What are optical illusions? They remind us that what we see is not always a true representation of the world but our brain's interpretation of it, which is a result of neural adaptation. Illusions can be categorized into several types, including motion, luminance and contrast, size constancy, color, geometric, and cognitive. Some illusions have been known for centuries, but many are being discovered today. Many of them have no explanation despite active research by vision scientists. One interesting illusion of motion is “Rotating Snakes” by Akiyoshi Kitaoka (2003) in which concentric rings of repeating pattern appear to rotate in different directions (Figure). This seems to be dependent on slight involuntary eye movements as the rotation slows or stops with steady fixation. Another more recent explanation is that changes over time in the neuronal representation of contrast are responsible for the illusion.2 What is known is that asymmetric luminance steps are required and their polarity determines the direction of illusory movement. The degree of movement depends on the background luminance; color enhances the illusion but is not required.3 Interestingly, different observers perceive illusions differently. For example, not everybody perceives the spinning movement of Rotating Snakes. Apparently, there is even a rumor on the Internet that the perception of movement in this illusion is determined by one's level of stress and therefore can be used as a test to assess it. This, of course, is not true. Professor Michael Bach has compiled a cornucopia of visual illusions on his Web site, a fascinating tour for everyone.
Rozenbaum I, Ritch R. Eye on the Web. Arch Ophthalmol. 2007;125(9):1268. doi:10.1001/archopht.125.9.1268