Curveballs: When What You See Isn't What You Get Baseball fans and players have long debated whether a curveball actually curves. New research suggests that the switch from using central vision to track a ball to peripheral vision creates the illusion of a curve. The combination of the spin and velocity of the ball also widens the gap between the ball's trajectory and its path.

Curveballs: When What You See Isn't What You Get

Curveballs: When What You See Isn't What You Get

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Dizzy Dean of the St. Louis Cardinals winds up for a pitch during spring training in 1936. AP hide caption

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Some of baseball's most iconic pitchers were notorious for their devastating curveballs. From Mordecai "Three Finger" Brown and Sandy Koufax, to Barry Zito and David Wells, the curveball and its signature look, where the ball bends and breaks, have long been part of the game.

And yet the curveball and whether it actually curves have been debated for decades, with some describing it more as an optical illusion. Of course, baseball players whose experience and eyes tell them otherwise often beg to differ. Hall of Famer Dizzy Dean famously once said, "Stand behind a tree 60 feet away, and I'll whomp you with an optical illusion."

New research suggests what you see when a curve is thrown isn't quite what you get, however.

"So the curveball is curving, but there's also a well-known perceptual puzzle. That is, instead of seeing a smooth curve, batters often see the ball as going through a big giant jump where it falls off the edge of a table or breaks off to the right," says visual scientist Arthur Shapiro of American University.

Shapiro describes the visual science of a breaking ball as NPR's Guy Raz takes pitches from Ben Webster, whose curveballs leave Raz catching only air with his bat. As Raz experiences the same frustration thousands of batters have before -- seeing a pitch clearly breaking and curving -- Shapiro tells him, "It certainly bent down, but we think the reason it looked so strong has to do with how our eye is organized."

"We think we see the world as one giant uniform spot, but the center of our vision, the fovea, is made up of really tightly packed cells, and the periphery is made up of loosely packed cells," he says. "It's like we have an HD camera in the center and a cell phone camera in the periphery."

This significantly affects how a batter sees an incoming pitch. "If you're in the batters box, at first you're seeing it in the central part of your visual system, but after a third of the way, the ball is going too fast to track it," Shapiro says. "So you take your eye off the ball and see it in an entirely different way."

In this interactive, a disk descends from the top of the screen to the bottom. If you track the disk in the periphery (for example, look to the right, but see the disk fall out of the corner of your eye), it appears to descend obliquely. Use the lever to adjust the angle of descent.

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It is during that switch from central vision to peripheral that creates the illusion of the curve, because peripheral vision isn't capable of differentiating the movements of a spinning ball. The combination of the spin and velocity of the ball is also confusing, so a widening gap between the ball's trajectory and its path is created.

The break in the pitch, or when it seems to "fall off the table," occurs when the ball comes to home plate and the batter reverts to central vision. This return to central vision has the batter seeing the ball in a different spot than expected, and thus the perception that the ball has suddenly changed positions is what creates the illusion of a rapid break as opposed to the gradual curve that physical measurements produce.

"Humans constantly shift objects between central and peripheral vision and may encounter effects like the curveball's break regularly," Shapiro says in his work. He believes that not only is his research relevant to baseball fans or hitting coaches, but also that "peripheral vision's inability to separate different visual signals may have far-reaching implications in understanding human visual perception and functional vision in daily life."

So the next time you're at a baseball game and a coach or parent tells a player to "keep his eye on the ball," consider helping the hitter out by yelling, "Keep your foveal vision on the ball!"