. Their tilted answers, added up around a ring, make a turn.

Baingio Pinna and Gavin Brelstaff described it in 2000. The two rings are tilted and shaded opposite ways, so their turns go opposite ways too. It also works with the phone held in your hand, moving your head in and out.

• Pinna–Brelstaff illusion — Wikipedia