This article describes the discovery of a default mode of brain function, a baseline state of brain activity that is suspended during goal-directed behaviors. The researchers used positron emission tomography (PET) to measure the oxygen extraction fraction (OEF), which is the ratio of oxygen used by the brain to oxygen delivered by flowing blood. They found that in the awake but resting state, the OEF is remarkably uniform across the brain. 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. They identified that areas of activation were conspicuous by their absence, and all significant deviations from the mean hemisphere OEF were increases, signifying deactivations, and resided almost exclusively in the visual system. These findings suggest the existence of an organized, baseline default mode of brain function that is suspended during specific goal-directed behaviors. The researchers also found that areas consistently exhibiting decreases in activity during goal-directed behaviors are those that are deactivated in the baseline state. This default mode of brain function is thought to be involved in processes such as self-referential thinking, memory retrieval, and mental simulation. The study highlights the importance of understanding the baseline state of brain function and how it is affected by various cognitive tasks. The findings have implications for understanding brain function in both healthy and diseased states.This article describes the discovery of a default mode of brain function, a baseline state of brain activity that is suspended during goal-directed behaviors. The researchers used positron emission tomography (PET) to measure the oxygen extraction fraction (OEF), which is the ratio of oxygen used by the brain to oxygen delivered by flowing blood. They found that in the awake but resting state, the OEF is remarkably uniform across the brain. 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. They identified that areas of activation were conspicuous by their absence, and all significant deviations from the mean hemisphere OEF were increases, signifying deactivations, and resided almost exclusively in the visual system. These findings suggest the existence of an organized, baseline default mode of brain function that is suspended during specific goal-directed behaviors. The researchers also found that areas consistently exhibiting decreases in activity during goal-directed behaviors are those that are deactivated in the baseline state. This default mode of brain function is thought to be involved in processes such as self-referential thinking, memory retrieval, and mental simulation. The study highlights the importance of understanding the baseline state of brain function and how it is affected by various cognitive tasks. The findings have implications for understanding brain function in both healthy and diseased states.