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The question of conscience – the supposed brain activity that distinguishes humans in their ability to reason and be aware – torments both neuroscientists and philosophers. Members of the First Camp appear to have uncovered a new clue in their search for a biology-based answer to this elusive question.
In an article published Monday in the journal JNeurosci, a team of researchers analyzed the brain activity of 37 participants as they slipped under anesthesia and then fell asleep. By examining different brain wave patterns during these trials, the researchers found identical patterns of activity in a shared brain network.
Why is this important – Because this brain activity has been observed regardless of the presence or concentration of anesthetic, the researchers report that this brain network must play a critical role in generate human consciousness.
Here is the background – Locating the link of human consciousness in the brain is one of the biggest challenges in neuroscience. The answer could fundamentally change the course of science and redefine how we humans understand ourselves. But even the very definition of this state of being is open to debate.
When comparing human consciousness to the consciousness of artificial intelligence systems, for example, “consciousness” could be used to describe the apparent spark of self-awareness and single purpose in living things. But for neuroscientists, plotting the exact division between consciousness and unconsciousness is a bit more complicated.
In this study, the researchers define “connected” and “disconnected” states of consciousness. A “connected” condition represents arousal and the ability to respond to external stimuli, such as answering a question, while a “disconnected” condition describes a state in which there is no behavioral response to external stimuli. , for example, not responding when spoken to, while generating internal markers of consciousness, like dreams.
The researchers sought to understand the differences between these separate states of consciousness, hypothesizing that examining brain activity during sleep and wakefulness, as opposed to states induced by anesthesia, might reveal some something new about the brain networks governing consciousness.
“We often claim to find neural correlates of consciousness by comparing brain activity data collected during two states: wakefulness and a presumed unconscious state. This paradigm is controversial, however. specifically to awareness of the overall effects of anesthesia and sleep, ”the authors write.
In order to compare the brain patterns of people in these disconnected and connected states of consciousness, the research team recruited 39 volunteers and pasted them into a PET brain scanner.
What did they do – The researchers first gave the 39 volunteers increasing doses of two different anesthetic drugs and placed them in PET scans for observation. Throughout the trial, participants were forced to wake up at different times and were interviewed with the research team about their state of consciousness. They were asked questions like: Were they aware of their surroundings before waking up?
Following this first experience, 37 of the participants then continued a similar sleep study. After 30 hours of sleep deprivation, they were also placed in PET scanners and were alternately allowed to dose, or forced to wake up, again, assessing their state of consciousness throughout.
What they discovered – By comparing participants’ brain activity in connected and disconnected states of consciousness, the researchers identified a network in the brain that showed certain patterns of activity in both states. More precisely, this network crossed these regions of the brain:
- The thalamus – which acts as a sensory signal center in the brain
- Anterior and posterior cingulate cortex – areas of the brain believed to play a role in cognition and memory
- Angular gyrus – believed to play a role in language processing, memory, and other forms of cognition
When a participant entered a more disconnected state, blood flow through that network decreased, but when the participant entered a more connected conscious state, blood flow increased again. This model remained stable in disconnected states induced by sleep and anesthesia, a result the researchers said suggests the mechanism was independent of the two – and instead correlated specifically with connected consciousness.
And after – The results of this study provide intriguing new insights into this important, but seemingly intangible, mental state. Nonetheless, the researchers write improvements to their experimental design, and further validation of their results, are needed to advance the field of this study.
One aspect of the study that reveals its limitations is the subjective nature of the connected and disconnected assessments carried out by the researchers. Ratings are self-reported by participants, which means they are subjective, rather than objective, measures and prone to bias. Future studies might try to include more objective measures of these states, as well as continue to focus makes us human.
Abstract: What happens in the brain when consciousness of the surrounding world wanes? We manipulated consciousness in two experiments in a group of healthy men and measured brain activity by positron emission tomography. Measurements were taken during wakefulness, increasing and constant levels of two anesthetic agents (experiment 1, n = 39) and during sleep-deprived arousal and non-rapid eye movement sleep (experiment 2, n = 37 ). In Experiment 1, subjects were randomized to receive propofol or dexmedetomidine until they no longer responded. In both experiments, forced awakenings were applied to achieve rapid recovery from an unresponsive state to a reactive state, followed by immediate and detailed talks of subjective experiences during the previous unresponsive condition. Lack of response rarely indicated loss of consciousness, as the majority of subjects had internally generated experiences. Unresponsive anesthetic states and verified sleep stages, where a subsequent report of mental content showed no sign of awareness of the surrounding world, indicated a disconnected state. Functional brain imaging comparing reactive and connected vs. non-reactive and disconnected states of consciousness under constant anesthetic exposure has revealed that the activity of the thalamus, cingulate cortices, and angular gyri is fundamental to human consciousness. These brain structures were affected regardless of the pharmacological agent, drug concentration, and direction of change in consciousness. Similar results were obtained when consciousness was regulated by physiological sleep. The condition-specific findings were distinct and separable from the overall effects of the interventions, which included generalized depression of brain activity in cortical areas. These findings identify a central brain network essential for human consciousness.
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