The fMRI Brain Scan: A Better Lie Detector?

It would seem that being honest is an absolute, undebatable state. A person is either truthful or he's not. Right?

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Consider this scenario: a shopkeeper mistakenly returns an extra $10 in change to a customer. In one outcome, the customer returns the money promptly, without pause. In another, he hesitates for just a second, thinks about pocketing the 10 bucks, then decides to give it back.

Which is true honesty?

That is the question that Joshua Greene, 35, an assistant professor of psychology at Harvard University is trying to answer. More specifically, Greene is trying to identify the particular pattern of brain activity that distinguishes people who are simply telling the truth from those who are resisting the temptation to lie. His findings, which are based on functional-magnetic-resonance-imaging (fMRI) data, shed light not only on the workings of the human mind but also on the controversy over using fMRI technology outside the lab in the detection of lies. (Check out a story about how to spot a liar.)

In a cleverly designed experiment, recently published in the Proceedings of the National Academy of Sciences, Greene recruited 35 volunteers and told them they would be participating in a study to find out whether people are good at predicting the future when they are paid for it. The real purpose, of course, was to get people to lie without asking them to lie — and image their brains committing an act of deception.

While inside an fMRI scanner, each participant was asked to predict the outcome — heads or tails — of about 210 coin tosses. The participants made their predictions privately, but after each toss, researchers asked them to reveal whether or not they had guessed accurately. A display mounted inside the scanner flashed the questions, and participants pressed a button in response. Each correct prediction was awarded up to $7; incorrect predictions were awarded nothing, but there was ample opportunity to lie and still win the money.

The researchers then divided the volunteers into groups on the basis of their answers. Those who reported an improbably high number of correct answers were labeled dishonest. Most of the others were classified as honest. Researchers then averaged the fMRI data — which monitors blood flow and, therefore, activity inside the brain in real time — for each group to try to establish a neural signature that represented truth-telling and one that characterized lying. (See the top 10 scientific discoveries of 2008.)

Compared with the lying group, honest volunteers had relatively quiet minds — that is, they showed no distinctive activity in the prefrontal cortex, an area of the brain responsible for planning and decision-making. In the dishonest group, however, areas within the volunteers' prefrontal cortices registered vigorous activity — and the activity persisted whether they were lying or not.

What does this mean? Greene suggests that in some circumstances, real honesty is not about overcoming the temptation to lie but about not having to deal with that temptation in the first place. On an fMRI image, at least, the lying brain may look no different from one that's simply contemplating whether to lie. "Within the dishonest group, we saw no basis for distinguishing lies from honest reports," says Greene.

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