Fig. 1.

Top: Schematic presentation of convexial and fissural currents in a slab of cortex. The main axis of pyramidal neurons, which are considered to be the main sources of the MEG signals, is perpendicular with respect to the cortical surface. Thus, currents in the walls of fissures are tangential with respect to skull surface and, therefore, are the main contributors of MEG signals. The current direction as such depends on the activation type (excitation, inhibition) of the neuron and the site (superficial, deep) of activation. For more details, see, e.g., Hari and Puce (2017). Modified from Hari and Puce (2017) with the permission of Oxford University Press. Bottom: Currents in the brain and “brain in a nutshell”. Panel (a) shows all possible current orientations in a sphere. The tangential source produces a magnetic field outside the sphere (corresponding to the MEG signals) and is the same as in panels (b–d) exactly because radial currents do not produce external magnetic fields (and as any current in the middle of the sphere is radial). Moreover, concentric inhomogeneities, as in (d) do not dampen nor smear the magnetic field. In other words, all situations (a–d) are equal from MEG's point of view. Modified from Hari and Puce (2017) with the permission of Oxford University Press; the original figure is from Hari et al. (2000).