Nowadays, it is commonly agreed that MEG signals originate mostly from postsynaptic currents in the apical dendrites of pyramidal neurons in the cortical mantle. Maxwell's equations in their quasi-static approximation describe how these currents translate into magnetic field (Hämäläinen et al., 1993). The MEG challenge lies into measuring such an extremely weak magnetic field, about one million times smaller than the Earth's magnetic field, in an often magnetically noisy environment due to nearby large metal objects or strong electric currents. Typically, the simultaneous activation of about tens of thousands of spatially aligned nearby neurons (i.e., a neuronal pool) is needed to generate a detectable MEG signal.
