Figure 4.

Radial and tangential sources and their associated magnetic fields.

The electrical current source is indicated by an arrow, and its associated magnetic field is indicated by a dashed line. (A) A radial current source located in the gyrus of the cerebral cortex (radial source) has its associated magnetic field oriented tangential to the scalp surface. (B) The magnetic field associated with a current source located in the sulcus (tangential source) is perpendicular to the scalp surface. Only tangential current sources generate neuromagnetic fields detectable by MEG.