The discovery of slowness--recent progress in DC-MEG research

The non-invasive electrical recording of Direct Current (DC) phenomena in the frequency range below 0.1 Hz, e.g., occurring in metabolic injuries to brain cells in stroke or migraine (anoxic depolarization, peri-infarct depolarization, spreading depression), is technically restricted due to large drift artifacts caused by electrochemical instabilities at the electrode-skin interface. This limitation could be overcome by invasive approaches only. However, as early as 1969 first magnetic fields in this frequency range have been recorded over the human torso by oscillating the subject vertically in front of a magnetic field detector using a see-saw. By this technique the DC field is conversed to a higher frequency, where the external noise level is less. In the last decade, the modulation based DC-magnetoencephalography (DC-MEG) has been methodically refined, which allowed monitoring low-amplitude magnetic fields in this frequency domain arising not only from injured tissue, but also generated by functional cortical activation. Furthermore, the combination of DC-MEG and NearInfraRed Spectroscopy (NIRS) opens up a new avenue to study cortical neurovascular coupling, as vascular and neuronal activations could be analyzed simultaneously even without averaging in a single-trial mode. Recordings inside the novel magnetically shielded room (BMSR-2 of the Physikalisch-Technische Bundesanstalt, Berlin) exhibiting an extremely low background noise level in the DC frequency range, and alleviating the need of sensor-to-source modulation, allow to resolve additionally the short-term (subsecond) dynamics of neuronal DC-processes.