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Zhong-Lin Lu, Arthur Shapiro, Chang-Bing Huang; Visual Illusion Contributes to the Break of the Curveball. Journal of Vision 2010;10(7):814. doi: 10.1167/10.7.814.
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In the game of baseball, the curveball follows a (physical) parabolic trajectory from the pitcher's hand to home plate, but batters often report that the path of the ball appears discontinuous. The perceived discontinuity is referred to as the “break”. The discrepancy between the perceptual and physical trajectories suggests that the break of the curveball is a perceptual illusion. A curveball contains two orthogonal motion signals: a global motion toward the batter (second-order motion), and a local spinning (first-order motion). We have created a simplified visual display to simulate the two orthogonal motion signals in the curveball. In our display, a spinning disk descends vertically on a screen; when viewed foveally, the disk appears to descend vertically, but when viewed with peripheral vision, the disk appears to descend obliquely. We found that the perceived motion direction of the disk deviated from vertical by about 0.67x eccentricity. We computed the moment-by-moment perceived velocity of the curveball from an actual trajectory (Bahill and Baldwin, 2004) by assuming that the batter's gaze shifts from the ball to the expected point of bat/ball contact when the ball is 0.2 sec from home plate, and by adding a 0.67x eccentricity (degrees) deviation to the physical velocity. The results predict an observer's perception of a discrete shift from the physical parabolic path traveled by a curveball and suggest that the misperception of the curveball's path may be attributable to a transition from foveal to peripheral visual processing of the image of the ball.
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