Bert Blyleven threw a wicked curveball, and whatever batters called it – the deuce, the yakker, the hook or Uncle Charlie – it was pretty tough to hit.

Why?

“It’s a figment of your imagination,” he kidded.

Ever since someone snapped off the first curve, and there’s been a century-old debate whether it was Candy Cummings or Fred Goldsmith, there’s been equal discussion over exactly what the pitch does in flight.

“There’s something physical about it and something illusory about it,” Bucknell University professor Arthur Shapiro said.

A diehard New York Mets fan well versed in the field of visual sciences, Shapiro has studied curves from every angle, and reached the same conclusion as many other experts.

“They look like they jump or break or do all these funky things, but they don’t. The idea that the bottom falls out isn’t so,” he said.

“I’m not saying curveballs don’t curve. I emphasize that, yes, they curve. They just do so at a more gradual rate. Instead of making a sudden hook, they would form a really big circle.”

That might’ve pleased Dizzy Dean. He had a favorite line for those who doubted the ball moved at all.

“Stand behind a tree 60 feet away and I’ll whomp you with an optical illusion!” ol’ Diz liked to say.

Shapiro, however, offers a new theory on why hitters might think a ball bends so drastically: The eye exaggerates the break.

Shapiro said the brain processes objects it sees in peripheral vision differently from things it observes looking straight on. As in, a batter tracking a pitch from the corners of his eyes might throw himself for a curve.

To illustrate his point, Shapiro presented a tantalizing design that recently was judged the world’s best visual illusion by a group of neuroscientists and psychologists.

It depicts a spinning ball that quickly changes direction, depending at which angle it’s viewed. Straight on, it appears to simply drop; from the side, it seems to veer.

“I’m not saying this is it,” Shapiro said. “It’s a hypothesis.”

Shapiro developed the illusion with three collaborators – Southern California professor Zhong-Lin Lu and former students Emily Knight and Robert Ennis.

“I would agree there is an illusion taking place when a batter visually tries to deal with a curving pitch, but not due to peripheral vision,” Hall of Famer Mike Schmidt told The Associated Press in an e-mail.

“Curveballs are like snowflakes, none are the same. No two curveballs have the same rotation speed, velocity toward the hitter, arm delivery angle or break. Nolan Ryan and Bert Blyleven had the tightest rotation and velocity combination,” he wrote.

Schmidt batted .348 (8 for 23) lifetime against Blyleven, hitting two home runs and striking out five times.

“Hitters are seeing the ball with both eyes, not out of the side of front eye as suggested. I believe the illusion is a result of the speed with which the action takes place, not a peripheral view,” Schmidt wrote. “Then again, I’m not a scientist, just a hitter.”

Blyleven won 287 games and two World Series rings. He is now a Minnesota Twins announcer.

Blyleven said he used to listen to Dodgers broadcaster Vin Scully describe Sandy Koufax’s curveball on the radio as a ball that was almost tumbling.

“I visualized the dropping motion,” he said.

Blyleven said he worked off the third-base side of the rubber, especially to right-handed hitters, to make it harder to pick up. He saw the differences in Shapiro’s illusion, and focused more on his own his grip and motion.

Shapiro will move from Bucknell, in Lewisburg, Pa., to American University in Washington this fall and will keep rooting for the Mets.

He admits that when he watches games with 10-year-old twins Benjamin and Sarah and 7-year-old son Joel, his “critical apparatus goes down.” As in, when Mets ace Johan Santana gets in a jam, he becomes more of a fan and less of an expert.

“Oh, sure,” he said. “I’ll be like, ‘C’mon, break one off right here.”‘