Multiplicity in the high-frequency signals during the short-latency somatosensory evoked cortical activity in humans

OBJECTIVE Recent studies using electroencephalography or magnetoencephalography have shown that peripheral nerve stimulations produce short-latency high-frequency signals in the human somatosensory cortex. The present study tested whether they consist of more than one distinct type of signal. METHODS Somatic evoked magnetic fields (SEFs) elicited by electrical stimulation of the median nerve were measured in 12 healthy volunteers. They were analyzed using a time-frequency analysis method based on Gabor filters and another based on autoregressive moving average, and also with bispectrum and bicoherence techniques and a new dispersion curve method. RESULTS Signals in two separate high-frequency bands (200 and 600 Hz) were distinguished from the main signal in the low frequency (LF) range during the time period of N20m and P25m. The novel 200 Hz-band signal was seen reliably in those channels where the LF band signal was weak, so that the former was not masked by the latter. The 600 Hz signal consisted of two distinct components or parts (p1 and p2) in 10 out of 12 subjects, one peaking during ascending slope and the second during the descending slope of the N20m. The latency of the p1 was shorter than the latencies of the 200 Hz and LF signals according to the dispersion curve analysis. The inter-peak interval of p1 became shorter for later peaks in all 12 subjects. Bicoherence analysis revealed a significant phase coupling between the 200 and 600 Hz bands. CONCLUSIONS There are three distinct types of signal during the time period of the short-latency cortical components of the SEF -- LF which gives rise to the commonly seen waveform of the SEF, the newly found 200 Hz signal and the 600 Hz signal which consists of two components. The possible origins of the high frequency signals are discussed in light of the new set of evidence found in the present study.