One of the greatest pitchers in Major League Baseball, Sandy Koufax of the Dodgers, could strike out batter after batter even though he lacked the throwing power of his teammate Don Drysdale. The source of Koufax's skill was his legendary curveball, which some players claimed could suddenly dip by as much as 30 centimeters. Other pitchers have gained fame over the years because of what are called their breaking fastballs, which seem to suddenly rise on their way to the catcher. But a team of researchers has now concluded that the erratic behavior of pitched baseballs is just a trick of the eye.
Fans and scientists alike have debated over the years about whether and how much the arc of a baseball can be affected by a pitcher. And plenty of pitchers have claimed to gain an edge by gripping the ball in a certain way before throwing it, or even slathering the ball with spit or other substances. Instead of focusing on how the pitcher manipulates the ball, a team of researchers led by neuroscientist Arthur Shapiro of American University in Washington, D.C., attacked the question from the other side: how the human eye and brain perceive the ball's movement.
Using five volunteers in the laboratory, the researchers devised a simple experiment using the images of two disks on an otherwise blank computer screen. They asked the volunteers to shift their attention between the two disks. One, a light-and-dark mix to simulate a spinning ball, slowly descended from the top of the screen to the bottom. The other disk was blue and stationary. The researchers found that, in all cases, when the volunteers focused on the "spinning" disk, they saw it drop straight down. But when they focused on the stationary disk, thereby using their peripheral vision to keep track of the spinning disk, the spinning disk appeared to veer away from the vertical. And shifting attention between the two disks made the spinning disk seem to behave even more erratically, the researchers report  online today in PLoS ONE. (Try it for yourself here .)
The key to the phenomenon, says Shapiro, is understanding how the human visual system works. One of its two components, the fovea, or central visual area, can track motion very well. The region around the fovea provides our peripheral vision; it can detect only motion and can't track it very well. "We often confuse different signals in peripheral vision," adds neuroscientist and co-author Zhong-Lin Lu of the University of Southern California (USC) in Los Angeles. "For example," he says, "if we see a moving car with our peripheral vision, we may confuse [its movement] with any movement in its immediate background."
Regarding baseballs, the problem is that the fovea can focus on only a very small area—only about 2 degrees of the visual field (or an area smaller than your thumbnail held at arm's length)—so as a pitched ball moves closer it can easily slip into your peripheral vision as it becomes larger. When that happens, Shapiro explains, the movement and spin of the ball combine in the hitter's mind and create the perception that the ball is veering off track. Hence, curveballs seem to curve more, fastballs seem to break, and the best hitters in baseball succeed in getting a hit only about three times in every 10 at bats.
So what's the solution for the hitter? "Don't take your eyes off the ball," Lu says. "Always track it with your central vision."
The paper presents "an interesting new hypothesis regarding the perceived break in the curveball," says Robert Cormack, a psychologist at the New Mexico Institute of Mining and Technology in Socorro and an expert on visual perception. "I agree with the conclusions 100%," adds psychologist Tom House, who is also at USC but was not involved in the research. House, who coaches the school's baseball team and runs a motion-analysis facility, says the study confirms "what we as coaches have recognized all along," that the way to pitch successfully is to throw every ball through the same "tunnel," or track, and force the batter to deal with whether the pitch is a fastball or a curve only at the last possible moment.