Abstract
We carried out measurement of somatosensory evoked fields (SEFs) by applying an electric stimulus to different right fingers (the thumb, fourth finger, and little finger), using three-dimensional second-order gradiometers connected to 39-channel SQUIDs, which can detect magnetic components perpendicular to the scalp (Br) and tangential to the scalp (Bθ, Bφ) simultaneously. A singular value decomposition method was applied to the SEF data for preprocessing. To discriminate the activities of multiple nerves (the median nerve and ulnar nerve) in the fourth finger, time-frequency analysis (wavelet transform) was applied to each set of SEF data. The dominant distribution of power spectrum to the thumb (median nerve) stimulation was different from the distribution of the little finger (ulnar nerve) stimulation (thumb: 16-18 Hz, little finger: 13-15 Hz). However, the fourth finger showed the distribution that combined the results of the thumb and little finger. Furthermore, coherence between each set of SEF data showed a high value in that frequency range. Source localization was also examined by the SEF data with different frequency ranges. We conclude that the proposed method is useful for detecting the frequency component of the SEF data obtained by stimulation of different fingers and for discriminating multiple sources of SEF data in the fourth finger.
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