By Michelle Roberts
Health reporter, BBC News
Conventional detectors monitor changes in vital signs like heart rate
Brain scans could be useful as lie detectors to show if a witness lies when identifying a suspect in a crime investigation, US researchers believe.
Scientists at Stanford University were able to tell when a person recognised a mug shot by reading their brain waves.
Functional magnetic resonance imaging (fMRI) revealed tell-tale brain activity during the memory recall task, Proceedings journal reports.
But experts warn the technology is not foolproof and can give false results.
And such false positives could have serious legal consequences, say Dr Jesse Rissman and his team, who conducted the research.
Put to the test
In their trial, they asked 16 volunteers to study hundreds of faces in an images database.
Next, each was shown a series of pictures that included some of the faces they had just seen and some that were new.
The researchers asked the volunteers to say which of the mug shots they recognised, while rigged up to the brain-scanning fMRI device.
Using computer software to analyse the brain scan data, the researchers were able to recognise distinct patterns that appeared to reflect what the individual was thinking.
Specifically, from the scans alone, they were able to tell if the volunteers recognised the faces as old or new and whether this recognition was accompanied by recollection.
But the technique was not perfect.
It was unable to reliably distinguish between subjects who accurately reported recognising a face and those who mistakenly claimed to recognise a previously unseen face.
Lead researcher Dr Jesse Rissman explained: "It was only as good as a person's memory and their memory may or may not be accurate."
He said for the technology to be of use in a court room, it would need to tell you not just that the person was recalling a memory but that the memory was accurate.
And it may be possible to cheat the scanner, he said.
"We can't tell from our data, because our participants were asked to make honest judgements, but if someone wanted to fool the test they might be able to."
To withhold the identity of a guilty suspect, a witness could fixate on a novel image or think of something new that they planned to do that day, for example.
Or to incriminate someone, the witness could think of a strong image from their past or remember a recent event, he said.
"These are things that would need to be checked. We need to do more work and plan to look at more in-depth memories and witness accounts.
"So the practical application is far off yet."
Professor Geraint Rees, from University College London's Institute of Cognitive Neuroscience, said: "Very strong claims are often made about the power of brain imaging to detect deception, so it is vitally important that scientifically rigorous studies like this one are carried out to critically evaluate these claims."