Default network

In neuroscience, 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. The default network may correspond 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".

Anatomy
The default network is an interconnected and anatomically defined brain system that preferentially activates when individuals engage in 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, along with the adjacent ventral 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.

Function
In humans, the default network has been hypothesized to generate spontaneous thoughts during mind-wandering and may relate to 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, autism, and schizophrenia. 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 experienced long-term trauma and describe feeling 'dead inside' or have a fragmented sense of self or enter dissociative states. 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. Meditation practice (including transcendental meditation ) 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, 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 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.

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