Friday, October 02, 2015

The Default Network Mode; The Brain's Screensaver

The default mode network (DMN) is a network of brain components active when during daydreaming, self-generated thought, and when not attending to outside stimuli. Marcus Raichle, the discoverer of the DMN, has referred to it as "the orchestrator of the self". It is most active when the brain is at rest or involved in social communication.
The concept of brain resting-state network arose from observations made when comparing cerebral perfusion during cognitive processing to that measured during passive baseline conditions such as at rest, that is, when subjects lie in the dark and are instructed to think about nothing in particular (Mevel, 2011).

Raichle first used the term in 2001 to describe the nature of brain activity when it is not engaged in any specific, externally focused task. It's been considered quite an elaborate system, and while there are no definitive functions of the DMN as of yet, some proposed have included internal processes such as self-reflection to diffused passive attention. The DMN is generally inhibited in most cognitive tasks, however, tasks that involve episodic memory does not deactivate the DMN - suggesting a link.

The main hypotheses associated with the DMN and cognitive functions are, the Internal Mentation Hypothesis, and the Sentinel Hypothesis. The Internal Mentation hypothesis holds that DMN is important in introspection and internal attention. The Sentinel Hypothesis argues that the DMN supports a low level ''exploratory'' attention that surveys for unexpected stimuli.

Although some variation occurs, the default network mostly includes medial brain structures, i.e., the ventral medial prefrontal cortex, the posterior cingulate cortex, the inferior parietal lobe, the lateral temporal cortex, the dorsal medial prefrontal cortex, and the hippocampal formation. Probing the functional anatomy of the network in detail reveals that it is best understood as multiple interacting subsystems (Buckner, 2008).

The link between DMN and episodic memory is well established. It is now known that retrieval of episodic memories, whether internally or externally cued; relies on the DMN (Cabeza et al., 2011). Further, dysfunction of both grey matter of DMN nodes as well as white matter connections are implicated in Alzheimer's Disease, a disease with obvious prominent effects on episodic memory. People with early signs of Alzheimer's Disease have unusual resting state signatures, while in Autism; the resting-state networks can be 'hyperconnected'.

People who are depressed show an increase in DMN activity. This is likely to be precisely because what characterizes depression is a sense of constant rumination and negative self-referential mental activity – in neurological terms being stuck in the DMN. (Smith, 2015). Others researchers discovered findings that suggest increased default mode network activation during meditation (Xu et al., 2014), indicating that this activation is related to the relaxed focus of attention, which allows spontaneous thoughts, images, sensations, memories, and emotions to emerge and pass freely, accepting them as part of the meditation process (Xu et al., 2014). The DMN has also been linked with depression (Belleau et al., 2014), schizophrenia (Mingoia et al., 2012), and post traumatic stress disorder (Lanius et al., 2009).

While the functional significance of the DMN remains unknown, converging evidence suggests that the DMN might be critical for self-referential processing (e.g., introspection). Age differences in the ability to deactivate the DMN has been found between older and younger adults, which may reflect the cognitive change experienced in normal aging (Park et al, 2009). The mental activity of the DMN has still not been rigorously assessed to date. Despite the growing amount of knowledge regarding the DMN physiology and anatomy, the cognitive function of this network is still poorly understood (Mevel, 2011).
''Whatever resting activity is doing, its existence proves one thing - the brain only rests when you're dead'' ~ Miall (2009)

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