Well now let's turn our attention to the structure of receptive fields of the ganglion cells, Again these are the cells that generate the action potentials that sent signals from the retina to the brain. We talked about receptive fields but we introduced them in the somatic sensory system. And the definition of the receptive field that we gave, was a region of the body, that when stimulated can modulate the activity of nerve cells. Well the same basic principle can apply to our retina. Only now the stimulation of the body would be the retina itself. A patch of photoreceptors that would modulate the activity of the ganglion cell. Well, because that stimulation is coming from light in the environment, we could also define the receptive field in terms of the location. In the visual environment that when illuminated or when there's some change in the level of illumination, can lead to a modulation in the firing of retinal ganglion cells. One additional point about receptive fields for ganglion cells in the retina Is that they also bear the center surround structure that we saw in the somatic sensory system, that is, there is a central region that is specially sensitive to changes in luminance and then there's a surrounding region that has an antagonistic relationship with the center. Well, putting these things together we can recognize that there are these two kinds of ganglion cells, that have different center surround configurations. There's the on-center ganglion cell that responds best when light strikes the center of it's receptive field. And then there is the off-center ganglion cell That responds best when light strikes the surround. So let's see how these two cells would respond when we illuminate the center of their receptive fields. So at time t1, a little spot of light goes off that illuminates the center What we see is a nice barrage of action potentials in our on center ganglion cell. Meanwhile, in our off center ganglion cell, what activity might have been present in the dark condition goes away. And when the spot of light in the center goes off, now there's a bit of a rebound discharge. So we have an increase in action potentials when the light goes off. Now, because we're talking about light and we can alter the level of light, we an imagine now, decreasing the amount of illumination to the center of the receptive field. And when that happens, the on center cell decreases its firing. And when that dark sport is removed, For essentially there's now an increase in illumination to the center. There's this nice rebound discharge. So that's the on center. Now let's look at the off center cell. Off ganglion cell. Then there is an increase in discharge. And when you return to that higher level of elimination then the cell becomes less active. Well, this may seem a bit curious but putting these classes of[UNKNOWN] together we have cells that might be sitting right next to each other in the[UNKNOWN] One member of the pair will increase its firing when light goes on and the other will decrease its firing but light can go off and when that happens we see the converse. The on-center cell will decrease its firing while the off-center cell will increase its firing. Putting these 2. Kinds of cells together makes each location in the retina sensitive to changes in the levels of light. Would be, they increases or decreases. Now, 1 additional consequence of the center surround organization I should mention is shown here in the final panel, panel c. When we evenly eliminate both the center and the suround notice taht there is not nearly so much change in the firing patterns of the cells. While the small spot in the center gave rise to a nice robust discharge and more even elimination across the receptive field. Gets rise to a much lower level of activity on our[UNKNOWN] cell. And the same sorts of realtionships would be seen for the off-center cell. The off-center cell stops firing with the presence of light in it's center and with more even illumination, it now begins to fire just a little bit, but not as much as when the dark field is once again returned. When the light goes off. But let's look at this behavior in a different way. So now imagine that we have an on center cell, and we have a small spot of light that is passing across the receptive field from right to left. As we pass that spot of light... From the center of the receptive field, we noticed that there is a initially, a pretty strong response in the firing rate of this on center ganglion cell. But as this spot of light acquires an increasingly more peripheral position, notice that the response. Falls and when we are illuminating the surrounding part of the receptive field there's actulaly a decrease below spontaeous level and firing. So this is the classic behavior of the on center ganglion cell that I just described. How do. These center surround receptive fields really contribute to visual perception. While there's still a lot of work addressing the possible answers to that question, but one provisional answer seems to be that this center surround structure Gives us the ability to be sensitive to edges of light. Now imagine rather than a small spot of light going across your retina that perhaps might have been a satellite that you might be looking up into the night sky and seeing the satellite passing overhead. Now imagine the much more common experience of having light and shadow in more of a linear configuration falling across your retina. When that happens, we can see that the best response from this on-center ganglion cell. Is when there's shadow across a significant portion of its off surround, while its center is completely on the illuminated side of this boundary. So when there's a luminance boundary, when there's light and shadow. And that edge is completely aligned with the center surround boundary. Then we have the greatest modulation of activity coming out of that particular cell. Well, we're getting close to the end of this tutorial, there's a few more things to be said about the organization of these Ganglion cell receptive fields. I want to address why we have on and off center responses in the first place. It does seem a bit odd that we should have these dichotomously arranged ganglion cells, that respond in exactly opposite ways to the very same stimulus. Well, how is that possible? So the neurotransmitter that's released by the bipolar cell is glutamate. And the receptors that are expressed on these two different types of bipolar cells are quite distinct. In the on center bipolar cell the receptor is a metabotropic receptive for glutamate, and that metabotropic cascade that's initiated results in the hyperpolarization of that bipolar cell. For the off-center bipolar cell the receptors are ampa receptors and we glutamate binds to ampa receptor there is a depolarizing response. So this is what's happening in the dark when the cell is depolarized at the level of about minus 40 millivolts and glutamate is being released constantly in the dark. It's keeping that on center bipolar cell hyperpolarized. Meaning that it is not releasing its neurotransmitter on the on center ganglion cell. So the on center ganglion cell is essentially turned off in the dark. Meanwhile, glutamate being released, and interacting with the off bipolar cell. Is going to lead to the depolarizaion of that off bipolar cell, the release of it's own transmitter and the activation that of the off cell. So different receptors for the same transmitter provides the basis for our on and off responses in the center of the receptive fields. So now let's consider what happens when light strikes the center of the photo receptor receptive fields that are driving activity in these two parallel channels. So recall that when light strikes the photo receptor, there is hyper polarization. Hyper polarization means much less release of glutamate, the neuro transmitter of the photo receptor. When we release less glutamate, then the on center bipolar cell is going to depolarize. Remember the metabotropic receptor mediates hyper polarization. So if you withdraw the transmitter that's interacting with that receptor, then this cells going to depolarize. It will depolarize, it will release it's neurotransmitter, which leads to the activation of this on center ganglion cell. And that cell now responds to the illumination of the center of its receptive field. Now let's think about what happens to the off center bipolar cell. So when there's less glutamate being released, that means that there's going to be less glutamate to interact with the NMDA receptor, and that's going to lead to the hyperpolarization of this off bipolar cell. If there's hyperpolarization then there's going to be less excitatory neurotransmitter release, and that's going to have a suppressive effect on this off-center ganglion cell. Again, the key principle here is that we have 2 very different receptor systems available for the neuro transmitter photo receptor and those different receptors set up these dichotomous responses in these bipolar channels to the ganglion cells. So now let's consider what happens when we decrease the illumination of the center of the receptive field. If we do that then we imagine that we are withdrawing photons from that photoreceptor, allowing it then to depolarize even more than where it was with background illumination. So now with greater depolarization, there is going to be an increase in the release of synaptic glut amide from the terminals of that photo receptor. So with an increase in synaptic glutamate, the on center bipolar cell is going to hyper polarize due to the activity of the metabotropic receptor system, whereas the off center bipolar cell. Is going to depolarize because glutamates activating its ample receptors. And as a consequence, the off-center cell is going to fire a barrage of action potentials with this decrease in light level of the center. Meanwhile the on-center cell is going to be supressed. Well, there's one more issue to consider here with respect to the organization of the receptive fields of ganglion cells, and that is the center surround interaction. Well, I'm not going to discuss in detail how all this works. What I'll rather simply say is that the horizontal cells are key to mediating. The center surround interaction. There is a bidirectional signal that is antagonistic or we call this a sign inverting signal. The glutamate that's released by the photoreceptor activates horizontal cells which in turn release GABA on the terminal of the photoreceptor as well as surrounding terminals over some considerable spatial extent of the retina. And this antagonistic interaction between the photo receptor and the horizontal cell is what gives rise to this antagonistic relationship between center and surround, with the surround signal suppressing the center signal.
