Learners will be introduced to the problems that vision faces, using perception as a guide. The course will consider how what we see is generated by the visual system, what the central problem for vision is, and what visual perception indicates about how the brain works. The evidence will be drawn from neuroscience, psychology, the history of vision science and what philosophy has contributed. Although the discussions will be informed by visual system anatomy and physiology, the focus is on perception. We see the physical world in a strange way, and goal is to understand why.
The Primary Visual Pathway (part 1)
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Visual Perception and the Brain
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- Dale PurvesM.D.
Duke Institute for Brain Sciences
Having talked about the eye and retina, the next obvious question is, well,
what about the rest of the visual system?
Where do things go from the eye?
So, the easiest way I think to introduce this is in
terms of what is called the primary visual pathway.
And, Let me explain that using this diagram.
In the simplest terms the problem with the pathways is
just the eye a way station called the thalamus.
We will talk about the details of this in a minute and the thalamus here,
and then there is visual cortex which is here which is the region.
Coming through the thalamus reach the visual system and
basically all the information that comes into the visual cortex.
Which is where we presume the processing that ends up with perception to occur
comes through the thalamus into the Primary visual cortex.
Before getting into the details of that I want you to be sure you
recognize that there are at least two other pathways that are shown
in this dotted lines here and here.
But don’t concern perceptions so
the reason that super [INAUDIBLE] pathway Is called what it is.
It's taken as primary because it's the root from the eye to what we perceive,
what we end up seeing, and that's of course what the course is mainly about,
what we're going to be talking about.
But the visual system does a lot more than this.
The information from the retina passes to two other centers.
The first of these Is called
the superior colliculus, and it's here.
The colliculus means little hill.
These are two hills on the dorsal surface of the mid brain and the brain stem.
And their concerned with eye movements.
So, I guess it should be obvious that this is a big part of vision.
I mean when something happens in visual space you want
to turn your body and your eyes in particular, you want to
turn towards the source of that happening and see what's going on there.
So you have to do this by
movements that are coordinated between the left and the right eye.
And the rather complicated circuitry that does this
is information from the retina that reaches the superior colliculi.
As I said, these two little bumps on the back surface of the mid brain so their
concern with coordinating eye movements with what's going on in visual space.
With body movements, with reflexes that keep your
eyes trained on something if you want to follow a moving object and so on.
There are many facets that the superior is concerned with.
The other pathway that is not the primary visual pathway but
very important is the autonomic pathway that controls the pupillary light reflex,
so it's this pathway shown in the dotted line here.
And this is going to an autonomic nucleus again in the midbrain
region that is concerned with pupilary dilation.
So you probably noticed that when you're looking at the mirror you shine
a flashlight in your eye the pupil constricts.
And it constricts a lot of strongly and
obviously in the presence of light or dilate in the presence of darkness.
Why is that?
The reason is that when you operate in different amounts of light as we talked
about last time there's a huge range of light from star light to bright sun light.
You don't want the apparatus in the retina damaged by to much light coming in.
You want to modulate the amount of light coming in and by Contracting or
dilating the pupil and that's done by
the nucleus that receives information from the retinal started line pathway.
And dilates or contracts the pupil according to the level of ambient light.
So let's go back to a little bit different view of the primary visual pathway,
which is again, the eye going to the thalamus.
And from the thalamus to the visual cortex, and talk in a little bit more
detail about each of these three stations in the primary visual pathway.
So what I've added to this diagram, you can think of this as a piece of paper, or
an object in visual space, half of which is blue and half of which is red.
Think of a piece of paper colored in that way.
And imagine further that you're looking at a central point on that piece of paper.
And my reason for emphasizing this, and this is really the visual field.
It's an object in the visual field that you're looking at and
you're looking at a particular point.
By focusing the line of sight, remember, coming to the phobia from the two eyes,
you're focusing that on a particular point.
So, what's color coded in the rest of the visual system in the red and
the purple is the pathway that the information on the right,
and left side of this piece of paper take in getting your cortex and
vision perception that follows.
So this is visual space.
This is an object in visual space, but you'll see by the color coding
in a primary visual pathway that it's a little bit complicated.
And the bottom line is that all of the information on one side of visual space,
let's say the right side of visual space on this piece of paper that you're
looking at, goes to different sides of the two retinas.
So, the blue information coming from the object in visual space goes to this side
of this retina, and this side of this retina.
And that information needs to be brought together, and it is brought together by
having the information cross at this region that's called the optic chasm.
So that all of the blue information comes to the same thalamus, and
from the thalamus, on to the visual cortex.
So you're keeping the right side of space together,
and you're keeping it together through the stations of the primary visual pathway.
Conversely, for the left side, that information is going to this side
of this eye's retina to this side of this eye's retina as this color coding shows.
And again crossing the red stuff is crossing at the eye optic chiasm,
or not in this case in this eye on the same side,
reaching the thalamus and hence keeping all this stuff.
On the other side of visual space, on the other side of this piece of paper that
you're looking at together in the opposite hemisphere.
So, this seems like a crazy way of doing vision.
We talked last time about the odd way that
the retina was set up with the photoreceptors at the back of the eye.
The information from light going through all the junk that's in
front of the photoreceptors.
And the reason why that was having to do with retinal
rod and cone receptor metabolism.
Here's another thing that seems odd to say the least.
I mean, why would you want to have the information crossing at the optic chiasm
and reaching the hemispheres in this rather strange way.
Why wouldn't you want to keep the information from one eye going to one
hemisphere and the other eye going to another hemisphere.
Well lots of animals in fact are set up like that,
animals with eyes on the sides of their heads.
Horses, cows, for example that are wall-eyed don't have this crossing or
at least they have much less of this crossing.
And the question arises from primates like us and other non-human primates.
Why is this?
Well, the short answer is that nobody really knows.
The best guess is that this has to do with the course of evolution from our fishy
ancestors way back when who climbed out on land,
but when they were in the sea they used a tail flip response to avoid predators.
And people who've looked carefully at the tail flip response have shown that it's
much more effective if the information
goes to the opposite side of visual system in fish.
And the general idea is that the evolutionary carry over
into us from the tail flip affecting this to
our other strangely laid out visual system is on this evolutionary basis.
You probably are aware that many people have pointed out
that evolution is a tinkering process.
It's not logical.
It's just using whatever works and
carrying on whatever works into the next evolutionary stage.
So we're presumably the inheritors of this way that fissures behave, but
the answer for why this setup of the visual system is the way it is is
just a best guess about that.
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