Right now you’re watching me in a video on 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 interpret what that means.
But how does your brain let you do all of that?
They say the eyes are the window to the soul.
But this week, they're actually the window...to the visual system.
The eye is a really fascinating and complex structure.
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 happens after 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’s how.
When light comes in, it hits this first layer of cells, called photoreceptors.
These photoreceptors are actually a type of neuron.
In one of our earlier videos, we talked about how 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 common name: 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 seeing fine 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, it activates a type of protein called an opsin,
which changes its structure to set off a chain of events
that results in the cell becoming “hyperpolarized”.
That is, the inside of the cell becomes more negative than normal.
“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 only is 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. When the photoreceptor is more positively charged,
or depolarized, it blocks these bipolar cells from firing.
So when light hits the photoreceptor and hyperpolarizes it,
making the photoreceptor more negatively charged on the inside,
this allows the bipolar cell to depolarize and pass the message along.
These are called “on” bipolar cells, since they’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” signals are important for creating contrast and
allow your eye to detect edges.
No matter which kind of bipolar cell they are, they all connect to
retinal ganglion cells, or RGCs.
These RGCs sit in a layer on top of all of the other cells in your retina,
and these are the cells that project to your brain.
All of the RGCs’ axons bundle together to pass through a single, narrow channel in the retina.
This bundle of fibers is called the optic nerve.
It follows this path back to a structure called the optic chiasm
It’s a “criss cross” shape.
This is because each of your eyes can only 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 all be integrated.
These axons go all the way back to this structure called the “lateral geniculate nucleus”
which acts as a relay center for visual information.
Here, your brain is able to start understanding
and sorting the visual data it’s receiving, and passing the information
on to the primary visual cortex, or V1 as people in the biz call it.
V1 has a “map” of visual space imprinted on it.
This means that there is a precise correspondence between
a point in your 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”:
the dorsal stream, which is processed up on top of your brain in the parietal lobe,
and the ventral stream, processed down below in the 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 parietal lobe, the dorsal stream, is sometimes called
the “where” pathway, because it’s where your 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 spatial information.
The temporal lobe, on the other hand, is important for recognizing objects.
That’s why the ventral stream is often called the “what” pathway.
It’s responsible for object and face recognition.
The processing here is 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 complete picture of what you see.
Whew! We’ve covered a lot today, haven’t we?
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.
If you liked it, please subscribe, give a thumbs up, and come back next time to learn
about how you’re able to hear my beautiful voice when we cover the auditory system!
Thanks for watching Neuro Transmissions. I’m Alie Astrocyte. Over and out!