RENEE MONTAGNE, HOST:
Of all our senses, hearing is the first to tell our brains what's going on. That's because our brains are finely tuned to use sound to interpret what goes on around us. Today, NPR's Christopher Joyce and audio producer Bill McQuay with the Cornell Lab of Ornithology conclude our series "Close Listening: Decoding Nature Through Sound" with this story on why we've evolved to listen.
CHRISTOPHER JOYCE, BYLINE: Evolution has a habit of teaching us what's important.
BILL MCQUAY: Or what not to step on.
JOYCE: And what we hear has always been high on that list. That's why parents are acutely attuned to the sound of their offspring. Animals that hunt use sound to collaborate. Animals that are hunted prick up their years.
MCQUAY: Listening is an almost universal sense, and it's become the subject of a relatively new field of science as a tool for understanding the world. The stethoscope, for example, in nineteenth-century technology, began to open up the unheard world. Scientists also learned you could listen to the Earth with a seismometer that measured vibrations in the ground.
JOYCE: That was right about the time of Krakatoa, a volcano that erupted in Indonesia. It was 1883. The sound of Krakatoa reached people 4,000 miles away. Scientists also tuned their ears to living things - sounds we can hear and some we cannot normally hear.
MCQUAY: Chris and I collected many of these animal sounds for NPR's radio expeditions program.
JOYCE: We traveled with biologists who showed us that there's more to wild sound than just, hey, here I am or hey, I need a mate. From insects to elephants, animals use sound to function and to converse in social groups. Biologist Laurel Symes, who listens to crickets, says there's a cacophony all around us that's full of information.
LAUREL SYMES: We think that we really know what's going on out there, and we're getting this tiny slice of all of the sounds in the world.
MCQUAY: Recently, scientists have been recording whole environments, not just the animals that live there. Some call it acoustic ecology - listening to the rain, streams, wind through the trees - in addition to the animals and insects. A particular forest has its own sound - so does a desert. Even a stream has its own distinct sound, depending on the time of year and what's living in it. If we listen, we can often hear how those environments are changing.
JOYCE: That's true. Every place has its sound. We're all swimming in it from fish to humans, and we are all shaped by it, which brings us to Seth Horowitz, a neuroscientist and author of a book on the hearing brain. He studied how sound - essentially vibration - has shaped the evolution of the brain.
SETH HOROWITZ: Vibration sensitivity is found in even the most primitive life forms.
JOYCE: Even bacteria.
HOROWITZ: Sound gives you a sensory input that is not limited by field of vision.
JOYCE: Sound is all around you, and you're listening all the time. Sound can travel a long way.
HOROWITZ: And it will propagate through anything.
JOYCE: Through the ground, through water even around corners. Given how well sound reflects what's going on around us, the brains of vertebrates - backboned animals like us - evolved to be exquisitely sensitive to it.
HOROWITZ: I mean, you hear anywhere from 20 to 100 times faster than you see, so that everything that you perceive with your ears is coloring every other perception you have and every conscious thought you have. Sound gets in so fast that it modifies all other input and sets the stage for it.
JOYCE: It can do that because the auditory circuitry in the brain is less widely distributed than the visual system, so sound signals don't have as far to travel in the brain. And sound gets routed to parts of the brain that deal with very basic functions - places where emotions are generated.
HOROWITZ: We're emotional creatures. Emotions are evolutionary, fast responses - things you don't have to think about.
JOYCE: Avalanche, landslide.
HOROWITZ: Hear that sound - get ready to run from it. Emotions become rapid response systems, and sound drives emotion.
JOYCE: So sound hits you in the gut.
MCQUAY: As the brain listens - and it's always listening - it's wired to seek patterns.
(SOUNDBITE OF MUSIC)
HOROWITZ: The brain is really a wet, sloppy drum machine. It's desperately seeking rhythms.
MCQUAY: We learn to recognize patterns in rhythms and pitch in voices or in nature. And when the pattern is broken...
(SOUNDBITE OF SCREAM)
MCQUAY: It gets our attention because that sound is different.
HOROWITZ: It's no longer regular, and it's becoming pseudo-periodic. And one of the examples of sounds like this is the fingernails-on-the-blackboard phenomenon.
(SOUNDBITE OF FINGERNAILS ON BLACKBOARD)
HOROWITZ: A sound that everybody hates.
JOYCE: And even as the brain is listening to the outside world, it's resonating with that sound like a tuning fork. You can actually hear the brain if you have the right equipment.
HOROWITZ: If you play a sound to a frog - frogs are very, very dependent on sound - you drop an electrode into their auditory nerve, you will hear the sound that the frog is hearing because it is so absolutely represented - change in the frequency or change in the pitch will be represented in how the nerves fire.
JOYCE: In fact, the working brain makes its own sounds continuously. Horowitz calls it a neuronal symphony.
HOROWITZ: It's sort of sounds like a well-tuned, old-school radio noise or crackling sound. You actually start hearing tonality, and you start hearing little songs.
JOYCE: Horowitz can even identify parts of the brain by their particular sounds.
MCQUAY: He's listened to just about anything you can hear on Earth. Now he started thinking about sound that's unearthly, literally. Space is full of electromagnetic radiation - just another form of vibration. Scientists have taken to turning it into sound to study it. This is the microwave radiation from the Big Bang when the universe was created.
JOYCE: And Horowitz wonders if there's intentional sound out there as well.
HOROWITZ: If we do find life on other planets - anything more complex than the simplest of microbes or viruses - we're going to find them having vibrational sensitivity.
MCQUAY: And they might just be making noise.
JOYCE: Maybe, says Horowitz, noise that we can recognize.
HOROWITZ: (Imitating instrument).
(SOUNDBITE OF MUSIC)
JOYCE: Keep listening.
MCQUAY: Who knows what you'll hear. I'm Bill McQuay.
JOYCE: And I'm Christopher Joyce, NPR News.
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