[SOUND] Okay, so let's figure out how visual perception happens. What we've, we've done is we've gotten information through the eye, which is really big piece of the journey, to the retina, thalamus, and into the primary visual cortex. And another name for the primary visual cortex is the striate cortex because it has a stripe that was discovered in 1776 by a Italian medical student named Gennari. It's the stripe of Gennari. And if you look at one of the videos the horizontal section video, you'll see the stripe of Gennari, it's really cool. So, the reason I mentioned that the primary visual cortex is also called the stripe cortex is because everything outside of the striate cortex that processes vision is called extrastriate cortex. And that's what we're going to talk about now. And extrastriate cortex is where perception happens. So remember that the, that V1 or the striate cortex, V1, is back here. Mostly on the medial surface of the brain. And from here, its information is going to go in two basic pathways. A ventral stream and a dorsal stream. A ventral stream is the what pathway. And the dorsal stream is the where pathway. So, let's, let's take that apart. Down in this ventral stream, we figure out what things are. So, we figure out what all these different lines and, and edges and colors. What do they actually represent? This is what Oliver Sacks described in The Man Who Mistook His Wife For a Hat. So, he described an individual with, what's called, visual agnosia. So, in other words, the, the title character of that story, didn't understand what it was that he was looking at. So he thought his wife was a hat, and when Doctor Sack showed him a rose this individual could describe all the components of the rose. He could say there's a, a green cylinder and then these polygons of red and, but he didn't know that it was a rose. When he smelled it, he immediately identified it as a rose. And so visual agnosia has these properties of all of visual processing is fine. He sees that it's a green cylinder, he sees the red polygons, he understands all that, but he can't put it together to say what it is. And that's the, that's the essence of this visual agnosia, is that the pathway through until the final perceptual interpretation event that pathway's fine. But that interpretation can't be made. And there's another form of a visual agnosia called prose, prosopagnosia. And that is a, an agnosia that is specific for faces. These are individuals who can't tell they can't tell a face. They can't recognize their own face. Their their family member's faces. They may not know that they're in fact looking at a face. So prosopagnosia is, is another form of this. And there are there are interesting stories by people who have prosopagnosia, as well as tests to show where on the on the scale of severity you, you may fall, so some people have a little bit of prosopagnosia. If you take away, for instance, all the hair, all the identifying characteristics, how well can you tell one face from another face. Okay, and then, [COUGH] this dorsal stream is more about where things are. What direction are they moving in? How would one-half, you, you might think about it as how would you have to move in order to reach something? And so it makes sense to a certain extent that this where is moving off towards here, where motor cortex is. So this is information that's going to feed in to the motor cortex. There this may also actually have a big role in using tools and understanding how tool use used is used. There is an interesting case that, that was recently popularized in the press about an individual who could see rain. And this individual had, she had a lesion back here in V1. So she had no primary visual cortex. But what was intact is an area up here, where direction move, and movement is perceived. Not just sensed, but perceived. And the trick is, that there is a, there is a very small, but present pathway from a, that does not require V1. It goes to this area of, that detects movement called MT and and this person had that pathway intact, so she could see the rain although she could say nothing about what she was seeing, she just saw movement, she could see movement. So this is these two different pathways, and one final point to make which is that viewing things and seeing them actually perceiving the visual world takes attention and there is a syndrome called hemispatial neglect, which typically happens with the, with lesions in the right hemisphere in this area here. And in, in patients with this hemispatial neglect, what happens is that the entire left side of the world does not exist. Okay? So this is a, this is a one-sided thing. This does not happen so that people ignore the right half of the world. It happens so that they ignore the left half of the world. And it's associated with a lesion here. And what's really remarkable is you can't, you can't actually get them to turn to look that way. But in fact, there's nothing wrong with their visual pathways. It's simply that they will not attend to the information coming in, in the left half of the world. So not only do we need all of the visual information coming in from the eyes, but we also need information to be processed and interpreted by these extrastriate pathways, these extrastriate streams and finally we need to actually attend to what we're looking at, in order to understand it. Okay, great, so in the final module, we're going to just talk about how we go from being a baby, who cannot do any of this, cannot interpret what we're seeing to learning how to see. [MUSIC]
