New research suggests when a pitcher appears to make a curveball suddenly break or a fastball rise, it's nothing but an optical illusion.
In fact, the ball is following a smooth arc for the whole of its journey, and the sudden drop is just a mirage.
"The curveball does curve, but the curve has been measured and shown to be gradual," says Arthur Shapiro of American University. "It's always going to follow a parabolic path. But from a hitter's point of view, an approaching ball can appear to break, drop or do a whole range of unusual behaviors."
The cause of the illusion, he says, is the shifting of the batter's eye between central and peripheral vision.
"If the batter takes his eye off the ball by 10 degrees, the size of the break is about one foot," said Zhong-Lin Lu of the University of Southern California.
He says batters tend to switch from central to peripheral vision when the ball is about 20 feet away. The eye's peripheral vision is unable to separate the motions of the spinning ball, and in particular gets confused by the combination of the ball's velocity and spin.
The result is a gap between the ball's trajectory and the path as perceived by the batter. The gap starts small while the batter is using peripheral vision, but gets larger as the ball travels the last 20 feet to home plate. As the ball arrives at the plate, the batter switches back to central vision and suddenly sees it in a different spot.
A similar illusion explains the rising fastball, says Lu.
The study was carried out with volunteers tracking the movement of a disk on a computer monitor. Lu and his team plan to build a physical device to further test the curveball illusion.