The locus of electrically excitable tissue where action potentials are induced can be controlled using the physiological principle of electrotonus. In one embodiment, first and second pulses are applied to first and second electrodes, respectively, to generate first and second subthreshold potential areas, respectively, within the tissue. The locus within the tissue where action potentials are induced is determined by a superposition of the first and second subthreshold areas according to the physiological principle of electrotonus. In another embodiment, a two-dimensional array of electrodes are formed. The cathode may be positioned near the center of the two-dimensional array or may be left out. The first and second subthreshold areas may thereby be steered. An array of anodal rings may be used to contain the field of excitation.