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

Transcriber: Joseph Geni Reviewer: Camille Martínez

Translator: Ellie Siu Reviewer: Thomas Tam

Can you hear me OK?

你哋聽到嘛？

Audience: Yes.

觀眾：聽到

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

講者：聽到已係唔簡單

because my voice is changing the air pressure where you sit

因為我嘅聲音正喺到改變緊 你哋周圍嘅空氣壓力

by just a few billionths of the atmospheric level,

但只係大氣壓力十億分之幾

yet we take it for granted

咁就係理所當然

that your ears can capture that infinitesimal signal

耳仔會接收到呢啲微弱嘅聲音訊號

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

仲將呢啲聽覺嘅體驗，傳畀大腦

the human voice, music, the natural world.

而裡邊夾雜咗人聲 音樂同大自然嘅聲音

How does your ear do that?

耳仔究竟係點樣聽到嘢？

And the answer to that is:

答案就係：

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

透過某啲細胞 亦即係呢次演講嘅英雄——

the ear's sensory receptors,

佢哋係喺耳仔裡嘅聽覺接收器

which are called "hair cells."

稱為「毛細胞」

Now, these hair cells are unfortunately named,

睇嚟佢呢個名無啦啦咁改咗

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

但係同我啲頭髮越來越少

of which I have less and less.

係完全無關係

These cells were originally named that by early microscopists,

呢個名係由早期顯微鏡學家改嘅

who noticed that emanating from one end of the cell

佢哋發現咗啲細胞嘅一邊

was a little cluster of bristles.

有執小鬣毛

With modern electron microscopy, we can see much better

如果我哋用而家嘅電子顯微鏡 仲會睇得更清楚

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

睇到符合「毛細胞」呢個名嘅特徵

That's the hair bundle.

就係呢執毛

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

佢哋係由二十、到多達數百條 纖細嘅圓柱條所組成

that stand upright at the top end of the cell.

直立喺細胞嘅頂頭

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

你哋家陣可以聽到我嘅聲音 就係拜呢啲毛細胞所賜

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

我一定要講我已愛上咗毛細胞喇﹗

I've spent 45 years in their company --

我同佢哋，已經相處咗四十五年——

(Laughter)

（笑聲）

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

部分原因、因為佢哋真係好鬼靚

There's an aesthetic component to it.

有種美感

Here, for example, are the cells

譬如話

with which an ordinary chicken conducts its hearing.

喺雞用嚟聽嘢嘅細胞

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

喺蝙蝠聲納裝置所用到嘅細胞

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

我哋喺好多實驗度 都用到嘅青蛙大型毛細胞

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

甚至乎最簡單嘅魚類身上 都搵到呢啲毛細胞

and those of reptiles often have this really beautiful,

而嗰啲爬蟲類嘅毛細胞 通常都有呢種好靚

almost crystalline, order.

近乎結晶嘅排列

But above and beyond its beauty,

除咗靚之外

the hair bundle is an antenna.

毛束都係一種天線

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

佢都係一種機械，將聲音振動

that the brain can then interpret.

轉變成大腦能夠解讀嘅電流反應

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

由呢張相，睇到毛束嘅頂頭

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

有一條細絲將所有嘅細毛連結起嚟

the stereocilia.

呢條叫靜纖毛

It's here marked with a little red triangle.

就係圖上用紅色三角形指住嗰部分

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

靜纖毛嘅底部，有啲離子嘅通道

which are proteins that span the membrane.

係由橫跨細胞膜嘅蛋白質所組成

And here's how it works.

佢嘅運作係咁

This rat trap represents an ion channel.

呢個捕鼠器代表一條離子通道

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

佢有個小毛孔 能夠俾啲鉀離子同鈣離子通過

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

又有個小型嘅分子閘門 可以打開，也可以關閉

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

閘門狀態係由蛋白質彈性帶所帶動

Now, imagine that this arm represents one stereocilium

想像呢隻手臂係代表一條立體纖毛

and this arm represents the adjacent, shorter one

呢隻手臂代表另一條 鄰近比較短嘅立體纖毛

with the elastic band between them.

兩條纖毛中間、又有彈力帶連結

When sound energy impinges upon the hair bundle,

喺聲音能量撞擊到毛束時

it pushes it in the direction towards its taller edge.

聲音嘅能量會推向比較高那邊

The sliding of the stereocilia puts tension in the link

立體纖毛嘅滑動 會使連接處產生張力

until the channels open and ions rush into the cell.

直至通道打開，離子湧入細胞度

When the hair bundle is pushed in the opposite direction,

喺毛束被推向反方向時

the channels close.

通道就關閉

And, most importantly,

最重要嘅係

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

喺有聲波嗰時

as ensues during the application of acoustic waves,

毛束會產生來來回回嘅運動

alternately opens and closes the channel,

構成通道嘅開開合合

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

每次開啟就會俾 數百萬嘅離子進入細胞

Those ions constitute an electrical current

嗰啲離子就會構成電流

that excites the cell.

刺激啲細胞

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.

然後大腦

Notice that the intensity of the sound

要注意嘅係，聲音嘅強度

is represented by the magnitude of this response.

會造成唔同程度嘅反應

A louder sound pushes the hair bundle farther,

較大嘅聲音會將毛束推得更遠

opens the channel longer,

令通道打開較長時間

lets more ions in

等更多離子進入通道

and gives rise to a bigger response.

造成更大嘅反應

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

呢種模式嘅優勢，就係運作速度好快

Some of our senses, such as vision,

我哋嗰啲感官，例如視覺咁

use chemical reactions that take time.

要靠化學反應，係需要時間

And as a consequence of that,

造成嘅結果就係

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

如果我給大家看一連串嘅圖片 每一秒鐘播放二十或三十張

you get the sense of a continuous image.

各位就會覺得影像係連續嘅

Because it doesn't use reactions,

因為毛細胞無使用化學反應

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

佢會比其他感官，快足足一千倍

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

我哋可以聽到每秒鐘 高達兩萬次嘅聲音頻率

and some animals have ever faster ears.

有啲動物嘅耳仔更加靈敏

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

譬如，蝙蝠同鯨魚嘅耳仔

可以接收每秒鐘十五萬次 聲納脈衝嘅振動同時作出反應

at 150,000 cycles a second.

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

但呢種速度都無辦法解釋 點解耳仔嘅性能會咁好

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

研究結果顯示我哋嘅聽覺 仲受惠於一種放大效果

something called the "active process."

叫做「主動接收過程」

The active process enhances our hearing

主動接收過程會加強我哋嘅聽覺

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

造成我剛才提到，非同凡響嘅效能

Let me tell you how it works.

究竟佢係點樣運作嘅？

First of all, the active process amplifies sound,

首先，聲音會被放大

當聲音達到臨界點嘅時候 你可以聽到

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

連微小到只將毛束移動 十分之三奈米嘅聲音

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

That's the diameter of one water molecule.

嗰個長度等如一個水分子嘅直徑

It's really astonishing.

咁你話係唔係好驚人呢﹗

The system can also operate

呢個系統能夠運作喺

over an enormously wide dynamic range.

好寛闊嘅動態範圍

Why do we need this amplification?

我哋點解需要咁樣嘅放大效果？

The amplification, in ancient times, was useful

古時，呢種放大嘅功能好有用

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

能夠喺老虎聽到我哋之前 我哋就先聽到老虎嘅聲音

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

家陣佢嘅重要，係喺警覺性 能夠聽到遠程嘅預警系統

It's valuable to be able to hear fire alarms

包括聽到火警鐘喇

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

或者係現代嘅危險，譬如 超速行駛嘅消防車、或者警車之類

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

喺呢啲聲音嘅放大效果失靈嗰陣 我哋聽覺嘅敏感度會直線下降

and an individual may then need an electronic hearing aid

嗰時，我哋就需要一個電子助聽器

to supplant the damaged biological one.

替代受損咗嘅生理聽覺

This active process also enhances our frequency selectivity.

助聽器嘅主動接收過程 亦會加強我哋嘅頻率辨識度

Even an untrained individual can distinguish two tones

既使我哋沒有受過訓練都能夠分辨 兩個頻率差別只有 0.2% 嘅音調

that differ by only two-tenths of a percent,

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

呢個差別等同鋼琴上 兩個音符差距嘅三十分之一

and a trained musician can do even better.

受過訓練嘅音樂家 能分辨更微細嘅變化

This fine discrimination is useful

呢種細緻嘅分辨能力

in our ability to distinguish different voices

等我哋可以區別出唔同嘅聲音

and to understand the nuances of speech.

同感受到說話中微細嘅差別

And, again, if the active process deteriorates,

同樣咁，如果主動接收過程變差

it becomes harder to carry out verbal communication.

亦會增加語言溝通嘅難度

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

嚟到最後，主動接收過程 仲有一個價值

就係能夠設定我哋嘅耳仔 忍受廣濶嘅聲音範圍

of sound intensities that our ears can tolerate,

從你可以聽到最微弱 譬如一枝筆跌落地上嘅聲音開始

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

to the loudest sound that you can stand --

到可以忍受最強大嘅——

say, a jackhammer or a jet plane.

譬如電鑽或者噴射機發出嘅聲音

The amplitude of sounds spans a range of one millionfold,

聲音振幅跨越咗一百萬倍嘅範圍

which is more than is encompassed by any other sense

其他感官都比唔上

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

就我所知嘅任何人造裝置一樣比唔上

And again, if this system deteriorates,

同樣咁，如果呢個系統變差咗嘅人

an affected individual may have a hard time

可能會好難聽見最微小嘅聲音

hearing the very faintest sounds

or tolerating the very loudest ones.

或者無辦法忍受到最強大嘅聲音

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

而家為咗了解毛細胞點樣運作

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

我哋就必須了解 喺耳仔入面係乜嘢嘅環境

We learn in school that the organ of hearing

喺學校，我哋學到聽覺嘅器官

is the coiled, snail-shaped cochlea.

係盤繞蝸牛形嘅耳蝸

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

得鷹嘴豆咁大

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

喺頭骨兩側嘅骨頭中間位置

We also learn that an optical prism

我哋有學過光學棱鏡

can separate white light into its constituent frequencies,

將白光拆開成不同組成嘅頻率

which we see as distinct colors.

就係我哋所見到嘅唔同顏色

In an analogous way,

同樣咁

the cochlea acts as sort of an acoustic prism

耳蝸嘅功能就類似聲學棱鏡

that splits apart complex sounds into their component frequencies.

將複雜嘅聲音拆開做組成嘅不同頻率

So when a piano is sounded,

所以，彈鋼琴嗰時

different notes blend together into a chord.

唔同嘅音符會混合成一個和音

The cochlea undoes that process.

耳蝸會逆轉呢個過程

It separates them and represents each at a different position.

將每個音符分開 用唔同嘅位置來代表佢哋

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

喺圖上大家睇到三個音符——

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

中央 C 音同鋼琴兩個極端嘅音符——

are represented in the cochlea.

喺耳蝸靜止時對應音域嘅位置

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

最低嘅頻率會直達耳蝸嘅頂頭

The highest frequencies, down to 20,000 Hz,

最高嘅頻率，二萬赫以上

go all the way to the bottom of the cochlea,

會直達耳蝸嘅底部

and every other frequency is represented somewhere in between.

其他頻率則落喺兩者之間嘅某處地方

And, as this diagram shows,

喺圖中見到

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

相鄰嘅音調喺耳蝸表面上

距離會係幾十個毛細胞

along the cochlear surface.

Now, this separation of frequencies

呢種頻率嘅距離

is really key in our ability to identify different sounds,

就係我哋能辨別唔同聲音嘅關鍵

because very musical instrument,

因為每一種樂器

every voice,

每一個聲音

emits a distinct constellation of tones.

都會發出獨一無二嘅音調組合

The cochlea separates those frequencies,

耳蝸能將呢啲頻率分辨出來

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

然後，一萬六千個毛細胞 會向大腦報告

how much of each frequency is present.

每個頻率應該有多少

The brain can then compare all the nerve signals

接著，大腦比較所有嘅神經訊號

and decide what particular tone is being heard.

然後判定聽到嘅係邊一種特定嘅音調

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

但仲係無講清楚我想解釋嘅一切

Where's the magic?

魔法喺邊度？

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

我已經講過給大家聽 毛細胞能夠做到了不起嘅嘢

How does it carry out the active process

咁佢要如何執行主動嘅接收過程

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

才能做到我一開始提到 所有特出嘅功能？

The answer is instability.

答案係：變化無常

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

我哋之前認為毛束係被動嘅

it just sat there, except when it was stimulated.

無受到刺激嘅時候，佢就係喺處等

But in fact, it's an active machine.

事實上，毛細胞係主動嘅機器

It's constantly using internal energy to do mechanical work

佢一直使用內部嘅能量 嚟做機械嘅工作

and enhance our hearing.

加強我哋嘅聽覺

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

所以，即使係休息緊 無任何聲音進入

an active hair bundle is constantly trembling.

活躍嘅毛束經常響處震動

It's constantly twitching back and forth.

不斷咁來回抽動

But when even a weak sound is applied to it,

但當佢接收到一個弱小嘅聲音，

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

就會開始配合嗰個聲音，

以一對一嘅方式，一齊移動

in a one-to-one way with it,

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

咁樣做，會將訊號放大一千倍

This same instability also enhances our frequency selectivity,

呢個不穩定嘅變化 同時增強咗我哋嘅頻率辨識度

for a given hair cell tends to oscillate best

因為每個毛細胞嘅振盪

at the frequency at which it normally trembles

都傾向喺未受刺激嗰時嘅震動頻率

when it's not being stimulated.

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

所以，呢個器官不僅造成 我哋擁有無比嘅敏銳聽覺

but also gives us the very sharp tuning.

仲可以準確咁辨別音調

I want to offer you a short demonstration

我想做個有關嘅簡短示範給大家睇

of something related to this.

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

我會請控制音響嘅人

to turn up its sensitivity at one specific frequency.

調高一個特定頻率嘅敏感度

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

就好似毛細胞咁 被調教到一個頻率一樣

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

家陣放大器會加強 聲音中嘅一個特定頻率

Notice how specific tones emerge more clearly from the background.

請你哋留心聽呢個特定嘅音調 點樣喺背景度突顯出嚟

This is exactly what hair cells do.

呢個就係毛細胞嘅功能

Each hair cell amplifies and reports one specific frequency

每一個毛細胞會針對 一個特定嘅頻率做放大同回報

and ignores all the others.

但係忽略其他頻率

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

將所有毛細胞嘅接收集合起嚟 就可以向大腦報告

exactly what frequencies are present in a given sound,

聽到邊啲聲音頻率

and the brain can determine what melody is being heard

接著大腦會判別 聽見係乜嘢嘅旋律

or what speech is being intended.

或者講緊啲係乜嘢

Now, an amplifier such as the public address system

譬如公共廣播系統嘅放大器

can also cause problems.

都可能會造成問題

If the amplification is turned up too far,

如果聲音放大過頭

it goes unstable and begins to howl

就會變得唔穩定，開始出現爆音

or emit sounds.

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

你可能會問點解 毛細胞嘅接收過程唔會變成咁？

Why don't our ears beam out sounds?

點解我哋嘅耳仔唔會發出聲音？

And the answer is that they do.

答案係，其實佢地會

In a suitably quiet environment, 70 percent of normal people

喺適當安靜嘅環境度 有七成嘅正常人

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

耳仔會聽到一種或多種聲音

（笑聲）

(Laughter)

I'll give you an example of this.

等我舉個例子給你聽

You will hear two emissions at high frequencies

各位會聽到喺正常人耳仔裡邊 發出嚟兩種高頻率嘅聲音

coming from a normal human ear.

You may also be able to discern background noise,

仲有可以察覺到背景嘅噪音

like the microphone's hiss,

譬如麥克風嘅嘶嘶聲、胃嘅咕嚕聲

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

心跳聲、衣服嘅沙沙聲

（嗡嗡聲、麥克風嘶嘶聲 水龍頭滴水聲、同衣服沙沙聲）

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

This is typical.

係好典型嘅聲音

Most ears emit just a handful of tones,

多數耳仔只會發出少量音調

but some can emit as many as 30.

但係也有發出多達三十種音調

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

每隻耳仔都係獨一無二

無論我嘅左耳同右耳 我嘅耳仔同你嘅都係唔同

my ear is different from your ear,

but unless an ear is damaged,

除非佢哋受損

it continues to emit the same spectrum of frequencies

否則，喺數年、甚至數十年間

over a period of years or even decades.

佢都會發出相同嘅聲波頻譜

So what's going on?

咁係乜嘢嘅一回事呢？

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

研究結果顯示，耳仔可以控制 佢自己嘅敏感度

its own amplification.

同決定要放大嘅程度

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

如果喺好嘈雜嘅環境裡邊 譬如喺比賽緊嘅運動場或者音樂會

or a musical concert,

you don't need any amplification,

就唔需要放大聲音

and the system is turned down all the way.

系統就會完全關閉

If you are in a room like this auditorium,

如果你喺呢個講堂嘅環境

you might have a little bit of amplification,

聲音可能需要放大啲

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

但係公共廣播系統 都會幫你放大聲音，令你哋聽得到

And finally, if you go into a really quiet room

最後，如果你進入寧靜到 連枝針跌落地都聽得到嘅房間

where you can hear a pin drop,

the system is turned up almost all the way.

呢個系統就會幾乎全部開晒

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

但係若你進入到超寧靜嘅房間 譬如隔音室裡邊

the system turns itself up to 11,

呢個系統就會自動調節到最大

it goes unstable

會變得好唔穩定

and it begins to emit sound.

同時開始發出聲音

And these emissions constitute a really strong demonstration

嗰啲發出嚟嘅聲音，清楚咁話俾你知

of just how active the hair cell can be.

毛細胞嘅主動性能

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

最後，我想要講一個 可能會有人提出嘅問題

which is: Where do we go from here?

跟著會點？

And I would say that there are three issues

我會講有三方面

that I would really like to address in the future.

將來我會好想研究

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?

背後嘅分子摩打係乜嘢？

Somehow, nature has stumbled across a system

大自然以某種方式 偶然發現咗一個系統

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

佢每秒鐘能夠振盪或放大到

兩萬次頻率、或者更高

or even more.

That's much faster than any other biological oscillation,

呢個速度比任何其他生物嘅 振盪頻率都更快

and we would like to understand where it comes from.

我哋好想知道佢嘅來頭

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

第二方面，係毛細胞 如何針對聲音環境

to deal with the acoustic circumstances.

嚟調節佢嘅放大程度

Who turns the knob to increase or decrease the amplification

喺寧靜或者嘈雜嘅環境裡邊

in a quiet or in a loud environment?

係乜嘢喺到控制佢嘅放大程度？

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

第三方面同大家都有關

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

就係對於人類嘅聽力退化 我哋可以做啲乜嘢？

Thirty million Americans,

三千萬美國人同全世界至少四億人

and more than 400 million people worldwide,

都係日常生活度 面對一個明顯嘅問題：

have significant problems on a daily basis

with understanding speech in a noisy environment

喺嘈雜嘅環境裡邊、或係電話度 好難聽清楚對方講啲乜

or over the telephone.

Many have even worse deficits.

好多人嘅狀況仲會更差

Moreover, these deficits tend to get worse with time,

此外，隨著年齡嘅增長 情況會繼續惡化

because when human hair cells die,

因為當人類嘅毛細胞死咗

they're not replaced by cell division.

佢哋唔會用細胞分裂嚟再次生長

But we know that nonmammalian animals can replace their cells,

但係我哋知道非哺乳類動物 佢哋嘅毛細胞可以再生

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

呢啲生物嘅毛細胞 死咗後會一直再生

so the animals maintain normal hearing.

所以佢哋能夠一直維持正常嘅聽力

Here's an example from a little zebra fish.

我用小斑馬魚做個例子

The cell at the top will undergo a division

最上方嘅細胞會進行分裂

to produce two new hair cells.

產生兩個新嘅毛細胞

They dance for a little bit,

佢哋會先搖擺一陣

and then settle down and go to work.

之後就會定下嚟，開始工作

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

我哋相信，若果能了解 呢啲動物嘅毛細胞

by these other animals to regenerate their hair cells,

再生嗰時使用邊啲分子訊號

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

我哋就能夠喺人類做到同樣嘅事

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

我哋同好多其他團體，正努力研究

trying to resurrect these amazing hair cells.

嘗試創造嗰啲不可思議 可重生嘅毛細胞

Thank you for your attention.

多謝各位

(Applause)

（掌聲）