PNAS<\/a><\/em> shows that our brains apply an algorithm, known as a Kalman filter, when tracking an object\u2019s position. This algorithm helps the brain process less than perfect visual signals, such as when objects move to the periphery of our visual field where acuity is low.<\/p>\nHowever, the same algorithm that helps our brain track motion can be tricked by the pattern motion of an object, such as the seams on a spinning baseball, which causes our brain to \u201csee\u201d the ball suddenly drop from its path when, in reality, it curves steadily.<\/p>\n
Though we often rely on Global Positioning System (GPS) to get us to our destination, the accuracy of GPS is limited. When the signal is \u201cnoisy\u201d or unreliable, your phone\u2019s GPS uses algorithms, including the Kalman filter, to estimate the location of your car based on its past position and speed.<\/p>\n
\u201cLike GPS, our visual ability, although quite impressive, has many limitations,\u201d said the study\u2019s coauthor, Duje Tadin, associate professor of brain and cognitive sciences at the University of Rochester.<\/p>\n
We see an object\u2019s position with great accuracy when it\u2019s in the center of our visual field. We do poorly, however, at perceiving position when it shifts into our visual periphery; then our estimate of its position becomes unreliable. When that happens, our brain gives greater emphasis to our perception of the object\u2019s motion.<\/p>\n
\u201cAnd, this is where we start seeing fascinating phenomena like the curveball illusion,\u201d said Tadin. \u201cWe\u2019ve found that the same algorithm that is used by GPS to track vehicles also explains why we perceive the curveball illusion.\u201d<\/p>\n
\u201cA curveball pitch does indeed curve,\u201d said the first author Oh-Sang Kwon, assistant professor at Ulsan National Institute of Science and Technology, South Korea. \u201cBut when it is viewed in the visual periphery, the spin of the ball\u2014the motion of the seam pattern\u2014can make it appear to be in a different location than it really is.\u201d<\/p>\n
\u201cHere, the brain \u2018knows\u2019 that position estimates are unreliable in the periphery, so it relies more on other visual cues, which, in this case, is the motion; the spin of the ball,\u201d said Kwon, who led the study while serving as a research associate in the Center for Visual Science at the University of Rochester.<\/p>\n
The perceived motion and position of the curveball depends on where it is in your visual field. So, when the ball enters your periphery, it appears to make an abrupt shift: The infamous and sudden \u201cbreak\u201d of the curveball as it nears home plate.<\/p>\n