The peripheral drift illusion is a perception of movement in a static image; the illusion is caused by the brain’s interpretation of patterns seen outside of the eye’s area of focus.
Certain repeated patterns of black and white and simple color patterns can elicit the illusory effect. Typically, people see the motion moving from high luminance to low luminance areas of the pattern. Larger images usually produce a stronger effect and some color combinations and intensities work better than others. If one stares at a fixed point in the image the motion disappears in the area of focus. The intricacies of how these illusions work are still debated, and theories that explain one type of illusion may fail to account for others.
One of the best-known illustrations of peripheral drift illusion is Kitaoka Akiyoshi's Rotating Snakes image, an adaptation of which is pictured below. As you read this text, you probably see the snakes below rotating. If you look directly at one snake, it appears to hold still, while those in your peripheral vision seem to be in constant movement; if you look close to the image but not right at it, it looks like a nest of stealthily stirring snakes. The obtruding tongues do not move, however, even in the illusion, another indication that none of the snakes are actually moving. (Definition continues below the image.)
Peripheral drift and other types of illusions are often presented purely for amusement or as psychedelic art. However, optical illusions are also used to study how the human brain interprets visual data. From studying how these patterns create perceived motion, we can better understand how the brain interprets motion. We can also gain knowledge about how a damaged brain fails to guide an individual’s movement when they have difficulties perceiving motion in their environment.
Motion illusions can be used in virtual reality (VR) and augmented reality (AR) applications to properly guide individuals with problems tracking movement. Various types of optical illusions are used for a number of effects in VR and AR. The peripheral drift illusion and other motion illusions can not only lead viewers to see movement but also to perceive themselves to be in motion while remaining stationary.
Michael Abrash, the chief scientist for Facebook's Oculus, explains how optical illusions are what make virtual reality work: