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 goaldirected behaviors. F unctional brain imaging studies in normal human subjects with positron-emission tomography (PET) and functional MRI (fMRI) have consistently revealed expected task-induced increases in regional brain activity during goal-directed behaviors (for brief reviews see refs. 1 and 2). These changes are detected when comparisons are made between a task state, designed to place demands on the brain, and a control state, with a set of demands that are uniquely different from those of the task state.Researchers have also frequently encountered task-induced decreases in regional brain activity even when the control state consists of lying quietly with eyes closed or passively viewing a stimulus. Whereas cortical increases in activity have been shown to be task specific and, therefore, vary in location depending on task demands, many decreases ( Fig. 1) appear to be largely task independent, varying little in their location across a wide range of tasks (3). This consistency with which certain areas of the brain participate in these decreases made us wonder whether there might be an organized mode of brain function that is present as a baseline or default state and is suspended during specific goal-directed behaviors.The primary issue this paper will address is whether these unexplained decreases merely arise from unrecognized increases (i.e., activation in the jargon of functional brain imaging) present only in the ''control state.'' Thus, on this argument, any control state, no matter how carefully it...

Practice of a novel task leads to improved performance. The brain mechanisms associated with practice-induced improvement in performance are largely unknown. To address this question we have examined the functional anatomy of the human brain with positron emission tomography (PET) during the naive and practiced performance of a simple verbal response selection task (saying an appropriate verb for a visually presented noun). As a control state, subjects were asked to repeat the visually presented nouns. Areas of the brain most active during naive performance (anterior cingulate, left prefrontal and left posterior temporal cortices, and the right cerebellar hemisphere), compared to repeating the visually presented nouns, were all significantly less active during practiced performance. These changes were accompanied by changes in the opposite direction in sylvian-insular cortex bilaterally and left medial extrastriate cortex. In effect, brief practice made the cortical circuitry used for verbal response selection indistinguishable from simple word repetition. Introduction of a novel list of words reversed the learning-related effects. These results indicate that two distinct circuits can be used for verbal response selection and normal subjects can change the brain circuits used during task performance following less than 15 min of practice. One critical factor in determining the circuitry used appears to be the degree to which a task is learned or automatic.

Patients with blepharospasm have abnormal sensorimotor processing in response to lower face vibration. They may also have abnormal brain responses to stimulation of clinically uninvolved parts of the body, but this requires confirmation.

Summary: To further our understanding of the best way to measure regional CBF with positron emission tomog raphy (PET), we directly compared two candidate tracers W50]water and e IC]butanol, administered intrave nously) and two popular implementations of the one compartment (IC) model: the autoradiographic imple mentation representing a single PET measurement of tissue radioactivity over I min and a dynamic implemen tation representing a sequence of measurements of tissue radioactivity over 200 s. We also examined the feasibility of implementing a more realistic, and thus more complex, distributed-parameter (DP) model by assigning fixed val ues for all of its parameters other than CBF and tracer volume of distribution (V d)' a requirement imposed by the low temporal resolution and statistical quality of PET data. The studies were performed in three normal adult human subjects during paired rest and visual stimulation. In each subject seven regions of interest (ROIs) were seVarious methods for measuring regional CBF (rCBF) with positron emission tomography (PET) have been proposed, both for functional brain map ping and for determination of cerebral metabolic supply-demand relationships. Many of these have employed e 5 0]water as the tracer, primarily due to the short half-life of 1 5 0, which allows sequential studies and exposes the subject to a relatively low radiation dose. Although the methods of tracer ad ministration vary , including continuous inhalation of e 5 0]carbon dioxide (Frackowiak et aI. , 1980; Brooks et aI. , 1986; Lammertsma et aI. , 1989), sin gle-breath inhalation of e 5 0]carbon dioxide (Kanno et aI. , 1984), ramp intravenous infusion of [1 5 0]wa_ ter (Ginsberg et aI. , 1982), and intravenous bolus injection of e 5 0]water (Herscovitch et aI. , 1983; Huang et aI. , 1983 ; Raichle et aI. , 1983), most anal yses of the resultant data are based on a one-