Simulation von tangentialer und radialer elektrischer Gehirnaktivität: unterschiedliche Empfindlichkeit in EEG und MEG
Based on the main direction of the neuronal currents with respect to the local skull curvature, it is common to distinguish between tangential brain activity originating mainly from the walls of the sulci and radial brain activity originating mainly from the gyri or the bottom of the sulci. It is well known that MEG is more sensitive to tangential activity while EEG is sensitive to both radial and tangential activity. Thus, it is surprising that studies in epileptic patients report cases were spikes are visible in MEG but not in EEG. Recently, it was discussed that a lower sensitivity of MEG to background activity might be the reason for the spike visibility in MEG but not in EEG. Consequently, we analyze the signalto-noise ratio (SNR) of simulated spikes at varying orientations and with varying background activity in realistic head models. For a fixed realistic background activity, we find a higher SNR for spikes in the MEG as long as the spike orientation is not more than 30 degrees deviating from the tangential direction. Vice versa the SNR for spikes in the EEG is higher as long as the spike orientation is not more than 45 degrees deviating from the radial direction. Our simulations provide a possible explanation for the experimentally observed differences in EEG and MEG signals.