Friday Illusion: Ghostly spiral appears from nowhere

Sandrine Ceurstemont, editor, New Scientist TV

Don't worry, the mesmerising swirls in this video won't hypnotise you. But once the moving pattern disappears, you may be surprised when a ghostly spinning spiral appears before your eyes.

The illusion was accidentally discovered by game designer Hjalmar Snoep while he was creating an animation late at night. "I realised that the after-image that appeared wasn't part of the animation," he says. "I've never come across a moving after-image even though I have collected lots of optical illusions, so it piqued my interest."

After-image effects are caused by the overstimulation of photoreceptors in the eyes after staring at an image for a long time. Typically, they take on the original shape, often appearing in its complementary colours. But a recent study by Hiroyuki Ito from Kyushu University in Japan showed for the first time how an after-image can vary in shape as well as hue.

Ito is now studying moving after-image illusions, since they aren't easy to explain with existing theories. After viewing Snoep's animation, he suggests that the averaged or accumulated retinal exposure to light over the clip's duration could cause the spiral after-image. "The hypothesis could be tested by calculating the averaged luminance for each pixel during the viewing period," he says. "It's a splendid demonstration as entertainment, as well as from a scientific point of view."

Were you able to see the illusion? Do you have a suggestion for why the effect might occur? Let us know in the comments section below.

If you enjoyed this post, check out our previous Friday Illusions to see, for example, how to colour an image with your eyes or how a moving background can induce a disappearing act.

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