NEW YORK (AP) — No one knows what led a Florida neighborhood watch captain to shoot Trayvon Martin, a teenager carrying no weapon.

But a new study raises an intriguing question: Could the watch captain have been fooled into thinking the youth was armed in part because he himself was holding a gun?

In the study, volunteers who held a toy gun and glimpsed fleeting images of people holding an object were biased toward thinking the object was a gun.

It's another indication that the brain shapes what we perceive in the world beyond the information that comes in through our eyes, said James Brockmole of the University of Notre Dame, who did the work with psychologist Jessica Witt at Purdue University.

In a telephone interview, Brockmole stressed he had no inside information on the Feb. 26 shooting of 17-year-old Martin, who was shot and killed in a gated community in Sanford, a suburb of Orlando. The neighborhood watch captain, George Zimmerman, said he shot the teen in self-defense because the youth attacked him. The case has drawn outrage and protests, and the federal Department of Justice said Monday it will investigate.

Brockmole said it's possible that Zimmerman's perception might have been skewed by being armed.

Race may have also played a role. Martin is black; Zimmerman's family says he is Hispanic. Past research suggests that people can be more likely to perceive a poorly seen object as a gun if it's held by a black person than by a white person, experts say.

Zimmerman has not spoken publicly. The police report does not mention whether he thought Martin had a firearm. But during his patrol of the neighborhood in his SUV, Zimmerman called 911 and told a police dispatcher that he was following Martin. "We've had some break-ins my neighborhood. ...There is a really suspicious guy."

Then a bit later, he said the youth was approaching and "he's got something in his hands."

In the study which was carried out well before the shooting, undergraduates at Notre Dame and Purdue glimpsed scenes of people holding objects and had to decide quickly whether the object was a gun. The results showed they were biased toward thinking so if they themselves were holding a toy gun, rather than a plastic ball. Just having a gun nearby didn't make a difference, researchers found.

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Why is that? Brockmole said people are primed to act in the world rather than just passively see it. So their minds have to contain information both about what they see and what they might do in response. Evidently, each kind of information can influence the other, he said.

He said the work, which is set to be published in the Journal of Experimental Psychology: Human Perception and Performance, is not intended to support gun control. But he said it suggests that people should know that when they hold a gun "that might change how you're going to interpret what's around you."

Brockmole's findings make sense, said Evan Risko, who studies perception and attention at Arizona State University. "Our perception is influenced by a number of different factors, and that can have important consequences," he said.

Dennis Proffitt, who studies visual perception at the University of Virginia, said there are many reasons why one person might think another is armed, such as if he is worried about his own safety or if he thinks the other person is a robber. The effect of holding a gun oneself "could be part of the story" in Florida, he said.

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Associated Press writer Mike Schneider in Sanford, Fla., contributed to this report.

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Malcolm Ritter can be followed at http://www.twitter.com/malcolmritter