Electromagnetism

H.C. Orsted observed in 1820 that an electric current in a wire generates a surrounding magnetic field. In the brain, impulses are generated by electric currents which in turn produce local magnetic fields associated with each current. Although the resistive skull blocks us from seeing the electric activity directly, the magnetic fields (the ones which dont cancel each other out) produced by the electric currents in the brain pass through the skull with no interference. The magnetic fields do get exponentially weaker at a distance, but can be detected without contact with the scalp or or body with highly sensitive detection devices (SQUID).

The electromagnetic signals in the brain are due to the net effect of ionic currents flowing through the dendrites of neurons that are actively transmitting ions across their synapses. Although action potentials are moving charges, the fields associated with them cancel out so the only externally detectable activity will be due to these major cortical neurons or groups of neurons firing at the same time, a form of resonance.

Unfortunately MEG doesn't on its own offer a perfect view of all the activity. The orientation of the neuron makes a difference to its detectability. The SQUID detectors are detecting changes in magnetic flux which can only be measured across a specific two dimensional area. It is therefore only going to detect the tangential components of brain activity, this reduces the brain imaged by MEG to that of the Sulci. But what MEG does detect it can localise and model with extremely high accuracy offering an exceptional (though arguably incomplete) view of the brain.