Default mode network

The default mode network (DMN), is a group of specific brain regions that are functionally-connected. The regions become active in the resting state (not doing any active task), and inactive when someone is engaged in any attention-demanding tasks ¹; this phenomenon has been termed task-induced deactivation, 

Function

The default mode network demonstrates less activity when a person is focused on an active task, but high activity when a person is not doing any active task. It is thought that these brain regions become active when people experience a state of daydreaming or are merely lost in their own thoughts ¹. At present, DMN is widely recognized for its function in internally directed thoughts ².

This emphasizes the idea that the brain is not idle when a person is not engaged in a specific task (i.e. restful awakeness), but there are some areas that are active. DMN regions are highly active during wakeful rest and wandering thoughts. Many cerebral activities can occur during free-thinking, e.g. visualizing the future, thoughts about oneself, processing of emotions, and remembering the past.

Default mode network regions

• precuneus/posterior cingulate cortex (PCu/PCC)
• hippocampus
• medial prefrontal cortex (mPFC)
• anterior cingulate cortex (ACC)
• some other regions are also mentioned in the literature

Radiographic features

The default mode network was first accurately studied with PET functional scans, and later fMRI was also widely employed. 

Nuclear medicine

PET

In the early PET studies, it was possible for some scientists to observe a set of brain regions that were less active during active tasks and more active during rest periods (control conditions).

In a meta-analysis ³, Shulman et al. investigated the results of several PET experiments for various visual and language-related tasks. Those previously collected data were not originally acquired for studying the DMN. 

This meta-analysis showed a persistent decrease in blood flow in some areas of the brain (such as the posterior cingulate cortex) during active conditions. They explained their findings of the increased activity in DMN during rest as “an exploratory state” during which the volunteers monitor their immediate environment. 

MRI

fMRI

Also, this network has been highly studied in fMRI experiments ² because MRI is considered a safer technique than PET due to the lack of ionizing radiation exposure. 

fMRI experiments demonstrate similar observations to PET, and are characterized by a decrease in BOLD signal within the set of brain regions during active (attention-demanding) tasks, and conversely an increase in the BOLD signal during rest conditions ^1,6. 

History and etymology

The discovery of the DMN in the brain was accidental and unexpected. It built on earlier work from the 1950s when it was first noted on cerebral blood flow studies before either PET or MRI had been developed, that the brain remains active during restful waking states ⁴.

In early work, PET researchers had noticed activation in some brain regions during rest blocks when a participant was not engaged in any purposeful activity. 

In fact, at that time, neither the term “DMN” nor its concept existed, even though the presence of scientific evidence from different techniques strongly implied that the brain is active during rest ^4,5.

There was no specific recognized name for this set of brain regions until Raichle et al. (2001) coined the term default mode because these regions become active when people are not engaged in any explicit task, and can be considered as a baseline or a default of the brain. 

The term network is used because these regions are functionally related and can be clearly discriminated from other brain networks.