Hi, I'm Michael Corayer and this is Psych Exam Review. In this video I'm going to explain the process of transduction for sound.
So how we go from sound waves around us in the air to brain activity inside our head that tells us what sounds we're hearing?
In order to understand this we're going to have to look at the parts of the ear and describe what each part does.
So here's a diagram of an ear.
We start with the sort of fleshy outer part of the ear that's sticking out of your head here and it's
essentially a funnel and this catches sounds or it helps to catch the sounds around you.
So sounds are directed through this funnel, the pinna,
into the auditory canal.
When they get to the end of the auditory canal
what happens is they vibrate the eardrum. So the eardrum is this thin stretched piece of skin essentially, you can also call it the tympanic
membrane and so the sound waves actually cause that to move. When the eardrum knows it vibrates these
bones that are behind it. So now we're in the inner ear.
These bones are called the ossicles and this is Latin for "little bones" and these are the smallest bones in your body. So you have three
of these in each of your ears. What happens is the hammer hits the anvil, the anvil then vibrates the stirrup
and now the sort of interesting part of what happens to the vibration occurs at the cochlea. So the cochlea is this spiral curled up structure
and cochlea is Latin for snail because this looks a lot like a snail.
So what happens in the cochlea is that it has a structure called the basilar membrane.
It's essentially a long rolled up a tube and it's filled with fluid.
When the stirrup vibrates that sends waves of vibration through the fluid.
Then lining the basilar membrane are little tiny hair cells, these are called stereocilia,
and these little hair cells are moved, right? They sort of blow in the breeze of the fluid moving and when they move they trigger
neurons to fire. Then those messages get sent out the auditory nerve
and then they go to the brain. They go to the thalamus and then from the thalamus they go out to the temporal lobes on either side of your
brain where we have the primary auditory cortex, or the area called A1. This is the area that
begins processing these sounds to figure out what they are.
OK so let's talk about this area of the cochlea in a little more detail. Before I close this picture you
probably notice there's a section called the vestibular system next to the cochlea here.
This actually has nothing to do with hearing so I'm going to talk about this in a future video. This has to do with
with your sense of balance and so it works in a similar way, it's filled with fluid,
but it doesn't have to do with hearing so I'm not going to go into detail in this video.
OK so
let's imagine that we unraveled the cochlea here. We have this sort of long
section here
this is probably not the best drawing of it and it's filled with fluid and it's
lined by these little tiny hair cells so
these are the stereocilia.
And this, sorry I should have written, is the
basilar membrane.
So an important idea for understanding the basilar membrane is that when the fluid moves
different wavelengths, different frequencies of sound,
are going to stimulate different areas of the basilar membrane. The way that it works is that very high pitches stimulate
the hair cells at the beginning of the basilar membrane, near the stirrup, and then the low frequency sounds do most of the stimulation
at the end of the basilar membrane.
So this is an idea called place code.
So this is the idea that part of the way we know what frequency we're hearing, how high pitched a sound is, is actually physically in our basilar
membrane where are the hair cells moving? If they're moving here it's a high pitched sound if
they're moving here it's a low pitched sound. If they're somewhere in the middle it's somewhere in the middle.
So that's place code. It's the idea that it's the location of
of the stereocilia.
The location of the hair cells that move tells us about the pitch.
This isn't the only way that we know about pitch. It's combined with another approach called temporal code.
Temporal code is the idea that
the neurons that are sending these messages
can fire at different rates.
So the idea of temporal code is that different frequencies can stimulate these neurons to fire at different rates. Now initially this idea
was kind of dismissed because people said "well we can hear very high pitches, 20,000 Hertz and that's very very fast vibration and
our neurons can't keep up, we don't have neurons that can fire 20,000 times a second.
So how could we possibly use temporal code?". Then the idea was well, actually you don't need to have neurons that fire 20,000 times per
second. If you have neurons that can fire about 1000 times per second, which is about the fastest that we have neurons that can fire, well
if you have 20 of them and you stagger their firing, you have a synchronized pattern of firing across a group of neurons then you can
actually tell about very very different rates of frequency. You can actually get up to, you know, telling about something that's
20,000 Hz based on that. So the idea of temporal code
has to do with the firing rate.
The idea is that we use a combination of both of these and the information then gets to our temporal lobe where it's processed
where we then sort of have the role of learning and experience, right? You learn to recognize certain patterns of frequencies as your mother's
voice or your friend's voice or the sound of a violin or the sound of a clarinet. That's a process of learning
because those aren't pure sounds. They have many frequencies in at once and there's particular
patterns that you learn, that's what a piano sounds like.
Ok so that's temporal code.
OK two other things I want to briefly mention. One of these is, I said we can hear up to 20,000 Hertz, how low can we hear?
We can hear speeds as low as 20 hertz all the way up to
about 20,000 Hz.
This a very broad range of sounds that we can hear. But you might not be able to hear 20,000 Hz. In fact I can't hear 20,000 Hz
anymore. By your mid-twenties you probably won't be able to hear them either and maybe even lower.
I'll post a link in the video description of a hearing test where you can try playing some very high frequency sounds. They're not very
pleasure to listen to but you can start, I think it starts around 8000 Hz and goes on up to 22 kHz, you can see if you can hear it. As for me
after about 14,000 Hz I can't hear it anymore. This is fairly common as you age. So what happens is these hair cells at the base of the
basilar membrane here essentially stop functioning properly. They get damaged and you can't just grow new ones very easily. So
essentially if they're damaged there's not much you can do, you just don't get to hear those sounds anymore.
So you can try that out, remember also it's not a perfect test, the speakers that you're using or the headphones
that you use will play a role in how well they can transmit sound of those frequencies.
But you can try it out and see how well your hearing is for those frequencies and actually this relates
to students use what are called mosquito ring tones.
These are ring tones on their phone or notification sounds that are very very high pitched so that students can hear them but
teachers most likely can't because they're older and their ears aren't working quite as well. Not that I'm encouraging that kind of behavior.
OK one final point is why do we have two ears?
Well we have two of most things in our body
we have this very symmetrical structure (That doesn't really look like a 2)
we have this very symmetrical structure to our body, so why is it we have 2 ears, is it just to be symmetrical or is this some other advantage?
You could probably figure out, if you thought for a second that yeah, of course there's an advantage to having two ears
beyond just in case you injure one you still have the other one. The other huge advantage is that you can now locate the direction of
sounds. So two ears gives us a huge boost in directionality.
So when I make a noise over here what's happening is it's hitting this ear
slightly earlier than this ear and it's louder in this ear than in this ear and that tells me about the location that the sound is coming from.
So if I were to close my eyes and you were to make noises around my head, it would be very easy for me to tell where the noise was coming
from. Of course if we think in evolutionary terms there's a huge advantage to this. If you're wandering in the woods and you hear the
sound of a predator behind you. Well you want to know it's behind you or in front of you or beside you.
You want to immediately recognize the location of the sound, that's going to help you to escape.
So this is a big advantage
and it's something that happens, you don't have to think about. You don't have to consciously say "ok which ear did it get to first, was it a little
bit louder in this ear." It's an automated process, you hear a sound, you immediately know where it came from.
Ok, so that's the basics of transduction for hearing that's really all the detail we're going to go into
for hearing. If you have further questions maybe I can make a future video
but I hope you found this helpful. If so, please like the video and subscribe to the channel for more.
Thanks for watching!