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  • 1
    ISSN: 1573-6792
    Keywords: MEG ; SQUID ; Frontal midline theta rhythm (Fmθ) ; Source model ; Computer simulation
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Summary In this paper, we report on our study of frontal midline theta (Fmθ) activity in human subjects, recorded during mental processes such as arithmetic calculation. The Fmθ is a 6–7 Hz rhythmic wave with a duration of few seconds. The Fmθ activity is observed in the central region at the front of the head. EEGs and MEGs of Fmθ were measured simultaneously during mental calculation, and we analyzed these waveforms based on both topographic EEG maps and magnetic fields measurements. A single dipole simulated the EEG topography adequately, but there are many other dipole models which can generate a similar EEG pattern. It is difficult to estimate the source location of the Fmθ from the EEG topography alone because the EEG technique has a certain ambiguity associated with source estimation. Therefore, we considered the spatial relationships between the sources and the patterns of EEG and MEG that were simulated. Although it is not possible to obtain a unique solution for the source location of Fmθ from the EEG data alone, the simultaneous recording of MEGs from a large scalp area may result in an unambiguous solution. We therefore conclude that the simultaneous recording of both MEG and EEG data is more useful for accurate localization, than the EEG alone.
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 1573-6792
    Keywords: Middle latency auditory evoked magnetic fields ; MLR ; AEFs ; MEG ; Human brain ; SQUID
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Summary The magnetic equivalents of SN10, Po, Na, Pa, Nb and Pb (SN10m, Pom, Nam, Pam, Nbm and Pbm) in short and middle latency auditory evoked potentials were measured with a 7-channel DC superconducting quantum interference device (SQUID). The sources of Pom, Nam, Pam, Nbm and Pbm responses were estimated to be located in the auditory cortex, while the source of SN10m was considered to be in a deeper part of the brain. In addition, the source of Pam was estimated to be in the vicinity of the moving N100m source. The source of Pbm was considered to be in a separate area, anterior to the source of Pam and N100m, which suggested that the source of Pam was located in the primary auditory cortex, while the source of Pbm was located in the secondary auditory cortex. The source of N100m was considered to spread from the primary auditory cortex to the secondary auditory cortex.
    Type of Medium: Electronic Resource
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  • 3
    ISSN: 1573-6792
    Keywords: EEG ; MEG ; Sleep spindle ; DC-SQUID ; Sources
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Summary MEG measurements can detect brain sources that are difficult to detect with EEG measurements. The purpose of this study was to investigate models of sleep spindles using both MEG and EEG activity that had been recorded simultaneously. The components of magnetic fields perpendicular to the surface of the head were measured using a DC-SQUID with a first-derivative gradiometer. We propose three models for sleep spindles. In the first model, the source slides into the superficial region of the head so as to be perpendicular to it's surface, and with this model, the power spectrum of the MEG is decreased. In the second model, the source slides into the deeper structures, so that it is perpendicular to the surface. Here, the power spectra of both the MEG and the EEG are decreased. The third model has the source perpendicular to the surface, leaning and sliding into the deeper structures. Here, the power spectrum of the EEG is decreased but that of the MEG is not.
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    Springer
    Brain topography 3 (1990), S. 151-165 
    ISSN: 1573-6792
    Keywords: MEG ; Neuromagnetism ; Source localization ; Modeling ; K-complexes ; Delta waves
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Summary During the past decade, substantial advances in the understanding of the functional organization of the human brain have been made through the technique of MEG topographic mapping. Most of these investigations were concerned with the estimation and localization of sources which were modeled as single current dipoles positioned in a semi-infinite volume conductor with homogeneous conductivity. However, the sources in the brain are complex, and the head as a volume conductor consists of different materials with different electrical conductivities. The influence of these inhomogeneities on the MEG topography is studied by a computer simulation, modeling the sources as single or multiple dipoles located in inhomogeneous volume conductors. The computer simulation suggests some important aspects in estimation of source localization. The sources of MEG activities in human subject during sleep are also studied. A comparison of simulated MEG topographic patterns with measured data suggests that the sources of K-complexes can be modeled by two current dipoles. Sources for delta waves are analyzed by the FFT technique. The results show that the frequency distributions are different for delta waves measured by MEG and EEG techniques, leading us to conclude that at least two different sources are present. The MEG measurements have an advantage to provide important information concerning brain function which cannot be obtained using the EEG measurements.
    Type of Medium: Electronic Resource
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  • 5
    ISSN: 1573-6792
    Keywords: MEG ; SQUID ; Sleep ; K-complex ; Auditory evoked responses ; N100 components
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Summary This study focuses on source estimation of spontaneous MEG activity and auditory evoked responses during sleep. Sources of K-complexes and auditory evoked responses were investigated by magnetoencephalograph (MEG) and electroencephalograph (EEG) measurements, simultaneously. Sources of K-complexes during stage 2 sleep were investigated. The MEG results suggested that the sources of K-complexes can be modeled by two current dipoles. Dipoles for the K-complexes were estimated to be located 5 mm away from the sources of the N100 components of auditory evoked responses during wakefulness. Sources of auditory evoked responses during each sleep stage were also investigated to clarify the origins of the K-complex, the vertex sharp transient, and delta waves. Estimated dipoles for the N100 component for each sleep stage were estimated to be at slightly different locations in the auditory area. Based upon results of the MEG measurements and the EEG topographies, sources of the N330 component can be modeled by multiple current dipoles, which are seen to be distributed diffusely throughout the cerebral cortex.
    Type of Medium: Electronic Resource
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