Flummoxed by those wrong "out" calls tennis referees make? Blame it on the brain's sluggish visual-processing system: It makes the ump — and all humans — likely to perceive moving objects as farther along in their trajectory than they actually are, a new study says.

The glitch produces a visual illusion that makes refs' erroneous calls overwhelmingly more likely to be on balls they call "out" than on ones they judge as "in," according to new research in today's Current Biology. Because the brain is constantly playing catch up with visual reality, it can become particularly taxed in situations in which an object is moving extremely quickly or unpredictably; in the case of a bouncing tennis ball, the brain may perceive it as landing beyond where it actually did.

"This is not a problem with referees," says study co-author David Whitney, a vision scientist at the Center for Mind and Brain at the University of California, Davis. "It’s a consequence of human visual processing … a visual illusion caused by a mechanism that allows the system to localize a moving object."

The problem involves a 100 millisecond-lag between the moment an object appears on the retina, the part of the eye that translates light into images, and when our brains make us aware of that object. "To compensate, the visual system takes the image and shifts it forward and allows us to perceive it in a more physically accurate position so we can live in the present," Whitney says. That trick "works great for continuously moving systems, but if something changes abruptly, there can be mistakes; the visual system has a hard time with that abrupt change."

In Whitney's study, three scientists independently reviewed video and instant replay of 4,457 randomly selected points from the 2007 Wimbledon championships. Of the 83 calls that the video and instant replay showed were wrong, 70 were "out" calls. The scientists found the same bias when they reviewed player challenges in this year's Wimbledon: 69 percent of those calls were judged by refs as being out when they were really in.

Players could use the findings to strategize about which calls to challenge (they should object more often to those called "out" than "in," Whitney says). The results are also likely to apply in other sports in which the visual localization process plays a make-or-break role, such as synchronized diving — in which athletes  quickly change positions — or baseball, Whitney says. "When a ball is thrown to first base there's a perceptual challenge there: Does the ball hit the glove first or the foot hit the base first?" he says. "The visual system may be taxed to its limit."

You can find our podcast on the study here.

(Image by iStockphoto/Stephen Morris)