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Mind wandering is a phenomenon that can be described as a drifting of one’s thoughts away from a current task towards internally focussed thoughts. This phenomenon is widespread, and deeply impacting on the daily lives of all modern citizens, from affecting the retention of lecture material, to increasing the likelihood of traffic fatalities and even death. Research into the characteristics of this phenomenon have largely been behavioural and psychophysical, but recent attempts to understand how this phenomenon occurs neurologically have made great progress in our understanding of mind wandering. It has become clear over the past few years that the network involved in mind wandering is actually the network that is involved in the “resting state” of the brain, the default network.

Why the Mind Wanders
Mind wandering has long been thought to be due to extra attentional capabilities present during a task, as per the Executive Control Model. This leftover attention and working memory capacity can be devoted to task unrelated thought, or mind wandering. This is supported by the fact that mind wandering occurs more frequently during simple, easy tasks than during difficult, complex tasks. There have been results, however, that show that those with larger working memories and higher intelligence will actually mind wander less, suggesting that mind wandering is likely caused by a failure of some mechanism to keep one’s attention focussed on the task at hand, suggesting there is more to the story.

The Default Network
The default network is the area of the brain that remains active when people are not currently working on a task. The result is a network that is usually statistically eliminated for activity during regular fMRI studies. However, recent research suggests that this network actually increases in Blood-Oxygen-Level-Dependent (BOLD) activity when someone is in the process of engaging in mind wandering. The areas involved in this network include the medial prefrontal cortex (mPFC), superior frontal gyrus (SFG), parts of the posterior cingulate, left angular gyrus, parts of the insula, and portions of the superior and middle temporal gyri.

Though the default network shows increased activity during mind wandering, some of the brain areas in the default network are active during other functions as well. Parts of the PFC have been shown to be active both during mind wandering, and during externally directed thought, or periods devoid of mind wandering

Related Disorders
Current hypotheses suggest that those with disorders of attention may suffer from low-frequency activation of the default network, as well as atypical connections between the respective parts of the default network. Abnormal connectivity, frequency, or activity have been shown to correlate with alzheimers, attention deficit and hyperactivity disorder, and even schizophrenia. Damage to the mPFC has been shown to cause a reported loss of internal thought in some cases. While these cases are rare, this disorder has been shown to cause a decrease in emotional stability.