11Why 22 Electrodes Behave Like 8 Channels

TContacts are not channels

Modern arrays carry 12–22 platinum-iridium contacts (Cochlear 22, AB 16, MED-EL 12), full-banded (current radiates in all directions) or half-banded (focused toward the modiolus). But more electrodes do NOT mean more channels.[2008]

CThe 4–8 channel ceiling

Performance plateaus at ~4–8 effective channels regardless of physical contact count (Dorman & Loizou 1997; Fishman 1997; Friesen 2001). The limit is not poor electrode discrimination but channel interaction, and counterintuitively broader monopolar configurations often perform as well as or better than narrower ones.[2012]

TChannel interaction

Channel interaction arises from overlapping current spread: monopolar stimulation spreads widely, so neighbouring electrodes excite overlapping neural populations and blur the spectral pattern. Bipolar and tripolar (focused) configurations narrow the field but require higher current and raise thresholds; closer spacing and faster rates both increase interaction.[2001]

CInterleaving the pulses

CIS and its descendants combat interaction by interleaving pulses so no two channels fire simultaneously (temporal separation) — the design principle that lets a fixed array behave more predictably (cross-ref Ch.8, Module 13).[2014]

TSpacing and modiolar distance

Contact spacing is itself a trade-off: wider spacing reduces channel interaction but reduces the number of available channels; placing contacts closer to the modiolus reduces interaction without sacrificing contact number — the device-level argument for perimodiolar design (cross-ref Module 12).

CThree kinds of channel

Distinguishing physical contacts (12–22), processing channels (~12–16) and functional channels (~4–8) is essential vocabulary; current steering attempts to add spectral resolution between physical contacts, but the perceptual ceiling remains the recurring constraint on CI performance.