Science in Sports

"Hawk-Eye" vs. Trick of the Eye

A team of researchers at University of California, Davis have shown that the unfortunate umpire may have been a victim of a common trick of the eye. Scientists have shown that tennis referees are vulnerable to an optical illusion that makes balls appear to be out, when they actually land just inside the court


By MARKO ANDREJIC
from Belgrade, SERBIA


The start of new tennis season has reminded of an interesting topic mentioned so many times - referees and their mistakes. New computer system "Hawk-Eye" has offered players a chance to 'challenge' umpires and ask for review of their decisions through instant replay; they used this opportunity many times with different rate of success. But Hawk-Eye also brought new comments - that even machine is not perfect, because some players are still suspicious about this system (even Roger Federer was talking about that).

Anyway, referees are just normal people and they make mistakes, less or more... A team of researchers at University of California, Davis have shown that the unfortunate umpire may have been the victim of a common trick of the eye. According to report in the October 28th issue of "Current Biology", scientists have shown that tennis referees are vulnerable to an optical illusion that makes balls appear to be out, when they actually land just inside the court.

Like all visual illusions, the new discovery provides visual neuroscientists with a window on how the brain processes information, explained David Whitney, leader of researching team.

"The visual system faces a big challenge when trying to code the locations of objects so that we can perceive them," Whitney said. "Consider one of the difficulties: every time we move our eyes, the image on our retina moves. Even if our coffee cup is actually stationary on our desk, we move our eyes and head while reaching to pick it up, so the image of the cup will move on our retina. This is a problem because the visual system is sluggish - it takes us a hundred or more milliseconds to become aware of an image that strikes our retina. So, by the time we perceive an object like the coffee cup in one location, it will have already changed location as we move towards it. Our perception lags behind reality. The visual system has mechanisms that help alleviate this problem of living in the past, but these mechanisms are not perfect and occasionally result in visual illusions - like the misperception of tennis ball location we discovered."

People consistently misperceive moving objects as shifted in the direction of their motion, so that at any moment they appear to be farther along their path than they are. Whitney said for "Science Daily" that he realized it might be possible to study this in the context of tennis when he saw a referee call overturned by a player's challenge during a Wimbledon match.

On a tennis court, a ball could physically bounce in the court but be called out, or a ball could physically bounce out of the court but be called in. If tennis referees were bias-free, they would be equally likely to make each of these two kinds of errors. But, because objects generally appear to be shifted in the direction of their motion, referees should incorrectly judge balls as being out more often. Whitney's team confirmed that prediction. In a review of more than 4,000 randomly selected Wimbledon tennis points, the researchers uncovered 83 incorrect calls. Of those, 70 of the errors were of the type predicted.

Further laboratory study of the phenomenon confirmed that the refs' mistakes are not the result of poor refereeing. Rather, the errors are a general artifact of the way the human brain processes visual information about motion.

Indeed, the researchers said, tennis players and audience members surely make the same mistakes that refs do. The new findings suggest, however, that players could maximize their opportunity to challenge calls by focusing on balls that are called "out," since they are more likely to be incorrect.

The report also suggests that every shot in professional tennis should perhaps be reviewed by instant replay. "If that proves prohibitively time-consuming, the rules allowing players to challenge referee judgments should be scrutinized, at least in light of the current findings," they wrote. "If all else fails," they added, "perhaps professional tennis venues should follow the French and universalize the clay court, where skid marks on the clay reduce reliance on the referees' motion perception."

Sounds like an interesting proposal, but scientists probably did not take into consideration the fact that clay court is the favorite type of Rafael Nadal! If tennis association accepts this solution, it is very possible he would stay # 1 till the end of his career and tennis matches wouldn't be interesting at all.


(Published: 10.01.2009.)