We see things along the way like lenses, you know and by the way why most of us
need reading glasses when we get a little older is because these lenses in the eye,
this is what we bend when we squint. And when we're young we're really good at
bending this and we can, we can focus light on our eyes really well.
But as we get older this lens gets harder, more brittle.
It won't bend as easily. And eventually we just can't bend it
enough to focus on the eye and then we need another lens.
literally glasses or something like that to help us with the focusing process.
But for a young, healthy eye, the light gets focused thanks to this lens onto the
retina and the retina is really the point of contact.
Okay. This is the point where right up until
now it's light energy but at this little magical area at the back of the eye that
light energy is going to be translated into a neural signal and then sent out
the optic nerve. Let's go there.
These are the two critters that really make the action happen.
Now there's actually three kinds of these cones.
there are ones that relate to red light, green light and blue light.
Okay, so they react to different things. I sometimes try to think of these guys
almost like they're vampires. Because this is kind of what happens.
They go through a process called bleaching.
What that means is when light of the right type hits them, it causes this
chemical reaction. So I think of a vampire you know,
dissolving into a bunch of pieces. Well, these don't dissolve into a bunch
of pieces but a chemical reaction happens and it's that chemical reaction that
literally produces. So they, they are what's called
photosensitive. So, light causes this reaction.
And it's the reaction which stimulates the nucleus and starts an action
potential. Okay?
So these are just, you know, light hits them.
And if enough of the right kind of light hits them, that's what kicks off the
action potential. Now, what do I mean by the right kind?
Well, for these kinds, these rods, it's really just brightness of light that
matters. Anytime there's a suitably bright light,
they will cause their little chemical reaction and will start a neural signal.
But you're going to see these are kind of like a shades of gray thing.
These, these are what we use at night. I don't know if you noticed.
But when I asked you to turn the lights down.
If you ever try that again. Turn the lights down and then look around
you. And what you'll see is if there's no
bright light anywhere everything kind of looks shades of grey.
But when there's lots of light, that's when we see color.
And that's what these rods do. They react to colored light.
As long as it's sufficiently bright. So they're really good in the daytime but
they're kind of useless at night. but if it's sufficiently bright, they
allow us to kind of take that energy and not only sense the energy but also sense
the wavelength of that energy. Was it, which is ultimately what we
experience as color. There's a whole lot to this story and I'm
going to have to go kind of quick. but I want you to know the players and
get a feel for the process. So these are really the ones that start
things happening but here's this kind of interesting process.
So when we look at how these rods and cones are distributed on the eye this,
this isn't really looking at an eye. This is more like looking at the back of
the screen of, of the eye. But if you're focusing on something and
you're looking at it. That area where you're focusing we call
the fovea, okay, that's the sweet spot of the eye.
And what you see is that sweet spot has a whole lot of rods and cones.
That's what's depicted by these red, green and blue things.
There's a lot of rods and cones in there. and so what that means is when you really
focus your attention, when you really look at something very carefully.
Your casting that light on your fovea and you're seeing rich detailed color.
information about what you're looking at. So you can really see it really well.
around the outside of the fovea you know, where you're not focusing so much.
More at what we call your periphery that's where a lot of the rods are.
They're really useful in, in the night when all of these things become kind of
useless. If there's not enough light, the ro-, the
cones are, are useless and the rods take over.
And it's the rods that give us a, a sense of night vision, as it were.
an ability to kind of detect shades all be them kind of shades of grey, but we
still can kind of see things in the light.
So, you know if you look at what, what this fancy thing here is.
If you look at the eye and how these cells respond, the cones are responding
very heavily to information that's presented right at the fovea.
and the, and the rods aren't there's no rods at the fovea.
But outside on each side of the fovea as we go out, that's when the rods really do
their thing. Notice here there's optic disc.
There's a point on the retina where there's neither rods nor cones.
There's, you actually have a black spot on your retina at all times.
But what the brain does is it kind of fills it in, it knows what's on either
side of that black spot and it fills it in for you.
And you don't even notice that you have that which is you know, another whole
story. It's kind of cool.
Hey, I tell you, everything's cool [LAUGH] about this stuff, it really is.
alright, so we have all these rods and cones.
They pick up the light. They turn it into a neural signal.
But then other cool stuff happens and, and I want to tell you about this because
I want you to understand that a lot of our sensory perception sometimes happens
before we even get to the brain okay. At the sensory organ itself and the eyes
are like this so for example and this is a really strange setup that the eye uses.
We have light coming in here, to the eye. And so, this is like, the back of the
eye. that's the optic nerve going out.
And so, light, when it strikes the retina, it actually does this, it goes
through this layer of nerve cells. It goes through these neurons and it goes
through the rods and cones as well. Sorry, it goes through the ganglion
cells, goes through the bipolar and then through the rods and cones and then
strikes the back of the retina here. And causes these photo receptors to
bleach and starts the signal, which then goes through the rods and cones, through
the bipolar cells, through the ganglion cells and then out the optic nerve on its
way to the brain. So its kind of a weird thing the light
goes all the way through these things without stimulating anything.
Hits the back and then starts a process that comes back through this way.
Why I don't know, that just seems to be how the eye is designed.
but you know its kind of interesting and weird.
