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  • 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,

  • that stand upright at the top end of the cell.

    就是這簇纖細的圓柱條 多達二十到數百根。

  • 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。

  • There's an aesthetic component to it.

    它們有種美感的成分。

  • 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

  • almost crystalline, order.

    很美麗且幾乎是結晶般的順序。

  • 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

  • that the brain can then interpret.

    轉換成大腦能夠解讀的電子反應。

  • 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,

  • Now, imagine that this arm represents one stereocilium

    彈力帶代表蛋白質細絲。

  • and this arm represents the adjacent, shorter one

    想像這隻手臂代表一條靜纖毛,

  • with the elastic band between them.

    這隻手臂代表鄰接的 靜纖毛,它比較短,

  • When sound energy impinges upon the hair bundle,

    兩者之間有彈力帶連結。

  • it pushes it in the direction towards its taller edge.

    當聲音能量撞擊到毛束時,

  • The sliding of the stereocilia puts tension in the link

    會把它推向比較高的那個邊緣。

  • until the channels open and ions rush into the cell.

    靜纖毛的滑動會讓連結產生張力,

  • When the hair bundle is pushed in the opposite direction,

    直到通道打開,離子湧入細胞中。

  • the channels close.

    當毛束被推向反方向時,

  • And, most importantly,

    通道就會關閉。

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

    最重要的是,

  • as ensues during the application of acoustic waves,

    在有聲波時,毛束會產生

  • alternately opens and closes the channel,

    來來回回的運動,

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

    讓通道開開關關,

  • Those ions constitute an electrical current

    每次開啟就會讓 數百萬個離子進入細胞。

  • that excites the cell.

    那些離子會構成電流,

  • The excitation is passed to a nerve fiber,

    刺激細胞。

  • and then propagates into the brain.

    刺激訊號會被傳送到神經纖維,

  • Notice that the intensity of the sound

    接著傳導到大腦。

  • is represented by the magnitude of this response.

    要注意到,聲音的強度

  • A louder sound pushes the hair bundle farther,

    會造成此反應有不同的程度。

  • opens the channel longer,

    較大的聲音會把毛束推得更遠,

  • lets more ions in

    讓通道打開較長的時間, 讓更多離子進入,

  • 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

  • at 150,000 cycles a second.

    對每秒十五萬次振動的

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

    聲納脈衝做出反應。

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

    但是這種速度無法完全解釋

  • something called the "active process."

    耳朵的性能為什麼這麼好。

  • The active process enhances our hearing

    結果發現,我們的聽覺 還受惠於一種放大器,

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

    叫做「主動接收過程」。

  • Let me tell you how it works.

    主動接收過程會強化我們的聽覺,

  • First of all, the active process amplifies sound,

    促成我剛才提到的那些非凡特色。

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

    讓我告訴各位它是怎麼運作的。

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

    首先,主動接收過程 會將聲音放大,

  • That's the diameter of one water molecule.

    連微弱到只將毛束移動 十分之三奈米的聲音,

  • It's really astonishing.

    你也可以聽見。

  • The system can also operate

    十分之三奈米等同於一個 水分子的直徑。非常驚人。

  • over an enormously wide dynamic range.

    在一個非常廣大的動態範圍內,

  • Why do we need this amplification?

    這個系統都能運作。

  • The amplification, in ancient times, was useful

    我們為什麼需要這種放大?

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

    古時,這種放大功能很有用,

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

    因為有了它,我們就能在 老虎聽見我們之前,先聽見老虎。

  • It's valuable to be able to hear fire alarms

    現今,它的重要性則是 遠距的早期警報系統。

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

    珍貴之處在於可以聽見火災警報,

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

    或者現代的危險,比如

  • and an individual may then need an electronic hearing aid

    加速行駛的消防車、警車等等。

  • to supplant the damaged biological one.

    放大功能失效時,

  • This active process also enhances our frequency selectivity.

    我們的聽覺敏感度會大大下降,

  • Even an untrained individual can distinguish two tones

    我們就需要電子助聽器

  • that differ by only two-tenths of a percent,

    來補足受損的生理聽覺。

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

    主動接收過程也會強化 我們的頻率辨識度。

  • and a trained musician can do even better.

    沒有受過訓練的人也可以區別出

  • This fine discrimination is useful

    頻率差別只有 0.2% 的兩種音調,

  • in our ability to distinguish different voices

    這個差別等同於鋼琴上 兩個音符差距的三十分之一,

  • and to understand the nuances of speech.

    受過訓練的音樂家 還可以區別得更細。

  • And, again, if the active process deteriorates,

    這麼細緻的分辨力,用途在於

  • it becomes harder to carry out verbal communication.

    我們可以區別出不同的聲音,

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

    並了解言談中的細微差別。

  • of sound intensities that our ears can tolerate,

    同樣的,如果主動接收過程變差,

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

    進行言語溝通的難度也會增加。

  • to the loudest sound that you can stand --

    最後,主動接收過程 還有一個價值,

  • say, a jackhammer or a jet plane.

    就是設定我們的耳朵 能忍受的廣大聲音強度範圍,

  • The amplitude of sounds spans a range of one millionfold,

    從你所能聽見最微弱的聲音, 比如筆掉在地上的聲音,

  • which is more than is encompassed by any other sense

    到能忍受的最大聲音——

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

    比如電鑽或噴射機。

  • And again, if this system deteriorates,

    聲音的振幅橫跨了 一百萬倍的範圍,

  • an affected individual may have a hard time

    超越任何其他感官所包含的範圍,

  • hearing the very faintest sounds

    或任何我所知道的人造裝置。

  • or tolerating the very loudest ones.

    同樣的,如果這個系統變糟,

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

    受到影響的人可能會 很難聽見最微弱的聲音,

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

    或無法忍受最巨大的聲音。

  • We learn in school that the organ of hearing

    為了瞭解毛細胞如何運作,

  • is the coiled, snail-shaped cochlea.

    我們就必須要把它 放到耳朵內的環境中。

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

    在學校,我們學到聽覺器官

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

    是盤繞式蝸牛形的耳蝸。

  • We also learn that an optical prism

    這個器官的大小只有鷹嘴豆那麼大。

  • can separate white light into its constituent frequencies,

    它位在頭骨兩側的骨頭中。

  • which we see as distinct colors.

    我們也學過光學棱鏡

  • In an analogous way,

    可以把白光拆開成 它的不同組成頻率,

  • the cochlea acts as sort of an acoustic prism

    也就是我們所見的不同顏色。

  • that splits apart complex sounds into their component frequencies.

    同樣的,

  • So when a piano is sounded,

    耳蝸的功能就類似聲學棱鏡,

  • different notes blend together into a chord.

    它能把複雜的聲音 拆開其組成頻率。

  • The cochlea undoes that process.

    所以,彈鋼琴時,

  • It separates them and represents each at a different position.

    不同的音符會混合成一個和音。

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

    耳蝸會逆轉這個過程。

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

    它會把每個音符分開, 用不同的位置來代表它們。

  • are represented in the cochlea.

    在這張圖上,各位可以 看見三個音符——

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

    中央的 C 以及鋼琴上 兩極端的音符——

  • The highest frequencies, down to 20,000 Hz,

    在耳蝸中的對應位置。

  • go all the way to the bottom of the cochlea,

    最低的頻率會直達耳蝸的頂端。

  • and every other frequency is represented somewhere in between.

    最高的頻率,兩萬赫茲以上,

  • And, as this diagram shows,

    會直達耳蝸的底部,

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

    其他的頻率則落在兩者之間某處。

  • along the cochlear surface.

    如圖所示,

  • Now, this separation of frequencies

    相鄰的音調在耳蝸的表面上

  • is really key in our ability to identify different sounds,

    會相距幾十個毛細胞的距離。

  • because very musical instrument,

    這種頻率分離

  • every voice,

    就是我們能辨別不同聲音的關鍵,

  • emits a distinct constellation of tones.

    因為每一種樂器,每一個聲音,

  • The cochlea separates those frequencies,

    都會發出獨一無二的音調組合。

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

    耳蝸能把那些頻率分離開來,

  • how much of each frequency is present.

    接著,一萬六千個毛細胞

  • The brain can then compare all the nerve signals

    會向大腦報告每種頻率出現的量。

  • and decide what particular tone is being heard.

    接著,大腦可以比較 所有的神經訊號,

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

    判定聽到的是哪一種特定音調。

  • Where's the magic?

    但是這還無法解釋

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

    我想解釋的一切。魔法在哪裡?

  • How does it carry out the active process

    我已經告訴各位毛細胞 能做到哪些很了不起的事。

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

    它要如何執行主動接收過程,

  • The answer is instability.

    並做到我一開始 提到的那些非凡特色?

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

    答案是:不穩定性。

  • it just sat there, except when it was stimulated.

    我們以前認為毛束是被動的物體,

  • But in fact, it's an active machine.

    沒有受到刺激的時候, 它就只是待在那裡。

  • It's constantly using internal energy to do mechanical work

    但是事實上,它是種主動的機器。

  • and enhance our hearing.

    它經常在使用內部的能量

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

    來做機械式工作 並強化我們的聽覺。

  • an active hair bundle is constantly trembling.

    所以,即使在休息時, 沒有任何輸入時,

  • It's constantly twitching back and forth.

    主動的毛束也經常在震顫。

  • But when even a weak sound is applied to it,

    它經常會來回抽動。

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

    但當它接收到一個微弱的聲音時,

  • in a one-to-one way with it,

    它就會開始配合那聲音, 以一對一的方式很整齊地移動,

  • 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.

    我會請控制聲音系統的人

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

    調高一個特定頻率的敏感度。

  • to turn up its sensitivity at one specific frequency.

    就如同毛細胞會被 調整到一個頻率,

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

    現在,放大器會強化 我聲音中的一個特定頻率。

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

    留意去聽特定的音調如何 在背景中更清楚地浮現出來。

  • Notice how specific tones emerge more clearly from the background.

    這就是毛細胞的功能。

  • This is exactly what hair cells do.

    每一個毛細胞會針對 一個特定頻率做放大及回報,

  • Each hair cell amplifies and reports one specific frequency

    忽略其他頻率。

  • and ignores all the others.

    所有的毛細胞集合起來, 便能向大腦報告

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

    我們聽到的聲音中有哪些頻率,

  • exactly what frequencies are present in a given sound,

    接著大腦會判定 聽見的是什麼旋律,

  • and the brain can determine what melody is being heard

    或者這聲音是在說什麼。

  • or what speech is being intended.

    比如公共廣播系統 (擴音系統)這類放大器,

  • Now, an amplifier such as the public address system

    也可能會造成問題。

  • can also cause problems.

    如果放大過頭了,

  • If the amplification is turned up too far,

    就會變得不穩定,開始出現爆音。

  • it goes unstable and begins to howl

    你可能會納悶,主動接收過程 為什麼不會變成這樣?

  • or emit sounds.

    為什麼我們的耳朵不會發出聲音?

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

    答案是,其實會。

  • Why don't our ears beam out sounds?

    在安靜程度適當的環境中, 有七成的正常人

  • And the answer is that they do.

    耳朵會有一種或多種聲音出來。

  • In a suitably quiet environment, 70 percent of normal people

    (笑聲)

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

    讓我舉個例子。

  • (Laughter)

    各位會聽到從正常人耳朵 發出來的兩種高頻率聲音。

  • I'll give you an example of this.

    各位可能也能夠察覺到背景噪音,

  • You will hear two emissions at high frequencies

    如麥克風的嘶嘶聲、胃的咕嚕聲、

  • coming from a normal human ear.

    心跳、衣服的沙沙聲。

  • You may also be able to discern background noise,

    (嗡嗡聲、麥克風嘶嘶聲、 濕的水龍頭、衣服沙沙聲)

  • like the microphone's hiss,

    這很典型。多數耳朵 只會放出少量音調,

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

    但有些能放出多達三十種。

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

    每一個耳朵都獨一無二, 我的右耳和我的左耳不同,

  • This is typical.

    我的耳朵和你的不同,

  • Most ears emit just a handful of tones,

    但是除非耳朵受損,

  • but some can emit as many as 30.

    不然,在數年或甚至數十年間,

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

    它會放出的聲波頻譜都不會變。

  • my ear is different from your ear,

    所以,這是怎麼回事?

  • but unless an ear is damaged,

    結果發現,

  • it continues to emit the same spectrum of frequencies

    耳朵可以控制它自己的敏感度,

  • over a period of years or even decades.

    自己決定放大程度。