Neuroscientists using so-called functional magnetic resonance imaging (fMRI) or positron emission tomography (PET) are frequently confronted with one uneasy question: What did neuroimaging teach us what we did not already know before the invention and use of this technology?

The default mode network: What is the default function of the human brain?

The default mode network: What is the default function of the human brain? | ©Alena Hovorkova @shutterstock.com

Although there might be no simple answer to this question, one may mention intriguing brain network that emerged about 15 years ago from neuroimaging research as a textbook example of a serendipity effect: the discovery of the “default mode network”. Not less of an unanticipated accident as the discovery of Penicillin antibiotics, brain-imaging neuroscientists identified a set of brain regions that do actually systematically decrease in brain activity during a broad set of different mental activities.

“The default mode network are the metabolically most active brain regions that underlie everyday thinking“

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Two of the regions that make up the default mode network are even the biggest consumers of metabolic energy in the human brain (namely, the medial prefrontal cortex and the posteromedial cortex). This begs the question about the nature and anthropological meaning of the neural processes that are subserved when the human mind is currently not solving a defined goal-directed task – during mind wandering. Some investigators believe that the default mode network probably subserves an evolutionarily adaptive function, such as the continuous prediction of possible upcoming events in the immediate environment based on knowledge from past events. This has been put into question by the observation that monkey and rats also appear to have a default mode network and, thus perhaps, a default mode of brain function. Thousands of papers have since been published on this mysterious brain network. A large number of psychiatric and neurological disorders has furthermore been repeatedly and convincingly associated with the default mode of human mental life. It is hardly neurobiologically conceivable how the exact same set of brain regions can be involved in the genesis of a contrasting array of brain disorders.

Conclusion

In this way, the default mode network challenges common neuroscientific intuitions and the conventionally employed methodological arsenal of neuroscientists. To unveil some of its fascinating riddles, I believe that we need to combine efforts from investigators in diverse disciplines and come up with novel research methods to more elegantly access this neurobiological phenomenon.

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