The following content is provided
under a Creative Commons license.
Your support will help MIT OpenCourseWare continue
to offer high-quality educational resources for free.
To make a donation or view additional materials
from hundreds of MIT courses, visit MIT OpenCourseWare
at ocw.mit.edu.
PROFESSOR: This is the first class since the midterm,
but the chemical senses I want you just to read.
And I posted questions, I hope.
I think I remember posting some questions.
My plan is not to give a quiz today
and to come up with some homework.
I think I should include something
on chemical senses, particularly the olfactory system,
but I hadn't settled on exactly what to do yet.
So I'll get that done tonight or tomorrow.
Because I'd like to spend the next three
classes on the visual system, and then
two classes on the auditory system.
If you bring your books to the class,
I guess it's a little heavy to lug around,
but I want to give you-- unfortunately,
I've found small errors, one just a typo,
in the auditory system chapter.
It's a long chapter, so that's why we need two classes.
So I'll specify those to you.
It's inevitable, they told me, at the MIT press,
that when you publish a book this big,
there's always a few things like that that you find out.
So they will fix them in the next printing if there is one.
I hope there will be.
So where are we now in the outline of this class?
We've already done seven parts, ending with the brain
states, which is the only part with only one chapter.
And so now with sensory systems, readings only for gustatory
and olfactory, and now three classes on the visual systems.
And notice I used that in the plural,
because there's multiple systems for vision.
Some of them operate pretty much independent of the others.
So we'll start that today.
We'll start with a little bit on the origins of vision,
with light detection.
And then finally, image formation
and the major functions it serves-- predatory escape,
which we've been talking about before, orienting
towards objects, and then identifying
patterns and objects and individuals.
And then I want to start going over
the retinal projections in more detail.
And next time, we'll look at that in a different way
in a little more detail, because I'd
like you to be able to remember some of that.
You guys have pretty good memories.
Just a little aside here-- I was pleased
with the results of the midterm.
You figure that 85 to 100 is an A, there were a lot of As.
And Bs go all the way down to 70,
so that means nobody got below a B. All As and Bs.
So I should be clapping up here for you.
So keep it up.
Keep working hard, and you'll all get As and Bs for sure.
All right.
So what in all likelihood was the first functional role
of the visual sense?
And I want you to be able to describe the nature of the most
primitive projection of the eyes to the brain.
And I point out here that it was actually
the last of the major retinal projections to be discovered.
Even after this one was discovered,
there's been a few more, and I'll point some of that
out in today's class.
So what do I claim is the most primitive function of vision?
It was there, even in amphioxus.
Sorry?
AUDIENCE: Avoidance of predators?
PROFESSOR: Predators?
No.
AUDIENCE: [INAUDIBLE].
PROFESSOR: Well, the pineal eye is
one of the ways this was expressed,
that's right-- just detection of light.
Think of the most primitive chordate
we talked about, amphioxus.
He doesn't have a lens.
But he can detect light.
I'll show you that again today.
So we think that's got to be the most primitive sense.
So then we look at, well, what are
the pathways in the mammal that just do that?
OK, so we'll look at that here, two of them.
Why was it so important to detect light?
It long preceded image formation and topographic projections.
It took a long time to evolve a lens.
And we don't know all the details of how that happened.
So that means the only topography
in primitive, bilaterally symmetric animals,
the only topography was they had a light detector
on the right and a light detector on the left.
They could see differences in light intensity
on the right and left sides.
So if some animal was approaching them,
attacking them, they might be able to detect,
oh, there's something over there.
So I need to go the other way.
So we know this is just on the primitive origins
that we've gone over before.
It was very adaptive for organizing the night and day
for efficiency and for safety.
If you're a day-active animal, you
need a place to get away from other animals at night
in order to sleep.
If you're a night-active animal, you
need a place to sleep during the day.
You're inactive.
But then if you use vision for predation,
you have to have vision in very low lights,
and you're more likely to use other senses
in addition to vision that might be even more important.
Those would be olfactory and auditory.
Somatosensory less so, but somatosensory
does play an important role in predation too
in attack behavior.
We'll talk a little more about that later on.
OK.
So this is just a reminder.
This is amphioxus, the rostralmost end of amphioxus.
You find cells with pigment in them.
But more importantly, right next to the pigment cells,
there are cells that are light receptors.
And next to them are cells that have axons that project
to other parts of the nervous system of amphioxus.
And you can compare the arrangement
of cells in that light spot in amphioxus
to the cells in the developing retina.
Normally the retina develops as sort
of an out-pouching of the sides of the diencephalon.
What causes that out-pouching to occur?
It's actually induced by things at the surface.
Placode, in this case, the lens placode.
And that causes the neural tube to extend out
towards the surface.
So here you see that.
And you see pigmented cells and the cells
of the central nervous system here,
including the retinal ganglion cells, bipolar cells,
and the visual receptor cells.
And this moves back here, so the receptors are actually
in the back.
The light comes in.
It forms a cup-like arrangement.
The light comes in this way, goes through the receptors.
But they're very transparent.
They don't cause much distortion.
Seems like a strange arrangement,
but that's the way it evolved.
So now this is just another representation
of the same thing, where they compare a primitive vertebrate
with the amphioxus.
And I pointed out before that we know from gene expression data
now that even amphioxus does have a hindbrain, a midbrain,
and a forebrain, but its forebrain
is only a diencephalon.
OK.
You want to behave differently in day and night.
But what if you're spending time underground?
Or what if the light is very dim?
You don't get the normal signals.
Well, it's an amazing thing that evolved that supplements that.
You don't need just light, which can vary a lot.
You determine your activity by an endogenous process,
the cells that have a circadian rhythm of activity.
But they are normally every day, they're
affected by the light-dark cycle.
So we say they're entrained by the light-dark cycle.
So they oscillate at 24 hours instead of 24 hours and 20
minutes, or 30 minutes, or whatever--
sometimes as long as 25 hours, including in humans,
sometimes a little less than 24 is their natural rhythm,
but they're all of them, no matter what
the natural rhythm of those cells
is, it's entrained to the light-dark cycle, normally
the light onset.
That's why if you have trouble-- if you're
circadian rhythm drifting, you can mess about at MIT
by staying up all night and not-- it can be a problem.
And one way to help with that is to exposure yourself
to bright light in the morning, especially sunlight,
especially effective.
Some people like to sleep by open windows because of that.
And if you're an MIT student, you probably
hate to sleep by an open window.
You want to be able to sleep in.
So the only thing that's been found
that you can counter some of the problems that can cause
is to take low doses of melatonin.
And I'll say a little more about why
I said low doses of melatonin.
So light controls the daily rhythm
of secretion of melatonin.
It controls daily rhythm of secretion, actually,
of a number of things.
But one of them is melatonin.
It's secreted by the pineal gland.
So how does light effect that rhythm of secretion.
You can say that, well, it's just by the endogenous rhythm.
But actually, no, it's affected by light.
And there's two different pathways in different animals.
One is very ancient that you mentioned
at the beginning of the class-- the oldest pathway
to affect secretion of melatonin.
What was it?
The pineal eye.
A number of animals have that eye.
A number of fish have it, some amphibians have it,
and I think even some reptiles.
It's an eye on the top of the head, a third eye.
It's led to all kinds of fantasies
in science fiction, everything else,
but all it is is a light detector that
affects the pineal, it has close connections to the pineal.
What about the other pathway?
We don't have a pineal eye.
We've got a thick skull.
Now, if you're a mouse, the skull is thin,
and some light can get right through that skull
and affect the pineal.
And yes, there are cells that are
sensitive to that light coming right through the skull.
But a lot of vertebrates have a thicker skull, like us.
So even the rat's got a pretty thick skull.
So light can't really get through-- or at least,
very, very little.
So let's talk about that pathway.
But first, let me show you the mammalian suprachiasmatic
nucleus, where the cells have this endogenous circadian
rhythm.
It means circa, dium-- about a day in length,
because it varies a little bit.
And then we'll talk about the pineal gland and melatonin
production, the pathway.
So this is from one of the studies
that I did with a postdoc in my lab.
We were using a method that's very sensitive compared
to earlier methods for tracing pathways
in the central nervous system.
And here we were tracing them from the retinal ganglion
cells.
This is the low power picture at the bottom.
So what it's showing here is the suprachiasmatic nucleus
and the number of nuclei-- that's
what SCN is, suprachiasmatic nucleus.
But notice that it spills outside that
and goes to these other areas around it as well.
In fact, there's even a few axons, to our surprise,
went all the way into the olfactory areas of cortex.
None of that was known before these methods got
so sensitive-- so sensitive that we could trace
individual axons.
They look like they were stained with the Golgi method,
except we get them all, all the retinal ganglion cell axons.
And then these are enlargements.
The suprachiasmatic nucleus would be here.
And here's a sagittal view.
This would be suprachiasmatic.
But you can see, they're in the anterior hypothalamic nucleus.
They're in the nucleus right above, even
a little bit in the ventromedial.
If you look at this slide, it's from a rat.
And here's the frontal section.
This is a Nissl stain, so you see
how clear that suprachiasmatic nucleus is.
These are the axons down here.
They're not stained in this, like they were here.
These are the axons at the bottom here.
There you'll see them in the chart
and here in the photograph.
So they're down here.
So we know then, the retina projects heavily--
oh, one other thing I left it.
Notice that it's going to both sides.
I labeled the axons from only one eye.
But they're going pretty much equally to the two sides.
And that's true in all the animals that have been studied.
This you don't need to pay attention to.
It's looking at a more specific stain for a particular peptide
that is found in some of the neurons
of the suprachiasmatic nucleus.
They've found that they can actually subdivide that nucleus
into a number of different parts that
do have slightly different projections,
and we believe, probably different functions.
Some of that is still not fully known.
But look at this complicated pathway
that influences of light on the retinal ganglion cells
take in order to reach the pineal.
Here they go to the suprachiasmatic,
and short axons go from there to a more dorsally located
hypothalamic nucleus.
Paraventricular hypothalamic nucleus--
that's what PDH means.
And there are long axon from that nucleus that
connect to the sympathetic nervous system
by axons that go right down to the lateral horn
in the upper thoracic cord.
Remember, the lateral horn cells are the preganglionic motor
neurons of the sympathetic nervous system.
They're found all the way from T1 down to L2 or L3.
So they get the influence of light.
They're activated.
Here the axon goes out into the ganglion.
SCG means the superior cervical ganglia.
It's part of that chain of sympathetic ganglia
on either side of the spinal cord.
Superior cervical ganglia is right up here.
It's way up in your neck.
That's where the cervical cord is, of course,
below the skull, uppermost spinal cord.
And axons from the superior cervical ganglia
provide the sympathetic innervation of the head region.
So some of those axons-- now, remember,
axons are going to all over from the head.
They go to the iris of the eye, for example,
and cause pupillary dilation with sympathetic activation.
But this is just one pathway here
that goes directly to the pineal gland.
And that is how light controls the daily rhythm of secretion
of melatonin.
OK.
So how about taking melatonin pills?
It's sort of popular and all kinds of beliefs about it.
Unfortunately, they're not produced by the drug industry.
They're produced by various companies that
aren't regulated by the Food and Drug Administration.
Therefore, the government, NIH, has no say
over what they can put in the pills.
And people have this tendency, and I'm
sure you've noticed it in yourself,
that if a little bit is good, more is better.
So people take-- they see these things on the shell,
oh, 1 milligram of melatonin, two, three, five, 10.
They buy the 10.
Here's what happens.
You take those big pills, you swamp the receptors,
you totally saturate them, and mess up their normal secretion.
So you can end up having more problems.
So people aren't helping themselves,
but they don't know it.
They're so convinced that more is better.
So if you want to take melatonin,
get the 300 microgram pills, the low-dose melatonin pills.
And I've taken them when I go on trips,
and it helps me recover from jet lag quite rapidly.
So if I go to China, I'm 12 hours off,
I take the melatonin pills 20 minutes
or 15 minutes before I go to bed.
And within a couple days, I'm on Chinese time.
Without that, it takes five or six days
before I'm fully on-- and even then,
I get sleepy at odd times.
So that's just a bonus.
I probably didn't put that in the book.
I'm sure I didn't.
I probably should.
But not enough people will read it
to make much of a difference.
All right.
Second main function of vision, of course,
involves image formation, animals with a lens.
And I've included the function of predator avoidance
and escape here, even though probably before there
were lenses, even when animals just could detect something
different on the right and left sides of [INAUDIBLE].
But that's the most primitive kind of image you can get.
It's also very useful for detecting novelty.
If you can discriminate differences--
if you can use those images to see differences in patterns,
differences in shapes, you can detect novelty.
Novelty is critically important, because if something
is different, it could be dangerous,
especially if you're a preyed upon animal.
Hence, our visual system is highly
attuned to detecting novelty and responding to it.
So next in importance, orienting towards novel objects
to explore them.
So there's a dual role of novelty there.
We have a tendency to explore them.
We have a tendency to be afraid of them.
So it depends on how novel they are,
and the situation, of course.
But obviously we orient in order to find food,
find potential mates or rivals, and so forth.
And finally, identifying animals and objects,
which when we think of vision, that's normally
what we're thinking about, not these other functions.
It doesn't mean humans don't have them.
We do.
All right.
So now I want you to recall the hypothesis, using
Darwinian logic, concerning the evolution of predominantly
crossed representation of visual space.
Can any of you remember that when we talked about it before?
Let's just go over it.
I claim it was an early evolution of connections
that carry information, of things
like a sudden appearance of shadows on one side,
or triggering escape movements.
The most direct, relevant connection
from the eye to the escape mechanism for turning away
was a cross projection.
And this is the way I represent it.
These are the pictures.
Here I put the spinoreticular pathway.
I discovered this morning that I had this reversed,
so I did a mirror image and changed the words.
That's why spinoreticular I think in the book
is on the left.
But anyway, here's the spinoreticular.
It's an ipsilateral pathway, but to a lesser degree,
it's bilateral.
It goes to both sides, but mainly ipsilateral.
Trigeminoreticular is similar-- mostly ipsilateral for input
from the face, ipsilateral but some fibers going
to the other side.
Now, in this picture, on the left,
here I show the primitive retinal projections.
They've just reached the hypothalamus.
They're going to that suprachiasmatic,
and I show them just as they are now.
I don't think there's been any change.
A wide range of animals, they've looked at this kind of pathway.
It's always like this, bilateral.
And I show it the way it is in the rodents and other mammals
they've studied.
Pretty much equally to the two sides.
And here's the trigeminoreticular.
So to escape from something over here,
you want the first movement to be turning away,
and you want, at almost the same time,
you want it to trigger locomotion,
rapid locomtion, this.
Turning away and swimming rapidly.
And that's exactly-- there's been some detailed studies
in the sea lamprey, a very primitive vertebrate,
at Northeastern University and some other places as well.
And if you look at their-- they take these rapid video images,
you'll see this bending of the animal first,
and the tail began to lash to propel the animal forward.
So because it's a cross pathway that
can reach those neurons first, that
gave the evolutionary advantage to the cross projections.
And we know that when they eye got bigger, generally,
the bigger a structure is, the more
connections it forms in development.
So some of the axons extended further.
Some of them reach structures that
can trigger escape movements.
And we'll talk in a minute about what
those neurons were early on.
But the main thing is that it was faster across.
OK.
Now, this is a simple picture on that mammalian brain diagram
I've used before in the class, just
to show you all of the retinal projections in vertebrates.
When I say all, I'm not including these very sparse
ones that we know exist here around the suprachiasmatic
nucleus, and they do exist around other nuclei too.
And there's a few that vary a bit from species to species,
and they vary even among animals within a species.
So I am not talking about those.
These are the ones you keep seeing in animal after animal
after animal-- all the vertebrates.
Here's the one to the hypothalamus.
This is the lateral geniculate body.
And you notice that there's two nuclei there.
The one we always hear about is the dorsal nucleus.
But that's the one that projects it to the visual cortex
up here, and I show that pathway.
Then the axons continue.
They reach the area in front of the superior colliculus,
or optic tectum in most animals.
And then they reach the optic tectum, labeled SC here
for superior colliculus, because this is a mammal.
Pretectal area has a number of different cell groups
that have different functions.
But we can lump them together.
It's part-- at least, in the way I treat it in my book,
it's part of the epithalamus because there's
various structural data that say that both
the nuclei and the pretectal cell groups and the posterior
commissure region are all part of the epithalamus.
But modern molecular studies, they vary a lot.
But these people tend to say epithalamus is just
[INAUDIBLE], and they call the pretectal area something else.
But being an anatomist person first and foremost and not
a molecular person that skips over the first 500
years of neuroscience, I say that no, it's all epithalamus.
Sorry.
OK.
So what is the structure that first
controlled escape reactions?
Well, here the shortest axons are
reaching the suprachiasmatic nucleus.
And we've talked about tectum being important for escape
movements, but look at all these other structures
they go through first.
So was the tectum really the first?
So I began to look at this nucleus, the first one
beyond the suprachiasmatic nucleus
if I don't include this little offshoot here.
We know quite a bit about the functions of that.
It's the accessory optic tract, and we'll
hear more about that next time.
Those are axons that leave the main optic tract,
so let's just stick with the main optic tract
and look at that.
What structure first controlled escape reactions?
So we know the midbrain tectum with the superior colliculus,
is the dominant structure now.
I believe that's true even in humans.
If you have the instinctive escape reaction to a novel
object.
Most of us don't have any memory of experiencing that,
but I was a pretty observant little kid,
and I remember suddenly-- I used to also think a lot.
I thought too much, so sometimes I wasn't paying attention.
And I got right in front of a car once,
and he was coming pretty fast right at me.
I had a sudden, instinctive reaction.
I froze, and I could not move the handlebars at all,
even though I wanted to.
My cognitive system said turn away, get out in front of this.
But I was frozen.
Freezing is a typical anti-predator response
of young animals, even big ones, like us.
I was pretty young.
I froze.
I could not move.
Thank God the guy at the wheel didn't just freeze.
He put his brakes on and he stopped.
And he didn't hit me.
All right.
So that's a tectal response, and that's what animals do.
I've studied it in animals.
But then they reach the LGV here,
the ventral nucleus of the lateral geniculate.
And I discovered that it's part of the subthalamus--
well, the more medial part of the subthalamus.
And there are connections there, from the visual part,
that if you stimulate, you can get very rapid running
movements.
Everybody's ignored it.
Visual people just focus on the popular things--
superior colliculus, lateral geniculate body, visual cortex.
But I love these things that people ignore.
In evolution, these things are very important.
It turns out, that area of the subthalamus
has direct projections to the midbrain locomotor area.
That's how the locomotion, the very short latency pathway,
is produced-- from this area.
But now we know that it's the pathways from the tectum that
also have a rapidly conducting ipsilateral pathway
to the midbrain locomotor area that
has become dominant, because the tectum is so much bigger.
So often something becomes dominant because of its size.
It doesn't mean that the ventral lateral geniculate
body is important, but it's become
overshadowed by the larger structure.
They both have connections to that area.
So next, orienting towards novel objects, for exploring,
for finding food, for finding a mate,
finding a rival, for finding the doorknob
so you can get out the door.
All right.
This is the question I wrote-- contrast
the type of visual orienting for which the midbrain
tectum or colliculus has become dominant in most species
and the visual orienting for which the pretectal area is
important.
Now, the problem here is that it's only
been discovered in a few species-- mainly two.
Discovered first in frogs, verified for a mammal.
But let's go through these.
First, the main type of orienting.
You see something here, you turn towards it.
You do that with a hamster.
Remember, this is, I think, chapter 13, I
talked about the regeneration of the optic tract,
and I showed you these videos of hamsters turning.
All those turning movements are controlled by the tectum.
The hamster can't turn at all without the projection
from retina to tectum.
There is input coming from visual cortex,
but it comes in through this same route,
from the front of the tectum into those superficial layers.
So the left eye we want to control something's out here,
say a sunflower seed.
It's mainly a crossed pathway because of that evolution
we talked about earlier.
So it's going to the right superior colliculus.
So to control, turning to the left,
he has to contract the muscles, activate motor
neurons on the left side of the spinal cord.
He does that by the crossed pathway, tectospinal pathway.
So to turn left, it crosses once here, crosses again here.
And was the tectum always dominant?
You can ask that same question we asked before.
I think the LGV was probably important early on,
but it clearly is not so dominant now.
So it's this tectum that's the major structure for that.
Now, whatever it was in very early evolution.
It reaches subthalamus first.
Then it goes through the thalamus.
But that, the dorsal thalamus, the lateral geniculate body,
doesn't have these same projections.
It only projects to the endbrain,
mainly to the visual cortex.
So let's talk about the pretectal area.
And this is an interesting structure, because remember,
I said it had multiple cell groups.
And they had different functions.
We know, for example, tat there's
a superficial part of it that projects
into the nucleus of the parasympathetic system that
then projects to the ciliary ganglion behind the eye,
that when that's activated it causes pupillary constriction.
That's the pupillary light reflex.
You shine a light in the eye, the pupil constricts.
You're in the dark, that is not active, the pupils expand.
Or if you get angry, the pupils expand.
You bring in more light at the expense of acuity.
It's important to get the maximum light.
And if you're talking to a girl or you're talking to a man,
you want to know how interested they are in you,
watch their pupils.
The pupils will get larger if the sympathetic nervous
system-- if they really start to get excited,
the pupils will expand.
If you're a salesperson, there are salespeople
that become sensitive to that, and they
can tell how interested someone is.
And that's important, because they can make a better
deal if they can detect pupils.
But anyway, another function discovered here
in frog and treeshrew-- very interesting,
because it's also a kind of orienting.
If these animals are running along, and there are barriers,
there are poles or sticks or trees in front of them,
they sidestep them.
That's a kind of orienting.
They're sidestepping the barrier.
They're running this way, they go around.
That's not controlled by the tectum.
It's controlled by the pretectal area,
according to some pretty good studies
of the frog and the treeshrew.
So similarly, when the animal has
to run rapidly into an opening, he
uses that system to control his limbs
to get his body aligned during locomotion.
It's not been studied very much.
Certainly deserving a lot more study.
And then the one other function that we know about
is a vestibular light function that when you start to fall
or when you're locomoting and moving your head,
there's stimulation of the entire retina
by the flow of visual images over the retina.
That flow, things that affect the whole retina,
are detected by cells in the accessory optic system
and in a nucleus of pretectal area,
the nucleus of the optic tract.
And that has an important function
that we'll be talking about after we finish audition.
So I say here, distinguish between two very different
functions in the midbrain tectum,
each involving a different output pathway.
But which of these functions is precise acuity more important?
Well, what are the two functions?
We've been talking about them.
Orienting towards something and escaping from something.
Those are the two functions.
We know they have different pathways.
The outputs-- and I talk here about the nature
of the pathways.
Escape from predators involves an ipsilateral pathway
from that tectum.
Remember, the fibers have already crossed.
The retinal fibers have crossed to get to the tectum.
But then there's a short ipsilateral pathway
to the local motor regions that control the escape reaction.
It's that cross pathway that I showed
in this diagram, this one.
This controls turning towards things.
So those are the two functions.
Now, I ask which of those things requires more acuity?
And here, I mean the precision of location, how precisely
does the animal know in the sense that his brain is
detecting the difference, between a thing
in the temporal retina here in front of him,
or in the temporal field or nasal retina
here or two regions close to each other here?
Well, it depends on how precise that topography is, right?
We talk about acuity in the visual cortex likewise.
And it's basically visual acuity.
How fine a difference you can detect
in details of the visual field depends on how much brain there
is representing that part of the visual field.
So we have a lot more brain representing a lot more area
in the cortex devoted to the phobia than areas
off the phobia.
Periphery gets much smaller representation.
But the same thing is true here for the orienting movements,
even though the functions are quite
different in the midbrain.
You still have a kind of acuity.
And here I'm asking, what are the two main methods that
have been used by neuroscientists to map
the topography of the representations
of the visual field and retina in the superior colliculus?
How did I get that map?
This comes from my work.
And this is actually a summary of a lot of work,
with two very different methods.
What are they?
One's anatomical, the other uses electrophysiology.
The most common method is to use electrophysiology,
to put the electrode in, record from the neuron
there in the superficial tectum, and find out
where in the visual field the stimulus has
to be in order to activate those cells.
That's mapping the receptive field of the cells.
And if we're interested in just location,
we don't need to worry about the properties of the stimulus.
We just have to find usually a little dark spot
on the light background.
Or it can be a bright spot on the dark background.
The dark spots tend to work the best.
You can easily map the visual field.
But you can also make-- if you want
to see at once the projection throughout the entire optic
tract, you can make a lesion in the eye
or an injection of a tracer.
You've got to label axons from one part of the retina.
So here's the retina, here's the optic disc.
Let's say you make a lesion here in the retina.
The fibers from out here, though, are all damaged too.
So the effective lesion is like that
when you make a hole in the retina like that.
You can then trace the axons from the retina
to a corresponding area.
You'll find the axons in the superior colliculus
to be in an area like that.
So it's only by putting together a number of different cases
like that you can come up with the same map.
But you will then come up with the map simultaneously
for the geniculate body, for the tectum,
for all the structures of the optic tract.
That's the advantage of the anatomical method.
So finally the function of identifying
animals and objects.
And I point out here that the colliculus
of the midbrain and the visual cortex areas
are each important for identification
of the [? stimulus ?].
I know we say it's always cortex,
and I want to explain why that is.
Because identification in the tectum and the pretectum
too involves what we call innate releasing [INAUDIBLE].
It's innate behavior, instinctive identification.
For example, the best example is the bug detectors in the frog.
A small little area in the visual field
that's dark and moving around.
There are neurons in the frog tectum that respond.
And the frog sits there still and [INAUDIBLE],
because his neurons are responding to that fly.
But he won't snap with his tongue until it stops.
The neurons keep firing for a little while.
Then his mouth opens, and the tongue zaps the fly.
It's an innate mechanism.
Whereas learned identification abilities with greater acuity
follow the invasion of the endbrain
by the visual pathways to the striatum and the cortex,
especially the cortex.
And these are just examples.
The hamster learns to use visual landmarks
to locomote towards its home or to find food and water.
He can't just inherit all that.
He might have innate preferences.
He has innate preferences to go out at a certain time of day
and maybe to go to certain places.
But he also has to learn about where the dangerous places are,
where the safe places are, where's the water,
or where are the fruits that he can get liquid from,
and things like that.
Where are the grains?
Where's the farmer's grain storage
that he can go rob and fill his cheek pouches.
And we know that some animals learn
to recognize members of their own species
as individuals, just by visual differences.
So I'm asking here, why?
Relatively recently in evolutionary time
have these visual identification abilities evolved so much.
And I point out that with these larger topographic maps, when
you have precise topography, you then
in the projections from that structure,
you just have to combine inputs in specific ways
to get some kind of shape and pattern recognition.
And that has been studied most extensively for visual cortex
in work-- initially it was done in the frog by Jerry Lettvin
here at MIT, and then it was done at Harvard
in the visual cortex of cats and later in monkeys
by [INAUDIBLE].
And in your vision classes, you always
learn about those studies.
OK.
Visual pathways to the endbrain follow multiple roots.
I want you to be able to describe more than just
the two traditional roots, because everybody
knows about two of them.
There's actually, like, six, and I describe them
all in the book.
And I just summarize them here.
Tectum and pretectum each project
to the older parts of the thalamus, which
project to both cortex and striatum.
And they also project to the newer parts
of the thalamus of mammals.
And then the retina projects directly to the thalamus also,
mainly geniculate body.
So that gives multiple projections for visual stimuli
to get to the endbrain.
So that's just summarized here.
And this just shows a picture of the projection
from a cell in one of those intralaminar nuclei.
And I picked this one because it gets input from the colliculus
that I've seen in my studies of hamsters.
And here it shows the axon going to corpus striatum
and to the neocortex.
Most of them don't go to both places.
They go to one of those two.
But some of them actually go to both.
And then I just go through the adaptive advantages
of this for reaching the striatum,
reaching the neocortex, and the cognitive functions
in the projections of those areas.
And then [INAUDIBLE] to your vision also,
which developed cortical projections better
than any place.
So you can read the rest of this.
I think it's very clear in the book.
And we'll do the retinal projections.
We'll look at pictures other than this next time
to go through that.
I'll post all these slides.
I've already posted them.
I may add a few notes to them after the class.
So now for next time, read that next chapter.
You were supposed to read chapter 20, was it, for today?
The first vision chapter.
And then, yeah, 19 was the olfactory chapter.
So read chapter 21 also for next time.
And I'll come up with some homework.
AUDIENCE: [INAUDIBLE].
PROFESSOR: No, I don't plan on giving a quiz,
but I plan on giving you some homework.
I don't know.
My impression is that you're happy with the homework
and you probably learn more.
I can give you an assortment of things.