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Baseball Curveballs `Break' Only in Batters' Minds, Scientists Suggest

San Francisco Giants pitcher Tim Lincecum’s curveball, which helped him win two Cy Young awards, doesn’t break near home plate. It just appears to move when the batter switches modes of seeing, scientists suggested.

The break, which appears to be a sudden change from the ball’s curved path, may come from the way the human eye shifts between central and peripheral vision, according to the research, released today by the journal PLoS One. The scientists explained the “rise” in a fastball the same way.

The work is the first to explain the break and rise as illusions, according to the authors. Previous explanations include the idea that the hitter underestimates a ball’s speed, said Zhong-Lin Lu, a neuroscientist at the University of Southern California in Los Angeles.

“The brain is tricked,” Lu said in a telephone interview.

He and his group used a flash animation of a descending circle with a moving shadow that mimicked spin. When five observers stared directly at the circle, it fell straight, and when they focused their vision on something else, the ball appeared to move to the side of the screen. That’s because the brain couldn’t process both the spin and the vertical motion, Lu said.

The researchers used the observers’ reports to figure out the size of the break. If the eye is off the curveball by about 10 degrees, the size of the break is about a foot, Lu said.

A fastball “rises” for the same reason, even though in reality, the ball is dropping, he said.

Middle Two Degrees

The illusion is possible because the eye is structured to best perceive the middle two degrees of an image, using so- called central vision. The area covered is about the size of a thumb when a person holds their arm directly in front of them, Lu said.

Anything outside that is peripheral vision. Most distortion that appears when objects go from central to peripheral vision isn’t something people notice; the shift is usually seamless, the authors wrote in their paper.

“When we look at a big field, everything looks continuous, you don’t see a break between the two visions,” Lu said. “That’s an illusion.”

Many batters tend to switch to peripheral vision when the ball is about two-thirds of the way to the home plate. That type of vision isn’t as good at sensing motion, and the brain gets confused by the combination of velocity and spin, and doesn’t track the ball’s trajectory well, according to the scientists. When the batter switches to central vision as the ball arrives back at the plate, the ball is in a different spot than the batter expected, Lu said.

“What happens in baseball is that the curveball comes out of the pitcher’s hand and gives the batter two motion signals,” Lu said. “If you take your central vision of the ball, the periphery vision gets confused and can’t separate the signals. You combine them.”

The Break

The perceived abrupt change when the central vision focuses again on the ball is the break, Lu said. Batters can be trained to keep their central vision on the ball for the entire pitch, so they aren’t vexed by the shift in vision, Lu said.

Lincecum, 26, is listed as the probable starter for the Giants against the Philadelphia Phillies in the first game of the best-of-seven National League Championship Series on Oct. 16. He won the last two Cy Young awards, the highest honor for pitchers, in Major League Baseball’s National League.

To contact the reporter on this story: Elizabeth Lopatto in New York at elopatto@bloomberg.net.

To contact the editor responsible for this story: Reg Gale at rgale5@bloomberg.net.

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