Magnetoencephalogram ( MEG )

Physicist David Cohen was the first to measure the magnetic field present around the head in 1968. This magnetic field is an outcome of electrical activities of the brain. Though EEG and MEG both have to do with the electrical potential of the brain, EEG measures the average frequency with electrodes separated from the brain by thick skull. On the other hand  MEG records the subtle magnetic field generated by the brain.  The electrical signals generated by the neurons create weak magnetic field which is about one billionth the strength of the earth's magnetic field. The superconducting neuromagnetometer placed near a persons head picks up the magnetic field emerging from the brain and a computer maps it. The video demonstrates the process of MEG recordings. The MEG image of the brain is also given below.

Although both MEG and EEG are sensitive to electrical flow in the brain MEG measures intercellular current while EEG measures volume extracellular current. Further, MEG primarily responds to tangential generators whereas EEG responds to tagential as well as ratial generators. Further, EEG signals get distorted because of the effect of the varying conductivity of the scalp. This makes precise localization of the source difficult or impossible. On the other hand, the scalp is transparent to the magnetic signals thus making it suitable for accurate localization. The advantage of MEG is its high resolution in space (2-3mm) as well as time (<1ms). However, it suffers a constraint as magnetic field decay very fast over distance. Although initially copper induction coil was used for MEG recordings, now-a-days superconducting material (SQUIDS) such as niobium is used. There has been a recent trend to use both EEG and MEG . CTF MEG /EEG system is used to have a relatively more accurate picture of the working brain.