This is actually the auditory pathway, take a look, from here, let's take a look at this one, this is the cochlear, right? This is the organ actually to transduce a sound signal to the electrical signal from here. And then will be relayed by those axons. Those axons actually coming from this ganglia, spiral ganglion. You remember those red axons connect with the inner there. Those axons are actually coming from this spiral ganglion. And this spiral ganglion will further relay this auditory information to the brain stem. [FOREIGN] okay? But in the brain stem, it's quite interesting. So for example, this is the left side input. And there in the brain stem, you can see the signal will target to the left side and also the right side. So in the brain stem the signal already make the left and the right targeting, okay? Just [FOREIGN] with the brain stem, [FOREIGN]. And then of course from this brain stem the information will further be relayed to the mid brain. [FOREIGN]. To mid brain, then will further relay to the. [FOREIGN]. In the visual system, we talk about the LGN, right? And this also [FOREIGN] okay? But here, this MGN, we can just had [FOREIGN] lateral geniculate nucleus in the visual system. This is a medial geniculate nucleic, MGN. And the front is Q now. The first relate to the auditory cortex. [FOREIGN]. This is actually the auditory pathway, so quite similar actually compared to the visual system. But there was once more step is the here. [FOREIGN]. This is the brain stem and it's a midbrain. [FOREIGN]. Why? You need a such a more complicated processing here. >> Good, so you think actually auditory information need more computations. >> [INAUDIBLE] >> Ignores those noises. So, okay that's a good point. But also, it's important. So remember in the eye, the outer [INAUDIBLE] actually. Not the front the photoreceptor. In this auditory system the output actually is directly from the hair cell to spiral ganglion, right? [FOREIGN]. We talk about issue in the retina there was lots of computations, centers around [FOREIGN]. So the brain stem and the midbrain so [FOREIGN]. Of course we will talk about it later. Actually there are more computations indeed actually happen in the brain stem. Okay so this is the auditory pathways. Lets take a look further about some processing of the auditory system. So for auditory system, we talked about the key features is the frequency and the intensity and the location, right? So, let's first take a look at the frequency coding. For the frequency coding, we talking about in the cochlea already there was some structural basis for the coding. What is the structure there? The basilar membrane, right? So, at the base the frequency actually is high or low? High. And the tip of the cochlea, then the frequency is slow. Okay, how did the brain. You see this is a cochlea, right? This is a cochlea and this is the base, the base is actually, is quite large, right? Just the base of the membrane is narrow in this region. So, high frequency happens here. And they go into the inside. Actually, then go to the low frequency. So, if you have a hail cell sit in this region, what's this hail cell's properties? Of course this [INAUDIBLE] cell where high frequency was lost here, okay. And then this region maybe the frequency is not so large but this region, the frequency is quite low. Quite interesting. So this inner cell was connected by the spiral ganglion and the spiral ganglion connection is quite specific. That is actually this one axon only connect to one cell. It's one connected to one. Of course it's quite strange when we actually talk about this is axon. It's actually dangerous to lose sight with a signal. Yeah, in some textbooks they call this axon. This is kind of special cell type, okay? Because, actually, people found, maybe, this one, the process is really quite long. So, for a signaling trail without the action potential, then you have problem. The signal will decrease, because this process is so long, without any action potential, [INAUDIBLE] and then the signal will receive from here, come to the spiral ganglion. Cell body will be very, very tiny. So in this case, the spiral ganglion, actually they can fire action potential somehow along this way. To actually really guaranteed the signal information can go down through this process. But is important to think is this frequency coding already preserved in the spiral ganglion. This one you see is the high frequency, this one compare this one is the low frequency, this one is 2 kilohertz frequency. And also, from this spiral ganglion then we'll go to the brain stem, right? And then to the mid brain and then to the sediments and then to the cortex. All these steps, this frequency, is a relation is maintained. That means there is a map in the brain. All of this steps. There was a map there. The map is actually correlate with the different frequency. At each different region the cell respond to a specific range of the frequency, okay?
