When you whack yourself with a hammer, it feels like the pain is in your thumb. But really it's in your brain.
That's because our perception of pain is shaped by brain circuits that are constantly filtering the information coming from our sensory nerves, says David Linden, a professor of neuroscience at Johns Hopkins University and author of the new book Touch: The Science of Hand, Heart, and Mind.
"The brain can say, 'Hey that's interesting. Turn up the volume on this pain information that's coming in,' " Linden says. "Or it can say, 'Oh no — let's turn down the volume on that and pay less attention to it.' "
This ability to modulate pain explains the experiences of people like Dwayne Turner, an Army combat medic in Iraq who received the Silver Star for valor.
In 2003, Turner was unloading supplies when his unit came under attack. He was wounded by a grenade. "He took shrapnel in his leg, in his side — and he didn't even notice that he had been hit," Linden says.
Despite his injuries, Turner began giving first aid and pulled other soldiers to safety. As he worked, he was shot twice — one bullet breaking a bone in his arm. Yet Turner would say later that he felt almost no pain.
"Soldiers in the heat of the moment don't recognize the pain that's happening," Linden says. But once that moment is over, those same soldiers may feel a lot of pain from something minor, like a hypodermic needle, he says.
The brain also determines the emotion we attach to each painful experience, Linden says. That's possible, he explains, because the brain uses two different systems to process pain information coming from our nerve endings.
One system determines the pain's location, intensity and characteristics: stabbing, aching, burning, etc.
"And then," Linden says, "there is a completely separate system for the emotional aspect of pain — the part that makes us go, 'Ow! This is terrible.' "