Using a sophisticated imaging test to probe for higher-level cognitive functioning in severely brain-injured patients provides a window into consciousness, scientists say, adding that the view it presents is one that is blurred in fascinating ways.
In a novel study of six patients ranging in their function from minimally conscious state to the locked-in syndrome (normal cognitive function with severe motor impairment), the researchers looked at how the brains of these patients respond to a set of commands and questions while being scanned with functional magnetic resonance imaging (fMRI).
They found there was a wide, and largely unpredictable, variation in the ability of patients to respond to a simple command and then using that same command to answer simple yes/no or multiple-choice questions. This variation was apparent when compared with their ability to interact at the bedside using voice or gesture.
Some patients unable to communicate by gestures or voice were unable to do the mental tests, while others unable to communicate by gestures or voice were intermittently able to answer the researchers' questions using mental imagery. And, intriguingly, some patients with the ability to communicate through gestures or voice were unable to do the mental tasks.
The researchers say these findings suggest that no exam yet exists that can accurately assess the higher-level functioning that may be, and certainly seems to be, occurring in a number of severely brain-injured patients but that progress is being made.
"We have to abandon the idea that we can rely on a bedside exam in our assessment of some severe brain injuries. These results demonstrate that patients who show very limited responses at the bedside may have higher cognitive function revealed through fMRI," said the study's corresponding author, Nicholas D. Schiff, of the Weill Cornell Medical College.
While progress has been made in elucidating the range of brain function in those who are severely injured, Schiff urges caution.
"Although everyone wants to use a tool like this, fMRI is not yet capable of making clear measurements of cognitive performance. There will be a range of possible responses reflecting different capabilities in these patients that we have to further explore and understand," he said.
"Thousands of people suffer debilitating brain injuries every year, and there is a clear ethical imperative to learn as much as possible about their ability to communicate," says the study's lead author, Jonathan Bardin, a third-year neuroscience graduate student at Weill Cornell Medical College.
"These findings caution us against giving too much weight to negative results and open our eyes to the diversity of responses one might expect from the wide-ranging group of severely brain-injured people," he says.
The findings have been published in the online edition of the journal Brain.