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 Visual Cortex (part 2)
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Do curso por Duke University
Visual Perception and the Brain
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- Dale PurvesM.D.
Duke Institute for Brain Sciences
Okay, now let's talk about the higher order visual areas.
Of course vision doesn't stop with V1,
that's just where the information from the thalamus comes into the visual system.
But there's much more, much, much more to tell about that.
So, just to put you in the sort of general
picture of what we're going to be talking about now.
First we're going to look at the brain from it's lateral surface and
see what part of the primary visual cortex we can see there.
That's this diagram.
Then we're going to look and see from the medial surface, so
turning the brain around and looking at it from the medial surface from the midline.
That's this diagram here.
And then we're going to discuss a clever way of getting rid of the sulci and
gyri so they can see the relationships of these higher order areas much better.
So let's first look at the brain, the visual,
the whole brain seen from the lateral perspective.
So this is the front of the brain, this is the frontal cortex, the parietal cortex,
the temporal cortex.
And as I said before, everything behind this dotted line of
the parieto-occipital sulcus is the occipital cortex.
So this whole back of the brain is the occipital cortex.
Is it all a vision?
No, there's other stuff in there, and
we'll have a chance to talk about some of this stuff as the course goes on.
But this is the part of the visual system that you can see from the lateral side.
You might well ask, well, how are these areas being revealed?
Well I talked about the radioactive sugar technique in the last little bit.
And now I'm going to introduce you to something that you probably heard about
and that is magnetic resonance imaging.
And the great utility of functional magnetic resonance imaging
which is how these areas have been in the last let's say 15,
20 years defined to enable me to tell you about their location here.
So what does functional magnetic imaging consist of?
Well, again it's similar in principle to the monkey
sugar technique that I told you about a few minutes ago.
You want to reveal active regions of the brain but
of course you want to do it in normal human beings that can be looking at visual
stimuli of various kinds to reveal what parts of the brain are active
when the visual system is doing whatever the experimenter asks it to do.
So functional magnetic resonance imaging is again,
using metabolism, the same principle, but it's using a different
index of metabolism, which is the blood flow to the brain.
And suffice to say that this really beautiful and
complex technique that came to the fore in the 1990s and
is now widely used for all kinds of purposes.
That this functional magnetic resonance imaging technique of
revealing active areas of the visual brain by metabolic changes that
activity introduces has come to be terribly important.
And in any event, I'm not going to have time to say more about it here,
but in any event, it's the basis for revealing these different areas.
So here is what you see from the lateral part of the brain and you can see
another area called V3, another area called VP, another area called MT.
But you don't really see much from the lateral part of the brain and
if you look at the medial part of the brain.
Here again we flip the brain around.
Here is the definition of the occipital lobe.
And here are all these regions that are higher order visual regions that
are adjacent to and extend from the primary visual cortex which is V1,
primary visual cortex indicated here.
But again, it's very hard to see how these areas are related because they're all
going down into the valleys and coming up on the peaks of the sulci and gyri.
And this brings me to a very clever way that was
invented at the Massachusetts General Hospital Imaging Center
by imagining specialists there.
Which is effectively to blow up the brain.
Not to blow it up of course in a sense that you blow up your bicycle tire.
But to blow it up computationally so that you get rid of these sulci and gyri.
And can see much better how these higher order areas are related to each other.
So here's the brain with all its sulci and gyri intact.
Here is the brain after being blown up by computer software that does this job
very effectively that was invented by these people, the MGH as I just said.
And here is a higher resolution look at what the result of that is.
So again, you can see V1, the primary visual cortex is in green here.
And now you can see how V2, the secondary visual area, V3,
the tertiary visual area,
V3a which is another area, VP, which is down here, V4, another area,
how these are laid out in relation to each other.
And what can be said about this,
also these areas called MT and MST, I'll talk about these in a minute.
So what can be said about the functional role of these higher order areas.
This shows you their anatomical relationship.
And again, they are getting information.
They're getting all of their information or
at least their lower level information from V1.
They're also getting a lot of other information from
other parts of the brain and from other regions, they're all interrelated.
But you can ask, what’s known about what these things are doing?
And the answer is, less than you might think despite
intensive study and labs around the world for many years now.
The reason just for that is not lack of techniques or lack of dedication but
just the complication of the organization of the visual system which, so I
said before in introducing the course, is really the best known part of the brain.
So what I'm saying for the visual system is going to be
even less definitive for other regions of the brain.
It's just a hard challenge and what one can say about these regions is,
first of all, that how information passing from V1 to these other regions,
what the function of that is is not entirely clear.
But obviously it's going on with further processing of the V1
information in each of these regions.
Are they devoted to a specific type of processing?
Well, the answer to that is kind of yes and no.
So for example, if you take a region like V4 that
people have studied very intensively,
it's pretty clear that that region is biased to
color coded information, to color information.
Not all animals have color vision but we certainly do, and
V4 is biased to cells that respond to color.
Is that all it's doing?
No, there are lots of cells in V4 that are not responsive to color.
And exactly what it's doing with respect to color is not so clear, but
this is a bias that's become evident.
There is another bias in neurons that are recorded in these other higher
order areas, again, getting information from the V1 that are the cells
when they're recorded from in these regions, are biased to motion.
They're more sensitive to the change in the retinal image, to motion of objects
in visual space, and for example, cells in V4 or V2 or V3.
But it's not possible to assign a simple function to
any of these regions because they all have cells that are not related to their bias.
They're all interconnected, and it's just a complicated
challenge to figure out what these higher order areas are doing.
There's a lot known about it.
Of course a ton more then I'm telling you here,
but the bottom line is that the challenge is still out to figure a simple way of
saying what these regions are doing and how they are related to each other.
Other then the simple facts that they're getting their information from V1.
That there are biases in these regions that suggest different functions,
but not in a simple way.
The other thing that's obvious, that I should emphasis to you,
is we said that in V1, the retinotopy
that's evident in the eye and is carried through the thalamus.
And that's equally equivalent in V1 is more or
less degraded as you go on to these higher order visual areas.
So that seems peculiar because you might think for
example that well retinatopy is all about representing an image in the brain,
that the visual brain is kind of like a camera.
The image is represented and you end up receiving an image
that is somehow laid out retinotopically in the visual
processing higher order areas of the visual system.
But that's not the case.
The retinotopy in V1 is basically lost in these other areas.
So that their receptive fields, which we'll talk about in a minute,
are much bigger and cells respond to more or less when you get to some of these
higher order areas, more or less the entire visual field.
Well that's not retinotopic representation,
that's kind of the opposite of retinotopic representation.
The processing is including information from the whole visual field,
not just a little part of it,
meaning that the image could be maintained in these higher order areas.
It's clearly not.
That's again, I mean the philosopher might have wished it otherwise or
a logical thinker might have wished it otherwise, it's just not the case.
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