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In, Out or a Visual Illusion?

October 31, 2008

Photo: tennis ball

Great news for those who always question a referee's call during a tennis match. You might not be so crazy after all.

New research, led by David Whitney, associate professor at the UC Davis Center for Mind and Brain and the Department of Psychology, shows that professional tennis referees are vulnerable to an optical illusion when they see balls bouncing on or close to a line. The common flaw in the visual system -- also known as a "perceptual bias"-- has rarely been documented in sports.

The error occurs because our brain's perception of the world lags a few milliseconds behind reality. While the brain is processing the image of a moving object received from your eyes, the object has already moved on. To compensate, your brain estimates where the object should go next, based on its speed and direction of travel. Mostly those guesses work very well, but if the object is moving very fast and making sudden changes of direction -- like a bouncing tennis ball -- they can give the wrong answer.

Whitney and his team reviewed a random set of 4,457 points from the 2007 Wimbledon tournament. All instances when the tennis ball landed close to or on the line were recorded, and three trained observers individually watched each play. They also examined instant replays.

Whitney expected referees to see the balls bounce in the direction of their motion, which would make them see more balls land off the line. He was proven correct -- 84 percent of the 83 wrong judgments observed were errors in which the ball was called "out" when it was actually in.

Does this mean that a referee's call is never to be trusted? No, according to Whitney -- most of the time referees are accurate. And, both players and referees can have the same perceptual bias with bouncing balls. However, this does mean that the player has an advantage in challenging a call, an already common practice in professional tennis.

And what would this mean for the everyday person who doesn't happen to play tennis for a living? Whitney said that these results give important information on the workings of everyone's visual systems.

"[These experiments] shed light on how the brain processes information. This knowledge can help us design better artificial visual systems and can also be applied in areas like computer science, medicine, and data visualization, where the limits of visual processing are centrally important," he said.

The research is published in the Oct. 28 issue of the journal Current Biology.

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