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The procedure uses a scan of the way in which the brain reacts to sounds to distinguish between people who are in an entirely vegetative state from those who are still minimally conscious.
According to lead researcher Mélanie Boly from the University of Liège in Belgium, as many as 40% of patients presumed to be vegetative in fact have some awareness, but are "locked-in" to their bodies and unable to communicate because of the brain damage.
Using a newly discovered signalling pathway in the brain, Boly's team has paved the way to a clinical test for levels of consciousness in brain-damaged patients.
Such a test would aid medical staff and relatives in decisions about whether or not to turn off life-support machines, for example.
The Belgian research team played irregular sounds to eight people previously diagnosed as being in a vegetative state, 13 people in a minimally conscious state and a healthy control group.
They monitored subjects' brain activity using an electroencephalograph (EEG) machine, which uses electrodes placed on the scalp to read brain activity.
The patients in a minimally conscious state generated similar signals to the the healthy control group, while the signals of the vegetative patients were much shorter.
Using dynamic causal modelling, the team were able to locate a feedback mechanism between the temporal cortex, which is responsible for auditory processing, and a higher-level brain area called the frontal cortex.
The study, published in the journal Science, found that signals were passed to and fro between these areas in both healthy and minimally conscious subjects.
However, they only passed in one direction - towards the frontal cortex - in the vegetative state patients.
The lack of top-down processes in the brain seems to lie behind unconsciousness in brain-damaged patients, according to Boly.
In future, severely brain-damaged patients could be given bedside EEG scans to diagnose their state of consciousness, she said.
Some experts said the findings were still ambiguous, however, urging further tests in more patients.
According to neuroscientist Nicholas Schiff at Weill Cornell Medical College in New York, not enough patients were involved in Boly's study for it to be conclusive.
Story property of hc2d.co.uk: http://www.hc2d.co.uk/content.php?contentId=18460
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