1. Introduction
Some images seem to contain spirals when they are composed of concentric circles. Others seem to rotate continuously when they are static. These illusions are not brain errors: they reveal the inference strategies the visual system uses to interpret a three-dimensional world from a two-dimensional retinal image.
2. The Fraser Spiral
The Fraser spiral (1908) is the most famous illusion in this category. It is composed of concentric circles made of twisted segments. The brain interprets the local orientation of the segments as a cue for global direction, and perceives a spiral where there are only circles. By tracing a circle with a finger, one finds that it closes on itself.
3. Illusory Motion
Illusory rotational motion (like Pinna's "wheels" or Kitaoka's spirals) exploits contrast and orientation differences between adjacent regions. The visual system, which detects motion by comparing signals between neighboring cells, interprets these static contrasts as a motion signal. The effect is amplified by involuntary eye movements (microsaccades).
4. Neural Mechanisms
The primary visual cortex (V1) contains neurons selective for orientation and direction of motion. Spiral illusions activate these neurons in a way inconsistent with the physical reality of the image. Areas MT/V5, specialized in motion perception, receive contradictory signals and generate a perception of rotation.
5. Other Spiral Illusions
- Plateau's spiral: continuous rotation after fixating a spinning disk
- Waterfall effect: after watching a waterfall, static objects seem to rise
- Münsterberg spiral: straight lines appear curved through context effect
- Kitaoka's spirals: illusory rotational motion through luminance contrast
6. Conclusion
Spiral illusions demonstrate that visual perception is an active construction, not a passive recording. The brain applies heuristics — "segments oriented this way indicate a spiral", "this contrast indicates motion" — that are generally correct but can be fooled by artificially constructed stimuli.