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OK, so you're in your car and you're heading for an intersection. The light turns yellow. But you think you can make it, so you decide to hit the gas. Then you see a police car. What do you do next? That depends on a sophisticated communications system in your brain, as NPR's Jon Hamilton reports.
JON HAMILTON, BYLINE: The moment you see that police car, you know that stomping on the accelerator is a big mistake. But Susan Courtney of Johns Hopkins University says there's a good chance you'll do it anyway.
SUSAN COURTNEY: Even if you haven't actually started moving your foot, your brain has already initiated that plan.
HAMILTON: Courtney says even as one area of your brain is recognizing that police car, other areas are carrying out your original plan to accelerate.
COURTNEY: So there's the decision area that recognized the yellow light. And that part of the brain then sends a signal to the foot moving part of the brain that then sends a signal to your foot.
HAMILTON: Courtney wanted to understand what it takes to change this sort of plan and when it's too late to be changed, so she had a team monitor the brain activity in 21 people as they encountered a situation that's a bit like approaching an intersection when the light turns yellow. Courtney says participants were asked to make a certain eye movement while they watched dots appear on a screen.
COURTNEY: One color dot meant continue to make that eye movement and the other color dot meant don't make that eye movement.
HAMILTON: In other, words don't do the thing you were planning to do. And it turns out that's complicated. Courtney and her team report in the journal Neuron that it took three key areas of the brain communicating with eight other areas to stop a planned movement. And all that communication had to occur within about a tenth of a second to prevent the wrong signal from heading for a muscle.
COURTNEY: So if the signal has already been sent, you can watch it happen without being able to stop it.
HAMILTON: Which is why we have that awful fleeting moment when our brain knows we shouldn't stomp on the gas, but our foot does it anyway. And Courtney says the brain's stop system appears to be involved in a lot more than just controlling our bodies.
COURTNEY: It's not just about stopping your foot or your eyes. It's about changing your plan about anything.
HAMILTON: Like blurting out to your boss, that's a stupid idea. Russ Poldrack of Stanford University has spent years studying the brain systems involved in stopping an action. He says one thing they seem to do is help us avoid danger.
RUSS POLDRACK: People now think that some of these same systems are involved in being more cautious in making choices, taking fewer risks, not just in kind of stopping yourself from making a motor action.
HAMILTON: And Poldrack says there's growing evidence that these circuits are faulty in people who have suffered certain kinds of brain damage or who abuse drugs. Poldrack says years ago, his lab did a study of people who take methamphetamine.
POLDRACK: Methamphetamine abusers were worse at stopping themselves on these very simple tasks. And their ability to stop themselves related to how much craving they had for the drug.
HAMILTON: The craving seems to be interfering with the brain's ability to change a plan it knows is a bad idea. Ultimately, Poldrack says, understanding these brain circuits is important because they have probably contributed to the success of our species.
POLDRACK: The thing that humans do better than any other species is sort of adapt ourselves to the world when things change. How is the brain wired up to let us do that?
HAMILTON: Poldrack says that's a question scientists are now closer to answering. Jon Hamilton, NPR News.
(SOUNDBITE OF GRAMATIK'S "JUST JAMMIN'")
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