A Default Mode of Brain Function, by Raichle et al.

Launched by Marcus Raichle in 2001 (Raichle et al. 2001, Raichle and Snyder 2007), the notion of a “default mode” of brain function has come to describe a “resting state” characterized by very slow neural oscillation (see Callard and Margulies 2011 for a history and larger significance of these notions).

The resting state is the “state” of large-scale networks that are active when the subject is awake but not focused on the external environment; their activity is therefore driven neither by tasks nor by external stimuli. Neuroimaging studies of the relationship between the default mode network and mental disorder began in the early 2000s and have shown, for example, that the network is functionally overactive in schizophrenia and hypoactive in Alzheimer’s disease (Buckner et al. 2008).

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Abstract

A baseline or control state is fundamental to the understanding of most complex systems. Defining a baseline state in the human brain, arguably our most complex system, poses a particular challenge. Many suspect that left unconstrained, its activity will vary unpredictably. Despite this prediction we identify a baseline state of the normal adult human brain in terms of the brain oxygen extraction fraction or OEF. The OEF is defined as the ratio of oxygen used by the brain to oxygen delivered by flowing blood and is remarkably uniform in the awake but resting state (e.g., lying quietly with eyes closed). Local deviations in the OEF represent the physiological basis of signals of changes in neuronal activity obtained with functional MRI during a wide variety of human behaviors. We used quantitative metabolic and circulatory measurements from positron-emission tomography to obtain the OEF regionally throughout the brain. Areas of activation were conspicuous by their absence. All significant deviations from the mean hemisphere OEF were increases, signifying deactivations, and resided almost exclusively in the visual system. Defining the baseline state of an area in this manner attaches meaning to a group of areas that consistently exhibit decreases from this baseline, during a wide variety of goal-directed behaviors monitored with positron-emission tomography and functional MRI. These decreases suggest the existence of an organized, baseline default mode of brain function that is suspended during specific goal-directed behaviors.

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