Figure 1:

Biphasic current pulses on a main electrode ELn (the second line from the top), the adjacent basal electrode ELn+1 (the top line), the adjacent apical electrode ELn-1 (the third line from the top), and the external ground (the bottom line) for monopolar (MP; panel a), partial tripolar (pTP; panel b), and steered pTP modes (panel c). The MP mode returns all the current to the external ground. The pTP mode and steered pTP mode return part of the current (e.g., σ = 0.75) to the basal and apical electrodes. The middle virtual channel (VC; solid lines in panel c) is the same as the pTP mode and returns an equal amount of current to the basal and apical electrodes (i.e., α = 0.5). The basal VC (short-dashed lines in panel c) returns less current to the basal electrode than to the apical electrode (e.g., α = 0.2), while the apical VC (long-dashed lines) returns more current to the basal electrode than to the apical electrode (e.g., α = 0.8).