A default mode of brain function

A baseline or control state is fundamental to the understanding of most com plex systems. Defining a baseline state in the human brain, arguably our mo st complex system, poses a particular challenge, Many suspect that left unc onstrained, its activity will vary unpredictably. Despite this prediction w e 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 o f oxygen used by the brain to oxygen delivered by flowing blood and is rema rkably uniform in the awake but resting state (e.g., lying quietly with eye s closed). Local deviations in the OEF represent the physiological basis of signals of changes in neuronal activity obtained with functional MRI durin g a wide variety of human behaviors. We used quantitative metabolic and cir culatory measurements from positron-emission tomography to obtain the OEF r egionally throughout the brain. Areas of activation were conspicuous by the ir absence. All significant deviations from the mean hemisphere OEF were in creases, signifying deactivations, and resided almost exclusively in the vi sual 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 p ositron-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.