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 th e afferent neural volley when entering the brainstem and leaving the t halamus, were found only in the SEP. The analysis of SEF and SEP revea led a minimum of four overlapping source activities in the region of t he contralateral post-and precentral cortical projection areas. Two so urces in the depth of the central sulcus could not be resolved unambig uously. The third, more superficial source, which probably reflects ac tivation of area 1, was better defined in the source analysis of the S EP, because dipole orientation was close to radial. The fourth source was more posterior. Its initial activity around 30 ms was seen consist ently in SEP and SEF. Several problems observed in the analysis of the present MEG and EEG data suggest that the simultaneous measurement an d analysis of multichannel EEG and MEG data will substantially increas e spatio-temporal resolution.