To develop a theory of deception requires parsing the subject into its most basic components so it can be studied one element at a time. That's what Daniel Langleben has been doing at the University of Pennsylvania. Langleben, a psychiatrist, started an experiment on deception in 2000 with a simple design: a spontaneous yes-no lie using a deck of playing cards. His research involved taking brain images with a functional-M.R.I. scanner, a contraption not much bigger than a kayak but weighing 10 tons. Unlike a traditional M.R.I., which provides a picture of the brain's anatomy, the functional M.R.I. shows the brain in action. It takes a reading, every two to three seconds, of how much oxygen is being used throughout the brain, and that information is superimposed on an anatomical brain map to determine which regions are most active while performing a particular task.

There's very little about being in a functional-M.R.I. scanner that is natural: you are flat on your back, absolutely still, with your head immobilized by pillows and straps. The scanner makes a dreadful din, which headphones barely muffle. If you're part of an experiment, you might be given a device with buttons to press for "yes" or "no" and another device with a single panic button. Not only is the physical setup unnatural, but in most deception studies the experimental design is unnatural, too. It is difficult to replicate the real-world conditions of lying -- the relationship between liar and target, the urgency not to get caught -- in a functional-M.R.I. lab, or in any other kind of lab. But as an early step in mapping the lying brain, such artificiality has to suffice.

In Langleben's first deception study at Penn, the subjects were told at the beginning of the experiment to lie about a particular playing card, the five of clubs. To be sure the card carried no emotional weight, Langleben screened out compulsive gamblers from the group. One at a time, the subjects lay motionless in the scanner, watched pictures of playing cards flash onto a screen and pressed a button indicating whether they had that card or not. When an image of a card they didn't have came up, the subjects, as they had been instructed, told the truth and pressed "no." But when an image of the five of clubs came up, they also pressed "no," even though the card was in their pockets. That is, whenever they saw the five of clubs, they lied.

According to Langleben, certain regions of the brain were more active on average when his 18 subjects were lying than when they were telling the truth. Lying was associated with increased activity in several areas of the cortex, including the anterior cingulate cortex and the superior frontal gyrus. "We didn't have a map of deception in the brain -- we still don't -- so we didn't know exactly what this meant," Langleben said. "But that wasn't the question we were asking at the time in any case. What we were asking with that first experiment was, 'Can the difference in brain activity between lie and truth be detected by functional M.R.I.?' Our study showed that it can." He said that the prefrontal cortex -- the reasoning part of the brain -- was generally more aroused during lying than during truth-telling, an indication that it took more cognitive work to lie.

Brain mappers are just beginning to figure out how different parts of the brain function. The function of one region found to be activated in the five-of-clubs experiment, the anterior cingulate cortex, is still the subject of some debate; it is thought, among other things, to help a person choose between two conflicting responses, which makes it a logical place to look for a signature of deception. This region is also activated during the Stroop task, in which a series of words are written in different colors and the subject must respond with what color the ink is, disregarding the word itself. This is harder than it sounds, at least when the written word is a color word that is different from the ink it is written in. If the word "red" is written in blue, for instance, a lot of people say "red" instead of "blue." Telling a spontaneous lie is similar to the Stroop task in that it involves holding two things in mind simultaneously -- in this case, the truth and the lie -- and making a choice about which one to apply.

Langleben performed his card experiment again in 2003, with a few refinements, including giving his subjects the choice of two cards to lie about and whether to lie at all. This second study found activation in some of the same regions as the first, establishing a pattern of deception-related activity in particular parts of the cortex: one in the front, two on the sides and two in the back. The finding in the back, the parietal cortex, intrigued Langleben.

"At first I thought the parietal finding was a fluke," he said. The parietal cortex is usually activated during arousal of various kinds. It is also involved in the manifestation of thoughts as physical changes, like goose bumps that erupt when you're afraid, or sweating that increases when you lie. The connection to sweating interested Langleben, since sweating is also one of the polygraph's hallmark measurements. He looked at existing studies of this response, and in all of them he found activity that could be traced back to the parietal lobe. Until Langleben's observation of its connection to brain changes, the sweat response (which the polygraph measures with sensors on the palm or fingertips) had been thought to be a purely "downstream" change, a secondary effect caused not by the lie itself but by the consequences of lying: guilt, anxiety, fear or the excess positive emotion one researcher calls "duping delight." But Langleben's findings indicated that it might have a corollary "upstream," in the central nervous system. This meant that at least one polygraph measurement might have a signature right at the source of the lie, the brain itself.