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  • Transcriber: Joseph Geni Reviewer: Camille Martínez

  • Can you hear me OK?

  • Audience: Yes.

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

  • because my voice is changing the air pressure where you sit

  • by just a few billionths of the atmospheric level,

  • yet we take it for granted

  • that your ears can capture that infinitesimal signal

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

  • the human voice, music, the natural world.

  • How does your ear do that?

  • And the answer to that is:

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

  • the ear's sensory receptors,

  • which are called "hair cells."

  • Now, these hair cells are unfortunately named,

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

  • of which I have less and less.

  • These cells were originally named that by early microscopists,

  • who noticed that emanating from one end of the cell

  • was a little cluster of bristles.

  • With modern electron microscopy, we can see much better

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

  • That's the hair bundle.

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

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

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

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

  • I've spent 45 years in their company --

  • (Laughter)

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

  • There's an aesthetic component to it.

  • Here, for example, are the cells

  • with which an ordinary chicken conducts its hearing.

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

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

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

  • and those of reptiles often have this really beautiful,

  • almost crystalline, order.

  • But above and beyond its beauty,

  • the hair bundle is an antenna.

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

  • that the brain can then interpret.

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

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

  • the stereocilia.

  • It's here marked with a little red triangle.

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

  • which are proteins that span the membrane.

  • And here's how it works.

  • This rat trap represents an ion channel.

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

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

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

  • Now, imagine that this arm represents one stereocilium

  • and this arm represents the adjacent, shorter one

  • with the elastic band between them.

  • When sound energy impinges upon the hair bundle,

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

  • The sliding of the stereocilia puts tension in the link

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

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

  • the channels close.

  • And, most importantly,

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

  • as ensues during the application of acoustic waves,

  • alternately opens and closes the channel,

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

  • Those ions constitute an electrical current

  • that excites the cell.

  • The excitation is passed to a nerve fiber,

  • and then propagates into the brain.

  • Notice that the intensity of the sound

  • is represented by the magnitude of this response.

  • A louder sound pushes the hair bundle farther,

  • opens the channel longer,

  • lets more ions in

  • and gives rise to a bigger response.

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

  • Some of our senses, such as vision,

  • use chemical reactions that take time.

  • And as a consequence of that,

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

  • you get the sense of a continuous image.

  • Because it doesn't use reactions,

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

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

  • and some animals have ever faster ears.

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

  • at 150,000 cycles a second.

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

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

  • something called the "active process."

  • The active process enhances our hearing

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

  • Let me tell you how it works.

  • First of all, the active process amplifies sound,

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

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

  • That's the diameter of one water molecule.

  • It's really astonishing.

  • The system can also operate

  • over an enormously wide dynamic range.

  • Why do we need this amplification?

  • The amplification, in ancient times, was useful

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

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

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

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

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

  • and an individual may then need an electronic hearing aid

  • to supplant the damaged biological one.

  • This active process also enhances our frequency selectivity.

  • Even an untrained individual can distinguish two tones

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

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

  • and a trained musician can do even better.

  • This fine discrimination is useful

  • in our ability to distinguish different voices

  • and to understand the nuances of speech.

  • And, again, if the active process deteriorates,

  • it becomes harder to carry out verbal communication.

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

  • of sound intensities that our ears can tolerate,

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

  • to the loudest sound that you can stand --

  • say, a jackhammer or a jet plane.

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

  • which is more than is encompassed by any other sense

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

  • And again, if this system deteriorates,

  • an affected individual may have a hard time

  • hearing the very faintest sounds

  • or tolerating the very loudest ones.

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

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

  • We learn in school that the organ of hearing

  • is the coiled, snail-shaped cochlea.

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

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

  • We also learn that an optical prism

  • can separate white light into its constituent frequencies,

  • which we see as distinct colors.

  • In an analogous way,

  • the cochlea acts as sort of an acoustic prism

  • that splits apart complex sounds into their component frequencies.

  • So when a piano is sounded,

  • different notes blend together into a chord.

  • The cochlea undoes that process.

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

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

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

  • are represented in the cochlea.

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

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

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

  • and every other frequency is represented somewhere in between.

  • And, as this diagram shows,

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

  • along the cochlear surface.

  • Now, this separation of frequencies

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

  • because very musical instrument,

  • every voice,

  • emits a distinct constellation of tones.

  • The cochlea separates those frequencies,

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

  • how much of each frequency is present.

  • The brain can then compare all the nerve signals

  • and decide what particular tone is being heard.

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

  • Where's the magic?

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

  • How does it carry out the active process

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

  • The answer is instability.

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

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

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

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

  • and enhance our hearing.

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

  • an active hair bundle is constantly trembling.

  • It's constantly twitching back and forth.

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

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

  • in a one-to-one way with it,

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

  • This same instability also enhances our frequency selectivity,

  • for a given hair cell tends to oscillate best

  • at the frequency at which it normally trembles

  • when it's not being stimulated.

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

  • but also gives us the very sharp tuning.

  • I want to offer you a short demonstration

  • of something related to this.

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

  • to turn up its sensitivity at one specific frequency.

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

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

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

  • This is exactly what hair cells do.

  • Each hair cell amplifies and reports one specific frequency

  • and ignores all the others.

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

  • exactly what frequencies are present in a given sound,

  • and the brain can determine what melody is being heard

  • or what speech is being intended.

  • Now, an amplifier such as the public address system

  • can also cause problems.

  • If the amplification is turned up too far,

  • it goes unstable and begins to howl

  • or emit sounds.

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

  • Why don't our ears beam out sounds?

  • And the answer is that they do.

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

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

  • (Laughter)

  • I'll give you an example of this.

  • You will hear two emissions at high frequencies

  • coming from a normal human ear.

  • You may also be able to discern background noise,

  • like the microphone's hiss,

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

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

  • This is typical.

  • Most ears emit just a handful of tones,

  • but some can emit as many as 30.

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

  • my ear is different from your ear,

  • but unless an ear is damaged,

  • it continues to emit the same spectrum of frequencies

  • over a period of years or even decades.

  • So what's going on?

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

  • its own amplification.

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

  • or a musical concert,

  • you don't need any amplification,

  • and the system is turned down all the way.

  • If you are in a room like this auditorium,

  • you might have a little bit of amplification,

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

  • And finally, if you go into a really quiet room

  • where you can hear a pin drop,

  • the system is turned up almost all the way.

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

  • the system turns itself up to 11,

  • it goes unstable

  • and it begins to emit sound.

  • And these emissions constitute a really strong demonstration

  • of just how