Even the most advanced techniques, such a fMRI, can be fooled.
Finally, there are ways of fooling a brain scanner, just as there are countermeasures for other lie-detection techniques. Ganis says, “I’ve done a study showing that you can play mental tricks with fMRI. You mentally associate the important events of your life to items that are shown during the test.” By bringing those events to mind at the right time, volunteers could bamboozle the scans, and slash their accuracy from 100% to just 33%.“If you ever want to apply a technique like this in real cases, where people have motivation to beat the test, that’ll become an important issue,” says Ganis.
FMRI scanners will undoubtedly improve, but the problems of countermeasures and the subjectivity of memory, may be harder to solve. A report from the Royal Society on neuroscience and the lawsaid that these problems were “seemingly insuperable”. Ganis agrees: “If you want a general lie detector, that’s definitely science fiction right now.”
Elizabeth Phelps, a prominent cognitive neuroscientist at N.Y.U., who studies emotion and the brain, questions another basic assumption behind all lie-detection schemes--that telling a falsehood creates conflict within the liar. With the polygraph, the assumption is that the conflict is emotional: the liar feels guilty or anxious, and these feelings produce a measurable physiological response. With brain imaging, the assumption is that the conflict is cognitive:the liar has to work a little harder to make up a story, or even to stop himself from telling the truth. Neither is necessarily right. "Sociopaths don't feel the same conflict when they lie," Phelps says. "The regions of the brain that might be involved if you have to inhibit a response may not be the same when you're a sociopath, or autistic, or maybe just strange.