Friday Illusion: How colour can warp a wheel's motion

Sandrine Ceurstemont, editor, New Scientist TV

Watching a moving car can sometimes be mind-bending. At certain speeds, the wheels of a forward-moving vehicle can appear to turn backwards due to a common brain trick called the wagon wheel effect. But to confuse your brain even more, a new variation by Arthur Shapiro and his team from the American University in Washington DC shows how adding colour to a wheel can further alter the motion perceived.

In the first example in the video, some of the dots in the wheel are coloured, allowing us to perceive the actual clockwise motion as well as the reverse at the same time. The effect is maintained when all of the dots are coloured using hues of the same brightness. However, by changing the brightness of the background, only one type of motion is perceived once again.

When the animation is made up of both bright and dark colours, two types of motion are perceived simultaneously once again. But by applying a bright background, the animation appears to flash as both types of motion seem to cancel each other out. ?

The classic monochrome wagon wheel effect occurs since our brain doesn't perceive motion continuously, but instead breaks it down into a series of snapshots, just like a video camera. When a wheel rotates clockwise, anti-clockwise information is sometimes generated after each step, causing our brain to misinterpret the direction of motion.

According to Shapiro, the colourful version juxtaposes some of the different ways used by our brain to reassemble motion to warp our perception. "Objects in the world can move from one location to another but in order for us to perceive motion, the brain has to do some tricks," he says. "The brain doesn't just have one means of constructing motion, it has several."

The illusion was awarded third place at this year's Best Illusion of the Year contest.?

If you enjoyed this post, check out a colour-blinding wheel or watch a turning wheel appear to jump backwards.

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