Scientists have developed mind-reading technology that can understand people's thoughts without the need for them to speak or even move.

The new non-invasive technology, which is able to interpret answers to questions by detecting changes of blood oxygen levels in the brain, offers the possibility that patients paralysed by injuries, motor-neurone diseases and a range of other conditions will be able to communicate again.

In a pioneering experiment, four people who were incapable of moving their eyes, were able to respond with "yes" or "no" answers. On seven out of 10 occasions the patients said they were happy despite their condition, which requires round-the-clock care.

"The striking results overturn my own theory that people with complete locked-in syndrome are not capable of communication," said Professor Niels Birbaumer, from the Wyss Center for Bio and Neuroengineering in Geneva.

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"If we can replicate this study in more patients I believe we could restore useful communication in completely locked-in states for people with motor neuron diseases."

Locked-in syndrome is classified as having mental awareness but complete paralysis except for up and down eye movements and blinking. In such cases, patients can communicate using simple blinking systems.

However, if all eye movements are lost, as in the case of patients in the study, the condition is referred to as complete locked-in syndrome.

But the new device was able to pick up the changes happening in the brain and read thoughts. All four patients were suffering from ALS - a progressive motor neuron disease that leads to complete destruction of the part of the nervous system responsible for movement.

Scientists asked questions which they already knew the answer to.

The researchers said they were amazed to find that all the participants reported being happy most of the time.

Birbaumer added: "All four had accepted artificial ventilation in order to sustain their life when breathing became impossible so, in a sense, they had already chosen to live.

The research was published in the journal PLOS Biology.