Cochlear Implants: 120,000 Cyborgs and counting

I'm currently taking an aural rehabilitation class, and one of the coolest things I've learned about so far has been cochlear implants. The FDA approved the current multi-electrode version of the device in 1990, and over 120,000 have now been implanted.

Here's a brief run-down on normal hearing and how cochlear implants work:

In normal hearing, sound energy is transmitted through your ear drum and bones of the inner ear to fluid in the cochlea, a tiny pea-sized cavity in your skull shaped like a snail. In fact, your whole outer ear, ear canal, ear drum and bones of the inner ear all simple function to amplify and transmit sound vibrations to your cochlea. The inside of the cochlea contains membranes covered with tiny hairs, and when sound waves pass through the fluid, these hairs undulate like beach grass in the wind. They act as transducers, converting mechanical sound energy into nerve signals. Moving a hair cell literally pulls open mechanically sensitive ion channels, which alters the voltage level within the cell and causes it to fire, sending an impulse up the auditory nerve. Pretty crazy, huh? The spiral shape of the cochlea serves to break the sound signal into its constituent frequencies, because low-frequency sounds can penetrate deeper into the spiral than high frequency sounds.

A large percentage of profound hearing loss is caused by damage to the hair cells in this delicate, totally badass system. A cochlear implant basically works by cutting out the hair cells middle man, and instead sends electrical impulses straight to the auditory nerves via electrodes:






The whole system consists of a microphone (replacing the hair cells as the transducer), a speech/ sound processor that converts the signal into very specific map of electrical amplitudes mapped onto the electrodes in the cochlea, a radio transmitter, and the actual implant itself, which consists of a radio receiver and the actual electrode array. The array is placed within the cochlea by drilling a hole straight through the skull, and a shallow depression is also drilled out in the cranium so the remainder of the implant can sit flush against the skin. The implant has a magnet attached to it as well, and the external radio transmitter just sticks right on like a refrigerator magnet. Three different companies make this sort of system in the US, and they all cost about $50,000 to implant. Here's a diagram:




The cool thing is that you end up with a cyborg whose auditory perception of reality is created by computer software. I've just finished a book by Michael Chorost called Rebuilt, where he talks about what it was like to go through the process of learning to use his implant. It wasn't simply a matter of altering the software until it fit his the organization of his auditory cortex; he simultaneously had to rewire his auditory cortex to make use of the new input. For him, one of the most valuable aspects of the experience came from switching between versions of speech processing software. This showed him in a literal sense how constructed his perceptions of reality are; natural selection had set up one program for interpreting sound, but here were a number of others he could flip through with a flick of a switch, each one as "true" as the next.

I think it's absolutely astounding that these implants have become so common-place. When implanted early enough, profoundly deaf children now have the potential to up with completely typical spoken language.

If anyone's interested, I can get into the controversies regarding this technology in the deaf community, but today I just wanted to focus on the sci-fi aspects.