The bit about multiple neurons with synapses to each inner hair cell is really kind of bogus the way it was described here, there is a lot going on, including the tuning of the firing of outer hair cells, etc, and the 0-10, 10-20 ... thing is just a great big nope.
Sounds like you caught me on oversimplification. Guilty as charged
What I meant is that, according to prevalent current understanding, there are three types of auditory nerve fibers: High|Medium|Low Spontaneous discharge Rate (HSR|MSR|LSR). Indeed, mappings of dB HL to Spiking Rate (SR) in those are different for different species, individuals, center tuning frequency, OHC condition and adaptation via efferent fibers etc. However, majority of such mappings do appear to share the following characteristics:
- HSR useful dynamic range is narrow, on the order of 10 dB, starts at the hearing threshold, and HSR HL->SR mapping function is very steep.
- MSR useful dynamic range overlaps with HSR's, is shifted up the dB axis, is as narrow or a bit wider than HSR's, with less steep mapping function.
- LSR useful dynamic range overlaps with MSR's, is shifted further up the dB axis, and is significantly wider than MSR's, with shallow mapping function.
Kind of what's shown here:
https://www.sciencedirect.com/science/article/pii/S0378595517303477#fig5.
Evolutionary, it makes perfect sense: the effective resolution of a bio-ADC connected to a fiber doesn't usually exceed 8 bit, due to limitations of the "serial link", so it makes sense to use three ADC units, with two of them digitizing attenuated input signals, to cover with sufficient resolution the range of sound levels needed for survival.
It wasn't completely clear, let's say ten years ago, how exactly the tracking and encoding of envelope happens. Phenomenologically, researchers understood that there ought to be some kind of rectifier there, followed by a low-pass filter. Yet what are all those diodes, resistors, capacitors, and how they are wired together was a mystery. (I'm oversimplifying again
)
There were theories building the required behavior from the mechanical and electrical properties of the basilar membrane and OHC. There were theories putting these elements into IHC soma - after all, neurons are known for implementing quite sophisticated functions, as they may contain internal analogs of diodes, resistors, and capacitors. There were theories, and rather convincing ones, postulating that most of the observed properties emerge from stochastic patterns of discharges in the "excess" fibers, cross-correlated by a cluster of neurons somewhere deeper in the brain.
Now, the article we are discussing claims, and rather convincingly, that the actual mechanism is split between mechanics of IHC stereocilia and electronics of IHC soma. Moreover, that the observable sophisticated yet stable mapping HL->ADC input, as well as parameters of envelope tracking, are mostly determined by the geometrical and mechanical properties of the stereocilia.
Why is that significant? Well, its significance to different people differs. For me, it finally resolves a couple of auditory system mysteries important for what I do. If you can't think of any, then I guess it is not all that important to you.