The increasing sophistication of brain imaging techniques means that researchers are better equipped to decode the mystery that is the brain’s inner workings. It’s understood now that our brain — the meaty slab that rolls around in our skulls — consists of a complex network of neural systems, that operate in harmony to keep it ticking away.
But despite this advanced, recent understanding, the phenomenon that continues to perplex scientists is consciousness: Why we experience it, its underlying neurological mechanisms, what it even is.
Luckily, a new study brings us one step closer to cracking consciousness once and for all. Researchers have finally provided convincing proof that a relationship between two brain networks — the default mode network, or DMN, and the dorsal attention network, or DAN — may be fundamental to keeping us conscious.
The findings were published Wednesday in the journal Science Advances.
These findings could have clinical applications, though lead author Zirui Huang, a researcher at the University of Michigan Medical School says, for now, they probably won’t be directly applicable.
“Indirectly, we have some kind of novel scientific insight into the neurobiology of consciousness,” he tells Inverse.
Meet the DMN and the DAN
The DMN is a group of brain regions that are active when the brain is at rest; it tends to have lower activity levels when an individual is engaged in a specific mental exercise. While the actual function of the DMN has yet to be determined, it’s thought to have a hand in cognitive processes such as self-reflection, forming autobiographical memories, and playing out potential future events. It is at play when thoughts stream through our mind as we gaze out of a window, or reflect on past memories.
It is also thought to be crucial to the maintenance of consciousness.
Conversely, DAN springs into action when one is actively engaged in a task (this mode of attention is sometimes called the “task-positive system”). It’s responsible for focusing your attention on external stimuli and lets you solve problems and make decisions.
During cognitive tasks that require externally-focused attention, the DMN shuts down, and the DAN comes alive. Their relationship is a reciprocal but competing one; in science, this is referred to as an “anti-correlated” relationship. When one is active, the other is not; like a seesaw in your brain.
“Studies have shown that the anti-correlation of the two networks is vital for maintaining ongoing interaction between self and the environment, [and] that contributes to consciousness,” says Huang.
In patients who are unconscious, either by anesthesia or due to neurological disorders that affect consciousness, this symbiotic relationship has been shown to also be affected, suggesting that it’s the relationship’s smooth functioning that’s needed to stay conscious.
However, all the research that has supported this theory up until now has used experimental techniques that have been subject to contentious criticism, leading to it still being considered up for debate.
A "give-and-take" relationship
This new research may finally have given us solid evidence on just how integral the interplay between the two brain networks is for maintaining consciousness, and demonstrates that when this connection is disrupted, so is consciousness.
Huang and his colleagues used functional magnetic resonance imaging (fMRI) to scan the brains of 98 human participants who were in varying states of consciousness or unresponsiveness. Some patients were rendered unresponsive using anesthetic agents, such as propofol and ketamine. Others suffered from neuropathological conditions that caused disorders of consciousness, such as unresponsive wakefulness syndrome (UWS), also known as a vegetative state.
The team observed that the degree to which the two brain networks were suppressed in the unresponsive patients – both via anesthesia and due to neuropathological disorders – was similar. This suggests that “the give-and-take relationship of the two systems may be particularly important for normal levels of consciousness,” they write in the paper.
The researchers also examined another cohort of 248 conscious participants, including both healthy control patients, as well as patients with diagnosed psychiatric disorders such as schizophrenia, bipolar disorder, and ADHD, which are known to alter brain networks. Like the patients under sedation, the transition to the DAT and DMN in the brains of these individuals was also significantly reduced.
Taken together, these results suggest that it’s these specific structured patterns of brain changes that are necessary for consciousness to happen — and are why each of us process and are aware of what we experience. You may know that you love a friend, or enjoy a pizza. Now scientists are one step closer to knowing the neural correlates that make that possible.
Abstract: The ongoing stream of human consciousness relies on two distinct cortical systems, the default mode network and the dorsal attention network, which alternate their activity in an anticorrelated manner. We examined how the two systems are regulated in the conscious brain and how they are disrupted when consciousness is diminished. We provide evidence for a “temporal circuit” characterized by a set of trajectories along which dynamic brain activity occurs. We demonstrate that the transitions between default mode and dorsal attention networks are embedded in this temporal circuit, in which balanced reciprocal accessibility of brain states is characteristic of consciousness. Conversely, isolation of the default mode and dorsal attention networks from the temporal circuit is associated with unresponsiveness of diverse etiologies. These findings advance the foundational understanding of the functional role of anticorrelated systems in consciousness.