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I know about the hair cells in our Cochlea and it is the movement of the fluid that makes them vibrate. And it is this that activates the transmission of electrical signals to the brain that become sound.

But I've heard that hair cells are each built to detect specific pitches of sound. And if they don't get that particular pitch then they don't transmit. So as I understand it, we have cells that each respond to one particular pitch and nothing else. Since we have many that each respond to a different pitch, this is what gives us a wide spectrum of hearing.

What I've been wondering is this, if each hair cell responds to one particular pitch, what happens if we try to hear a sound that is in between two hair cells?

Imagine a hair cell that only responds to 1 Hz (and based on my understanding they do) And next to it a cell that responds to a sound at 2 Hz. What happens if there is a sound that is 1.5 Hz? Can we not hear that sound because it exists in the gap between our hair cells? Or can hair cells cover a wide range so they overlap and cover the gap?

As our ability to hear sound quantised or continuous?

I know that there are sounds so low or high they are beyond our range of hearing but what about within our range of hearing? If someone played a sound that was right in between two hair cells could we not hear it? Is there a finite number of different sounds that we can hear within our hearing range?

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  • $\begingroup$ Good question. I think clearly the range of the hair cells do overlap, because if I listen to a tone that slowly changes in pitch, such as slowly tuning a guitar or piano string I don't notice gaps where the sound goes away and comes back as I tune the string. Although it would be interesting to know exactly how much they do overlap. $\endgroup$ – Karl Jun 23 '19 at 17:14
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To add to the good answer by @AliceD, take a look at some tuning curves for hair cells, the sound receptors in the cochlea (these happen to be fairly low frequencies and a turtle cochlea, though those details aren't all that important):


IHC tuning curves

Fettiplace, R. (1987). Electrical tuning of hair cells in the inner ear. Trends in Neurosciences, 10(10), 421-425.


The horizontal axis is frequency; the vertical axis is the response of the neurons in voltage. The data are produced by recording responses to a bunch of different pure tones.

What you see is that the cell that is "tuned" to 131 Hz doesn't only respond to 131 Hz, that's just where it responds most. Same for another cell that is tuned "best" to 314 Hz. There would be many cells in between with different best frequencies, and you could take a weighted average of the activity across that population of cells to precisely identify the actual tone.

Generally, the hair cells that actually monitor sounds are some of the most poorly tuned cells in the early auditory system. At higher levels of processing in the auditory brainstem, lateral inhibition improves tuning.

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  • $\begingroup$ Great answer! And +1 for that ancient graph of Fettiplace ;-) $\endgroup$ – AliceD Sep 26 '19 at 8:14
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Short answer
No, there are no between-hair cell induced tonotopical gaps in frequency perception.

Background
Young people are able to hear over a frequency range of about 10 octaves, with a frequency resolution of about 0.3% of an octave. So we would have about 2300 resolvable frequencies. This number is dependent on the sound level, as excitation spreads at higher sound levels, but it gives a reasonable starting point to answer your question.

The human cochlea contains on the order of 3,500 inner hair cells. Hence, according to this (highly simplified) calculation, it seems that the number of hair cells is redundant.

In other words, the notion of in-between hair-cell stimulation is stretching the limits of physiological frequency resolution a tad too far.

Reference
- Elliott & Shera, Smart Mater Struct (2012); 21(6):064001

Source
- Cochlea.eu. Cochlea: function

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