Magnetoencephalography (MEG) | How Can We Eavesdrop on the Brain's Magnetic Whispers?

Magnetoencephalography (MEG) is a non-invasive neurophysiological technique that measures the magnetic fields generated by the brain's electrical activity. Every time a neuron fires, it creates a tiny electrical current. Basic principles of physics dictate that any electrical current produces a corresponding magnetic field. While the magnetic fields produced by a single neuron are infinitesimally small, the synchronized activity of thousands of neurons working together generates a field that is weak yet detectable outside the head. MEG uses an array of highly sensitive sensors called SQUIDs (Superconducting Quantum Interference Devices) to measure these fields. A key advantage of MEG is that magnetic fields are not distorted or blocked by the skull and scalp tissues. This is a significant difference from electroencephalography (EEG), which measures electrical potentials that are smeared by these tissues. Because the magnetic fields pass through unaffected, MEG can locate the source of the brain's activity with much higher precision. The entire measurement process must be conducted within a magnetically shielded room to eliminate interference from the much stronger magnetic fields present in the ambient environment, such as the Earth's magnetic field or signals from electronic equipment.

The primary strengths of MEG lie in its exceptional temporal resolution and very good spatial resolution. Temporal resolution refers to the precision of a measurement with respect to time. MEG can record brain activity on a millisecond-by-millisecond basis, which is the actual speed at which neural communication occurs. This allows researchers and clinicians to track the sequence of brain events with incredible accuracy, observing the flow of information processing in real-time. This is essential for studying cognitive functions that unfold rapidly, such as understanding language, reacting to a stimulus, or making a quick decision. Spatial resolution refers to the ability to distinguish the location of activity. While not as precise as functional magnetic resonance imaging (fMRI), MEG's spatial accuracy is typically within a few millimeters. This combination of "when" and "where" information makes MEG a uniquely powerful tool for creating dynamic maps of brain function, bridging the gap between the structural detail of MRI and the timing information of EEG.

During an MEG scan, the individual sits comfortably or lies down, placing their head inside a helmet-shaped detector that houses the array of SQUID sensors. The procedure is completely passive, silent, and painless. Depending on the purpose of the scan, the person may be asked to simply rest, or they might perform specific tasks. These tasks can include viewing images on a screen, listening to sounds through earpieces, or pressing buttons in response to stimuli. This allows for the measurement of brain activity related to sensory, motor, or cognitive processes. The entire session is conducted inside a magnetically shielded room to ensure that only the brain's magnetic signals are detected. No radiation is used, and there are no known risks associated with the procedure.

MEG, EEG, and fMRI each provide a different window into brain function. EEG measures electrical activity, whereas MEG measures the corresponding magnetic fields. Because the skull distorts electrical signals but not magnetic ones, MEG generally offers superior spatial localization of neural sources. fMRI measures brain activity indirectly by detecting changes in blood oxygen levels (the BOLD signal), which is a slower physiological response to neural firing. Consequently, fMRI has excellent spatial resolution but poor temporal resolution (on the order of seconds). In contrast, MEG directly measures the postsynaptic potentials of neurons, providing millisecond-level temporal resolution. In summary, if the primary question is "where" in the brain activity occurs with high precision, fMRI is often preferred. If the question is "when" it occurs, MEG and EEG are superior. MEG provides a robust combination of both.

The most prominent clinical application of MEG is in the field of epilepsy. For patients with drug-resistant epilepsy who are candidates for surgery, MEG is used to precisely identify and locate the source of epileptic discharges in the brain. This is known as pre-surgical mapping. By pinpointing the epileptic focus, neurosurgeons can plan a more targeted resection, which increases the chances of seizure freedom while minimizing the risk of damaging healthy tissue responsible for critical functions like language, memory, or motor control. Similarly, MEG is also used for pre-operative functional mapping in patients with brain tumors. By identifying the location of essential brain functions relative to the tumor, surgeons can navigate more safely during its removal. Beyond clinical practice, MEG is an invaluable research tool used to investigate a wide array of neurological and psychiatric conditions, including Alzheimer's disease, Parkinson's disease, schizophrenia, and autism spectrum disorder, by analyzing neural network dynamics and brain wave patterns (neural oscillations).