This episode originally aired on Unexplainable

In the same way that optical illusions can trick our eyes, auditory illusions can trick our ears. This raises a serious question: what is hearing, and how much of it is actually made up by our brains? In this story from the Unexplainable podcast, we explore how our brains create the sonic world around us, and what it takes to harness that mysterious power. Featuring psychologist Diana Deutsch, hearing researcher Matthew Winn and author Mike Chorost.

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Original music by Wesley Slover

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View Transcript ▶︎

You’re listening to Twenty Thousand Hertz.

[music in]

When we look at colors, we all see things a bit differently. For example, remember that dress that became an internet phenomenon back in 2015?

[sfx clip: CNN - It’s the dress that has divided opinion online, pitting Taylor Swift against Julianne Moore.]

Some people swore it was blue, while others were convinced it was gold.

[sfx clip: TODAY - Everyone weighed in, from actors to restaurants… Kim Kardashian tweeting, “I see white and gold. Kanye sees black and blue. Who is color blind?”]

And no matter how much people argued about it, they just couldn't agree.

[sfx clip: Ellen DeGeneres - I know there are people out there that say they see blue and black, and those people are crazy [cheers].]

And that's because when it comes to colors, it's all a matter of perception. But for a long time, scientists didn't realize that the same is true for sounds. They thought if a bird was singing [sfx: bird song] or a piece of music was playing [pause for music] our brains would process that in the same way.

But then, in the 1970s, a researcher made a pretty surprising discovery about how we all hear sounds differently. I'll let Noam Hassenfeld from the podcast Unexplainable take it from here.

[music out]

Noam: For a lot of people figuring out what you're meant to do with your life is a long winding process. But for some lucky ones, a career path becomes clear in an instant.

Diana: Well, I've always been very interested in music. I spent all my time playing the piano and composing and so on.

Noam: For Diana Deutsch that moment happened back in the fifties, but it didn't go exactly how she imagined it.

[sfx: piano music]

Diana: My music teacher performed at the, um, BBC third program in the mornings.

Noam: She was playing piano in a trio.

Diana: And I was asked to be a page turner.

Noam: Essentially, she'd be turning the pages of the sheet music so her teacher wouldn't have to stop playing.

Diana: So I went up to BBC house and I was all of 16 at the time. And very excited about doing this. Noam: Diana had always dreamed of being a musician. So even just turning pages on the BBC felt like the big time.

Diana: What happened was I turned the first page. No problem. I turned the second page. No problem. When it came to the third page. Unfortunately my hand jerked on all the pages flew down onto the floor. The poor lady had to while playing the piano with one hand, pick up the pieces with the other one. I just, yeah, it was, it was a terrible experience.

Noam: Diana came face to face with her dream. And she knew with complete clarity that it wasn't for her.

Diana: It certainly made me realize that being a performing musician was probably not a good idea for me.

Noam: Instead of aiming for a career as a performer, Diana got into researching the psychology of music. Particularly how different people perceive sounds. And she was one of the first people to study this by generating synthesized tones, using enormous mainframe computers. One day in 1973, she was experimenting with playing two sequences at the same time.

Diana: And I had no idea what would happen, but I thought it would be interesting to try.

Noam: You can actually hear exactly what Diana heard back then, but only if you're listening on headphones. So if you have a pair around, now would be a good time to put 'em in

Diana: I started off with a high tone alternating with a low tone in one ear. And at the same time, a low tone alternating with a high tone in the other ear.

Noam: High- low on one side, low- high on the other.

Diana: And what I heard seemed incredible. I heard a single high tone in my right ear, that alternated with a single low tone in the left ear.

Noam: Both ears were getting high, low sequences, but she wasn't hearing them in both ears. She only heard high tones on the right and low tones on the left.

Diana: Just as a kind of knee jerk reaction, I switched the headphones around. And it made no difference to what I perceived. The high tones remained in my right ear and the low tones remained in my left ear.

Noam: If you have headphones on flip 'em around, there's probably no difference.

Diana: I went out into the corridor, pulled in as many people as I could. And by the end of that afternoon, I must have tested or I, I don't remember how many, but probably dozens of people. And most of them heard exactly what I heard.

Noam: Diana, literally couldn't believe it.

Diana: I was beside myself. It seemed to me that, you know, I'd entered another universe or I'd gone crazy or, or something. It just seemed that the world had just turned upside down.

[music out]

To understand what causes this effect, it’s important to remember what sound actually is.

Matthew: Sound is rapid changes in air pressure that happen when something is vibrating.

Noam: Matthew Wyn, Audiologist, University of Minnesota.

Matthew: So you can think of it in the same way that you think of a wave in a pond. None of the water particles move very far. They just sort of Bob up and down, but they set a whole wave into motion and it's like a domino effect moving through space.

Noam: This pressure wave travels through the air.

Matthew: And then, you know, a whole chain of events will set into motion in your ear.

Noam: The wave passes through the ear canal.

Matthew: The ear drum vibrates back and forth.

Noam: And a few little bones amplify that vibration, sending it deeper toward the cochlea. This spiral shaped organ in the inner ear, that's covered with thousands of hair cells.

Matthew: The cochlea is where the sensory cells are that pick up the sound and turn it into something the brain can use.

Noam: Pressure waves become electrical impulses, which are eventually interpreted as sound.

Matthew: So this sounds like a long, complicated process, but it's extremely fast. I mean, there's, there's no sense that's faster than hearing your ear can do this whole process thousands of times per second.

Noam: All of that, the pressure waves, the ear vibrations, the transformation to electrical impulses. That's the simple part. The part we know. The complicated part is pretty much gonna take up the rest of this episode. Because there's a difference between the pressure waves that enter our ears and what we actually end up hearing.

Matthew: If we actually perceived every different sound that came in, we would be utterly confused. Noam: Take Matthew's voice, for example.

Matthew: Even in, in the room that I'm in right now, I'm just in a room in my house. There are echoes All around me because anytime you have a flat surface on a table, a wall, a computer screen, anything the sound will in fact reflect off of it.

Noam: of these echoes bouncing around should theoretically make sounds really hard to locate in space.

Matthew: And so if we hear that and then hear another echo [sfx] coming from the wall on my right, and then I hear an echo coming off the ceiling and then my table.

Matthew: How would I know which direction the sound is coming from, it's coming from all directions. Noam: But our brain has an answer

Matthew: Thankfully, our brain knows, sounds only come from one direction and that's the only way the world makes sense.

Noam: In order to function in the real world our brain makes a guess.

Matthew: It perceives that first wave of sound coming in. And then every subsequent reflection of that sound it's like saying, "Okay, I can suppress you." Which is why a lot of people aren't even aware that there are echoes because our brain is so good at suppressing them.

Noam: Our brain essentially edits our auditory experience.

Matthew: The way I like to phrase it is that the brain is being nudged in a direction rather than just straight out reading the world.

Noam: Which is exactly what Diana stumbled across that day in the seventies, when she was flipping her headphones back and forth.

Diana: It just seemed that the world had just turned upside down.

Noam: These days, auditory, illusions aren't as unheard of as they used to be. But Diana's a big reason why. She's now a psychology professor at UC San Diego. And she's been using computer generated sounds to study the brain's editor for decades. With that first illusion, she discovered Diana thinks two parts of your brain are disagreeing. The parts that determine pitch and location. That's why you hear a high tone on one side and a low tone on the other, even though they're really on both sides. And after finding that first illusion, Diana, couldn't stop thinking about it.

Diana: Of course, I didn't sleep much that night. This can't be the only illusion that does this kind of thing.

Noam: Diana started wondering whether she could design other illusions to learn more about the brain's internal machinery.

Diana: In the same way as you know, if a piece of equipment such as a car breaks down, you can find out a lot about the way the car works just by fixing what went wrong.

Noam: So she started brainstorming.

Diana: I was sort of half asleep and I was imagining notes jumping around in space. And by The next morning they had sort of crystallized into what I named the scale illusion.

Noam: The scale illusion, like before this illusion consists of two tone sequences, one in each ear.

Diana: So there's one channel alone.

Noam: high notes, some low notes.

Diana: And then the other channel alone.

Noam: Some more high notes, some more low notes.

Diana: And then you hear them together again.

Noam: If you're listening on headphones, you're probably hearing all the high notes on one side and all the low notes on the other, even though those notes are actually jumping from left to right. That's your brain editing the sounds it's separating them to reflect the way the world usually is.

Diana: In the real world, one would assume that sounds that are in a higher pitch range are coming from one source and sounds in a lower pitch range are coming from another source.

Noam: So that's what the brain assumes is happening here.

Diana: The brain reorganizes the sounds in space in accordance with this interpretation.

Noam: Just like removing echoes, this kind of brain editing would normally help you make sense of the world. But Diana's illusion is explicitly designed to fool the brain into making a wrong guess and not everyone's brain makes the same guess.

Diana: Lefthanders as a group are likely to be hearing something different from righthanders as a group.

Noam: Righthanders tend to hear high tones on the right side, but for lefthanders it's more complicated. They're likelier than other people to hear high tones on the left or in even weirder ways. All of this reorganization, the way the brain edits are hearing to help us navigate the real world it's sometimes called top down processing.

Diana: Top down processing occurs when the brain uses expectation, experience and also various principles of perceptual organization to influence, um, what is perceived.

Noam: Instead of bottom up processing, which is sensing the world and then having that travel up to the brain top down processing means that our brain is influencing how we hear.

Diana: To some extent, our brain is hearing what we are expecting to hear.

Noam: In a sense, a lot of what we perceive, isn't actually us hearing sound waves hit our eardrum. It's a prediction of what those waves should be. To illustrate this Diana uses something called the mysterious melody.

Diana: This is a well known tune, but the notes are presented in different octaves.

Noam: For all the non-music folks out there, an octave is basically a standard range of musical notes. In this illusion the notes stay the same, but which range they're played in changes. So instead of playing do-re-mi in the same range with all the notes next to each other [sfx], you could play do-re-mi with the notes, jumping into a different range [sfx].

Noam: So Diana takes a well known tune, doesn't change the melody, just changes the range.

Diana: And the question is, can people recognize this melody [sfx] and in fact people can't recognize the melody.

Noam: Now listen to a simplified version of the same sequence [sfx].

Diana: In this case, all the notes are in the same octave.

Noam: Same range [sfx].

Noam: You know what it is.

Diana: Yeah, indeed it's Yankee Doodle.

Noam: And a lot of times when people go back and listen to the scrambled version, they can hear Yankee Doodle in there [sfx]. Noam: When you have a frame of reference for what you're hearing, when you have an expectation, it actually changes what you're hearing. Illusions like this tend to circulate around the internet every once in a while. Like this one where, depending on which word you're thinking of, you might be able to hear either Laurel or Yanny.

[sfx clip: Jimmy Kimmel - Remember last year when that Laurel versus Yanni thing, everybody's going nuts over?. Well, there's a kiddie version of it making the rounds right now.]

Noam: This is from Jimmy Kimmel's show and he starts by pulling up a clip from Sesame Street, of all places.

[sfx clip: Sesame Street: I move it to follow you. Move the camera. Yes! Yes! That sounds like an excellent idea!

Jimmy Kimmel - And pay attention to this. And tell me if you hear a Grover say one of two things. "That sounds like an excellent idea," or "That's a f-ing excellent idea." Are you ready? Okay.

Sesame Street: Yes! Yes! That sounds like an excellent idea!

Jimmy Kimmel: Guillermo, what did you hear? //It's a f-ing excellent idea. //You heard that?// Yes, I did. 'Cause the first time I heard it, I didn't hear a curse word at all. And then the next 12 times I watched it, the F word was all I heard, but]

Noam: Just in case you want one more go at it, here's Grover, maybe making a lot of parents upset.

[sfx clip: Sesame Street - Yes! Yes! That sounds like an excellent idea!]

Noam: This type of misperception is true to an extent with all our senses, we've all seen visual illusions, or you might remember the debate around the dress, but Diana eventually found that the various ways our brain edits the world, they're not just due to hard coded differences like whether you're right or lefthanded.

Noam: Brain editing can vary from person to person based on life experience. To prove this, she asked listeners to determine whether a pattern is going up or going down [sfx]. For people who know a bit of music theory, this interval is a tritone, which is exactly half of an octave. So to get from note to note, you travel the same distance, whether you're going up or down. If you don't know that much about music, all you need to know is that this is a particularly ambiguous pattern [sfx].

Noam: But Diana does something really interesting in her experiment here. She plays the melody in a bunch of registers at the same time. So you might have an extra hard time figuring out if it's rising or falling.

Diana: And sure enough, you get huge differences from one individual to the other. And this is something that really does surprise people [sfx].

Noam: I hear it going up. And Diana found that other people hear it going up, but some people hear it going down. What's truly mind boggling is that Diana's found that the difference in how two people perceive this pattern, it might come down to where you grew up. Believe it or not, when Diana compared two groups, people from Southern England and people from California, she found that the English people tended to hear this pattern as rising [sfx].

Noam: Whereas the Californians heard that same pattern as falling [sfx]. Diana's hypothesis is that based on where you grow up, you tend to hear different pitches as low or high.

Diana: It has to do with the pitch range of the speech, to which you have been most frequently exposed, particularly in childhood.

Noam: So, if you hear that first pattern, which goes from the notes D to G sharp as falling [sfx], you probably hear this second pattern, which goes the exact same distance from the notes, A to D sharp, as rising or vice versa [sfx]. But ultimately the mechanics of all this are still pretty much a mystery scientists don't really know how all this brain editing happens.

Diana: I mean, we know that the brain does that, but we don't really know how.

[music in]

Noam: In a sense, it's almost like we're all listening to a play performed in our heads, just for us. There's a script, the entire world of pressure waves bouncing around, but how we actually hear it all is up to the performers.

If you give the same script to two different actors, they're going to perform it in two different ways. And if you give the same sound waves to two different brains, the way they interpret those signals might be vastly different from one another.

Most of the time, these performances happen without us ever noticing. But for some people, it becomes necessary to direct the performance themselves. So how do you train your own brain to interpret the sonic world the way you want it to?

That's coming up, after the break.

[music out]

MIDROLL

[music in]

Until the 1970s, most researchers believed that we all experienced the sonic world in basically the same way. Of course, they knew that hearing could get damaged, and fade with age. But when it came down to it, a car horn was a car horn, no matter who was hearing it.

But then, psychologist Diana Deutch discovered that hearing was far more subjective than anyone had thought. It turns out, our brains take the messy, complicated sounds around us, and translate them into a world we can understand. And since no two brains are exactly alike, that translation is a little different for everyone.

It's a highly complex process that we're still trying to understand. But when the world doesn't sound the way you know it should, being able to harness that magic becomes crucial.

[music out]

Mike: Okay. Testing one. And two, three testing.

Noam: This is Mike Chorost.

Mike: So I'd like to take the word chorus.

Mike: just add a T at the end. Noam: Mike's a science writer who was born with severe hearing loss, but he was able to use hearing aids and starting from when he was 15, he became obsessed with "Bollero," the famous piece by Maurice Ravel.

[music clip: Bolero by Maurice Ravel]

Mike: It was this riotous melange, with such a fascinating drumbeat underneath it all. It really thrilled me and fascinated me.

Noam: He particularly loved the way the melody would gradually evolve over the course of the piece.

Mike: Each repetition is on a higher level. It's louder. The resonance is deeper. Until you reach the climax, so it's a very auditorily, overwhelming piece of music. Noam: He would listen to Boléro over and over and over.

[music clip continued: Bolero by Maurice Ravel]

Mike: It was kind of my piece of music that I would come to again and again and again, to test out new hearing aids. So it's always been an auditory touchstone for me. Noam: And then one day in 2001, the limited hearing he still had started disappearing.

Mike: I was standing outsidea rent-a-car and I suddenly thought that my batteries had died. My hearing aid batteries.

Noam: Suddenly the traffic on a nearby highway started sounding different.

Mike: It was just that sound that you associate with cars going by, you know, vroom, vroom, vroom.

Mike: But all of a sudden it sounded, more like, woosh, woosh, woosh.

Mike: If somebody had dumped a whole bunch of cotton onto the highway.

Noam: Pretty soon Mike found out he was quickly losing what was left of his hearing.

Mike: It was like, my hearing was pouring outta my head, like water out of a cracked jar. So after about four hours after that initial realization, I was essentially completely deaf. It was just such a shocking experience.

Noam: But Mike was eligible to receive a cochlear implant. It's a surgically implanted device that can offer a form of hearing in some deaf people. Many people in the deaf community prefer to communicate using sign language or lip reading rather than using a cochlear implant. But for some people, especially people who've lost their hearing later in life and want to continue using their native spoken language, cochlear implants can be helpful tools.

Matthew: The cochlea is this tiny spiral shaped organ inside your head. And a cochlear implant is a string of electrodes that's carefully inserted inside that spiral organ.

Noam: This is Matthew again, the audiologist who actually works with cochlear implant users to help them understand their experience.

Matthew: There's this external part that looks like a hearing aid, but is not a hearing aid. It's a microphone and a computer that analyzes the sound and sends instructions to those electrodes that are inside the ear.

Noam: The implant, essentially bypasses a lot of the ear. It directly activates the cochlear, which then passes an electric signal onto the brain. But cochlear implants don't just reproduce normal hearing. Mike says that reducing sound to digital ones and zeros and beaming them directly into your brain, it can sound strange.

Mike: It was shocking is not at all what I expected.

Noam: When Mike's implant was turned on the first thing he did was listen to his own voice.

Mike: And my voice sounded really weirdly high pitched. I almost sounded like, mee-yaw, mee-yaw, mee-yaw. It was that kind of sound. It was like a, it was like, it was like a listening to a dimented mouse.

Noam: Matthew actually gave me a program he uses as an audiologist to simulate various types of cochlear implant sounds. So here's a general idea of what it might have sounded like to Mike [sfx].

Mike: It was very upsetting. I thought the world would sound pretty much like I heard with hearing aids just fuzzier. I was completely unprepared for the huge difference in pitches.

Noam: Because of the way the implants are designed, they tend to make everything seem a bit high pitched.

Mike: So when you send a signal to any part of the cochlear implant, the brain will interpret that as a high pitched sound, even if it's a low pitch. Noam: Which is why everything can sound all mousey.

Mike: But the interesting thing is within just a day or two, I started to hear low pitches again. And part of that, it was my brain adapting to it. My brain was saying, "Okay, this is my voice. I know it's supposed to be a low pitch. However, right now I'm hearing it as a high pitch. Never mind that because I know the low pitch, I'm going to interpret it as a low pitch."

Noam: Essentially Mike's brain was editing the world for him.

Mike: So very quickly my brain started figuring out, “Okay, the world sounds really weird, but I'm gonna try to fit that into my preconception, into what the world is supposed to sound like.”

Noam: He was taking command of his own top down processing.

Mike: So within hours, I stopped sounding like Mickey Mouse to myself. Noam: And then Mike started training.

Mike: I, I got, um, the audio books of the winnie the pooh books. And I remember the first time I put the tape into the cassette player and play winnie the pooh and some bees. I think that's the one.

Mike: [sfx] I couldn't make it out at all. It was just complete jibberish.

Noam: But he also had the physical book. So he read along with the tape.

Mike: So I was able to start matching up the weird input that I was getting with the words on the page that told me what that input meant.

[sfx clip: What about a story said, Christopher Robin. Could you very sweetly tell Winnie the Pooh one?]

Mike: This is what the S sounds like. This is what the phoneme Pooh sounds like [sfx].

Mike: So it is a process of re-mapping.

Noam: According to Matthew, this process of brain remapping is a pretty normal experience for cochlear implant users.

Matthew: Any good audiologist would say to someone, if they're thinking about a cochlear implant, that when you first get it and it first is activated, you probably won't understand much at all, but over the first six months, maybe the first year, your brain learns to reorganize how it associates sound with meaning.

Noam: Training's more accessible these days. It's certainly not as DIY as it was for Mike 20 years ago, but this kind of improvement can still be hard to believe.

Matthew: A lot of the people that I've worked with will say, now, when I listen to my spouse, it sounds like her voice, which baffles all of us who work in this field. Because if you look at how the ears being activated there's no explanation. I mean, not to be too on the nose, but it's unexplainable, right? So it's, there's no way that that could possibly be true. And yet a lot of people say it.

Noam: Tweaking settings on the implant does make it work better, but that doesn't account for most of this incredible improvement.

Matthew: a lot of the success of the cochlear implant is really a testament to how strong the brain is working rather than a reflection of high quality of the sound input. Noam: Our brains have an almost uncanny ability to predict language and fill in gaps even when we hear something muffled or distorted. But while cochlear implants work pretty well for speech, they don't work nearly as well for music. Music is just a much more complicated kind of sound you need to distinguish melodies and harmonies and textures and most fundamentally pitches and an implant only has a small number of electrodes.

Matthew: You have to simplify all the frequencies and you can think of it as like pixelating the sound.

Noam: Making this even harder because the cochlea is filled with fluid it's hard to use electrical pulses to stimulate the exact part, that codes for the right frequency. Instead, the pulses kind of spread out around the part that codes for that frequency.

Matthew: Let me make an analogy. Suppose you're playing a note on the piano [sfx]. You can be really careful and hit the exact key you want, or you can be kind of crude and put your whole hand down on the piano [sfx]. Like you're gonna be in the right ballpark of the note, but you're not gonna hit the exact note very clearly.

Matthew: So a cochlear implant is more like putting your whole hand down on the note. It's not a very press concise frequency you're hearing.

Noam: When you take all of this into account, translating music with a cochlear implant can seem almost impossible.

Matthew: The current design of cochlear implants, isn't set up really for music. It's set up to understand speech.

Mike: But I want my Boléro back.

[music clip: Bolero by Maurice Ravel]

Noam: Even though Mike's brain had learned how to edit those high pitched tinny sounds to understand speech music still wasn't the same.

Mike: It just sounded awful. I'm like, oh my God. You know, it was really shocking because like, even if it gets twice as good as this, it's still gonna be awful. Even if it gets three times, this it's still gonna be awful. It was really bad.

Noam: Mike upgraded the hardware of his cochlear implant. He upgraded the software. He even volunteered as a Guinea pig for some tests on new equipment.

Mike: So I would put on a set of headphones, I'd hear the set of beeps and boobs and like, okay, which song is that? [sfx: staticy Twinkle, Twinkle Little Star]

Mike: Like, I don't know. [sfx: staticy Twinkle, Twinkle Little Star] It was like, could anybody know. And for me, this was a very deeply frustrating time experiment, cuz I know twinkle, twinkle, little star. I was like that doesn't sound like twinkle, twinkle, little star to me. How could this sound like twinkle, twinkle, little star to anybody else?

Noam: Researchers I spoke to told me that some cochlear implant users just don't enjoy music that much. It's certainly harder to get used to than speech. And because patients are often told to focus more on improving, listening to speech, music can get left by the wayside. But appreciating music through an implant can sometimes be presented as an insurmountable obstacle.

Noam: You can see this in the movie, the sound of metal, where a musician gets a cochlear implant after losing is hearing, and then goes to this performance, listening to the song you're hearing right now [sfx]. In this scene, the movie shows what other people at the performance hear, and then it gradually shifts perspectives to highlight what the main character hears through his cochlear implant [sfx].

Noam: The performance is so upsetting for the main character that he ultimately takes his processor off. He essentially decides not to use his implant anymore.

Noam: You can find a lot of simulations online like this. So I asked Mike if these kind of simulations or even ones like the simulations I created of a distorted voice or a distorted Boléro for this episode, if they seem like accurate representations of what music sounds like through an implant.

Mike: I think you have to be extremely careful when listening to these simulations because, basically what those simulations are telling you is this is what the software is giving to the user. Okay. That's not the same thing as what the user hears. These are two very different things. You know, when I listen to these simulations and I have listened to them, it does sound a lot like what I heard on day one, it does not sound like what I hear in year 20.

Noam: For Mike, this was a combination of training himself with careful listening, but also tweaking the settings of the implant because with a lot of practice and effort and time, the experience of listening to music can improve.

Mike: Yeah, I would listen to music over and over again. And I would try tweaking different settings and I would go to my audiologist and I would say these pictures sound really fuzzy to me, can you do something about that? And so she would tweak how much electricity went to different electrodes. And so this was an iterative process that went on and it's still going on.

[music clip: Bolero by Maurice Ravel]

Noam: After years of upgrades, tweaks, training, Mike's noticed some real improvement, but not for all music.

Mike: Most of the pieces of music that I enjoy is music that I heard with hearing aids. It's familiar to me.

Noam: Mike does listen to some new music, but preferring familiar music, it's a pattern that Matthew notices with his patients too.

Matthew: And I think it's a testament to the brain filling in those gaps, conjuring the memory of what the sound quality should be. The implant sort of gives you just enough that the brain can put together the whole puzzle. Noam: And of course, Mike is listening to Boléro again.

Mike: Well, it sounds good. I really enjoy it, but there are things that I know that I'm missing. I know that I'm still not getting some of that, the intensity and the purity where the music is reaching for a crescendo in each of its iterations. So I know I'm missing that.

Noam: In a sense Boléro is so familiar, it's almost like language for Mike.

Mike: Boléro sounds really good to me because I know exactly what it's supposed to sound like. Noam: This new Boléro is certainly different from the version he remembers, but Mike loves the new version.

Mike: Even though the input I'm getting of Boléro is incomplete. And I can hear that it's incomplete. It is still a source of pleasure to me.

Noam: Ultimately, we don't really know exactly how our brain is able to do this. It can almost feel like magic, how it filters out echoes, how it shifts high tones to one ear and low tones to the other. How it can take a tinny, noisy input and rebuild a new version of Boléro.

Diana: We do this very complex calculation, but I don't think that we really know exactly how it's done.

Noam: Psychologist, Diana Deutsch again.

Diana: There are an awful lot of things about our hearing that we don't understand. And what we hear is often quite different from what in point of fact is being presented.

Noam: But we do know that the brain is constantly editing, shaping, and building the world that we hear. Our brain, our life experience, our familiarity with a piece of music, it all shapes how we hear and what we hear, which raises a pretty fundamental question.

Diana: When an orchestra performs a symphony, what is the real music?

Noam: Is it in the mind of the composer?

Diana: Or Is it in the mind of the conductor who has worked long hours to shape the orchestral performance?

Noam: Is it in the mind of someone in the audience, who's never heard it before and doesn't know what to expect?

Diana: And the answer is surely that there's no one real version of the music, but many, and each one is shaped by the knowledge and expectations that listeners bring to their experiences.

Noam: The idea that to a very real extent, our brains conjure different individual realities inside our heads, on the one hand, it's a clear reminder to be humble. Not just for hearing, no matter how certain we are, what we perceive isn't unfiltered reality. So it's worth questioning ourselves at our most stubborn moments. At the same time, though, how cool are brains? I know there are this perfect reminder of our own subjectivity and humility, but I also just can't get over the of fact that our brain puts on this firework show every day. And that a lot of people using a cochlear implant can tap into this almost magic ability to translate a few electrodes into this new emotionally satisfying experience without scientists really knowing how the whole thing works.

That story came from Unexplainable, a science podcast from Vox about everything we don't know. This episode in particular was part of their new series Making Sense, where they explore all six of the human senses. That's right, I said six. Every episode is intriguing, enlightening, and super highly produced. Subscribe to Unexplainable right here in your podcast player, or click the link in the show notes.

Twenty Thousand Hertz is produced out of the sound design studios of Defacto Sound. To hear our latest sonic creations, visit Defacto Sound dot com.

Noam: This episode was edited by Katherine Wells, Meredith Hoddinott and Brian Resnick. It was produced and scored by me, Noam Hassenfeld. Cristian Ayala handled the mixing and sound design with an ear from Efim Shapiro. Richard Sima checked the facts. Tori Dominguez is our audio fellow. Mandy Nguyen is keeping things sunny. And Byrd Pinkerton is dreaming of bioluminescence.

[music out]

We'll end the show on one final auditory illusion. Our brains are really good at recognizing patterns, which can make us hear things even when they're not really there. For instance, if you take a well known song and convert it into nothing but piano notes, people will start to think they're hearing vocals. That's because your brain fills in the words it expects to hear.

To show you what I mean, here's a clip of "We Will Rock You," by Queen.

[music clip: We Will Rock You by Queen]

Now, we can take that audio, and convert it into notes on a virtual piano. But after we do, there's a good chance you'll still perceive some ghostly vocals mixed in. Here it is.

[sfx clip: piano version of We Will Rock You]

Thanks for listening.