Objective: Some basic characteristics of magnetoencephalographic (MEG) and electroencephalographic (EEG) signals were studied by comparing somatic evoked fields (SEFs) and potentials (SEPs) elicited by electrical stimulations of different areas of the snout in piglets.
Methods: SEFs were measured with and without an intact skull, whereas SEPs were measured on the skull and cortex (Electrocorticograms - ECoG) and within the cortex of the same animal.
Results: The SEFs above the skull and dura were very similar to each other in temporal waveform and spatial topography, indicating small effects of the skull. They both revealed very similar somatotopic projections of the snout. The SEPs on the skull and cortex were, in contrast, clearly different in their amplitudes as well as temporal and spatial morphologies, indicating significant effects of the skull. However, an early component of the SEP on the skull revealed a somatotopic representation of the snout, indicating that EEG can be also useful for inferring cortical projection areas. Discrepancies in their maps were due to predominance of the potentials produced by currents in the gyral cortex. The projection sites inferred from SEFs were quite accurate in comparison to those inferred from ECoGs and intracortical SEPs.
Conclusion: The similarities and differences clearly point out the complementary nature of MEG and EEG.