We’re going to get into now
that the light has come in,
it’s being detected by the eyes
and it’s going to the brain.
We need to somehow process
And this is in itself a fairly complicated
and robust thing to understand.
So you’re going to have to probably spend
some time looking at this figure that you
see here and familiarize yourself with the
structures and the way light goes in.
So a couple of key things to realize,
you know we have binocular visions.
Two eyes, okay?
So each eye actually detects a
bit from both visual fields.
So we can have two
So imagine this.
If you were to take the world
and you were to cut it in half,
you would have your left visual
field and your right visual field.
Now within each visual field,
you’ll have two more hemispheres.
And information from both sides of the
world go into both eyes bilaterally.
So that’s a lot of crisscrossing happening.
So we’ll walk through a couple of
examples and it will start to make sense.
Once it enters the eye,
it crosses yet again.
And it is processed by the
opposing side of the brain.
And that’s done in the primary visual cortex
which is found in the occipital lobe.
So the right and left visual fields
are processes -- are processed
the visual cortex.
What we’re saying is what
you see in the right
visual field is processed
by the left and vice versa.
Nerves cross at
the optic chiasm.
So if you look at this diagram,
it’s where the tracks are coming
in passed the eye and that you
see them crossing before they enter
the actual lobe of the brain.
That is called optic
chiasm as it’s labeled.
And it goes through the lateral
geniculate nucleus, the LGN.
The eye is further divided by
field, like I’ve mentioned,
and visual information is processed
ultimately at the primary visual cortex.
So what we’ll do
is we’ll look at
the best way we do to understand these types
of things is to look at damage, right?
We love damage.
So let’s see what happens when there’s
damage within the visual pathway.
So here’s another orientation
of the exact the same diagram,
and I want you to note the
left eye, the right eye.
Now let’s focus on the left eye.
You’ll notice two colors.
There’s a pinkish color and
kind of a bluish color.
Now on the left eye,
the left eye will actually get information
from both fields, left field, right field.
And if you look at the sort of the
left eye, you have this blue track
and it crosses the optic chiasm
and it ends up on the right side.
So we call that the nasal passage
because it’s closes to the nose
and you have one on either side.
You also have, on the outside,
you have the optic
nerve that’s going
and it’s going to stay on
the same side of the brain.
So we get information coming
to both sides of the brain.
But what happens in
terms of processing
is the left side of the brain
will acquire all the information
coming from all the right fields
but coming from both eyes.
So just again,
follow the colors.
If you look at the left eye, you
see this pink color coming down
and it does not cross the optic chiasm,
it actually stays on the same side,
and you have information
on the blue, staying on
the blue side but then
you also have crossing.
let’s take a look at lesions
to the right optic nerve.
So where I have -- where
I’ve put the number one
is where I’m going to go
in and cut that nerve.
I want you to tell me what do you think
is going to happen to your view.
What are you going
to be able to see?
So I have vision and blind.
So if it’s filled in black, that means it’s
And if it’s white, it’s vision.
You tell me what do
you think you’ll see.
Just take a moment,
mull it over.
Okay. So if you picked
complete blindness in the right field,
in the right eye, that’s right.
So if you just follow the lines of where
the lesion is made, you can see that
that information won’t go to the appropriate
lobe where it can get processed.
Let’s take a look another
one that might help.
Lesion of the midline
of the optic chiasm.
So now what we’re doing
is we’re actually cutting
right at this midline and
so information can’t cross.
Where are you going
to have vision?
Where are you going
to have blindness?
On the outside, you
On the midline, you have vision, and it’s
because that we’ve cut that midline chiasm.
Let’s do one more.
We have now a lesion at the
right side passed the chiasm.
What do you think you’ll have in
terms of vision and blindness?
So follow the lines and look at where
that light will continue to follow.
There you go.
So we have a detriment, a vision for your left visual field
-- sorry, the left hemisphere of the
field while maintaining your right
based on the damage that we have just
prior to the lateral geniculate nucleus.
feature detection and
Now when we are talking about how
the signal actually gets detected
and then we establish
the different features,
that’s really important.
Again, like sound, vision is detecting
a lot of different attributes,
things like, again, we
don’t really think about.
So it includes certain
things like color, right?
We know that’s mediated by the
cone cells, but their shape,
the lines, the edges,
that is done by a specific type of
cell called the parvocellular cell.
And we have movement, which is telling
us, you know, something is moving.
That’s magnocellular cells.
So these are just three attributes
that I’m talking about,
but there’s even more than this.
And all of this
happens in parallel.
So we call that parallel
So I’m going to highlight two
of these specific cells.
It’s important that
we know these.
And that’s the parvocellular cells
and the magnocellular cells.
It’s also called
They’re both located in the lateral
geniculate nucleus and they both have
specific roles in terms of vision,
but one is greater spatial acuity
or spatial resolution and the other
is lower spatial resolution.
One is lower temporal resolution because
it’s not really detecting movement.
While the one -- the m-cells that detect
movement, they want to be good at time.
One is sensitive to color and one has large
fast-conduction neurons because again,
it’s trying to detect speed, because
it’s trying to detect time.
You want them to be fast.
Okay. So in terms of
conduction and neurosignals,
the larger something is,
typically, the faster it is
and it’s also in terms
So highly myelinated, really large,
it’s going to be really fast.
again, what are some of the things
that we want to talk about?
The brain processes all aspect of
visual information in parallel.
So we don’t look at something and
first realize that it’s red,
oh and then realize that it’s moving,
oh and realize that it’s round and
realize that it’s actually really close.
It’s going to hit me in the face.
We realize that a red ball is coming, screaming
out my face and I need to move, right?
So all of that happens
You know, it has to get
integrated at some point.
So color, emotion, shape, depth
The occipital lobe constructs all
of these into one overall image.
So we call that a
It integrates all
There’s also stored
information that’s accessed
to speed up the processing
and the add context.
So if you know like
cherries are red,
then you don’t have to
reestablish that every time.
And if you’re thinking
that, you know,
that you have black hair again, you don’t
need to process that every single time.
So by storing these concepts, it
allows us to speed things up.
And this is a very resource
intensive process that requires
30% of the energy
utilization of your cortex.
That’s a lot considering the
amount of information that you
actually process just by
interacting with somebody.
Vision really takes the lion’s
share of the resources.
So we just walked all the way through
the different components of the eye.
We talked about all the
how we have rods and cone
cells, how those are --
the process of
and then we finished off
on looking in how all
this information is
processed and detected.