Retina


We did also look before about how the
cones are not evenly distributed, right? There are many, many more red and
green ones than there are blue ones, remember we were talking about that? And we see that most of your color
vision is based just upon the ratio between the red and the green, and
the blue is just there to give you a little bit of,
sort of sensitivity to blue, but you don’t really have high
resolution in the blue channel. If you were to take a look in your eye,
well, not your eye because this
has to be a dead eye. So, some other eye, or
your eye after you’re dead. If you want to donate your eye when
you’re done just send Megan a postcard. If you peal the retina out and
you stick it under electron micrograph, if you take a look at that,
you’d see two, you’d see that as you moved along
the eye it was not the same everywhere. In the middle, which is the area
referred to as a ‘fovea’, about one and a half, degrees, you see this magnificent hexagonal
packing of photo receptors, okay? So, hexagonal pack, so i, if you will, the pixels in the human eye are not on
square grid, but they are on a grid. They’re on a hexagonally packed grid,
and the regularity of this is
pretty remarkable, right? I mean, it, it, it gives you a very
good sampling, a ve, very dense and regular sampling of
the underlying visual field. Which is one of the reasons that you
have such good high resolution up here and really lousy resolution out here. You see, so
I’m looking straight at Megan, wake up. Okay, good. So I’m looking at, I can’t really
tell how many fingers I’m holding up. I mean, I happen to know, of course,
because they’re my fingers. So I can tell there’s seven of them,
but or, whatever. But I can’t, if you do that
yourself just do it this way. If you want to have a partner, look,
look them straight in the eye and have them bring their, their fingers in. You’ll see their fingers are there. You can have them wiggle them, but
if you, if they ask you how many fingers are they holding up, you cant really
tell until it gets pretty close. And yet, you feel like you got all
this high resolution vision out here. That’s an illusion that your
brain is creating for you, out here in your periphery you just have
good sensitivity in motion, in fact, wiggle your fingers you’ll,
you’ll see them right away. Why might you want to do that? Well, maybe you want to
saccade your eyes over there. So, the system is very good at
detecting motion in the periphery, and controlling the eye to look at it and
foveating on what’s there. But you actually only have high
resolution in this very small area. In the periphery, as shown here,
you have a few large cones. So, the cones are actually big. And they’re giving you
overall color information. And then you have this high density,
again, of rods so, with low levels of illumination,
you can still see out really well. There, let’s take
a look at this picture. So this picture is degrees from center,
so temporal is off to the side,
nasal is towards the nose. And the first thing you’ll see is
that you’ve got all these cones that are mostly in the middle,
in the fovea. That’s exactly what we said. Notice, the same way we have
rods out here in the periphery, notice you don’t have
any rods in your fovea. Over here’s your blind spot, right? You don’t have anything there. That’s why it’s blind. So, what would one of the implications
be of having no cone, no rods in your fovea? It would mean that you don’t have very
good low light perception in your fovea. And here’s another
experiment you can do. Some night, go outside when
there’s a bunch of stars, some that are really bright,
some that are less bright. See if you can find a star that’s less
bright that stays there ’til you look at it, and when you look at it directly,
it disappears. And that’s because your fovea can’t
see the low light that your rods do.

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