It's probably a good idea to keep your opinions to yourself if your friend gets a terrible new haircut - but soon you might not get a choice.

That's because scientists at the University of Texas at Austin have trained an artificial intelligence (AI) to read a person's mind and turn their innermost thoughts into text.

Three study participants listened to stories while lying in an MRI machine, while an AI 'decoder' analysed their brain activity.

They were then asked to read a different story or make up their own, and the decoder could then turn the MRI data into text in real time.

The breakthrough raises concerns about 'mental privacy' as it could be the first step in being able to eavesdrop on others' thoughts.

Lead author Jerry Tang said he would not provide 'a false sense of security' by saying that the technology might not have the potential to eavesdrop on people's thoughts in the future, and said it could be 'misused' now.

But he said: 'We take very seriously the concerns that it could be used for bad purposes.

'And we want to dedicate a lot of time moving forward to try to try to avoid that.

'I think, right now, while the technology is in such an early state, it's important to be proactive and get a head-start on, for one, enacting policies that protect people's mental privacy, giving people a right to their thoughts and their brain data.