What is MEG?

The word magnetoencephalography (MEG) can be roughly translated to magnetic brain recording. MEG is a non-invasive, passive, and silent method that measures weak magnetic fields that occur as the result of brain activity. By measuring these magnetic fields at multiple locations around the head one can infer where in the brain the activity originates from.

MEG shares many similarities with EEG, which measures the same underlying activity. Like EEG and in contrast to fMRI, MEG excels at temporal resolution. Since we measure magnetic fields, which propagate at the speed of light, the temporal resolution is only limited by the sensor technology (see instrumentation section) and signal origins (see signal section) resulting in a resolution on the order of a millisecond. Compared to EEG, the main advantage lies in the fact that MEG is less effected by the conductivity of the surrounding tissues – which can result in higher spatial precision.

The main application areas of MEG are cognitive and clinical neuroscience and pre-surgical mapping. Cognitive neuroscience includes, but is not limited to, development, learning/memory, language, and sensory processing. Clinical neuroscience aims to investigate the mechanisms behind and identify neurological markers for neurological diseases such as Parkinson’s disease, Alzheimer’s disease, and Autism. Pre-surgical mapping can be divided into two main applications: mapping of important functional areas, like those responsible for language or motor, to avoid in surgery and localization of epileptogenic zones (where seizures originate) for resection in drug-resistant cases of epilepsy. Traumatic brain injury, another potential clinical application for MEG is currently being investigated.

Signal origin

Neurons, the cells that form the basic processing unit of the brain, produce a weak electric current when activated. Just like any other current, this neural current generates a magnetic field that curls around the current direction according to the right-hand curl or grip rule, where the extended thumb of the right hand represents the current direction and the fingers “grabbing the current” show the direction of the magnetic field. Unlike electric fields, magnetic fields extend relatively undisturbed through the brain, skull, scalp and into the space surrounding the head. A single neuron does not produce strong enough magnetic fields to be measurable outside the head. The magnetic fields we measure in MEG are instead the summation of the magnetic fields generated by tens of thousands of aligned neurons that fire at approximately the same time. Due to this spatial and temporal summation, MEG is most sensitive to pyramidal cells in the cortex (which are roughly aligned perpendicular to the surface of the grey matter) and postsynaptic potentials (which last approximately 10x longer than action potentials). When measuring outside the head we are most sensitive to tangential sources, that is sources in the cortex that are oriented parallel to the head surface, and less sensitive radial sources, that is sources pointing out or into the head surface. One should keep in mind that the head is not a perfect sphere, and most sources neither entirely radial nor tangential. 

Figure 1: MEG signal generated by summation of magnetic fields from simultaneously active neurons.