Brain scans revealed which event a person was remembering
Scientists say they have been able to tell which past event a person is recalling using a brain scan.
The University College London researchers showed people film clips and were able to predict which ones they were subsequently thinking about.
The research, published in Current Biology, provides insight into how memories are recorded.
The authors hope the findings will ultimately contribute to development of treatments for memory loss.
Previous research has shown brain scans can predict simpler thought processes such as distinguishing between colours, objects or places.
The UCL researchers say recalling memories of past events is a more complex process.
The study builds on a previous discovery by the same team that they could tell where a person was standing in a virtual reality room using a brain scan.
"In our previous experiment, we were looking at basic memories, at someone's location in an environment," says Professor Maguire, from the Wellcome Trust Centre for Neuroimaging at UCL, who led the study.
"What is more interesting is to look at 'episodic' memories - the complex, everyday memories that include much more information on where we are, what we are doing and how we feel."
The researchers asked 10 volunteers to watch three short film clips of people doing everyday activities such a posting a letter or throwing a coffee cup in a bin.
The volunteers were then asked to remember each of the films in turn while inside a specialist MRI scanner, which recorded brain activity that was then studied by a computer programme.
The researchers found that in subsequent scanning sessions, the computer algorithm could predict which film the volunteers were thinking about from the pattern of their brain activity.
They said it was the first time that brain scans had been used to distinguish between memories of past events.
Experts praised the research. Richard Morris, professor of neuroscience at the University of Edinburgh, said: "These findings are a really valuable advance on traditional ways of analysing brain images. They look not just at the strength of the signal, but the actual pattern of activity across the brain.
"By doing this in memory areas, it is possible for the first time to distinguish one memory from another - even if both memories are equally strong."
But he pointed out that the research does not mean it is possible to literally know what a person is remembering.
"The computer algorithm doesn't really 'read' memories - it merely distinguishes one from another."
He said the findings could lead to a whole range of interesting developments.
"All manner of next steps come to mind - such as distinguishing a true memory from a false one, or a recent memory from one long ago."
The scientists say their research contributes to an understanding of how memories are formed and recalled. They hope it will eventually help develop treatments for people with memory loss though ageing or brain injury.
Dr Susanne Sorensen, Alzheimer's Society's head of research said: "This research is interesting because it tells us more about how memories are stored and recalled in normal healthy people.
"It is not directly related to the study of the memory problems that are one of the symptoms of dementia, but the methods developed in this study may in the longer term help us investigate what goes wrong in brains that are developing the diseases which cause dementia."