Peripheral Drift Illusion

The peripheral drift illusion is a motion illusion generated by a static image containing a sawtooth pattern of light and dark bands, called a sawtooth luminance grating, that produces an illusory sense of continuous movement when viewed in the visual periphery, even though the image itself never actually moves. The vision scientists Jean Faubert and Andrew Herbert first documented the effect in 1999. Two mechanisms have been proposed to explain why it occurs: Faubert and Herbert's original explanation held that differences in the timing with which the visual system processes different levels of luminance generate false motion signals that the brain's motion-detecting neurons then misread as real movement, and later research published in 2005 supported this general account while also emphasizing the importance of contrast perception to the effect. The illusion is strongest when a viewer looks toward the pattern peripherally, out of the corner of the eye, rather than directly at it, and it is intensified by blinking rapidly; the perceived direction of motion consistently runs from the darker regions of the pattern toward the lighter ones. The most widely recognized variant of the illusion is the Rotating Snakes image created by Akiyoshi Kitaoka in 2003, which achieved wide popularity online after Kitaoka reversed the placement of the intermediate gray tones within the original sawtooth pattern; research on the variant has found that the speed of the illusion's perceived motion depends on the frequency of the viewer's own small, involuntary eye movements called microsaccades.