Default mode network
In neuroscience, the default mode network (DMN), also default network, or default state network, is a network of interacting brain regions known to have activity highly correlated with each other and distinct from other networks in the brain.
The default mode network is most commonly shown to be active when a person is not focused on the outside world and the brain is at wakeful rest, such as during daydreaming and mind-wandering. But it is also active when the individual is thinking about others, thinking about themselves, remembering the past, and planning for the future. The network activates "by default" when a person is not involved in a task. Though the DMN was originally noticed to be deactivated in certain goal-oriented tasks and is sometimes referred to as the task-negative network, it can be active in other goal-oriented tasks such as social working memory or autobiographical tasks. The DMN has been shown to be negatively correlated with other networks in the brain such as attention networks. Thinking about others also could include guessing their thoughts, emotions, and psychological motivations.
The default mode network is known to be involved in many seemingly different functions:
It is the neurological basis for the self:
- Autobiographical information: Memories of collection of events and facts about one's self
- Self-reference: Referring to traits and descriptions of one's self
- Emotion of one's self: Reflecting about one's own emotional state
Thinking about others:
- Theory of Mind: Thinking about the thoughts of others and what they might or might not know
- Emotions of other: Understanding the emotions of other people and empathizing with their feelings
- Moral reasoning: Determining just and unjust result of an action
- Social evaluations: Good-bad attitude judgments about social concepts
- Social categories: Reflecting on important social characteristics and status of a group
Remembering the past and thinking about the future:
- Remembering the past: Recalling events that happened in the past
- Imagining the future: Envisioning events that might happen in the future
- Episodic memory: Detailed memory related to specific events in time
- Story comprehension: Understanding and remembering a narrative
The default mode network is active during passive rest and mind-wandering. Mind-wandering usually involves thinking about others, thinking about one's self, remembering the past, and envisioning the future.Electrocorticography studies (which involve placing electrodes on the surface of epileptic patient's brains) have shown the default mode network becomes activated within an order of a fraction of a second after participants finish a task.
Studies have shown that when people watch a movie, listen to a story, or read a story, their DMNs are highly correlated with each other. DMNs are not correlated if the stories are scrambled or are in a language the person does not understand, suggesting that the network is highly involved in the comprehension and the subsequent memory formation of that story. The DMN is shown to even be correlated if the same story is presented to different people in different languages, further suggesting the DMN is truly involved in the comprehension aspect of the story and not the auditory or language aspect.
The default mode network has shown to deactivate during external goal-oriented tasks such as visual attention or cognitive working memory tasks, thus leading some researchers to label the network as the task-negative network. However, when the tasks are external goal-oriented tasks that are known to be a role of the DMN, such as social working memory or an autobiographical task, the DMN is positively activated with the task and correlates with other networks such as the network involved in executive function.
People with Alzheimer's disease show a reduction in glucose (energy use) within the areas of the default mode network. These reductions start off as slight decreases in mild patients and continue to large reductions in severe patients. Surprisingly, disruptions in the DMN begin even before individuals show signs of Alzheimer's disease. Plots of amyloid-beta, which is thought to cause Alzheimer's disease, show the buildup of the protein is within the DMN. This prompted Randy Buckner and colleagues to propose the high metabolic rate from continuous activation of DMN causes more amyloid-beta protein to accumulate in these DMN areas. These amyloid-beta proteins disrupt the DMN and because the DMN is heavily involved in memory formation and retrieval, this disruption leads to the symptoms of Alzheimer's disease.
DMN is thought to be disrupted in individuals with autism spectrum disorder. These individual are impaired in social interaction and communication which are tasks central to this network. Studies have shown worse connections between areas of the DMN in individuals with autism, especially between the mPFC (involved in thinking about the self and others) and the PCC (the central core of the DMN). The more severe the autism, the less connected these areas are to each other. It is not clear if this is a cause or a result of autism.
Lower connectivity was found across the default network in people who have experienced long term trauma, such as childhood abuse or neglect, and is associated with dysfunctional attachment patterns. Among people experiencing posttraumatic stress disorder, lower activation was found in the posterior cingulate gyrus compared to controls. Hyperconnectivity of the default network has been linked to rumination in depression and chronic pain.