I'm going to talk with you today about things the brain does. Together with Dan, we decided to call this lecture Sensations, Perception, Actions, and Emotions but really, the sub-title is the Story of a Sound because I'm going to emphasize the auditory system, which is my research subject. Now what are neuronal processing mechanisms for? So what is sensation? Sensation is the transformation of external events into neural activity. Perception, has to do with the processing of the sensory information. And we believe that the end result of the conception is the useful representation of the central information in terms of external object, objects that produce the sensations and I want to illustrate these starting with a well know quote of David Marr. From his book Vision David Marr was one of the premiers of computational neuroscience. in his book he defines vision as seeing is knowing what is where by looking. The idea being that you have a two, 2-dimensional image, which represents the whole visual world on the retina, and the brain has to reconstruct the three-dimensional representation of the visual scene. Today, we will say it a little bit differently. We have a 2-dimensional image on the retina and what we want to do is recover the latent variables. Latent variables as the course is for the things that fall on the retina. So for example if this is the, the, the image that falls on the retina you have the eye and you have the brain. Behind the eye, that processes this image and eventually should come out with a list of latent variables like the fact that the image included a house, the sun, a tree, and maybe that it was drawn by a bad artist. They result the representation that produced by perception is a base for actions and the idea is that organisms use the representation a of that, of the world that was produced by the sensory systems. in order to act on it, and the goal is to produce a lot of reward, rewards and to avoid as much as possible punishment. The view of a linear order, sensation, perception, actions, is actually a little bit naive and probably wrong. About the years, people helped in discussing the concept of a perception action loop were found that going to the brain and produce back options that changed the world, changed world and changed perception and these actions and perceptions. Are not separate things. They act in a, in a loop that and they modify each other. Eh, this beautiful illustration comes from a meeting in which eh, we tried to eh, bring up eh, people who were eh, studying perception and action a few years ago. but the idea is therefore, has been there for many years and there are many people who study it. In this lecture, I cannot go into the complexity of the perception action loop, so, we leave it at that. The last subject I'm going to discuss is emotions and within the context of these sensation perception actions, a pathway or loop. Emotions are a little bit strange and we'll discuss them at the end. So, at this point, I would like to start the main subject of the talk. And what I will, the subject that I will deal with, in this lecture and Sensory Transduction. Okay, how do sound become electrical activity. I will discuss one example of the early processing of sensory formation and this example is the case of auditory localization and more specifically, the calculation of the minute. A difference between time arrival to the two ears. I will show how this sensory information guides motion. And in this case again it will be a simple, I will discuss a simple test case, the case of addition guiding head turns to an auditory target. Then, I will discuss a little bit, try out all the processing of sensory formation specifically, what surprised does to neurons in brains. And finally, I'll discuss a little bit emotions that are sound induced. Okay, so lets start with sensory transduction. How do sound become electricity? As the, the, scri, the standard description of the way that sound become electricity is simple. We have hair cells. Hair cells are specialized cells, cells they sit in the ear, and in a moment I will show you where they sit in the ear. And these cells have hills, this is why they are called hill cells. The notion is that when sounds come, they cause movements of the hairs. Easily said, the mechanic sensitive ocean signals the hills and the motion of the hills open and close. These mechanosensitive ion channels okay, this cause currents into the neuron and the currents, into the hair cell, and the currents into the hair cell cause depolarization in the hair cell. This depolarization, like in all neurons, acts on a synapse. It modulates the amount of transmitter that is released in sinups between the hair cell and the auditory nerve fibers. And this cause changes in the spiking activity of the auditory nerve fiber. Now this auditory nerve fiber has one end. contacting the hair cell in the ear, and its other end is inside the brain. And so the brain gets to know about the motion of the hairs. Okay, so up to here, this is, this sounds pretty simple, but this begs the question of how does sound, sound cause vibration of the hair, of the hairs? So, the sounds cause vibrations of the hairs because the hair cells themselves sit on top of a membrane, called the baslar membrane, again in the ear. The hair cells are embedded In a structure which is very stiff. It's called the organ of corti, the name is not important but the structure is very stiff and vibration in the ear caused by vibration of the basilar membranes. So, the basilar membranes sometimes goes up and sometimes goes down. So, the hair cells are embedded in the organ of corti and when the, the basilar membrane go up or down, where this causes a relative motion of the hair we suspect to this huge huge copula this kind of a roof that lies above them. And, this causes a [UNKNOWN] forces that moves the, moves the hairs around. So the motion of the basilar membrane is translated into motion of the hair bundle and this causes the opening and closing of ion channels and the polarization, the transmitter release and spikes. So next, the next question is, why does the baslar membrane vibrate in response to sound? So it turns out that the baslar membrane itself sits inside a long tube. This long tube eh, is shown here in cross section So it the, the tube is a divided in the middle by the basilar membrane so here is the basilar membrane and the organ of quality. Now the hair cells are so small that you can hardly see them in this illustration and, and the, this whole tube. This whole tube is so long that whoever designed this sensory apparatus, coiled round, coiled this tube around itself, forming a snail like structure which is called the Cochlea. So, we have the Cochlea. Its a long tube and if I cut the cross section to the cochlea, I see here this tube at the number of, of location along its length and at each one of these cross sections I see the basilar membrane with the organ of coat on top of it. And in this illustration you can see also this all of these fibers that contacted the hair cells. Which joined together to form the cochlear nerve onto nerve that goes out of the ear and into the central nervous system. Now how do sound cause vibrations of the basilar membrane? We have the external ear that sits somewhere here and then we have the ear canal and the ear canal ends in the tympanic membrane. Vibration in the ear causes vibration of this tympanic membrane and this vibrations are. It transfers to the fluid that fills up the, the cochlea, through a chain of three bones. These are the tiniest bones in the human body. The vibrations, of this bone into the fluid cause pressure waves. And these special waves in the fluid, cause the vibration of the [UNKNOWN] membrane. And then the basilar membrane vibrates, the organ of corti moves with it, the hairs move because of the vibration of the organ of corti, ion channels open, the polarization transmitter release and so on. One important feature of the Cochlea that has been discovered already in the 19th century is the fact that it is mechanically homogeneous. Close to the middle ear, close to the bones of the middle ear. The basilar membrane is narrow and stiff, whereas close to the end of the, of the tube the basilar membrane is wider and less stiff. And this led researchers in the Nineteenth century, mostly Hermann von Helmholtz, Helmholtz. So to suggest that the basilal membrane vibrates best when excited by different frequencies along its length. So if this is the whole tube of the cochlea, uncoiled, and this is the representation of the basilal membrane, if you are going to play a low frequency sound. It will cause vibrations best close to end, to the far end of the basilar membrane. If you use a high frequency sound, then it will cause vibrations with the loudest amplitude close to the beginning of the of the basilar membrane, close to the middle ear. And the frequency in between. For example, 1,000 Hertz in this case, will cause vibrations somewhere in the middle. And finally, when you have a complex sound, a complex sound will include a lot of different frequencies and the different frequencies that are included in the complex sound will Will cause vibrations of the baslar membranes at different places along its length. This turned out to be more or less correct. I'm going to show, to show to you here now a short, a movie. That is based on actual measurements of the movement of a real basilar membrane. This axis is the length of the basilar membrane and this represents its width and the stimulation here its the middle ear by a full sine wave by a full frequency, whose frequency correspond to the best. Frequencies that excite, that vibrates best the middle of this part of the basilar membrane here. And you can see the way that the vibrations increase in size as they reach the best location and then they start to decre, to decrease in size again. So you can imagine that the wave, traveling, this peak traveling along the basilar membrane, becoming larger and larger until it reach the, its best, the location that best fits its frequency, and from there on it is reduced in amplitude as it progresses beyond that point. so you see that the you can, you can see here that the way that energy travels in the basilar membrane is by so-called traveling wave. This peak of activity shifts around, okay, it moves From the middle here to the edge of the, to, to the far end of the cochlear max, with maximal size, at its best location. The movie that's shown here was also recorded from the same cochlea except that this is a dead cochlea. This movie comes from a cochlea that's alive and well, and this movie was taken from the same cochlea except that it got, it was damaged, and the living tissue in this cochlea is dead. What you can notice, is that the amplitude of vibrations here, is substantially smaller than here. So the fact that the cochlear includes a living tissue, something in the living tissue of the cochlear increases substantially the amplitude of the of the vibration of the, of the basilar membrane and for many years. They, there was a, a serious search for what are the mechanisms that cause these, same, eh, these, say, additional increase in amplitude the so called action of the cochlea amplifier and it turned out in a that they a, the main source of a, a energy to the cochlea amplifier actually the hair cells themselves. The hair cell come into a, in 2 types of flavel and, and flavels and one of these flavels is so called outer hair cells are piezoelectric. When they change their electricity, they change their length and this transformation of energy of the polarization into mechanical energy, supplies the energy to the cochlea amplifier. So, the question of how the hairs of the hair cells move down the out to include many, many, many details that will worked out over many years as I said initially Hermann von Helmholtz as suggested that the cochlea frequency, the composition of sound. But eh, he thought that the cochlea is basically like eh, a piano. So you have a lot of strings that are eh, connected eh, eh, from, from one side of the tube to the other, and they vibrate with eh, with the sounds like The sympathetic vibration of eh, of the strings of a string instrument, with eh, with sound. Early in the 20th century, Georg von Bekeshy discovered that this is not a correct eh, representation of what happens in the, in the cochlea. And he discovered the traveling wave He got a Nobel Prize for it in 1961, and he is the only auditory scientist to ever who ever won the Nobel Prize. And finally in the late 20th century, and the research is still ongoing by many researchers, the cochlear amplifier was discovered and its relationship to the outer hair cells. This is Herman von Helmholtz. The drawing is by Hans [UNKNOWN] and this is Georg von Bekesy, the picture from the Nobel Prize website. To summarize, auditory transduction is performed by specialized cells, that sit in a specialized organ. With many different specialization the organ of quality its extreme a, a, extreme stiffness the basilar membrane with its mechanical properties the change as the function of its length. Along its length and finally this specialized organ is coupled to the stimulus to the, to the middle ear to the external ear and the a lot of fascinating details that I couldn't touch upon in this lecture.
