Background: We investigated the characteristics and correlates of high-frequency oscillations (HFOs) of somatosensory evoked potentials (SEPs).
Methods: Subjects were 26 healthy volunteers. SEP was recorded from the hand sensory area contralateral to the median nerve stimulated at the wrist. HFOs were obtained by digitally filtering raw SEPs from 500 to 1000 Hz, and their amplitudes and area-under-curve, duration, and number of negative peaks were measured. We also measured amplitudes of the N20 onset-peak (N20o-p), and N20 peak-P25 peak (N20p-P25p).
Results: In normal subjects, several oscillation potentials were observed at the latency of 0 to 10 ms after the onset of N20. The mean number of negative peaks of total HFOs was 6.96 +/- 1.20 (early phase 3.36 +/- 0.62; late phase 3.60 +/- 1.14). The mean maximal amplitude of total HFOs was 0.16 +/- 0.07 microV (early phase 0.14 +/- 0.05 microV; late phase 0.15 +/- 0.07 microV). The mean duration of total HFOs was 10.19 +/- 1.98 ms (early phase 4.89 +/- 1.04 ms; late phase 5.31 +/- 1.95 ms). The mean area of total HFOs was 567.54 +/- 227.86 microV x ms (early phase 268.46 +/- 98.40 microV x ms; late phase 299.08 +/- 183.44 microV x ms). The amplitude ratio was 7.30 +/- 3.32% of HFOearly/N20o-p, 3.19 +/- 1.55% of HFOlate/N20-P25, and 3.54 +/- 1.84% of HFOtotal/N20p-P25. During the test, the amplitude of HFO was significantly reduced by drowsiness.
Conclusions: In waking subjects, a burst of low-amplitude HFO can be extracted from the N20.