Estimation of multiple sources using time Frequency analysis of 3-D SEF to finger stimulation

In order to discriminate multiple sources of human hand somatosensory cortex we recorded somatosensory evoked field (SEF) after stimulation of three fingers (the thumb, fourth finger, and little finger), respectively. We applied time-frequency analysis (wavelet transforms) to examine the frequency component of each SEF data. Dominant power spectrum to the thumb (median nerve) stimulation appeared at higher frequency range than that of the little finger (ulna nerve) stimulation (thumb: 15-20 Hz, little finger: 10-15 Hz). In the result of fourth finger stimulation, the frequency of dominant power spectrum existed between that of the thumb and little finger stimulation. Furthermore, the coherence method was applied to these SEF data. Source localization was done by the SEF data to the fourth finger stimulation, having different frequency ranges (13-15 Hz and 16-18 Hz band-pass filter). We conclude that the proposed method, combining time-frequency analysis with the inverse problem of magnetoencephalogram, is useful for discriminating multiple sources of the SEF elicited by two nerve activities in the fourth finger overlapping in time. Index Terms--Magnetoencephalogram, somatosensory evoked field, SQUID, time-frequency analysis.