The research breaks controversial new ground in scientists' ability to probe people's minds and eavesdrop on their thoughts, and raises serious ethical issues over how brain-reading technology may be used in the future.
The team used high-resolution brain scans to identify patterns of activity before translating them into meaningful thoughts, revealing what a person planned to do in the near future. It is the first time scientists have succeeded in reading intentions in this way.
"Using the scanner, we could look around the brain for this information and read out something that from the outside there's no way you could possibly tell is in there. It's like shining a torch around, looking for writing on a wall," said John-Dylan Haynes at the Max Planck Institute for Human Cognitive and Brain Sciences in Germany, who led the study with colleagues at University College London and Oxford University.
The research builds on a series of recent studies in which brain imaging has been used to identify tell-tale activity linked to lying, violent behaviour and racial prejudice.
The latest work reveals the dramatic pace at which neuroscience is progressing, prompting the researchers to call for an urgent debate into the ethical issues surrounding future uses for the technology. If brain-reading can be refined, it could quickly be adopted to assist interrogations of criminals and terrorists, and even usher in a "Minority Report" era (as portrayed in the Steven Spielberg science fiction film of that name), where judgments are handed down before the law is broken on the strength of an incriminating brain scan.
"These techniques are emerging and we need an ethical debate about the implications, so that one day we're not surprised and overwhelmed and caught on the wrong foot by what they can do. These things are going to come to us in the next few years and we should really be prepared," Professor Haynes told the Guardian.
The use of brain scanners to judge whether people are likely to commit crimes is a contentious issue that society should tackle now, according to Prof Haynes. "We see the danger that this might become compulsory one day, but we have to be aware that if we prohibit it, we are also denying people who aren't going to commit any crime the possibility of proving their innocence."
During the study, the researchers asked volunteers to decide whether to add or subtract two numbers they were later shown on a screen.
Before the numbers flashed up, they were given a brain scan using a technique called functional magnetic imaging resonance. The researchers then used a software that had been designed to spot subtle differences in brain activity to predict the person's intentions with 70% accuracy.
The study revealed signatures of activity in a marble-sized part of the brain called the medial prefrontal cortex that changed when a person intended to add the numbers or subtract them.
Because brains differ so much, the scientists need a good idea of what a person's brain activity looks like when they are thinking something to be able to spot it in a scan, but researchers are already devising ways of deducing what patterns are associated with different thoughts.
Barbara Sahakian, a professor of neuro-psychology at Cambridge University, said the rapid advances in neuroscience had forced scientists in the field to set up their own neuroethics society late last year to consider the ramifications of their research.
"Do we want to become a 'Minority Report' society where we're preventing crimes that might not happen?," she asked. "For some of these techniques, it's just a matter of time. It is just another new technology that society has to come to terms with and use for the good, but we should discuss and debate it now because what we don't want is for it to leak into use in court willy nilly without people having thought about the consequences.
"A lot of neuroscientists in the field are very cautious and say we can't talk about reading individuals' minds, and right now that is very true, but we're moving ahead so rapidly, it's not going to be that long before we will be able to tell whether someone's making up a story, or whether someone intended to do a crime with a certain degree of certainty."
Professor Colin Blakemore, a neuroscientist and director of the Medical Research Council, said: "We shouldn't go overboard about the power of these techniques at the moment, but what you can be absolutely sure of is that these will continue to roll out and we will have more and more ability to probe people's intentions, minds, background thoughts, hopes and emotions.
"Some of that is extremely desirable, because it will help with diagnosis, education and so on, but we need to be thinking the ethical issues through. It adds a whole new gloss to personal medical data and how it might be used."
The technology could also drive advances in brain-controlled computers and machinery to boost the quality of life for disabled people. Being able to read thoughts as they arise in a person's mind could lead to computers that allow people to operate email and the internet using thought alone, and write with word processors that can predict which word or sentence you want to type . The technology is also expected to lead to improvements in thought-controlled wheelchairs and artificial limbs that respond when a person imagines moving.
"You can imagine how tedious it is if you want to write a letter by using a cursor to pick out letters on a screen," said Prof Haynes. "It would be much better if you thought, 'I want to reply to this email', or, 'I'm thinking this word', and the computer can read that and understand what you want to do."
· FAQ: Mind reading
What have the scientists developed?
They have devised a system that analyses brain activity to work out a person's intentions before they have acted on them. More advanced versions may be able to read complex thoughts and even pick them up before the person is conscious of them.
How does it work?
The computer learns unique patterns of brain activity or signatures that correspond to different thoughts. It then scans the brain to look for these signatures and predicts what the person is thinking.
How could it be used?
It is expected to drive advances in brain-controlled computers, leading to artificial limbs and machinery that respond to thoughts. More advanced versions could be used to help interrogate criminals and assess prisoners before they are released. Controversially, they may be able to spot people who plan to commit crimes before they break the law.
What is next?
The researchers are honing the technique to distinguish between passing thoughts and genuine intentions.