Electro-encephalography (EEG) helped doctors realize that several patients diagnosed as being in a permanently vegetative state were in fact aware, according to a study published on Thursday in The Lancet.

The technique could be developed as a portable, cheaper way of helping doctors make more accurate diagnoses and establish contact with patients who are immobile but aware, its authors say.

A persistent or permanent vegetative state is defined as "wakefulness without conscious awareness of self and environment." During a coma, by contrast, a patient lacks both awareness and wakefulness.

Researchers led by Adrian Owen and Damian Cruse at the Centre for Brain and Mind at the University of Western Ontario, Canada, carried out a test involving 16 brain-damaged patients in the vegetative state and 12 healthy controls.

EEG entails placing sensors on the scalp to record electrical signals that result from activity by brain cells.

Three of the 16 patients accurately and persistently showed a clear EEG response when they were asked to imagine movements of their right hand and toes, according to the paper.

Electrical signals on the top of their scalp matched those of the controls when the patients were asked to carry out this motor movement, even though their bodies did not make any motion.

The authors say they do not presume to draw conclusions about the "inner worlds" of the three patients on the basis of this experiment.

But they note that understanding the request and processing it in the brain was complex, requiring sustained attention, selecting the right response and understanding language.

"Despite rigorous clinical assessment, many patients in the vegetative state are misdiagnosed," the researchers say.

"The EEG method that we developed is cheap, portable, widely available and objective. It could reach all vegetative patients and fundamentally change their bedside assessment."

The technique is considered less sensitive than functional magnetic resonance imaging (fMRI), which monitors brain flow across the brain and has been used in several important experiments to determine awareness among vegetative patients.

But fMRI scanners are very expensive and cannot be used on patients with metal in their body, often the case with patients whose severe brain damage occurs in a car accident.

EEG diagnosis, if refined, could move beyond simple "yes/no" responses to include methods of communication that are more expressive, the authors say.

Development of techniques for real-time classification of different forms of mental imagery "will enable two-way communication with some of these patients, allowing them to share information about their inner worlds, experiences and needs," they conclude.

As opposed to brain death, persistent vegetative state is not recognised by legal systems as death.

"Lock-in" syndrome describes a condition in which mental functioning is normal, but the body inert and incapable of responding to commands.