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High frequency oscillations in early cortical somatosensory evoked potentials
I Ozaki1, C Suzuki, Y Yaegashi
1Third Department of Medicine, Hirosaki University School of Medicine, Japan. isamu@cc.hirosaki-u.ac.jp
Objective:
To evaluate the characteristics of high frequency (HF) components of the early cortical somatosensory evoked potentials (SEPs).
Methods:
We recorded 8-channel SEPs from the frontal and left centro-parietal scalp after right median nerve stimulation with a wide band-pass (0.5-2000 Hz) and digitized at 40 kHz sampling rate in 12 healthy subjects. HF components were analyzed after digital band-pass filtering (300-1000 Hz). The power spectrum was obtained by a maximum entropy method.
Results:
HF oscillations (maximum power at 600-800 Hz) consisting of 5 to 8 peaks were discriminated from the preceding P14 far-field in all cases and their phases were reversed between the frontal and contralateral parietal regions. In addition, in subjects with a high amplitude central P22 potential in original wide-band recordings, a single HF oscillation with a maximum at the central region was present. Furthermore, this component showed no phase reversal over the centro-parietal area.
Conclusion:
We therefore conclude that HF oscillations are superimposed not only on the tangential N20-P20 but on the radial P22 potential, and are generated from both tangential (area 3b) and radial (area 1) current sources.