Default mode network

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. But in the 1950s, Louis Sokoloff and his colleagues noticed that metabolism in the brain stayed the same when a person went from a resting state to performing effortful math problems, suggesting active metabolism in the brain must also be happening during rest. In the 1990s, with the advent of positron emission tomography (PET) scans, researchers began to notice that when a person is involved in perception, language, and attention tasks, the same brain areas become less active compared to passive rest, and labeled these areas as becoming "deactivated". Later, experiments by neurologist Marcus E. Raichle's lab at Washington University School of Medicine and other groups the concept rapidly became a central theme in neuroscience. Around this time the idea was developed that this network of brain areas is involved in internally directed thoughts and is suspended during specific goal-directed behaviors. In 2003, Greicius and colleagues examined resting state fMRI scans and looked at how correlated different sections in the brain are to each other. Their correlation maps highlighted the same areas already identified by the other researchers. Until the mid-2000s, researchers labeled the default mode network as the "task-negative network" because it was deactivated when participants had to perform external goal-directed tasks. DMN was thought to only be active during passive rest and inactive during tasks. However, more recent studies have demonstrated the DMN to be active in certain internal goal-directed tasks such as social working memory and autobiographical tasks. Around 2007, the number of papers referencing the default mode network skyrocketed. In all years prior to 2007, there were 12 papers published that referenced "default mode network" or "default network" in the title; however, between 2007 and 2014 the number increased to 1,384 papers. One reason for the increase in papers was the robust effect of finding the DMN with resting-state scans and independent component analysis (ICA). Another reason was that the DMN could be measured with short and effortless resting-state scans, meaning they could be performed on any population including young children, clinical populations, and nonhuman primates. A third reason was that the role of the DMN was now understood to be more than just a passive brain network. ==Anatomy==

The default mode network is an interconnected and anatomically defined Information regarding the self • Posterior cingulate cortex (PCC) & precuneus: Combines bottom-up (not controlled) attention with information from memory and perception. The ventral (lower) part of the PCC activates in all tasks which involve the DMN including those related to the self, related to others, remembering the past, thinking about the future, and processing concepts plus spatial navigation. The dorsal (upper) part of PCC involves involuntary awareness and arousal. The precuneus is involved in visual, sensorimotor, and attentional information. • Medial prefrontal cortex (mPFC): Decisions about self-processing such as personal information, autobiographical memories, future goals and events, and decision making regarding those personally very close such as family. The ventral (lower) part is involved in positive emotional information and internally valued reward. • Angular gyrus: Connects perception, attention, spatial cognition, and action and helps with parts of recall of episodic memories. Dorsal medial subsystem: • Posterior inferior parietal lobe (pIPL): Junction of auditory, visual, and somatosensory information and attention The default mode network is most commonly defined with resting state data by putting a seed in the posterior cingulate cortex and examining which other brain areas most correlate with this area. The DMN can also be defined by the areas deactivated during external directed tasks compared to rest. It has been shown that the default mode network exhibits the highest overlap in its structural and functional connectivity, which suggests that the structural architecture of the brain may be built in such a way that this particular network is activated by default. In the infant's 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 ==

The default mode network is thought to be involved in several different functions: It is potentially the neurological basis for the self: • Autobiographical information: Memories of collection of events and facts about one's self • Self-reference: Referring to traits and descriptions of one's self • ''Emotion of one's self:'' Reflecting about one's own emotional state Thinking about others: Remembering the past and thinking about the future: The default mode network is active during passive rest and mind-wandering Electrocorticography studies (which involve placing electrodes on the surface of a subject's cerebral cortex) have shown the default mode network becomes activated within a fraction of a second after participants finish a task. Additionally, during attention demanding tasks, sufficient deactivation of the default mode network at the time of memory encoding has been shown to result in more successful long-term memory consolidation. Studies have shown that when people watch a movie, listen to a story, or read a story, their DMNs are highly correlated with each other. DMNs are not correlated if the stories are scrambled or are in a language the person does not understand, suggesting that the network is highly involved in the comprehension and the subsequent memory formation of that story. further suggesting the DMN is truly involved in the comprehension aspect of the story and not the auditory or language aspect. The default mode network is deactivated during some external goal-oriented tasks such as visual attention or cognitive working memory tasks. Meditation, which involves focusing the mind on breathing and relaxation, is associated with reduced activity of the DMN. Starr et al. (2019) suggests that the DMN is related to the perception of beauty, in which the network becomes activated in a generalized way to aesthetically moving domains such as artworks, landscapes, and architecture. This would explain a deep inner feeling of pleasure related to aesthetics, interconnected with the sense of personal identity, due to the network functions related to the self. == Clinical significance ==

The default mode network has been hypothesized to be relevant to disorders including Alzheimer's disease, autism, schizophrenia, major depressive disorder (MDD), chronic pain, post-traumatic stress disorder (PTSD) and others. In particular, the DMN has also been reported to show overlapping yet distinct neural activity patterns across different neurodevelopmental conditions, such as when directly comparing attention deficit hyperactivity disorder (ADHD) and autism. People with Alzheimer's disease show a reduction in glucose (energy use) within the areas of the default mode network. These individuals are impaired in social interaction and communication which are tasks central to this network. Studies have shown differences in connections between areas of the DMN in Autistic individuals, especially between the mPFC (involved in thinking about the self and others) and the PCC (the central core of the DMN). The more severe the autism (or the higher the support needs if using a neuroaffirming lens), the less connected these areas are to each other. Rumination, one of the main symptoms of major depressive disorder, is associated with increased DMN connectivity and dominance over other networks during rest. Such DMN hyperconnectivity has been observed in first-episode depression and chronic pain. Altered DMN connectivity may change the way a person perceives events and their social and moral reasoning, thus increasing their susceptibility to depressive symptoms. Lower connectivity between brain regions was found across the default network in people who have experienced long-term trauma, such as childhood abuse or neglect, and is associated with dysfunctional attachment patterns. Among people experiencing PTSD, lower activation was found in the posterior cingulate gyrus compared to controls, and severe PTSD was characterized by lower connectivity within the DMN. Adults and children with ADHD show reduced anticorrelation between the DMN and other brain networks. The cause may be a lag in brain maturation. More generally, competing activation between the DMN and other networks during memory encoding may result in poor long-term memory consolidation, which is a symptom of not only ADHD but also depression, anxiety, autism, and schizophrenia. This begs the question, is this difference related to neurodivergence or the impact of living in an unaccommodating world. == Modulation ==

The default mode network (DMN) may be modulated by the following interventions and processes: • Acupuncture – Deactivation of the limbic brain areas and the DMN. It has been suggested that this is due to the pain response. • Antidepressants – High levels of DMN connectivity are impaired following treatment with antidepressant medications in PTSD. • Deep brain stimulation – Alterations in brain activity with deep brain stimulation may be used to balance resting state networks. • Meditation – Structural changes in areas of the DMN such as the temporoparietal junction, posterior cingulate cortex, and precuneus have been found in meditation practitioners. Various forms of nondirective meditation, including Transcendental Meditation and Acem Meditation, have been found to activate the DMN. • Physical Activity and Exercise – Physical Activity, and more likely Aerobic Training, may alter the DMN. In addition, sports experts are showing networks differences, notably of the DMN. • Psychedelic drugs – Reduced blood flow to the PCC and mPFC was observed under the administration of psilocybin. These two areas are considered to be the main nodes of the DMN. One study on the effects of LSD demonstrated that the drug desynchronizes brain activity within the DMN; the activity of the brain regions that constitute the DMN becomes less correlated. • Psychotherapy – In PTSD, the abnormalities in the default mode network normalize in individuals who respond to psychotherapy interventions. • Sleep deprivation – Functional connectivity between nodes of the DMN in their resting-state is usually strong, but sleep deprivation results in a decrease in connectivity within the DMN. Recent studies suggest a decrease in connectivity between the DMN and the task-positive network as a result of sleep loss. • Sleeping and resting wakefulness • Onset of sleep – Increase in connectivity between the DMN and the task-positive network. • Stage N2 of NREM sleep – Decrease in connectivity between the posterior cingulate cortex and medial prefrontal cortex. • Stage N3 of NREM sleep – Further decrease in connectivity between the PCC and MPFC. == Criticism ==

Some have argued the brain areas in the default mode network only show up together because of the vascular coupling of large arteries and veins in the brain near these areas, not because these areas are actually functionally connected to each other. Support for this argument comes from studies that show changes in breathing alters oxygen levels in the blood which in turn affects DMN the most. measuring electrical activity on the surface of the brain, and in MEG by measuring magnetic fields associated with electrophysiological brain activity that bypasses the hemodynamic response. 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. == Nomenclature ==

The default mode network has also been called the language network, semantic system, or limbic network. == See also ==