In order to evaluate the topography of high-frequency oscillations (HFOs>300 Hz) superimposed on the early cortical responses, we recorded somatosensory evoked potentials (SEPs) and magnetic fields (SEFs) following median nerve stimulation. Using a digital band-pass filter (300–1000 Hz), HFOs were extracted from the original wide-band recordings. In SEP study for 12 healthy subjects, HFOs (five to eight peaks, maximal at 600–800 Hz) were discriminated from the preceding P14 far field in all cases, and their phases were reversed between the frontal and contralateral parietal regions. In subjects with a high amplitude P22 potential in original recordings, a single HFO with a maximum at the central region was present, showing no phase reversal over the centro-parietal scalp. In SEF data for 14 normal subjects, we found that the orientations of the current sources for the HFOs and underlying N20m are different and that the HFO source orientations show a more divergent pattern. HFOs are superimposed not only on the tangential N20–P20 but also on the radial P22 (area 1) potential. We suggest that the somatosensory HFOs are possibly generated from a non-pyramidal neuron population while the underlying N20m is generated from the pyramidal neuron population in area 3b. © 2002 Elsevier Science B.V.
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