Separation of overlapping activity in first and second somatosensory evoked fields with 3-D MEG measurement

This paper describes a new method with singular value decomposition (SVD) for discriminating multiple sources overlapping in time. We carried out a three-dimensional (3-D) vector measurement of somatosensory evoked field (SEF) with the electric stimulus to the right thumb, using a 3-D second-order gradiometer connected to 39 channel SQUIDs, which can detect magnetic field components perpendicular to the scalp (Br) and tangential to the scalp (B(theta), B(phi)) simultaneously. The SVD method was applied to spatio-temporal 3-D MEG data with different frequency ranges. Time course of singular value corresponded to magnetic field waveforms. Second singular value increased at the latency of SII activity. In 15-40 Hz, it was also confirmed that a good agreement between measured magnetic fields and theoretical maps with second signal subspace. Proposed method using dynamic SVD method with different frequency ranges is useful for discriminating multiple sources overlapping in time.