Simultaneous measurements of somatosensory evoked AC and near-DC MEG signals

Biomed Tech (Berl). 2011 Apr;56(2):91-7. doi: 10.1515/BMT.2011.007. Epub 2011 Mar 30.

Abstract

Magnetoencephalography measurements of somatosensory evoked brain activity taken inside an extremely magnetically shielded room are reported. The massive low frequency shielding in combination with a high sampling rate enabled the simultaneous observation of AC and near-DC effects. Neuronal activation was achieved by repetitive electrostimulation of the right median nerve above motor threshold using repetition rates from 3 Hz to 12 Hz. Stimulation sequences lasted for 10 s and were interspersed with periods of rest of equal length. The recorded magnetic fields, inferred for the N20m and a sustained near-DC component, revealed mainly dipolar patterns with mutually rotated orientations with angles of rotation of 30° and 75°. At the start of the stimulation we observed a fast rise within 100 ms in the evoked magnetic near-DC fields for which a maximum equivalent current dipole strength of 65 nAm was obtained. The sustained fields decayed by a factor of ∼4 to a lower DC-level B(γ) with a time constant τ of order of seconds. For 12 Hz repetition rate B(γ) was decreased. We suggest that the sustained neuronal activity evoked by repetitive electrostimulation could provide a suitable scheme to realize the direct detection of DC effects of neuronal currents via low field magnetic resonance.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Adult
  • Algorithms*
  • Brain / physiology*
  • Diagnosis, Computer-Assisted / methods*
  • Evoked Potentials, Somatosensory / physiology*
  • Female
  • Humans
  • Magnetoencephalography / methods*
  • Male
  • Reproducibility of Results
  • Sensitivity and Specificity