Catching fly balls on the line.

Unless they're staring up at a domed ceiling or a glaring sun, baseball outfielders at all levels of the game typically catch routine fly balls with ease. A new theory holds that outfielders perform this task by running or positioning themselves so their visual image of the moving ball's path follows a straight line.

Several predatory animals move toward their prey in this way, contends Michael K. McBeath, a psychologist at Kent State University in Kent, Ohio, and his colleagues. This visual strategy simplifies such tasks by turning them into two-dimensional problems, McBeath's group concludes.

The scientists first videotaped two male college students with baseball experience catching fly balls and analyzed the paths they took to snag the projectiles. The same students then shagged flies while wearing a lightweight video camera on their shoulders that tracked the ball from their perspective. These outfielders typically took a slightly curved path toward the baseball, shifting speeds along the way, in a fashion that made the ball's trajectory appear straight, the researchers report in the April 28 Science. The need to track a fly ball continuously while keeping in proper position relative to its flight occasionally causes outfielders to crash into fences or each other, they note. It also explains why outfielders have a harder time catching balls hit straight at them than those they can approach at an angle.