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In baseball, a good curveball can turn a hitter's legs to jelly, traveling on a devastating arc that causes him to wave his bat awkwardly at where the baseball used to be. In science, a good curveball can tell researchers a lot about the differences between what a person sees when an object is viewed via the eye's narrow band of central (or foveal) vision, compared with what the object looks like when spotted through one's peripheral vision.

"The differences between peripheral and foveal vision are important whenever you discuss eye diseases like glaucoma, which affects your peripheral vision, or macular degeneration, which affects foveal vision," says Arthur Shapiro, an associate professor of psychology at American University in Washington, D.C. (Until recently, Shapiro researched and taught at Bucknell University in Lewisburg, Pa.) "You really want to understand what people suffering from these are able to see."

To get a sense of the scope of human foveal vision, Shapiro says to hold your thumb directly in front of you at arms length. The thumb will fit into your foveal vision, but looking to the left or right of it engages your peripheral vision and makes the thumb's details more difficult to distinguish.

Shapiro hatched the idea of using baseball to illustrate this when trying to explain the differences between foveal and peripheral vision to his 10-year-old twins during a game of catch. When the 2.9-inch (7.4-centimeter) diameter baseball is in the pitcher's hand, 60 feet (18.3 meters) away from home plate, it appears small enough to fit in the batter's foveal vision. By the time a 75-mile (121-kilometer) per hour curveball spinning at about 1,500 rotations per minute reaches the plate, it is too big to be seen only in the foveal and moves to the batter's peripheral vision, according to Shapiro.

In the optical illusion Shapiro created to demonstrate this effect, the perception of a change in the curveball's path may be related to physiological differences between foveal and peripheral vision. The curveball illusion consists of a single oval that drifts from the top of the screen to the bottom. To see Shapiro's optical illusion, Click Here.

When the observer tracks the oval foveally, the motion will follow the oval (i.e., the oval appears to descend vertically). However, if the observer shifts his gaze to a blue dot to the right of the screen (thus viewing the falling oval out of peripheral vision), the oval appears to fall at a diagonal angle.

Image ©iStockphoto.com/ Joshua Blake

10 Comments

1. 1. rockjohny 03:18 AM 5/19/09

   SO WHAT CONCLUSIONS FOR THE BATTER IS THERE? Should he keep his eyes on the ball or only to a point? I know when you deer hunt, if you're scanning the tree line for deer (or turkey, whatever), it's best to relax your eyes and just look for movement rather than focus very hard. Your eyes will automatically pick up the movement.

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2. 2. BigMacFrys 01:47 PM 5/19/09

   Interesting observations, but what implications does this actually have? Are you suggesting that the ball doesn't *actually* move, or that the curveball's movement is simply enhanced by this visual phenomenon? It also doesn't explain the perceived (or actual) movement as seen by the pitcher who just released the ball, or a camera in centerfield showing the pitch on TV. I don't know how you would measure this, but you could simply use a ball with no actual laces, paint them on in the same pattern as a baseball, and then somehow determine if the perception of the curveball is the same, or not, by the hitter. Interesting article though :)

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3. 3. posthumandeus 03:45 PM 5/19/09

   Pray tell...in what matter does this article benefit humanity????

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4. 4. djk_wff in reply to posthumandeus 04:24 PM 5/19/09

   Or the words, how does "pray tell" benefit humanity? sounds like a semantic opiate of the masses ...

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5. 5. sumitsahdev@in.com 07:55 AM 5/20/09

   Interesting article about the foveal and the peripheral vision of an eye.
   With this concept, I guess most of the illusions can be explained.
   Would like to read more articles on optical illusion.

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6. 6. wiffleball 02:34 AM 5/21/09

   I would like to see the actual flight of a curveball compared with this hypothesis. Because a baseball appears to have the same movement from the pitchers, center fielders and batters perspective (and also from the center field TV camera). Also a batted ball hit 350' down the line also has the same perspective. A baseball appears to curve more when it's at the end of it's flight from all perspectives.
   
   I have some questions for the writer. Why does this optical illusion work the same for the batter, pitcher, center fielder and TV camera? Did they do the experiments with a ball that has laces? Did they actually take video of the flight of a baseball? Did they look at video from different perspectives? Has he ever played baseball and can he throw a good curveball? Does a baseball curve more at the end of it's flight because it's slowing down and dropping at an increased rate? Does it curve more at the end because the air builds up around it more?

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7. 7. The Engineer 06:51 AM 5/21/09

   Presumably there are also implications for road safety. For example motorcyclists often have car drivers pull out in front of them. Do you think the reasons are similar?

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8. 8. 3X UROUT 02:42 PM 7/16/09

   I am an Engineer and a professional Baseball Umpire. I have been in awe of the artistry of pitchers for 25 years which previous to this simulation was a mystery. I see thousands of curve balls every year. This is important information, not only for batters, but for the umpires as well. We umpires stand in a position with our heads fixed in one position to see the ball leave the pitchers' throwing arm and use only eye movement to track the ball into the catchers' mitt. The slight angle that we use for that is about 10 degrees off center. (A little bit of Engineer slips into my discussion.) That slight angle gives us almost the same view as the batter, not quite as dramatic, but is affects the instantaneous decisions we make every pitch. I have been experimenting with angles and distortions since I read the article and if I stand facing the pitcher with no offset, there is no distortion, but I can't see the catchers mitt at the end of the pitch. Seeing the position of the mitt at the catch is where the decision chain begins, followed by the ball in the zone decision. So there is more information which we use to call strikes and balls. This simulation is a good training tool for training umpires on what to expect and how to make the right calls.

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9. 9. usaforwarding 11:09 PM 5/23/11

   This analogy is amazing! I loved the example in the Curveball Shapiro app. After making adjustments with the orientation slider, you can control the perception of the pitched curve ball. This is a great way to teach kids.
   Nice work!
   Kristin
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10. 10. usaforwarding 11:00 AM 6/13/11

    I hate curved balls. They always get me. lol Even in Wii Sports, I always get a strike.
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