Researchers in Belgium have identified different brain responses linked to states of consciousness.
It is hoped the findings could improve the treatment and pain control of patients with brain damage.
Electrical responses in the brain to a series of simple tones were measured to potentially provide doctors with a test to determine different states of consciousness.
The scientists from the University of Liege identified a specific neural 'fingerprint' in the brain responses of healthy volunteers and minimally conscious individuals, but not in vegetative patients.
Studies have shown that severely brain damaged patients displaying few outward signs of awareness may still have some perception of what is happening around them.
The Belgian team combined electroencephalography (EEG), which measures electrical activity in the brain, with a sophisticated mathematical model.
EEG recordings were gathered from 22 healthy volunteers and 21 patients with severe brain damage.
Eight patients were vegetative, in a state of non-responsive wakefulness, and 13 minimally conscious, a condition in which there may be some deliberate movement such as eye-tracking.
None of the patients was in a coma, a state of deep unconsciousness lasting more than six hours.
Previous studies have shown that the brain's electrical response to the sound alters when there is a change in pitch. But this does not happen during sleep and under anaesthesia, suggesting that it may be an indicator of consciousness.
Both healthy volunteers and minimally conscious patients reacted to the changing tone frequency. However, only a fleeting response was seen in vegetative patients whose EEG readings altered for less than one tenth of a second.