The Role of default network in human behavior

We have a perspective on the world, we respond emotionally to what happens, we are capable of introspection, we can remember the past, we can experience the present and we can project our future, we decide what to do and sometimes we learn from our mistakes thanks to our brain, the most sophisticated known organ.
A new theory is emerging in neuroscience that the brain is a sophisticated hypothesis-making and testing mechanism for predicting the future, constantly engaged in minimizing the error of its predictions based on past and present environmental sensory inputs and which will direct immediate behavior. It is an attractive theory because it is supported by many theoretical arguments and experimental evidence. (Hohwy, J. 2013)
An important role in all this is played by the Default Mode network discovered by chance in humans (Raichle et al., 2001) and then in monkeys (Kondo et al., 2005) and rodents (Lu et al., 2012).
Studies have shown that the Default Mode network has neuroplasticity, developing through synaptic plasticity and neurogenesis through exposure to new experiences.
“Synaptic plasticity, i.e. changes in the strength of synapses (Synaptogenesis) takes place along 5 phases. The initial phases are exclusively controlled by genes, while in the late phases the control gradually passes to epigenetic factors” (Cîrneci, 2014 p.36, 43), respectively the information coming from the environment can use the genetic potential in our favor or not. Neurogenesis is a form of neural plasticity that contributes to the brain’s ability to process, respond and adapt to stimuli, including learning and memory. […] Factors that have a positive effect on neurogenesis are a combination of social interactions, learning and behavioral activity. Exposure to stressful experiences decreases the number of new neurons” (Cîrneci, 2014 p.40, 44).

Neuroimaging research has found that the Default Mode network is altered in numerous psychiatric and neurological diseases, including major depression, anxiety, social phobia, post-traumatic stress disorder, schizophrenia, attention deficit hyperactivity disorder (ADHD), autism, Tourette syndrome, Alzheimer’s, stroke, chronic pain, cirrhosis, lateral sclerosis, progressive multiple sclerosis, persistent vegetative state. During research studies, using rs-fcMRI (Resting-State Functional Connectivity Magnetic Resonance Imaging) techniques of disturbances in Default Mode network activity in various diseases, it has also been shown that the network exhibits changes in task-related activations and deactivations, in changes in brain and in the development of molecular pathology. (Andrews-Hanna, 2012).
This network uses more energy than any other network in the brain, consuming 20% of the body’s energy while resting. This network does anything but “rest,” even though it operates largely under the radar of consciousness. It retrieves memories, connects ideas so that we become more creative and also feel more connected. Although the Default Mode network is involved in representing and understanding the self, it also helps us “read the minds” of others.

Default Mode network functions
In the book “Neuroscience: Exploring the brain”, regarding the functions of the Default Mode network, the authors consider two hypotheses: the sentinel hypothesis and the internal mentalization hypothesis. The hypothesis of the sentinel function is based on the fact that, even when we are resting, we must monitor (be attentive to) our environment; in comparison, when we are active, we focus our attention on the activity at hand. They say that if we think about our ancestors who lived in a world full of constant threats, it makes sense that we might have evolved to always be “on standby.” The function hypothesis in internal mentalization holds that the Default Mode network supports thinking and remembering, the kind of “dreaming” we do while sitting still. (Bear et al., 2016 p.722-723) and is involved in:

• (A) Autobiographical memory
• (B) Predicting the future
• (C) Theory of mind (mentalizing)
• (D) Moral decision making

Plasticity is an intrinsic property of the brain throughout life. The world we live in changes rapidly, sometimes faster than the time required to implement genetic or even epigenetic changes. Brain plasticity allows function and structure to change in response to environmental demands by strengthening, weakening, cutting, or adding synaptic connections and through neurogenesis. This means that the brain does not remain static, but continues to change as a consequence of sensory, motor, associative inputs, reward signals, action plans and awareness. (Pascual-Leone et al. 2011).
Grayson and Fair (2017) in their article “Development of large-scale functional networks from birth to adulthood: A guide to the neuroimaging literature” present the following aspects regarding the development of the Default Mode network:

• The research literature on infant network topology supports that there is a robust network structure at birth, but in a primitive form. In neonates, numerous anatomically localized modules in the infant brain are correlated with functional organization in adults.
• In adults the Default Mode network is a well-integrated circuit comprising several distinct and distant areas of the cortex. In newborns and very young infants, these individual brain areas demonstrate locally correlated activity but fail to synchronize into a coherent network.
• During the first two years of life, these regions undergo a precise evolution to gradually bring the network “online”. By two years of age, the network has adult-like qualities, although the coupling between distant anterior and posterior nodes of the network remains relatively low. Research suggests that sensory networks develop at a much earlier age than those involved in higher-level cognition. These results are consistent with the presence of basic somatosensory and visual functions from birth and help contextualize findings that primary sensory regions are generally the first cortical areas to reach growth peaks.
• And the Default Mode network shows, from birth to the first two years of life, a substantial increase in synchronization.
  Sato et al. (2014) conclude that the 7–15-year period is crucial for the development of both the Default Mode network and the control networks, with integration between posterior and anterior neural modules and an increase in the degree of centralization of hub regions.

The Role of the Default Mode Network in Behavioral Training
The higher power consumption of the Default Mode network compared to other brain networks and its connection to self-awareness points to an adaptive evolutionary role using remembering past experiences and imagining possible developments to anticipate the future.
Predictive coding mechanisms are frequently used in the context of the Default Mode network function. A prediction error at one of the processing levels induces changes in the plasticity of neural projections to enable progressive improvement in future prediction of the environment. Neuroscientific evidence suggests that the human brain is a “statistical organ” with the biological purpose of generalizing from the past and incorporating new experiences.
Understanding the workings of the human brain and mind in such a way makes sense because the relationship between cause and effect in our complex world is not one-to-one, similar causes can have different effects and similar effects can have different causes.

“An interesting possibility is that engaging in spontaneous thinking may allow us to construct and simulate alternative scenarios, mentally organize our plans, and prepare for what will come next. In addition, spontaneous thoughts can facilitate the organization and structuring of daily events, promoting the consolidation of the most important personal information in long-term memory. Humans have the unique ability to rapidly retrieve stored episodic, conceptual, and contextual representations and use this information to construct or ‘simulate’ future versions before they happen.” (Andrews-Hanna, 2012).

There are studies proving the involvement of the Default Mode network in moral decision making, empathy and theory of mind (mentalization). (Reniers et al., 2012). There is connection between the Default Mode network and the Mirror Neuron System (MNS). (Molnar-Szakacs & Uddin, 2013). Default Mode network activity is impaired under stress conditions. “Chronic stress and depression affect broad areas of the brain, thus affecting the connectivity between hubs in the brain and the exchange of information between distal areas carried out by the fronto-parietal and Default Mode networks.” (Cîrneci et al., 2017).
Through moral decisions, through stress response, through empathy, through how we construct our Selves, through how we use past experiences to create patterns and anticipate, we are actually preparing our behavior. The existence and functions of the Default Mode network support the idea that a human brain is indeed a predictive machine, continuously producing an abundance of probabilistic models to explain and, in a sense, “create” both the self and the world, the good news, the prediction can be altered depending on the input and the deliberate desired cognitive reframing.

Bibliography:
Andrews-Hanna, J. R. (2012). The Brain’s Default Network and its Adaptive Role in Internal Mentation. The Neuroscientist : a review journal bringing neurobiology, neurology and psychiatry, 18(3), 251–270. https://doi.org/10.1177/1073858411403316  

Bear, M. F., Connors, B. W., & Paradiso, M. A. (2016). Neuroscience: Exploring the brain (Fourth edition). Wolters Kluwer.

Cîrneci, D. (2014). Introducere în neuroştiinţe: Curs în tehnologie ID-IFR. Editura Fundaţiei “România de Mâine. Cîrneci, D., Pleș, S., & Cătău, G. (2017). A functional diagnosis of depressive disorders. Psychiatrist.Ro.

Grayson, D. S., & Fair, D. A. (2017). Development of large-scale functional networks from birth to adulthood: A guide to the neuroimaging literature. NeuroImage, 160, 15–31. https://doi.org/10.1016/j.neuroimage.2017.01.079  

Kondo, H., Saleem, K. S., & Price, J. L. (2005). Differential connections of the perirhinal and parahippocampal cortex with the orbital and medial prefrontal networks in macaque monkeys. Journal of Comparative Neurology, 493(4), 479–509. https://doi.org/10.1002/cne.20796

Lu, H. & Collab. (2012). Rat brains also have a default mode network. Proceedings of the National Academy of Sciences of the United States of America, 109(10), 3979–3984. https://doi.org/10.1073/pnas.1200506109  

Molnar-Szakacs, I., & Uddin, L. Q. (2013). Self-processing and the default mode network: Interactions with the mirror neuron system. Frontiers in Human Neuroscience, 7. https://doi.org/10.3389/fnhum.2013.00571  

Pascual-leone & Collab. (2011). Characterizing Brain Cortical Plasticity and Network Dynamics Across the Age-Span in Health and Disease with TMS-EEG and TMS-fMRI. Brain Topography, 24(3–4), 302–315. http://dx.doi.org.am.e-nformation.ro/10.1007/s10548-011-0196-8  

Raichle, M. E. & Collab., (2001). A default mode of brain function. Proceedings of the National Academy of Sciences of the United States of America, 98(2), 676–682.

Reniers, R. L. E. P. & Collab., (2012). Moral decision-making, ToM, empathy and the default mode network. Biological Psychology, 90(3), 202–210. https://doi.org/10.1016/j.biopsycho.2012.03.009  

Sato, J. R. & Collab. (2014). Age effects on the default mode and control networks in typically developing children. Journal of Psychiatric Research, 58, 89–95. https://doi.org/10.1016/j.jpsychires.2014.07.004   van Oort, J. & Collab., (2017). How the brain connects in response to acute stress: A review at the human brain systems level. Neuroscience & Biobehavioral Reviews, 83. https://doi.org/10.1016/j.neubiorev.2017.10.015