How Come: When wheels appear to be spinning backward

When you are driving at highway speeds and you look at the wheels of some adjacent cars, they sometimes appear to be rotating backwards. What causes this visual effect? asks David Berkof Melville

Scientists call it the Wagon-Wheel Effect, because it's familiar from scenes of runaway stagecoaches in Western movies. As a wagon -- or a steam-powered train engine -- picks up speed, its wheels suddenly seem to shift from frantic forward spinning to a lazy backward turn.

But we don't just notice the effect on film. The optical illusion also occurs in real life -- on highways, and even in your own living room. Any kind of "wheel" will do -- even the spokelike blades of a ceiling fan.

When we watch backward-turning wheels on film, we're seeing a joint production of the movie camera and our own brain. A motion picture camera that uses film takes many snapshots per second of a scene. The result is thousands of individual static images on a film strip.

In the finished movie, as the film moves through the projector, snapshots of movement flash on-screen. Thankfully, our brains fill in the blanks, keeping one split-second image in mind until the next one appears. And the action appears not jerky, but smoothly continuous, as it was in reality.

When it comes to backward-moving wheels, however, we can also blame our brains. Imagine a wheel spoke as the minute hand on a clock. If one film snapshot is taken when a turning spoke is in the 6 o'clock position, the same spoke may have spun around to 5:59 by the next snapshot. The third may catch the spoke in the 5:58 position, and so on. Our eyes and brain obediently splice the motion together. And instead of spinning quickly forward, the wheel seems to turn slowly backward.

The same thing can happen when we use a digital camera, which records a series of images at 25 to 50 frames a second. But the illusion can also occur in everyday life, when we're using only our eyes. Watch a ceiling fan turn, and if it's lit by a bulb that cycles on and off 60 times a second, like an older fluorescent, the "strobe-light" effect can make the blades appear to spin backward.

But even in daylight we sometimes see the illusion, whether on turning fans or spinning car wheels. How come? Some say that when the brain sees a pattern moving, say, from right to left, more of its leftward than rightward motion-detector neurons are activated.

But the rightward detectors occasionally win out, and then a pattern in motion appears to reverse.