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...
Analyses of intrinsic fMRI BOLD signal fluctuations reliably reveal correlated and anticorrelated functional networks in the brain. Since the BOLD signal is an indirect measure of neuronal activity, and anticorrelations can be introduced by preprocessing steps such as global signal regression (GSR), the neurophysiological significance of correlated and anticorrelated BOLD fluctuations is a source of debate. Here, we address this question by examining the correspondence between the spatial organization of correlated BOLD fluctuations and correlated fluctuations in electrophysiological high gamma power (HGP) signals recorded directly from the cortical surface of 5 patients. We demonstrate that both positive and negative BOLD correlations have neurophysiological correlates reflected in fluctuations of spontaneous neuronal activity. Although applying GSR to BOLD signals results in some BOLD anticorrelations that are not apparent in the ECoG data, it enhances the neuronal-hemodynamic correspondence overall. Together, these findings provide support for the neurophysiological fidelity of BOLD correlations and anticorrelations.
A variety of clinical and experimental findings suggest that parkinsonian resting tremor results from the involuntary activation of a central mechanism normally used for the production of rapid voluntary alternating movements. However, such central motor loop oscillations have never been directly demonstrated in parkinsonian patients. Using magnetoencephalography, we recorded synchronized and tremor-related neuromagnetic activity over wide areas of the frontal and parietal cortex. The spatial and temporal organization of this activity was studied in seven patients suffering from early-stage idiopathic Parkinson's disease (PD). Single equivalent current dipole (ECD) analysis and fully three-dimensional distributed source solutions (magnetic field tomography, MFT) were used in this analysis. ECD and MFT solutions were superimposed on high-resolution MRI. The findings indicate that 3 to 6 Hz tremor in PD is accompanied by rhythmic subsequent electrical activation at the diencephalic level and in lateral premotor, somatomotor, and somatosensory cortex. Tremor-evoked magnetic activity can be attributed to source generators that were previously described for voluntary movements. The interference of such slow central motor loop oscillations with voluntary motor activity may therefore constitute a pathophysiologic link between tremor and bradykinesia in PD.
iELVis promises to speed the progress and enhance the robustness of intracranial electrode research. The software and extensive tutorial materials are freely available as part of the EpiSurg software project: https://github.com/episurg/episurg.
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