 We're going to talk about visual physiology in this video, like how do we actually see? So the key is that we have to get, you know, the photons of light that are bouncing off the things that we're looking out in the world have to get to our photoreceptors. We have a whole system designed to do that, and then we'll talk about what happens once those rods and cones are stimulated. So the beginning here is light has to be bent, light has to be bent or refracted as it passes through the eye. So normally when we think about bending of light rays, we think of the lens, but about 70% of this light refraction is actually occurring by the cornea and the aqueous humor of the eye. The lens, which is full of these cool transparent proteins called crystallines, it's going to do the rest of the job. It's going to finish off the refraction or bending of light rays to focus the image on the retina, on the photoreceptors. And I'll talk in the next video what happens if that doesn't work, where you're going to be near sighted or far sighted. So the lens's job is to focus a visual image on these photoreceptors. So the lens provides that extra bit of refraction we need after the photons of light from the image have already gone through the cornea and the clear aqueous humor. All right, so the lens will change shape in order to do that, depending on what we're looking at, right? The idea of accommodation, looking at far and near images, it takes a different shape lens to do those things. A flat lens is good for far vision. A more rounded or thicker lens is good for near vision. Okay, so we've gotten the image that we're looking at through our cornea, aqueous humor. It's gone through the pupil, right? That opening, depending on how much light there is, the lens has finished off that refraction. And now the signal is traveled through the posterior chamber of the eye, which is full of that clear, vitreous humor. So notice the word clear or transparent constantly. Everything has to be clear. Or also we're looking at can't reach these photoreceptors. So here's the photoreceptors. I've already done a separate video on them. But as big picture, we have rods and cones. Rods doesn't take very much light to stimulate them, but they don't discern color at all. Rods see the world in grayscale, and they're also not very clear. So if you're in a low light situation, rods are quickly important. They'll sense shadows and movement and diffuse edges. You can tell some things there, but not exactly sure what it is. Cones need a lot more light to work. That's their big downside. But they can see things in colors, right? Using the three different types of cones we have, red, green, and blue, we can see up to a million different colors. And they see clear edges, defined edges. So if you want to clearly see something, it takes a lot of light to trigger these cones. And if you don't have one or more of these cones, you would be color blind. So you can go back and watch this other video about rods and cones there. So once these photoreceptors are actually triggered, now the visual pathway is going to begin. So the visual pathway begins at these photoreceptors. Now real quickly, not a big deal, but you'll notice light's coming from the bottom. Light actually travels through where the ganglion and bipolar cells are to strike the photoreceptors, and then the signal actually gets sent back. It's kind of weird. It's almost like the eye is backwards there. But once this message triggers the photoreceptors, it's going to synapse in two places, from the photoreceptors to those bipolar cells, then the bipolar cells to the ganglion cells. And I covered the two different types of ganglion cells in that rod and code video as well. So this message has to cross those two synapses. So from the rods and cones to the bipolar cells to the ganglion cells, these ganglion cells are going to collect there and converge on the optic disc, which is called your blind spot because there's no photoreceptors there. And they're going to travel through the wall of the eye and they'll proceed here, as you can see, as cranial nerve three, the optic nerve. So the optic nerve is going to travel, and it's going to, you're going to reach that optic chiasm, which is in the diencephalon. So you'll notice there that at the optic chiasm, only some of the nerves cross. They don't actually all cross. So the optic chiasm is where some of these nerves are going to cross. Now, after the optic chiasm, these bundles of neurons are now called the optic tract. This has to do with the fact that the word nerve is used with the peripheral nervous system, tract with a central nervous system. So the optic tract is going to carry information from the eyes to really three different locations. So most of it's going to go to the thalamus, specifically, as you can see here, the lateral geniculate, geniculate, hard saying, lateral geniculate nucleus of the thalamus. And that's going to be the relay center that's going to send the information on to the visual cortex in the occipital lobe. So that's one place it's going. But then some of this information is also going to the superior colliculus, which helps track movement. So it's how we track moving targets, these kinds of things. And then lastly, a very small number of these retinal ganglion cells are going to project to the suprachiasmatic nucleus of the hypothalamus. And that's going to, these do not actually perceive images. They're photosensitive. They can tell if it's light or not, but they don't actually tell you what you're looking at. So the presence or absence of light is going to tell your hypothalamus, basically what time of day it is and how long the days are. So the suprachiasmatic nucleus takes this information and uses it to set your day-night cycles, your circadian rhythms. Now that's great for most of you in history. That's great. And I won't go into it all in detail here. But the problem now is we use so much technology and we have so much light that even at midnight, your brain can be convinced that it's new, right? And that is a problem. Okay, so we've talked about how we've gone from the image traveling through the eye to the retina. We've talked about how the retina has these photoreceptors that are going to send the signal to the bipolar cells, ganglion cells. They're going to leave the eye and become the optic nerve. At the optic chiasm, some of them will cross. We'll talk about why they all don't at the end of this video. Most of those nerve signals are going to the lateral geniculate nucleus of the thalamus where they're going to be relayed onto the visual cortex and the occipital lobe. Some go to the colliculus for tracking movement and then some go to this suprachiasmatic nucleus to tell you what time of day it is. But now we're actually at the conscious mind here, the visual cortex. So notice we have these what are called combined fields of vision because when you're looking at something, the left side of both eyes are going to send information to the right side of the brain. And the right side of both eyes are sending information to the left side of the brain. So you'll notice that some of these nerves are going to cross and some do not. So you see the purple and green here. Everything you're looking at with the photoreceptors that are purple are going to end up on the left side of the brain. Everything it's green on the right side of the brain. So we need two eyes to see things clearly because everything we're looking at is actually a combination of both eyes looking at them. So some of these nerves do not cross which is very rare in the nervous system. So why do we need two eyes? This binocular vision, the key is that it gives us our depth perception, right? The left, the images that are entering the brain from the left and right eye, we can put that information together and figure out about how far something is away. So I wanted to talk about why do images show up in reverse, upside down and backwards? So I've got a picture of all over here it is kindergarten graduation. So light from the top of an image hits the bottom of the retina. Light from the bottom of an image hits the top of the retina. Light from the left side of an image hits the right side of the retina. And light from the right side of an image hits the left side of the retina. I think I get you get where I'm going here. So the image is going to be reversed upside down and backwards because what was on the top of an image hits the bottom of the retina and was on the left side of an image hits the right side of the retina. So actually the retina At the retina, this image of Oliver is going to look like this. It's going to be upside down and backwards. It's going to be reversed. And you see that with everything. So here we see our field of view. So notice that the image striking the retina is reversed upside down and backwards. Your brain has to take all that information using both eyes. And it has to figure out what's coming from the left visual field and right visual field. And it turns it into a picture that we can see. And then your brain will integrate that image and put the two fields of view together and then flip it back around so you're seeing it normally. OK, so that's why images strike the retina upside down and backwards. All right, lots of stuff going on there, I get it. But this is how we see this. This is our visual physiology. I hope this helps. Have a wonderful day. Be blessed.