Voiceover: In order to distinguish between the sounds of a base drum and something that has a much higher frequency, such as the sound of a bee's wings flapping in the air, your brain is relying on the cochlea, in order to differentiate between the two different sounds. So, the difference between a base drum and a bee's wings flapping in the air, is the frequency. So a base drum has a very low frequency, whereas the wings of a bee, when they're moving through the air very quickly, have a very high frequency. So as the information from a base drum beating, or a bee's wings flapping, comes into the ear, they eventually hit the cochlea. And we went into a lot of detail about how exactly the sound wave is converted into a neural impulse by the cochlea, that eventually reaches the brain. But now we're gonna go into how the cochlea distinguishes between sounds of varying frequencies, and how this distinction is maintained all the way to the brain, in order for the brain to be able to perceive different sounds. So this is known as "Auditory Processing." Your brain needs to be able to distinguish between sounds of varying frequencies, and you're actually able to hear things with a frequency of 20 hertz, all the way up to a frequency of 20,000 hertz. So this is a huge range, and in order to distinguish between sounds of low and high frequencies, the brain uses the cochlea, and particularly, something known as "Basilar Tuning." And the term "basilar" comes from the basilar membrane, which is inside the cochlea. So inside the cochlea, there's actually a membrane that contains a bunch of hair cells. And if we were to unroll this cochlea, if we took the cochlea and we unrolled it, so it's normally rolled up like this, if we unrolled it, so now it's flat, there are varying hair cells. So this would be the very base, this is the base of the cochlea, and this is the very apex, the very tip. So the base would be right here, the apex would be right here. Now if we unrolled it, and looked at which hair cells were activated, given different sounds, we would notice that hair cells at the very base of the cochlea were actually activated by very high frequency sounds, and hair cells at the very apex of the cochlea are stimulated by very low frequency sounds. So let's look at another picture, just to make things a little bit clearer. So this picture basically just shows the cochlea unrolled. And as I mentioned before, this would be the base of the cochlea, I'll use a darker color. This would be the base of the cochlea, and this would be the very tip, or the apex of the cochlea. And hair cells are found all along the basilar membrane, so this membrane right here is the basilar membrane, and there are hair cells implanted inside of it, there are a whole bunch of these hair cells. And hair cells closer to the very base respond to a very high frequency, so this is 1,600 hertz. And hair cells closer to the apex respond to a lower frequency, so 25 hertz. So this would be something like a base drum, and something with a very high frequency, would be something like a bee's wings flapping in the air. So as sounds with varying frequencies reach the ear, they will stimulate different parts of the basilar membrane. So if we have a base drum being played, it has a pretty low frequency, and it'll eventually go into the ear, reach the cochlea, and it'll actually travel along this basilar membrane, until it reaches the hair cell that is attuned to that particular frequency. So let's say, that this is a frequency of 100 hertz for example. The sound waves eventually cause fluid inside the cochlea to travel in such a way, that the hair cells that are very sensitive to a frequency of 100 hertz, which looks like it's right around here, will actually activate. And these hair cells will fire an action potential, and this signal will eventually reach the brain, and it will be mapped to a very particular part of the brain. So this right here is the brain, and if you lift up this little piece of brain, there is something known as the "Primary Auditory Cortex." And the primary auditory cortex is this blue region over here, and it's basically responsible for receiving all of the information from the cochlea. And you can see that it's actually separated, similar to how the cochlea separated to various frequencies, it's sensitive to various frequencies, this primary auditory cortex is also sensitive to sounds of various frequencies. So, for example, this would be a part of the cortex that receives information from hair cells that are sensitive to a frequency of .5 hertz. And this part of the auditory cortex receives information from hair cells that are sensitive to a frequency of 16 hertz. And the reason that this is important, is because the brain needs to be able to distinguish between various sounds. So if we had all the hair cells sensitive to every single sound, then whenever you heard any sound, then all the hair cells would fire at once, and they would send this huge signal to the brain, and the brain wouldn't be able to distinguish between different sounds. So by having this basilar tuning, the brain is able to differentiate between sounds with a very high frequency, and sounds with a very low frequency. And this mapping, so this mapping of sounds with a higher frequency versus sounds of a lower frequency, is known as Tonotypical Mapping." And just to summarize, we have sounds waves coming into the ear, and different sound waves have different frequencies. And we need to be able to distinguish between the different frequencies. So the sound waves come in, they hit the cochlea, and they will activate hair cells in different parts of the cochlea. So if it's a very high frequency sound, it'll activate a hair cell over here; if it's a very low frequency sound, it'll activate a hair cell over here. And these hair cells will actually send axons, and these axons eventually all bundle together to form the auditory nerve. And the auditory nerve carries axons from each hair cell inside the cochlea. And the auditory nerve eventually reaches the brain, and will again separate its fibers, and reach different parts of the brain.