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Watch: The optical illusion that makes it so hard to hit a curveball

In baseball, the curveball is a monumentally difficult pitch to hit. It turns out there's a very good scientific reason why.

Right when a curveball crosses the plate, the spinning of the seams tricks a hitter's brain into thinking the ball is diving at a steeper angle than it really is. In the video above, you can see the same illusion at work — the circle is dropping straight down the screen, but its spin makes it seem like it's moving to the left.

This is a well-known phenomenon called the curveball illusion. In a recent paper, a group of University of Rochester cognitive scientists conducted some tests to propose a new model of how the human brain uses motion to estimate the location of an object — and explain why it can sometimes be tricked.

They did so by carefully tracking the eye positions of study participants who watched videos of the curveball illusion and the related phenomenon of "peripheral slowing" (in which the circle about 1:10 into the video appears to spin more slowly when you're not looking directly at it).

They argue that both these illusions — which only occur when an object enters your peripheral vision — are caused by assumptions your brain makes about position when it doesn't have good data and can't see the object clearly. In essence, it uses what's called a Kalman filter: an algorithm that involves collecting a bunch of noisy data and making the best possible estimate based on all of it.

As the video notes, this is the same sort of algorithm GPS software uses to determine your location when you don't have a clear line of sight to a satellite. In response, it uses data on where you'd been a minute ago and your previous speed to make an estimate.

Of course, it doesn't always work perfectly: this is why the dot on your phone's map sometimes shows up blocks away from where you actually are. It seems a similar process goes on in the mind of a batter — and when a ball enters the field of peripheral vision, previous data on location and apparent spin can lead the brain astray.