Cortical Auditory Evoked Magnetic Fields: Mapping of Time and Frequency Domain Aspects

  • M. Hoke
  • K. Lehnertz
  • B. Lütkenhöner
  • C. Pantev
Conference paper

Abstract

Less than a decade has passed since the auditory evoked magnetic field was first described by Reite et al. (1978). Since then, not only auditory evoked magnetic fields, but also magnetic fields evoked by stimuli of different sensory modalities as well as endogenous and motor fields have been studied, not to mention spontaneous brain magnetic activity as recorded in the magnetoencephalogram (MEG), the magnetic complement of the electroencephalogram (EEG). Before dealing with brain magnetic activity, one important question to be posed is whether, why and to what extent mapping of brain magnetic fields is superior to mapping of brain electric potentials. Basically, biomagnetic fields and bioelectric potentials have the same source, are generated by the same (populations of) current dipoles and thus reflect the same physiological processes. The decisive difference is that the electric potential distribution over the surface of the scalp is brought about by volume currents flowing in the conductive medium of the head from the source to the sink of the individual dipole generators, whereas the radial magnetic field component measurable outside the head is essentially generated by the intracellular (“impressed”) current flowing within the dipole. At least for spherical bodies — and the head is a rough approximation of a sphere — volume currents do not contribute to the radial magnetic field component outside the skull, nor do secondary sources originating at boundaries between media of different conductivity.

Keywords

Entropy Coherence Nism 

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Copyright information

© Springer-Verlag Berlin Heidelberg 1989

Authors and Affiliations

  • M. Hoke
  • K. Lehnertz
  • B. Lütkenhöner
  • C. Pantev
    • 1
  1. 1.Institut für Experimentelle AudiologieWestfälischen Wilhelms-Universität MünsterMünsterFederal Republic of Germany

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