The Break of Curveballs

Visual illusions are a fun and entertaining way to find out more about our perceptual system. Illusions occur when the brain is presented with conflicting sensual information that it cannot easily interpret. What results is the perceptual experience of illusions. This paper will focus on the illusion known as the “Break of the Curveball,” discussing how the illusion is achieved and how it offers insight into the mechanics of human visual perception.

The curveball illusion is one commonly observed in a game of baseball by batters on home plate. The pitcher stands on a mound and throws a baseball towards home that is 60.5 feet (18.44 meters) away. The pitcher throws the ball in such a way that makes the baseball spin. The ball’s trajectory is a curved path created by the imbalance of the forces on different sides of the ball. Although the ball’s curved path towards home should appear gradual, batters often experience a sudden and dramatic drop of the ball just as they swing the bat. The ball’s change in position is often called a curveball’s “break.”

This phenomenon is reproduced in a research lab led by Arthur Shapiro and Zhong-Lin Lu. They created an interactive computer display that shows a disk descending vertically (see Figure 1). This vertical descent is referred to as the global motion. The disk is shaded with a moving gradient known as local motion. When looking directly at the descending stimulus, the central view, the viewer sees both global and local motion. When viewing the stimulus at an angle by staring at a blue dot off the side of the screen and using peripheral vision to attend to the disk, the internal gradient appears stationery and the disk seems to descend at an angle rather than vertically. Furthermore, the angle of the perceived descent increases when the viewer attends to the stimulus even further in the periphery. The most dramatic effect of the illusion occurs when the viewer shifts his gaze from the blue dot back to the disk while it is in mid-descent. The apparent disk will snap back from its angled path and continue its original vertical path. This change in gaze creates a jump in the direction of the disk and is analogous to the break of the curveball (Shapiro et al., 2010).

Researchers propose that the break behind the curveball illusion lies in the shift from foveal to peripheral vision processing. When the baseball is in focus, its image falls on the fovea and is processed in the central visual processing pathway. All other visual input falling outside of the fovea belongs to peripheral processing (Goldstein, 2009). In terms of the curveball simulation illusion, both global and local motion can be perceived. That is, viewers are able to separate both the vertical motion of the disk as well as the right-to-left movement of the shaded gradient inside the disk. Sensory input hitting the receptors outside of the fovea are represented holistically in the visual system. When attending to the falling disk using peripheral vision while looking at the static blue dot, viewers are unable to separate the two motions. The vertical motion and the internal gradient motion are jumbled. The viewer ends up seeing the disk descending at an oblique angle and the internal movement of the disk as flashing white and black. One can no longer differentiate between the drop and the spin of the disk. Shapiro and his colleagues termed this perceptual combination of features as “feature blur,” showing the peripheral system as lacking the ability to represent multiple features of a stimulus (Shapiro et al., 2010). Once the disk hits the fovea again, all the details of the descending disk will be made clear again, each feature of the disk as its own entity.

The computer simulation of the curveball illusion produces a very pronounced effect of the break seen in a real curveball pitch. By adjusting the manuals that control the orientation, spatial frequency, and speed of the internal gradient, the researchers can measure just how strong of a breaking effect can be achieved. Shapiro et al. (2010) found that, when seen in the periphery, the higher the moving speed of the internal gradient, the bigger the disk seems to deviate from the vertical. This results in a stronger illusion effect. On the other hand, when the internal gradient speed is slowed down close to 0, the effect is not as strong. Increasing the spatial frequency to 40 or more dramatically reduces the effects of the break. When viewing the disk peripherally, it seems to fall as is expected – in a vertical path. Spatial frequencies smaller than 30 bring out the effect even more. The curveball illusion is seen regardless of whether the gradient was oriented at 90° or 45° but it creates the strongest effect when the orientation of the gradient was moving horizontally while the disk moves down vertically. The illusion most noticeably fails when the gradient was seen to be rotating too slowly or to be not rotating at all. This happens when the spatial frequency increases, independent of the speed or orientation of the gradient.

In a real game of baseball, the curveball is a pitch dreaded by many. A curveball pitched right will more often than not induce the illusion of the ball “breaking” and end up being higher or lower than the batter estimated. The gap between the ball’s actual trajectory and the batter’s perception of the ball’s path starts out small and then increases as the ball approaches the last 20 feet to home plate. The ball is first seen in the batter’s peripheral vision, but as it approaches home plate, the batter sees it with center vision again and the ball is suddenly at a different spot than estimated. The distance of the ball is misperceived, and this jump is substantial. Batters cannot completely keep up with the ball as it approaches them and effectively zips across their field of vision. As Professor Zhong-Lin Lu said, “If the batter takes his eye off the ball by 10 degrees, the size of the break is about 1 foot” (Marziali, 2010). The best way to avoid striking out is to keep the eye on the ball. If the ball was kept in focus and stayed in central vision throughout its trajectory, then the illusion should not be perceived. As all players know, of course, keeping the eye on the ball is easier said than done.

The visual system’s physiology may help explain why batters see the curveball break. The retina contains two types of visual receptors, cones for color vision and rods for light detection. When light hits photoreceptors in the retina, the cones and rods perform nonuniform filtering and sampling of the visual image. Stimuli are filtered into the central or peripheral visual processing path. Light hitting the cone-rich fovea proceed to the central path where it reaches a large area of the striate cortex in the occipital lobe dedicated to processing information from the fovea; this is called cortical magnification (Goldstein, 2009). This large area preserves the details of the retinal image into separate mental representations, such as motion and shape, and these representations are consciously perceived. All light hitting outside the fovea belong to the peripheral path. Periphery information, however, is not processed in the same way as those for fovea vision. Information from rod photoreceptors is believed to be processed in such a way that concentrates on the holistic image rather than individual details, resulting in feature blurring (Shapiro et al., 2010).

Knowing these underlying physiological mechanisms behind the visual system, it is clear that the curveball illusion further illustrates what is known and unknown about the central and peripheral vision. The differences between the two types of processing are significant. Neural signals projected from the primary visual cortex to other cortical areas differ dramatically depending on whether the signals originated from the fovea or peripheral regions of the retina (Shapiro et al., 2010). More importantly, the curveball illusion reveals when the perceptual system may falter. The normal visual perception of the world is one of a continuous visual space. As one looks from one object to another, the image on the retina is seamlessly pieced together, from the first object of attention to the next object in the periphery. Yet, once there is motion in the picture, the image falls apart at the seams, which the curveball illusion demonstrates. The transition from central to peripheral vision in the illusion created a perceived discontinuous breaking effect. The differing capabilities of the central and peripheral visual systems suggest that they are separate and independent systems working in parallel. When the stimulus moves from central to peripheral and then back to central vision too quickly, such as when batters try to track the trajectory of a baseball in flight, the perceived motion shifts as the central visual system processes motion in the fovea and the peripheral visual system processes motion in the periphery (Marziali, 2009). Likewise in the curveball illusion simulation, the perceived break could result from the peripheral system’s inability to maintain separate representations of different motions. The disk’s vertical descent and the internal spin are clearly represented in central vision. Once this stimulus is switched to the periphery, these details are lost; the disk seems to fall at an angle and no longer seems to spin. These results, both from the computer simulated illusion and actual accounts of curveball breaks, support the idea that peripheral vision operates differently from central vision and that the visual cortex must integrate information coming in from both ends.

The curveball break illusion is commonly perceived by baseball players when served a curveball. It is one of the few illusions where the illusion is perceived first in real life and then is created in a computer program to investigate why the illusion occurs, which is what makes this illusion interesting to research about. The illusion may be perceived outside of a baseball game because humans constantly shift their gaze when looking at objects static and moving. But it may not be so common to acknowledge this illusion outside of baseball because it is not every day when an object is projected straight at us at such high speeds. This illusion certainly adds to the literature of what is known about the central and peripheral visual centers and points out what still needs to be researched, such as how the two visual parts are actually integrated together in the visual cortex.

Screenshot of the falling disk with internal moving gradient and the blue dot off to the right of the screen as the focal point for peripheral viewing of the disk.


Goldstein, E. B. (2009). Introduction to Vision. In Hague, J. & Perkins, J. (Eds.), Sensation and Perception (8th ed.). Belmont: Cengage Learning.

Marziali, C. (2010). Breaking Curveball Too Good to Be True. Retrieved from USC News: good_to_be_true.html.

Shapiro, A., Lu, Z. L., Huang, C. B., Knight, E., & Ennis, R. (2010). Transitions between Central and Peripheral Vision Create Spatial/Temporal Distortions: A Hypothesis Concerngin the Perceived Break of the Curveball. PLoS One , 5 (10).


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