[MUSIC] Okay, so now we're going to leave the retina and we're going to get visual information into the cortex. And of course, from retina we actually, before we can talk to cortex, we have to go through thalamus. Remember, thalamus is our translator. So, what we're going to do here is just follow the pathway from retina to primary visual cortex. And this shows you the two visual fields. This is a person who's foveating, whose two foveas are fixated on this center spot. And this is the right visual field, in red. And, and in blue is the left visual field. And what you can see because of the pathway that light takes, this left visual field is going to hit this half of the right et, retina and this half of the left retina. Now you could memorize that this is the right temporal and the left nasal. But, there's a much easier way to remember these pathways. I've never memorized this, I just draw it out every time because the easy thing to remember is that this left visual field has to end up on the right visual cortex. So, if it already is on the right, it doesn't have to cross. So information from this retina, this half of the retina which is getting information from the other side's visual field, it's just simply going to go to the thalamus on the same side, and from the thalamus on the same side to the visual cortex on the same side. On the other hand, this information from this half of the retina, which is getting information from this half of the visual field, it's on the wrong, it's on the opposite side from where it has to end up. So where does visual information cross? It all crosses at the optic chiasm. That's this structure right here, the optic chiasm. So it's going to go down, across, over into the thalamus, and from there to the, to the visual cortex. So there are a few points that we're going to make with this. One is that if you, if you test visual fields you test one eye at a time. So normally in the left eye, if you are looking at this center point you can see the right half of the world, and the left half of the world. Normally, if you are looking at the world with your right eye, you can also see the left and right halves of the world. What would happen if, let's just take a few, we're going to take a few cuts. What would happen if there was some, something that prevented this optic nerve from from working? And what would happen is that in, from the right eye we would see nothing, this would all be nothing. But left eye, we, it would be completely normal. Okay? So we'd just be blind in the right eye. Blind in the left visual field of the right eye and right visual field of the right eye. Now what would happen anywhere with a lesion anywhere back here? From optic tract, optic radiation, or primary visual cortex? Well in that case, what would happen is that on the, in the right eye we would lose the left visual field. So this part would be no, and this would be fine. And on the, on the left eye, we would also lose. The, the, left visual field. But the right visual field would be fine. And that is so that the, what we lose on the left and the right is coordinated, it's, it's the same. And then we call that in, in medical terms that's homonymous. It's a homonymous visual field loss. The same on the two sides. Now, we're going to look at one more lesion, and that is right here. You know what's right here? If you've been watching the video, the lab videos, you remember that right here is the pituitary, it actually sits underneath the optic chiasm. And if there's a space-occupying tumor in the pituitary, a pituitary adenoma, it can come and push up on that optic chiasm and, and block those fibers. So what would the, what would the visual field of somebody with a pituitary tumor that, that impacted the optic chiasm be? Well on the right side, they would lose the right visual field. So this would be no good, this would be fine. And on the left side, they would lose the left visual field, this stuff that is crossing, and this would be fine. And that's, so, basically, what you're losing is the two peripheral halves of the, of the visual field, the two temporal halves of the visual field. Now, one final point. Let's go over to, to here to the tablet. And what this shows is an original photograph of a lovely anhinga. This is a beautiful bird. And what this shows is, all I've done is I've taken the, the two halves of the, I've taken the picture, I've made it into 50% transparency, and I ever, either overlaid it perfectly as in here. Or I overlaid it with a with a, a jiggle so that the two overlays are not completely on top of each other. They're either displaced up and down, side to side, or one's been rotated a bit. So, what this tells you is that if the two eyes are not perfectly aligned, things will look blurry. And the point here is, if we go back to here, we can see that you can have poor vision either because there is some problem in the visual pathway or because the eyes are not perfectly aligned. And understanding that those two things are, are possibilities is very important. Because oftentimes a blurry vision is the result of misaligned eyes. I mean certainly even late at night if you're really tired, you're kind of, sort of o, beyond tired, you'll find that your eyes don't perfectly align and, and things can, you can see double vision. That's actually more likely to come from a, a problem of misa, misaligned eyes than it is from a sudden stroke in eh, somewhere in the visual pathways. And one prediction that you can make is that if it is a problem with eye alignment, then, if you close each eye, vision should be normal in each eye individually. But it's not normal when both eyes are open. Okay. So in the next segment we're going to look at, at how visual information is processed at various points in this, in this pathway. [MUSIC]
