After having talked about auditory transduction, the way that sounds become electrical activity, I'm going to show you now, one case of an interesting processing of sounds and specific of the electrical activity that results from sounds. And specifically, I want to discuss the issue of auditory localization. We can identify the direction of sound source of, of a, of objects by the sound that they emit. So we hear a sound, and we'll turn our heads towards the, towards the sound. This is a very interesting use of sound, and the reason is that there is no explicit representation of sound direction in the ear. The ear has well, a sound. So identification of sound direction requires comparing the sound to each of the two ears. And therefore the auditory localization requires computation. It requires information from the two ears to which the central nervous system and get processed there. So how do we know which the direction of sound, what are the physical queues that we can, we can use. in order to identify the direction of a sound source. these cues have to do with the sounds reaching the two ears and so they are called Binaural cues, Binaural means things that happen in the two ears separately. W are going to discuss only the localization of pure sine waves, pure frequencies and when it feel sine waves reaches the ears, it has two types of physical differwences. It produces two types of physical differences in the two ears. First of all it reaches first the near ear, and only then the far ear. So there is a slight shift in time, a minute shift in time of arrival of sounds to the two ears. And secondly the head acts roughly as a shadow. So the sound is going to be louder in the near ear and slightly softer in the other ear. That's if I put microphones in the two ears and I present a sine wave, and I record a sound in the two ears I will find two sign waves. The sine wave in the near ear is going to be earlier and, earlier and louder. We call the time differences between the two waves, the interaural time difference, it can again, interaural between the two ears. So interaural time difference or ITDs, and the level differences, are the interaural level differences, ILDs. I'm going to discuss mostly the calculation of the ITDs, of the interaural time differences. This time different cells between the eh, peak of the sine waves reaching eh, each of the ear are really small. We are talking, when we are talking about eh, human heads, the distance between the two ears is something like 25 centimeters. And the time that takes the sounds to move from one ear to the next to the other is less than one millisecond. If we are talking about the ability to descriminate between the location of two sound sources in front of you. We can identify differences on the order of two degrees. And the, the time differences between a sound that is straight, the, it's, a sound that is straight ahead relay to the ear, the two ears at the same time. And, and the, a sound that is shifted by a few degrees from the center Will reach the close yield few times of micro seconds a, earlier than a a the other yield. So we're capable of disseminating interaural time difference of few tens of micro seconds that's hundreds of a millisecond. The important property that allows the audotory system to know when these peaks of the sound wave occur is called Phase Locking. It turns out that when the auditory nerve fibers, that are sensitive to the element range of frequencies, low frequencies, when the auditory nerve fibers fire, they fire at a fixed relationship to the shape of the incoming sound. So if I excite the, if, if I play a sine wave, to the ear, and I, eh, record the spikes that occur during, the presentation of this sound, these spikes are going to happen, in this case, mostly it's the peak of the sign waves. So the time which spikes occur, mark the peaks, the pressure maximum in the incoming sound. And this is true for both ears. So now when the sound reaches the two ears at slightly different times. These spikes are going to occur at slightly different times in one ear relative to the other. This is a complex slide, I'm going to explain to go over it very slowly and it describes the pathway. That eh, eh, is eh, involved in the calculation of the inter aural time differences. So first of all, we have the ears, right ear, and left ear, and the sound reaching the two ears. Eh, the ear canal, the tympanic membrane, and here's the cochlea. These eh, large ci, circular structures are part off the vestibular system that's involved in eh, eh, eh, balance. So the auditory nerve come out of the cochlea, of the cochlea, goes into the, the central nervous system, and ends in a structure called the cochlear nucleus. So we have the ear, the auditory nerve and the cochlear nucleus on this one side. And the ea, the ear, the cochlear nucleus from the other side. The auditory nerve fiber terminate in the cochlear nucleus. They make eh, very large synapses on the relevant neurons in the cochlear nucleus, and these neurons now send the axons to a third structure that sits not far from the cochlear nuclei called the medial superior olive. I wanted to show you a little bit a rough view of the brain in order to for you to have an idea of where these structures lie. So this is a presentation of part of the brain called the brain stem. The brain stem comes, comes out of the skull so from here down we have the spinal chord, the spinal chord goes into the skull and becomes brain stem. On the brain stem we have the two cochlear nuclei. So the ear, one ear sits here the other ear sits here. And, the cochlear, the auditory nerves come of the ear, crosses the bone and goes into the cochlear nucleus. The right ear to this cochlear nucleus and the left ear to that cochlear nucleus. Slightly further forward in the brainstem, we have the nuclei that together form the so called superior olivary complex, and the MSO, the medial superior olive is part of the superior olivary complex. The neurons of the MSO have dendrites pointing, to, to the two sides. One side gets the inputs from the axons of cochlear nucleus of the same side. And the pther side, the dendrites of the other side of the neuron get input that come on axons, that come from the other side from the other ear. How does this work? The point to notice in this diagram, the crucial point to notice in this diagram is that the axon from this ear is a bit shorter than the axon from the other ear. The reason for that is simply distance. The, this superior olive sits closer to its cochlear nucleus, than it sits to the cochlear nucleus from the other side. So we have slightly longer axons. Next we know that action potential propagation along an axon takes time. There's a finite velocity, so the longer the axon the longer it will take for the spike to go from the cell body to the axon terminus. In consequence, a spike that occurs in, in this, on this side of the brain, will reach this, the neuron in the MSO slightly earlier than its, its spike that occurs slightly faster than it spikes that occurs on this side. Okay, now assume that a sound a occurs a on the right side. This sound will reach the right ear fills. And the left ear a little bit later, 300 microseconds later, let's say. The spikes that are locked to the peaks of the sine wave will occur 300 microseconds earlier here, than here. However, this axon is longer. And if its length is such that the travel time from this cochlear nucleus to MSO is exactly 300 microseconds longer, than the travel time from this cochlear nucleus to the MSO. The spikes that originated from this, it will reach the MSO at exactly the same time as the spikes that originated from that ear. In consequence, the spikes reaching the MSO neurons from both sides will arrive at the same time and this MSO neurons have a special property, their coincidence detectors. The fire mostly when spikes occur simultaneously on both sides. So, the MSO will see the spikes at the same time, and the reason that it will see the spikes at the same time is because of this internal delay caused by the axon length and it will fire. On the other hand, for this specific neuron if the sums come exactly from the middle, the spikes will occur at the same time, locked to the peaks that occur at the same time in both ears. The delay due to the longer pass lengths here, the axon lengths here, will cause the spike to reach the MSO neuron at slightly different times on the two dendrites. And then the MSO neuron will not see the spikes at the same time and so it will not fire. In summary we got here a circuit that a insensitive to the time of arrival, to the, to the interval or time difference. It fires in this case, this neuron fires when they. sounds arrives at the right ear first then its the left ear later and it doesn't first when the sound reaches the two ears at the same time. So this neuron can tell us that the sound came from the right rather than from the center or, or obviously from the left. This circuit requires extreme specialization of the bio physics of the neuron that participate in it. The reason is that it needs to keep the timing of the spikes extremely, precisely all the all the way from the auditory nerve until they reach the MSO. And this is done by neuron in the cochlear nucleus with the specialized not to do anything to the input for relying it to the output. You learned with a done a lot about what neurons can do. That did calculations, temporal summation, partial summation and so on and so forth. The neuron in the cochlear nucleus participate in this eh, in this silqeek are designed so that they don't do anything of eh, eh, eh, any of these things. The only thing that they do is to produce a spike with very short, very small jitter for every spike that reaches them from the auditory nerve. Secondly, the, the neurons that do the coincidence detection in the medial superior olive also very special in terms of, most of the structure and of the bio physical mechanisms. And the reason that again, is that they needed a, this coincidence detection process that they participate in, requires sensitivity to time differences of a tens of micro seconds. You learned with [INAUDIBLE], that spike wave is order of one millisecond, that's 1000th micro second. So, we have spikes whose width is 100's of micro seconds, maybe 1000th micro second. And with these extremely wide things, we need to be able to discriminate into all time differences of a few tens of microseconds. So we cannot allow our self any jitter, we need a new one who's temple, eh, eh, temporal summation properties work with time concerns that are extremely short. There are other problems, other issues that these neurons have to solve as well. And in fact it turns out that some of these a problems that a are due to the to, to, to the attempt to the discriminate tens of micro seconds we say [INAUDIBLE] life will whose time constants are much longer, some of these problems. probably solved by using tricks that that have to do with the properties [INAUDIBLE]. Ehh, the, this is as far as I want to go into the description of the, eh, ITD interaural time difference all-time difference calculation. But I want to remark that similar mechanisms can be observed throughout the sensory systems. Sensory systems have, each has its own specialized needs and specialized calculations that needs to be done. I showed you here, the comp, the comparison of timing between the two ears. There's a parallel pathway that calculates the inter aural level differences, the differences in the amplitude of the two sound that reaches, reach the two ears. And in other sensory systems you find other such eh, computational tasks that are done by neurons. For example, the retina contains neurons that calculates motion, that they're sensitive to motion and that they act to motion in one direction but not in other directions. Eh, so this is a kind of calculation that's done by the, by the retina. Eh, early processing of smells also has its own share of eh, specialized eh, eh, calculations and needs to be done, and all of these specialized needs are catered in different sensory systems by different specialized circuits. So, for example, in the auditory system, we have the specialized circuit in the auditory brain stem. They're specialized in terms of the neurons that live there. They're specialized in terms of the connectivity between them, and they'll specialize in terms of the computations that they perform. Similarly, in the retina, there are a lot of different [UNKNOWN], which are now starting get eh, eh, eh, dissected out. And that perform all of these specialized calculations that are specific to the visual system. And similarly in the eh, eh, for the smell eh, sense, there's eh, a structure called the olfactory bulb, which gets the input from the nose. And eh, again, has a specialized circuitry that eh, performs the early processing that's eh, that is specific the olfactory system. And all of these, again, come with a lot of important fascinating details, and I urge you to go and look for them. So this is what I wanted to discuss with the spectral early processing of sensory information.
