Scientists have devised a mind-reading technique which could possibly allow them to visualise someone's dream by analysing their brain activity with a medical scanner.

Scientists in the United States led by Professor Jack Gallant of the University of California, Berkeley, have built a computer that can 'decode' the brain activity signals from a scanner and match them to photographs of what a person has seen.

"Our results suggest that it may soon be possible to reconstruct a picture of a person's visual experience from measurements of brain activity alone," Prof Gallant wrote in journal Nature.

The study raises the possibility in the future of the technology being used to visualise scenes from a person's dreams or memory.

"Our data suggests that there might potentially be enough information in brain activity signals measured using functional magnetic resonance imaging to do this in the future... In fact, so much information is available in these signals that one day it may even be possible to reconstruct the visual contents of dreams or visual imagery," Prof Gallant stressed.

According to experts, the technique could also be useful for understanding the mental state of a person who is in a coma. However, it inevitably raises the fear that such a technology could be used for interrogating a person for 'thought crimes'.

The decoder works by analysing the patterns of activity within the visual centre of the brain, detected by an fMRI machine as a person looks at a set of randomly-arranged photographs, one at a time.