Large scale brain networks
Large scale brain networks are collections of widespread brain regions showing functional connectivity by statistical analysis of the fMRI BOLD signal or other signal fluctuations. An emerging paradigm in neuroscience is that cognitive tasks are performed not by individual brain regions working in isolation, but by networks consisting of several discrete brain regions that are said to be "functionally connected" due to tightly coupled activity. Functional connectivity may be measured as long-range synchronization of the EEG, MEG, or other dynamic brain signals. Synchronized brain regions may also be identified using spatial independent component analysis. The set of identified brain areas that are linked together in a large-scale network varies with cognitive function. When the cognitive state is not explicit (i.e., the subject is at "rest"), the large scale brain network is a resting state network (RSN). As a physical system with graph-like properties, a large scale brain network has both nodes and edges, and cannot be identified simply by the co-activation of brain areas. In recent decades, the analysis of brain networks was made feasible by advances in imaging techniques as well as new tools from graph theory and dynamical systems. Large scale brain networks are identified by their function, and provide a coherent framework for understanding cognition by offering a neural model of how different cognitive functions emerge when different sets of brain regions join together as self-organized coalitions. Disruptions in activity in various networks have been implicated in neuropsychiatric disorders such as depression, Alzheimer's, autism spectrum disorder, schizophrenia and bipolar disorder.
The following six networks have been identified by at least three studies.
- Default mode: The default mode network is active when an individual is awake and at rest. It preferentially activates when individuals focus on internally-oriented tasks such as daydreaming, envisioning the future, retrieving memories, and theory of mind. It is negatively correlated with brain systems that focus on external visual signals. It is the most widely researched network.
- Dorsal attention: voluntary deployment of attention and reorientation to unexpected events.
- Ventral attention:  responds when behaviorally relevant stimuli occur unexpectedly.
Several other brain networks have also been identified: auditory, motor, right executive, posterior default mode, left fronto-parietal, cerebellar, spatial attention, language, left executive, sensorimotor network, somatomotor, visual, and temporal . There are also models suggesting that “components of memory representation are distributed widely across different parts of the brain as mediated by multiple neocortical circuits”.
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