Default Mode Network
Shift to attention by insula
Default_Mode_Network_Connectivity (1).jpg
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.[3]
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.[3][4] 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,[5] it can be active in other goal-oriented tasks such as social working memory or autobiographical tasks.[6] The DMN has been shown to be negatively correlated with other networks in the brain such as attention networks.[7] Thinking about others also could include guessing their thoughts, emotions, and psychological motivations.
Evidence has pointed to disruptions in the DMN with people with Alzheimer's and autism spectrum disorder.[3]


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Function

The default mode network is known to be involved in many seemingly different functions:
It is the neurological basis for the self:[8]
  • 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:[8]
  • 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:[8]
  • 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.[3] Mind-wandering usually involves thinking about others, thinking about one's self, remembering the past, and envisioning the future.[8] 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.[9]
Studies have shown that when people watch a movie,[10] listen to a story,[11] or read a story,[12] 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,[13] 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.[5] 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.[6]

Anatomy[edit]

external image 300px-Default-network-graph-maturation.jpeg

Graphs of the dynamic development of correlations between brain networks. (A) In children the regions are largely local and are organized by their physical location; the frontal regions are highlighted in light blue. (B) In adults the networks become highly correlated despite their physical distance; the default network is highlighted in light red.[14]

The default mode network is an interconnected and anatomically defined[3] set of brain regions. The network can be separated into hubs and subsections:
Functional hubs:[15] Information regarding the self
  • Posterior cingulate cortex (PCC) & precuneus: Combines bottom-up (not controlled) attention with information from memory and perception. The ventral (lower) part of PCC activates in all tasks which involve the DMN including those related to the self, related to others, remembering the past, thinking about future, and processing concepts plus spatial navigation. The dorsal (upper) part of PCC involves involuntary awareness and arousal. The precuneus is involved in visual, sensorimotor, and attentional information.
  • Medial prefrontal cortex (mPFC): Decisions about self processing such as personal information, autobiographical memories, future goals and events, and decision making regarding those personally very close such as family. The ventral (lower) part is involved in positive emotional information and internally valued reward.
  • Angular gyrus: Connects perception, attention, spatial cognition, and action and helps with parts of recall of episodic memories
Dorsal medial subsystem:[15] Thinking about others
Medial temporal subsystem:[15] Autobiographical memory and future simulations
The default mode network is most commonly defined with resting state data by putting a seed in the posterior cingulate cortex and examining which other brain areas most correlate with this area.[16] The DMN can also be defined by the areas deactivated during external directed tasks compared to rest.[17]
It has been shown that the default mode network exhibits the highest overlap in its structural and functional connectivity, which suggests that the structural architecture of the brain may be built in such a way that this particular network is activated by default.[1] In the infant brain, there is limited evidence of the default network, but default network connectivity is more consistent in children aged 9–12 years, suggesting that the default network undergoes developmental change.[7]
Function connectivity analysis in monkeys shows a similar network of regions to the default mode network seen in humans.[3] The PCC is also a key hub in monkeys; however, the mPFC is smaller and less well connected to other brain regions, largely because human's mPFC is much larger and well developed.[3]
Diffusion MRI imaging shows white matter tracks connecting different areas of the DMN together.[8] The structural connections found from diffusion MRI imaging and the functional correlations from resting state fMRI show the highest level of overlap and agreement within the DMN areas.[1] This provides evidence that neurons in the DMN regions are linked to each other through large tracks of axons and this causes activity in these areas to be correlated with one another.

Pathophysiology[edit]




The default mode network has been hypothesized to be relevant to disorders including Alzheimer's disease, autism, schizophrenia, depression, chronic pain and others.[3]

People with Alzheimer's disease show a reduction in glucose (energy use) within the areas of the default mode network.[3] 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.[3] Plots of amyloid-beta, which is thought to cause Alzheimer's disease, show the buildup of the protein is within the DMN.[3] 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.[3] 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.[3] 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).[18][19] The more severe the autism, the less connected these areas are to each other.[18][19] 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.[20]Hyperconnectivity of the default network has been linked to rumination in depression[21] and chronic pain.[22]




Modulation[edit]

The default mode network (DMN) may be modulated by the following interventions and processes:

  • Sleep deprivation – Functional connectivity between nodes of the DMN in their resting-state is usually strong, but sleep deprivation results in a decrease in connectivity within the DMN.[29] Recent studies suggest a decrease in connectivity between the DMN and the task-positive network as a result of sleep loss.[30]
  • Psychedelic drugs – Reduced blood flow to the PCC and mPFC was observed under the administration of psilocybin. These two areas are considered to be the main nodes of the DMN.[31] One study on the effects of LSD demonstrated that the drug desynchronizes brain activity within the DMN; the activity of the brain regions that constitute the DMN becomes less correlated.[32]