Does the Nyquist frequency of the Cochlear nerve impose the fundamental limit on human hearing?
A quick run-through the human auditory system:
- The outer ear (pinnae, ear canal), spatially "encodes" the sound direction of incidence and funnel the sound pressure towards the
- ear drum, which converts sound into physical motions, i.e. mechanical energy
- The middle ear (ossicles) is a mechanical transformer (with some protective limiting built-in) that impedance matches the air-loaded ear drum to the liquid-loaded oval window of the
- Cochlea (inner ear). The vibration excites a bending wave on the basilar membrane. The membrane is highly resonant and transcodes frequency into location: for any given frequency the location of the resonance peak is in a different spot. High frequencies wiggle very close to the oval window, low frequencies towards the end of it. This motion is picked up by the
- Cochlea neurons, which transmit the intensity of the excitation at their location to the brain. About 20% of the neurons are efferent (come out of the brain) and are used to actively tune the resonance with a feedback loop (which causes tinnitus if misadjusted)
So in essence the Basilar Membrane performs sort of a mechanical Fourier transform. The frequency selectivity of the Neurons is NOT determined by the firing pattern but simply by their location. A neuron at the beginning of the basilar membrane is sensitive to high frequencies and a neuron at the end detects low frequencies. But they are more or less the same type of Neurons.
The Nyquist criteria doesn't come into play at all since no neuron is trying to pick up the original time domain waveform. The couldn't anyway: human neurons have a maximum firing rate of less than 1000 Hz and average firing rates are way below that. The firing rate of a cochlea neuron represents "Intensity at a certain frequency" where that frequency is determine by the location of that specific neuron.
So you can think of it as a short term Fourier Transform. Instead of a single time domain signal you get a parallel stream of frequency domain signals where each individual signal has a much lower bandwidth.
A cochlea implant basically does the short term Fourier transform internally and then connects the output for each frequency range to the "matching" neurons in the cochlea nerve. Theoretically you can create ">20 kHz" hearing with an implant that can actually receive and process higher frequencies and simply routes them to existing neurons, i.e. you could feed 40 kHz activity to the 10 kHz Neuron. The human would have a sensation when exposed to 40 kHz but it's unclear what they could do with that: they would have "relearn" how to hear. Aside from the highly questionable practical and ethical issues, it probably wouldn't be useful. In order to get to 40 kHz you'd have to give some other frequencies, and chances are that evolution has chosen the current "normal" range for humans pretty carefully.