What is MEG?

magenetoencephalography (MEG): non-invasive investigation of human brain activity at high temporal resolution

Magnetoencephalography (MEG) allows us to measure the ongoing brain activity with millisecond time resolution. Since the neuronal activity is detected by ~300 sensors distributed over the head, it is possible to identify where in the brain the activity is produced with reasonable accuracy. This makes MEG suited for studying the human brain as a network of interacting brain areas during performance of various tasks. The main applications of MEG are clinical investigations and cognitive neuroscience research.

MEG basics: Synaptic input to a neuron results in a small post-synaptic current that is associated with a very small magnetic field. When a sufficiently large population of neurons receives synaptic inputs within a short time-window, the dendritic currents will sum up, producing a field which is large enough to be detected outside the head. The neuromagnetic fields of the brain are typically in the order of 50-500 fT (10^-15 Tesla), which is about 100 million times weaker than the earth's magnetic field. Magnetoencephalography (MEG) is a technique, which allows us to measure the magnetic fields produced by the brain.

MEG is based on SQUID technology. The superconducting quantum interference device (SQUID), which was introduced in the late 1960s, is a sensitive detector of magnetic flux. Today's whole-head MEG systems contain a large number of SQUIDs (between 100 to 300) connected to sensor coils in a configuration roughly following the curvature of the head. The technique is completely non-invasive. Since the environmental magnetic noise level due to traffic, elevators etc is several orders of magnitude higher than the neuromagnetic signals, the MEG system needs to be placed in a magnetic shielded room.

Since the magnetic field measured by MEG is produced directly by electrical neuronal activity, it is possible to detect signals from the brain on a sub-millisecond time scale. This makes MEG fundamentally different from imaging technologies like functional magnetic resonance imaging (fMRI), which measures blood flow changes occurring on a much slower time-scale. The activity measured by MEG may be the result of an evoked response, such as visual stimulation, or spontaneous oscillatory activity such as alpha rhythms.

When analyzing the data measured by a whole-head MEG system it is often possible to identify the brain areas generating the  measures MEG signals (source localization). Magnetic fields are less distorted by tissues of different conductivity compared to the electric potentials measured with electroencephalography (EEG). Consequently, source localization can be obtained with simpler volume conduction models and may result in more accurate source models. In addition, MEG does not require a reference electrode that is needed for EEG recordings. Another important difference between MEG and EEG is that MEG is insensitive to current flows oriented perpendicularly to the scalp. Only the tangential component of a current flow will produce a measurable field. This in fact makes EEG and MEG complementary techniques, and MEG and EEG signals are often recorded simultaneously.