The locus of electrically excitable tissue where action potentials are induced can be controlled using the physiological principle of electrotonus. Substantially non-simultaneous first and second pulses are applied to first and second electrodes, respectively, to generate both first and second action potentials and first and second subthreshold potential areas, within the tissue. The locus within the tissue where additional action potentials are induced may be determined by a superposition of the first and second subthreshold areas according to the physiological principle of electrotonus. Superposition of the first and second subthreshold areas provides deep tissue suprathreshold potential areas of adjustable locus wherein additional action potentials are induced.