Now the correspondence problem is not the only challenge in understanding vision with two eyes as opposed to one. The other challenge, and it's a major one, is how binocular fusion occurs. That is, how do the two eye views get put together in any event whether you're talking about stereopsis or anything else. So let's review first a bit of anatomy and physiology that we talked about to some degree before but now I'm going to go into a little bit more detail. So let's look at this diagram, the left retina and the right retina. And you remember that the left retina goes to the thalamus through the optic nerve. Optic nerve being here. Then through the optic radiation being here, to the visual cortex. And the other eye joins it by traversing the optic chasm, going to the thalamus, and again, the halves of the views of the left eye and right eye view come together. We talked about this before, you can go back and look if you need to be reminded about that. Come together in the visual cortex. But where do they come together, and how do they come together? We didn't really talk about that. What we did discuss, is that at the level of the thalamus, and here remember. So we have the views of the right eye in blue and the left eye in green are kept apart at the level of the lateral vernacular nucleus of the thalamus. They are in different layers, and you may recall that we looked at those different layers. And described them as magnocellular and parvocellular, and some interesting aspects that we discussed earlier. But now we're considering them from a little bit different view, where do the two eyes come together? Where does the information from the two eyes come together? Well obviously it's not in the thalamus. We have to look further at the projections from the thalamus to the visual cortex. So this is the primary visual cortex, obviously in diagrammatic form, and these are the layers that we talked about before. And it is simply the case that in mammals the input from the two eyes come together in layer four of the six layers of the neocortex. It's the neocortex, the covering of the cerebral hemispheres in primates, carnivores, and other mammals come together in layer four. But they come together in a very special way, and they are still initially kept apart. They are kept apart because the blue from the right eye and the green inputs from the left eye come into layer four in what are called stripes, or columns, ocular dominance columns, ocular dominance stripes. We kind of skirted around this a little bit earlier, but now it becomes central in asking the question where do the views of the two eyes actually come together in neurons, in the individual cortex. Well, they don't come together in layer four because they're kept apart, just as they are in the thalamus in these ocular left eye, right eye stripes or columns. That's an interesting phenomenology in its own right that's been very useful in studying neuro-development but it's not our concern today. So where do they come together? Where does the input from the two eyes come together? Well they come together in the layers above and below layer four, so all of these triangles are binocular neurons above and below layer four and that's where the views of the two eyes come together in cells that are monocular. That is you record from them, you stimulate the left and the right eye in layer four. You get a right eye view or a left eye view, some limited combination of them. So they come fully together in the binocular cells that are above and below layer four. And that raises an expectation that oddly is not fulfilled. So you record from one of these cells. In these other layers and the cell responds to the left eye view and the right eye view. So from that coming together. In binocular neurons, cells that respond to input from both the left eye and the right eye, you would expect to see something that was sort of a grid if you present it to the left eye. And to the right eye, images like these that are vertical stripes and horizontal stripes. And again, you can see this for yourself by free fusing these, or taking a piece of cardboard, putting it between and looking, limiting the right eye view to the horizontal grid. And the left eye view to the vertical grid. You can see this phenomenon for yourself, and it's called binocular rivalry. You don't just see a grid which would be the, I think common sense expectation from the fact that they're numerous binocular cells in the visual cortex that get input from both the left and the right eye. So, fine, they get input from the left eye. Vertical input from the right eye, horizontal. Wouldn't that just make a superimposed grid? A mesh. Well, that is not what you see. And again, I invite you. It is really quite interesting to look at this and see it for yourself by fusing or using the trick with the cardboard. What you see is, sometimes vertical, sometimes horizontal. But for the most part you see a binocular percept that is dynamically changing all the time. There are patches that are more or less vertical. There are patches that are like this that are more or less horizontal. But it's dynamic, it's moving all the time as if the brain, the visual brain, is simply very uncomfortable with images like this that are fundamentally different in the left and the right eye. It's as if the visual brain doesn't really know what to do with them. And is going back and forth in a phenomenon that's been referred to for years, decades, as binocular rivalry. This has been known for of course a very long, long time, since Wheatstone's Era.
