Every so often, a nasty confrontation shatters the genteel atmosphere of tennis, when a player angrily challenges the call of a line judge. Such dramas might make for good television, but a new analysis shows that the judges are usually right. Although the findings might not stop athletes from venting their frustrations, they do provide a hard slice of insight into the capabilities of human visual perception.
To help resolve disputes, pro tennis adopted a video system called Hawk-Eye in 2006 that tracks the position of the ball and gives players a resource with which to challenge official calls. Psychologist George Mather of the University of Sussex in the U.K realized that Hawk-Eye data could help evaluate how well our perception performs in a situation that demands almost instantaneous decisions--balls hit by the pros often travel 50 meters a second.
Mather obtained data from nearly 1500 challenges made to calls in 15 pro tennis tournaments during 2006 and 2007. He organized them according to how close the tennis ball landed to the line, as reported by the Hawk-Eye system, and whether the dispute involved the side lines or the baseline at the back of the court. "I was surprised at just how closely challenges and errors depended on ball position," he says. If the challenges were due largely to attention lapses by officials or gamesmanship by players, Mather says he would have expected little relation between ball location and challenges or errors. Instead, he found that almost all of those challenges--95%--occurred when the ball was within 10 centimeters of the line, meaning the challenges had to be due to conflicting perceptions. In such cases, the Hawk-Eye data upheld the judges about 60% of the time. The data show the performance of the officials "was remarkably good," says Mather, who reports his findings in today's Proceedings of the Royal Society B.
Behavioral neuroscientist Saumil Patel of the University of Texas Medical School in Houston says Mather's paper "takes many, many years of laboratory research on how the brain processes information" about bodies in motion and applies it to something that arises routinely in tennis. Patel, who specializes in visual perception studies, calls it "amazing" that a complex decision-making problem such as making tennis line calls "fits in a simple statistical framework."