The Men's Final of the 1981 Wimbledon Tennis Championships is one of the most memorable events in sporting history. John McEnroe, who was playing against Bjorn Borg, famously challenged one of the referee's calls by throwing a tantrum, during which he shouted the immortal line "You cannot be serious!"
McEnroe's outburst was controversial, and he was almost eliminated from the championship because of it. But he may have been right to challenge the referee after all: according to a new study published in Current Biology, in such close calls, professional tennis referees consistently misjudge the location of a ball's bounce because of a perceptual error caused by an inherent property of the visual system.
David Whitney and his colleagues at the University of California, Davis examined how accurately referees perceive the position at which a tennis ball bounces. First, they watched video footage of randomnly selected tennis matches from the 2007 Wimbledon Tennis Championship, and reviewed more than 4,500 individual points in those matches. They recorded each case in which the ball landed on or close to the line, and asked three trained to independently rate whether the ball landed in or out of play, using all available instant replay video to analyse each point.
In tennis, two kinds of errors are possible: a ball can bounce in the court and be called out, or it can bounce out and be called in. (A ball that touches even the tiniest fraction of the baseline should be ruled as "in", hence McEnroe's argument that "chalk flew up".) Previous experiments had established that the perception of a moving object is shifted in the direction of motion. The researchers therefore predicted that there should be a significant perceptual bias, so that balls that bounced on the line would be called "out" when they should have been called "in". If, on the other hand, the referees are free of perceptual bias, then they would be equally likely to make both types of errors. The data obtained confirmed the researchers' prediction. Of the 83 points in which the referee made an error, 70 of the calls were in the predicted direction: the position of the ball was misperceived to be shifted in the direction of motion, so that the referees judged them to be out of play when in fact they had bounced on the line.
The researchers then examined this phenomenon in the laboratory setting. They carried out a psychophysics experiment, using a stimulus consisting of a moving dot which moved diagonally across a computer screen while expanding or contracting to simulate motion toward or away from the observer (right). The dot mimicked the trajectory of a tennis ball - it quickly changed direction, or "bounced", when located at one of seven vertical positions on or near the horizontal line. 6 participants presented with this stimulus were asked to fixate their gaze on the horizontal line and to judge whether the change in direction occurred above or below the horizontal line (corresponding to in or out of play, respectively).
In these experiments, the participants exhibited a strong bias in their perception of the position in which the ball bounced. The perception was shifted in the direction of motion, mirroring the perceptual bias seen in the professional referees' calls during the first part of the study. When the dot appeared to be moving toward them, the participants consistently judged it to have bounced under the line, or out of play. The effect was strongest in this direction, but was also observed when the dot appeared to be moving away.
Why does this perceptual error occur? A likely explanation has to do with the functioning of the visual system. It takes time to transmit information from the photoreceptors in the retina to the visual cortex, where processing of the image takes place. Thus there is a significant delay - of about 100 milliseconds, or one tenth of a second - between the time at which an image impinges upon the retina and the conscious perception of it. This delay would cause the perceived location of an object moving at around 100 miles per hour, such as a tennis ball, to appear retarded by nearly 20 feet in its trajectory.
The brain therefore compensates for this time lag inherent in visual processing by extrapolating the trajectory of an object, so that it is misperceived as being closer than it actually is. Localizing moving objects is one of the most important functions of the visual system, as it could be crucial for the survival of the organism. The brain probably evolved this forward extrapolation mechanism because extrapolating in the other direction, so that a moving object is perceive as being further away than it actually is, could in some circumstances be dangerous, for obvious reasons.
All of this has implications for tennis players. The findings show that error rates are highest with balls that bounce on the line - in these situations, referees are likely to call the ball "out", when in fact it was "in". Players should therefore maximize their challenges, but should focus on challenging those balls that are called "out". As for John McEnroe, he may well have been correct to challenge the referee during the 1981 Wimbledon Finals, but he was wrong to assert that the referee was incompetent. If his call was incorrect, it is not because he was a bad referee, but rather because of a perceptual error that arises as a result of mechanisms evolved by the brain.
Whitney, D. et al (2008). Perceptual mislocalization of bouncing balls by professional tennis referees Curr. Biol.18: R947-R949. DOI: 10.1016/j.cub.2008.08.021
Nijhawan, R. (1994). Motion extrapolation in catching. Nature 370: 256-257.
Sorry if I am naive, but the referees in Tennis sit mid court, and all base line calls would therefore involve balls moving away from the observer, right? This statement:
The brain therefore compensates for this time lag inherent in visual processing by extrapolating the trajectory of an object, so that it is misperceived as being closer than it actually is. . . . The brain probably evolved this forward extrapolation mechanism because extrapolating in the other direction, so that a moving object is perceive as being further away than it actually is, could in some circumstances obviously be dangerous.
seems contradictory when dealing with an object moving away. If the brain is compensating by extrapolating towards the viewer (i.e., things appear closer than they are), then the expected error would be too many balls called "in". However, if the extrapolation is always along the direction of movement, then the preference for "out" is explained. Sorry if I am not understanding something basic.
It's the umpire who sits mid-court. The referees stand at each end - they're the ones who constantly have to swerve out of the way to avoid being hit by the balls when the players serve!
The error does occur in the direction of motion, as I say at the end of paragraph 4. In the segment you quoted, I used motion toward the observer to illustrate the evolutionary advantage of such a mechanism.
I see another reason why close in shots can be called out. Slow motion replay shows the ball distorting (flattening out) when it hits the court much more than normal speed does (if it does at all to human visual perception). This means that even if the initial contact with the court is actually outside the line, as the ball flattens part of it can then make contact with the line. Since even if one loose strand of fuzz just grazes the line, the shot is in. If the ball remained perfectly spherical it would indeed be out. But humans watching this at full speed don't see this flattening out and so make the wrong call.
On the other hand, will in all likelihood receive these calls the same. If the calls were perfect, the same players would still in all likelihood still be the same winners and losers. If these calls were disproportionate though...