When it comes to lying, the brain may be our Achilles’ heel, rather than physiological indices like blood pressure, pulse, respiration and skin conductivity.

A new study from researchers at the University of Pennsylvania suggests functional magnetic resonance imaging (fMRI) is significantly more effective at spotting lies than a traditional polygraph test. In the study, neuroscience experts with no experience in lie detection were 24 percent more likely to spot falsehoods than professional polygraph examiners reviewing polygraph recordings.

An fMRI is a functional neuroimaging procedure using MRI technology that measures brain activity by detecting changes associated with blood flow. The technique relies on the fact that cerebral blood flow and neuronal activation are coupled. When an area of the brain is in use, blood flow to that region also increases.

This fMRI vs. polygraph study is the first to compare the two technologies in the same individuals in a blinded and prospective fashion.

Twenty-eight participants were given the Concealed Information Test (CIT), which is used by polygraph experts to establish a baseline. Next, each participant wrote down a number between three and eight and were administered both the CIT while hooked up to a polygraph or lying inside an MRI scanner. All participants took both tests, in different orders, a few hours apart.

Participants were instructed to answer “no” to all questions about what number they wrote down, making one of the six answers a lie. The results were then evaluated by three polygraph and three neuroimaging experts.

Using a logistic regression, the authors found fMRI experts were 24 percent more likely to detect the concealed number than the polygraph experts.

In one example, fMRI clearly shows increased brain activity when a participant, who picked the number seven, is asked if that is their number. Experts who studied the polygraph counterpart incorrectly identified the number six as the lie. The polygraph associated with the number six shows high peaks after the participant is asked the same questions several times in a row, suggesting that answer was a lie.  The scenario was reversed in another example, as neither fMRI nor polygraph experts were perfect, which is demonstrated in the paper.

Photo: University of Pennsylvania

“Polygraph measures reflect complex activity of the peripheral nervous system that is reduced to only a few parameters, while fMRI is looking at thousands of brain clusters with higher resolution in both space and time,” explained Daniel Langleben, the study’s lead author and professor of psychiatry at the University of Pennsylvania. “While neither type of activity is unique to lying, we expected brain activity to be a more specific marker, and this is what I believe we found.”

The researchers made a second unexpected, but equally important, discovery. In the 17 cases where the polygraph examiners and neuroscience experts agreed on the concealed number, they were 100 percent correct—suggesting a possible concurrent technique in the future.

“These data justify further evaluation of fMRI as a potential alternative to polygraphy,” the study’s conclusion reads. “The sequential or concurrent use of psychophysiology and neuroimaging in lie detection also deserves new consideration.”

The polygraph is currently inadmissible in court in most U.S. states based on it’s unclear performance rate and subjectivity. There are, however, special circumstances that allow the results of a polygraph in court if both parties agree to its use ahead of time.

The study "Polygraphy and Functional Magnetic Resonance Imaging in Lie Detection: A Controlled Blind Comparison Using the Concealed Information Test" was published recently in The Journal of Clinical Psychiatry.