Magnetic Fields from the Human Auditory Cortex

  • K. Saermark
  • J. Lebech
  • C. K. Bak
Part of the Springer Series in Brain Dynamics book series (SSBD, volume 1)


Since the first observation by Reite et al. (1978) of auditory evoked magnetic fields (AEF) from the human cortex, several research groups have been actively engaged in such studies (Reite et al. 1978; Elberling et al. 1981, 1982a, b; Hari et al. 1980, 1982; Arlinger et al. 1982; Romani et al. 1982; Zimmerman et al. 1983; Tuomisto et al. 1983; Pellizone et al. 1985; Bak et al. 1985). It appears well established by now that the experimentally measured AEF perpendicular to the skull surface can be accounted for in terms of a model consisting of an equivalent current dipole (ECD) embedded in a volume conductor (for ECD, see for example Williamson and Kaufman 1981), with a source strength, the dipole moment measured in units of amperes times meters, determined by the intracellular axial currents in the neural tissue in question (Plonsey 1981). However, for a dipole oriented in an arbitrary direction, only the tangential component generates the field normal to the skull. The measured magnetic fields are, of course, not caused by a single active neuron, but rather to an array of neurons, which may be modelled as a distribution of elementary current dipoles. If these can be regarded as independent, noninteracting units, excited synchronously, the field may be calculated as if its origin were that of a single equivalent current dipole. The dipole moment, P (t), will in general be time dependent and the magnetic field is given by \(B(\overline r ,t) = P(t)G(\overline r ,\overline {{r_s}} )\), where G is a geometrical factor that depends on the position of the point of measurement \(\overline r\), the position of the source \(\overline {{r_s}}\), and on instrument properties, but not on the time t. It follows that spatial mapping studies allow for a determination of both the source location and the source strength P; we note that it also follows that the latency τ, for an ECDE model, is independent of the position of the point of measurement.


Source Strength Tone Burst Measured Magnetic Field Sylvian Fissure Current Dipole 
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Copyright information

© Springer-Verlag Berlin Heidelberg 1988

Authors and Affiliations

  • K. Saermark
  • J. Lebech
  • C. K. Bak

There are no affiliations available

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