Sound test separates vegetative states from minimally conscious ones

Using EEG, researchers hope to develop a tool that can diagnose states of consciousness at the bedsides of patients who have suffered brain damage.

When faced with life-support decisions, how can you tell how conscious someone is or isn’t?

Neuroscientists have spent a long time trying to nail down the clinical difference between vegetative and minimally conscious states.

New research reveals a neural signature present in healthy people and brain-damaged patients who retain some awareness… but not in people who are in a true vegetative state.

As Nature News describes, a person in a vegetative state will open their eyes spontaneously and make reflexive movements, but has no cognitive function and likely does not feel pain. Recovery is possible, but the chances of improvement greatly diminish after a year. In contrast, someone in a minimally conscious state has intentional, non-reflexive but inconsistent responses to stimuli. They might speak a few words or track their image in a mirror, and they feel pain.

Using an electroencephalogram (EEG), a team led by Melanie Boly of the University of Liège in Belgium measured electric signals through the scalp in 3 groups of people: 8 in vegetative states, 13 in minimally conscious states, and 22 healthy volunteers.

The subjects were played a series of tones that changed occasionally in pitch, Nature News explains.

The test suggests that 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.

The differing tone constituted a surprising event in the environment – something that the frontal cortex has to consider. So in all subjects, the temporal cortex would send the frontal cortex a message.

In minimally conscious and healthy people, the frontal cortex would then send a message back to the temporal cortex. But for people in a vegetative state, the communication was one-way: signals passed from the temporal to frontal area, but not back (see diagrams).

Then using a technique called dynamic causal modeling, the team made their best guesses of what inner-brain activity could have yielded the EEG. "It uses the information we know about the brain – how neurons are connected for example – to see which processes might explain this activity at the scalp level,” Boly explains.

So to rephrase, when healthy or minimally conscious brains registered unexpected noises, electrical signals passed between the brain’s auditory centers to the frontal cortex in a long-lasting loop. In vegetative patients, the loop screeched to a halt: signals could get into the frontal cortex, but neurons there couldn’t send messages back. “The bottleneck, if you will, is the top-down connection,” says Boly.

Doctors have been wrong more than 40% of the time compared to the Coma Recovery Scale. Currently the gold standard, it scores patients on the basis of whether they are capable of conscious behaviors.

If further research backs up these findings, EEG could be used at the bedside of severely brain-damaged patients to diagnose their state of consciousness, Boly says.

It could also lead to a new diagnostic tool to help doctors with life-support decisions in cases such as that of Terri Schiavo, the Florida woman who was in a vegetative state for 15 years before a court ordered her feeding tube removed.

The study was published in Science this week.

Image: University of Liège, via Science News

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