EEG/MEG measurements in a macroscopic level are insensitive to action potential sources with short-duration bidirectional current flows. As action potentials propagate along an axon, the associated electrical currents flow in opposite directions (i.e. along and opposite to the direction of action potential propagation) in both intracellular and extracellular spaces. Signals generated by such action potential sources are only recordable from a sensor at a close vicinity to the neuron. At a relatively remote sensor (such as on the scalp surface), the electromagnetic signals generated by opposing currents are essentially cancelled out. Moreover, the brief duration of the action potential also requires a very strong degree of synchrony with great temporal precision in order to be observable from the scalp surface. However, high-frequency (>120Hz) components associated with neuronal spiking activity are significantly underrepresented in EEG/MEG.
