Cochlear implants

The cochlea is tonotopic: position along it maps to frequency. So an electrode array threaded into it can, in principle, stimulate the auditory nerve in a frequency-ordered way. Split the incoming sound into bands, take the envelope of each band, and use it to modulate pulse trains on the corresponding electrode.

This is the most successful neural prosthesis by a wide margin — hundreds of thousands of users, most of whom understand speech well enough to use a telephone.

Why the interleaving mattered

Early multichannel implants stimulated electrodes simultaneously, and performance was disappointing. The reason is physical: current from one electrode spreads through the conductive fluid of the cochlea, so simultaneous stimulation on neighbouring contacts sums in the tissue unpredictably and smears the frequency separation the array was built to provide.

Continuous interleaved sampling fixed it by never stimulating two electrodes at once — pulses are staggered in time so their fields do not superpose. The improvement in speech recognition was substantial, and it came from a scheduling decision rather than from better electrodes or more channels.

What it says about coding

The result is startling from an information point of view. The healthy cochlea has some 3,000 inner hair cells and roughly 30,000 auditory nerve fibres, with exquisite frequency selectivity and phase locking. An implant has around 12 to 22 electrodes, no phase information, and crude spatial selectivity — and speech comes through.

So speech must be extremely redundant, and most of what the intact ear transmits is not required for it. That is an efficient-coding argument arrived at by prosthesis rather than by theory, and it also explains the honest failure mode: music and speech in noise, which depend on the fine spectral and temporal detail the implant discards, remain much harder for implant users than quiet speech.