A form of functional neuroimaging that uses very sensitive magnetometers to measure magnetic fields that occur naturally in the brain. Because the magnetic field of the brain is very weak compared to the magnetic noise present in the environment, all such measurements are taken in a magnetically shielded room. Uses of MEG include research into brain activity and certain clinical assessments before brain surgery is performed. It can observe much shorter and better localized brain activity events than is possible with functional magnetic resonance imaging (fMRI); in practice, the techniques are often used together.