Activity in the hippocampus was monitored
Scientists say for the first time they have understood someone's thoughts by looking at what their brain is doing.
The hippocampus is widely known to be integral to memory, but researchers say they now see just how images are stored and recalled in this part of the brain.
Wellcome Trust scientists trained four participants to recognise several virtual reality environments.
Discernible patterns in brain activity then signalled where they were, they wrote in the journal Current Biology.
Neurons in the hippocampus, also known as "place cells", activate when we move around to tell us where we are.
The team, based at University College London, then used specialised scanning equipment which measures changes in blood flow in the brain.
This allowed them to examine the activity of these cells as the participants - all young men with experience of playing videogames - moved around the virtual reality environment. The data was then passed through a computer.
"We asked whether we could see any interesting patterns in the neural activity that could tell us what the participants were thinking, or in this case where they were," said Professor Eleanor Maguire.
Are you lying?
"Surprisingly, just by looking at the brain data we could predict exactly where they were in the virtual reality environment. In other words we could 'read' their spatial memories."
"By looking at activity over tens of thousands of neurons, we can see that there must be a functional structure - a pattern - to how these memories are encoded."
But they stressed that the prospect of genuinely reading someone's most intimate thoughts - or working out if they were lying - was still a long way off.
Their participants were all willing subjects who allowed their brains to be trained and monitoring to take place.
"It would be very easy not to co-operate, and then it wouldn't work," said Demis Hassabis, who developed the computer programme to read the data. "These kind of scenarios would require a great technological leap."
Participants were asked to navigate between virtual reality rooms
It is brain diseases such as Alzheimer's which could stand to benefit from such research.
"Understanding how we learn and store memories could aid our understanding of conditions in which memory is compromised and potentially help patients in the rehabilitation process," said Professor Maguire.
Professor Clive Ballard, director of research at the Alzheimer's Society, said: "This exciting development will boost our understanding of the hippocampus, a key area affected in Alzheimer's disease and the most important part of the brain for memory.
"Learning more about how the brain works could help us work out which types of nerve cells are lost in Alzheimer's."
Rebecca Wood, of the Alzheimer's Research Trust, said the research was "fascinating".
She said: "Understanding how memories are formed may help researchers discover how this process goes wrong in diseases like Alzheimer's."