Dipole orientation differs between high frequency oscillations and N20m current sources in human somatosensory evoked magnetic fields to median nerve stimulation.

The dipole orientation of equivalent current source for the high frequency oscillations (HFOs) above 300 Hz and that for the underlying N20m were compared. Somatic magnetic fields were recorded over the left hand somatosensory area to right median nerve stimulation at the wrist with a wide-bandpass (0.1-2000 Hz). The HFOs and underlying N20m were extracted by digital filtering of 300-900 Hz and 1-300 Hz, respectively. We found that the orientation of the HFOs and underlying N20m current sources differs and that the HFO source orientation shows a more divergent pattern than the N20m. These results suggest that the somatosensory HFOs are not generated from the pyramidal cell population in area 3b which produces the underlying N20m and that they may reflect activities of the non-pyramidal neuron population.