G+Connections



=Default Network=

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]] 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 believed 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 mental disorders including [|Alzheimer's disease], [|autism], and [|schizophrenia].[|[3]] In particular, reduced default network activity has been associated with autism [citation?], overactivity with schizophrenia,[|[5]] and the default network is preferentially attacked by the buildup of beta-amyloid in Alzheimer's disease[|[6]]. Lower connectivity was found across the default network in people who have experienced long term trauma, such as childhood abuse.

Among people experiencing [|Post Traumatic 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]. Children who been traumatised often lack an inner world of imagination and show little symbolic play, this too is likely to be due to interuptions across the default network. [|Mindfulness] practice is recommended for reactivating these networks. Impaired control of entering and leaving the default network state is correlated with old age.[|[7]] The idea of a "default network" is not universally accepted.[|[8]] 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.[|[9]]

[|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) becomes active.[|[10]] Later, experiments by the group of neurologist Marcus E. Raichle's 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.[|[10]] Raichle coined the term "default mode" in 2001 to describe resting state brain function;[|[11]] the concept rapidly became a central theme in [|neuroscience].[|[12]] The brain has other Low Frequency Resting State Networks (LFRSNs), such as visual and auditory networks.[|[2]]

White matter

is one of the two components of the [|central nervous system] and consists mostly of [|myelinated] [|axons]. White matter tissue of the freshly cut brain appears pinkish white to the naked eye because [|myelin] is composed largely of [|lipid] tissue veined with [|capillaries]. Its white color is due to its usual preservation in [|formaldehyde]. A 20 year-old male has around 176,000 km of myelinated axons in his brain.[|[1]] The other main component of the brain is [|grey matter] (actually pinkish tan due to blood capillaries). A third colored component found in the brain that appears darker due to higher levels of [|melanin] in [|dopaminergic] [|neurons] than its nearby areas is the [|substantia nigra]. Note that white matter can sometimes appear darker than [|grey matter] on a [|microscope] [|slide] because of the type of [|stain] used. There are three different kinds of tracts within the white matter: 1. Projection tracts, which send action potentials from the cortex to other brain regions, out of the brain to muscles, or into the brain from sense receptors. 2. Commissural tracts, which carry information between the left and right hemispheres of the brain over bridges known as commissures. These tracts allow the two hemispheres of the brain to communicate with one another. 3. Association tracts, which carry information between lobes within the same hemisphere. Long association fibers connect different lobes of a hemisphere with one another and short association fibers connect different gyri within a single lobe.[|[5]] The brain in general (and especially a child's brain) can adapt to white-matter damage by finding alternative routes that bypass the damaged white-matter areas, and can therefore maintain good connections between the various areas of gray matter. Unlike gray matter, which peaks in development in a person's twenties, the white matter continues to develop, and peaks in middle age (Sowell et al., 2003). This claim has been disputed in recent years, however. A 2009 paper by Jan Scholz and colleagues[|[6]] used [|diffusion tensor imaging] (DTI) to demonstrate changes in white matter volume as a result of learning a new motor task (juggling). The study is important as the first paper to correlate motor learning with white matter changes. Previously, many researchers had considered this type of learning to be exclusively mediated by dendrites, which are not present in white matter. The authors suggest that electrical activity in axons may regulate myelination in axons. Similarly, the cause may be gross changes in the diameter or packing density of the axon[|[7]].

=**Diffusion MRI**= is a [|magnetic resonance imaging] (MRI) method that produces //[|in vivo]// images of [|biological tissues] weighted with the local microstructural characteristics of water [|diffusion]. The field of diffusion MRI can be understood in terms of two distinct classes of application—diffusion weighted MRI and diffusion tensor MRI. Diffusion weighted MRI can provide information about damage to parts of the nervous system. Diffusion tensor MRI can provide information about connections among brain regions. In **diffusion weighted imaging** (DWI), each image [|voxel] (three dimensional pixel) has an image intensity that reflects a single best measurement of the rate of water diffusion at that location. This measurement is more sensitive to early changes after a stroke than more traditional MRI measurements such as T1 or [|T2 relaxation] rates. DWI is most applicable when the tissue of interest is dominated by isotropic water movement e.g. [|grey matter] in the [|cerebral cortex] and major brain nuclei—where the diffusion rate appears to be the same when measured along any axis. More extended diffusion tensor imaging (DTI) scans derive neural tract directional information from the data using 3D or multidimensional vector algorithms based on six or more gradient directions, sufficient to compute the diffusion [|tensor]. The diffusion model is a rather simple model of the diffusion process, assuming homogeneity and linearity of the diffusion within each image voxel. From the diffusion tensor, diffusion anisotropy measures such as the fractional anisotropy (FA), can be computed. Moreover, the principal direction of the diffusion tensor can be used to infer the white-matter connectivity of the brain (i.e. [|tractography]; trying to see which part of the brain is connected to which other part). Recently, more advanced models of the diffusion process have been proposed that aim to overcome the weaknesses of the diffusion tensor model. Amongst others, these include q-space imaging and generalized diffusion tensor imaging
 * Diffusion tensor imaging** (DTI) is important when a tissue—such as the neural [|axons] of [|white matter] in the brain or muscle fibers in the heart—has an internal fibrous structure analogous to the [|anisotropy] of some crystals. Water will then diffuse more rapidly in the direction aligned with the internal structure, and more slowly as it moves perpendicular to the preferred direction. This also means that the measured rate of diffusion will differ depending on the direction from which an observer is looking. Traditionally, in diffusion-weighted imaging (DWI), three gradient-directions are applied, sufficient to estimate the trace of the diffusion tensor or 'average diffusivity', a putative measure of [|edema]. Clinically, trace-weighted images have proven to be very useful to diagnose vascular [|strokes] in the brain, by early detection (within a couple of minutes) of the hypoxic edema.