The default-mode network
It seems evident that the default-mode network is anatomically and functionally distinct from networks involved in sensory- and executive- functions.
Another robust finding in neuroimaging is the presence of low-frequency fluctuations of spontaneous brain activity organized in specific and distinct spatial patterns, or sets of resting-state networks.
The neuroanatomy of the default mode network is not entirely set, but studies have repeatedly linked some major regions to the network. Including the posterior cingulate-, medial frontal-, superior frontal- cortex, and the Angular gyrus.
These regions are almost never activated during sensory, motor or cognitive tasks.
The posterior cingulate cortex seems to play an important integrative role. While it is functionally connected to all other default-mode network regions, it projects to and receives input from the anterior cingulate-, prefrontal-, lateral parietal-, and parahippocampal- cortex, all regions not associated with the default-mode-network.
Other studies also linked the hippocampal formation and the Thalamus, but these are both regions which are utilized during a wide range of sensory-, motor- and cognitive- tasks.
In sum, the multi-synaptic structure of the default mode network suggests that it draws multiple, specialized processing system that integrate their information via the posterior cingulate cortex.
What does it do?
The internal Mentation hypothesis
According to the “internal mentation hypothesis”, default network activity triggers spontaneous introspective processing, also known as “mind-wandering”.
Some studies have even shown a correlation between the activation of the posterior cingulate- and medial prefrontal- cortex and the frequency of spontaneous thought.
It was repeatedly demonstrated that default network activity increases prior to errors during sustained attention tasks, and can be manipulated by meditation practices.
Patients with damage to portions of the network, particularly the medial prefrontal cortex, have reported a sensation of “mental emptiness”.
One of the first functions which have been attributed to the default mode network was the utilization of memory wherein the hippocampal formation is the most important area.
In sum,
the internal mentation hypothesis proposes that the default mode network allows one to shift from perceiving the immediate environment to an alternative, imagined perspective that is largely based on memories of the past.
The Sentinel Hypothesis
The sentinel hypothesis proposes that the default mode network maintains a low-level focus of attention when one monitors the environment for unexpected events. A relative lack of external stimulation is then thought to drive us towards internally generated information or representations.
The default mode network and conscious experience
Many influential theories have proposed that consciousness is highly linked to the integration of information through thalamo-cortical and cortico-cortical loops. (I would highly appreciate if you also check out my earlier post on: Why humans think?!)
The posterior cingulate cortex seems to be crucial for conscious thought, as seen by the lack of consciousness in patients with lowered or no activation in the PCC.
In sum,
the default mode network may provide integrated internal representations of the outside world and of our organism. These representations can be regarded as the central function of conscious experience, offering important advantages for survival such as planning, decision making, and behavioral control in response to external demands.