This paper introduces the use of magnetic field tomography (MFT), a noninvasive technique based on distributed source analysis of magnetoencephalography data, which makes possible the three-dimensional reconstruction of dynamic brain activity in humans. MFE has a temporal resolution better than 1 msec and a spatial accuracy of 2-5 mm at the cortical level, which deteriorates to 1-3 cm at depths of 6 cm or more. MFT is used here to visualize the origin of a spatiotemporally organized pattern of coherent 40-Hz electrical activity. This coherence, initially observed during auditory input, was proposed to be generated by recurrent corticothalamic oscillation. In support of this hypothesis, we illustrate well-defined 40-Hz coherence between corticalsubcortical sites with a time shift that is consistent with thalamocortical conduction times. Studies on Alzheimer patients indicate that, while a similar activity pattern is present, the cortical component is reduced in these subjects.In the past decade significant advances have been made in noninvasive technology capable of imaging brain activity with sufficient spatial resolution to complement the structural imaging analysis offered by magnetic resonance imaging (MRI) (1) and computerized tomography (2). However, all imaging techniques available to date fall short of the optimal temporal resolution.The utilization of electrical or magnetic signals has been known to afford the necessary temporal resolution but, until very recently, magnetoencephalography (MEG) analysis was based solely on the assumption of a single point source, the current dipole (3-7), which is only valid when the underlying activity is highly localized and, in general, cannot support imaging capabilities. The development of inverse problem algorithms with primary sources specified by continuous current densities confined to a well-defined region referred to as the source space (8-10) removes the limitations of point source models in the spatial domain without imposing restrictions on the temporal resolution. In earlier work, some of us used a two-dimensional surface as the source space (8,9). In this paper, we solve the inverse problem by using a three-dimensional source space (a cylinder). This threedimensional solution allows the generation of a set of twodimensional images that provide a sequence of slices through the source space. Each image shows the square of the magnitude of the current density at points in the appropriate slice, represented by a color map ranging from black (low activity) to yellow (high activity). By analogy with computerized tomography and positron emission tomography (11), we call this technique magnetic field tomography (MFT). The basic methodology utilized here has been extensively tested and the results have shown excellent reproducibility with computer-generated data (refs. 8 and 9; see Fig. 1C). The two-dimensional paradigm was initially tested on spontaneous and evoked activity in normal subjects and in patients (8,12).We describe here, as an example of the ap...
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A general method for obtaining continuous solutions to the biomagnetic inverse problem is outlined and illustrated with a wide range of test cases, in a variety of experimental geometries. Magnetic sources are discussed briefly, but the main emphasis is on ionic flows, both in free space and in a homogeneous conducting sphere. We describe a way of obtaining depth information from measurements taken in a single plane and show how instrumental noise affects the quality of our reconstructions. An iterative scheme is introduced, capable of pinpointing a number of localised sources with a minimum of prior assumptions. However, our method is most naturally adapted to distributed sources. A number of inversions of distributed sources demonstrate that the method is poweiful, accurate and convenient.
This paper describes a protocol designed to help students taking their first undergraduate physics course in acquiring the basic skills of physics problem-solving. The educational effectiveness of this protocol for mature students in distance education has been extensively evaluated, and it is now being used as the basis for an interactive multimedia learning package.'I ll put a girdle round the Earth in forty minutes. '
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