CI Atlas · Devices & Electrode Arrays · Module 13

13Stimulation Modes: From Monopolar to Tripolar

Once an array is placed, the clinician chooses how current flows: out to a distant ground, between two intracochlear contacts, or focused by flanking returns. Each configuration sits on a continuum trading threshold against selectivity, quantified by classic spatial-tuning data — and underpinned by the charge-balanced biphasic pulse that protects the tissue from every one of those modes.

TThe charge-balanced pulse

Every modern implant stimulates with charge-balanced biphasic current pulses (equal cathodic and anodic phases) so no net DC charge accumulates to corrode the electrode or damage neurons. Magnitude is set by two independent knobs — per-phase amplitude (microamperes) and pulse width (tens to hundreds of microseconds); doubling either roughly doubles delivered charge.[2017]

CMonopolar — the default

Monopolar (MP) mode is the clinical default: one intracochlear active contact returns to a distant extracochlear ground (Nucleus MP1 ball and MP2 case, usable singly or together). The far return spreads current broadly, giving low thresholds but poor spatial selectivity (~3.1 dB/mm tuning, Kral et al. 1998).[2006]

TBipolar and tripolar focusing

Bipolar (BP) mode flows current between two intracochlear contacts, narrowing the field at the cost of higher thresholds (~8.5 dB/mm); radial orientation (aimed at the modiolus) is far more selective than longitudinal. Tripolar (TP) uses a central active flanked by two returns and is the sharpest common mode (~20.8 dB/mm, roughly 7× monopolar), restricting excitation to about one critical band; partial-tripolar returns a fraction intracochlearly to tame the high tripolar thresholds.[2008]

CCommon-ground mode

Common-ground (CG) mode ties all other intracochlear electrodes together as the return; it is especially diagnostic for shorted or open electrodes, with flat abnormally low impedances flagging a problem (cross-ref Ch.16 Programming, Ch.23 Objective Measures).[2008]

TPhased-array focusing

Phased-array / focused-multipolar stimulation generalises tripolar focusing by solving for current weights across many electrodes to null the voltage at off-target contacts, requiring the array's measured trans-impedance matrix; it is the conceptual endpoint of the focusing continuum but remains largely investigational due to high charge demand.

CSequential versus simultaneous

Sequential, non-simultaneous stimulation (the core of CIS) avoids uncontrolled summation of overlapping fields; current steering deliberately reintroduces controlled simultaneity on just two adjacent contacts as a managed exception to synthesise a virtual channel (cross-ref Module 6, Ch.8).

Each stimulus is a biphasic pulse — a cathodic phase that depolarises the nerve, followed by an equal-and-opposite anodic phase. The two phases carry equal charge (equal area), so no net charge accumulates at the contact: this charge balance protects both tissue and the platinum electrode from electrochemical damage. Loudness is encoded by the charge per phase (amplitude × width), so amplitude and pulse width can be traded against each other to reach a comfortable level within compliance. Schematic.

Case 13.13 · Focusing the field

To reduce channel interaction, a clinician considers tripolar instead of monopolar stimulation.

What is the trade-off?

Self-assessment — Module 132 questions

Question 1

Which mode is the most spatially selective?

Question 2

Why are pulses charge-balanced and biphasic?

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