Different brain responses linked to states of consciousness have been identified by researchers in Belgium.
They have discovered a test that can distinguish between states of consciousness using a simple electroencephalogram (EEG) and some mathematics.
According to the researchers, the key difference between minimally conscious and totally unconscious non-coma states is communication between the frontal cortex- the planning, thinking part of the brain - and the temporal cortex, where sounds and words are processed, reports Nature.
This technique could help doctors to make accurate diagnoses about consciousness, and better predict how a patient will recover.
The Coma Recovery Scale is the current 'gold standard', said co-author Melanie Boly, a neuroscientist at the University of Liege in Belgium.
The scale scores patients on the basis of whether they are capable of conscious behaviours such as speech and directed gaze.
Boly and her colleagues sought a biological tool to complement the behavioural scale. They used EEG to measure electrical signals from the brains of 8 people in vegetative states, 13 in minimally conscious states and 22 healthy participants.
In minimally conscious and healthy people the frontal cortex would then send a message back to the temporal cortex. The reason for this is uncertain; it may be to let the temporal cortex know what to expect in the future. But for people in a vegetative state, the communication was one-way: signals passed from the temporal to frontal area, but not back.
EEG data are measured through the scalp, so the technique provides at best a fuzzy approximation of what is going on inside the skull.
Garrido and her colleagues took their analysis a step further. They used dynamic causal modelling, a mathematical method developed by several members of the team, to create a computer model showing the best guess of what inner-brain activity could have yielded the EEG.
The researchers have spent several years validating the technique, but it is only a model. A more direct experiment would be to use electrodes implanted in the brain, perhaps in animal studies, Boly suggested.
The study has been published in Science.