Magnetoencephalography (MEG) is a noninvasive technique used to measure magnetic fields generated by small intracellular electrical currents in neurons of the brain. In this way MEG can provide direct information about the the dynamics of evoked and spontaneous neural activity and the location of their sources in the brain.

MEG and EEG are closely related, the latter detecting the electric potentials generated by neural currents instead of the corresponding magnetic fields. However, it turns out that the task of inferring the sites of brain activation is often more straightforward with MEG than with EEG. This is due to the electric and magnetic properties of the tissues in the cranium as well as to the fact that MEG is selectively sensitive to currents flowing tangential to the scalp, corresponding to sulcal activations. In contrast, the interpretation of EEG is often complicated by the simultaneous presense of both sulcal and gyral sources, the latter corresponding to radial currents.

Clinically, MEG is used to detect and localize epileptiform spiking activity in patients with epilepsy. Also, when planning for removal of brain tumors, surgeons will use it to localize brain areas important for speech that should be avoided.

At the Martinos Center, researchers use MEG - often in conjunction with EEG, MRI, fMRI and optical imaging - to obtain maps of brain activity in cognitive neuroscience studies carefully designed to investigate the workings of the normal and damaged brain.