Brain scans decode an elusive signature of consciousness

A conscious brain hums with elaborate, interwoven signals, a study finds.

Scientists uncovered that new signature of consciousness by analyzing brain activity of healthy people and of people who were not aware of their surroundings. The result, published online February 6 in Science Advances, makes headway on a tough problem: how to accurately measure awareness in patients who can’t communicate.

Other methods for measuring consciousness have been proposed, but because of its size and design, the new study was able to find a particularly strong signal. Conducted by an international team of researchers spanning four countries, the effort “produced clear, reliable results that are directly relevant to the clinical neuroscience of consciousness,” says cognitive neuroscientist Michael Pitts of Reed College in Portland, Ore.

Consciousness — and how the brain creates it — is a squishy concept. It slips away when we sleep, and can be distorted by drugs or lost in accidents. Though scientists have proposed many biological explanations for how our brains create consciousness, a full definition still eludes scientists.

By finding a clear brain signature of awareness, the new work “bring us closer to understanding what consciousness is,” says study coauthor Jacobo Sitt of INSERM in Paris.

Sitt and his colleagues scrutinized functional MRI data that captured the brain activity of 125 people as they rested inside scanners at research institutes in Paris, New York City and Liège, Belgium. Forty-seven of these people were healthy. The rest had unresponsive wakefulness syndrome, in which their eyes were open but they showed no signs of awareness, or were in a minimally conscious state, in which they could follow simple instructions such as moving their eyes on command.

Two distinct patterns of brain activity emerged. The first was a complex pattern characterized in part by opposites. When one neural spot was active, others were not. This complex pattern also didn’t follow the anatomy of the brain; signals ping-ponged far away from their anatomical connections. The second pattern was simpler, and more closely constrained by the anatomy of the brain. (The scientists found two other patterns, but those didn’t correspond to consciousness.)

The brains of people who were fully conscious spent more time exhibiting the complex pattern, Sitt and his colleagues found. People who were diagnosed with unresponsive wakefulness syndrome spent more of their time in the simple pattern, while those in the minimally conscious state split the difference, spending time in both states to varying degrees.
 
The researchers looked for these signals in a group of 11 patients in London, Canada, some of whom were conscious but unable to communicate. Sure enough, patients who were aware of their surroundings spent more time in the complex state of brain activity. What’s more, when a different group of 23 patients was anesthetized, their brains spent less time in the complex state, suggesting that it does come along with consciousness.

People flitted between the two distinct states occasionally, the researchers found. Healthy people’s brains sometimes slipped into the simple form of behavior, perhaps representing temporary “mind blanks,” the team writes. And unconscious patients exhibited the complex pattern now and then. Researchers don’t yet know whether the brief blips come with a temporary increase in consciousness in those patients, Sitt says. If so, “is it a window of opportunity of communication?” he asks.