Nerve impulses and sound perception

[MayIKnow?]

Hello to all,

 

I do not have any background in medical or science field and therefor this question may sound rather naïve to you guys. I hope you would tolerate my ignorance and shed some light on something that has been in my mind for a long time.

 

My understanding is the fastest transmission of nerve signals is about 268 miles per hour or just about 120m/s. The transmission of nerve impulses from the hair cells may be much lower than the figures mentioned here. Leaving that aside, let's take 120m/s as the speed of nerve impulses (SIC) from the ears hearing mechanism to the brain for the purpose of this question.

 

Now, if sound waves travel at 340 m/s then when they reach our ears they would be slowed down to not more than 120m/s. Does this mean some information is lost after reaching our ears due to the bottleneck created by our slow nerve impulses compared to the speed of the sound?

 

 

Thank you.

[MayIKnow?]

Thanks to Mr Glasgal of Ambiophonics.org, I got the answer. It doesn't matter.

[StringJunky]

Thanks to Mr Glasgal of Ambiophonics.org, I got the answer. It doesn't matter.

What was the answer you found?

[MayIKnow?]

According to him, and I quote" ...the speed of the sound is not material since it is the change in pressure at the ear from millisecond to millisecond that matters."

 

I am beginning to see the relevance here when you listen to underwater speakers in the swimming pool. However, I am still wondering if the pressure at ear level which is due to the oscillation of air molecules below or above the speed of the nerve transmission speed. If the are above the speed of our nerve transmission then we may still lose some information.

[StringJunky]

Yes, I read a bit, and thought about it, and came to a similar conclusion because the fluid-enclosed hair-cells in the cochlear record timing, amplitude and frequency information. The resulting fluid-vibrations are sampled and transmitted 600 times a second by each of 30 000 nerve fibres, so that's 18 million impulses a second it's receiving in total being received by the auditory cortex. It occurred to me that it's not the speed of sound and nerve-signal speed that matters, regarding information, but sampling rate and cochlear hair density, or rather the number of signal pathways from them. What the slower nerve-signal speed actually limits (relative to the speed of sound) is the brains' fastest possible reaction-time to the presence or variation of an acoustic signal sequence.

 

I sourced my data from this nice article by Yamaha.

Edited February 28, 2015 by StringJunky

[studiot]

Our ears did not evolve to listen to continuous sounds such as music.

 

They evolved to react to change.

For example the sudden rush of sound caused by a predator.

 

The important factor in this is the time the change takes to reach its destination.

 

Now in the time it takes for a predator to travel say 10 metres towards you, according to your speed figures which I have no reason to doubt, a nerve signal can travel about 3 metres.

 

But the distance from your ears to your brain centre is only about 0.07 metres.

 

So there is ample time for a change signal to arrive.

Edited February 28, 2015 by studiot

[CharonY]

Ultimately the problem that was solved is how can I transmit information over long distances, without losing details. The solution was to use action potentials which basically allow a mostly loss-less transmission. The strength of a stimulus is encoded via frequency. The other thing is how to differentiate and sense minutiae of the stimulus itself (such as frequency of sound for example) here the encoding is often at or near the sensor levels. For example you have stereocilia in your ear and the interesting bit is that they some react stronger to a certain frequency. Hence if air pressures reaches them (transduce and transmitted via eardrum and associated bones) the frequency is decoded via the type and area of stereocilia reacts to them.

[MayIKnow?]

Ultimately the problem that was solved is how can I transmit information over long distances, without losing details. The solution was to use action potentials which basically allow a mostly loss-less transmission. The strength of a stimulus is encoded via frequency. ......

 

I do read quite a bit about ears and how they work. Anyway, a little knowledge can be a dangerous thing. I hope you would indulge my ignorance here.

 

My understanding is the ears and eyes discard many "bits" before processing them. The best that our eyes could resolve is just about 7 to 10 bits. I believe our ears too fall within this range. Furthermore, once the signal reaches the ear it is transmitted via pulses of the hair cells nerve in in batches of 500Hz at max by each (something?).

 

The other reason why I am interested in this point is because the best of the loudspeaker could only produce about 8 to 10 bit that is far less than the a recording made with 16 or 24 bit. It is also evident that we are in most cases unable to distinguish a 16/44.1 recording vs 24/96 which still far less than the resolution of actual sound. Doesn't this mean we are losing a lot of info before reaching the ears?

 

 

?..... It occurred to me that it's not the speed of sound and nerve-signal speed that matters, regarding information, but sampling rate and cochlear hair density, or rather the number of signal pathways from them. What the slower nerve-signal speed actually limits (relative to the speed of sound) is the brains' fastest possible reaction-time to the presence or variation of an acoustic signal sequence.

 

.

 

I am trying to get the speed of molecules vibration which is measure in frequency to determine the speed of information hitting out ear drum. Meanwhile, correct me please if my thinking is that you cannot transfer information faster than the weakest link irrespective number of extra connections.

 

It is like a bullet train reaching a cliff at 300 mph and the same bullet train miles before reaching the station splits each coach to a many different tracks but only travel at 100mph. Although, all of them going to end up at the bottom of the cliff but the speed of them differs. Unless, as already been explain in the earlier post, the spread doesn't matter.

 

 

 

Our ears did not evolve to listen to continuous sounds such as music.

 

They evolved to react to change.

For example the sudden rush of sound caused by a predator.

 

The important factor in this is the time the change takes to reach its destination.

 

Now in the time it takes for a predator to travel say 10 metres towards you, according to your speed figures which I have no reason to doubt, a nerve signal can travel about 3 metres.

 

But the distance from your ears to your brain centre is only about 0.07 metres.

 

So there is ample time for a change signal to arrive.

I have been told about this in another forum that music only evolved about 10 thousand years ago and evolution did not make our ears for listening music. However, I did not get a satisfactory answer when I pointed out there are many ancient artefacts of musical instruments dating as far as 400000 years and besides using our hearing sense for survival it also the first natures tagging to establish mother and newborn. In nature, the mother's voice is registered while the unborn is still in the womb. it take a little bit of sophistication to able to distinguish a mother voice among thousands of , say, noisy penguins.

[CharonY]

I would dismiss evolutionary speculations as they are unlikely to hold significant explanatory power and concentrate on the physiology of our sense instead. I had a lengthy reply but lost it and have not the time to retype everything.

Just some short notes:

The dynamic range of our sense is huge, eyes can resolve about 24 bit of contrast, but not the whole range at once. Instead our sense shift to the apparent level (e.g. adapt to bright/loud situations differently than to dim/silent ones). That allows us to see something in a dim room at candlelight as well as under sunlight, for example.

There are several mechanisms that allow that, and typically dissect the signal via the use of different sensory neurons and different ways of how they encode the info as action potentials (which basically encode everything as frequencies). Example are for auditory neurons that some are not very sensitive (need strong stimulus to activate) but when they activate they fire at high frequency. Others have a low threshold, but fire slowly. Frequency processing at the afferent pathways and finally location in the brain are used to figure out how loud something is (to put it very simply).

 

Speakers can get to 120 dB and louder but may start distortion (but I am not sure how that is really relevant, especially that loudness would be highly unconfortable). However for most music it does not make much sense to go much beyond 16bit (96 dB). 24 bit is more relevant for processing rather than playback. That coincides with the total dynamic range of our hearing which goes up to 120 dB. While this is a bit more than 16 bit, it really hurts and I would assume that there is no good reason to listen to something at that level.

Edited March 1, 2015 by CharonY

[studiot]

 

Today, 03:16 AM

I would dismiss evolutionary speculations as they are unlikely to hold significant explanatory power and concentrate on the physiology of our sense instead. I had a lengthy reply but lost it and have not the time to retype everything.

 

Sorry you lost your typing.

That happens to me all too often.

 

 

Surely auditory sensors evolved pre hominid?

Do we know for (reasonable certain) that the speed of our nerve signal transmission is significantly different from that in the first hominid (who came equipped with ears)?

I suspect there is not enough difference to affect the simple estimate of time and distance I made.

I don't see how faithful transmission of a signal over long distance would have been initially important?

[CharonY]

In the oldest organisms signaling occurs chemically, where diffusion is the time limiting step. Once distances became longer, different solutions were needed. Neurons are a rather early in the evolutionary tree and has remained mostly unchanged. (I am not sure why you think transmission speed would be significant? Unless you are misunderstanding my point of signal transmissions pre-neurons).

 

My objection to trying to explain mechanisms based on evolutionary usefulness is that it creates an interesting narrative, which usually is not testable and typically is more a distraction. Ultimately they are also not terribly useful as the mechanism itself and their properties tell us much more.

Edited March 1, 2015 by CharonY

[studiot]

 

Charon Y

In the oldest organisms signaling occurs chemically, where diffusion is the time limiting step. Once distances became longer, different solutions were needed. Neurons are a rather early in the evolutionary tree and has remained mostly unchanged. (I am not sure why you think transmission speed would be significant? Unless you are misunderstanding my point of signal transmissions pre-neurons).

 

My objection to trying to explain mechanisms based on evolutionary usefulness is that it creates an interesting narrative, which usually is not testable and typically is more a distraction. Ultimately they are also not terribly useful as the mechanism itself and their properties tell us much more.

 

 

Then please look more carefully at the facts of what I actually said.

 

Yes I offered some pleasant chitchat background about a possible evolutionary connection.

 

But the main point I wanted to make was entirely contemporary.

 

If you find it difficult to follow the logic try it the other way round.

 

Work in terms of distance.

 

Given that the distance from you ears to your brain is about 70mm, something travelling three times as fast will arrive at the same time from a distance of 210mm.

 

It would be impossible for you to react to any change in audible signal that originated within this distance, within the transit time.

[CharonY]

 

Then please look more carefully at the facts of what I actually said.

 

Yes I offered some pleasant chitchat background about a possible evolutionary connection.

 

But the main point I wanted to make was entirely contemporary.

 

If you find it difficult to follow the logic try it the other way round.

 

Work in terms of distance.

 

Given that the distance from you ears to your brain is about 70mm, something travelling three times as fast will arrive at the same time from a distance of 210mm.

 

It would be impossible for you to react to any change in audible signal that originated within this distance, within the transit time.

 

While this is true, it offers no explanation on the properties of the system. Merely that it is fast enough to deal with that particular example. I would agree that if the question was whether the transmission speed is too slow to deal with external stimuli. However, the question stated in OP is actually complicated as it conflates several properties of the stimulus, including information depth which technically is not related to signal speed at all. (except maybe localization)

The evolutionary comment was not directed specifically to your comment though, as on this board many explanations about physiology is waved way by "we evolved that way" or we have selective pressure X that made things that way, which still does not explain how it works.

 

Related to the question by OP the relevant mechanism is the fact that our sensory neurons are tuned within to specific properties of the stimulus in question (wavelength, amplitude, frequency etc). at that the combined action of these as well as processing on the way to the brain as well as in the brain itself offers the dynamic range that we are able to perceive. Granted, this is also a very simple explanation, but the discussion of specifics would easily fill a lecture. My main point is thus: read up on physiology. It is an absolutely fascinating topic.

Edited March 1, 2015 by CharonY

[studiot]

The OP was right, there is a long term transmission bottleneck due to mismatch of transmission speed. The best speed attainable is obviously that of the slowest link in the chain.

This must result in information loss during sustained transmission.

 

Hence my point about our auditory system being sensitive to change, which is short term.

 

Note that I am using physics rather than biology (which you are far more expert in than I) and continuing that theme, I mentioned music.

 

The above is rather similar the the (IMHO sneaky) american method of rating audio power amplifiers in terms of 'music power'.

 

The american method rates the amplifier at power levels it can only attain for milliseconds and cannot possibly sustain. This allows the advertisers to claim hugely unrealistic power output ratings.

This has some justification in that much of music depends on sudden large amplitude changes that only last for short periods.

 

So just like our auditory system is fast enough for sudden large (important) changes, so the amplifier can respond correctly to say a drum.

[CharonY]

If I understand you correctly you are talking about sampling rate (or the equivalent)? Now this is a hugely interesting aspect, but my knowledge is very limited. You may think that the transmission speed is the limiting factor in distinguishing time-dependent differences, yet it is not so. To simplify a lot (and keep in mind that this is outside my expertise, so it may be incorrect), the brain is able to integrate information with a higher resolution than the transmission speed. The trick is that unlike an amplifier it does not in real-time. Instead the brain integrates information and then uses cues in the signal to backdate the stimulus when it happened. Of course you would not be able to react to that, but you would be able to distinguish two stimuli that were given shortly after each other. Data suggest that we are able to distinguish visual stimuli 5ms seconds apart, which is close to the theoretical maximum firing speed of neurons (I believe). However, this is only possible if sufficient cells are stimulated, indicating that the answer of cell population has to be taken into account and not just the activation time. Hence, for complex stimuli the required time can be longer (something for like 20-30 ms for faces, I believe).

 

However the latency of the afferent pathways (i.e. from eyes to the visual cortex) seems to be around 30-100 ms, IIRC. In other words, the discriminatory power is higher than the speed of transfer (and much faster than the ability to respond with an action). This is some of the stuff that I have referred to as trickery because biology does not like to do things easy (bloody nature).

[MayIKnow?]

Lost 45 minutes of reply to CharonY.

Studiot wrote: .. there is a long term transmission bottleneck due to mismatch of transmission speed. The best speed attainable is obviously that of the slowest link in the chain.

This must result in information loss during sustained transmission.

 

 

The other reason why I think there must be significant amount of data lost due to the bottleneck is working of 24fps motion picture. While in nature our eyes receive light the whole time (second, in this context) the eyes only need slices of 24 frames in a second to reconstruct indistinguishable normal motion. Though we can see the lag or missing information of a rotating fan blades in a 24fps movie those information could be reconstructed by increasing the frame rate. The increase of frame rate can go one until the fastest event is captured and played back exactly how we would perceive them in nature. However, no matter how many frame rate we use, we will reach a point that our eyes will reach a point where it cant utilize the higher frame rate. This is the point why I think why our eyes and ears discard many information. If that is the case, what we are seeing and hearing may not look or sound as they appear to our brain.

When it comes to sound perception, we cannot use the about example related to visual decoding because we cannot freeze sound like a picture. There must be continues stimulation for us to perceive sound. This stimulation comes in the form of sound pressure.

Sound pressure is “Sound pressure or acoustic pressure is the local pressure deviation from the ambient (average, or equilibrium) atmospheric pressure, caused by a sound wave –Wiki” . This pressure is the product of vibration of molecules in wave like motion. Speed of sound is determined by the speed of the waves travel through the medium. Finally these waves in the form of vibrating molecules hit our ear drum.

1) The speed they hit our ear drum would be 340m/s.

2) The millions of nerve cells would transfer the information with the speed of not more than 120m/s to the brain.

3) The distance of ear to brain doesn’t matter as the information gets slowed before reaching the brain. It doesn’t matter how many cells are undertaking the job to transfer the sound to the brain.

4) Sound is vibration and when the rate of vibration changes should it also change in the characteristic of the sound?

On item (1) above, I am not sure if the speed of the sound and vibration of the molecules frequency are the same. I believe they must be lower. So if the vibration frequency is lower than what contributes to the speed of the sound?

Thank you.

[Acme]

...On item (1) above, I am not sure if the speed of the sound and vibration of the molecules frequency are the same. I believe they must be lower. So if the vibration frequency is lower than what contributes to the speed of the sound?

 

Thank you.

What 'contributes' to the speed of sound is the medium in which it propagates. The frequency is a function of how fast the vibration is that initiates the sound. If we suffer any lost information in hearing, it is in the frequencies and amplitudes that our ear is not responsive to. Edited March 2, 2015 by Acme

[StringJunky]

 

...When it comes to sound perception, we cannot use the about example related to visual decoding because we cannot freeze sound like a picture.

I think this is not correct. The cochlear is the exact equivalent of the retina. Think of the sum total of vibrational perturbations in the cochlear-fluid in any instant in time - this is the aural 'picture'. All the cilia register information about those perturbations in concert with each other simultaneously. The speed of sound is 4 times (1500 metres/second) in a fluid than it is in air, so it can propagate and be registered faster at the cilia than it is coming in.from outside the cochlear.

 

 

On item (1) above, I am not sure if the speed of the sound and vibration of the molecules frequency are the same. I believe they must be lower. So if the vibration frequency is lower than what contributes to the speed of the sound?

Propagation of Sound in Fluids. (PDF download) . Density of the medium the wave is travelling through.

Edited March 2, 2015 by StringJunky

[MayIKnow?]

........ The speed of sound is 4 times (1500 metres/second) in a fluid than it is in air, so it can propagate and be registered faster at the cilia than it is coming in.from outside the cochlear.

 

How about sound in when you are underwater? I have not experience underwater speakers in a swimming pool but to a person who claimed to have experienced it says they sound identical to the music that we hear in normal environment(air).

 

 

 

 

What 'contributes' to the speed of sound is the medium in which it propagates. The frequency is a function of how fast the vibration is that initiates the sound. If we suffer any lost information in hearing, it is in the frequencies and amplitudes that our ear is not responsive to.

 

This is another mystery like electricity that I will never able to understand. Even though electricity travels at almost the speed of the light the electrons hardly move. How and where these waves originates? Please don't answer this! Right now, I am interested to know if the slow conduction of our nerve system affects the tonal characteristics of the sound. Perhaps, 1 kHz may sound different to a cat since their nerve transmission is much faster than human.

Edited March 2, 2015 by MayIKnow?

[Acme]

...

[Acme]This is another mystery like electricity that I will never able to understand. Even though electricity travels at almost the speed of the light the electrons hardly move. How and where these waves originates? Please don't answer this! Right now, I am interested to know if the slow conduction of our nerve system affects the tonal characteristics of the sound. Perhaps, 1 kHz may sound different to a cat since their nerve transmission is much faster than human.

Roger rhetorical question; proceeding to not answer.

 

As to the rest, yes the perception of tonal character etcetera is dependent on our nervous system and yes cats [undoubtedly] hear differently. Even humans hear differently among individuals and as hearing is a function of our bodies then aspects of our bodies affect what we hear. This includes -but is not limited to- the speed at which nerves conduct impulses.

[StringJunky]

 

How about sound in when you are underwater? I have not experience underwater speakers in a swimming pool but to a person who claimed to have experienced it says they sound identical to the music that we hear in normal environment(air).

 

Yes it will sound the same, but the sound will reach him four times faster. To illustrate this: if the underwater speaker sends a series of short single-frequency pulses, one second apart, the first pulse would reach hime FOUR TIMES quicker than in air BUT the rate of the pulses will STILL be one pulse per second.

 

 

1 kHz may sound different to a cat since their nerve transmission is much faster than human.

No, because the received oscillations will still be a 1000 cycles PER SECOND, regardless of the transmission speed of a nerve. Kilohertz is frequency over time, and time or frequency does not change just because the medium, sample rate or nerve transmission speed does.

Edited March 2, 2015 by StringJunky

[MayIKnow?]

.........Even humans hear differently among individuals and as hearing is a function of our bodies then aspects of our bodies affect what we hear. This includes -but is not limited to- the speed at which nerves conduct impulses.

..........

No, because the received oscillations will still be a 1000 cycles PER SECOND regardless of the transmission speed of a nerve..

 

We can't agree on this?

[StringJunky]

We can't agree on this?

 

It appears not.

[Acme]

We can't agree on this?

You can no more know what something 'sounds like' to a cat then you can know what something 'sounds like' to me. The perception is dependent on the perceiver, not on the phenomena perceived.

[MayIKnow?]

You can no more know what something 'sounds like' to a cat then you can know what something 'sounds like' to me. The perception is dependent on the perceiver, not on the phenomena perceived.

 

With respect, I am not referring to subjective perception of an individual. What I am told by someone whose expertise is in the field of psychoacoustics that the speed of nerve transmission limits our ability to hear above 20kHz. Scientifically, could this imply that whatever sound we hear is processed and playback in our head at own biological predetermined speed?

 

Meaning what supposed to sound like a 33rpm record would sound like 10rpm to us. Since time is essentially the same then there must be significant amount of information discarded. Do I make sense?