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From what I have read, outer hair cells in the human ear amplify incoming signals and inner hair cells "pick-up" the signals and generate action potential. However, neurons have refractory periods during which they cannot fire again. Does this mean that the human ear has a "sampling period" within which it cannot "pick-up" sounds?

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Inner hair cells (IHC) do not fire action potentials themselves. It's the auditory-nerve that synapses with IHC that generates action potentials. The firing rate of the auditory nerve can be as high as few hundred Hz with a refractory period as short as 1 ms or so (depends on the animal).

However, it is important to note that the signal is not sampled at this rate. As you may know, the auditory signal is first transformed to the "frequency" domain through the physical structure of cochlea. Each inner hair cell therefore is roughly only encoding the relative strength of a frequency band. There are presumably inner hair cells in the human cochlea that correspond to a range centered around 18 kHz for example, but neither the neurotransmitters of the corresponding IHC nor the auditory nerve can fire at 18 kHz. Nevertheless, the amplitude modulation at this high frequency is what is transmitted.

Also, thinking of neural firing as a "sampling period" is not always a good analogy. There are debates about this, but it could be that precise timing of action potential carries large amount of information about the stimulus (perhaps not so much so in early auditory system.)

If you want to see some computational modeling work for inner ear, IHC, and auditory nerve, I recommend the Meddis IHC model:

  • C. J. Sumner, E. A. Lopez-Poveda, L. P. O'Mard, R. Meddis. A revised model of the inner-hair cell and auditory-nerve complex, J. Acoust. Soc. Am. 111 (5) 2002
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I agree with your explanation. I think cochlea can be modeled as a bank of band-pass filters giving a spectrogram-like analysis of the audio. Lets consider the 18KHz IHC for example. If the auditory nerve connected to this IHC fires(sensing a strong 18KHz band), it cannot fire for another 1ms. Does this mean that I can hear the 18KHz signal only after 1ms again? – user1155386 Jul 4 '13 at 14:52
@user1155386 No. Signal is ENCODED by the train of action potentials. The information is there and can be DECODED if needed. This is more so because the amplitude envelope of the 18kHz signal will not change very fast. The brain is capable of interpolating through time, and it has many mechanisms for short-term memory. Things do not have to be represented instantaneously. – Memming Jul 4 '13 at 15:14
It probably bears mentioning that there is an extensive feedback stream from the auditory cortex to the thalamus that serves as a sifting mechanism for the incoming signal. – jonsca Jul 4 '13 at 16:40

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