Default network
Encyclopedia
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
Coherence (physics)
In physics, coherence is a property of waves that enables stationary interference. More generally, coherence describes all properties of the correlation between physical quantities of a wave....

 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
Introspection
Introspection is the self-observation and reporting of conscious inner thoughts, desires and sensations. It is a conscious and purposive process relying on thinking, reasoning, and examining one's own thoughts, feelings, and, in more spiritual cases, one's soul...

, 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".

Anatomy

The default network is an interconnected and anatomically defined brain system that preferentially activates when individuals focus on internal tasks such as daydream
Daydream
A daydream is a visionary fantasy, especially one of happy, pleasant thoughts, hopes or ambitions, imagined as coming to pass, and experienced while awake. There are many types of daydreams, and there is no consistent definition amongst psychologists. The general public also uses the term for a...

ing, envisioning the future, retrieving memories
Memory
In psychology, memory is an organism's ability to store, retain, and recall information and experiences. Traditional studies of memory began in the fields of philosophy, including techniques of artificially enhancing memory....

, 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
Theory of mind
Theory of mind is the ability to attribute mental states—beliefs, intents, desires, pretending, knowledge, etc.—to oneself and others and to understand that others have beliefs, desires and intentions that are different from one's own...

, and the posterior cingulate cortex for integration, along with the adjacent precuneus
Precuneus
The precuneus is a part of the superior parietal lobule hidden in the medial longitudinal fissure between the two cerebral hemispheres. It is sometimes described as the medial area of the superior parietal cortex. It is involved with episodic memory, visuospatial processing, reflections upon self,...

 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.

Function

In humans, the default network has been hypothesized to generate spontaneous thoughts during mind-wandering
Mind-wandering
Mind-wandering is the experience of thoughts not remaining on a single topic for a long period of time, particularly when people are not engaged in an attention-demanding task...

 and to be an essential component of creativity. Alternatively, default mode activity may represent underlying physiological processes going on in the brain that are unrelated to any particular thought or thoughts. It has been hypothesized to be relevant to disorders including Alzheimer's disease
Alzheimer's disease
Alzheimer's disease also known in medical literature as Alzheimer disease is the most common form of dementia. There is no cure for the disease, which worsens as it progresses, and eventually leads to death...

, autism
Autism
Autism is a disorder of neural development characterized by impaired social interaction and communication, and by restricted and repetitive behavior. These signs all begin before a child is three years old. Autism affects information processing in the brain by altering how nerve cells and their...

, and schizophrenia
Schizophrenia
Schizophrenia is a mental disorder characterized by a disintegration of thought processes and of emotional responsiveness. It most commonly manifests itself as auditory hallucinations, paranoid or bizarre delusions, or disorganized speech and thinking, and it is accompanied by significant social...

. In particular, reduced default network activity has been associated with autism, overactivity with schizophrenia, and the default network is preferentially attacked by the buildup of beta-amyloid in Alzheimer's disease. 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
Dissociative identity disorder
Dissociative identity disorder is a psychiatric diagnosis and describes a condition in which a person displays multiple distinct identities , each with its own pattern of perceiving and interacting with the environment....

. 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. Mindfulness
Mindfulness-based stress reduction
Mindfulness-Based Stress Reduction is a structured complementary medicine program that uses mindfulness in an approach that focuses on alleviating pain and on improving physical and emotional well-being for individuals suffering from a variety of diseases and disorders. The program was established...

 practice is recommended for reactivating these networks. Impaired control of entering and leaving the default network state is correlated with old age.

The idea of a "default network" is not universally accepted. 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.

History

Hans Berger
Hans Berger
Hans Berger was born in Neuses near Coburg, Bavaria, Germany. He is best known as the first to record human electroencephalograms in 1924, for which he invented the electroencephalogram , and the discoverer of the alpha wave rhythm known as "Berger's wave".- Biography :After attending...

, 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.

Later, experiments by neurologist
Neurology
Neurology is a medical specialty dealing with disorders of the nervous system. Specifically, it deals with the diagnosis and treatment of all categories of disease involving the central, peripheral, and autonomic nervous systems, including their coverings, blood vessels, and all effector tissue,...

 Marcus E. Raichle's
Marcus Raichle
Marcus Raichle is a neurologist at the Washington University School of Medicine in Saint Louis. He is a professor in the Department of Radiology with joint appointments in Neurology, Neurobiology and Biomedical Engineering...

 lab at Washington University School of Medicine
Washington University School of Medicine
Washington University School of Medicine , located in St. Louis, Missouri, is one of the graduate schools of Washington University in St. Louis. One of the top medical schools in the United States, it is currently ranked 4th for research according to U.S. News and World Report and has been listed...

 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.

Raichle
Marcus Raichle
Marcus Raichle is a neurologist at the Washington University School of Medicine in Saint Louis. He is a professor in the Department of Radiology with joint appointments in Neurology, Neurobiology and Biomedical Engineering...

 coined the term "default mode" in 2001 to describe resting state brain function; the concept rapidly became a central theme in neuroscience
Neuroscience
Neuroscience is the scientific study of the nervous system. Traditionally, neuroscience has been seen as a branch of biology. However, it is currently an interdisciplinary science that collaborates with other fields such as chemistry, computer science, engineering, linguistics, mathematics,...

. The brain has other Low Frequency Resting State Networks (LFRSNs), such as visual and auditory networks.

External links

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