Curveballs Do Not Break

In Award-Winning Demo, Neuroscientists at USC and American University Suggest Curveballs Do Not Break. The answer to the question of whose curveball breaks harder - that of the Yankees' A.J. Burnett or the Phillies' Cole Hamels - may be neither. Zhong-Lin Lu, a professor of cognitive neuroscience at USC, along with USC alumni Emily Knight and Robert Ennis and Arthur Shapiro, associate professor of psychology at American University, developed a simple visual demo that suggests a curveball's break is, at least in part, a trick of the eye.

Their demo won the Best Visual Illusion of the Year prize at the Vision Sciences meeting earlier this year. The idea is that the effect is due to the batters being forced to switch between peripheral vision and central vision during a swing.

Detailed Description of the Phenomenon

A baseball pitcher creates different pitches by throwing at different velocities, with different spins. A typical major-league curveball travels at about 75 mph with a 1500 rpm spin; a typical fastball, about 90 mph with a 1500 rpm backspin. Although the physics of curveballs and fastballs is well understood, perceptual puzzles remain. Physically, the ball's spin creates an imbalance of forces on different sides of the ball and therefore a substantial (yet gradually) curved trajectory. However, for batters, the curveball's flight often appears to undergo a dramatic and nearly discontinuous shift as the ball approaches home plate (the "break"). Similarly, a fastball descends during flight, yet batters often perceive a rise approaching home plate.

The new research suggests that this is a perceptual illusion due to the inability of peripheral vision (looking at something indirectly) to maintain separate representations of different motion signals.

A curveball contains two motion signals: movement toward the batter, and the spinning of the ball. In the demonstration, the drop and spin of the disk represent these two motion signals. When observed directly ("in central vision"), the spinning disk will appear simultaneously to fall vertically and spin. That is, you can separate the two motion signals. If you look instead at the blue point ("in peripheral vision"), the disk will appear to fall at a 20 degree angle (more or less, depending on viewing distance and how far the disk is from central vision), because you can no longer separate the drop and spin. Shifting your gaze during the disk's descent will make the disk appear to abruptly change direction ("break").

For a batter who can track a curveball's entire flight, a good portion of the ball is in his/her peripheral vision when the ball approaches home plate. For a batter who cannot maintain eye tracking, the change from central to peripheral vision (and vice versa) is more dramatic. The researchers contend that the change in eccentricity (how far the ball is from central vision) underlies the curveball's break. Similar principles may also explain the fastball's rise. Research with realistic baseball simulators or field studies with eye-tracking equipment is necessary to completely understand the curveball's break.

Humans constantly shift objects between central and peripheral vision and may encounter effects like the curveball's break regularly.

The principle that peripheral vision cannot separate different visual signals may have far-reaching implications in understanding human visual perception and functional vision in daily life.

Contact: Carl Marziali
(213) 740-4751
marziali@usc.edu