What are rotation illusions?
Rotation illusions show a single motion that can look clockwise or counterclockwise. As you watch, the direction of spin flips partway through. The image never changes direction. What changes is how you interpret it.
Illusions in this category
- Reversing Ring: a flat, silhouette-like ring spins. Nothing tells you which side is near and which is far, so it can turn either way.
- Spinning Point Clouds: a solid made only of dots rotates. The motion of the dots gives you the shape, but not which side faces you.
- Wireframe solids: a solid drawn only in lines turns slowly. By changing how you read it, you can flip its spin yourself.
- Necker Cube: a motionless line drawing of a cube. The face that looks nearest swaps places, so you can feel the same two readings of depth as in the rotation illusions, in a still image.
Why does the direction flip?
A rotation shown on a flat screen has no cues, such as overlap, shading or change in size, that decide which side is in front. The brain picks one reading, front or back, and shows you that. But both are about equally plausible, so after a while it switches to the other. When what you see keeps shifting between two or more readings like this, it is called multistable perception. The famous silhouette dancer who seems to spin both ways works the same way.
Check it yourself
- Fix your gaze on one edge and tell yourself “this side is in front”. That makes it easier to flip the direction on purpose.
- The direction often flips after a blink or after you look away.
- Press “Pause” to stop the rotation. At the moment it stops, the shape contains no cue for either direction.
Another figure with two readings, but without any rotation, is the Hidden and ambiguous images group.