Is my yellow the same as your yellow? Does your pain feel like my pain? The question of whether the human consciousness is subjective or objective is largely philosophical. But the line between consciousness and unconsciousness is a bit easier to measure. In a new study of how anesthetic drugs affect the brain, researchers suggest that our experience of reality is the product of a delicate balance of connectivity between neurons—too much or too little and consciousness slips away.
“It’s a very nice study,” says neuroscientist Melanie Boly at the University of Wisconsin, Madison, who was not involved in the work. “The conclusions that they draw are justified.”
Previous studies of the brain have revealed the importance of “cortical integration” in maintaining consciousness, meaning that the brain must process and combine multiple inputs from different senses at once. Our experience of an orange, for example, is made up of sight, smell, taste, touch, and the recollection of our previous experiences with the fruit. The brain merges all of these inputs—photons, aromatic molecules, etc.—into our subjective experience of the object in that moment.
“There is new meaning created by the interaction of things,” says Enzo Tagliazucchi, a physicist at the Institute for Medical Psychology in Kiel, Germany. Consciousness ascribes meaning to the pattern of photons hitting your retina, thus differentiating you from a digital camera. Although the brain still receives these data when we lose consciousness, no coherent sense of reality can be assembled.
In order to look for the signature of consciousness in the brain, Tagliazucchi and his colleagues used a drug called propofol—an anesthetic drug used in surgery—to induce loss of consciousness in participants while they were inside a functional magnetic resonance imaging (fMRI) machine’s scanner. fMRI works by tracking blood flow in the brain, which can be used as a real-time proxy for electrical activity when neurons fire. The team recorded data from 12 participants in states of wakefulness, ongoing sedation, unconsciousness, and recovery.
The results, published online today in the Journal of the Royal Society Interface, show that brain activity varies widely between conscious and unconscious states. The difference may come down to how the brain “explores the space of its own possible configurations,” Tagliazucchi says.
During wakeful consciousness, participants’ brains generated “a flurry of ever-changing activity,” and the fMRI showed a multitude of overlapping networks activating as the brain integrated its surroundings and generated a moment to moment “flow of consciousness.” After the propofol kicked in, brain networks had reduced connectivity and much less variability over time. The brain seemed to be stuck in a rut—using the same pathways over and over again.
The results suggest that, in the brain, there is an optimal level of connectivity between neurons that creates the maximum number of possible pathways. If each neuron can be thought of as a node in the network, consciousness might result from exploring the network as thoroughly as possible. But the most diverse networks—the ones with the largest possible number of arrangements—don’t necessarily have the maximum amount of neuronal connectivity.
If every neuron in the brain were directly connected to every other neuron, however, the brain would become too homogenous, and one signal would become indistinguishable from the next, Tagliazucchi explains. “They all fire or they all stay quiet.” Instead, consciousness might emerge from a careful balancing that causes the brain to “explore” the maximum number of unique pathways to generate meaning, he says. The researchers call this balance point “a critical point.”
“[It’s] like cars exploring the streets of the city,” Tagliazucchi says. “If the cars move always in the same restrictive manner, if they move from point A to point B and back, at the end of the day you don’t really understand the city. But if the cars are thorough explorers and go through all possible parts of the city, you get a map that’s very close to the actual map of the city. At the critical point, the cars are exploring the streets in the optimal way.”
Unlike cars, though, the flow of electricity through our brains is not driven by some sentient force with will or intent. And, in fact, what causes the brain to move between states of consciousness and unconsciousness—to and from the critical point—remains unknown. “If you’re in a critical point, the brain is really chaotic,” Boly says. “If you’re far from there, it’s too monotonous or stable.”
That stability could explain what makes it hard to wake people from a coma. Tagliazucchi hopes that by understanding where the critical points lie, and how they’re maintained, we might eventually rouse coma patients from unconscious states, coaxing the brain to begin exploring its streets in just the right way.