hand, it travels a smooth, consistent, parabolic arc. There’s no disjointed change in its motion from beginning to end.

Yet as the ball nears home plate, the batter observes a sudden jump in its trajectory, the notorious “break.” A new study in PLoS ONE argues that the discrepancy between the physics and the perception of the curveball may be all in the mind — or, more specifically, an optical illusion created by the batter’s eyes and brain.

The human visual system dedicates more of its resources to processing images in the center of our field of view than in our peripheral vision. Larger numbers of photoreceptors and retinal ganglion cells in the fovea — the center part of our eyes — help produce extremely high-res, three-dimensional static images. And as the images processed by our retinas head to the brain, larger numbers of neurons in the visual processing centers (lateral geniculate nucleus and primary visual cortex) are responsible for helping make sense of what we see when looking at something straight on as compared to out of the corner of our eye.

A curveball is a unique pitch, in that the ball exhibits two distinct types of motions. The ball approaches the batter, dancing across their field of view — from peripheral to center (or vice versa) — all the while rapidly spinning on its own axis.

During a very small pilot study, Arthur Shapiro’s team created a computer simulation to determine how the motion of a curveball could create an optical illusion as it skates across our entire visual field. If the observers tracked a spinning gray disc while directly looking at the falling object, it moved as intended. But if people tracked the spinning disc out of the corner of their eye — in their peripheral vision — discs that dropped straight down appeared to fall at an angle, while discs that followed a smooth arc as they descended seemed to plunge straight down.

The illusion grew stronger if participants shifted their gaze while the disc was mid-flight, so that the disc rolled from their peripheral vision to their center vision (or vice versa). If observers watched a disc falling at an angle in their peripheral vision, and suddenly shifted their focus so that the object was in their central vision, they perceived the disc to initially fall at an angle, then unexpectedly drop like dead weight to the bottom of the screen. Conversely, if they started the trial by looking squarely at the disc, and shifted their focus a few inches to the right as the object curved downward, it seemed as if the disc was initially dropping straight down, and then suddenly peeled away at a sharp angle.

Previous studies have shown that batters’ eyes make small saccadic movements during the flight of the ball that would alter their point of focus during a pitch. Most noticeably, the batter switches their gaze off the ball toward to the expected point of contact fractions of a second before the bat and the ball meet. By fitting their experiments to the typical motion of a curveball, Shapiro’s team concluded that if a batter shifts focus at any time during the flight of a curveball, it could appear to instantaneously jump as much as 1.25 feet, which would explain the perceived break of the pitch.

Despite the beefed-up capabilities of our fovea compared to our peripheral vision, we don’t experience fuzzy images on the fringes of our vision. Rather, as the authors put it, we view the world “as a seamless visual space composed from different views of the high-resolution portions of the (same) image.” Yet when an arced, spinning ball comes speeding toward us, our high- and low-resolution visual signals get crossed, jumbled up as they bounce from our eyes to different parts of our brain, and back again.

And to the chagrin of batters and the thrill of the crowd, it seems the curveball plays on our senses as well as our emotions.

Citation: Shapiro, A., Lu, Z., Huang, C., Knight, E., & Ennis, R. (2010) Transitions between Central and Peripheral Vision Create Spatial/Temporal Distortions: A Hypothesis Concerning the Perceived Break of the Curveball. PLoS ONE, 5(10), e13296. DOI: 10.1371/journal.pone.0013296

Image: Flickr/ableman, CC

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Authors: Brian Mossop

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