Forget the eyes; a liar’s brain activity may be what gives him away, a study published Tuesday suggests.

Using a form of brain imaging called functional magnetic resonance imaging (MRI), researchers found that certain brain regions “light up” when a person tells a lie. In fact, twice as many brain areas were active when study participants practiced deception than when they told the truth.

“The brain has to work so much harder in lying than in telling the truth,” Dr. Feroze B. Mohamed, the study’s lead author, told Reuters Health.

Though it’s too early to judge the practical implications of the findings, he speculated that functional MRI could offer a more reliable alternative to the traditional polygraph.

Brain activity would presumably be “less susceptible to control” than the nervous system activities polygraph tests measure, explained Mohamed, an associate professor of radiology at Temple University School of Medicine in Philadelphia.

The polygraph is used in the criminal justice system, but is not infallible. Its “lie detection” is based on certain changes in a person’s sympathetic nervous system activity - increases in heart rate, breathing and perspiration.

A shortcoming is that those changes are not unique to lying. General anxiety and anger, for instance, can spur the same physiological responses, and polygraph results, Mohamed noted, appear less accurate when people are telling the truth than when they are lying.

Functional MRI measures real-time changes in blood flow in the brain. The results indicate which brain areas are firing during a given activity.

In the current study, published in the medical journal Radiology, the researchers had 11 adults undergo both polygraph testing and functional MRI after they had or had not committed a “crime.”

The crime in this case was firing a starter pistol, carrying only blanks, in the research setting. Half of the study participants fired the pistol, while the rest did not. All were told they would be questioned afterward as suspects in the hospital “shooting.”

Five of the participants were instructed to tell the truth and the other six were told to lie. Along with MRI scans, the subjects underwent a standard polygraph during questioning.

Overall, the MRI scans showed that 14 brain areas were significantly activated when study participants were being deceptive, compared with seven that were activated when the subjects were telling the truth.

Many of these brain areas, according to the researchers, are associated with planning, emotion and inhibition - which, in the case of lying, would mean inhibiting the truth.

When a person tells the truth, they note, there should be less anxiety, and no need for inhibition, so it makes sense that fewer brain regions would be active.

As for the polygraph results, they were accurate 92 percent of the time when study participants were lying. The test was less adept at spotting the truth - showing 70 percent accuracy.

It will be a “couple years,” Mohamed said, before researchers know whether brain scans can take the place of, or be used along with, polygraph tests.

Larger studies that look at lying in different scenarios are needed to determine whether the same brain regions are consistently activated during deception.

SOURCE: Radiology, February 2006.