Default 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]

The default network 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 mode network (DMN), default state network, or task-negative network (TNN), it 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. It is thought that the default network corresponds to task-independent introspection, or self-referential thought, while the TPN corresponds to action, and that perhaps the TNN and TPN may be "considered elements of a single default network with anti-correlated components".^[2]

Anatomy

The default network is an interconnected and anatomically defined brain system that preferentially activates when individuals focus on internal tasks such as daydreaming, envisioning the future, retrieving memories, and gauging others' perspectives. 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,^[3] along with the adjacent precuneus and the medial, lateral and inferior parietal cortex. 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.^[2]

Function

In humans, the default network has been hypothesized to generate spontaneous thoughts during mind-wandering and to be an essential component of creativity.^[3] Alternatively, default mode activity may represent underlying physiological processes going on in the brain that are unrelated to any particular thought or thoughts.^[4] It has been hypothesized to be relevant to disorders including Alzheimer's disease, autism, and schizophrenia.^[3] In particular, reduced default network activity has been associated with autism^[5], overactivity with schizophrenia,^[6] and the default network is preferentially attacked by the buildup of beta-amyloid in Alzheimer's disease.^[7] 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 (Dr. Ruth Lanius, Brain Mapping conference, London, November 2010). The posterior cingulate gyrus discerns emotional and self-relevant information; this interacts with 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. This appears to correlate with the experience of people who have experience long-term trauma and describe feeling 'dead inside' or have a fragmented sense of self or enter dissociative states.^{[citation needed]} Children who have been traumatised often lack an inner world of imagination and show little symbolic play, this too is likely to be due to interruptions across the default network.^{[citation needed]} Mindfulness practice is recommended for reactivating these networks.^[8] Impaired control of entering and leaving the default network state is correlated with old age.^[9]

The idea of a "default network" is not universally accepted.^[10] 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.^[11]

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.^[12]

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.^[12]

Raichle coined the term "default mode" in 2001 to describe resting state brain function;^[13] the concept rapidly became a central theme in neuroscience.^[4] The brain has other Low Frequency Resting State Networks (LFRSNs), such as visual and auditory networks.^[2]

References

1. ^ Fair, Damien A.; Cohen, Alexander L.; Power, Jonathan D.; Dosenbach, Nico U. F.; Church, Jessica A.; Miezin, Francis M.; Schlaggar, Bradley L.; Petersen, Steven E. (2009). Sporns, Olaf. ed. "Functional Brain Networks Develop from a 'Local to Distributed' Organization". PLoS Computational Biology 5 (5): e1000381. doi:10.1371/journal.pcbi.1000381. PMC 2671306. PMID 19412534. http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=2671306. 
2. ^ ^{a b c} Broyd, Samantha J.; Demanuele, Charmaine; Debener, Stefan; Helps, Suzannah K.; James, Christopher J.; Sonuga-Barke, Edmund J.S. (2009). "Default-mode brain dysfunction in mental disorders: A systematic review". Neuroscience & Biobehavioral Reviews 33 (3): 279–96. doi:10.1016/j.neubiorev.2008.09.002. PMID 18824195. 
3. ^ ^{a b c} Buckner, R. L.; Andrews-Hanna, J. R.; Schacter, D. L. (2008). "The Brain's Default Network: Anatomy, Function, and Relevance to Disease". Annals of the New York Academy of Sciences 1124: 1–38. doi:10.1196/annals.1440.011. PMID 18400922. 
4. ^ ^{a b} Raichle, Marcus E.; Snyder, Abraham Z. (2007). "A default mode of brain function: A brief history of an evolving idea". NeuroImage 37 (4): 1083–90. doi:10.1016/j.neuroimage.2007.02.041. PMID 17719799. 
5. ^ Kennedy, DP; Redcay, E; Courchesne, E (2006). "Failing to deactivate: resting functional abnormalities in autism.". Proc Natl Acad Sci 103: 8275–8280. 
6. ^ Whitfield-Gabrieli, S.; Thermenos, H. W.; Milanovic, S.; Tsuang, M. T.; Faraone, S. V.; McCarley, R. W.; Shenton, M. E.; Green, A. I. et al. (2009). "Hyperactivity and hyperconnectivity of the default network in schizophrenia and in first-degree relatives of persons with schizophrenia". Proceedings of the National Academy of Sciences 106 (4): 1279–84. doi:10.1073/pnas.0809141106. PMC 2633557. PMID 19164577. http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=2633557. 
7. ^ Kolata G (2010-12-13). "Insights give hope for new attack on Alzheimer's". New York Times. http://www.nytimes.com/2010/12/14/health/14alzheimers.html. Retrieved 2010-12-14. 
8. ^ Jang, Joon Hwan; Jung, Wi Hoon; Kang, Do-Hyung; Byun, Min Soo; Kwon, Soo Jin; Choi, Chi-Hoon; Kwon, Jun Soo (2011). "Increased default mode network connectivity associated with meditation". Neuroscience Letters 487 (3): 358–62. doi:10.1016/j.neulet.2010.10.056. PMID 21034792. 
9. ^ Lehrer J (2008-08-31). "Daydream achiever". Boston Globe. http://boston.com/bostonglobe/ideas/articles/2008/08/31/daydream_achiever/. Retrieved 2009-01-23. 
10. ^ Fair, D. A.; Cohen, A. L.; Dosenbach, N. U. F.; Church, J. A.; Miezin, F. M.; Barch, D. M.; Raichle, M. E.; Petersen, S. E. et al. (2008). "The maturing architecture of the brain's default network". Proceedings of the National Academy of Sciences 105 (10): 4028–32. doi:10.1073/pnas.0800376105. PMC 2268790. PMID 18322013. http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=2268790. 
11. ^ Poldrack, Russell A.; Devlin, Joseph T. (2007). "On the fundamental role of anatomy in functional imaging: Reply to commentaries on 'In praise of tedious anatomy'". NeuroImage 37 (4): 1073–82. doi:10.1016/j.neuroimage.2007.06.019. PMID 17681817. 
12. ^ ^{a b} "The Brain's Dark Energy", Scientific American, March 2010
13. ^ Raichle, M. E.; MacLeod, AM; Snyder, AZ; Powers, WJ; Gusnard, DA; Shulman, GL (2001). "Inaugural Article: A default mode of brain function". Proceedings of the National Academy of Sciences 98 (2): 676–82. doi:10.1073/pnas.98.2.676. PMC 14647. PMID 11209064. http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=14647. 

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