Chapter 9 — Cochlear implants

Electrode arrays, processing strategies, electric hearing.

When the inner hair cells of the cochlea are damaged severely enough that even maximum hearing-aid amplification cannot produce audibility — when there are simply no working sensors left to transduce the acoustic signal into neural activity — the only remaining intervention is to bypass the cochlea entirely. A cochlear implant is a surgically implanted device that delivers electrical pulses directly to the auditory nerve via an array of platinum-iridium electrodes threaded into the scala tympani. The electrical stimulation activates surviving spiral ganglion cells (whose central processes form the auditory nerve fibres) without requiring functional hair cells.

The device is in two parts. The internal component — implanted under the skin behind the ear with the electrode array threaded into the cochlea through the round window or a small cochleostomy — contains a receiving coil, current-driver electronics, and the 16–22 platinum electrodes (manufacturer-dependent). The external component — a behind-the-ear processor — contains the microphones, the speech-processing DSP, and the transmitting coil that magnetically couples to the implanted receiver. Power and data flow from outside-to-inside via the magnetic coupling; the implant has no battery.

By 2026 over 1.2 million people worldwide have received cochlear implants. Pediatric implantation has expanded from a procedure reserved for profoundly deaf children to one routinely performed on infants under 12 months of age. Adult indications have expanded from bilateral severe-profound loss to single-sided deafness and even moderate-to-severe sensorineural loss in some patients. The procedure is one of the most successful neural prosthetics in clinical use.

This chapter develops the cochlear implant as an electroacoustic device: the electrode array’s geometry and current-spread physics, the speech-processing strategies that convert acoustic input to electrode pulse trains, and what “electric hearing” — the perceptual experience of a CI user — actually sounds like.

Three lessons:

• 9.1 The electrode array and current spread — geometry, monopolar / bipolar / tripolar stimulation, current spread along the array, the channel-resolution limit.
• 9.2 Processing strategies: CIS, ACE, FSP — how acoustic input is decomposed and mapped to electrode pulses; the trade-off between temporal continuity and spectral focus.
• 9.3 What electric hearing sounds like — pitch coding, temporal vs place pitch, simulation, outcome variability, and the sensitive-period justification for early implantation.