MEG vs EEG: The Sarvas Dipole Model

Compute external magnetic fields from Sarvas’ formula and contrast radial versus tangential source sensitivity with idealized EEG.

Chapter 7
MEG
Sarvas
EEG complementarity

Radial MEG peak

143.6 fT

Full-vector axial norm

145.9 fT

EEG arc peak

1.74 µV

Tangential moment

10.00 nA·m

EEG scalp potential — infinite medium

−1.717 to +1.717 µV · blue − / red +

MEG radial magnetometer component

−142.372 to +142.372 fT · blue − / red +

Electric potential angular profile

EEG x–z arc

Magnetic radial-component profile

MEG y–z arc

Model scope

MEG is the exact external Sarvas field for spherical symmetry; radial silence is not universal in real heads. EEG uses an infinite homogeneous 0.33 S/m medium (no skull). The two curves traverse different arcs: EEG x–z, MEG y–z, avoiding a tangential source’s nodal plane. The maps use identical upper-hemisphere projection; no map-extremum localization is inferred.

Preparing the simulation…

Conceptual bridges & related modules

Volume Conduction & Spherical Head Models
physical contrast · Chapter 7
Contrast skull-dependent electric conduction with spherical magnetic-field symmetry.
Minimum-Norm Source Localization
companion · Chapter 7
Distinguish forward source visibility from inverse localization validity.
Helical Diffraction Simulator (Photo 51)
physical contrast · Chapter 9 · Sections 4–6; Chapter 8 bridge
Contrast wave-scattering symmetry with quasi-static source-field symmetry.