Comparison of MEG and EEG on the basis of somatic evoked responses elicited by stimulation of the snout in the juvenile swine

Objective: Some basic characteristics of magnetoencephalographic (MEG) and electroencephalographic (EEG) signals were studied by comparing somatic evo ked 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 with in 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 th e skull. They both revealed very similar somatotopic projections of the sno ut. The SEPs on the skull and cortex were, in contrast, clearly different i n their amplitudes as well as temporal and spatial morphologies, indicating significant effects of the skull. However, an early component of the SEP o n the skull revealed a somatotopic representation of the snout, indicating that EEG can be also useful for inferring cortical projection areas. Discre pancies in their maps were due to predominance of the potentials produced b y currents in the gyral cortex. The projection sites inferred from SEFs wer e quite accurate in comparison to those inferred from ECoGs and intracortic al SEPs. Conclusion: The similarities and differences clearly point out the compleme ntary nature of MEG and EEG. (C) 1999 Elsevier Science Ireland Ltd. All rig hts reserved.