What does MEG see that EEG does not, and the other way round? · Grey Matter

Both record the same synchronous currents of pyramidal cells, but the skull treats them differently: it smears the electric potential that EEG measures and leaves the magnetic field almost untouched, so MEG maps sources more sharply.

What does MEG see that EEG does not, and the other way round?

Both record the same synchronous currents of pyramidal cells, but the skull treats them differently: it smears the electric potential that EEG measures and leaves the magnetic field almost untouched, so MEG maps sources more sharply. MEG is mostly blind to currents pointing straight out of the head (the crowns of the cortical folds) and sees best those running parallel to the scalp (the walls of the folds), while EEG picks up both orientations. EEG is cheap and portable; MEG needs a shielded room and, until optically pumped sensors arrived, a helium-cooled helmet. Recording both together gives two views of the same sources and constrains the inverse problem better than either alone.