What MEG detects
Neuromagnetometer detects electric activities of pyramidal cells in cerebral cortex. When neurotransmitters excite pyramidal cells, electric current occurs in their dendrites (Excitatory PostSynaptic Potentials). Neuromagnetometer detects magnetic field induced by this intracellular electric current. Extracelluar currents also occur magnetic fields, however, which are reduced by each other.
Neuromagnetometer can't detect a single pyramidal cell activity, but synchronized activites of tens of thousands of pyramidal cells.
Neuromagnetometer can't detect electric activities in direction normal to the pick up coil plane. Electric activities of pyramidal cells located in gyri are difficult to detect.
Isomagnetic field maps simulated using Biot-Savart's rule. 0 degree indicates the current's direction is perpendicular to this schema, and 90 degree indicates the current's direction is right-left.