Study of comatose brains finds changes to highly connected hub areas

Scientists have uncovered a key property of comatose brains that differentiates them from normal brains and may explain what goes wrong during severe brain injury.

The report, published Monday in the Proceedings of the National Academy of Sciences, utilizes graph theory, which uses data to determine how well connected each part of a network is to every other part of the network. The approach has been used to study social networks like Facebook and circuit engineering for electronics.

From such an analysis comes a plethora of measures and descriptions that help researchers understand how a large network fits together. A small-world network, for example, is one in which each “node” of the network -- whether a person in a social network or an anatomical region in the brain -- can be connected to any other node with relatively few connections (similar to the “six degrees of separation” concept).

In the new study, the researchers set out to see whether any measures of brain connectivity used by graph theorists would shake out differently in people with coma compared with those with uninjured brains.

Numerous studies have shown that, much like social networks, the brain is a small-world network, a finding confirmed in the new report. Interestingly, the study found that even after severe injury, comatose brains still were small-world networks. And comatose brains held on to most of the other global graph theoretic properties of the normal brain too.


But when the researchers looked more closely at the properties of individual brain areas, they found some striking differences. In the normal brain, certain areas are considered “hubs” because, much like LAX or other major airports, they are highly connected to many other parts of the brain. And when the researchers looked at these hubs, they found that the areas that are hubs in normal brains were no longer hubs in comatose brains. Instead, other areas had taken over as hubs.

The finding suggests that the brain’s rapid response to severe injury can lead to a dramatic reorganization of how strongly different brain areas are connected to one another, completely changing the brain’s traffic patterns.

More generally, the authors write, the analysis reveals that some of the more common graph theory measures, when applied to the entire brain, may not be so important for consciousness after all. Instead, it is the connectivity of specific brain areas that may support consciousness -- the LAXs of the brain. If this proves true, it might give doctors a new measure they can use to determine whether patients are likely to recover.

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