In fact, while scientists have been preoccupied with understanding consciousness for centuries, it remains one of the most important unanswered questions of modern neuroscience. Now our new study, published in Science Advances, sheds light on the mystery by uncovering networks in the brain that are at work when we are conscious.
It's not just a philosophical question. Determining whether a patient is "aware" after suffering a severe brain injury is a huge challenge both for doctors and families who need to make decisions about care. Modern brain imaging techniques are starting to lift this uncertainty, giving us unprecedented insights into human consciousness.
For example, we know that complex brain areas including the prefrontal cortex or the precuneus, which are responsible for a range of higher cognitive functions, are typically involved in conscious thought. However, large brain areas do many things. We therefore wanted to find out how consciousness is represented in the brain on the level of specific networks.
The reason it is so difficult to study conscious experiences is that they are entirely internal and cannot be accessed by others. For example, we can both be looking at the same picture on our screens, but I have no way to tell whether my experience of seeing that picture is similar to yours, unless you tell me about it. Only conscious individuals can have subjective experiences and, therefore, the most direct way to assess whether somebody is conscious is to ask them to tell us about them.
But what would happen if you lose your ability to speak? In that case, I could still ask you some questions and you could perhaps sign your responses, for example by nodding your head or moving your hand. Of course, the information I would obtain this way would not be as rich, but it would still be enough for me to know that you do indeed have experiences. If you were not able to produce any responses though, I would not have a way to tell whether you're conscious and would probably assume you're not.
Scanning for networks
Our new study, the product of a collaboration across seven countries, has identified brain signatures that can indicate consciousness without relying on self-report or the need to ask patients to engage in a particular task, and can differentiate between conscious and unconscious patients after brain injury.
When the brain gets severely damaged, for example in a serious traffic accident, people can end up in a coma. This is a state in which you lose your ability to be awake and aware of your surrounding and need mechanical support to breathe. It typically doesn't last more than a few days. After that, patients sometimes wake up but don't show any evidence of having any awareness of themselves or the world around them — this is known as a "vegetative state". Another possibility is that they show evidence only of a very minimal awareness — referred to as a minimally conscious state. For most patients, this means that their brain still perceives things but they don't experience them. However, a small percentage of these patients are indeed conscious but simply unable to produce any behavioural responses.
We used a technique known as functional magnetic resonance imaging (fMRI), which allows us to measure the activity of the brain and the way some regions "communicate" with others. Specifically, when a brain region is more active, it consumes more oxygen and needs higher blood supply to meet its demands. We can detect these changes even when the participants are at rest and measure how it varies across regions to create patterns of connectivity across the brain.