[MUSIC] Okay, so the, this pathway is very important, but it's not sufficient for perception. But what it's doing is it's going to hand off to regions of, of cortex that are going to enable us to perceive, to say, oh, I see my mother there. I, enable us to have visual perceptions. It's going to hand off a series of information about edges. Really this pathway is built to tell the difference between dark and light. There are large expanses where everything's light or everything's dark. Everything's of the same luminance, the same brightness. The pathway does not care. These neurons do not care about, things of the same luminous. So in the retina and in the thalamus, the, the way that the neurons respond is that they're very excited by stimulation in one center, in one place, and then they're actually inhibited by light around that. This is called a center-surround organization. So center-surround, and what that means is that their either very excited by light here, or, or excited by dark here. I'm, I'm sorry. Light in the center, or light in the surround. And then inhibited by light in the surround or inhibited by light in the center. So they're making a distinction between the, the center and the surround. And once you get to visual cortex these spots become longer. They get elongated. Maybe what happens is that a, a few of these line up and add up to become something where there are actual edges. So these are this is the basic response properties of neurons in the primary visual cortex. Where their responsive to, a slit of light in a, in a specific orientation. This orientation, this orientation, that orientation, and, and inhibited by light outside of that slit of light. So, how can we can we see this in, in our in, in a visual illusion. And this is the, called the Cornsweet illusion. And it basically demonstrates this this property of the visual pathways of, of really highlighting edges. And so what you see is that this left half looks darker than the, than the right half. In fact, they are the same luminance. If I put a, a barrier right there, you can see that they're the same brightness. And what's happened instead is that the, right at the edge, the darkness goes down, and then it goes up. And then it goes back to the same level. So this, this level, and this level, are equal. And we, but, we detect this edge, because we detect this. And essentially what you can think about, imagine a center-surround here, the plus and the minus [COUGH] equal each other out. But one that's on the edge here there's an, there's more of, one of them, one of the center of the surround will outweigh the other if part of the receptive field is in this half and part is in that half. If it divides the barrier. This Cornsweet illusion is a really good example of how we need edges in order to perceive in, texture. And we need edges either in space, as this, shows you, or in time. So what do I mean by, a edge in time? What I mean is that, if you were perfectly still and the visual scene was perfectly still. Neither of those conditions is, is typically met, but let's say you were perfectly still and the scene was perfectly still after a, a while. The scene would fade, you would no longer see it because there is no change across time. So, one of the things that we do when we're fixating on something is we make little eye movements in order to juggle the visual scene so that we have edges that cut across time. And we'll look at that more when we look at the gaze control. Okay, so in the final part of this module, the final, the final segment of this module, we're going to look at what we do with a higher processing of this basic visual information. [MUSIC]
