Magnetoencephalography (MEG) | How Do We Listen to the Brain's Whispers in Real-Time?

Magnetoencephalography, or MEG, is a non-invasive neuroimaging technique that directly measures the magnetic fields produced by the brain's electrical activity. Every time a neuron fires, it generates a tiny electrical current. According to the fundamental principles of physics, any electrical current produces a corresponding magnetic field that radiates from it. While the magnetic fields generated by a single neuron are too small to detect, the synchronized activity of thousands of neurons working together creates a magnetic field that is strong enough to be measured outside the head. These fields are, however, exceedingly weak—about a billion times weaker than the Earth's magnetic field. To detect them, MEG systems use highly sensitive magnetometers known as SQUIDs (Superconducting Quantum Interference Devices). These sensors must be kept at extremely cold temperatures (around -269°C) using liquid helium to achieve the necessary sensitivity. A helmet-shaped device containing hundreds of these SQUID sensors is placed over the subject's head. The system is housed in a magnetically shielded room to eliminate interference from external magnetic sources. By measuring the patterns of these magnetic fields, scientists can calculate the location, orientation, and strength of the neural currents that produced them with remarkable precision. MEG is entirely passive; it does not apply any energy to the brain but simply "listens" to its natural activity.

MEG's primary strength lies in its exceptional temporal resolution, which is on the order of milliseconds. This means it can track brain activity as it unfolds in real-time, capturing the rapid neural dynamics underlying complex cognitive processes like understanding language, perceiving a visual scene, or making a quick decision. This high-speed recording capability provides a significant advantage over other functional imaging methods like fMRI, which have a much slower response time. Another key advantage is that the skull and scalp tissues are transparent to magnetic fields. Unlike the electrical signals measured by Electroencephalography (EEG), which get smeared and distorted as they pass through these tissues, magnetic fields pass through them undisturbed. This property allows MEG to localize the sources of neural activity within the brain with a higher degree of spatial accuracy than EEG. Therefore, MEG provides a powerful combination of "when" and "where" information about brain function, making it an invaluable tool for both neuroscience research and clinical applications.

One technique is not inherently "better" than the other; instead, MEG and fMRI offer complementary information about brain function. MEG directly measures the magnetic fields from neuronal currents, providing millisecond-level temporal resolution. This makes it ideal for studying the precise timing of brain processes. fMRI, conversely, measures changes in blood oxygenation (the BOLD signal), which is an indirect and much slower measure of neural activity. However, fMRI offers superior spatial resolution, allowing it to pinpoint the location of activity with greater accuracy. In practice, researchers often combine MEG and fMRI to get a more complete picture of brain function, leveraging MEG's temporal precision and fMRI's spatial detail.

An MEG scan is a completely painless, non-invasive, and silent procedure. The individual sits or lies down comfortably inside a magnetically shielded room, which is necessary to block external magnetic noise. A large, helmet-shaped device containing the sensors is lowered over their head, but it never makes direct contact. Unlike an MRI scan, which can be very loud, an MEG scan is silent. During the recording, which can last from a few minutes to over an hour, the person may be asked to rest quietly or to perform specific tasks, such as looking at images, listening to sounds, or pressing buttons, depending on the goal of the study.

In a clinical setting, MEG is most prominently used for the pre-surgical evaluation of patients with epilepsy. For individuals with seizures that do not respond to medication, surgery to remove the seizure focus—the specific area of the brain where seizures originate—can be an effective treatment. Before surgery, it is crucial to pinpoint this focus precisely. MEG is exceptionally well-suited for this task because it can detect the abnormal magnetic signals generated by epileptic activity (known as interictal spikes) and localize their origin with high accuracy. This helps guide the surgical planning. Furthermore, MEG is used to map critical brain functions, such as the areas controlling language and motor movement. By identifying these eloquent cortices in relation to the seizure focus or a brain tumor, surgeons can plan their approach to maximize the removal of pathological tissue while minimizing the risk of causing functional deficits for the patient.