Doctor Marcus Raichle
Lecture Abstract
A Default State of the Brain
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 utilized 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 magnetic resonance imaging (fMRI) during a wide variety of human behaviors. We employed quantitative metabolic and circulatory measurements using positron emission tomography (PET) 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 area in this manner attached new meaning to a group of areas that consistently exhibit decreases from this baseline during a wide variety of goal directed behaviors monitored with PET and fMRI. These decreases suggest the existence of an organized, baseline default mode of brain function that is suspended during specific goal-directed behaviors.
The neural correlates of consciousness: An analysis of cognitive skill learning
A default mode of brain function
Emotion-induced changes in human medial prefontal cortex I
Emotion-induced changes in human medial prefontal cortex II
