Default mode network

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.^[1]

Default Mode Network Connectivity. This image shows main regions of the default mode network (yellow) and connectivity between the regions color-coded by structural traversing direction (xyz -> rgb).^{[2] [3]}

In neuroscience, the default mode network (DMN) is a network of brain regions that are active when the individual is not focused on the outside world and the brain is at wakeful rest. Also called the default network, default state network, or task-negative network, the DMN is characterized by coherent neuronal oscillations at a rate lower than 0.1 Hz (one every ten seconds). During goal-oriented activity, the DMN is deactivated and another network, the task-positive network (TPN) is activated. The DMN may correspond to task-independent introspection, or self-referential thought, while the TPN corresponds to action, and thus perhaps the DMN and TPN may be "considered elements of a single default network with anti-correlated components".^[4]

Anatomy

The default mode network is an interconnected and anatomically defined^[5] brain system that preferentially activates when individuals engage in internal tasks such as daydreaming, envisioning the future, retrieving memories, and gauging others' perspectives.^[5] It is negatively correlated with brain systems that focus on external visual signals. Its subsystems include part of the medial temporal lobe for memory, part of the medial prefrontal cortex for theory of mind, and the posterior cingulate cortex for integration,^[5] along with the adjacent ventral precuneus^[6] and the medial, lateral and inferior parietal cortex. 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) as long as no other mental activities are in process (i.e. in a task-absent state).^[7] 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.^[4]

Function

The function of the default mode network remains unclear; however, activity in the network negatively correlates with activity in regions involved in attention and executive function.^[8] In humans, the default mode network has been hypothesized to generate spontaneous thoughts during mind-wandering and may relate to creativity.^[5][9] Alternatively, default mode activity may represent underlying physiological processes going on in the brain that are unrelated to any particular thought or thoughts.^[10] The posterior cingulate gyrus discerns emotional and self-relevant information; this interacts with both the anterior cingulate gyrus, which integrates emotional information with cognition, and the medial prefrontal cortex, which allows for self-reflection and the regulation of emotion and arousal. Attribution of arousal and emotion regulation to the DMN appears to correlate with the experience of people who have experienced long-term trauma and describe feeling 'dead inside' or have a fragmented sense of self or enter dissociative states.

Pathophysiology

The default mode network has been hypothesized to be relevant to disorders including Alzheimer's disease, autism, schizophrenia, depression, chronic pain and others.^[5] In particular, a failure to deactivate default network activity during cognitively demanding tasks is associated with autism,^[11] overactivity with schizophrenia,^[12] and the default network is preferentially attacked by the buildup of beta-amyloid in Alzheimer's disease.^[13] Lower connectivity was found across the default network in people who have experienced long term trauma, such as childhood abuse. Among people experiencing posttraumatic stress disorder, lower activation was found in the posterior cingulate gyrus compared to controls.^[14] Hyperconnectivity of the default network has been linked to rumination in depression^[15] and chronic pain.^[16]

Modulation

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

• Acupuncture - Deactivation of the limbic brain areas and the DMN.^[17] Tactile control stimulation results in deactivation patterns that are considerably weaker and not statistically significant.^[18]

• Meditation - Structural changes in areas of the DMN such as the temporoparietal junction, posterior cingulate cortex, and precuneus have been found in meditation practitioners.^[19] There is reduced activation and reduced functional connectivity of the DMN in long-term practitioners.^[19]

• Sleeping and resting wakefulness
  • Resting wakefulness - Functional connectivity between nodes of the DMN is strong.^[20]
  • Onset of sleep - Decrease in connectivity between the DMN and the task-positive network.^[20]
  • Stage N2 of NREM sleep - Decrease in connectivity between the posterior cingulate cortex and medial prefrontal cortex.^[20]
  • Stage N3 of NREM sleep - Further decrease in connectivity between the PCC and MPFC.^[20]
  • REM sleep - Possible increase in connectivity between nodes of the DMN.^[20]

• 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.^[21] Recent studies suggest a decrease in connectivity between the DMN and the task-positive network as a result of sleep loss.^[22]

• Psychedelic drugs - Reduced blood flow to the PCC and mPFC were observed under the administration of Psilocybin. These two areas are believed to be involved in the DMN.^[23]

History

Hans Berger, the inventor of the electroencephalogram, was the first to propose the idea that the brain is constantly busy. In a series of papers published in 1929 he showed that the electrical oscillations detected by his device do not cease even when the subject is at rest. However, his ideas were not taken seriously, and a general perception formed among neurologists that only when a focused activity is performed does the brain (or a part of the brain) become active.^[24] In the 1970s, intrinsic oscillatory behavior in vertebrate neurons was observed in cerebellar Purkinje cells, inferior olivary nucleus and thalamus.^[25]

Later, experiments by neurologist Marcus E. Raichle's lab at Washington University School of Medicine and other groups showed that the brain's energy consumption is increased by less than 5% of its baseline energy consumption while performing a focused mental task. These experiments showed that the brain is constantly active with a high level of activity even when the person is not engaged in focused mental work. Research thereafter focused on finding the regions responsible for this constant background activity level.^[24]

Raichle coined the term "default mode" in 2001 to describe resting state brain function;^[26] the concept rapidly became a central theme in neuroscience.^[10] In later work, the default mode network was demonstrated by measuring correlated functional activity using resting state fMRI. The brain has other Resting State Networks (RSNs), such as visual and auditory networks.^[4]

Tom Eichele, a neuroscientist at the University of Bergen in Norway, as part of an international team of researchers, identified a change in brain scans up to thirty seconds before a mistake, associated with brain activity changes. The researchers suspect the abnormal behavior is the result of the brain attempting to save effort on a task by entering a more restful state.^[27][28]

Criticism

The idea of a "default network" is not universally accepted.^[29] In 2007 the concept of the default mode was criticized as not being useful for understanding brain function, on the grounds that a simpler hypothesis is that a resting brain actually does more processing than a brain doing certain "demanding" tasks, and that there is no special significance to the intrinsic activity of the resting brain.^[30]

References

External links

• Healy, Melissa "An idle brain may be the self's workshop", Los Angeles Times, August 30, 2010
• Healy, Melissa "Brain's default mode network may hold key to better psychiatric diagnoses", Los Angeles Times, August 30, 2010