Cochlear Outer Hair Cells - responding to frequencies up to 79kHz

ack

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In an effort to attempt to answer once and for all the hotly debated question of whether we can be stimulated by frequencies higher than the commonly accepted 20kHz - motivated by the HDCD patent's claim that there are nerve receptors up to about 80kHz - I have started reading medical research papers on the ear... This is probably just the beginning of a series of findings that I think I will be able to dig up, and is provided as food for thought, not definitive proof...

First of all, a little primer from Wikipedia:

Research of the past decades has shown that outer hair cells [OHCs] do not send neural signals to the brain, but that they mechanically amplify low-level sound that enters the cochlea. The amplification may be powered by movement of their hair bundles, or by an electrically driven motility of their cell bodies...

In mammalian outer hair cells, the receptor potential triggers active vibrations of the cell body. This mechanical response to electrical signals is termed somatic electromotility and drives oscillations in the cell’s length, which occur at the frequency of the incoming sound and provide mechanical feedback amplification. A movie clip showing an isolated outer hair cell moving in response to electrical stimulation can be seen here....

without functioning outer hair cells the sensitivity decreases by approximately 50 dB. Outer hair cells extend the hearing range to about 200 kHz in some marine mammals
In my initial research on OHCs - the amplifiers - I ran into this article http://physrev.physiology.org/content/88/1/173.full on Outer Hair Cell motility, which claims OHC length variations - thus stimulation - up to 79kHz, although the acceptable human hearing threshold is still defined at ~20kHz:

From the Abstract:
Normal hearing depends on sound amplification within the mammalian cochlea. The amplification, without which the auditory system is effectively deaf, can be traced to the correct functioning of a group of motile sensory hair cells, the outer hair cells of the cochlea. Acting like motor cells, outer hair cells produce forces that are driven by graded changes in membrane potential.
The paper - using guinea pigs, whose hearing characteristics resemble that of humans, they claim - then goes on to claim that:

[ack: He's now describing his new technique using lasers for measuring motility] Long cylindrical cells, such as OHCs from the apical end of the guinea pig cochlea, can be drawn into a suction pipette and stimulated with a current passed down the pipette. The method is noninvasive in the sense that the plasma membrane is not punctured and intracellular contents remain undisturbed even though the suction pipette internal diameter and the cell diameter have to be very closely matched. The technique offers good stability for the recording of cell length changes even though it does not permit the intracellular potentials or currents to be measured directly. The microchamber also permits different solutions to be presented to the apical and the basolateral surfaces of the cell, thus more closely mimicking the situation in vivo. The configuration allows an elegant demonstration that deflection of the stereocilia does indeed lead to the electromotile changes in an OHC, for the projecting stereocilia can be manipulated while motion of the cell body is measured (85)...
Such experiments [ack: Referring to similar but older experiments, using now-considered-limiting techniques using photodiodes] reported that the upper limit of electromotility was above 22 kHz. That higher rates could not be measured more precisely was probably a consequence of the limited bandwidth of the instrumentation, and in that case the photodiode system employed. Such limitations have subsequently been removed and, by enhancing the signal-to-noise ratio of the optical measurement by using a laser vibrometer to measure light reflected from an atomic force microscope cantilever placed on the cell axis, a much higher frequency limit of the electromotility has been determined. The 3-dB point of the frequency response was found to be 79 kHz for cells from the basal, high-frequency end of the cochlea, and at a slightly lower frequency for cells from the apical, low-frequency end of the cochlea.
This raises some important questions: Does this motility, or lengthening and shortening of OHCs, directly affect hearing up to those frequencies? If it can do it for marine mammals, and as high as 200kHz, why not humans? If we still don't hear beyond ~20kHz as measured by an audiologist, can we still be stimulated by higher frequencies by some other means that affect perception of sound and/or its location? If not, what's the significance of this research finding?

The paper appropriately asks:

Is OHC motility really the explanation for "cochlear amplification"?
As you can tell, they are not as assertive as Wikipedia...

As Hallowell Davis presciently remarked in 1983: "The mechanism of the CA (cochlear amplifier) is unknown, and the problem remains of how its action can be triggered by submolecular movements near threshold."
Finally, a few more claims:

Inner hair cells (IHCs), of which there are ~3,500 in each human cochlea, are innervated by dendrites of the auditory nerve and are considered to be the primary sensory hair cells of the cochlea. Outer hair cells (OHCs) number ~11,000 in each human cochlea and lie in 3 or 4 rows... Only 5% of the dendrites of the auditory nerve are associated with OHCs.
From this I take that OHCs directly relate to the auditory nerve (the so-called 8th cranial nerve) , and that there are indeed "nerve receptors up to about 80kHz" if this is what the patent refers to; and the quest continues...
 

Kal Rubinson

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Are you (or they) inferring that direct electrical stimulation of OHC's showing motility above commonly accepted hearing limits is a factor in hearing? Is the graded amplitude of the stimulation correlated with the frequency of response or is the frequency of the stimulation correlated with the frequency of the response?

I have not yet read the paper (but thanks for the link).

Kal
 

ack

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Are you (or they) inferring that direct electrical stimulation of OHC's showing motility above commonly accepted hearing limits is a factor in hearing?

The paper is strictly about the motility of OHCs in relation to frequency... And they and I raise the questions posed above. Clearly, unless one believes this stimulation is a dead function of the ear (a possibility, but see wiki wrt marine mammals), there has to be some relation to "hearing" - the question remains 'What' and to what degree. The paper does not attempt to answer this. It simply claims that certain parts of the ear "respond" to such high frequencies, with no information on what this means. If research ends up answering the question they pose, 'Is OHC motility really the explanation for "cochlear amplification"?' then yes it would have a direct impact on our hearing. Again, as I Ipointed out, they still claim the hearing threshold is 20kHz, for now.

The main point here is that OHCs connected to the nerve are just not inert above 20kHz; but we just don't know yet what happens with information above that threshold. I am looking for research on that subject... Clearly, this paper and others I am hoping to uncover aim to challenge common perceptions about hearing.

Is the graded amplitude of the stimulation correlated with the frequency of response or is the frequency of the stimulation correlated with the frequency of the response

If I understand your question correctly, the latter, assuming that by 'response' you mean - as the paper does - 'reaction to'
 

JackD201

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I remember a thread from the earlier days of WBF about audibility. I've taken hearing tests twice and accompanied my father-in-law to get fitted with hearing aids. In any case, we know the drill. If you hear something push a button, etc. That's how audibility is measured. Another member, I believe he's a neuroscientist said that there's more to it than that and ultimately the best indicator would be to see if stimulus actually changed brain activity whether you hear it (in the traditional sense) or not. You can imagine how this was not taken well by the audibility crowd. I don't think he ever posted again. Too bad.
 

rbbert

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The standard hearing test administered by audiologists is really to determine the need or possible benefit of a hearing aid. There is no way a test like that will truly reveal whether or not we "sense" higher frequencies, since as you note it requires a specific type of learned response by the examinee.
 

Kal Rubinson

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Is the graded amplitude of the stimulation correlated with the frequency of response or is the frequency of the stimulation correlated with the frequency of the response?

If I understand your question correctly, the latter, assuming that by 'response' you mean - as the paper does - 'reaction to'
Then I wonder what is going to stimulate them in vivo at anything near those frequencies?
I know it is not fair to ask you and I will refrain from further comment until I read the paper.

Kal
 

fas42

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I remember a thread from the earlier days of WBF about audibility. I've taken hearing tests twice and accompanied my father-in-law to get fitted with hearing aids. In any case, we know the drill. If you hear something push a button, etc. That's how audibility is measured. Another member, I believe he's a neuroscientist said that there's more to it than that and ultimately the best indicator would be to see if stimulus actually changed brain activity whether you hear it (in the traditional sense) or not. You can imagine how this was not taken well by the audibility crowd. I don't think he ever posted again. Too bad.
A possibility for checking out some of this stuff in a meaningful way, for audiophiles, would be to take a top notch 24/96 or better recording, one that has genuine musical content created by instruments that create ultrasonic overtones, and create a second version where the track, same format, has that content filtered out, only ultrasonic noise is left. Set up a high quality system with supertweeters, run proper measurements at that driver's output to make sure that the ultrasonic overtones are indeed being reproduced cleanly. Then the obvious follows, run DBTs on people with various qualities of hearing, to see if they can hear a difference, not whether one is "better" than the other.

Or perhaps this has already been done?

Frank
 

JackD201

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There's a reason 20Hz to 20k is the accepted limit of audibility Frank even as in vision 20/20. They're the accepted statistical average of the human population. What you propose requires finding a whole lot of outliers!
 

Kal Rubinson

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I remember a thread from the earlier days of WBF about audibility. I've taken hearing tests twice and accompanied my father-in-law to get fitted with hearing aids. In any case, we know the drill. If you hear something push a button, etc. That's how audibility is measured. Another member, I believe he's a neuroscientist said that there's more to it than that and ultimately the best indicator would be to see if stimulus actually changed brain activity whether you hear it (in the traditional sense) or not. You can imagine how this was not taken well by the audibility crowd. I don't think he ever posted again. Too bad.
Well, as another neuroscientist, I will concur. Not all perceptual processes are conscious but they can influence memory and behavior without the awareness of the subject.
 

rbbert

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There's a reason 20Hz to 20k is the accepted limit of audibility Frank even as in vision 20/20. They're the accepted statistical average of the human population. What you propose requires finding a whole lot of outliers!

I think if you look at the research for hearing you will find very little that has even investigated whether or not we "hear" (in the context of audiology testing as you described) above 20kHz, much less "sense" it through hearing plus (presumably) touch.
 

fas42

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There's a reason 20Hz to 20k is the accepted limit of audibility Frank even as in vision 20/20. They're the accepted statistical average of the human population. What you propose requires finding a whole lot of outliers!
But what we're looking at is whether when a cross section of people are exposed to these acoustic frequencies whether a significant number can sense the presence, and absence, of them: differentiate between the two situations. Whether you want to call it hearing, or body sensing, or ESP doesn't really matter: what counts is whether that people can detect that is there is something that distinguishes one replay experience from another.

Frank
 

ack

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Then I wonder what is going to stimulate them in vivo at anything near those frequencies?
I know it is not fair to ask you and I will refrain from further comment until I read the paper.

Kal

Anticipating this, I quoted such language above, repeated here again

The microchamber also permits different solutions to be presented to the apical and the basolateral surfaces of the cell, thus more closely mimicking the situation in vivo.
 

JackD201

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I think if you look at the research for hearing you will find very little that has even investigated whether or not we "hear" (in the context of audiology testing as you described) above 20kHz, much less "sense" it through hearing plus (presumably) touch.

Exactly :)
 

Kal Rubinson

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Anticipating this, I quoted such language above, repeated here again
I would so assume since, otherwise, one cannot establish a potential across the cell. OTOH, what I was asking what how such high frequencies can pass from the environment into this system in vivo. Most biological fluids are LP filters.
 

ack

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I would so assume since, otherwise, one cannot establish a potential across the cell. OTOH, what I was asking what how such high frequencies can pass from the environment into this system in vivo. Most biological fluids are LP filters.

I don't know, but will try to look it up... But thinking about it from another angle, if kids can demonstrably be shown to actually hear sounds up to ~32kHz and considering that hearing degrades thereafter, the question to ask is, is this degradation attributed mostly to cochlear hair loss, changes to the bone and/or membrane, _as well as_ degradation to the fluid? I've heard all of the above as being attributed to hearing loss, but not the fluid - and I can certainly be misinformed, hence would be interesting to investigate this also.
 

ack

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Here's WUSTL's web site on cochlear fluid research. I haven't dug in yet, but I found some other interesting research papers with respect to OHCs and *infrasound* frequencies... Here are some highlights from http://oto2.wustl.edu/cochlea/windmill.html

Our group has for years been using infrasonic tones to study how the ear works. These are often described as “biasing tones”, because they allow the structures of the ear to be displaced slowly while measurements are made. For almost 10 years we have been using infrasonic 5 Hz bias tones at levels as low as 85 dB SPL (shown as the green diamond in the graph at the right) to manipulate cochlear responses in guinea pigs. The guinea pig is LESS sensitive to low frequencies than the human, so this makes you realize that low frequency infrasonic sounds ARE AFFECTING THE FUNCTION OF THE EAR at levels well below those that are heard by humans.
...
The outer hair cells of the cochlea are stimulated by low frequency sounds at levels much LOWER than the inner hair cells. It is likely that the OHC are stimulated in some people by infrasounds at the levels generated by wind turbines. Thus the infrasound component of wind turbine noise may be the cause of the increased annoyance of some individuals to wind turbine noise. It also has to be considered that if there are health effects in some individuals, then the infrasound component of wind turbine noise could be involved.
Stimulation of the OHC occurs at infrasound levels substantially below the levels that are heard. We calculate that stimulation of the OHC occurs at approximately 30-40 dB below sensation level depending on frequency. The concept that sounds that you cannot hear can have no influence on the inner ear is incorrect. Infrasounds that cannot be heard DO influence inner ear function.
And finally, referring to OHCs:

another function of these cells [OHCs] may be to mechanically counteract very low frequency, infrasonic vibrations - to help make sure you don't hear them. This would represent a biological form of active noise cancellation.

I am sure some of us have heard claims before that infrasounds can make us dizzy...
 

ack

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I think if you look at the research for hearing you will find very little that has even investigated whether or not we "hear" (in the context of audiology testing as you described) above 20kHz, much less "sense" it through hearing plus (presumably) touch.

Though the paper I cited includes 357 references related to this subject alone...
 

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