title: Simultaneous measurements of somatosensory evoked AC and near-DC MEG signals.
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.
authors: Physikalisch-Technische Bundesanstalt, Berlin, Germany. rainer.koerber@ptb.de
authors: Rainer
authors: R
authors: Körber
authors: Gabriel
authors: G
authors: Curio
authors: Stefan
authors: S
authors: Hartwig
authors: Ingo
authors: I
authors: Hilschenz
authors: Nora
authors: N
authors: Höfner
authors: Hans-Jürgen
authors: HJ
authors: Scheer
authors: Lutz
authors: L
authors: Trahms
authors: Jens
authors: J
authors: Voigt
authors: Martin
authors: M
authors: Burghoff
mesh_terms: D000328:Adult; D000465:Algorithms; D001921:Brain / Q000502:physiology*; D003936:Diagnosis, Computer-Assisted / Q000379:methods*; D005073:Evoked Potentials, Somatosensory / Q000502:physiology*; D005260:Female; D006801:Humans; D015225:Magnetoencephalography / Q000379:methods*; D008297:Male; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity