Although security services the world over have used lie detectors to lock many people up over the years, they are not very reliable. However, a new way of scanning brain activity may change that.
The long-established polygraph test measures a person's respiration rate, heart rate, and perspiration. The idea is to detecting anxiety associated with guilt or lying.
But simply being anxious – about, say, being interrogated – can produce similar signals, and some people may be able to learn to beat the test. By some accounts, the results from such tests are no more accurate than guesswork.
Measuring brain activity directly is though to be a more promising approach. This is currently achieved using technologies such as functional magnetic resonance imaging or EEGs.
But these methods have disadvantages of their own. For example, EEGs provide only a very low resolution picture of brain function, and fMRIs are hugely expensive, while scans are ruined by small movement of the subject.
Scott Bunce, at Drexel University's College of Medicine in Philadelphia, thinks a better solution is to send near-infrared light through the scalp and skull into the brain and see how much is reflected back. And he has designed a special headband that does just that
The amount of reflected light is dependent on the levels of oxygen in the blood, which in turn depends on how active the brain is at that point.
This, he says, gives a detailed picture of real-time activity within the brain that can be used to determine whether the subject is lying. The technique is both cheaper and easier to apply than fMRI and gives a higher resolution than an EEG.
A similar approach is being used to investigate Alzheimer's disease.
Of course, nobody knows whether brain activity can reliably be decoded to reveal deception, but that's another question.