Last year two patients were able to write messages on a computer via electrodes implanted in their brains but the new method does not require risky surgery, simply placing electrodes on the top of the head.

"We have got patients writing messages who couldn't communicate at all," said Edward Taub of the University of Alabama at Birmingham in New Scientist magazine.

Motor cortex signals

The scientists, led by Niels Birbaumer of the University of Tubingen, Germany, has to train the three patients to harness their brain power. The two electrodes, the size of contact lenses, picked up signals from near the motor cortex.

The on-screen panel has 32 letters and punctuation marks

The patients had to learn to make their cortical potentials more negative or positive to move a cursor up and down a computer screen. At each successive session the researchers made the task more difficult, requiring the patients to generate bigger and bigger changes in their cortical potentials.

Alphabet choice

When the patients could control the cursor well, they began to write. They selected each letter by whittling down the alphabet. First they choose one half of the alphabet, then half of that half and so on.

It took the patients an average of 80 seconds to pick a letter, meaning a short sentence could be written in about 30 minutes.

Dr Taub recognises that a system based solely on either-or choices will always be limited. But he believes it will be possible to train patients to make choices between more than two options if they can create several levels of positive or negative cortical potential.

In the meantime, the "thought translation device" could be speeded up by using the context of the sentence to make accurate guesses of the word after the first letters are typed.