BBC Science Correspondent Pallab Ghosh reports

Although details of the breakthrough were first reported by BBC News Online in January, the researchers have now agreed to talk about their work for the first time and have released pictures of those involved in the study.

Electrodes are attached to the head

A letter written by one of patients has also been published in this week's science journal Nature.

The computer system depends on an individual's ability to control their brainwaves. Two electrodes, the size of contact lenses, are taped on to the head.

This allows an electroencephalogram to detect brain signals, which can be passed to a computer. By using the power of thought alone, patients can then drive a cursor on a video screen that selects letters of the alphabet.

Brain activity

"First the patient has to learn consciously to control a particular kind of brain activity which is called slow cortical potential, which everybody has," says Professor Niels Birbaumer of the University of Tübingen in Germany.

"It comprises slow changes in the excitation level of the brain. Patients learning to control this see their own brain activity on a computer screen in the form of a trace that moves up and down - so they can observe it continuously.

Professor Niels Birbaumer: We can make life better for patients

"Then the computer or a therapist asks the patient to control the shape of the trace and to use it to move a cursor on the screen.

"The computer helps by saying 'That's good, that's perfect', and so on and so on."

Patients can write on the screen at a rate of one letter every six seconds, and by using letters to represent key words that rate can be speeded up enormously. It can be used to control household appliances such as television sets.

Trapped butterfly

The device costs about $20,000 to produce.

It will benefit people who suffer from a progressive nervous disease called Amyotrophic lateral sclerosis (ALS), in which the nerve cells controlling movement, so-called motor neurons, progressively die off with the result that patients lose all control over their bodies.

Patients learn to control a cursor on the screen

There is no cure or effective treatment. The frustration and horror experienced by patients who find themselves in such a state was famously described by Jean-Dominique Bauby's book 'The Diving Bell and The Butterfly'. Bauby dictated the book, letter by letter, by winking.

"It describes the terrible locked-in state he faced before he died," Professor Birbaumer. "Such a state can be brought about by strokes or accidents as well as Amyotrophic lateral sclerosis.