Consciousness is at the center of human existence, the ability to see, hear, dream, imagine, feel pain or pleasure, dread, love and more. But where precisely does this reside in the brain? That is a question that has long confounded scientists and clinicians. A new study is offering fresh insight.
In a quest to identify the parts of the brain underpinning consciousness, neuroscientists measured electrical and magnetic activity as well as blood flow in the brains of 256 people in 12 laboratories across the United States, Europe and China, while the participants viewed various images. The measurements tracked activation in various parts of the brain.
The researchers found that consciousness may not arise in the "smart" part of the brain - the frontal areas where thinking is housed, which progressively grew in the process of human evolution - but rather in the sensory zones at the back of the brain that process sight and sound.
"Why is any of this important?" asked neuroscientist Christof Koch of the Allen Institute in Seattle, one of the leaders of the study published this week in the journal Nature.
"If we want to understand the substrate of consciousness, who has it - adults, pre-linguistic children, a second trimester fetus, a dog, a mouse, a squid, a raven, a fly - we need to identify the underlying mechanisms in the brain, both for conceptual reasons as well as for clinical ones," Koch said.
The subjects in the study were shown images of people's faces and various objects.
"Consciousness is the way it feels like to see a drawing of a toaster or Jill's face. Consciousness is not the same as the behavior associated with this feeling, for example pushing a button or saying, 'I see Jill,'" Koch said.
The researchers tested two leading scientific theories about consciousness.
Under the Global Neuronal Workspace Theory, consciousness materializes in the front of the brain, with important pieces of information then broadcast widely throughout the brain. Under the Integrated Information Theory, consciousness emanates from the interaction and cooperation of various parts of the brain as they work collectively to integrate information that is consciously experienced.
The findings did not square with either theory.
"Where are the neuronal footprints of consciousness in the brain? Very crudely put, are they in the front of the cortex - the outermost layer of the brain - such as the prefrontal cortex, as predicted by the Global Neuronal Workspace Theory?" Koch asked.
It is this prefrontal cortex that makes our species uniquely human, driving higher-order cognitive processes such as planning, decision-making, reasoning, personality expression, and moderating social behavior.
"Or are the footprints in the back regions of the cortex, the posterior cortex?" Koch asked. The posterior cortex houses the regions where hearing and vision processing occurs.
"Here, the evidence is decidedly in favor of the posterior cortex. Either information pertaining to the conscious experience couldn't be found in the front or it was far weaker than in the back. This supports the idea that while the frontal lobes are critical to intelligence, judgment, reasoning, etc., they are not critically involved in seeing, in conscious visual perception," Koch said.
However, the study did not identify enough connections that last for as long as the conscious experience in the back of the brain to uphold the Integrated Information Theory.
There are practical applications in gaining a deeper understanding of the mechanics of consciousness in the brain.
Koch said it would be important for how doctors deal with patients in a coma or patients in a vegetative state or with unresponsive wakefulness syndrome, when they are awake but present no signs of awareness due to traumatic brain injury, stroke, cardiac arrest, a drug overdose or other causes.
"If the patient remains in this unresponsive state for longer than a few days without signs of recovery, the clinical team initiates discussion with the family around, 'Is this what they would have wanted?'" Koch said.
Of such patients, 70% to 90% die because a decision has been made to withdraw life-sustaining treatment.
"However, we now know that around a quarter of patients in either coma or vegetative state/unresponsive wakefulness syndrome are conscious - covert consciousness - yet are unable to signal this at the bedside," Koch said, referring to research published last year in the New England Journal of Medicine. "Knowing about the footprints of consciousness in the brain will let us better detect this covert form of 'being there' without being able to signal."
