Compared with healthy patients in the study, high-traffic hubs of brain activity are dark in coma patients while more quiet regions spring to life.
"Consciousness may depend on the anatomical location of these hubs in the human brain network," said study co-author Sophie Achard, a statistician at the French National Center for Scientific research in Grenoble.
The findings have several important implications, said Indiana University neuroscientist Olaf Sporns, who was not involved in the study.
"It gives us a handle on what may be different between healthy conscious people and people who have loss of consciousness," Sporns told LiveScience. "The traffic patterns have totally reorganized. And maybe it's the rerouting of the traffic patterns that underlies the loss of consciousness," or the mysterious ability to be self-aware that seems to set humans apart from other animals.
In the future, the research could also help doctors determine which coma patients are likely to recover based on activity in high-traffic brain regions, he said. The research could potentially even suggest ways to stimulate the brains of patients in a coma to improve their outcome, he added.
The study was published today (Nov. 26) in the Proceedings of the National Academy of Sciences.
Mystery of consciousness
Scientists still don't understand exactly how human consciousness works, but the twilight state of a coma could reveal some insight. Past research revealed that a person in a coma is closer to being anesthetized than being asleep. Other studies have found that vegetative and minimally conscious patients have very different brain activity.
But for the most part, it was hard to find obvious differences in brain functioning between healthy patients and those who have lost consciousness.
To tease out these differences, Achard and her colleagues took functional magnetic resonance imaging (fMRI) brain scans of 17 patients who were in a coma a few days after cardiac arrest and compared them with scans from 20 healthy volunteers who were at rest. Some patients, who had lost oxygen to the brain for up to 30 to 40 minutes, eventually recovered, but more than half died.
The team tracked 417 different brain regions for changes in blood flow - a marker of brain activity. They then correlated synchronized increases or decreases in activity between different regions.
In healthy patients, about 40 regions lit up in concert with many other parts of the brain. These high-traffic hubs, like busy airports, apparently process much of the electrical firing in the brain.
Rerouted brain traffic
But in the coma patients, many of these hubs were darkened, and other, normally peripheral regions took their place. Intriguingly, coma patients had fewer hubs in a region called the precuneus, which is known to play a role in consciousness and memory.
These central nodes of brain activity may hold the key to consciousness, Achard told LiveScience. Because they direct so much of the brain's traffic, they also require more oxygen and thus may be more vulnerable to its loss, the study authors write in the journal article.