Current focusing and current steering (virtual channels)

Two complementary electrode-engineering approaches try to beat channel interaction without changing the physical array. Current focusing sharpens the excitation field by returning current through neighbouring electrodes: partial tripolar and phased-array (matrix-inversion) stimulation add flanking compensating currents that narrow the effective spread, at the cost of higher current and reduced dynamic range. Current steering does the opposite — it deliberately co-activates two adjacent electrodes with a controlled current ratio to place a virtual channel at an intermediate cochlear location, potentially creating more pitch percepts than there are physical contacts (the basis of HiRes 120 / Fidelity 120).

Both are well demonstrated psychophysically — steering yields intermediate pitches, focusing sharpens spatial tuning — but neither has produced a large, reliable clinical speech benefit across users, because the ultimate limiter is the number and health of surviving spiral ganglion neurons and their distance from the array. Focusing and steering can present more distinct patterns than the degraded neural substrate can resolve.