We fall in and out of consciousness every day. It's called sleep. Brain injury can cause a more permanent state of unconsciousness that, on the surface, demonstrates an inability to respond to stimuli – but this inability in and of itself is not necessarily a sign that the patient is unconscious, scientists say.
A patient who appears to be in a vegetative state might display the same brain signals when asked to imagine chucking a football or playing the guitar as a person who is fully conscious, for example.
So, what is it that separates the conscious from the unconscious? (The latter being those unable to describe a subjective experience.) An international team set out to find out, using brain scans taken from healthy, partially conscious, and unconscious patients. Their results have been published in Science Advances.
In total, 159 subjects were studied – 47 healthy individuals under anesthesia, 59 patients with unresponsive wakefulness syndrome or UWS (an essentially vegetative state where they can open their eyes but cannot make voluntary movements), and 53 people in a minimally conscious state or MCS (a partially conscious state where they show additional behaviors that could indicate awareness).
The researchers collected data from MRI scans to determine the brain patterns in their subjects, amassing hundreds of images for each individual over a 20-minute period. Specifically, they studied the way fluctuations of the blood oxygenation level-dependent (BOLD) signal affected 42 cognitively important regions of the brain that, combined, represent six brain networks. In this instance, BOLD signals were used as a stand-in for neuronal activity.
There appeared to be four distinct patterns, ranging in complexity. The first (pattern one) was the most complex and appeared most often (but not exclusively) in the healthy group. The findings suggest the long-distance, brain-wide coordination that is displayed in pattern one is a key indicator of consciousness. In contrast, the last (pattern four) demonstrated the least coordination between brain regions and was the most prominent in patients with UWS.
Brain activity in people with MCS appeared to sit somewhere between the two extremes but patterns two and three were roughly consistent among all three groups, the researchers say.
What exactly does this all mean?
"We conclude that these patterns of transient brain signal coordination are characteristic of conscious and unconscious brain states, warranting future research concerning their relationship to ongoing conscious content, and the possibility of modifying their prevalence by external perturbations, both in healthy and pathological individuals, as well as across species," the study authors conclude.
We might not be there quite yet but the researchers hope that one day in the future, medics will be able to use this information to monitor brain activity in real time, using this knowledge to non-invasively restore consciousness.
A better understanding of what makes someone conscious or unconscious on a neurological level will also help doctors make decisions when it comes to patients who have impaired consciousness.