Cochlear Implant Electrode Current Spread

Stimulation Mode

Electrode Parameters

Active electrode (base → apex) 9 / 16

Current amplitude I 400 µA

Electrode-to-modiolus distance d 0.70 mm

Live Readouts

Spread width (FWHM)

— mm

Fibers excited

—

Peak field at nearest fiber

— mV

How it works

An intracochlear electrode is a current source sitting in the conductive perilymph fluid of the scala tympani. Treating it as a point source in a homogeneous resistive medium, the electric potential at distance r follows the point-source law:

V(r) = I / (4πσr)

where I is the injected current, σ is the perilymph conductivity, and r is distance from the contact. Auditory-nerve fibers embedded in the modiolus (the cochlea's central bony core) are recruited wherever this potential exceeds their firing threshold — so the current does not stimulate one clean "pitch channel", it spreads and recruits a whole neighborhood of fibers.

Monopolar — one intracochlear contact plus a distant extracochlear ground. The field falls off smoothly as 1/r with no nearby return path, producing a broad excitation spread.
Bipolar — current returns through an adjacent intracochlear contact. The two point-source potentials subtract, V(x) = (I/4πσ)·(1/r₊ − 1/r₋), which cancels the far field and sharply narrows the spread — at the cost of needing more current for the same loudness.
Electrode-to-modiolus distance — arrays hugging the modiolar wall sit closer to the nerve fibers than lateral-wall arrays, so the same current produces a narrower, more focused excitation region.

This spatial "spread of excitation" is why cochlear-implant listeners typically discriminate far fewer independent pitch channels than the 12–22 physical electrodes on their array: neighboring electrodes' current fields overlap on the nerve, a phenomenon called channel interaction.

Excited nerve fiber Resting nerve fiber Active electrode

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