Somatosensory evoked potentials and magnetic fields: separation of multiple source activities

Median nerve somatosensory evoked potentials (SEP) and magnetic fields (SEF) were recorded in two subjects with multichannel (32 SEP, 24 SEF channels) devices in Aachen and Helsinki. Single-moving- and multiple-stationary-dipole models were compared with the brain electric source analysis (BESA) program of Scherg. Subcortical sources, reflecting the afferent neural volley when entering the brainstem and leaving the thalamus, were found only in the SEP. The analysis of SEF and SEP revealed a minimum of four overlapping source activities in the region of the contralateral post-and precentral cortical projection areas. Two sources in the depth of the central sulcus could not be resolved unambiguously. The third, more superficial source, which probably reflects activation of area 1, was better defined in the source analysis of the SEP, because dipole orientation was close to radial. The fourth source was more posterior. Its initial activity around 30 ms was seen consistently in SEP and SEF. Several problems observed in the analysis of the present MEG and EEG data suggest that the simultaneous measurement and analysis of multichannel EEG and MEG data will substantially increase spatio-temporal resolution.