The basic science behind the idea is not much more difficult. Brain cells emit electronic signals in a rhythmic, up-and-down pattern. These signals can be recorded from a person's scalp, and the resulting sequence of peaks and dips is called a brainwave. One of these waves, the P300, swoops enormously when it recognizes an image. The "P" aspect stands for positive, and the "300" refers to the number of milliseconds the wave occurs after recognition.
In 1987, Rosenfeld tried his P300 test on ten subjects. Each subject "stole" one item from a box of nine desirables. By actually touching the item, subjects formed a bond with the object that would result in a P300 response, Rosenfeld predicted. The subjects then watched names of the items flash across a monitor. When non-stolen items appeared, the brainwaves showed up normal. But when the stolen item flashed on the screen, the subject's brainwave formed a distinct P300 response.
The main advantage of this method over the traditional polygraph is striking: deception is implied without the suspect saying a single word. In fact, the P300 cannot even be considered a lie detector. "You're looking at recognition, not lying," Rosenfeld says. "However, I think the inference is justified if you take the proper measures."
In the 1990s, a scientist named Lawrence Farwell combined the guilty knowledge test and the P300 technique to create a commercial lie detector called Brain Fingerprinting. In 2000, Brain Fingerprinting almost gained admission to the courtroom during an appeal of a murder case in Iowa. (A district court judge rejected the appeal but ruled that the technique could have been admissible. A State Supreme Court judge eventually upheld the appeal, but did not take Brain Fingerprinting results into account.)
But a drawback of lie detectors based on the P300 method is that investigators must work very hard to find unusual items that only the criminal would have seen. Take the case of the bright green dress. If that dress is truly unique to the crime, the suspect will produce a powerful P300 response. But if the criminal's wife happens to wear a lot of green dresses, the P300 wave could be blunted down to regular size.
Functional imaging, often called fMRI, allows scientists to monitor brain activity in real time. Subjects are wheeled on a padded platform into a noisy magnetic resonance imaging machine that scans their brains every two seconds in search of increased neural activity. A small mirror allows them to see and react to prompts shown on a screen outside the machine. Meanwhile, from another room, investigators collect brain activity for statistical analysis.
The first fMRI study of lie detection to receive widespread attention was published in 2002 by Daniel Langleben of the University of Pennsylvania. Langleben handed his subjects a playing card—the five of clubs—before sliding them into the MRI machine. He encouraged them to deny having the card, offering a $20 reward for those who successfully deceived the machine, which was more than enough incentive for his undergraduate subjects.
During the test, subjects saw various playing cards on a screen and pushed a button indicating whether or not they had the card being shown. Most of the time, when subjects denied having the card on the screen, they were telling the truth. Only when the five of clubs appeared was the response a lie.
Langleben compared truthful brain activity with deceptive activity and found that a person's mind is generally more active when lying. This result suggests that truthfulness might be our default cognitive status, and that deception requires additional mental effort.