Magnetoencephalography

Magnetoencephalography (MEG) is the recording of magnetic fields produced by the electrical currents of neurons in the brain. Unlike Electroencephalography (EEG), which measures the voltage differences on the scalp caused by neural currents, MEG measures the magnetic fields that are orthogonal to those currents. Because magnetic fields are not distorted by the skull or scalp — both of which are effectively transparent to magnetism — MEG offers superior spatial resolution compared to EEG, while maintaining the same millisecond temporal resolution. It is the only non-invasive technique that combines high temporal precision with reasonable spatial precision.

The magnetic fields produced by the brain are extraordinarily weak — on the order of femtotesla (10⁻¹⁵ T), billions of times smaller than the Earth's magnetic field. Detecting them requires superconducting quantum interference devices (SQUIDs) operating at liquid helium temperatures, housed in magnetically shielded rooms. The SQUID array, typically covering the entire head, records the magnetic field at hundreds of locations simultaneously, and sophisticated inverse algorithms (beamforming, minimum-norm estimation) are used to localize the neural sources in the brain.

MEG is used clinically for localizing epileptic foci, mapping cortical function prior to neurosurgery, and studying the timing of cognitive processes. In research, it is particularly valuable for studying oscillatory dynamics, because magnetic fields are less contaminated by muscle artifacts than EEG and because the spatial precision of MEG allows better separation of sources from different brain regions. The combination of MEG and EEG — measuring both the magnetic and electric consequences of the same neural currents — provides the most complete non-invasive picture of brain activity currently available. See also Electroencephalography, Neural Oscillation, Local Field Potential, Hodgkin-Huxley model.

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