This is the human brain while apparently doing nothing. When you think you're just sitting quietly letting your mind wander, a set of regions comprising 20% of your brain kicks into action. This is the default mode network. Wait a second...that's weird. Like what are those brain regions doing? They must be doing something. The mysterious pattern of activity inside this network indicates that the so-called resting brain is hard at work. Maybe it's not just an idling default state, but maybe it's involved in some more interesting processes. The discovery of this network has revolutionized our understanding of the human brain, and it possibly holds the secret to what makes you uniquely you. Obviously, a number of exciting questions remain. In the 1920s German psychiatrist Hans Berger invented the electroencephalogram or EEG, a machine, which records the brain's electrical activity. When Berger took readings of subjects at rest, the EEG revealed something unexpected. The brainwave patterns changed, but didn't stop. Berger theorized that the human brain was never truly inactive, but his ideas failed to gain much traction. 70 years later, a team at Washington University found an intriguing pattern of brain activity in neuroimaging studies of subjects performing active tasks like reading aloud. While the areas that are active varied, depending on the task demands and context, the areas that were deactivated were quite common. These inactive regions, including the medial prefrontal cortices, posterior cingulate cortex and the angular gyrus were already known to researchers and commonly associated with things like emotion, language, and memory. A second neuroimaging study led by neurologist Marcus Raichle confirmed the findings. These areas were turned off when the mind concentrated on something external, but became engaged when quietly at rest. So it's kind of doing the opposite of what most of the brain does. They called this resting activity the brain's 'default mode'. In 2001, when it came out, some people thought it was totally nuts. A few years later, neurologist Vinod Menon, and a team at Stanford studied subjects at rest using functional magnetic resonance imaging or fMRI. They found not only the same regional brain activation, but also in underlying connectivity between these regions. And these are not just isolated regions, but they coupled together in particular ways and they put all this stuff together and we said, well, this is something we should call the 'default mode network,' this actually functions as a network. Brain networks work like a symphony, a complex arrangement of coordinated individual parts which come together to perform a specific function which they couldn't produce alone. There's networks related to movement or vision or hearing, and then the sort of higher order ones related to things like attention. But the default mode networks function was enigmatic, at first. We didn't know what it was doing and why it was doing what it was doing. For the last two decades, researchers have chipped away at the enigma implicating the default mode network, or DMN, in several key areas of cognition, each of which involves a specific coordination between the DMN's different regions. There's sometimes regions that come in and out and maybe affiliate with the network in the service of certain goals. One important function involves memory. DMF network is preferentially connected to the hippocampus, which is the critical structure for making new memories. Including the formation and recall of what are called episodic memories, which are autobiographical memories about our past experiences. The sum of all your memories that are relevant to you. And also prospective memories or thinking about what we intend to do in the future. Sometimes we're basically drawing on the past events to imagine the future. The DMN is involved with another important form of memory, semantic memory. Semantic memory is knowledge that we have acquired as distinguished from individual events. These are long-term memories, general knowledge about the world we inhabit - the processing of facts, concepts, and language. The default mode network is also connected to self-awareness and social cognition. Things like thinking about your friends and the attributes of your friends and thinking about yourself and your place in society. Which requires an interior theory of mind. Thinking about what other people are thinking. Like you're constantly running a model of someone you're talking to about what they're thinking and feeling in response to what you're saying, and that's really important because if you're way off, you lose a friend or you could get punched. Absent external stimuli, the default mode network turns inward. The DMN switches or defaults to an internally focused thought process such as self-reflection, daydreaming, mind wandering. When mind wandering, we're often recalling personal experiences and envisioning the future. All of these kinds of complex and interesting introspective and evaluative types of processing tends to engage the default network. While the default mode network may switch on when the rest of the brain switches off, it isn't independent. The default mode network does not function in isolation. It's both excited and suppressed by other systems. Like the salience network, a key cognitive control network responsible for switching attention. Which would go off right now if the fire alarm went off in the building. That one seems to go up when the default goes down. The disruption of this normal push and pull, the switching between the DMN and other networks is implicated in abnormal brain function. Individuals may not be attending to external stimuli when they should. They may be attending to internal stimuli when they should not. It turns out that the default mode network is impacted in major ways in virtually every psychiatric disorder. Menon has developed a theory that a disruption to the dynamic between the DMN, the salience network and a third brain network plays a role in a range of psychiatric disorders including schizophrenia, Alzheimer's disease, and depression. The nature of the deficits is different across disorders. We don't fully understand them, but the notion that the DMN is disrupted and its interactions with other core cognitive control systems is disrupted, there's very broad evidence for that. However, recent research on the therapeutic effects of psilocybin, the active component of magic mushrooms has shown promise. It's got great potential for treating depression and other neuropsychiatric disorders, likely because it boosts neuroplasticity. An fMRI study by a team at Washington University revealed a desynchronization or scrambling of the default mode network while on psilocybin. One of the researchers participated in this study and experienced this firsthand. The boundaries become blurred. I lost my sense of space completely, and then I lost myself. The brain scans revealed massive temporary changes. Your functional brain organization, your functional networks, they're like your fingerprint or your face in the sense that truly, does not to be cheesy, they're unique like yours is unique. Giving people's psilocybin makes it so that their brain changes so much that my brain and your brain on psilocybin are more similar to each other than my brain, not on psilocybin to my brain on psilocybin. That's wild. And the experience of having a scrambled default mode network revealed something else. Part of the effects that People report, which are more strongly focused around effects on your sense of time, space, and sense of self and your memories, I think that solidifies this idea that that's one of the main functions or sets of functions of the default mode network. While many of the individual elements of the default mode network are now better understood, the larger question remains, what does the entire network accomplish? In some sense, the DMN is where I think, and we need certainly a lot more evidence, is where all of these critical components come together to create a unifying sense of the self. The default mode network enables us to weave together an internal narrative, a story of who we are that shapes how we perceive ourselves and interact with others. It's like the simulation that you're always running, that's you.