The problem is stimulating the nerves. While these implants are marvelous, they are still pretty crude in the stimulation portion.
The implant has an electrode array that is shoved into the cochlea. In diagrams of the ear, the cochlea is the thing that looks like a snail shell.
The cochlea itself wraps around the auditory nerve, and normally works with the auditory nerve as a pressure transducer. Pressure changes in the cochlea result in nerve stimulation.
Different frequencies of sound can propagate to different lengths of the cochlea and stimulate the nerves in the respective areas. You can think of the nerve bundle as a piano keyboard wrapped in a spiral. Different frequencies correspond to different positions. So having a bunch of electrodes matters for granularity, but positioning in the cochlea is just as important.
With the implants though, they don't try to stimulate via pressure, they just send electrical impulses through the electrodes. The resulting electric field is what stimulates the nerve.
> With the implants though, they don't try to stimulate via
> pressure, they just send electrical impulses through the
> electrodes. The resulting electric field is what
> stimulates the nerve.
Yes, it is true that electrical stimulation skips a few steps in the signaling cascade, but I don't think that matters, in principle, since the end result for both cases is the generation of action potentials.
In normal hearing, the pressure waves affect perception by mechanically deflecting the stereocilia (hairs) on the hair cells, which open ion channels, which lead to the production of action potentials. Electrical stimulation, on the other hand, opens the ion channels directly (either on the hair cells themselves or a synapse or two upstream), bypassing the mechanical action of the stereocilia entirely. This is actually a good thing since in sensorineural hearing loss it is often the hairs or hair cells that are damaged or malformed. By interfacing with the nervous system so peripherally, the vast majority of the neural processing in the auditory system is preserved, as opposed to auditory brainstem implant or stimulation of cortical auditory regions.
Of course there are technical limitations to electrical stimulation: the spatial and temporal spread of the electric field is only a very rough approximation of the pressure waves caused by sound, even with sophisticated acoustic models of the cochlea. But with smaller electronics and increased numbers of channels it should be possible to make the match closer, and perhaps someday indistinguishable for most individuals. These are differences in degree not in kind.
The implant has an electrode array that is shoved into the cochlea. In diagrams of the ear, the cochlea is the thing that looks like a snail shell.
The cochlea itself wraps around the auditory nerve, and normally works with the auditory nerve as a pressure transducer. Pressure changes in the cochlea result in nerve stimulation.
Different frequencies of sound can propagate to different lengths of the cochlea and stimulate the nerves in the respective areas. You can think of the nerve bundle as a piano keyboard wrapped in a spiral. Different frequencies correspond to different positions. So having a bunch of electrodes matters for granularity, but positioning in the cochlea is just as important.
With the implants though, they don't try to stimulate via pressure, they just send electrical impulses through the electrodes. The resulting electric field is what stimulates the nerve.
So it's pretty different from normal functions.