Patients thought to be in a permanently "vegetative" state may still be able to think and communicate, a study has confirmed.
Scientists using a device to record brainwaves found "meaningful" responses from one in five patients who appeared unaware of the outside world.
Experts say it could change the way consciousness disorders are classified and may be a practical method for discovering the needs of patients, even whether they still want to live.
Researchers used a portable electroencephalography (EEG) device to record electrical signals from the brains of 16 patients diagnosed as being in a vegetative state.
They were told to imagine moving their right hand and toes although this was physically impossible.
Three produced repeated and appropriate brainwave patterns which showed they were aware of the commands and able to respond to them.
The study – which was published in an online edition of The Lancet medical journal – involved patients at Addenbrooke's Hospital in Cambridge and University Hospital of Liege, Belgium.
Previous studies have shown that functional magnetic resonance imaging scans can uncover awareness in apparently vegetative patients.
However, fMRI scans are expensive, are often unavailable and can produce confusing results.
Many brain-damaged patients also have metal implants due to traumatic injuries, which rule out MRI scans.
The EEG method is a "considerably cheaper and more portable bedside technique", said one of the researchers, Professor Adrian Owen, from the Medical Research Council's Cognition and Brain Sciences Unit, Cambridge.
"This method could reach all vegetative patients", he added.
Patients diagnosed as vegetative have periods of wakefulness but seem unaware of themselves or their surroundings.
The findings raise ethical issues. Previous research shows some patients with "locked-in syndrome" who cannot move or speak still find life worth living.