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"Resting state networks" are defined as the brain areas that exhibit relatively strong activities in the absence of any external stimuli. They comprise over 40 networks (Littow et al., 2015), but only four of them are frequently mentioned: default mode network, central executive network, salience network and cerebellum network.
Default mode network
The default mode network includes three main areas (Figures 5-1 and 5-2): medial prefrontal cortex (mPFC), posterior cingulate cortex (PCC)/precuneus, and temporo-parietal junction, which show more activity at rest than during attention-demanding tasks (Vanhaudenhuyse et al, 2010). That means, without external stimuli, the network sets certain activity "by default". The external stimuli will reduce the default activity.
Why such a design? A possible reason is that the activities of resting state networks must reach a certain level to produce clear consciousness. Without external stimuli, the activities of other resting state networks are low. If the default mode network activity is not strong enough, it would be insufficient to produce clear consciousness. On the other hand, if the overall activities of resting state networks are too strong, it may cause mental illness (Chapter 14). Thus, the default mode network activity should be reduced when the external stimuli enhance other resting state network activities.
Other resting state networks
The central executive network, also called fronto-parietal network, includes dorsolateral prefrontal cortex (dlPFC) and posterior parietal cortex (PPC). This network has been found to be associated with intelligence. Individuals with higher IQ have stronger functional connectivity in the network (Li and Tian, 2014).
The salience network consists of anterior insula and anterior cingulate cortex (ACC), which are responsible for the amplification of salient features in sensory inputs. The cerebellum network is involved in movement.
Resting state networks and consciousness
Brain areas in resting state networks are widely distributed. Most of them do not have direct anatomical connections. However, they may be functionally connected in the form of synchronous oscillations. More synchrony indicates stronger "functional connectivity", which can be quantified by fMRI (Hsu et al., 2016). It has been demonstrated that consciousness is directly related to the functional connectivity of resting state networks.
"Consciousness" is not all or none. It has varying degrees. For example, during light sleep, you still retain a significant amount of consciousness, capable of hearing some voices, albeit only vaguely. In deep sleep, you would lose most consciousness, very hard to wake up. Brain damages can also cause varying degrees of coma. A growing number of studies have confirmed that consciousness depends on the functional connectivity of resting state networks (Figure 5-3). Whenever the connectivity is significantly disrupted, whether it arises from natural sleep, general anesthesia or brain damages, consciousness will be impaired (Vanhaudenhuyse et al., 2010; Heine et al., 2012; Lee et al., 2013; Picchioni et al., 2014; Wu et al., 2015).