David Shultz in Science:
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.