It is a common reaction -- hear the dentist's drill rev up and the sweat breaks out. Now scientists say the feeling is not only real, but they can show just what happens in the brain to cause it.

Reporting in the journal Science Thursday, Alexander Ploghaus and colleagues at Britain's Oxford University and in Canada say they hope their findings can lead to better ways to treat chronic pain.

They did brain scans of people who were anticipating pain and found a certain area is activated, which may may help prove a theory that the fear of pain is worse than the pain itself.

"One knows the situation where one clings to the dentist chair in anticipation of pain and probably because of that finds the pain subsequently gets more intense than if they relaxed," Ploghaus said in a telephone interview.

His team recruited 12 volunteers who agreed to be strapped to a machine that would nearly burn their hands, deliver a warm feeling, or do nothing at all.

Each painful pulse was preceded by a light of one color, warmth by another color and nothing by yet a third color. The volunteers quickly learned to associate a certain color light with pain.

"We measured brain activity throughout the experiment," Ploghaus said. They used functional magnetic resonance imaging, or fMRI, which measured which brain cells were activated by detecting how much oxygen was used.

Although there was some variation by individuals, for the most part one part of the brain lit up in anticipation of the painful burn and another lit up when the burn itself was delivered.

"Expectation of pain activated sites within the medial frontal lobe, insular cortex and cerebellum distinct from, but close to, locations mediating pain experience itself," Ploghaus's team wrote.

"The area that has been involved in anticipation of pain is an area that is believed to be involved in 'gut feeling' decision," Ploghaus said.

Previous experiments have shown that people often realize something unconsciously before they realize it consciously, and a certain part of their brain is activated.

This could be involved in pain response, too. "I think we are seeing here some autonomic conditioning -- the typical changes that you see when you have to run away from a threat," he said.

These include faster heartbeat, a quick look for an escape route, and the loosening of the bowels meant to lighten an animal for quick flight.

Ploghaus said experiments on rats have shown a similar pattern, so the response is something that is probably common to most animals.

Ploghaus, who experimented on himself before subjecting any of his volunteers to his pain machine, said the heat was bad enough to be distressing but not enough to cause a burn.

"I felt quite threatened when I saw the light preceding the pain," he said.

He hopes to use this research to help people with chronic pain. Recent studies suggest that the fear of pain is much worse than the pain itself for such patients.

"Our method may be used to identify effective treatments to reduce this pain," Ploghaus said.

"We want to screen drugs and psychotherapy for their ability to reduce the anticipation activation. If that activity is abolished, that shows us these treatments can be effective in reducing pain in chronic pain patients."