MANOUSH ZOMORODI, HOST:
And Mary Louise's hearing loss is actually pretty common.
JIM HUDSPETH: In our society, about 10% of the populace - that's 30 million people - have significant hearing problems. By the time that we're on the order of 70 years old, about a quarter of us have significant hearing loss. And by 80, it's more than half.
ZOMORODI: This is Jim Hudspeth.
HUDSPETH: I'm a professor at Rockefeller University in New York City, and I'm a neuroscience researcher. So I work particularly on hearing.
ZOMORODI: And Jim says to understand why hearing loss is so common, we need to understand how the ear works.
HUDSPETH: Oh, yeah. So here we go. Sound is, of course, a vibration in the air, and that's really obvious when a jet plane, for example, rattles a window.
(SOUNDBITE OF WINDOW RATTLING)
HUDSPETH: There's energy or power flowing through the air. Sound energy hits the eardrum. It moves three little bones in the middle ear. And finally, it causes pressures to change in the spiraling cochlea. That's our organ of hearing. The cochlea is about the size of a chickpea, a garbanzo bean. It's a little thing. There's one in each ear. And within the cochlea, there are 16,000 sensory receptors.
HUDSPETH: They're called hair cells.
ZOMORODI: Jim continues his explanation from the TED stage.
(SOUNDBITE OF TED TALK)
HUDSPETH: 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.
ZOMORODI: In a minute, Jim Hudspeth on how, over time, we lose those tiny hair cells in our ears and why chickens don't. On the show today, ideas about Sound and Silence. I'm Manoush Zomorodi, and you're listening to the TED Radio Hour from NPR. Stay with us.
It's the TED Radio Hour from NPR. I'm Manoush Zomorodi. And we were just hearing from neuroscientist Jim Hudspeth describing the sensory receptors in our ears - hundreds of microscopic hair cells that make up the hair bundle.
(SOUNDBITE OF TED TALK)
HUDSPETH: Now, I must say that I'm somewhat in love with these cells. I've spent 45 years in their company.
HUDSPETH: And part of the reason is that they're really beautiful. There's an aesthetic component to it. 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 a machine for converting sound vibrations into electrical responses that the brain can then interpret.
(SOUNDBITE OF MUSIC)
HUDSPETH: Those little bristles get tickled or moved by the sound energy. And when that happens, the cell develops an electrical response...
(SOUNDBITE OF ELECTRIC ZAPPING)
HUDSPETH: ...That it then communicates to the brain. So that's all the brain knows. These 16,000 cells each send information about a particular sound that flows into the brain. And the brain then says, OK, I heard a middle C.
(SOUNDBITE OF PIANO NOTE)
HUDSPETH: I heard whatever tone it happens to be.
ZOMORODI: Is it fair to say that these little hairs are antennas, or are they more like little amplifiers?
HUDSPETH: Yeah, well, now you've just exactly hit on the head what I've been doing for the last 40 years. It turns out that the hair cells - and indeed the hairs - are not just passive recipients of sound. Instead, each of them is a little amplifier that enhances the signals going in.
(SOUNDBITE OF TED TALK)
HUDSPETH: 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.
(SOUNDBITE OF WATER DROP)
HUDSPETH: 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.
(SOUNDBITE OF TIGER SNARL)
HUDSPETH: These days, it's essential as a distant early warning system. It's valuable to be able to hear fire alarms...
(SOUNDBITE OF DINGING)
HUDSPETH: ...Or contemporary dangers such as police cars or the like. 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. When the amplification fails, our hearing's sensitivity plummets.
ZOMORODI: And so what exactly is happening in Mary Louise's ear, for example - in all our ears - when we hear and then when we lose our hearing?
HUDSPETH: The answer is that that amplifier begins to burn out. Basically, any noise that's loud enough to be uncomfortable, to make your ears hurt is doing some damage to the cells of the ear. The little hairs no longer actively amplify the incoming sound, and therefore, hearing becomes harder and harder, particularly in places that are very - where sound is very faint or in crowded circumstances where sound is very confusing - consonants, for example. So the difference between buh (ph) and puh (ph) and things like that is somewhat subtle. And the high frequencies that are necessary to convey that information are the first thing to go. And so one begins to have trouble understanding speech. And then as lower and lower frequencies are affected, the difficulties become greater and greater.
ZOMORODI: So there is technology that can give people the choice as to whether or not they can resolve their deafness - cochlear implants - correct?
HUDSPETH: Exactly right. The idea of the implant is to replace the hair cells that have died. And I should say that when hair cells die in our ears, they are not replaced by cell division. So they're unlike the skin or the liver or other organs. When they're gone, they're not replaced. And that, of course, leads to cumulative damage. It all adds up. And this is also true for daily listening. People who listen for hours and hours a day continuously are quite likely to damage their ears.
ZOMORODI: You get one shot, basically.
HUDSPETH: You get one shot, as it stands. Of course, we're interested in trying to change that.
(SOUNDBITE OF MUSIC)
HUDSPETH: And this isn't totally ridiculous because mammals have the problem of no regeneration. But fish and amphibians, reptiles - all can regenerate their hair cells. In fact, some of these animals are losing them all the time and continuously growing new ones just as we grow new skin.
ZOMORODI: What about birds?
HUDSPETH: Birds can do it 100%. In fact, this is where it was first observed. People noticed in pigeons and then in chickens that they could take them to, say, a heavy metal concert...
HUDSPETH: ...Blast the ears really to oblivion and then, within days, new hair cells would begin to sprout. And within a few weeks, hearing was more or less back to normal. It's really extraordinary.
ZOMORODI: How would you explain why evolution would grant amphibians and reptiles, you said, and birds this quite extraordinary power?
HUDSPETH: Well, I think it's not the question so much of why they have it as why we've lost it because if you look, you know, at more primitive vertebrates - the earliest are fishes, then amphibians, then reptiles - each of them has it. Somewhere along the line, mammals lost it. And there is no certainty why we have lost that capacity. It's true of many forms of regeneration. We can't grow our hearts back. We can't grow nerve cells back. Part of the issue may simply be that we live longer. If you have cells that can readily regenerate, you also have cells that can more readily become cancerous because they have the chance to grow. So if you're a fish that's going to live two years, you may not spend much time worrying about cancer. But if you're a radio announcer who hopes to live 95 years, you have to take it into consideration.
ZOMORODI: As someone who understands how sound and hearing works more intimately than 99.9% of the rest of the population, what have you observed about your own hearing over the years?
HUDSPETH: Well, the principal thing as - I've observed is I was stupid in the 1960s...
HUDSPETH: ...But then, so was everybody. So I spent much too much time at loud concerts or with my head between two speakers turned all the way up, and I'm paying some of the price of that now.
ZOMORODI: That was you in the '60s. I'm thinking of me in the late '80s going - sitting front row at a Guns N' Roses concert and having that ringing in my ears for three days afterwards and - big mistake.
HUDSPETH: Yeah. It's worth it to hear Slash.
HUDSPETH: But I agree. You know, if that's the last thing you hear, you might rue the experience.
ZOMORODI: That's neuroscientist Jim Hudspeth. You can watch his full talk at ted.com.
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