**Fig. 1.3.** Lead field theoretical approach to describe the subdivisions of bioelectromagnetism.
The sensitivity distribution in the detection of bioelectric signals, the energy distribution
in electric stimulation, and the distribution of measurement sensitivity of electric impedance are the same,
owing to the principle of reciprocity. This is true also for the corresponding bioelectromagnetic
and biomagnetic methods. Maxwell's equations tie time-varying electric and magnetic fields together
so that when there are bioelectric fields there are also bioelectromagnetic fields, and vice versa.