Severe brain damage can leave patients unresponsive to stimuli, but they may be more aware than we realize. A new study of brain activity has shown that a quarter of unresponsive patients may exhibit “covert consciousness.”
After a severe brain injury, such as trauma or stroke, patients may enter a state in which they stop responding to stimuli in their environment, such as sound, light, pain, and instructions. This can include a coma, which is similar to a continuous deep sleep; a vegetative state, in which the patient may appear awake and even have open eyes, but still be unresponsive; and a minimally conscious state, in which patients can track movements with their eyes but are unable to respond to commands.
But that doesn’t mean these patients were at least partially unaware of what was happening. Over the past few decades, scientists have detected signs of “covert consciousness” in some unresponsive patients. In tests, patients were given instructions such as “imagine opening and closing your hand” and their brain activity was monitored with functional magnetic resonance imaging (fMRI) or electroencephalography (EEG).
And indeed, in some cases these measurements show that intelligent responses are taking place in the brain, even when there is no obvious physical sign, such as the patient opening and closing their hand. This suggests that in some patients the problem is not cognitive but motor, leading to a condition called cognitive-motor dissociation.
“Some patients with severe brain damage do not appear to process their external world,” said Yelena Bodien, lead author of the new study. “However, when assessed with advanced techniques such as task-based fMRI and EEG, we can detect brain activity in the opposite direction. These results raise critical ethical, clinical, and scientific questions—for example, how can we use this unseen cognitive capacity to build a communication system and support further recovery?”
The new study examined 241 participants from six different sites in the U.S., U.K. and Europe over the past 15 years. All were unresponsive, in a comatose, vegetative or minimally conscious state when tested with fMRI, EEG or both, from a few days to years after their injuries.
Interestingly, the study found that 25% of patients were able to follow instructions repeatedly for several minutes, which is higher than estimates in previous studies that suggested 15% to 20%.
There were some caveats, though. Because the data was collected by many different teams over a long period of time, the methods were not standardized, so there could be some variability. To complicate matters further, the study analyzed an additional 112 participants who were able to respond to verbal instructions. However, only 38% of these patients showed brain activity consistent with covert consciousness, suggesting that brain activity and the tests used to find it are still quite complex.
The team says developing guidelines for how to assess cognitive-motor dissociation will be a focus of future work. A better understanding of consciousness in these situations could improve outcomes for patients, with some promising early results in inducing consciousness using ultrasound or non-invasive neural implants.
“Families have told us that when a positive test result revealing cognitive-motor dissociation is shared with the patient’s clinical team, it can change the way the team interacts with their loved one,” Bodien said. “Suddenly, the team is paying more attention to subtle behavioral cues that may be under voluntary control, or talking to the patient or playing music in the room. Failing to detect cognitive-motor dissociation, on the other hand, can have serious consequences, including premature withdrawal of life support, missed awareness, and lack of access to intensive rehabilitation.”
The research was published in the journal: New England Journal of Medicine.
Sources: Mass General Brigham, Columbia University