The human brain tracks moving objects using an algorithm the GPS in your phone or car relies on, but that leaves us vulnerable to being fooled by some motion-based optical illusions, researchers have found.
That's why we think we see a sudden "break" in a pitcher's curve ball, something that doesn't actually happen, they say.
In a study appearing in the Proceedings of the National Academies of Science, the researchers explain how our brains make use of an algorithm known as a Kalman filter when it's tracking the position of a moving object.
Named after its co-inventor, mathematician Rudolph Kalman, the algorithm can find optimal and integrated solutions from noisy or unreliable data.
That allows our brains to process visual signals that are far from perfect, such as when an object comes into our peripheral vision where our visual acuity is reduced, researchers say.
A GPS system works in a similar way, using the Kalman filter when the satellite signals it works with are "noisy" or unreliable so that it can still present an estimate of your current position based on previous location and speed.
However, researchers say, in our brains, that same algorithm can be fooled by the motion of patterns on an object, such as the seams on a baseball that is spinning, which leads our brain to "see" the thrown ball suddenly drop from its observed path when, in reality, it followed a steady, regular curve.
"Like GPS, our visual ability, although quite impressive, has many limitations," says study co-author Duje Tadin, a professor of brain and cognitive sciences at the University of Rochester.
When a moving object shifts its position into our visual periphery, our estimate of its position begins to get unreliable, so our brains give more emphasis to the object's motion rather than its position, Tadin says.
"And, this is where we start seeing fascinating phenomena like the curveball illusion," he says.
Our brains "know" when its position estimates are starting to become unreliable, so it relies more on visual cues like the spin of the ball.
Since the perceived position and motion of the curveball is dependent on where it is located in your visual field, when it enters your periphery, it appears to make an abrupt shift: the infamous and sudden "break" of the curveball as it nears home plate — a break that doesn't really exist, the researchers explain.
"A curveball pitch does indeed curve," says first author Oh-Sang Kwon, who led the study while a research associate assistant at Rochester's Center for Visual Science. "But when it is viewed in the visual periphery, the spin of the ball — the motion of the seam pattern — can make it appear to be in a different location than it really is."
