Hawk-Eye virtual reality system can tell whether a ball is in or out
Tennis line judges are more likely to make mistakes when calling balls "out" rather than "in", say researchers.
Californian scientists found that of 83 incorrect calls, 70 of the errors were wrong "out" calls.
This was down to a time lag of a few hundred milliseconds between an image hitting the retina and the viewer processing it, the team said.
This bias, revealed in Current Biology, could enable players to exploit the "challenge" system, they suggested.
Decoding information about the position of objects is a complex task, as the brain has to allow not only for the movement of the object, but also for the movement of the eye relative to it.
The result is a time lag of a few hundred milliseconds between an image hitting the retina and our becoming aware of it.
If the object is moving fast, the brain produces an illusion that the object has moved slightly further than it actually has in order to overcome this lag.
The team from the University of California, Davis, says, in tennis terms, this means a ball which bounces on the line could actually be perceived by the line judge or umpire as slightly further away and called "out" as a result.
To test this, video clips of 4,000 random Wimbledon points were examined and any incorrect calls logged.
If this "bias" did not exist scientists would have expected the number of balls wrongly called "out" to equal the number wrongly judged "in".
In fact, of 83 calls, 70 of the errors were wrong "out" calls.
The researchers said the only ways to eliminate this were to either judge every shot using the "Hawk-Eye" radar method or via instant replay, or to make everyone play on clay, when the ball leaves a "skid mark" showing the exact point it bounced.
There is potential for players to use the knowledge of the brain bias to improve their success rate when challenging line calls, they suggested.
"As it stands, the skill of challenging referee calls is intertwined with the skill of playing tennis itself - those players who make better use of their challenges benefit more," they said.
"Players should maximise their challenges, because the referee error rate is rather high for close calls - it is a suboptimal strategy to leave unused challenges.
"Because players are allowed to continue challenging calls as long as the challenges are correct, players should predominantly challenge those calls consistent with the perceptual error revealed here."
Professor George Mather, from the University of Sussex, has conducted his own research into the accuracy of tennis line calls.
He said it was possible the bias was introduced not as the brain processed the visual information from the court, but as it decided how to respond.
"It may well be that there is a contribution from both the perception stage, and the response stage, with some other reason why the decision to call 'out' is made wrongly more often."
He said his own research found the risk of wrong decisions due to this kind of bias was a small one.