Discover the fundamental electromagnetic principle of human electrophysiology: why Magnetoencephalography (MEG) is selectively sensitive to tangential sulcal currents and completely blind to radial gyral currents, whereas Electroencephalography (EEG) detects both.
Physical Foundations of the EEG vs. MEG Sensitivity Difference:
1. The Biot-Savart Law in a Spherical Conductor: In 1853, Hermann von Helmholtz proved a remarkable theorem regarding electromagnetism in spherically symmetric conductors. Intracellular dendritic currents (primary current Jp) induce extracellular return currents through the brain tissue (volume current Jvol). For any current flowing purely radially (perpendicular to the concentric skull shells), the magnetic field created outside the sphere by the volume currents is exactly equal and opposite to the magnetic field created by the primary current. They completely cancel everywhere outside the head: Bradial ≡ 0!
2. Sulcal vs. Gyral Sensitivity:
Because the cerebral cortex is folded into gyri and sulci:
• Gyral Crowns: Pyramidal neurons are oriented perpendicular to the skull — creating radial dipoles. MEG is completely blind to this activity, while EEG records it prominently.
• Sulcal Banks: Pyramidal neurons are oriented parallel to the skull — creating tangential dipoles. Both EEG and MEG record this activity, but MEG records it with superior spatial resolution because magnetic fields pass undistorted through the skull.
3. Field Topography Orthogonality: Notice that for a tangential dipole in a sulcus, the EEG potential lobes (+ and −) align along the direction of current flow, while the MEG magnetic flux lobes (field exiting + and entering −) align 90° perpendicular to the current flow (governed by the electromagnetic Right-Hand Rule).
4. Depth Sensitivity Trade-off: The magnetic field of a dipole falls off with distance as 1/r2 in the near field and 1/r3 further away. Consequently, MEG sensor arrays (SQUIDs and OPMs) are predominantly sensitive to superficial neocortex within 2–3 cm of the scalp. EEG potentials fall off less precipitously with depth, giving EEG greater sensitivity to deep subcortical structures (e.g. hippocampus, thalamus).