Polygraph tests rely on measures of stress, such as heart rate and blood pressure, which can shoot up when one is telling a lie. But the stress of being accused of a crime can also trigger a stress response, making it difficult for examiners to interpret the results. FMRI-based lie-detection systems seek to assess a more direct measure of deceit: the level of activity in brain areas linked with lying. Previous studies have shown that the brain appears more active when someone is telling a falsehood, especially the brain areas involved in resolving conflict and cognitive control. Scientists think that lying is more cognitively complex than telling the truth, and therefore it activates more of the brain.

A few scientists say they have devised algorithms to identify deceit-specific patterns in individuals. In one study published in 2005, for example, subjects were asked to commit a fake crime--they stole a watch or a ring--and were then instructed to answer a series of questions, giving false answers to those about the crime but answering truthfully when asked about other things. Using such an algorithm, scientists were correctly able to detect lies 90 percent of the time.

But that's just not good enough, said Nancy Kanwisher, a neuroscientist at MIT who also spoke on the panel. She said that these studies don't recreate the real-world situation well enough to truly uncover lies. "Making a false response when ordered to do so is not a lie," said Kanwisher. "The stakes in the real world are much higher. Someone accused of a crime, guilty or not, will feel very anxious, and that will affect the data."

Emotion also affects the results of lie-detection tests, according to Elizabeth Phelps, a neuroscientist at New York University who spoke at the symposium. Previous research has shown that brain-activity patterns change when a person is asked to, say, read emotionally charged words rather than neutral ones. "The neural circuitry used for lie detection is significantly modified by emotion," Phelps said.

Those developing fMRI for lie detection say that the criticisms are too harsh. According to Steven Laken, CEO of Cephos Corporation, one of the companies that hopes to commercialize fMRI, "Too often, people present this as a done deal. We are continuing to do research and develop the technology as much as we can." He adds that Cephos's scientific collaborators, based at the Medical University of South Carolina and at the University of Texas Southwestern Medical Center, in Dallas, are already exploring some of the issues brought up by the panel. They are planning studies in which subjects must carry out tasks designed to elicit an emotional response, such as stabbing a dummy, and are tested with fMRI much later, as would happen in the real world.

One of the most important tests for the technology will likely be to identify the specific situations in which fMRI can reliably detect someone's honesty or deceit. Joy Hirsch, a neuroscientist at Columbia University, in New York, says that she agrees that real-world deceit is different than giving a false answer on request, as is done in the lab. "But the situation that I think fMRI, with its current technology, can speak to is innocence," says Hirsch. "If someone is telling the truth about something, we should be able to detect that."

Cephos does not yet offer the technology commercially, but when it does, Laken says the company will be "very selective on who it is and how it is we will scan people."