Do you know what a MEG (magnetoencephalography) is?

A Magnetoencephalography (MEG) is a technique for mapping brain activity by recording magnetic fields produced by electrical currents occurring naturally in the brain, using arrays of SQUIDs (superconducting quantum interference devices). Applications of MEG include basic research into perceptual and cognitive brain processes, localizing regions affected by pathology before surgical removal, determining the function of various parts of the brain, and neurofeedback.

Here, you will find a full explanation of what a MEG is.
http://en.wikipedia.org/wiki/Magnetoencephalography




And also an instructive video: