The Science of Everything Podcast - Episode 78: Hearing

Episode Date: September 17, 2016

An explanation of the process of human hearing, covering the structure and function of the outer, middle, and inner ears, with a focus on explaining how the hair cells of the cochlea transduce mechanical motion into electrical signals that are interpreted as sound. I also discuss the mechanisms of sound localisation, some of the causes of deafness, and briefly outline the workings of the cochlea implant. Recommended pre-listening is Episode 53: Sound and Music.

You're listening to The Science of Everything podcast episode 78, Hearing. And I'm your host, James Fodor. So in this episode, we're going to look at the human hearing system and explain how we perceive sounds and interpret them. We'll look at the structure of the ear, moving from the outer, through the middle and the inner ear, talk a bit about the neural pathways and some of the studies of audio processing, so how we understand and interpret audio signals with the thought. focus on how we perceive the sound localizations at the direction and location of sound.

And we'll talk a little bit about the different types of deafness and the cochlear implants as well. Recommended pre-listing for this is episode 53 on sound and music, which will give you a few of the background concepts that will help, although it's not strictly necessary, but it might be beneficial. All right, so let's get started and talk about the structure of the year, beginning with the outer ear, refers to essentially the external visible parts of the ear. So the folds of skin and cartilage surrounding the outer ear canal are called the pinner. And if you sort of look at the outer ear, you'll notice that there is a sort of intricate structure of folds and grooves and so on, which if you think about it is kind of weird, why would these shapes be necessary?

Why don't we just have sort of a flat surface there like other animals do, actually? Humans are interesting because we generally can't move our ears around nearly as much as other mammals can. And actually, mammals in turn, are interesting because they have external ear structures, which reptiles don't. So it's not something we think about very often, but the external ear structure, the pinner and so on, does serve a function. And its purpose is essentially to gather sound and sort of direct it into the ear canal so that we can hear better. and the structure of fold patterning on the surface of the pinner is thought to contribute to reflecting and attenuating the sound waves so that first of all they're received and sort of concentrated better into the ear canal

but also it helps us to provide additional information about the direction that sounds come from by the way they interfere with each other and so on So anyway, the sound waves reflect off the pinner and are sort of concentrated down into the auditory canal, so they enter through the auditory canal, and vibrate the tympanic membrane, which is also known as the eardrum. So the eardrum marks the sort of divide between the outer and the middle ear. The eardrum is basically a membrane, so it's a sort of a flat, fleshy organ, I suppose. It's not exactly an organ, whatever you want to call it. It is, it can vibrate to different frequencies depending upon the frequency. of the sound that's causing it to vibrate, obviously.

Humans can hear frequency between a range of about 20 to 20,000 hertz, so 20 hertz up to 20 kilohertz, although there's some variability there. And that's going to depend in part upon the range of frequencies at which the eardrum is able to vibrate. Now, of course, when we're talking about frequencies here from the eardrum and later on, it's very rarely going to be the case that the vibration only occurs at a single pure frequency that wouldn't be very useful for extracting information wouldn't be very interesting music either

rather what occurs in practice is a complicated pattern of many different frequencies with each a sort of the full sound wave that we hear is comprised of the superposition of all of these individual frequencies each again at a different amplitude so some more of a some sounds have more high frequencies some more frequencies and the pattern, the rich sort of textured pattern of the frequencies constitutes the unique sound that we hear as speech or music or whatever else it is,

and it's the job of our brain essentially to process that and make sense of it. So that's what's actually happening, but subsequently we'll often talk about sort of the frequency as if there's only one, but of course bear in mind that that's really going to be the case. So on the inside of the tympatic membrane or the the eardrum, we have the middle ear. The middle ear is basically a hollow space that contains three small bones called ossicles. I believe these are actually the smallest bones in the human body. The bones are called the Malleus, the Incas, and the Stapies, which are also referred to by the English translation of these essentially, which is the hammer, the anvil and the stirrup.

And they get their names basically from their shape because they kind of look like a hammer hitting an anvil, which is then attached to a stirrup, sort of. But anyway, I'll put up a diagram of this so you can see what they look like. Basically, the malice is attached to the tympatic membrane, the eardrum. The ingus connects the malleus to the stapies, and the stapies is connected in turn to the overwindow of the cochlea, which we'll get to. That's the inner ear. So basically the stapies is connected to the inner ear.

So together, the three bones, the three obstacles, bridge the connect the outer ear to the inner ear. And their purpose is to transmit the sounds from the vibrations, from the tympanic membrane through to the cochlear. And in doing so, they also further concentrate the sound energy. So the size of the bone sort of decreases as you move from the tympanic membrane through the cochlear. They're by concentrating the energy into a smaller place and providing, essentially more of a signal that can be detected by the cochlear, by the inner ear. So the middle ear is crucial because the way we hear in the inner ear, which we'll get to, the way we actually transduce sound into electrical signals, is through movements of a fluid,

or a couple of fluids that exist in the cochlear of our inner ear. So we need to be able to translate density changes in air to move, of an incompressible fluid in the inner ear. And the three ossicles serve that role. So they transduce energy from movements in the tympanic membrane through to motion in the fluid of the inner ear. That's their purpose. There are a number of muscles that are actually connected to the ossicles,

which can contract in response to loud sounds, thereby basically reducing the freedom of motion of the ossicles as sort of stiffening them. And this reduces the degree to which sound is transmitted through the inner area, not completely removing it, but reducing the degree of transmission. Basically, that's going to reduce the loudness of the sound because the size of the vibrations is going to correlate with perceived loudness of the sound. This tightening or this contraction of the muscles leading to tightening or sort of stiffening of the osicles is called the acoustic reflex, and it occurs in response to loud sounds,

so that if sounds get very loud, the osicles sort of stiffen up, and we won't hear very loud, or the perceived loudness will be reduced, and that's a way of protecting our hearing, especially the hair cells, because if there's too much vibration of the hair cells, which, again, we'll get to in the inner ear, but basically if they vibrate too much, they'll be damaged. And so therefore the acoustic reflex is a way of reducing the transmission of sound energy thereby reducing the vibration of the hairstyles and protecting them from very loud sounds. The osicles can also become stiffened with age, and that's one cause of, or can be one cause of hearing loss in the elderly. So the osicles become sort of stuck to each other in a sense.

It becomes hard of them to move around, and so they're not as good at transducing sounds, and therefore hearing is lessened. The middle ear is hollow, apart from the obstacles, obviously, the obstacles exist in a hollow space, essentially, which means, so it's not filled with fluid like the inner ear is. So that means if you move into a higher altitude or conversely dive into water, the pressure differences between the middle ear and the outside

will become quite noticeable. And this is why you feel that pressure change in your ears when you're going up in a plane or if you dive. And that can actually become dangerous if the pressure difference becomes large because there's a risk of bursting or damaging the eardrum, the tympanic membrane, if the pressure difference isn't relieved. So you want to keep the pressure difference between the middle ear and the outer ear sort of not too large, otherwise it poses a danger. The eardrum or tympanic membrane can become ruptured. One cause of that, as I've just mentioned, is if there's very large pressure differences. It can also become ruptured due to infection or trauma.

There's various causes of it becoming ruptured. Or very loud sound can also cause rupture. Thankfully, though, it seems to be quite good at healing itself because in the large majority of cases, the eardrum will heal completely within a few weeks. There can be cases where it won't heal, especially if there's ongoing infection. But for the most part, perforated eardrums will heal by themselves,

although it will take a little bit of time. Okay, so that's a bit about the middle ear. Now let's move on to talk about the inner ear, which is the most complicated part of the ear. So this is the most, obviously, inward part of the ear, and it consists mostly of the cochlear, which will mostly be the focus of what we talk about today. The inner ear also includes three semicircular canals

at right angles to each other, which constitute the vestibular system and are important for balance. I'm not going to talk about those today because they're not really related to hearing. Instead, I'm going to just focus on the cochlear. But bear in mind that the vestibular system, these canals are technically speaking part of the inner ear.

So, what is the cochlear? Cochlear derives from the Latin word, which essentially means snail. And it's appropriate because it looks like a snail. It's got a sort of a head and then a coiled sort of, well, tails-slash, shell. So it really does look like a snail. The starpiece, remember, that's the smallest of the osicles, the bones in the middle ear, connects to, if you like, the head of the snail. And then

moving away sort of towards the inside of the head is the coiled tail, if you like. As I've said, the cochlear is coiled up. It's actually a single tube. You can think of it as like a tapering tube that's hollow on the inside, although it is filled with fluid. So unlike the inner ear. It's not hollow. It does have fluid inside it. But the cochlear is essentially a tapering tube which is coiled up on itself and winds inwards. However, for ease of discussion, I'm going to talk about it as if it's unwound. So you could sort of imagine undoing the spiral. So the cochlear now consists of a tapering tube. It's quite small, only about three centimeters long that is uncoiled. So it is not very big at all.

but it is quite intricate in its structure, and so it'll take a little bit to discuss. So the basic idea of the cochlear is, as I said, that it's a tapering tube. It's also described as a membranous labyrinth, which is a bit of a mouthful. That means it's made of membranes or consists of membranes, and it's sort of wound up and complicated and its structure. But we're imagining unwinding it and trying to simplify a bit just to get the basic idea of what's going on here. The basic idea is inside the cochlear are fluid-filled tubes, the pressure is transmitted by the stappies to these tubes which causes motion of the fluid in the tubes. This in turn leads to movement of parts of what are called hair cells, and we'll get to those.

I've mentioned those, we'll get to discussing them. This movement in hair cells in turn leads to the generation of electrical signals which are transmitted to the brain and interpreted as sound. So this is the process by which mechanical motion is transduced into electrical signals, which are interpreted by the brain as sounds. But that's the basic idea of what the inner ear does, and now we'll go through and walk through a bit more detail as to how that works. So you may recall that I said that the starpeys, which is the smallest of the osicles, the final bone in the three-bone sequence,

it connects with what's called the over window. Basically, you can think of that as the opening, of the cochlea, its opening or its boundary with the middle ear. Now, I say opening, it's not like it opens up and the fluid spills out. The overwindows a membrane which keeps the fluid in place. But the Starpiece connects to the oval window vibrating it, which in turn transmits these mechanical vibrations. through to the fluid which occupies the tubes inside cochlear.

If you think of the overwindow is sort of the entry point, again, nothing is physically moving into it. It's vibrations that are being transmitted, but you can think of it as kind of like the entry point. There is also sort of down the bottom, so lower down than the over window, is the exit point, which is the round window. It's similar to the overwindow, it's just a bit smaller. it's also a membrane on the cochlear, and this is necessary because the fluid that fills the ducts inside that the cochlear is essentially incompressible. So you wouldn't be able to move it around

if there was no scope for it to sort of bulge out at the other end. If you're pushing something inwards and that thing is incompressible, then there has to be an outwards out bulging to offset the in-pushing. So you can think of it that way. When the Starpiece pushes inwards on the over window, the round window or the fluid pushes outward on the round window, the membrane there, offsetting that. So the vibrations

travel through the cochlear around from the oval window out to the round window. The sort of tube that connect the two actually run right down to the tip of the cochlear. Remember we're thinking, we're unrolling the cochlear, so we're thinking of it as a tapered tube, so the tubes run down right to the tip and then up the other side.

What I've been calling a tube is actually called the vestibular canal. So the oval window opens up onto or forms the boundary of the vestibular canal, which then runs right down to the end of the cochlear, end of this tapered tube, and then up the other side. But when it runs up the other side, it's not called the vestibular canal. It's called the tympanic canal. So it sort of flips between the vestibular canal and the tympanic canal,

which are connected at the very end of the cochlear. I am simplifying the structure here, I should note, but I'm trying to get you to understand the basic idea. So what we've got, again, to summarize, is cochlear unwound, unrolled. It's a tube, a couple of tubes within a tube, really. It's a tapered tube. At the entry point where the startpiece connects, we've got the overwindow, which it's a membrane which marks the boundary of the vestibular canal,

which extends out to the end of our tapered tube, the cochlear, and connects into the tympanic canal, which then runs back up the cochlear, and exits at the round window, which is another membrane. sitting just sort of below the over window. So we go in and around and back up again. Now, I've talked about the vestibular canal and the tympanic canal, forming the sort of inward and outward segments, if you like, of the cochlear. But there is also a duct that sits in between these two

that's not connected either to the over window or to the round window. So what is this duct called and why is it there? This is the cochlear duct, and this is essentially where the hair cells live. So these hair cells are crucial for the transduction of the. actual electrical signals or the mechanical signals into electrical signals, which we'll talk about. So the hair cells don't sit in either the vestibular canals or the tympanus. They actually sit in between them in a third canal, if you like, or space called the cochlear duct.

And the auditory nerves, all of the neurons which transmit the signals to the brain, connect with the hair cells in the cochlear duct. Now, how does that mechanical vibration or movement get translated into the electrical signals which they're interpreted as sound? Well, that all happens through the hair cells which sit in the cochlear duct. More specifically, the hair cells sit on top of a membrane, which basically is, you can think of it as the roof of the Tepanic Canal. It's called the basilar membrane. It's the base, basically. It's the base where the hair cells sit.

Above the hair cells is something called the tectorial membrane. So you can think of it as if the hair cells are wedged between two membranes, tectoral at the top and basilar at the bottom. So basilar at the base. So what are these hair cells doing and what does the tectoral membrane that I keep talking about have to do with it? Well, as I said, the hair cells sit on, so the cell bodies are anchored to the basilar membrane,

which, remember, sits on top of the tympanic canal. So, of course, as the tympanic canal vibrates, as the fluid moves around in there, the hair cells are going to move as well. The basilar membrane is moved, the hair cells move. But it's not exactly the motion of the cell body of the hair cell that's crucial. Rather, there are these projections from the cell body, which form these crucial organelles called stereocilia. So I've talked about cilia in some of the episodes on cell structure

and function. I don't remember exactly when. But basically, these are projections of the cell. They kind of look like hairs, really, and that's why they're called these cells are called hair cells, because it looks like they have this hair on top of them. It's not literally hair, but it kind of looks like that, that these stereocilia kind of look like hair. So that's why they're called hair sets. Now, these stereocilia are lodged or stuck, if you like, or project into the tectoral membrane.

So this is really crucial here. The cell bodies of these hair cells just sit on the basal membrane. The cell bodies don't contact the tectorial membrane, but the stereocilia, which stick out up the top of them, do. So, I mean, the way I think of it as the hair cells are kind of like these heads that are sort of sitting on the basal membrane, and then their hair sticks up and is stuck into the tectorial membrane on top of them, which is a little bit of a weird way of thinking about it, sort of disembodied heads with their hair sticking out into the tectorial membrane. But, you know, it's a picture that you remember, hopefully.

So, hair cells between these two membranes. The tectorial membrane is held more or less in place. So when the basilar membrane vibrates as a result of the fluid moving about and the tympanic canal beneath it, the hair cell body moves. But the stereocilia stuck in the tectorial membrane are more or less fixed in place. Again, I'm simplifying here, but this will give you the sense of what's happening. So if the cell body is moving and the stereocilia are fixed in place,

obviously the stereocilia are going to be moved in relation to the body. There's going to be a shearing motion there. You can think of this again very loosely as if the, as if, you know, when you move the toothbrush back and forth on your teeth, the bristles are, the bristles bend and move back and forth. That's sort of what the stereocilia are doing as the, as the hair cells are moved on as a result of the motion of the basilar membrane beneath them. It could be up and down or side to side either way. It's moving around. So what happens is these bristles, as we think of them, or the stereocilia, are moving or bending in relation to the cell body of the hair cells. Well, basically, there are ion channels which sit on the seria cilia. Remember, ion channels are basically proteins that stick in the cell membrane, which can open and close allowing ions to move in and out. And this is crucial to the production of graded potentials and depolarization,

which is necessary for transmitting neural signals. If you don't know what I'm talking about here, I'd recommend consulting episode 38 on neurons and synapses. Probably should have mentioned that as a prerequisite. It would help for this part anyway. So these ion channels that I mentioned, particularly potassium ion channels, are mechanically gated, which means they're literally pulled open as the stereocilia move. They're actually connected by these sort of thin wire-like sort of things,

so if one is pulled open or sort of pull open the next one, it's almost like kind of like pulling a rope to open a trap door. Iron channels are physically pulled open by the shearing motion of the stereocilia moving in relation to the cell body. Now, when the mechanically gated potassium ion channels open, the potassium comes in, enters the cell, causes membrane polarization, so a differential charge between the inside and the outside of the cell. This, in turn, causes the opening of calcium ion channels that are located down in the body of the cell. So the calcium ion channels are different to the potassium ion channels. Potassium ion channels are up in the stereocelia, and they're mechanically gated. They're physically pulled open as the stereocytes.

very easily a move in relation to the cell body. These potassium ions don't directly lead to the transduction of electrical signals. Rather, they cause calcium ion channels that are voltage-gated, so respond to changes in electrical potential. The potassium causes these calcium-voltage-gated ion channels to open in the cell body, causing calcium ions to come in. These calcium ions then fuse with synaptic vesicles,

which are located in the cell body. These vesicles are just essentially a little sphere. inside the cytoplasm, which contain neurotransmitters. The calcium signals the vesicles and causes them to fuse with the membrane, releasing the neurotransmitter, which then travels across the synaptic duct and interacts with the synapse of an aphoran neuron, which carries the signal to the brain. So the crucial process here to understand is that the hair cells themselves don't fire action potentials. You remember action potential is a sort of a progressive change, sort of like a wave traveling along a wave of change potential along the membrane of an axon.

Sort of like the tail of a neuron, so the action potential travels along the tail of the axon and then synapses with another neuron, causing it to then fire an action potential. Hesnels don't fire action potentials, they don't have axons. Instead, they just directly release neurotransmitter to the affluent neuron, which then fire an action potential and takes the signal to the brain. So, again, that process is motion of the stereocilia, leading to opening of mechanically gated ion channels, leading to potassium flowing into the cell, leading to depolarization, leading to opening of calcium voltage-gated iron channels, leading to calcium interacting with the vesicles, which then fuse with the cell membrane,

which then leads to the release of the neurotransmitters which were sitting in the vesicles, which in turn leads to a signal firing in the synaptic. neuron which then carries the signal to the brain. So that's how mechanical vibrations are transmitted into electrical signals. It's all the hair cells. The hair cells do the job through there. Two different types of ion channels mechanically gated and then voltage gated, leading to a release of neurotransmitters and then setting the signal to lead to the firing of an action potential of the synapsed aphrine neuron, and these afferate neurons in turn are bundled into the cochlear nerve, which carries a signal to the brain.

Now that we finally managed to transduce an electrical signal from mechanical motion, let's talk about what happens to this signal and how it's processed. As always from sensory systems, we understand the sort of mechanical transduction part better than we understand the higher-level processing because that's a lot more complicated. But there's a bit we can say about it, and I'll just give a few interesting points here. There's much more that we could discuss, but this is just an introductory discussion, so we'll just hit on a few key points.

So you remember I said that once we've transduced the mechanical motion into electrical signals via the nerves, sorry, via the hair cells, those signals are sent to the brain. That's not quite correct because they don't transmit directly to the cortex, so the higher regions of the brain. They first synapse with the brain stem, which is part of the brain, but not sort of part of the higher brain. So first, this sound information is transmitted to the brain stem, which is responsible for a lot of sort of autonomic. processes, so things that we like digestion and breathing, for example, which we don't consciously think about, but nonetheless need neural signals to happen. So a lot of the auditory information is first transmitted here and then up to the higher regions of the brain, the cortex, the outer region of the brain, which is responsible for the more complex processing, including conscious

thought and decision-making and so on. The primary auditory cortex is located in the temporal lobes. That's sort of on the side of the head, and it's responsible for processing auditory information or the primary audio information processing region in humans. Neurons in the auditory cortex are organized according to the frequency of sound that they respond best to that they respond best to that frequency of sound. So this is called tonotopic mapping. So there's actually a direct relationship between the location of the neuron in the auditory cortex and the frequency response to. So higher frequencies, cells that respond best to higher frequencies will be located to one end and then sort of middle frequencies in the middle and then lower frequencies

on the other end. So there's a direct mapping. This is similar to what we saw in the visual cortex with mapping of the angles or the orientation of lines that cells respond to or of colors and things like that. We see similar tonatopic mapping in the auditory cortex. And this is thought to directly reflect the fact that the cochlear is also arranged according to sound frequency. So I didn't mention this before, but let's jump back for a moment. And if you recall the basilar membrane that I mentioned, that's the membrane that sits on top of the tympanic duct, and it's where the hair cells live. So the hair cells sit on top of the basilar membrane. The basilar membrane, however, is not the same along the entire length of the cochlear. Remember,

the cochlear is a sort of a tapered tube. The basilar membrane actually gets wider moving from the base, sort of near the, remember the round and the oval windows, that's the base of the cochlear, if you like, up to its apex or the tip. It actually widens, and as it does, it gets more flexible. So it's narrow and stiff at the base of the cochlear, and wider and more flexible at the apex of the cochlear. And this means that the basilar membrane vibrates better in response to different frequencies as you move along. So it responds best to high-pitched sounds, high-frequency sounds at the base, and lower-frequency sounds at the apex. This means, in turn that the hair cells located at different places along the basal of membrane will tend

to respond better to different frequencies of sound. And in turn, therefore, that the signals coming from those particular hair cells will then respond better to different frequencies of sound. And it's thought that that's directly reflected in terms of a mapping from these hair cells in particular locations along the basal membrane to the neurons that respond in the primary auditory cortex best to particular frequencies of sound. So this tone-topic mapping seems to have its origins in the basal membrane and then is further reflected in the auditory cortex itself, which is quite interesting. And this is one way that we perceive different frequencies. It's not the only way, it's quite complicated, but this is one way.

It's basically depends on the location of the hair cells that is most active in response to a particular sound. That tells you what the frequency of that sound is based on, again, how stiff or flexible the basilar membrane is in that location. coming back to neural pathways and auditory processing surrounding the primary auditory cortex are the secondary and tertiary audio cortexes which are thought to respond more to complicated sounds so many of the cells or the neurons in the primary auditory cortex respond best to sounds of a particular frequency this is a tonatopic mapping we talked about neurons at higher levels of secondary and tertiary cortex however seem to respond to more complicated

patterns of sounds in particular these areas in the secondary and cortex have been associated with language processing and music as well. There's been a lot of study about processing and music, which we mostly won't talk about. But so again, likewise, as we had in the visual system where there's a hierarchy of levels of processing from the sort of simple up to the more complex as you move through the cortex. It's similar in the auditory cortex, it seems, that you start with sort of lower level processing of individual frequencies and then move up to combine those into more complicated patterns of sounds, particularly with a focus on language and music. Okay, let's now talk a little bit about some of the more detailed aspects of auditory processing with a focus on sound localisation. So how do we know where sounds come from? Nothing we've talked about so far really gives us any indication of that.

Regardless of where the sound comes from, it is detected in the same way by those hair cells, you know, depending on the vibration of the basilar membrane in the cochlear, and transmitted to the brain. So there's no correlate of sort of the place in the visual field, of sounds like there is obviously for vision. So we see where things are, but based on where in the visual field they're detected, but there's no correlate to that in the auditory system. So how do we know where sounds come from?

We can tell quite accurately to within about one degree, I think, where around us the sound is coming from. So how do we do that? This is called sound localisation. It's been extensively studied in mammals, and it seems that there's no one way we do it. There's many different ways, which makes sense because it's a complicated problem, and you wouldn't want only one way of doing it,

Because if that method breaks down due to disease or something like that, then you're going to have no ability to localize sound. It's much better to have a variety of methods which can complement each other. It seems that there are two important mechanisms that we rely on are what are called interoral differences in timing and phase. So basically this means the fact that our brains compare the sounds between our two different ears and use that to make a determination about which direction it's coming from.

and also how far away it is potentially. So there's a couple of ways that you can do this. First of all, you can look at the timing difference between either the onset of the sound or looking at the phase difference in the wave patterns that hit one ear versus the other. That can tell you the difference in time that elapsed between when the sound first hit one ear

and when it hit the other ear, and that will give you an indication as to potentially the direction but also how far away the sound was. That's more useful for higher frequencies because if the frequencies get too large, the distance between the two ears is actually very small compared to the size of the wavelengths of sound that are actually hitting us.

So therefore, the phase differences isn't very useful when the wavelengths get that long. So for lower frequencies, determination of direction is more dependent, not on the timing difference or the phase difference, but just on the difference in loudness of the sound. So that is, from one end to the other, obviously, the ear that's further away will have,

be slightly softer, to the dissipation having moved further away from its source. So essentially for longer frequencies, we're more reliant on differences in volume between one and the other, and for higher frequencies, we're more reliant on differences in phase or the timing, the onset of the sounds, the wavelengths from one ear to the other. There are many other things that we use, many other tools we use, our brains use rather, because we're not conscious of all this, to determine the origin of sounds. So it's thought that the pin of the structure of the outer ear actually is partly developed in order to help us determine the direction based on the pattern of reflections and so on that we have on the ear, which is interesting.

It's also been shown that vision plays a role in helping us to determine where the sound is coming from. Essentially, this has been determined by you disrupt one of the ears, so disrupt the ability to localize sound. This is done in animals and humans, obviously. So you disrupt one of the ears, thereby disrupting sound localization ability. then if you wait a while, you'll see that it tends to be corrected over time. However, if you do something to the vision, so you can fit these lenses on an animal, which will rotate light by a certain number of degrees,

you can show that they still sort of offset, they correct the defect to the sound localization over time, but the correction is 10 degrees off, just as their vision is 10 degrees off. So this tells us that the visual information is used to calibrate the various mechanisms, that are used to for sound localisation. So that's just quite interesting finding, I think.

So the summary there is that we use a large number of different means to methods for sound localization, including the difference in the intensity or the volume of the sound from one or to the other, the difference in phase or timing between one ear and the other, structures of the pinner and the reflections that they create, as well as visual information and other mechanisms as well. So to finish out the episode, I want to finish out the episode, to talk a little bit about deafness. Just very briefly, one of the most important things to

understand is that there are many, many, many different causes of deafness. So there's no, you know, there can never be a cure for deafness because there's no single cause of deafness. There are three main types or classes of hearing loss that I want to talk about, which are called conductive hearing loss, sensory neural hearing loss, and central deafness. And there are also sort of a fourth class, which consists of combinations of those. But we'll focus on the first three. Conductive hearing loss is caused by sound not reaching the inner ear, so not reaching the cochleas. This is no vibrations in the cochlea. So your cochlear could be completely fine, the hair cells are fine, all of the neural aspects of hearing is completely fine. It's just the vibrations aren't

reaching the cochlea, so you're not going to hear anything. This can be due to malformation of the external ear canal, so the outer ear, or some sort of dysfunction of the eardrum or the bones in the middle ear. So this form of conducting hearing loss is the most, as I understand it, most common type of hearing loss as a result of aging, either because the eardrum stiffens up or has problems, or there's some infection there, or the ossicles stiffen up and have difficulty conducting the sound to the inner ear. Conductive hearing loss is in some sense the easiest, or at least the simplest form of hearing loss to deal with, because the outer and middle ear are the sort of simplest aspects of the ear, and we understand them the best. Any means

of bypassing these structures and just sort of directly stimulating the cochlear. would restore hearing in these people, and people are working on that sort of thing. The next type of hearing loss is called sensory neural hearing loss, and it's caused by some sort of dysfunction of the inner ear, so the cochlear, or the nerves that transmit the impulses from the cochlear to the brain. So most common is some sort of damage or disruption or dysfunction of the hair cells. If they're not working properly, obviously, you're going to get the vibrations to the cochlear, but they're not going to be transduced and carried to the brain.

The final type of deafness is called central deafness. This is when the outer ear, the inner ear, the middle ear, the transduction, everything's working fine. It's just there's some damage to the brain which leads to inability to properly interpret the signals and hear. Now, this type of deafness is interesting because since all of the peripheral aspects of hearing are working correctly, but the patient is unable to hear, unable to perceive sound, some of these patients who centrally deaf are actually able to react to sounds that they perceive unconsciously. So remember I said that a lot of these auditory signals go directly to the brain stem and only then to the auditory cortex. If there's some sort of damage either to the auditory cortex or to the transduction from the brain stem to the cortex, there'll be no ability to consciously perceive sound because that's all the auditory cortex doing that, sort of processing.

But the person may be able to react reflexively to sounds that they nevertheless perceive through, perceive is the wrong word, that they detect in the brain stem. So that's actually quite interesting. Central deafness is really a defect of the brain, whereas the other two are defects of the ear or the nerves connecting the brain to the ear. Now, you may have heard of cochlear implants. Cochlear implants are used to treat the second type of hearing loss,

so sensory neural hearing loss. Actually, come to think of it, I think it could be used, there's no reason it couldn't be used to treat conductive hearing loss as well. It would not work on central deafness because the cochlear implants don't do anything to the brain directly. So if the ability to process the auditory signals is disrupted, then cochlear implants isn't going to help with that. But at least for some types of deafness, cochlear implants can be helpful.

Essentially what they do is bypass the middle ear and large parts of the inner ear completely. Cochlear implants are an implant inside the inner ear, inside the cochlear, which artificially stimulate the cochlear nerve by providing electrical impulses. So the cochlear nerve still needs to be functional for cochlear implants to work, but you don't need a functioning hair cells because it bypasses those. You don't need the osicles in the middle ear, those are bypassed as well, the eardrum bypassed, all of that's bypassed, basically. The way they work is their external microphones, which pick up sound.

They usually mounted somewhere sort of behind the ear. Those transmit signals in a wire that passes through and essentially through the middle they're through the cochlear and connect up in a complicated way which we're going to talk about to the cochlear nerve. Now of course it's not just one why because there's needs to send different signals for different frequencies and so on. So the way it's connected up is complicated and that's the difficulty in producing these devices and implanting them properly. But the key, the reason that these devices work as well as they do is because we don't actually need to sort of replicate the patterns of neural, sorry, the patterns of electrical activity that hair cells produce.

All we need to do is provide the brain with pattern of electrical impulses, which in some logical way map to changes in frequencies in the air. The magic of the brain is that it will figure out the rest in a sense. Not perfectly, obviously, because the cochlear implants don't restore hearing in the same way that people who are hearing via transduction of signals by hair cells do, because it's a different mechanism, it's not as precise. But their brains are able to make sense of the input in a way that still allows them to perceive sounds and to understand spoken speech and so on.

So that's one reason why cochlear implants, well, actually, that's the main reason why cochlear implants work best when they're implanted very young, because the brain is more plastic at that age and is better able to adapt to the, and make sense of the patterns of input coming from the implants. That's the crucial insight of the cochlear implants, is that we actually don't sort of need to understand exactly how the input of the output of the electrical signals from hair cells normally works. We just have to understand the basic idea that it's related to the frequencies of sound. And then if we can provide sort of substitute electrical output, which is also related to frequencies of sound,

the brain will figure out a way of making sense of that and turning it into sort of conscious perception of sound. We don't have to do that for it. The brain does it. So the way I've heard this described is that cochlear implants work so well, not because, because they are really smart in any way. It's because the brain is really clever. That's not to diminish the achievement of developing the cochlear implant, mind you, but I think it does put in perspective as to how the technology actually works.

Largely, it relies on the brain's plasticity. If it was just an issue of us providing the same input that the hair cells do, we'd have no chance. We don't understand in enough detail how that works. Okay, so before we finish out the episode, let's pass through one quick trip through the ear, from the outer to the inner so that we make sure we've got in our minds, or I've properly explained, how this process works.

So we begin with the pinner of the outer ear and the vibrations in the air, changes in pressure of air molecules, which interact with the folds and the cartilage of the pinner, producing patterns which help the patterns of sounds which are detected by the brain and help us to localize sound and so on. The pinner also collects sound from a larger area and focuses it down through the external auditory canal, the ear canal, which then passes to the tympatic membrane or the eardrum.

The eardrum then vibrates in accordance with the external signals that are being received, or the sound waves that are being received. The ear drum then causes motion, the vibration of the eardrum causes motion of the malleus, which is one of the three ossicles or the small bones inside the middle ear. Malleus is connected to the incus, which is in turn connected to the star. which finally is connected to the oval window, which forms the outer part of the inner ear.

So the obstacles, remember, are part of the middle ear. So they transmit the vibrations of the tympanic membrane or the eudrum through to the inner ear via the oval membrane. Now, as the oval membrane vibrates, it causes motion of the fluid inside the vestibular canal, which then sort of moves around and extends out at the tip. of the cochlear and comes back up through the tympanic canal. Remembering, of course, that the cochlear is actually wound up, but we're imagining that it's unwound for ease. So these vibrations travel up the vestibular canal around and up the tympanic canal. And as they travel up the tympanic canal, they cause motion of the basilar membrane. On the basilar membrane sit

hair cells, which have projections called stereocilia, which are stuck into an overlying membrane, which is called the tectorial membrane. As the basilar membrane moves in relation to the tectorial membrane, which is held more or less in place, the hair cells move in relation to the stereocilia, causing the stereocilia to move, essentially in a shearing motion like the bristles of a toothbrush. As the stereocilia moved in that way,

mechanically gated potassium ion channels that sit on the stereocilia are opened, literally pulled open. Potassium ions flow into the cell, depolarization, which causes voltage-gated calcium ion channels to open up, causing calcium ions to flow into that this is now in the body of the hair cell. These calcium ions cause synaptic vesicles that are sitting around inside the hair cell to fuse to the cell membrane, causing these vesicles to release their contents of neurotransmitter, which then diffuses through the synaptic cleft and synapses with the, or fuse with the membrane of the

aphrant neurons, thereby passing electrical signals. to the brain. Well, first of all, generally to the brain stem, and then in turn, the neurons transmit the signals up to the auditory cortex, the primary auditory cortex, which is located in the temporal lobes, where it's then processed on the basis of signal, and then in higher secondary and tertiary auditory cortex areas to interpret speech and music and other things like that. So, that's how we hear. Hopefully you found this episode interesting. If you did, I'd appreciate it if you'd go on to Facebook and give out the podcast page a like. I also appreciate reviews on iTunes or other podcast aggregators that you might use. If you're interested in contacting me, sending a message or giving some feedback about the show or an

idea for a future topic, my email address is FOD12 at gmail.com. That's FODDS12 at gmail.com. Thanks for listening and I'll talk to you next time.