Rotating Snakes Illusion

A picture that won't sit still

The rotating snakes illusion is a completely still image. There's no animation and no flicker, and printed on paper it does exactly the same thing as it does on screen. As soon as your eyes leave the centre, its rings seem to crawl and turn. Each "snake" is a ring of repeating segments, and every segment cycles through the same four tones: black, dark blue, white and light yellow. The version above is drawn by your browser with no animation whatsoever, and I've added two toggles, "Greyscale" and "Reverse the colour order", so you can test which parts of the design really matter.

Brightness, not colour

The rotating snakes are a peripheral drift illusion, and what makes them so strong is the order of the four brightness steps. Kitaoka and Hiroyuki Ashida found that the illusory motion runs in a consistent direction through the sequence: from black towards dark grey (the blue, in the colour version) and from white towards light grey (the yellow) (Kitaoka & Ashida, 2003). Reverse the order, which you can do in the demo, and the rings turn the other way. Remove the colour but keep the brightness order, and you'll find the effect survives, because it's the brightness steps that drive it, not the hues.

The leading explanation is all about timing. Your visual system doesn't process every contrast level at exactly the same speed, so whenever the image is refreshed on your retina, by a blink, a glance or one of the tiny eye movements you make constantly, the different steps of the pattern are signalled with slightly staggered delays. To a motion detector, a feature appearing at one point and then, a few milliseconds later, at the point next to it looks just like something moving. Recordings in monkeys support this. Bevil Conway and his colleagues found that direction-selective neurons in the visual cortex respond to these brightness sequences as if they were moving, in the same direction people report (Conway et al., 2005). Repeat that false signal across dozens of segments in a ring, and the whole ring seems to spin.

Why looking straight at it helps

The effect is strongest in your peripheral vision, which leans more on the fast, motion-sensitive pathways this pattern exploits, while the fovea at the centre of your gaze is better at seeing fine, still detail. Eye movements matter too. Jorge Otero-Millan, Stephen Macknik and Susana Martinez-Conde tracked people's eyes while they looked at the snakes, and found that the illusory rotation tended to start just after tiny involuntary eye movements called microsaccades, and after blinks (Otero-Millan, Macknik & Martinez-Conde, 2012). That's why fixing your gaze firmly on one point can calm the ring you're looking at while the others keep on turning.

Not everyone sees the same thing

How strongly the snakes move varies a lot from person to person, and some people see very little movement at all. The repeating high-contrast pattern can also be uncomfortable for some viewers, so I've included a photosensitivity note on this page, even though nothing on it is actually moving.

Kitaoka's snakes

Akiyoshi Kitaoka, a professor of psychology at Ritsumeikan University in Japan, created the rotating snakes in 2003, building on his research with Hiroyuki Ashida into the peripheral drift effect. It became by far the most famous version, reproduced in textbooks, museums and all over the internet, and Kitaoka has since published many variations on his illusion pages, which are well worth a visit. The motion aftereffect is a kind of opposite: real motion that leaves still things looking as if they move.