The most implanted neural interface
On the order of a million people worldwide now hear through a cochlear implant — more than carry any other neural implant, making it by a wide margin the most successful neuroprosthesis ever fielded. It works by an elegant shortcut: rather than repairing dead hair cells, it bypasses them entirely and injects charge directly into the surviving spiral-ganglion neurons of the auditory nerve, using an array of 12–22 intracochlear electrodes.
But success here is task-dependent, and honesty matters. Most experienced adult users understand open-set sentences in quiet remarkably well. The same users often struggle badly with music and with speech in background noise — and that gap has barely moved in decades. Naming that gap precisely is the whole point of this track: it is the boundary between what electric hearing has achieved and what it has not.
The tonotopic trick
The implant works at all because the cochlea is a spatial frequency analyser: high frequencies excite the base, low frequencies the apex, in a smooth logarithmic map. Tonotopic stimulation exploits this — an apical electrode is meant to signal 'low pitch', a basal one 'high pitch'. The classical Greenwood function makes the place-to-frequency mapping quantitative.
Greenwood's cochlear frequency–position function: characteristic frequency f as a function of normalized distance x along the cochlea. Electrode placement is, in effect, a discretization of this curve.
A map of the frontier
The auditory pathway is a ladder, and each rung is a place to interface. When the cochlea and auditory nerve are intact, you stimulate the periphery (the CI). When the nerve itself is gone, you must climb: the auditory brainstem implant sits on the cochlear nucleus, the auditory midbrain implant penetrates the inferior colliculus, and — still experimental — one could target auditory cortex directly.
And hearing is not the only sense the inner ear carries. The neighbouring vestibular organs encode head motion; when they fail bilaterally, the vestibular prosthesis tries to restore balance by the same logic — sense motion, encode it, stimulate the afferent nerve. Across five guides we will descend into the sound-coding biophysics, climb the pathway, cross into balance, and end at the honest limits.