Cochlear implant · Grey Matter

A cochlear implant is a prosthesis that gives a sense of sound to a deaf person by turning sound into electrical pulses on the auditory nerve inside the cochlea, and it is the most widely used neural interface, with about a million implanted by 2022.

Cochlear implant. A cochlear implant is a prosthesis that gives a sense of sound to a deaf person by turning sound into electrical pulses on the auditory nerve inside the cochlea, and it is the most widely used neural interface, with about a million implanted by 2022.

It works where the hair cells of the inner ear have died but their nerve fibres remain. A microphone behind the ear feeds a processor that splits incoming sound into frequency bands and sends them by radio across the skin to a receiver under the scalp. A flexible array threaded into the cochlea carries a row of contacts, and each contact stimulates the nerve fibres nearest to it. The cochlea is laid out by pitch (high frequencies at the base, low ones at the tip), so a band delivered further along the array is heard as a lower tone. The signal then reaches the primary auditory cortex by the ordinary route, and the cortex has to learn to read it.

It writes a coarse signal. A few dozen contacts replace about 3,500 inner hair cells, and current spreads between neighbouring contacts, so music and voices in noise stay hard while speech in quiet works well: implanted adults recognise on average 70 to 80 % of sentences in quiet.

It is old enough to have a history. The first device approved in the United States, the single-channel House/3M implant, was approved in November 1984; the multichannel Nucleus 22 followed in 1985 for adults who had lost their hearing after learning to speak.

The brain does much of the work. Results depend on how long the nerve went unused and on months of listening, and children implanted young usually do best.

It is the model every brain-computer interface is measured against, because it showed that a few channels of crude stimulation can restore a sense to a brain that adapts.

A cochlear implant succeeds by writing very little and letting the brain learn the rest.

The device sends a handful of frequency bands, and the auditory system turns them into speech over months of use.

Questions: Is a cochlear implant a brain-computer interface? In the broad sense, yes: it is a computer that writes into the nervous system, and with about a million users it is by far the most common neural interface. It only writes, and it stimulates the auditory nerve in the inner ear instead of the brain itself, which is why it is usually called a neural prosthesis. It still sets the benchmark for every implant in the brain, because it shows how much a few channels can achieve once the brain adapts to them. Why does a cochlear implant take months to sound like speech? The implant delivers a handful of frequency bands where a healthy ear sends the detailed output of thousands of hair cells, and each contact may sit at a slightly different place in the cochlea from the pitch its band is meant to carry. The auditory cortex therefore receives a coarse and partly shifted signal, and at switch-on many people describe what they hear as beeps or robotic voices. Over months of listening the cortex learns to map those patterns onto speech, which is why results depend on daily use and why implanted adults end up recognising most sentences in quiet.