英语听力：自然百科 Lie Detector 测谎仪的奥秘

It might be the cheeks flushing with embarrassment or a voice choking with emotion. Our physical reactions can reveal what we truly feel. And some think they can show when someone's lying.

Typically when a person tells a lie, they begin to sweat. I'm sure we've all seen the 1940s and 50s interrogation movies where the person in the hot seat has a sweaty brow or a sweaty upper lip or perhaps sweat dripping down from their sideburns.

"All right, if you lean forward in the chair, put your arms up (in) front of you, like you're gonna dive into a swimming pool. Put this around your stomach. "

Jack Trimarco uses physical sensations to probe for truth. His primary tool? The liedetector, or polygraph.

"This cuff is just like the one of the doctor."

Polygraph, its work's very similar to any other biofeedback instrument. It simply recordsdeviations from a person's physiological norm, such as someone's blood pressure going up and then coming back down, or perhaps a decrease or an increase in pulse rate, or sweat pore activity.

"Are you sometimes known as Bobbie?"

"Yes."

A polygraph examiner knows what to look for when questioning someone.

"D o you intend to be completely truthful with me?"

"Yes."

Any deviation from normal could signal deception. Our body can give us away.

"And are you sure that I won't try to trick you on this test?"

"Yes."

We may think we're being cool, but when we're stressed, our network of nerves starts to prepare our body for action as if it's under attack. It's a response that harks back to our ancient ancestors -- fight or flight.

It starts by triggering your stress hormone -- adrenalin. Your heart rate increases to pump more blood to your muscles as if you were about to go into battle. Your breathing deepens as vital oxygen reaches your muscles. And you begin to sweat -- the body cools so that you won't overheat when rushed into action. The brain won't let us stop the survival strategy. These basic nervous functions are automatic. That's why a polygraph can often uncover a liar.