At least a quarter of people who have severe brain injuries and are physically unable to respond to commands are actually consciously, as the first international study of its kind revealed 1.

Even though these people couldn't give, say, a thumbs up, they still showed repeated brain activity when asked to imagine themselves moving or exercising.

“This is one of the very important landmark studies” in the area of ​​coma and other disorders of consciousness, says Daniel Kondziella, a neurologist at Rigshospitalet, the teaching hospital at the University of Copenhagen.

The findings mean that a significant number of people with brain injuries who appear unresponsive can hear things happening around them and may even be able to use brain-computer interfaces (BCIs) to communicate, says study leader Nicholas Schiff, a neurologist at Weill Cornell Medicine in New York City. BCIs are Devices implanted in a person's head that record brain activity, decode them and translate them into commands that can, for example, move a computer cursor. “We should dedicate resources to finding these people and helping them,” Schiff said. The work was done todayThe New England Journal of Medicinepublished 1.

Scanning the brain

The study included 353 people with brain injuries caused by events such as physical trauma, heart attacks or strokes. Of those, 241 failed to respond to a battery of standard bedside responsiveness tests, including the test asking for a thumbs up; the other 112 could.

All study participants underwent one or both types of brain scans. The first was functional magnetic resonance imaging (fMRI), which measures mental activity indirectly by detecting blood oxygenation in the brain. The second was Electroencephalography (EEG), which directly measures brain wave activity using an electrode-covered cap on a person's scalp. During each scan, subjects were instructed to imagine themselves playing tennis or opening and closing their hand. The commands were repeated continuously for 15–30 seconds, then there was a pause; the exercise was then repeated for six to eight command sessions.

Of the physically unresponsive subjects, approximately 25% demonstrated brain activity throughout the entire EEG or fMRI examination period. The medical term for being able to respond mentally but not physically is cognitive motor dissociation. The 112 people in the study who were classified as responsive did slightly better on brain activity tests, but not by much: only about 38% showed consistent activity. This is probably because the tests set a high hurdle, says Schiff. "I've been in the MRI and done this experiment and it's hard," he adds.

It's not the first time a study has found cognitive motor dissociation in people with brain injuries who are physically unresponsive. For example, a paper published in 2019 showed this behavior in 15% of 104 people tested 2. However, the latest study is larger and the first multi-center investigation of its kind. The tests were carried out in six medical facilities in four countries: Belgium, France, the United Kingdom and the United States.

The 25% of unresponsive people who showed brain activity tended to be younger than those who did not, had injuries from physical trauma, and had their injuries longer than the others. Kondziella warns that further investigation of these relationships would require repeated assessments of people over weeks or months. “We know very little about the recovery curves of consciousness over time and across different brain injuries,” he says.

Opportunities for improvement

However, the study has some limitations. For example, not all medical centers used the same number or type of tasks during EEG or fMRI scans, or the same number of electrodes during EEG sessions, which could bias the results.

Ultimately, however, with such a high hurdle to register brain activity, the study likely underestimates the proportion of physically unresponsive people who are conscious, Schiff says. Kondziella agrees. Rates of cognitive-motor dissociation were highest in people tested for both EEG and fMRI, he notes, so if both methods had been used on every person in the study, the overall rates might have been even higher.

However, the tests used are logistically and computationally challenging, "so there are really only a handful or so centers worldwide capable of using these techniques," says Kondziella.

Schiff emphasizes the importance of identifying people with brain injuries who are unresponsive but conscious. “There will be people we can help get out of this condition,” he says, perhaps through the use of BCIs or other treatments or simply continuing to provide medical care. Knowing that someone is conscious can influence families and medical teams' decisions about life support and treatment. “It makes a difference every time you find out someone is responsive,” he says.