Researchers Identify 'Signature of Consciousness' in Brain-Damaged Patients

Researchers recorded EEG tracings of brain-damaged patients and fed them into a mathematical brain network model, allowing them to identify the “true signature of consciousness”.

Representation of consciousness from the seventeenth century.
Representation of consciousness from the seventeenth century (Robert Fludd).

Many folks who suffer severe head injuries can end up in a coma or in a so-called “vegetative state” in which the injured person may show few or no outward signs of awareness. But not all such patients in comas are in fact “vegetative”; some brain-damaged people are still “minimally conscious” and are able to perceive external stimuli, such as a doctor’s or loved one’s voice.

The challenge for doctors — and for family members who must make some difficult choices — has been how to determine if a person still possesses this minimal consciousness, or, if their brains are indeed irreparably damaged. But now thanks to a novel combination of Electro- Encephalograph (EEG) technology and mathematical modeling, researchers Melanie Boly and Stephen Laureys of the Coma Science Group (CSG), University of Leige, Belgium, have identified what they believe is the true “signature of consciousness”.

The breakthrough could help medical researchers better understand what is going on inside a damaged brain, so as to provide better care and guide treatment; it may also help family members who agonize over whether their injured loved-ones are still able to understand them, and not simply perceive them on some basic, mechanical level. Such knowledge can mean keeping a person on life-support, or not.

This tell-tale neural “signature” — produced as a response to auditory signals — is retained in healthy persons and also brain-damaged persons who still possess some minimum awareness, but it is absent in those who are truly in a vegetative state.

The CSG study involved 43 patients: 22 healthy volunteers and 21 brain-damaged patients; 8 of whom were previously diagnosed to be in a vegetative state (VS) , and the remaining 13 were in “minimally conscious state” (MCS; note: family members gave permission to use their relatives in the study). This minimally conscious state persists in less profoundly damaged brains and exhibits sporadic moments of awareness and responsiveness to external signals (such as eye-tracking movements and hand-squeezing on command).

A wave showing several ERP components, including the N100 and P300
A wave showing several ERP components, including the N100 and P300


While patients were presented with a set of tones, researchers made EEG recordings of their brain activity. Previous research on non-brain-damaged volunteers showed a characteristic up and down pattern of “blips”, known as event-related potentials (ERPs), as indicated by their EEG tracings (what some call “brain waves”). These ERPs lasted for several hundred milliseconds after each tone and before the next tone occurred (referred to generally as latency). These up/down oscillations diminish greatly during sleep and under anesthesia  and many researchers believe that they represent, or reflect, consciousness, or conscious awareness.

What researchers Boly and Laureys discovered was that in the vegetative patients, these blips lasted far shorter — less than 100 milliseconds. But why the marked difference?

To gain a better understanding of their findings, the researchers teamed up with neuroscientist Karl Friston and colleagues from the University College London who had developed various mathematical models that allowed them to locate the brain networks that correspond to specific EEG signals.

View of the frontal lobe (red) in the left cerebral hemisphere

Previous research had shown that the brain processes sounds and other external stimuli in a hierarchical (“bottom-up”, or feedforward ) manner — flowing from the brain stem to the temporal lobes and then to higher regions of the parietal and frontal cortices. These latter regions are believed to be responsible for conscious awareness (such as when we recognize a ring tone, or our spouse’s voice) and decision making (“I’ll let that go to voice mail.”).

But normal consciousness is a matter of these regions maintaining a continuous “conversation” with each other via feedback loops. So, in healthy brains, the processing and transmitting of signals works “top-down” as well. Signals that arrive in the parietal and frontal regions are then relayed back “down” to the temporal regions of the brain, completing the feedback loop.

The mathematical models revealed that this last part of the loop is missing in the EEG tracings made from patients in vegetative states. These results lend support to a model of consciousness advocated by French cognitive neuro-scientist Lionel Naccache which holds that perception is initially non-conscious, and that consciousness arises (or emerges) from a long-range “conversation” across several cortical regions.

View of the parietal lobe (red) in the left cerebral hemisphere (animation)
View of the parietal lobe (red) in the left cerebral hemisphere

Writing in the published paper, the researchers state:

“This suggests that  the level of consciousness may rest on the integrity of backward (top-down) connectivity.” *

So far, the new method has been shown only to distinguish differing groups from one another; it has not been refined to the point where it can be applied in a clinical setting. Never-the-less, the new technique, and the knowledge gained from its use, is shedding light on how consciousness arises in the brain.

And one day soon, it may also help families and loved ones in hospitals all over the world make that most difficult of all decisions.

* ‘Preserved Feedforward but Impaired Top-Down Processes in the Vegetative State’, Boly et al, Science, 13 May, 2011

Some additional source material for this article came the news report ‘Feedback from Frontal Cortex May be a Signature of Consciousness’ (Science, 13 May, 2011), by Greg Miller.

Diagram: (ERP) Gringer ; CC – By – SA 3.0

.GIFs: (frontal and parietal lobes) Database Center for Life Science (DBCLS); CC-BY-SA-2.1-jp

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