How do we see?
Right now you’re watching me in a videoon your computer screen. You can see my face and bask in my beauty. You can recognize the color of my shirt, and when I wave my hand, you can follow that movement and interpretwhat that means. But how does your brain let you do all ofthat? They say the eyes are the window to the soul.
But this week, they're actually thewindow...to the visual system. The eye is a really fascinating and complexstructure. The part that we care about most is way at the back, called the retina. I’m not going to go into how light works or the complete anatomy of the eyeball. What I want to talk about is what happensafter light passes through the lense. Strangely, the anatomy of the eye seems inside out. The cells that receive the light signal are all the way at the back of your eye. That signal then moves toward the front again before it can be transmitted to your brain. That’s weird, but it all works. Here’show.
How do we see?
When light comes in, it hits this first layerof cells, called photoreceptors. These photoreceptors are actually a type of neuron. In one of our earlier videos, we talked abouthow signals are transmitted from one neuron to another at the synapse, using neurotransmitters. In the case of the retina, these neurons essentially act like light is a neurotransmitter! You might know these cells by their more commonname: rods and cones. Why are they called rods and cones? Simple.
That’s the shape of the cell! Let’s break them down. The rod cells are in charge of low-light vision.They’re not very useful for high resolution images, but they are really important nonetheless. The cone cells are responsible for seeingfine detail and color. Humans have three subtypes of cones: red, green, and blue. All of our visible light is a combinations of these three colors. When the light hits the photoreceptor, itactivates a type of protein called an opsin, which changes its structure to set off a chainof events that results in the cell becoming “hyperpolarized”. That is, the insideof the cell becomes more negative than normal.
How do we see?
“Hang on a second. In order to send a signal, I thought the neuron has to be positively charged on the inside, not more negatively charged. That’s what you said before!” That’s true! But it turns out that not onlyis the eye built inside out, it also uses backwards signals! I’ll explain this as best I can, but it gets a little complicated, so hopefully this diagram will help us out. The photoreceptors connect, or synapse, onto the next layer of cells. These cells are called bipolar cells. These bipolar cells react to light in one of two ways. Some bipolar cells fire in response to light.
How do we see?
When the photoreceptor is more positively charged, or depolarized, it blocks these bipolarcells from firing. So when light hits the photoreceptor and hyperpolarizes it, making the photoreceptor more negatively charged on the inside, this allows the bipolar cellto depolarize and pass the message along. These are called “on” bipolar cells, sincethey’re on when the light is on. And some bipolar cells are the opposite. Instead, they hyperpolarize, or become more negative, in response to their photoreceptor becoming more negative. This means that they depolarize, or become more positive, when there is no light, because the photoreceptor is firing, which makes the bipolar cell fire. These are called “off” bipolar cells because they’re off when the light is on. The two different “on” and “off” signalsare important for creating contrast and allow your eye to detect edges.
No matter which kind of bipolar cell theyare, they all connect to retinal ganglion cells, or RGCs. These RGCs sit in a layeron top of all of the other cells in your retina, and these are the cells that project to yourbrain. All of the RGCs’ axons bundle together topass through a single, narrow channel in the retina. This bundle of fibers is called theoptic nerve. It follows this path back to a structure calledthe optic chiasm It’s a “criss cross” shape. This is because each of your eyes canonly see a part of your visual field. If you cover up one of your eyes, you’ll notice that you see a lot less from side to side.
So, in order to create a seamless picture,your brain came up with a way to combine the information from both eyes. At the optic chiasm, some of the axons cross and project to the opposite side so the information can allbe integrated. These axons go all the way back to thisstructure called the “lateral geniculate nucleus” which acts as a relay center forvisual information. Here, your brain is able to start understanding and sorting the visualdata it’s receiving, and passing the information on to the primary visual cortex, or V1 aspeople in the biz call it. V1 has a “map” of visual spaceimprinted on it. This means that there is a precise correspondence between a point inyour subjective visual field and a location in your brain. V1 then projects onto an area called V2, where cells have many of the same properties as V1, plus some more complex coding. In V2, visual information splits into two “streams”:
How do we see?
the dorsal stream, which is processed up ontop of your brain in the parietal lobe, and the ventral stream, processed down below inthe temporal lobe. Remember how I told you that the parietal lobe is responsible for the way you sense your body? Well, this stream to the parietallobe, the dorsal stream, is sometimes called the “where” pathway, because it’s whereyour brain processes where things are in space, including the location of your body. This stream is helpful for seeing motion, and consists of fast, mostly unconscious sorting of spatialinformation.
The temporal lobe, on the other hand, is importantfor recognizing objects. That’s why the ventral stream is often called the “what”pathway. It’s responsible for object and face recognition. The processing hereis slower and more deliberate. And it’s strongly connected with your memory systems. When the information comes in, your brain parses out details like color, size, shape, and orientation. Once that’s all figured out, your brain is able to put together a completepicture of what you see. Whew! We’ve covered a lot today, haven’twe?
The visual system is a complex and interesting area of neuroscience and probably one of those fields where we know the most information. I couldn’t cover all of the details here,but check out some of the links in the description if you want to learn more about visual processing. I hope that you learned at least a little bit about how your brain “sees” the world around you.
How do we see?
If you liked it, please subscribe, give athumbs up, and come back next time to learn about how you’re able to hear my beautifulvoice when we cover the auditory system! Thanks for watching Neuro Transmissions. I’m AlieAstrocyte. Over and out!
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