The beautiful, mysterious science of how you hear | Jim Hudspeth

Transcriber: Joseph Geni Reviewer: Camille Martínez

譯者: Lilian Chiu 審譯者: Lo Hsien Huang

Can you hear me OK?

各位可以聽見我的聲音嗎？

Audience: Yes.

觀眾：可以。

Jim Hudspeth: OK. Well, if you can, it's really amazing,

好，如果可以， 那是很不簡單的事，

because my voice is changing the air pressure where you sit

因為我的聲音會改變 各位所處的氣壓，

by just a few billionths of the atmospheric level,

改變幅度只有大氣壓力的 十億分之幾而已，

yet we take it for granted

但我們卻覺得我們的耳朵理所當然

that your ears can capture that infinitesimal signal

可以捕捉到極微小的訊號，

and use it to signal to the brain the full range of auditory experiences:

並用它來向大腦示意 各種聽覺的體驗：

the human voice, music, the natural world.

人聲、音樂、大自然的聲音。

How does your ear do that?

耳朵是怎麼辦到的？

And the answer to that is:

這個問題的答案是：

through the cells that are the real hero of this presentation --

透過細胞，也就是 這場簡報的真正英雄——

the ear's sensory receptors,

耳朵的感覺受器，

which are called "hair cells."

稱為「毛細胞」。

Now, these hair cells are unfortunately named,

毛細胞雖然叫毛細胞，

because they have nothing at all to do with the kind of hair

但和我越來越少的那種毛

of which I have less and less.

一點關係也沒有。

These cells were originally named that by early microscopists,

早期的顯微鏡學家 為這些細胞命名，

who noticed that emanating from one end of the cell

他們注意到，該細胞有一端

was a little cluster of bristles.

會有一小簇短毛冒出來。

With modern electron microscopy, we can see much better

有了現代的電子顯微鏡， 我們能更清楚看到

the nature of the special feature that gives the hair cell its name.

讓毛細胞得到這個名字的特徵。

That's the hair bundle.

那是毛束。

It's this cluster of 20 to several hundred fine cylindrical rods

直立在細胞頂端的，

就是這簇纖細的圓柱條 多達二十到數百根。

that stand upright at the top end of the cell.

And this apparatus is what is responsible for your hearing me right this instant.

各位現在能聽到我的聲音， 就是拜毛細胞所賜。

Now, I must say that I am somewhat in love with these cells.

我必須要說， 我愛上這些細胞了。

I've spent 45 years in their company --

我和它們相處了四十五年——

(Laughter)

（笑聲）

and part of the reason is that they're really beautiful.

部分原因是它們真的很美。

它們有種美感的成分。

There's an aesthetic component to it.

Here, for example, are the cells

比如，這些是雞在聽的時候

with which an ordinary chicken conducts its hearing.

所使用到的細胞。

These are the cells that a bat uses for its sonar.

這些則是蝙蝠的聲納要用的細胞。

We use these large hair cells from a frog for many of our experiments.

我們有許多實驗都使用 這些青蛙的大型毛細胞。

Hair cells are found all the way down to the most primitive of fishes,

一直到最簡單的魚類身上 都可以找到毛細胞，

and those of reptiles often have this really beautiful,

而爬蟲類的毛細胞通常都會有種

很美麗且幾乎是結晶般的順序。

almost crystalline, order.

But above and beyond its beauty,

但是除了美麗之外，

the hair bundle is an antenna.

毛束也是一種天線。

It's a machine for converting sound vibrations into electrical responses

它是一種機器，能把聲音振動

轉換成大腦能夠解讀的電子反應。

that the brain can then interpret.

At the top of each hair bundle, as you can see in this image,

在這張影像上可以看到， 在毛束的頂端，

there's a fine filament connecting each of the little hairs,

有一條細絲將所有的毛連結起來，

the stereocilia.

叫做靜纖毛。

It's here marked with a little red triangle.

就是圖上用紅色三角形 標示出來的部分。

And this filament has at its base a couple of ion channels,

在這細絲的底部有一些離子通道，

which are proteins that span the membrane.

是由橫跨細胞膜的蛋白質所組成。

And here's how it works.

它是這麼運作的。

This rat trap represents an ion channel.

這個捕鼠器代表一條離子通道。

It has a pore that passes potassium ions and calcium ions.

它有一個孔，能讓 鉀離子和鈣離子通過。

It has a little molecular gate that can be open, or it can be closed.

它有個小型的分子門， 可以開啟也可以關閉。

And its status is set by this elastic band which represents that protein filament.

它的狀態由彈力帶來決定，

彈力帶代表蛋白質細絲。

Now, imagine that this arm represents one stereocilium

想像這隻手臂代表一條靜纖毛，

and this arm represents the adjacent, shorter one

這隻手臂代表鄰接的 靜纖毛，它比較短，

with the elastic band between them.

兩者之間有彈力帶連結。

When sound energy impinges upon the hair bundle,

當聲音能量撞擊到毛束時，

it pushes it in the direction towards its taller edge.

會把它推向比較高的那個邊緣。

The sliding of the stereocilia puts tension in the link

靜纖毛的滑動會讓連結產生張力，

until the channels open and ions rush into the cell.

直到通道打開，離子湧入細胞中。

When the hair bundle is pushed in the opposite direction,

當毛束被推向反方向時，

the channels close.

通道就會關閉。

And, most importantly,

最重要的是，

a back-and-forth motion of the hair bundle,

在有聲波時，毛束會產生

as ensues during the application of acoustic waves,

來來回回的運動，

alternately opens and closes the channel,

讓通道開開關關，

and each opening admits millions and millions of ions into the cell.

每次開啟就會讓 數百萬個離子進入細胞。

Those ions constitute an electrical current

那些離子會構成電流，

that excites the cell.

刺激細胞。

The excitation is passed to a nerve fiber,

刺激訊號會被傳送到神經纖維，

and then propagates into the brain.

接著傳導到大腦。

Notice that the intensity of the sound

要注意到，聲音的強度

is represented by the magnitude of this response.

會造成此反應有不同的程度。

A louder sound pushes the hair bundle farther,

較大的聲音會把毛束推得更遠，

opens the channel longer,

讓通道打開較長的時間， 讓更多離子進入，

lets more ions in

and gives rise to a bigger response.

也就會促成更大的反應。

Now, this mode of operation has the advantage of great speed.

這種運作模式的優勢 在於速度很快。

Some of our senses, such as vision,

我們的一些感官，如視覺，

use chemical reactions that take time.

要靠化學反應，會花時間。

And as a consequence of that,

造成的結果就是，

if I show you a series of pictures at intervals of 20 or 30 per second,

如果我給各位看一連串的圖片， 每秒播放二十或三十張，

you get the sense of a continuous image.

各位就會覺得影像是連續的。

Because it doesn't use reactions,

因為毛細胞不用化學反應，

the hair cell is fully 1,000 times faster than our other senses.

因此會比其他感官 快到足足一千倍。

We can hear sounds at frequencies as great as 20,000 cycles per second,

我們能聽到的聲音頻率 可高達每秒兩萬次振動，

and some animals have ever faster ears.

某些動物的耳朵更靈敏。

The ears of bats and whales, for example, can respond to their sonar pulses

比如，蝙蝠和鯨魚的耳朵能夠

對每秒十五萬次振動的

at 150,000 cycles a second.

聲納脈衝做出反應。

But this speed doesn't entirely explain why the ear performs so well.

但是這種速度無法完全解釋

耳朵的性能為什麼這麼好。

And it turns out that our hearing benefits from an amplifier,

結果發現，我們的聽覺 還受惠於一種放大器，

something called the "active process."

叫做「主動接收過程」。

The active process enhances our hearing

主動接收過程會強化我們的聽覺，

and makes possible all the remarkable features that I've already mentioned.

促成我剛才提到的那些非凡特色。

Let me tell you how it works.

讓我告訴各位它是怎麼運作的。

First of all, the active process amplifies sound,

首先，主動接收過程 會將聲音放大，

so you can hear, at threshold, sounds that move the hair bundle

連微弱到只將毛束移動 十分之三奈米的聲音，

by a distance of only about three-tenths of a nanometer.

你也可以聽見。

That's the diameter of one water molecule.

十分之三奈米等同於一個 水分子的直徑。非常驚人。

It's really astonishing.

The system can also operate

在一個非常廣大的動態範圍內，

over an enormously wide dynamic range.

這個系統都能運作。

Why do we need this amplification?

我們為什麼需要這種放大？

The amplification, in ancient times, was useful

古時，這種放大功能很有用，

because it was valuable for us to hear the tiger before the tiger could hear us.

因為有了它，我們就能在 老虎聽見我們之前，先聽見老虎。

And these days, it's essential as a distant early warning system.

現今，它的重要性則是 遠距的早期警報系統。

It's valuable to be able to hear fire alarms

珍貴之處在於可以聽見火災警報，

or contemporary dangerous such as speeding fire engines or police cars or the like.

或者現代的危險，比如

加速行駛的消防車、警車等等。

When the amplification fails, our hearing's sensitivity plummets,

放大功能失效時，

我們的聽覺敏感度會大大下降，

and an individual may then need an electronic hearing aid

我們就需要電子助聽器

to supplant the damaged biological one.

來補足受損的生理聽覺。

This active process also enhances our frequency selectivity.

主動接收過程也會強化 我們的頻率辨識度。

Even an untrained individual can distinguish two tones

沒有受過訓練的人也可以區別出

頻率差別只有 0.2% 的兩種音調，

that differ by only two-tenths of a percent,

which is one-thirtieth of the difference between two piano notes,

這個差別等同於鋼琴上 兩個音符差距的三十分之一，

and a trained musician can do even better.

受過訓練的音樂家 還可以區別得更細。

This fine discrimination is useful

這麼細緻的分辨力，用途在於

in our ability to distinguish different voices

我們可以區別出不同的聲音，

and to understand the nuances of speech.

並了解言談中的細微差別。

And, again, if the active process deteriorates,

同樣的，如果主動接收過程變差，

it becomes harder to carry out verbal communication.

進行言語溝通的難度也會增加。

Finally, the active process is valuable in setting the very broad range

最後，主動接收過程 還有一個價值，

就是設定我們的耳朵 能忍受的廣大聲音強度範圍，

of sound intensities that our ears can tolerate,

from the very faintest sound that you can hear, such as a dropped pen,

從你所能聽見最微弱的聲音， 比如筆掉在地上的聲音，

to the loudest sound that you can stand --

到能忍受的最大聲音——

say, a jackhammer or a jet plane.

比如電鑽或噴射機。

The amplitude of sounds spans a range of one millionfold,

聲音的振幅橫跨了 一百萬倍的範圍，

which is more than is encompassed by any other sense

超越任何其他感官所包含的範圍，

or by any man-made device of which I'm aware.

或任何我所知道的人造裝置。

And again, if this system deteriorates,

同樣的，如果這個系統變糟，

an affected individual may have a hard time

受到影響的人可能會 很難聽見最微弱的聲音，

hearing the very faintest sounds

or tolerating the very loudest ones.

或無法忍受最巨大的聲音。

Now, to understand how the hair cell does its thing,

為了瞭解毛細胞如何運作，

one has to situate it within its environment within the ear.

我們就必須要把它 放到耳朵內的環境中。

We learn in school that the organ of hearing

在學校，我們學到聽覺器官

is the coiled, snail-shaped cochlea.

是盤繞式蝸牛形的耳蝸。

It's an organ about the size of a chickpea.

這個器官的大小只有鷹嘴豆那麼大。

It's embedded in the bone on either side of the skull.

它位在頭骨兩側的骨頭中。

We also learn that an optical prism

我們也學過光學棱鏡

can separate white light into its constituent frequencies,

可以把白光拆開成 它的不同組成頻率，

which we see as distinct colors.

也就是我們所見的不同顏色。

In an analogous way,

同樣的，

the cochlea acts as sort of an acoustic prism

耳蝸的功能就類似聲學棱鏡，

that splits apart complex sounds into their component frequencies.

它能把複雜的聲音 拆開其組成頻率。

So when a piano is sounded,

所以，彈鋼琴時，

different notes blend together into a chord.

不同的音符會混合成一個和音。

The cochlea undoes that process.

耳蝸會逆轉這個過程。

It separates them and represents each at a different position.

它會把每個音符分開， 用不同的位置來代表它們。

In this picture, you can see where three notes --

在這張圖上，各位可以 看見三個音符——

middle C and the two extreme notes on a piano --

中央的 C 以及鋼琴上 兩極端的音符——

are represented in the cochlea.

在耳蝸中的對應位置。

The lowest frequencies go all the way up to the top of the cochlea.

最低的頻率會直達耳蝸的頂端。

The highest frequencies, down to 20,000 Hz,

最高的頻率，兩萬赫茲以上，

go all the way to the bottom of the cochlea,

會直達耳蝸的底部，

and every other frequency is represented somewhere in between.

其他的頻率則落在兩者之間某處。

And, as this diagram shows,

如圖所示，

successive musical tones are represented a few tens of hair cells apart

相鄰的音調在耳蝸的表面上

會相距幾十個毛細胞的距離。

along the cochlear surface.

Now, this separation of frequencies

這種頻率分離

is really key in our ability to identify different sounds,

就是我們能辨別不同聲音的關鍵，

because very musical instrument,

因為每一種樂器，每一個聲音，

every voice,

emits a distinct constellation of tones.

都會發出獨一無二的音調組合。

The cochlea separates those frequencies,

耳蝸能把那些頻率分離開來，

and the 16,000 hair cells then report to the brain

接著，一萬六千個毛細胞

會向大腦報告每種頻率出現的量。

how much of each frequency is present.

The brain can then compare all the nerve signals

接著，大腦可以比較 所有的神經訊號，

and decide what particular tone is being heard.

判定聽到的是哪一種特定音調。

But this doesn't explain everything that I want to explain.

但是這還無法解釋

我想解釋的一切。魔法在哪裡？

Where's the magic?

我已經告訴各位毛細胞 能做到哪些很了不起的事。

I told you already about the great things that the hair cell can do.

How does it carry out the active process

它要如何執行主動接收過程，

and do all the remarkable features that I mentioned at the outset?

並做到我一開始 提到的那些非凡特色？

The answer is instability.

答案是：不穩定性。

We used to think that the hair bundle was a passive object,

我們以前認為毛束是被動的物體，

it just sat there, except when it was stimulated.

沒有受到刺激的時候， 它就只是待在那裡。

But in fact, it's an active machine.

但是事實上，它是種主動的機器。

It's constantly using internal energy to do mechanical work

它經常在使用內部的能量

來做機械式工作 並強化我們的聽覺。

and enhance our hearing.

So even at rest, in the absence of any input,

所以，即使在休息時， 沒有任何輸入時，

an active hair bundle is constantly trembling.

主動的毛束也經常在震顫。

It's constantly twitching back and forth.

它經常會來回抽動。

But when even a weak sound is applied to it,

但當它接收到一個微弱的聲音時，

it latches on to that sound and begins to move very neatly

它就會開始配合那聲音， 以一對一的方式很整齊地移動，

in a one-to-one way with it,

and by so doing, it amplifies the signal about a thousand times.

這麼做便能將訊號放大一千倍。

This same instability also enhances our frequency selectivity,

同樣的不穩定也會強化 我們的頻率辨識度，

for a given hair cell tends to oscillate best

因為每個毛細胞 振盪的頻率都傾向

at the frequency at which it normally trembles

是它平常沒有受到刺激時的

when it's not being stimulated.

震動頻率。

So, this apparatus not only gives us our remarkably acute hearing,

所以，這個器官不僅 讓我們有非常敏銳的聽覺，

but also gives us the very sharp tuning.

也讓我們能敏銳辨別音調。

I want to offer you a short demonstration

我想要做一個相關的 簡短展示給大家看。

of something related to this.

I'll ask the people who are running the sound system

我會請控制聲音系統的人

to turn up its sensitivity at one specific frequency.

調高一個特定頻率的敏感度。

So just as a hair cell is tuned to one frequency,

就如同毛細胞會被 調整到一個頻率，

the amplifier will now enhance a particular frequency in my voice.

現在，放大器會強化 我聲音中的一個特定頻率。

Notice how specific tones emerge more clearly from the background.

留意去聽特定的音調如何 在背景中更清楚地浮現出來。

This is exactly what hair cells do.

這就是毛細胞的功能。

Each hair cell amplifies and reports one specific frequency

每一個毛細胞會針對 一個特定頻率做放大及回報，

and ignores all the others.

忽略其他頻率。

And the whole set of hair cells, as a group, can then report to the brain

所有的毛細胞集合起來， 便能向大腦報告

exactly what frequencies are present in a given sound,

我們聽到的聲音中有哪些頻率，

and the brain can determine what melody is being heard

接著大腦會判定 聽見的是什麼旋律，

or what speech is being intended.

或者這聲音是在說什麼。

Now, an amplifier such as the public address system

比如公共廣播系統 （擴音系統）這類放大器，

can also cause problems.

也可能會造成問題。

If the amplification is turned up too far,

如果放大過頭了，

it goes unstable and begins to howl

就會變得不穩定，開始出現爆音。

or emit sounds.

And one wonders why the active process doesn't do the same thing.

你可能會納悶，主動接收過程 為什麼不會變成這樣？

Why don't our ears beam out sounds?

為什麼我們的耳朵不會發出聲音？

And the answer is that they do.

答案是，其實會。

In a suitably quiet environment, 70 percent of normal people

在安靜程度適當的環境中， 有七成的正常人

will have one or more sounds coming out of their ears.

耳朵會有一種或多種聲音出來。

(Laughter)

（笑聲）

I'll give you an example of this.

讓我舉個例子。

You will hear two emissions at high frequencies

各位會聽到從正常人耳朵 發出來的兩種高頻率聲音。

coming from a normal human ear.

You may also be able to discern background noise,

各位可能也能夠察覺到背景噪音，

like the microphone's hiss,

如麥克風的嘶嘶聲、胃的咕嚕聲、

the gurgling of a stomach, the heartbeat, the rustling of clothes.

心跳、衣服的沙沙聲。

(Hums, microphone hiss, dampened taps, clothes rustling)

（嗡嗡聲、麥克風嘶嘶聲、 濕的水龍頭、衣服沙沙聲）

This is typical.

這很典型。多數耳朵 只會放出少量音調，

Most ears emit just a handful of tones,

but some can emit as many as 30.

但有些能放出多達三十種。

Every ear is unique, so my right ear is different from my left,

每一個耳朵都獨一無二， 我的右耳和我的左耳不同，

my ear is different from your ear,

我的耳朵和你的不同，

but unless an ear is damaged,

但是除非耳朵受損，

it continues to emit the same spectrum of frequencies

不然，在數年或甚至數十年間，

它會放出的聲波頻譜都不會變。

over a period of years or even decades.

So what's going on?

所以，這是怎麼回事？

It turns out that the ear can control its own sensitivity,

結果發現，

耳朵可以控制它自己的敏感度，

its own amplification.

自己決定放大程度。

So if you're in a very loud environment, like a sporting event

若在很吵的環境中，

如運動比賽或音樂會中，

or a musical concert,

you don't need any amplification,

就不會需要放大，

and the system is turned down all the way.

系統自己會完全關閉。

If you are in a room like this auditorium,

如果你在這個講堂的環境，

you might have a little bit of amplification,

可能會需要放大一點點，

but of course the public address system does most of the work for you.

但是公共廣播系統 都幫你把放大工作做得差不多了。

And finally, if you go into a really quiet room

最後，若進入到連針落地 都可以聽到的極安靜房間中，

where you can hear a pin drop,

the system is turned up almost all the way.

這個系統就會幾乎完全啟動。

But if you go into an ultraquiet room such as a sound chamber,

但是若進入超安靜的房間中，

如隔音室，這個系統 會自動調到最大值，

the system turns itself up to 11,

it goes unstable

它會變得不穩定， 並開始發出聲音。

and it begins to emit sound.

And these emissions constitute a really strong demonstration

這些發出的聲音，清楚呈現了

of just how active the hair cell can be.

毛細胞能夠有多主動。

So in the last minute, I want to turn to another question that might come up,

在最後，我想要談一個 可能會有人提出的問題，

which is: Where do we go from here?

那就是：接下來呢？

And I would say that there are three issues

我會說，有三個議題

that I would really like to address in the future.

是我將來會很想要探究的。

The first is: What is the molecular motor

第一：毛細胞能做到放大，

that's responsible for the hair cell's amplification?

背後的分子馬達是什麼？

Somehow, nature has stumbled across a system

大自然以某種方式 偶然發現了一個系統，

that can oscillate or amplify at 20,000 cycles per second,

它每秒能夠振盪或放大到

每秒兩萬次振動或甚至更高。

or even more.

That's much faster than any other biological oscillation,

這個速度比任何其他 生物振盪都更快，

and we would like to understand where it comes from.

我們想要知道它的源頭。

The second issue is how the hair cell's amplification is adjusted

第二個議題則是毛細胞如何針對

聲音情境來調整它的放大作用。

to deal with the acoustic circumstances.

Who turns the knob to increase or decrease the amplification

在安靜或吵鬧的環境中，

是什麼在控制放大作用的程度？

in a quiet or in a loud environment?

And the third issue is one that concerns all of us,

第三個議題與大家都有關，

which is what we can do about the deterioration of our hearing.

那就是：對於聽力退化， 我們能做什麼？

Thirty million Americans,

三千萬美國人及全世界至少四億人

and more than 400 million people worldwide,

have significant problems on a daily basis

在日常生活中都會 面臨一個明顯的問題：

with understanding speech in a noisy environment

在吵鬧環境中或在電話中 很難聽懂對方的言談。

or over the telephone.

Many have even worse deficits.

許多人的狀況還更糟。

Moreover, these deficits tend to get worse with time,

此外，隨時間， 情況還會繼續惡化，

because when human hair cells die,

因為當人類的毛細胞死亡後，

they're not replaced by cell division.

它們不會因細胞分裂而再生。

But we know that nonmammalian animals can replace their cells,

但是我們知道非哺乳類 動物的毛細胞能再生，

and those creatures' cells are dying and being replaced throughout life,

那些生物的死亡毛細胞 都會一直再生，

so the animals maintain normal hearing.

所以這些動物能維持正常的聽力。

Here's an example from a little zebra fish.

用小斑馬魚為例來說明。

The cell at the top will undergo a division

最上方的細胞會進行分裂，

to produce two new hair cells.

產生出兩個新的毛細胞。

They dance for a little bit,

它們會先搖擺一陣子，

and then settle down and go to work.

接著就會定下來，開始工作。

So we believe that if we can decode the molecular signals that are used

我們相信，若我們能解出 這些動物的毛細胞

by these other animals to regenerate their hair cells,

再生時所使用的分子訊號為何，

we'll be able to do the same thing for humans.

我們就能夠讓人類 也做到同樣的事。

And our group and many other groups are now engaged in research

我們的團體及許多其他團體 現在致力於研究中，

trying to resurrect these amazing hair cells.

試圖讓這些不可思議的 毛細胞能復活。

Thank you for your attention.

謝謝各位聆聽。

(Applause)

（掌聲）