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  • Voiceover: In order to distinguish between

  • the sounds of a base drum

  • and something that has a much higher frequency,

  • such as the sound

  • of a bee's wings flapping in the air,

  • your brain is relying on the cochlea,

  • in order to differentiate between

  • the two different sounds.

  • So, the difference between a base drum

  • and a bee's wings flapping in the air,

  • is the frequency.

  • So a base drum has a very low frequency,

  • whereas the wings of a bee,

  • when they're moving through the air very quickly,

  • have a very high frequency.

  • So as the information from a base drum beating,

  • or a bee's wings flapping,

  • comes into the ear,

  • they eventually hit the cochlea.

  • And we went into a lot of detail

  • about how exactly the sound wave is converted

  • into a neural impulse by the cochlea,

  • that eventually reaches the brain.

  • But now we're gonna go into how the cochlea distinguishes

  • between sounds of varying frequencies,

  • and how this distinction is maintained

  • all the way to the brain, in order for the brain

  • to be able to perceive different sounds.

  • So this is known as "Auditory Processing."

  • Your brain needs to be able to distinguish

  • between sounds of varying frequencies,

  • and you're actually able to hear things

  • with a frequency of 20 hertz,

  • all the way up to a frequency of 20,000 hertz.

  • So this is a huge range, and in order to distinguish

  • between sounds of low and high frequencies,

  • the brain uses the cochlea,

  • and particularly,

  • something known as "Basilar Tuning."

  • And the term "basilar" comes

  • from the basilar membrane,

  • which is inside the cochlea.

  • So inside the cochlea, there's actually a membrane

  • that contains a bunch of hair cells.

  • And if we were to unroll this cochlea,

  • if we took the cochlea and we unrolled it,

  • so it's normally rolled up like this,

  • if we unrolled it, so now it's flat,

  • there are varying hair cells.

  • So this would be the very base,

  • this is the base of the cochlea,

  • and this is the very apex, the very tip.

  • So the base would be right here,

  • the apex would be right here.

  • Now if we unrolled it,

  • and looked at which hair cells were activated,

  • given different sounds,

  • we would notice that hair cells at the very base

  • of the cochlea

  • were actually activated by very high frequency sounds,

  • and hair cells at the very apex of the cochlea

  • are stimulated by very low frequency sounds.

  • So let's look at another picture,

  • just to make things a little bit clearer.

  • So this picture basically just shows the cochlea unrolled.

  • And as I mentioned before, this would be the base

  • of the cochlea, I'll use a darker color.

  • This would be the base of the cochlea,

  • and this would be the very tip, or the apex

  • of the cochlea.

  • And hair cells are found all along the basilar membrane,

  • so this membrane right here is the basilar membrane,

  • and there are hair cells implanted inside of it,

  • there are a whole bunch of these hair cells.

  • And hair cells closer to the very base

  • respond to a very high frequency,

  • so this is 1,600 hertz.

  • And hair cells closer to the apex

  • respond to a lower frequency, so 25 hertz.

  • So this would be something like a base drum,

  • and something with a very high frequency,

  • would be something like a bee's wings flapping in the air.

  • So as sounds with varying frequencies reach the ear,

  • they will stimulate different parts

  • of the basilar membrane.

  • So if we have a base drum being played,

  • it has a pretty low frequency,

  • and it'll eventually go into the ear, reach the cochlea,

  • and it'll actually travel along this basilar membrane,

  • until it reaches the hair cell

  • that is attuned to that particular frequency.

  • So let's say,

  • that this is a frequency of 100 hertz for example.

  • The sound waves eventually cause fluid inside the cochlea

  • to travel in such a way,

  • that the hair cells that are very sensitive

  • to a frequency of 100 hertz,

  • which looks like it's right around here,

  • will actually activate.

  • And these hair cells will fire an action potential,

  • and this signal will eventually reach the brain,

  • and it will be mapped

  • to a very particular part of the brain.

  • So this right here is the brain,

  • and if you lift up this little piece of brain,

  • there is something known as the "Primary Auditory Cortex."

  • And the primary auditory cortex

  • is this blue region over here,

  • and it's basically responsible for receiving all

  • of the information from the cochlea.

  • And you can see that it's actually separated,

  • similar to how

  • the cochlea separated to various frequencies,

  • it's sensitive to various frequencies,

  • this primary auditory cortex is also sensitive

  • to sounds of various frequencies.

  • So, for example, this would be a part of the cortex

  • that receives information from hair cells

  • that are sensitive to a frequency of .5 hertz.

  • And this part of the auditory cortex receives

  • information from hair cells that are sensitive

  • to a frequency of 16 hertz.

  • And the reason that this is important,

  • is because the brain needs to be able to distinguish

  • between various sounds.

  • So if we had all the hair cells sensitive

  • to every single sound,

  • then whenever you heard any sound,

  • then all the hair cells would fire at once,

  • and they would send this huge signal to the brain,

  • and the brain wouldn't be able to distinguish

  • between different sounds.

  • So by having this basilar tuning,

  • the brain is able to differentiate

  • between sounds with a very high frequency,

  • and sounds with a very low frequency.

  • And this mapping,

  • so this mapping of sounds with a higher frequency

  • versus sounds of a lower frequency,

  • is known as Tonotypical Mapping."

  • And just to summarize,

  • we have sounds waves coming into the ear,

  • and different sound waves have different frequencies.

  • And we need to be able to distinguish

  • between the different frequencies.

  • So the sound waves come in,

  • they hit the cochlea,

  • and they will activate hair cells

  • in different parts of the cochlea.

  • So if it's a very high frequency sound,

  • it'll activate a hair cell over here;

  • if it's a very low frequency sound,

  • it'll activate a hair cell over here.

  • And these hair cells will actually send axons,

  • and these axons eventually all bundle together

  • to form the auditory nerve.

  • And the auditory nerve carries axons

  • from each hair cell inside the cochlea.

  • And the auditory nerve eventually

  • reaches the brain,

  • and will again separate its fibers,

  • and reach different parts of the brain.

Voiceover: In order to distinguish between

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B1 中級

聽覺處理 (Auditory Processing)

  • 42 5
    Liao Jess 發佈於 2021 年 01 月 14 日
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