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Brown study uses virtual reality to challenge popular baseball lore

"Keep your eye on the ball" is probably the best advice a baseball coach can give to his or her fielders, according to the results of a recent Brown study.

The research disproves the popular notion that outfielders predict in full the trajectory of the ball, said the paper's senior author, Professor of Cognitive and Linguistic Sciences William Warren. Instead of running to where they think the ball will land, outfielders track the velocity of the ball and move in order to keep the speed constant from their point of view, in accordance with a model called optical acceleration cancellation.

In this model, fielders aim to keep their eyes still as the ball nears. If a fielder's eyes are moving up, the ball will land behind him or her, while downward movement indicates that the ball will land short, Warren said.

The study was published in last month's Journal of Vision and was funded by the National Institutes of Health.

The research deepens knowledge about the link between visual cues and movement in human cognition, Warren said. He said the study also raises an important question: Why are some people much better at catching fly balls than others? The study's conclusions could also have a big impact on how Little League players are taught to catch fly balls, Warren said.

The other two members of the team, which began its work in 2002, are Philip Fink, formerly a postdoctoral associate at Brown and now a lecturer in exercise and sport science at Massey University in New Zealand, and Patrick Foo, also a former postdoctoral associate at Brown, now an assistant professor of psychology at the University of North Carolina at Asheville.

Warren said he and his team employed virtual reality techniques to arrive at their conclusion. The eight varsity baseball players and four varsity softball players who participated in the experiment wore head-mounted displays, which allowed the researchers to monitor their movements as they were fed virtual fly balls. 

As the participants ran around the 40-by-40 foot experiment space, the researchers programmed the computer to change the downward course of some balls, having them travel in an impossibly straight trajectory as opposed to a parabola. By observing the participants' reactions to the balls' altered paths, the team was able to conclude that prediction has little to do with an outfielder's movements.

The team's research question proposed to investigate which of three models better describes how baseball players catch high balls. The first model, called the mental model of trajectory, maintains that baseball players view the initial trajectory of a ball's parabola and calculate a landing spot, to which they then run. The problem with this hypothesis is that "the human visual system can't see the ball's motion and velocity in three dimensions from that far away," Warren said.

If the prediction model were true, the participants would have shown no reaction to the altered path of the ball. Instead, the fielders adjusted immediately to the impossible trajectory, and even caught the ball, Warren said.

The second model the team considered is called linear optical trajectory. This hypothesis maintains that outfielders constantly keep their eyes on the ball and move to make their visual image of the ball form a straight line.

Warren said his team has "ruled out the linear optical trajectory" because of a subtle inconsistency. The theory contends that the fielder's effort to keep his visual image of the ball straight causes him to move forward and backward at the same time as he moves side-to-side. Though the two movements could be related, there were no side-to-side adjustments when the virtual trajectory of the ball was modified, indicating that the two movements are independent of each other, Warren said. This indicates that human vision more closely follows the optical acceleration cancellation model, he added.




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