MEG

Magnetoencephalography (MEG) measures the magnetic fields generated by neuronal currents outside the head. Conventional whole-head systems use superconducting quantum interference devices (SQUIDs), which require cryogenic cooling, while newer optically pumped magnetometer (OPM) systems are cryogen-free.

MEG offers millisecond-scale temporal resolution and generally better source localization than EEG because magnetic fields are less distorted by the skull and scalp than electric potentials. Source-localized resolution can reach the millimetre range in favourable conditions, although sensor-level localization is coarser.

MEG is widely used in research and in clinical settings such as presurgical epilepsy mapping.

Functional resolution & depth (note): MEG spatial resolution is better than EEG (~5 mm for superficial cortical sources) because magnetic fields are not blurred by the skull; the limit is the sensor–source distance, so on-scalp OPM arrays do better than helmet SQUIDs. Like all electromagnetic methods it follows the quasi-static e^(−kd) law — resolution scales roughly as depth/ln(SNR), so deep sources are localised far more coarsely than cortex.