The University of California scientists were able to “read” the electric recordings to predict what word was being heard.

Professor Robert Knight, co-author of a paper in the journal Plos Biology, was excited by the finsings.

“This is huge for patients who have damage to their speech mechanisms because of a stroke or Lou Gehrig’s disease and can’t speak,” he said.

“If you could eventually reconstruct imagined conversations from brain activity, thousands of people could benefit.”

Fears scientists will be able to listen in on people’s innermost thoughts were unfounded, he said.

“We can rest assured that our skulls will remain an impenetrable barrier for any would-be technological mind hacker for any foreseeable future.”

The research built on earlier studies, which recorded words in the brains of ferrets – even though the ferrets could not understand them.

The latest research was undertaken on epileptic patients undergoing surgery to find the area of seizures in their brains so it could be removed.

Neurosurgeons cut a hole in the skull and then placed 256 electrodes on the brain surface covering the temporal lobe to record activity and pinpoint the area of seizures.

Co-author Dr Brian Pasley visited the patients and recorded five to 10 minutes of  conversation. Then he used computers to match the recorded activity to sounds.

He compared the technique to a pianist who knows the sounds of the keys so well that he can look at the keys another pianist is playing in a soundproof room and “hear” the music just as composer Ludwig Van Beethoven was able to “hear” his music despite being deaf.