There are a number of issues around quality: the number of electrodes is not always the most important factor. The processing algorithm is also vital to good quality, and that can be improved in situ. There's a lot of research that has gone into algorithms that are best suited to speech, music, etc.
More importantly, many patients receiving implants do not have a healthy cochlear, in the sense many of their auditory nerves may be damaged. In this case the electrodes stimulate the surviving nerves, and additional electrodes may not provide much additional benefit.
Again, the audio quality is really down to the algorithms: because each patient is different the processor can require intensive tuning and individual work to get it sounding reasonably decent. Far more research goes into these audio processing algorithms increasing the number of electrodes (not least because improving the processing benefits both new and existing patients).
There are approximately 3,000 nerve endings in a working cochlea that need to be stimulated by a few (8, 10, 12, 24) electrodes. So no matter what, it not going to sound like what a fully functioning cochlea. You can't get the resolution. The algorithms are important, but so are the number of electrodes. Ideally you want 3,000 electrodes perfectly aligned with the nerve endings.
It must be a difficult choice for parents to implant their children. Children make the best candidates, but the technology is changing at such a fast rate. 24 electrodes today could be 3000 a few years from now. Once they are in, you can't replace the electrodes, only the signal processor.
This would be true for a consumer electronics device, but this is something that's inserted inside the body and is expected to function for many, many years. 24 electrode implants have been in the market for over twenty years. The major, major strides in implant technology have generally happened outside the body - speech processing packs have gone from shoulder worn packs to behind-the-ear models.
You need to bear in mind that we're not dealing with traditional technology here: we're dealing with implanted medical devices. This is not a situation where Moore's law applies. It is vastly cheaper to spend R&D developing speech/music processing algorithms than trying to cram more and more electrodes into implants which require extensive testing and certification by the likes of the FDA.
To be honest, it's not a difficult decision at all. If you're happy with your child having an implant[1], you would be crazy to wait a few years 'just in case' the technology improved. Children implanted at an early age can often adapt far more rapidly than those who are older.
But it's a moot argument, because like I say these implants are not like smartphones that get improved every year. It is simply too costly to not only re-certify, but to re-train surgeons and audiologists on the new devices. Developing improvements to the external processing units is many factors cheaper than upgrading the implants themselves.
[1] There are some parents who do not necessarily approve or want their children to get implants, but not for the reasons you suggest. This mainly happens with parents who themselves are deaf, or part of the deaf community. This is a whole other ethical kettle of fish though!
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.
