So, there are several subdivisions to the superior olivary complex, and I'd like to talk about two of them. Beginning first with a division that we call the medial superior olive. In the lower left of this slide, I have a bit of a photomicrograph that shows you what this medial superior olive actually looks like. this is from a primate brain stem. And this nucleus takes the shape of a spherical ball and the outline of the ball is roughly about there. And the reason why I'm showing you the nissl stain is because there's a very unusual distribution of cell bodies withing this spherical shape. The cell bodies are all found right along the equator of this ball and those cell bodies are all lined up in a very characteristics array. Such that they can extend their dendrites, in opposite directions, away from that equator. Well this is the arrangement that is shown schematically, in the figure from our textbook, that we have in front of us here. And what we see is that along this equator, there is a systematic distribution of inter oral latency differences from one end of the medial superior olive to the other. So, here's how this works. In the medial superior olive, neurons become precisely tuned to the timing difference between the encoding of sound in one ear, and the other ear on the other side of the head, obviously. Now, here in this figure, we see a schematic representation of this array of cells that we find here across the equator of the medial superior olivary nucleus. And I want you to notice that, that each neuron here in this medial superior olive receives two sources of input. One source of input is the cochlear nucleus coming from one side, and the other source of input is the complimentary cochlear nucleus on the other side of the brain stem. And in order for any one of these cells to fire an action potential, there must be the coincident arrival of the electrical signal, causing the release of neurotransmitter from both of these pathways onto the neuron. So when that happens, then the neuron can fire and transmitted signal to its targets. Well, let's take this neuron e for example in this illustration. In order for neuron e then to fire, there must be the simultaneous arrival of input from the left ear and the right ear. Well, because of the difference in the path length, from the cochlear nucleus on the left side of the brain stem, to neuron e, compared to the path length from the cochlear nucleus from the right side of the brain stem to Neuron e. The only way that inputs can arrive simultaneously would be if the sound source is shifted over here to the left somewhere. So, if that sound source is here on the left hand side, then the activation of the left cochlea is going to occur before. The activation of the right cochlea because of the temporal lag in the conduction of that sound wave from the left side of the head to the right side of the head. So even though the path from the left cochlear nucleus to neuron e is longer, with sound present on the left hand side of the midline, the signal is going to get started in advance on the left side compared to the right side. And with the sound source shifted to the left, neuron e is much more likely, then to receive coincident input from both sides of the brain stem. The same basic scenario would apply to each of these neurons that they're in a slightly different position along the equator of these nucleus. Finally, on the opposite side of the nucleus, we will find a cell that has exactly the opposite arrangement from what discussed for neuron e. It has a long path length from the right side, a short path length from the left side. So coincident activation of these inputs to neuron a would require a localization of sound over here to the right. So the path length is short to the right ear, long to the left ear allowing for the longer path length here to be started in advance of the shorter path lengths. Well this is really a beautiful system. it requires the precise development of connections that are specifically tuned for delays, between the arrival of sound energy on the two sides of the head. And, I wish we had time to talk in more in detail, about exactly how this system develops, and how it's tuned up to the environment. but that's a wonderful story that pertains to development and developmental plasticity. Which I would invite any of you interested to look up that literature and discover more about that for yourself. So this is the story of the medial superior olive. It encodes the localization of sound in space based on inter oral timing differences. Well, there's another division of the media superior olive that I'd like to tell you about. It's the lateral superior olive. And it encodes information about the location of the sound source based on inter oral intensity differences. Okay? So the medial superior olive bases its signals on inter oral timing differences. The lateral superior olive on inter oral intensity differences. Well here, in this part of the superior olive, the circuitry is just a little more complex. There is a second nucleus involved. so here is the lateral superior olive, and the secondary nucleus is a source of inhibitory input to the lateral superior olive. It's called medial nucleus of the trapezoid body. So here's how this circuit works. Input derives from the cochlea nucleus on one side of the brain stem, send projections on up to this superior olive. So they input the code directly to the lateral superior olive causing an exitation of that cell. But there's also collaterals that project across the midline to this medial nucleus at the trapezoid body. Neurons in the medial nucleus of the trapezoid body grow short connections then to the lateral superior olive. And function to inhibit that nucleus, when activated from the opposite side of the brain stem. So, you can imagine what would happen, if there were sound source localized over here. Just to one side, let's say this is the left side of the brain stem here. So this left lateral superior olive would be excited and fire at an increased rate, whereas the right lateral superior olive will be inhibited. And that's exactly what is plotted over here to the right hand side of the figure. We're looking at the output of the lateral superior olive or just the firing rate of neurons in this structure relative to the location of the sound source. Now, if the sound source is well to the left side of the head for example, we would expect the intensity of that sound to be much greater in the left ear compared to the right because of what's called an acoustic shadow that's cast by the head. Basically, the energy will be absorbed on the left-hand side of the head, more so than would be transmitted to the right side. Consequently, with a left sided sound source we would expect there to be very strong activity in the left lateral superior olivary nucleus. But also strong inhibition In the right superior olivary nucleus. So while activity is quite high with that left sided sound source in the left LSO, we would expect activity to be just about shut down in the right lateral superior olive, again because of this inhibitory connection. The the medial nucleus of the trapezoid body. Now, if we look on the opposite side of the head with a sound source localized to the right, we would expect there to be strong activity in the right lateral superior olive. But inhibition of the left lateral superior olive. So this difference between the activity of the two sides of the superior olivary complex is one means for encoding the location of sound and space. Based on this intensity differences that arises because of the acoustic shadow cast by the head. Now with the sound source directly in front of the listener that sound source would have equal intensity in the two ears. So, consequently the activation of this inhibitory circuit would essentially null out the difference in activity levels across the two sides of the lateral superior olive. Well the upshot of all of this is that the superior olivary complex of the inferior pons provides a dual means for encoding the localization of sound and space. The medial superior olive encodes the localization of sounds in space based on inter-aural timing differences, whereas the lateral superior olive and codes the location of a sound source based on intensity differences between the two sides of the head.
