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PROFESSOR: We had talked about the function of the auditory
system and anti-predator behavior, so it was probably
with the early predators that these other abilities of the
auditory system developed to identify and localized prey.
And I pointed out that anti-predator behavior doesn't
really require precise ability to localize with
the auditory system.
They can use the visual system to find the escape route.
So I'm going to start now by going through the basic
connections of the cochlear nuclei.
A little bit about transduction initial coding
and the channels of conduction into the
central nervous system.
It'll be like other systems.
And then we'll consider separately localization and
identification ability pattern detection, which is temporal
patterns, and a little bit about specializations.
So we're going to start with the cochlea
and the eighth nerve.
We'll start with variations in sound pressure
levels in the air.
And that has to be transduced to vibrations in the fluid of
the cochlea.
To get there, you need some kind of impedance
transformation, because the vibrations are very different
in air and in fluid.
So it requires some kind of impedance matching.
And there was a big change in early mammals that made that
more efficient.
Here's a skull of dimetrodon, a mammal-like reptile, and in
his middle ear he just has one bone to transfer vibrations
from the air to the fluid of the inner ear.
It corresponds to the stapes in modern mammals.
Mammals have--
besides the stapes, they have the malleus and the incus.
It looks like part of my slide maybe didn't copy,
but that's all right.
So what that did was with more efficient transduction, but
with would better impedance matching, it allowed mammals
to respond to much higher frequencies.
This ability to respond to higher frequencies has evolved
in other animals too.
But if you compare reptiles and amphibians with mammals,
you see there's a huge difference in the highest
frequency they can response to.
Birds are a little better, mammals are
considerably better.
So what that meant was when mammals were first evolving,
which was at the time of the dinosaurs, these lizards, both
the big ones and the small ones, couldn't hear the high
frequencies that mammals could.
And that means that the young of the mammals in their nests
could call for their parents with very high frequency
sounds and they wouln't be heard by the predators.
And they could also communicate this way and not
attract predators when they did so.
So it gave mammals a big advantage.
What was the other system that changed at that
transition to mammals?
What other system was so important in early mammals?
There was actually some regression in
vision in early mammals.
Loss of the postorbital bar there, around the eye,
indicates that.
The other one was olfaction.
There was a big increase in the number of receptors in the
olfactory system, allowing a greater range of olfactory
ability as well as auditory.
So here's a picture of the human with the external ear,
external auditory maze.
And then the tympanic membrane, the ossicles of the
middle ear.
Then the oval window where you get the transfer to the
vibrations of the inner ear.
The chamber of the cochlea is attached to the fluid of the
semicircular canal of the vestibular system.
So here I show the vestibular system off to the side here,
and this is an unrolled cochlea.
So just to show you that down the middle there's--
I think I have an enlarged picture here--
down the middle of the cochlea, if you cut this way
you'll see that there's bailar membrane.
Actually, the basilar membrane and the tectorial membrane.
And then the cells in between.
So the vibration of the fluid results in
changes in these cells.
These are the receptor cells of the auditory system.
The inner hair cells and the outer hair cells.
They're called hair cells because at the end of the cell
hair you have these little processes buried in the
tectorial membrane.
And these are not neurons, they're receptor cells,
specialized epithelial cells that generate receptor
potentials that then trigger action potentials in the
dendritic endings of the eighth nerve.
They show two connections there because there's a lot of
centrifugal connection in the auditory sytem too coming from
the brain and affecting hearing that way.
OK, one other thing.
If you unroll the cochlea--
here they show the unrolling--
you find that for steady tones, the maximum vibration
of that membrane changes as you go out along the cochlea,
as you go from the base to the apex here.
So way out at the end you get maximum vibration to the
lowest sounds.
Near the tympanic membrane--
near the oval window, rather, you have maximum vibrations
for the highest pitch sounds.
So there's a spatial layout, then, in the basilar membrane
of frequency responsiveness.
I mean, it's not that each area responds to only one
sound, but there's a maximum at different positions.
You can think of them as sort of scanning waves.
OK, we'll talk about frequency and intensity coding.
This is what we just were talking about, the place code
for the frequency spectrum.
So frequency is already then represented maximally in
different axons of the eighth nerve, coming into the
cochlear nuclei.
And if we record, and here's a picture here of the two
cochlear nuclei, the dorsal and the ventral.
Here's the eighth nerve fibers coming in.
And I've just shown three of them coming from different
parts of the cochlea.
I show them in different colors.
And I've indicated their
topographic termination patterns.
As each axon enters the ventral cochlear nucleus, it
branches and the axons terminate all along a plane in
the nucleus.
And similarly for all the other axons.
And then a branch goes up to the dorsal cochlear nucleus
and it repeats the same kind of pattern.
Most of this work has been done in the cat, but it's been
done in a few other animals also.
Here you have an electrode that's penetrating from dorsal
to ventral.
And I'm showing the recordings as they go through the ventral
cochlear nucleus.
And you can see as they advance the best frequency of
the neurons being recorded close to the electrode tip
changes very systematically.
It's a pretty precise map.
We call it a tonotopic map because it's responding to
different tones.
The tones have different frequencies.
Intensity is also encoded spacially in the sense that
different fibers, even with a similar response to frequency,
have different thresholds.
Some of them are higher thresholds, some of them lower
thresholds.
So just depending on how many of them are activated, you get
a representation of intensity as well.
And you have fibers then with best intensities as well.
So now let's talk a little bit about the sensory channels and
the pathways to the medial geniculate body.
And then we'll talk about the two major functional pathways
ascending to the endbrain.
This is the kind of medical school textbook figure of the
human auditory system.
So here's your peripheral apparatus.
The inner ear here.
The eighth nerve fibers coming in, contacting
the cochlear nuclei.
And from there, you have pathways that go directly to
the inferior colliculus and a few directly to the medial
geniculate body.
Many axons then connect to structures more ventral.
These other structures are located in the ventral part of
the hindbrain, in a region we call the trapezoid body.
There are nuclei of the trapezoid body, and then one
of the cell groups there's called the superior olive.
That's because there's an inferior olive--
a bigger one, actually, that connects to the cerebellum.
And this one is a little more rostral.
It's another little bump on the base of the hindbrain.
So it's the superior olive.
And these types of connections characterize the auditory
system processing right there in the hindbrain, which we'll
talk about.
Now, this one I can't really do on this computer because I
don't want to waste time trying to magnify it.
When you get this picture online, if you look at the
details here, you'll see the simple pathways, the eighth
nerve going from the cochlear nuclei right up to the
inferior colliculus, and then from there to the medial
geniculate body and up to the auditory cortex.
But then you'll see pathways that go from the cochlear
nuclei to the ventral hindbrain that then lead to
projections to the thalamus and to the inferior colliculus
and to these little nuclei in the midbrain.
it's a little too complicated because I've added there the
cerebellar pathway from the ventral cochlear nucleus.
I've even added some reflex pathways and
pathways to the pons.
So I shouldn't have put those all on the same diagram.
So it's not quite as bad as it looks.
So we have local reflex channels, which you can find
in that other diagram.
You have the cerebellar channel.
And most attention we give to the lemniscal channels, to
major routes, to the inferior colliculus, which then
projects to the medial geniculate body.
It also projects a few other places, but that's one of the
major routes to the neocortex.
And then a smaller route that goes directly from cochlear
nuclei to the medial geniculate body, also from the
inferior colliculus.
And I put that one picture of human.
Those weren't in the textbook picture, I've added those more
recently discovered pathways.
So just one closer look at those axons that come in to
the ventral cochlear nucleus.
Want to know why they branch and extend from the posterior
to the anterior end of the nucleus.
And when they do that, you can imagine these different cell
types distributed along that axon, with terminations of the
axon on these various cell types in the nucleus.
If we're stimulating a cat, say, with little tiny bursts,
we can record from the fibers in the eighth nerve and we'll
get a series of action potentials, as you should show
there in the post-stimulus histogram of the responses.
I want you to pay attention to this one because it's unusual
in that it's--
the action potentials fired are just like
in the eighth nerve.
One action potential comes in, one action potential goes out.
So how could that happen?
It can only happen if there's sort of a one-to-one transfer.
So this is set up for the journal computer, but I can
describe it to you without the help of the picture because
it's very simple.
The axon comes in and has a big ending when it's forming
in development.
It's initially attaching to many different parts of the
cell, and the cells are very simple.
If we look at them in the chick, they become bald cells.
Just big, bald cells, no dendrites.
They lose their dendrites in development.
And that ending develops this big, cup-like [INAUDIBLE].
It encloses quite a bit of the cell.
And if you look with an electron microscope at that
ending, you'll find it's just studded with synapses.
So the one axon has a lot of synapses from the one
axon to that cell.
So it's not surprising once you see that anatomy that you
only need one action potential coming in the eighth nerve to
fire the action potential in the cochlear nucleus cell,
these large cells which in the chicken are called the nucleus
magnocellularis because they're large.
So I won't go over all of this, this is just reviewing
some of these connections, the major types of connections
that we just talked about.
And then, to get up to the auditory cortex, we come from
the medial geniculate body, through the internal capsule,
up to the auditory areas of the neocortex.
They're in the temporal region of the cortex.
In the human and in the monkey, they're right at the
top of the superior bank of the superior temporal gyrus.
So to get to most of the auditory cortex, you have to
go into the sylvian fissure, and they're along that top
surface of the temporal lobe.
Now, there are additional auditory projections into the
thalamus, to the cell groups right around the medial
geniculate body.
There are the posterior nucleus of the thalamus and
also the ventral parts of LP.
They also get auditory projections, and they carry
information to several additional auditory areas.
There are multiple auditory regions just like there are
multiple visual regions.
Now, a lot of these studies have been done on the cat.
It's only recently that more attention's been paid to the
primate, and also to human, with
functional imaging methods.
And then I mentioned this additional projection,
probably a very early one that went directly from the medial
geniculate body to the amygdala.
So not to neocortex at all, but to the lateral nucleus and
the amygdala.
And there are projections like that in the visual system also
that come from parts of LP.
But it's more developed for the auditory system.
And that's this projection, which we showed before.
Cells here in the medial geniculate body that send
their axons out into the internal capsule, reach the
auditory cortex, and they also reach the lateral amygdala.
The amygdala projection is relatively
larger in the opossum.
Hedgehog, another fairly primitive brain mammal.
They're almost equal to cortex and amygdala.
But most of the other mammals, including the tree shrew and
the rat, the projection to the amygdala's smaller.
I want to go to the other file.
Do I hit this?
No.
This one?
Okay.
AUDIENCE: Hit the other ones, right next to it.
Right underneath that stuff.
PROFESSOR: This?
AUDIENCE: Underneath that.
Underneath.
Down.
PROFESSOR:Oh, this one
AUDIENCE: You've got to click on the picture.
Yeah yeah.
You got it.
Click on-- yeah yeah.
Double click.
PROFESSOR: OK--
That's good.
So continue with the ascending lemniscal pathways.
I've made this one slide just to summarize for you these
major pathways.
This is the one that you should leave out at the
beginning, because these are the main pathways that are
mostly talked about and described in the
books and so forth.
This is the main one.
Dorsal cochlear nucleus to the midbrain, mostly inferior
colliculus, but also these nuclei to the lateral
lemniscus, and then to the medial geniculate body.
And then the other one.
Similar route, but it goes from the cochlear nuclei,
mostly the ventral, to these more ventral structures in the
hindbrain, and from there to the midbrain, which projects
to the medial geniculate body.
And then the smaller one goes directly.
That was first discovered in a chimpanzee and hadn't been
seen in other animals, and it was thought that maybe only in
advanced primates, the big primates, that you even found
such a pathway.
But then methods for tracing axons got a little better and
they have even found it in the rat now, so I look at it as
more general.
It's been discovered in the rat and the
ferret and a few others.
And then finally, you have these pathways that just avoid
the interior colliculus all together.
Some of them go through the lateral lemniscus, some of
them may just go through the reticular formation.
And they travel up through the midbrain, not with the main
ribbon of fibers from inferior colliculus at all, but they
reach the posterior thalamus as well.
And that's been the most difficult to study projection
ever since its discovery by this man, [UNINTELLIGIBLE].
So two major types of projections, to the neocortex
from the medial geniculate body to the
auditory areas of neocortex.
And then from the posterior nucleus to several different
auditory areas.
And then the amygdala projection.
So now, let's take these ascending pathways and
separate them into the kinds of functions, the major
functions they're serving.
One of them is more concerned with where things are, the
other is more concerned with what it is, and for that we
have to discriminate template patterns in the sound.
So we'll deal first with the spatial localization pathway.
There's strong connections to the superior colliculus,
which, remember, is in control of orienting movements as well
as escape movements, and it also has
connections to the thalamus.
And then the pattern identification pathway, which
goes more directly to the thalamus, and from there to
the endbrain.
So this is the other diagram I'm using.
I show the separation of the eighth nerve into the two
cochlear nuclei.
And I'm showing it's the ventral cochlear nucleus, and
pathways from that nucleus, that are more concerned with
spatial localization, as we go from hindbrain to midbrain to
thalamus to neocortex.
So let's deal with that one first.
These structures in the hindbrain that relay that
input up to the superior colliculus.
One of the major ones in mammals is called the medial
superior olive, which I mentioned before.
It's got a different name in a chick, and in the chick, the
studies have been a lot easier to do.
Especially physiological studies, because the nucleus
isn't down in the base of the hind brain, which is much
harder to reach with an electrode.
It sits right up on top, where the cochlear
nuclei are in mammals.
So let's talk about how you use time of arrival to give
the azimuthal position--
that is, the position along the horizon.
Localization in the vertical axis.
There's other cues that animals use.
We know the projection of the superior colliculus is very
important, because if you oblate the colliculus you get
very defective orienting to sounds discovered in the cat
and in the hamster.
So let's go through those studies in chicken.
First of all, you have these large cells, the nucleus
magnocellularis, and the cochlear nucleus, where the
endings of the eighth nerve have these large calyces.
The Calyx of Held because Held was the discoverer.
Not the Held here in the department, but a much earlier
one a previous century before.
The method involves a simple kind of coincidence detection.
So what happens in nucleus laminaris in the chick happens
in the medial superior olive of the mammal.
So here's the location we're talking about in the ascending
pathways from the ventral cochlear nucleus, and here's
the diagram in the chick.
This is the cochlear nucleus cell, nucleus magnocellularis.
Here's the big calyx at the end of the eighth nerve cell.
So this is where one action potential in the eighth nerve
gives rise to one action potential with a very short
latency in the cochlear nucleus.
Now, the axons from that cell branch--
one goes to the other side, one stays on the same side.
And the ones that stay on the same side have a peculiar loop
that seems to function as a delay line.
They're projecting through this structure, the nucleus
laminaris, because the cells are laid out in a layer.
All in a line.
Now, just imagine--
let's say that the input from the other side always takes
the same amount of time.
It may not, but let's just, for the sake of argument at
the moment, say that the input from the other side is a
structure just like this.
It's projecting to these ventral dendrites.
The dendrite going up, the dorsal dendrite, is getting
inputs from cells from this cochlear nucleus with varying
amounts of delay.
And that delay is quite systematically changed as we
go from one end of the nucleus to the other.
The only other thing you need to know is that these cells in
nucleus laminaris require almost simultaneous time of
arrival in order to get the summation you
need to fire the cell.
So the result is cells at one end of the nucleus respond to
things right in front of the animal with nearly
simultaneous arrival of input at the two ears, and at the
other end of the nucleus, they respond to input way out here,
with maximal differences in the time of arrival.
Differences are very tiny, of course, but that's what the
system is set up to do, to respond to these tiny
differences in time of arrival to get localization.
So then the projections of that nucleus are already
mapped for position along the azimuth.
On one side of the brain and for the
other side of the brain.
Right side to the left side of space.
I actually said that wrong, but I think you get what I'm
talking about.
Just using time of arrival to get position.
You can imagine how that evolved.
All you need to pay attention to here is that--
all you needed at the beginning with certain cells
firing more when a sound came from one side
rather than the other.
Simple--
that crude localization.
And then if you imagine that with more precise timing in
the brain mechanisms, that mechanism became exploited,
became more precise until you finally could--
I mean, some animals localize by time of arrival much better
than we do.
For example, the owl.
Very precise.
He can also locate well on the vertical axis.
We can too, but we usually have to move our head to use
the directionality from the external ear, more out head a
little bit to get elevation.
And the owl can be very accurate, about enough to
precisely attack a mouse scurrying under the leaves on
the floor of the forest.
So the owl can't use his vision, but
he can use his audition.
OK, there is another mechanism using the lateral superior
olive that simply uses differences in amplitude,
because obviously something over here is louder in my
right ear than in my left ear.
And that information is also used.
It gives a separate type of map.
It also gives azimuthal position.
And whichever gives more precise localization, the
animal will use.
And as I mentioned, head movements are used for
vertical localization, but there is a different
attenuation of different frequencies depending on
elevation, and that's particularly enhanced in the
owl because of the arrangement of feathers around his
external auditory meatus.
So that information then reaches the colliculus, and I
think I mentioned before that the map of auditory space in
the colliculus matches the visual map, the map of the
retina, and it's a plastic map.
So if we put prisms on the animal and distort the visual
world, the auditory map will adjust until
it matches the visual.
It's an interesting kind of plasticity that's been studied
by a few different labs.
These are the oblation effects I mentioned.
Sorry for all the delays today.
We didn't get very far, but we'll go to the
neocortex next time.
And we'll start out by separating the two systems