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PROFESSOR: OK, I guess
we'll get started.

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00:00:28,830 --> 00:00:36,453
So last time we talked about
hearing loss, and deafness,

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and cochlear implants.

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00:00:39,040 --> 00:00:39,660
Any questions?

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00:00:44,660 --> 00:00:48,645
So did anyone read the Sunday
New York Times yesterday?

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My email really lit up when this
front page article showed up

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and it was titled, "Ground
Shaking Noise Rocks NFL

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00:01:03,410 --> 00:01:06,730
and Eardrums Take a Big Hit."

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00:01:06,730 --> 00:01:10,620
So the reason my email lit
up is because my lab director

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is quoted.

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So it's a big deal to be quoted
in the Sunday Times, right?

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So this is talking
about the NFL games

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00:01:18,130 --> 00:01:21,860
where they have a noise
meter down on the field

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00:01:21,860 --> 00:01:24,575
and they encourage the fans
to make a lot of noise.

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00:01:25,680 --> 00:01:32,700
And apparently at the Seattle
Seahawks recent game--

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00:01:32,700 --> 00:01:36,350
whatever-- the
crowd was recorded

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00:01:36,350 --> 00:01:40,445
at making 136 decibel
noise in September.

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00:01:41,630 --> 00:01:47,120
And so it says later on in
the article, "Fans accustomed

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00:01:47,120 --> 00:01:51,200
to hollering may--" is that what
they do at the Seahawks games?

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00:01:51,200 --> 00:01:54,540
"May scoff at the warnings
as nanny state silliness.

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But to auditory experts,
the danger is very real.

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People think it's cool
or funny, or whatever.

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00:02:02,820 --> 00:02:09,440
But there is increasing--"
this is a quote "Increasing

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00:02:09,440 --> 00:02:13,850
evidence that if your ears are
ringing, damage is happening,

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00:02:13,850 --> 00:02:18,330
said M. Charles Liberman, a
professor of otology at Harvard

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00:02:18,330 --> 00:02:22,210
Medical School, and the director
of a hearing research lab

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00:02:22,210 --> 00:02:24,260
at the Massachusetts
Eye And Ear Infirmary."

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00:02:24,260 --> 00:02:26,259
So he's a director
of the lab where

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00:02:26,259 --> 00:02:27,800
we're going to have
a lab tour later.

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00:02:29,305 --> 00:02:31,760
"There's something
irreversible going on,

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00:02:31,760 --> 00:02:35,770
and it's only going to get
worse as you get older.

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00:02:35,770 --> 00:02:38,870
Liberman's research shows that
even in the immediate effects

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00:02:38,870 --> 00:02:42,750
of noise exposure after the
immediate effects subside--

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00:02:42,750 --> 00:02:45,710
the ringing, the muffling,
the feeling of pressure-- ears

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00:02:45,710 --> 00:02:48,200
do not really recover."

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00:02:48,200 --> 00:02:51,360
So noise damage in the news.

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00:02:52,836 --> 00:02:54,312
I'll pass it around.

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All right.

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00:02:57,650 --> 00:02:58,539
So yeah, question?

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00:02:58,539 --> 00:03:00,163
AUDIENCE: I just have
a quick question.

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00:03:00,163 --> 00:03:03,225
So when I'm down at the
T, it's extremely loud.

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00:03:03,225 --> 00:03:05,350
Do you happen to know how
loud that is in decibels?

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00:03:05,350 --> 00:03:07,450
PROFESSOR: I've never
measured it on the T.

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00:03:07,450 --> 00:03:08,908
AUDIENCE: I had a
friend who tried.

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And the thing on his phone
wouldn't go high enough.

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PROFESSOR: Yes.

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00:03:13,440 --> 00:03:17,800
And a lot of folks have
a sound level meter app.

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00:03:17,800 --> 00:03:20,547
So how high was the maximum?

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AUDIENCE: The max was 100,
and it more than maxed out.

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PROFESSOR: Yeah.

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Well, so I wouldn't be
surprised if it's 110 dB

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00:03:26,660 --> 00:03:30,710
on the T. Certain
in between stops

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runs are louder than others
as you know if you ride the T.

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So I think that
that's not so damaging

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that it would hurt your hearing.

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But if you were the conductor
or the driver on the train,

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you really have
to worry about it.

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On the other hand,
if you're the driver,

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you have to have good
hearing to respond

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to some kind of signals.

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So it's not like they can
just wear hearing protection.

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00:03:55,800 --> 00:03:59,050
In this article
about NFL fans, they

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said in some games they hand
out those little foam ear

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00:04:01,710 --> 00:04:07,750
plugs, which attenuate the
noise as much as 20 dB.

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00:04:07,750 --> 00:04:12,020
So if you're listening to 120,
put the noise-- ear plug in

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00:04:12,020 --> 00:04:13,660
and it goes down to 100.

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00:04:13,660 --> 00:04:18,260
But it also said that some
people, like children,

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can't fit the foam ear
plugs in their ear canal.

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So it doesn't work for them.

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And some people, like
people who work on the T,

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need good hearing.

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So today, we're going
to move on and get back

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to the auditory brainstem.

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We had talked about the
auditory pathway that comes up

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from the cochlea through
the auditory nerve

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and into the cochlear
nucleus, which

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is the very lowest
level of the brainstem.

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And now we're going to
talk about some higher

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levels of the brainstem.

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And especially some
pathways that are

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called descending systems
and brainstem reflexes.

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So we'll define what
those two things are.

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We'll talk especially about
one descending system, which

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is a brainstem reflex called
the olivocochlear neurons.

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00:05:18,750 --> 00:05:22,180
Their anatomy, their functions,
and their reflex pathway.

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00:05:22,180 --> 00:05:27,390
And let me just stop and tie in
once again with hearing loss.

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00:05:28,540 --> 00:05:31,420
These two brainstem
reflexes that I'm

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going to talk about
clearly protect the ear

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from damage due to
high-level sound.

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00:05:37,690 --> 00:05:41,110
So that is certainly one of the
functions of the Olivocochlear

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neurons.

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And we'll see how
that takes place.

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The second brain stem
reflex that we'll talk about

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is the middle ear muscle reflex.

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00:05:51,600 --> 00:05:53,700
There are two muscles
in your middle ear.

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00:05:53,700 --> 00:05:57,400
When they contract, they
make your sense of hearing

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00:05:57,400 --> 00:05:58,260
less sensitive.

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00:05:59,610 --> 00:06:01,410
Why would you ever
want that to happen?

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00:06:01,410 --> 00:06:04,690
Well again, these
muscles contract

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00:06:04,690 --> 00:06:07,580
when you're in a high-level
sound environment,

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00:06:07,580 --> 00:06:10,060
like at the NFL game.

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00:06:10,060 --> 00:06:13,050
And one of their
functions is to reduce

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00:06:13,050 --> 00:06:14,775
the sound getting
into your inner ear.

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So to prevent damage from
the high-level sound that

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might otherwise damage
your hair cells.

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So that's the subject
of today's talk.

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Now, what do I mean by reflexes
and descending systems?

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00:06:30,110 --> 00:06:36,560
Well, this is a nice diagram
from Michael Slama, who

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00:06:36,560 --> 00:06:40,240
shows in the solid lines
the pathways going up,

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00:06:40,240 --> 00:06:42,020
which we've been talking about.

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00:06:42,020 --> 00:06:45,390
And sometimes those pathways
are called ascending systems

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00:06:45,390 --> 00:06:46,430
because they go up.

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00:06:47,450 --> 00:06:50,140
So it says here in
red, the solid lines

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00:06:50,140 --> 00:06:51,250
are ascending pathways.

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00:06:51,250 --> 00:06:56,130
So by ascending, we mean from
a lower level up to a higher.

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00:06:56,130 --> 00:06:59,250
And ultimately, to the highest
level in the neural pathway

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00:06:59,250 --> 00:06:59,905
to the cortex.

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There are analogous
descending systems,

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00:07:05,090 --> 00:07:06,909
which are shown here
in dashed lines.

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00:07:06,909 --> 00:07:08,450
And I think you can
see some of them.

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For example, here's
auditory cortex.

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And there's some
dashed lines, which

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00:07:13,680 --> 00:07:16,900
means cell bodies sitting
an auditory cortex that

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project their axons down and end
at the next lowest level, which

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00:07:21,810 --> 00:07:25,125
is the auditory thalamus
or the medial geniculate.

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And so that would be an
example of a descending system

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because the information is
starting at the higher level

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and going down to a lower level.

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And at every junction
between nuclei

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00:07:39,590 --> 00:07:42,080
you can find descending systems.

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And in the auditory pathway,
even at the lowest level,

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00:07:45,540 --> 00:07:47,740
you have a descending
system that

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00:07:47,740 --> 00:07:51,460
starts here in
the superior olive

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00:07:51,460 --> 00:07:54,200
and goes all the way
out to the cochlea.

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00:07:54,200 --> 00:07:57,660
And we talked about some
efferent nerve endings

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00:07:57,660 --> 00:07:59,565
on the hair cells
and the nerve fibers.

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00:08:00,880 --> 00:08:06,670
And that descending system is
called the olivocochlear system

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00:08:06,670 --> 00:08:10,200
of neurons because it
starts out in the olive

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00:08:10,200 --> 00:08:11,290
and goes to the cochlear.

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00:08:12,730 --> 00:08:15,170
OK, so that's what
descending systems

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is, starting from higher levels
and going down to lower levels.

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Now, how does that work out
with brainstem reflexes?

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Well, we've said that
one of the functions

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00:08:27,370 --> 00:08:29,290
of these olivocochlear
neurons would

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00:08:29,290 --> 00:08:32,030
be they prevent the
cochlea from being

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00:08:32,030 --> 00:08:33,735
damaged by high-level sounds.

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00:08:34,890 --> 00:08:38,059
Well, when would you
activate that system?

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00:08:38,059 --> 00:08:42,120
Well, obviously, when
you hear a loud sound.

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00:08:42,120 --> 00:08:46,460
So the ascending pathway
has to come into the brain,

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00:08:46,460 --> 00:08:52,300
synapse in the cochlear nucleus,
go up to the olivary complex,

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00:08:52,300 --> 00:08:56,270
and then come back down to the
cochlear to prevent the damage.

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00:08:56,270 --> 00:09:01,350
And so that little loop
that I just diagrammed

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00:09:01,350 --> 00:09:03,500
could be called
a reflex pathway.

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00:09:06,150 --> 00:09:08,355
So let's define
what a reflex is.

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What is a reflex?

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00:09:14,160 --> 00:09:14,900
Anybody know?

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00:09:16,250 --> 00:09:18,173
What does it mean
to act reflexively?

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00:09:23,730 --> 00:09:25,140
AUDIENCE: Automatic reaction.

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00:09:25,140 --> 00:09:25,848
PROFESSOR: Right.

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00:09:30,750 --> 00:09:32,215
So an automatic response.

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00:09:38,600 --> 00:09:41,060
And certainly, these
olivocochlear neurons

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00:09:41,060 --> 00:09:42,026
turn on automatically.

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00:09:43,390 --> 00:09:48,570
And sort of in between the
lines of that definition

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00:09:48,570 --> 00:09:52,420
means that the reflex pathway
is operating down here

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00:09:52,420 --> 00:09:55,880
in the automatic portion
of your auditory pathway

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00:09:55,880 --> 00:09:56,635
in the brainstem.

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00:09:58,890 --> 00:10:02,150
The part where you think
the auditory cortex

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00:10:02,150 --> 00:10:05,810
or the other regions of cortex
doesn't have to get involved.

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00:10:05,810 --> 00:10:09,910
You don't have to say, OK,
I'm in a loud environment.

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00:10:09,910 --> 00:10:12,290
Should I respond
with this reflex?

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00:10:12,290 --> 00:10:12,790
OK.

179
00:10:12,790 --> 00:10:13,910
Yeah, maybe I will.

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00:10:13,910 --> 00:10:15,230
OK, respond.

181
00:10:15,230 --> 00:10:17,240
You don't have to
think about it.

182
00:10:17,240 --> 00:10:20,000
It's automatic
because it happens

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00:10:20,000 --> 00:10:22,740
in the parts of the brain where
things happen automatically.

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00:10:22,740 --> 00:10:24,610
What other things
happen in the brainstem?

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00:10:25,740 --> 00:10:28,070
Well, control of breathing.

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00:10:28,070 --> 00:10:32,270
You have the motor neurons
that come to your chest muscles

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00:10:32,270 --> 00:10:35,580
and your diaphragm that
enable you to breathe.

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00:10:35,580 --> 00:10:38,060
Those motor neurons are
located in the brainstem.

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00:10:38,060 --> 00:10:42,430
And the control of respiration
is this brainstem function.

190
00:10:42,430 --> 00:10:44,770
It doesn't have to go
all the way to the cortex

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00:10:44,770 --> 00:10:46,850
and have to think about it.

192
00:10:46,850 --> 00:10:51,590
Now, that's not to say that
reflexes, like the breathing

193
00:10:51,590 --> 00:10:53,160
reflex, you can
say, OK, I'm going

194
00:10:53,160 --> 00:10:55,750
to stop breathing for a minute.

195
00:10:55,750 --> 00:10:57,470
OK, I'm going to hold my breath.

196
00:10:57,470 --> 00:11:00,930
So you can, via
your higher centers,

197
00:11:00,930 --> 00:11:03,800
say I'm going to send
information down here.

198
00:11:03,800 --> 00:11:06,030
Maybe some of these
other descending pathways

199
00:11:06,030 --> 00:11:08,810
come down to the
olivocochlear neurons and say,

200
00:11:08,810 --> 00:11:11,972
I'm not going to do this
reflex for a moment.

201
00:11:11,972 --> 00:11:14,305
Of course, you eventually
have to start breathing again.

202
00:11:15,780 --> 00:11:21,320
But you can have higher center
control of these reflexes.

203
00:11:21,320 --> 00:11:24,800
It's not that the higher
centers aren't involved.

204
00:11:24,800 --> 00:11:27,980
OK, so another reflex would be
your patellar tendon reflex.

205
00:11:27,980 --> 00:11:33,010
The physician tests when you
go to your doctor's office.

206
00:11:33,010 --> 00:11:36,750
They hit your patellar
tendon with a little mallet

207
00:11:36,750 --> 00:11:38,510
and see that your leg contracts.

208
00:11:41,310 --> 00:11:45,850
And apparently, everyone
passes that test, right?

209
00:11:45,850 --> 00:11:48,450
You can't walk into
the doctor's office

210
00:11:48,450 --> 00:11:50,060
without having some
kind of reflex.

211
00:11:50,060 --> 00:11:54,500
But they're actually testing
for hyper-reflex in that case.

212
00:11:54,500 --> 00:11:58,470
And so some of these
reflexes can be in overdrive.

213
00:11:58,470 --> 00:12:00,920
And that's what the
physician is testing for.

214
00:12:02,750 --> 00:12:06,660
OK, so we are really
interested in what all

215
00:12:06,660 --> 00:12:08,470
these descending systems do.

216
00:12:09,780 --> 00:12:12,230
What do the descending
systems do in general?

217
00:12:13,350 --> 00:12:17,170
Well, it actually ends up being
a very hard issue to study.

218
00:12:18,470 --> 00:12:25,170
We know that those systems are
there, but what good are they?

219
00:12:26,530 --> 00:12:29,280
What do they do in
our sense of hearing?

220
00:12:29,280 --> 00:12:34,250
So the classic, old-fashioned
way of studying such a system

221
00:12:34,250 --> 00:12:36,840
would be to go in
and make a lesion

222
00:12:36,840 --> 00:12:40,010
and see how the animal's
behavior, the person's behavior

223
00:12:40,010 --> 00:12:41,800
changes, or do
some kind of test.

224
00:12:43,270 --> 00:12:46,150
But it turns out most of
these descending systems

225
00:12:46,150 --> 00:12:50,790
are intertwined with their
corresponding ascending

226
00:12:50,790 --> 00:12:51,500
pathway.

227
00:12:51,500 --> 00:12:57,590
So it's very difficult to go
in and make a cut or a burning

228
00:12:57,590 --> 00:13:02,900
lesion that just lesions one
of the descending pathways

229
00:13:02,900 --> 00:13:04,805
and not affect the
ascending pathway, which

230
00:13:04,805 --> 00:13:06,305
would complicate
the interpretation.

231
00:13:08,340 --> 00:13:11,010
The reason that some of
these systems-- for example,

232
00:13:11,010 --> 00:13:14,930
the olivocochlear
neurons-- are amenable

233
00:13:14,930 --> 00:13:16,880
and we know a lot
about them-- are

234
00:13:16,880 --> 00:13:20,210
amenable to experimentation--
is we can selectively

235
00:13:20,210 --> 00:13:23,980
lesion and stimulate
them in isolation

236
00:13:23,980 --> 00:13:26,410
from the ascending systems.

237
00:13:26,410 --> 00:13:29,490
And let me show you a diagram
done of the olivocochlear

238
00:13:29,490 --> 00:13:32,790
system that illustrates that.

239
00:13:32,790 --> 00:13:37,910
So these are the olivocochlear
neurons, these stars here.

240
00:13:37,910 --> 00:13:39,360
There are two types.

241
00:13:39,360 --> 00:13:44,100
The red are the so-called
medial olivocochlear neurons

242
00:13:44,100 --> 00:13:46,870
because they're sitting
in the medial part

243
00:13:46,870 --> 00:13:47,865
of the superior olive.

244
00:13:50,100 --> 00:13:55,100
The green ones are so-called
lateral olivocochlear neurons

245
00:13:55,100 --> 00:13:58,120
because they're sitting
in the more lateral part

246
00:13:58,120 --> 00:13:59,060
the superior olive.

247
00:13:59,060 --> 00:14:03,900
And this diagram shows the cell
bodies of those olivocochlear

248
00:14:03,900 --> 00:14:08,110
neurons that are innovating
this cochlea-- sending axons out

249
00:14:08,110 --> 00:14:09,940
to this cochlea
on the right side.

250
00:14:11,260 --> 00:14:16,120
And that defines this cochlea
as the ipsilateral cochlea.

251
00:14:16,120 --> 00:14:17,860
Ipsi means the same.

252
00:14:17,860 --> 00:14:20,630
So that cochlea gets a name.

253
00:14:20,630 --> 00:14:22,870
That's the ipsilateral cochlear.

254
00:14:22,870 --> 00:14:26,730
And you can see that these
green lateral olivocochlear

255
00:14:26,730 --> 00:14:29,340
neurons are basically on
the same side of the brain

256
00:14:29,340 --> 00:14:31,395
stem as the cochlea
that they innervate.

257
00:14:32,510 --> 00:14:34,840
But these medial
olivocochlear neurons

258
00:14:34,840 --> 00:14:37,119
are distributed on both
sides of the brain.

259
00:14:37,119 --> 00:14:39,035
So there's a little bit
difference in anatomy.

260
00:14:40,380 --> 00:14:43,960
Now, these are a
little bit like neurons

261
00:14:43,960 --> 00:14:46,094
that we call motor neurons.

262
00:14:46,094 --> 00:14:48,010
Everybody knows what a
motor neuron is, right?

263
00:14:48,010 --> 00:14:52,590
It's going from the brain of the
spinal cord out to the muscle.

264
00:14:52,590 --> 00:14:54,600
That's why it's
called a motor neuron.

265
00:14:54,600 --> 00:14:57,014
And when it fires off,
it contracts the muscle

266
00:14:57,014 --> 00:14:57,930
that it's innervating.

267
00:14:59,850 --> 00:15:02,790
It turns out all the muscles
on the side of the body,

268
00:15:02,790 --> 00:15:05,000
let's say the
ipsilateral muscles,

269
00:15:05,000 --> 00:15:08,475
the motor neurons are located
on that same side of the brain.

270
00:15:09,960 --> 00:15:14,100
So right-side muscles are always
innervated by right-side motor

271
00:15:14,100 --> 00:15:14,600
neurons.

272
00:15:15,690 --> 00:15:20,760
So this anatomy is a little
bit different than the anatomy

273
00:15:20,760 --> 00:15:22,240
of motor neurons.

274
00:15:22,240 --> 00:15:25,576
Even though these
neurons are having

275
00:15:25,576 --> 00:15:26,950
an effect in the
cochlea, they're

276
00:15:26,950 --> 00:15:28,240
not innervating muscles.

277
00:15:28,240 --> 00:15:31,180
They're innervating
hair cells in this case.

278
00:15:31,180 --> 00:15:36,480
They do use the same
neurotransmitter as muscles.

279
00:15:36,480 --> 00:15:43,317
So the olivocochlear neurons
use the neurotransmitter

280
00:15:43,317 --> 00:15:43,900
acetylcholine.

281
00:15:45,570 --> 00:15:48,075
And so they would be
called cholinergic.

282
00:16:00,010 --> 00:16:03,180
OK, so they synthesize
the acetylcholine

283
00:16:03,180 --> 00:16:04,930
and their cell bodies
are transported down

284
00:16:04,930 --> 00:16:07,490
to the axons and
the nerve terminals.

285
00:16:07,490 --> 00:16:09,730
Out in the cochlea
use acetylcholine.

286
00:16:09,730 --> 00:16:10,910
They release it.

287
00:16:10,910 --> 00:16:13,520
Now, where do they
release it to?

288
00:16:13,520 --> 00:16:15,710
They release it
to their targets.

289
00:16:17,040 --> 00:16:21,160
And out in the periphery, these
Medial Olivocochlear Neurons,

290
00:16:21,160 --> 00:16:25,650
or the MOC neurons, target
the outer hair cells.

291
00:16:25,650 --> 00:16:27,700
And they release
acetylcholine directly

292
00:16:27,700 --> 00:16:31,720
onto the outer hair
cells via their synapses.

293
00:16:33,510 --> 00:16:36,510
The lateral
olivocochlear neurons

294
00:16:36,510 --> 00:16:40,210
come out to the periphery and
they release the acetylcholine

295
00:16:40,210 --> 00:16:45,420
in their synapses on
the auditory nerve fiber

296
00:16:45,420 --> 00:16:46,405
peripheral dendrites.

297
00:16:47,820 --> 00:16:49,620
OK, so the innervation
is very distinct.

298
00:16:52,129 --> 00:16:54,170
What are the auditory
nerve peripheral dendrites?

299
00:16:54,170 --> 00:16:56,800
Well, those are the ones
we've been talking about.

300
00:16:56,800 --> 00:16:59,680
We've been talking about
auditory nerve fibers.

301
00:16:59,680 --> 00:17:03,084
They start at the inner hair
cells for the most part.

302
00:17:03,084 --> 00:17:03,875
They send messages.

303
00:17:03,875 --> 00:17:05,569
That's what this
arrow is indicating.

304
00:17:05,569 --> 00:17:07,335
Send messages into the brain.

305
00:17:08,700 --> 00:17:12,140
When there's a sound,
the membranes move.

306
00:17:12,140 --> 00:17:14,420
The outer hair cells
are electromotile.

307
00:17:14,420 --> 00:17:15,685
The inner hair cells respond.

308
00:17:17,530 --> 00:17:20,609
They send messages,
synaptic messages,

309
00:17:20,609 --> 00:17:21,780
to their nerve fibers.

310
00:17:21,780 --> 00:17:24,872
The nerve fibers spike and they
send information to the brain

311
00:17:24,872 --> 00:17:26,080
through the cochlear nucleus.

312
00:17:28,119 --> 00:17:30,720
These efferent fibers
are sending messages

313
00:17:30,720 --> 00:17:31,530
the opposite way.

314
00:17:31,530 --> 00:17:34,680
They're starting in the brain
and going out to the cochlea.

315
00:17:34,680 --> 00:17:37,240
And out in the
cochlea, this arrow

316
00:17:37,240 --> 00:17:40,740
means that the information
is coming from the brain

317
00:17:40,740 --> 00:17:45,360
out to the outer hair cells or
the auditory nerve dendrites

318
00:17:45,360 --> 00:17:45,860
here.

319
00:17:46,880 --> 00:17:48,130
Is everybody clear about that?

320
00:17:49,880 --> 00:17:52,500
Now, the anatomy
works out so that you

321
00:17:52,500 --> 00:17:54,590
can do some pretty
interesting things.

322
00:17:54,590 --> 00:17:59,770
You can make a cut of this nerve
bundle, the olivocochlear nerve

323
00:17:59,770 --> 00:18:00,740
bundle.

324
00:18:00,740 --> 00:18:02,930
You can stimulate it.

325
00:18:02,930 --> 00:18:05,920
And right there underneath
where the word "brainstem" is,

326
00:18:05,920 --> 00:18:07,800
there's a great
place to make a cut

327
00:18:07,800 --> 00:18:10,970
or to electrically stimulate
and activate this system.

328
00:18:12,180 --> 00:18:14,864
Well, why don't we use sound
to stimulate the system?

329
00:18:14,864 --> 00:18:16,030
Well, it's a little messier.

330
00:18:17,240 --> 00:18:19,700
It's cleaner to
activate this bundle

331
00:18:19,700 --> 00:18:21,470
with electrical
stimulation because you

332
00:18:21,470 --> 00:18:24,130
can put your stimulating
electron right on it

333
00:18:24,130 --> 00:18:26,255
and only activate that system.

334
00:18:28,700 --> 00:18:31,250
So that's one big advantage.

335
00:18:31,250 --> 00:18:35,650
You can selectively activate
these olivocochlear neurons

336
00:18:35,650 --> 00:18:37,880
and then study, well,
what the heck do

337
00:18:37,880 --> 00:18:40,180
they do out in the cochlea?

338
00:18:40,180 --> 00:18:44,410
What changes do they have
when you activate them?

339
00:18:44,410 --> 00:18:47,000
And what changes happen
when you deactivate them

340
00:18:47,000 --> 00:18:49,740
and making a cut in the system?

341
00:18:49,740 --> 00:18:51,910
And de-efferent, if
you will, the cochlea.

342
00:18:54,460 --> 00:18:58,350
And as this list shows, there
are a number of functions.

343
00:18:59,810 --> 00:19:04,860
And if I had to pick one of
these, each of these functions

344
00:19:04,860 --> 00:19:06,350
has some experimental support.

345
00:19:07,790 --> 00:19:09,840
And I don't think I
would be able to pick

346
00:19:09,840 --> 00:19:11,410
which is the most important.

347
00:19:11,410 --> 00:19:14,070
Well, sure, if you're
at an NFL game,

348
00:19:14,070 --> 00:19:18,180
you are probably experiencing
a very high sound level.

349
00:19:18,180 --> 00:19:20,560
And so it becomes
important in that situation

350
00:19:20,560 --> 00:19:23,675
to protect the
cochlea from damage.

351
00:19:23,675 --> 00:19:25,440
So it depends on the situation.

352
00:19:26,890 --> 00:19:28,850
But we're going to
go through these.

353
00:19:28,850 --> 00:19:33,930
In turn, I think that
since I mentioned damage,

354
00:19:33,930 --> 00:19:36,800
and since I don't
have a slide on it,

355
00:19:36,800 --> 00:19:41,190
let me say how that
experimental evidence arises.

356
00:19:41,190 --> 00:19:43,959
It's a very simple
type of experiment.

357
00:19:43,959 --> 00:19:44,750
You take an animal.

358
00:19:46,200 --> 00:19:49,720
You put it in a high-level
sound environment

359
00:19:49,720 --> 00:19:52,425
comparable to an
NFL game, 120 dB.

360
00:19:53,740 --> 00:19:56,800
Take the animal out,
you study its cochleas.

361
00:19:56,800 --> 00:19:59,220
You can count the hair cells.

362
00:19:59,220 --> 00:20:01,430
You can measure the responses.

363
00:20:01,430 --> 00:20:04,010
The hearing has become
much less sensitive.

364
00:20:04,010 --> 00:20:05,975
Some of the hair cells
have been killed.

365
00:20:07,370 --> 00:20:08,400
OK, no big deal.

366
00:20:08,400 --> 00:20:10,470
We went over that last time.

367
00:20:10,470 --> 00:20:11,960
Take a second animal.

368
00:20:11,960 --> 00:20:18,550
And in that animal, cut this
olivocochlear bundle going out

369
00:20:18,550 --> 00:20:20,620
to, let's say, the
ipsilateral cochlea.

370
00:20:22,500 --> 00:20:25,430
The olivocochlear neurons
going to this other side,

371
00:20:25,430 --> 00:20:28,295
the contralateral cochlea,
leave that intact.

372
00:20:29,820 --> 00:20:34,910
OK, beautiful experiment
because within just one animal,

373
00:20:34,910 --> 00:20:39,455
you have one side's cochlea
has been de-efferented.

374
00:20:40,570 --> 00:20:43,840
You have cut off
these efferent fibers.

375
00:20:43,840 --> 00:20:46,095
The other side has a
normal innervation.

376
00:20:47,240 --> 00:20:50,400
Expose the animal to 120 dB SPL.

377
00:20:50,400 --> 00:20:53,590
Do the same response metrics.

378
00:20:53,590 --> 00:20:55,510
Test the responses
of the cochlea.

379
00:20:55,510 --> 00:20:58,380
Look at how many hair
cells have been killed.

380
00:20:58,380 --> 00:21:02,075
You find that there is a huge
difference in the two ears.

381
00:21:03,170 --> 00:21:06,480
Where the efference or
these olivocochlear neurons

382
00:21:06,480 --> 00:21:09,830
have been cut, there's
a lot of damage

383
00:21:09,830 --> 00:21:11,325
and a lot of loss
of sensitivity.

384
00:21:12,610 --> 00:21:15,880
On the intact side,
it has been protected.

385
00:21:15,880 --> 00:21:19,710
There is less damage
and better responses.

386
00:21:19,710 --> 00:21:21,040
It's more sensitive.

387
00:21:21,040 --> 00:21:23,970
So it's a very, very nice,
very elegant experiment

388
00:21:23,970 --> 00:21:25,145
that's been done many times.

389
00:21:27,250 --> 00:21:29,520
In probably both
of these systems,

390
00:21:29,520 --> 00:21:34,500
the medials and the laterals
provide such protection

391
00:21:34,500 --> 00:21:35,750
from damage.

392
00:21:35,750 --> 00:21:37,285
So we can say one
of the functions

393
00:21:37,285 --> 00:21:45,480
then is then protection.

394
00:21:57,740 --> 00:21:59,970
What do I mean by these
other functions-- shift

395
00:21:59,970 --> 00:22:02,070
the dynamic range
of hearing, reduce

396
00:22:02,070 --> 00:22:06,590
the effects of noise masking,
and reduce hearing sensitivity

397
00:22:06,590 --> 00:22:09,290
when paying attention
to visual tasks?

398
00:22:09,290 --> 00:22:11,400
OK, we're going to go
over those one by one.

399
00:22:14,210 --> 00:22:20,310
For the first one, the
experiment runs like this.

400
00:22:20,310 --> 00:22:24,085
You record from these afferent
auditory nerve fibers.

401
00:22:25,820 --> 00:22:29,210
And instead of cutting
these olivocochlear neurons,

402
00:22:29,210 --> 00:22:31,110
you stimulate them.

403
00:22:31,110 --> 00:22:34,240
You can put a stimulating
electrode right down there,

404
00:22:34,240 --> 00:22:36,130
right below the
word "brainstem,"

405
00:22:36,130 --> 00:22:38,590
and activate this bundle.

406
00:22:38,590 --> 00:22:41,860
What happens to the
responses of the nerve fibers

407
00:22:41,860 --> 00:22:45,020
when you activate this
olivocochlear system?

408
00:22:47,692 --> 00:22:48,900
And that's what's shown here.

409
00:22:48,900 --> 00:22:53,930
And this experiment has
been done since the 1970s.

410
00:22:53,930 --> 00:22:55,050
This is an old experiment.

411
00:22:57,790 --> 00:23:00,560
This is the response
in terms of the firing

412
00:23:00,560 --> 00:23:02,100
rate from the auditory nerve.

413
00:23:03,620 --> 00:23:06,040
And in this case, a tone is on.

414
00:23:06,040 --> 00:23:08,970
So you're really driving
the auditory nerve fiber.

415
00:23:10,500 --> 00:23:12,750
So the firing rate is high.

416
00:23:12,750 --> 00:23:14,350
Then, during the
second black bar

417
00:23:14,350 --> 00:23:17,544
here, you stimulate the
olivocochlear neurons

418
00:23:17,544 --> 00:23:18,960
that are going out
to the cochlea.

419
00:23:21,520 --> 00:23:23,290
And look what happens
to the firing rate.

420
00:23:23,290 --> 00:23:24,600
It goes almost down to zero.

421
00:23:26,300 --> 00:23:28,160
When you turn that
stimulation off,

422
00:23:28,160 --> 00:23:30,890
the firing rate comes back
to about what it was before.

423
00:23:32,610 --> 00:23:36,630
There's been a huge
inhibition of firing rate

424
00:23:36,630 --> 00:23:37,735
in the auditory nerve.

425
00:23:40,300 --> 00:23:45,190
If you plot the firing rate as
a function of the tone burst

426
00:23:45,190 --> 00:23:47,802
level-- so now we're going
to do different tones

427
00:23:47,802 --> 00:23:48,635
at different levels.

428
00:23:50,210 --> 00:23:54,077
At low tone levels where the
fiber is firing spontaneously,

429
00:23:54,077 --> 00:23:55,160
there's hardly any effect.

430
00:23:56,790 --> 00:24:00,240
In mid-levels, like we
saw illustrated here,

431
00:24:00,240 --> 00:24:01,940
this is without stimulation.

432
00:24:01,940 --> 00:24:03,430
This is with stimulation.

433
00:24:03,430 --> 00:24:06,716
There's a huge decrease
in firing rate.

434
00:24:06,716 --> 00:24:08,310
And up at the
highest levels where

435
00:24:08,310 --> 00:24:13,760
the fiber has become
saturated-- saturation means

436
00:24:13,760 --> 00:24:16,710
that even though you're
increasing the tone burst,

437
00:24:16,710 --> 00:24:21,180
you're not increasing the fire
rate coming from the fiber--

438
00:24:21,180 --> 00:24:23,350
there's very little effect
of this stimulation.

439
00:24:26,330 --> 00:24:27,900
How can that help us?

440
00:24:29,140 --> 00:24:31,730
We have a dynamic range
problem in hearing.

441
00:24:31,730 --> 00:24:35,440
Most auditory nerve fibers--
forget about the stimulation

442
00:24:35,440 --> 00:24:35,940
right now.

443
00:24:35,940 --> 00:24:36,940
Look at the solid curve.

444
00:24:38,240 --> 00:24:40,690
The dynamic range of most
auditory nerve fibers

445
00:24:40,690 --> 00:24:43,740
has just 20 or 30 dB
before saturating.

446
00:24:43,740 --> 00:24:46,315
That means the fiber
goes up and it saturates.

447
00:24:47,630 --> 00:24:50,160
What happens when we
get to the NFL game

448
00:24:50,160 --> 00:24:53,640
or in the better case,
what happens when

449
00:24:53,640 --> 00:24:55,870
we're in a restaurant
or a bar and we want

450
00:24:55,870 --> 00:24:59,120
to listen to the speaker
across the table from us?

451
00:24:59,120 --> 00:25:02,290
Well, that's a pretty
high sound level, 80 dB.

452
00:25:02,290 --> 00:25:04,910
It's not damaging,
but it's high enough

453
00:25:04,910 --> 00:25:07,740
to saturate our
auditory nerve fibers.

454
00:25:07,740 --> 00:25:10,920
That means when the
added sound level

455
00:25:10,920 --> 00:25:14,610
of the person across the
table from you, their voice,

456
00:25:14,610 --> 00:25:18,522
adds to the
background, they're not

457
00:25:18,522 --> 00:25:21,105
going to change the firing rate
of your auditory nerve fibers.

458
00:25:22,320 --> 00:25:25,250
And the brain won't
know except by seeing

459
00:25:25,250 --> 00:25:28,525
the person's lips move
that there is speech.

460
00:25:30,090 --> 00:25:33,320
It's much better to
understand any kind of signals

461
00:25:33,320 --> 00:25:37,460
to be within this rising
function of the firing rate

462
00:25:37,460 --> 00:25:40,210
curve, not in the
saturated function where

463
00:25:40,210 --> 00:25:42,990
there's no change
in firing rate.

464
00:25:42,990 --> 00:25:45,510
OK, so how can the
olivocochlear system

465
00:25:45,510 --> 00:25:48,685
help us with this
dynamic range problem?

466
00:25:50,170 --> 00:25:55,040
Well, I've been emphasizing
the decrease of firing rates.

467
00:25:55,040 --> 00:25:58,580
But as you can see from
this curve with stimulation

468
00:25:58,580 --> 00:26:02,035
of the OC neurons, the effect
is to shift this function over.

469
00:26:03,400 --> 00:26:05,460
In this case, it's
about a 20 dB shift.

470
00:26:06,750 --> 00:26:13,550
And now, at 70 dB, you're in the
dynamic range of the function.

471
00:26:13,550 --> 00:26:16,425
And you're not saturated
anymore as you were before.

472
00:26:18,390 --> 00:26:21,430
So one of the
functions, we think then

473
00:26:21,430 --> 00:26:26,120
from this type of experiment,
is the MOC efference kick in,

474
00:26:26,120 --> 00:26:28,940
reflexively kick in when
you're in a high-level sound

475
00:26:28,940 --> 00:26:30,200
environment.

476
00:26:30,200 --> 00:26:33,850
And they shift the firing
rate functions over

477
00:26:33,850 --> 00:26:36,630
so you can now
understand a speaker's

478
00:26:36,630 --> 00:26:38,285
voice in a high-level
background.

479
00:26:45,320 --> 00:26:46,770
Now, there are
some other factors

480
00:26:46,770 --> 00:26:50,800
that change an increase of
dynamic range of hearing, which

481
00:26:50,800 --> 00:26:52,760
I don't think are
important to the course.

482
00:26:52,760 --> 00:26:56,173
But we have talked about
two-tone suppression.

483
00:26:57,710 --> 00:27:00,850
This certainly does
the same kind of thing.

484
00:27:00,850 --> 00:27:03,470
We're going to talk about
contraction of the middle ear

485
00:27:03,470 --> 00:27:08,140
muscles, which also helps you
with the dynamic range problem.

486
00:27:08,140 --> 00:27:12,150
So this is clearly then,
one important function

487
00:27:12,150 --> 00:27:13,700
of the olivocochlear neurons.

488
00:27:13,700 --> 00:27:24,445
That is, shifting dynamic range.

489
00:27:26,820 --> 00:27:29,260
Now, is there
experimental evidence

490
00:27:29,260 --> 00:27:32,730
besides what you
see here looking

491
00:27:32,730 --> 00:27:35,290
at firing rates of
auditory nerve fibers

492
00:27:35,290 --> 00:27:39,780
to suggest that the
olivocochlear system really

493
00:27:39,780 --> 00:27:40,550
does this?

494
00:27:40,550 --> 00:27:42,170
There is a little bit.

495
00:27:42,170 --> 00:27:45,630
That is, with animals
trained to detect changes

496
00:27:45,630 --> 00:27:48,420
in tones in a
background of noise,

497
00:27:48,420 --> 00:27:51,250
they do it a little
bit better when

498
00:27:51,250 --> 00:27:54,660
they have an intact
olivocochlear system.

499
00:27:54,660 --> 00:27:56,740
You can't do these
experiments in humans

500
00:27:56,740 --> 00:28:00,080
because it's very difficult
to turn this system off.

501
00:28:00,080 --> 00:28:04,510
We don't know a way of turning
it off or interrupting it.

502
00:28:06,740 --> 00:28:10,550
And people probably call this
system into play reflexively.

503
00:28:11,730 --> 00:28:15,295
So it's difficult to do these
kinds of experiments in humans.

504
00:28:19,410 --> 00:28:22,460
Now, there's another
important function

505
00:28:22,460 --> 00:28:26,430
of the olivocochlear
system, which is probably

506
00:28:26,430 --> 00:28:33,465
to reduce the effects
of noise masking.

507
00:28:35,450 --> 00:28:42,430
And we haven't talked too much
about masking in this course.

508
00:28:42,430 --> 00:28:45,140
I mean, two-tone suppression
is a kind of masking.

509
00:28:47,270 --> 00:28:51,200
Masking is where you're
listening to one sound

510
00:28:51,200 --> 00:28:55,160
and a second sound comes in
and interferes your ability

511
00:28:55,160 --> 00:28:58,110
to detect that very first sound.

512
00:28:58,110 --> 00:29:00,090
So I have a
demonstration, though,

513
00:29:00,090 --> 00:29:04,260
that I think will convince you
that masking is very important.

514
00:29:04,260 --> 00:29:05,450
And it runs like this.

515
00:29:06,740 --> 00:29:11,580
So the demonstration
is going to be

516
00:29:11,580 --> 00:29:16,260
you're listening to tone bursts,
which are the pink things here.

517
00:29:16,260 --> 00:29:18,910
And they're at 2,000 Hertz,
so it's going to be kind

518
00:29:18,910 --> 00:29:23,500
of a little bit above
our 1,000 standard

519
00:29:23,500 --> 00:29:26,185
middle-of-the-hearing-range
frequency.

520
00:29:27,950 --> 00:29:30,245
And it's going to give
you 10 tone bursts

521
00:29:30,245 --> 00:29:34,090
and they're each going
to be successively softer

522
00:29:34,090 --> 00:29:35,630
in sound level.

523
00:29:35,630 --> 00:29:39,110
And you're supposed to count
how many steps you can hear.

524
00:29:39,110 --> 00:29:41,340
And I think there
are 10 of them.

525
00:29:41,340 --> 00:29:44,680
We should be able to hear
all 10 with the way I

526
00:29:44,680 --> 00:29:46,570
have set the level.

527
00:29:46,570 --> 00:29:49,170
The second part
of the demo is now

528
00:29:49,170 --> 00:29:51,320
the signal is masked
with broadband noise.

529
00:29:51,320 --> 00:29:54,040
So the broadband noise
will come on first.

530
00:29:54,040 --> 00:29:55,530
Shh.

531
00:29:55,530 --> 00:29:58,250
And these tone pips will
be on top of that noise.

532
00:29:59,270 --> 00:30:02,340
And you're supposed to
count now how many tone

533
00:30:02,340 --> 00:30:05,810
pips you can hear with
the broadband noise.

534
00:30:05,810 --> 00:30:09,640
And I think there's a
little verbiage before this.

535
00:30:09,640 --> 00:30:10,660
Just ignore that.

536
00:30:14,644 --> 00:30:16,138
[AUDIO PLAYBACK]

537
00:30:16,138 --> 00:30:19,126
-Critical bands by masking.

538
00:30:19,126 --> 00:30:23,608
You will hear a 2,000
Hertz tone in 10 decreasing

539
00:30:23,608 --> 00:30:25,600
steps of 5 decibels.

540
00:30:25,600 --> 00:30:28,588
Count how many
steps you can hear.

541
00:30:28,588 --> 00:30:30,082
Series are presented twice.

542
00:30:31,576 --> 00:30:43,030
[BEEPING]

543
00:30:43,030 --> 00:30:44,060
[END AUDIO PLAYBACK]

544
00:30:44,060 --> 00:30:46,460
PROFESSOR: OK, so could
everybody hear most of them?

545
00:30:46,460 --> 00:30:46,960
10?

546
00:30:46,960 --> 00:30:47,491
All 10?

547
00:30:47,491 --> 00:30:47,990
OK.

548
00:30:49,170 --> 00:30:49,995
Here's the masked.

549
00:30:49,995 --> 00:30:50,661
[AUDIO PLAYBACK]

550
00:30:50,661 --> 00:30:53,334
-Now the signal is masked
with broadband noise.

551
00:30:55,310 --> 00:31:07,660
[BEEPING]

552
00:31:07,660 --> 00:31:08,670
[END AUDIO PLAYBACK]

553
00:31:08,670 --> 00:31:10,640
PROFESSOR: OK, how many now?

554
00:31:10,640 --> 00:31:11,274
AUDIENCE: Five.

555
00:31:11,274 --> 00:31:11,940
PROFESSOR: Five?

556
00:31:13,500 --> 00:31:16,820
I mean, this is not a
bad illustration, right?

557
00:31:16,820 --> 00:31:20,076
When the pink tone bursts
get within the black noise,

558
00:31:20,076 --> 00:31:21,450
they're pretty
much disappearing.

559
00:31:22,920 --> 00:31:28,020
So clearly the noise
was an effective masker

560
00:31:28,020 --> 00:31:29,180
of the tone pip.

561
00:31:29,180 --> 00:31:36,030
Now, we had the example
earlier of the phenomenon

562
00:31:36,030 --> 00:31:37,530
called two-tone suppression.

563
00:31:51,700 --> 00:31:55,221
And that was measured in
an auditory nerve fiber.

564
00:31:55,221 --> 00:31:57,470
And everybody should be able
to draw an auditory nerve

565
00:31:57,470 --> 00:31:58,605
fiber tuning curve.

566
00:32:00,990 --> 00:32:03,510
On the x-axis, we
have the frequency

567
00:32:03,510 --> 00:32:04,675
of the sound in kilohertz.

568
00:32:05,940 --> 00:32:09,810
And the y-axis, we have
the sound pressure level

569
00:32:09,810 --> 00:32:12,325
for a response.

570
00:32:15,000 --> 00:32:20,040
An auditory nerve tuning curves,
at least those with high CFs,

571
00:32:20,040 --> 00:32:21,160
look like this.

572
00:32:21,160 --> 00:32:24,570
And the characteristic frequency
is the frequency right here.

573
00:32:26,160 --> 00:32:31,680
In two-tone suppression,
what we had is two tones.

574
00:32:31,680 --> 00:32:36,390
The first tone is
a probe tone, which

575
00:32:36,390 --> 00:32:39,905
is placed inside the nerve
fibers response area.

576
00:32:41,860 --> 00:32:45,450
And if you look at a
graph of firing rate--

577
00:32:45,450 --> 00:32:51,480
so this is firing-- and
you turn that tone on,

578
00:32:51,480 --> 00:32:53,770
the firing rate is
going to go way up.

579
00:32:53,770 --> 00:32:58,550
Maybe 100 spikes per
second because it's

580
00:32:58,550 --> 00:33:00,570
within the response area.

581
00:33:00,570 --> 00:33:05,440
Now, the second tone, sometimes
called the suppressor,

582
00:33:05,440 --> 00:33:09,410
is put outside the response
area but close to it.

583
00:33:14,610 --> 00:33:15,870
OK, so here's the probe.

584
00:33:25,940 --> 00:33:29,276
This is the probe
tone, high firing rate.

585
00:33:29,276 --> 00:33:30,650
And now a little
bit later, we're

586
00:33:30,650 --> 00:33:32,620
going to turn on a second tone.

587
00:33:35,900 --> 00:33:37,336
And this is called
the suppressor.

588
00:33:42,570 --> 00:33:44,410
And I haven't drawn
this very well,

589
00:33:44,410 --> 00:33:46,590
but when the suppressor
goes on, the firing rate

590
00:33:46,590 --> 00:33:47,755
can come back down.

591
00:33:49,300 --> 00:33:50,635
This goes off first.

592
00:33:52,356 --> 00:33:54,230
And come back up.

593
00:33:55,930 --> 00:33:56,950
OK.

594
00:33:56,950 --> 00:34:01,930
So clearly, at least in the
discharges of auditory nerve

595
00:34:01,930 --> 00:34:07,380
fibers, you can have suppressors
outside the response areas

596
00:34:07,380 --> 00:34:09,579
that decrease the
response to probes.

597
00:34:12,400 --> 00:34:16,820
This probe tone is going to
be signaled by auditory nerve

598
00:34:16,820 --> 00:34:20,020
fibers close to that frequency.

599
00:34:20,020 --> 00:34:24,745
So many auditory nerve fibers
have CFs close to 2 kilohertz.

600
00:34:26,190 --> 00:34:32,409
This noise has energy
throughout the frequency range,

601
00:34:32,409 --> 00:34:33,854
if it's absolutely white noise.

602
00:34:34,969 --> 00:34:39,310
Some of its noise will be
within the response area,

603
00:34:39,310 --> 00:34:42,239
but some of it will be
in suppression areas.

604
00:34:42,239 --> 00:34:46,409
And these suppression
areas can be

605
00:34:46,409 --> 00:34:48,979
either side of the
excitatory area.

606
00:34:50,050 --> 00:34:51,199
And they can be big.

607
00:34:51,199 --> 00:34:55,349
And in some cases, they can
overrule the excitation,

608
00:34:55,349 --> 00:34:56,349
or at least decrease it.

609
00:34:57,700 --> 00:35:01,740
So some of the reason you
couldn't hear the tone when

610
00:35:01,740 --> 00:35:03,320
it was masked by
the noise is because

611
00:35:03,320 --> 00:35:04,480
of two-tone suppression.

612
00:35:06,360 --> 00:35:08,745
This is sometimes called
suppressive masking.

613
00:35:19,160 --> 00:35:22,280
There's another kind of masking
that's important-- it may

614
00:35:22,280 --> 00:35:28,445
be equally important-- and
it's called adaptive masking.

615
00:35:33,570 --> 00:35:36,523
And it comes from the
process called adaptation.

616
00:35:37,960 --> 00:35:42,720
Almost all sensory
systems have adaptation,

617
00:35:42,720 --> 00:35:45,370
which means that when
you turn a stimulus on,

618
00:35:45,370 --> 00:35:46,895
you get a vigorous response.

619
00:35:47,900 --> 00:35:51,560
And even though the stimulus
stays on, after a while

620
00:35:51,560 --> 00:35:53,810
the response dies
down a little bit.

621
00:35:53,810 --> 00:35:56,085
And that process is
called adaptation.

622
00:35:57,140 --> 00:36:01,220
So I've illustrated the
process of adaptation

623
00:36:01,220 --> 00:36:04,750
for auditory nerve fibers
in this next graph.

624
00:36:06,800 --> 00:36:08,920
Here's that pink
tone burst at 2,000

625
00:36:08,920 --> 00:36:10,170
Hertz we were listening to.

626
00:36:11,180 --> 00:36:14,320
There's the auditory
nerve response to it.

627
00:36:14,320 --> 00:36:17,070
Right as the tone
burst goes on, there's

628
00:36:17,070 --> 00:36:20,020
going to be a vigorous
discharge, which

629
00:36:20,020 --> 00:36:22,870
die eyes down and
becomes a smaller-- still

630
00:36:22,870 --> 00:36:25,390
a discharge, but a
smaller discharge.

631
00:36:25,390 --> 00:36:27,250
And that process is
called adaptation.

632
00:36:29,290 --> 00:36:31,000
Where do you think
that process arises?

633
00:36:33,659 --> 00:36:35,200
OK, while you're
thinking about that,

634
00:36:35,200 --> 00:36:48,240
I'm going to draw a picture
of what's happening here.

635
00:36:48,240 --> 00:36:50,400
So you have the three
rows of outer hair cells.

636
00:36:54,660 --> 00:36:56,180
We have the inner hair cell.

637
00:36:58,040 --> 00:36:59,736
We have the auditory
nerve fiber.

638
00:37:00,870 --> 00:37:02,200
And we're recording here.

639
00:37:04,750 --> 00:37:07,290
And we're saying,
we turn the sound on

640
00:37:07,290 --> 00:37:08,950
and you get a whole
bunch of spikes

641
00:37:08,950 --> 00:37:11,070
from that single
auditory nerve fiber.

642
00:37:11,070 --> 00:37:18,190
But after a few milliseconds
or so, the response dies down.

643
00:37:18,190 --> 00:37:20,630
What experiment could you do?

644
00:37:20,630 --> 00:37:22,750
We don't know where
that process is arising.

645
00:37:22,750 --> 00:37:24,310
We can't explain it.

646
00:37:24,310 --> 00:37:28,970
What experiment would you do
to study where that comes from?

647
00:37:31,774 --> 00:37:32,274
Anybody?

648
00:37:37,010 --> 00:37:37,985
Here's the tone.

649
00:37:43,730 --> 00:37:45,590
Here's the response
from the nerve fiber.

650
00:37:50,592 --> 00:37:52,050
What do we need to
figure out where

651
00:37:52,050 --> 00:37:53,670
that adaptation is taking place?

652
00:37:57,430 --> 00:37:59,540
Well, how about recording
from somewhere else?

653
00:38:03,000 --> 00:38:04,670
What about recording
from the hair cell?

654
00:38:07,920 --> 00:38:12,260
OK, if you do that, the hair
cell doesn't fire spikes,

655
00:38:12,260 --> 00:38:15,125
but it has a receptor potential
in response to the sound.

656
00:38:17,715 --> 00:38:21,445
The receptor goes
on and stays on.

657
00:38:24,170 --> 00:38:27,870
OK, where is adaptation
taking place?

658
00:38:27,870 --> 00:38:32,130
Well, somewhere between the
hair cell and the nerve fiber.

659
00:38:33,300 --> 00:38:36,770
The hair cell doesn't adapt,
the nerve fiber does adapt.

660
00:38:36,770 --> 00:38:40,940
what could explain
adaptation then given that?

661
00:38:42,280 --> 00:38:42,780
Anybody?

662
00:38:46,750 --> 00:38:49,650
OK, what do we have here?

663
00:38:49,650 --> 00:38:53,445
We have a synaptic ribbon with
lots of synaptic vesicles.

664
00:38:57,420 --> 00:38:58,690
The tone goes on.

665
00:38:58,690 --> 00:39:00,560
You have a whole bunch
of synaptic vesicles.

666
00:39:02,760 --> 00:39:06,362
You release them because the
hair cell has depolarized.

667
00:39:06,362 --> 00:39:09,020
You have a burst of
auditory nerve firing.

668
00:39:11,200 --> 00:39:14,500
And you can make new
synaptic vesicles, right?

669
00:39:14,500 --> 00:39:16,690
Yeah, but it takes time.

670
00:39:17,910 --> 00:39:21,170
OK, so as time goes on,
you've released all of these

671
00:39:21,170 --> 00:39:22,800
or many of them.

672
00:39:22,800 --> 00:39:27,210
And you can make some new ones,
but maybe not quite as fast

673
00:39:27,210 --> 00:39:28,550
as you've released them.

674
00:39:28,550 --> 00:39:32,085
So you deplete your
synaptic vesicles.

675
00:39:33,130 --> 00:39:35,180
The hair cell's
still responding.

676
00:39:35,180 --> 00:39:38,570
There are fewer vesicles and
fewer neurotransmitter released

677
00:39:38,570 --> 00:39:41,275
to the nerve and so the
nerve firing dies out.

678
00:39:43,020 --> 00:39:55,500
So adaptation is often ascribed
to the diminished release

679
00:39:55,500 --> 00:39:56,476
of neurotransmitter.

680
00:40:08,960 --> 00:40:13,560
And so far, that's all
adaptation to a single tone.

681
00:40:13,560 --> 00:40:15,650
How does that explain masking?

682
00:40:15,650 --> 00:40:21,120
Well, if your nerve fiber
responded to the noise--

683
00:40:21,120 --> 00:40:24,430
at the very beginning you
heard that noise come on-- shh.

684
00:40:24,430 --> 00:40:27,400
And then the tone came
on a little bit later.

685
00:40:27,400 --> 00:40:29,530
If the tone is high
in level, sure, it's

686
00:40:29,530 --> 00:40:32,900
going to still have some
synaptic vesicles to release.

687
00:40:32,900 --> 00:40:37,840
But if the tone is very
soft, the hair cell

688
00:40:37,840 --> 00:40:41,760
is not going to be able to
release synaptic vesicles

689
00:40:41,760 --> 00:40:43,426
because they've
already been released

690
00:40:43,426 --> 00:40:44,675
at the beginning of the noise.

691
00:40:47,700 --> 00:40:51,900
So adaptive masking is where
you have, to a certain extent,

692
00:40:51,900 --> 00:40:54,086
run out of hair cell
neurotransmitter.

693
00:40:55,390 --> 00:40:58,490
And there's none, or much
less, left to release.

694
00:41:00,870 --> 00:41:05,420
Clearly, adaptive masking
means that this fiber is also

695
00:41:05,420 --> 00:41:09,150
responding to the mask or
the noise in this case.

696
00:41:09,150 --> 00:41:11,840
So in adaptive masking
you have to have

697
00:41:11,840 --> 00:41:13,660
a stimulus that excites.

698
00:41:13,660 --> 00:41:18,673
The masker has to excite the
nerve fiber and the hair cell.

699
00:41:18,673 --> 00:41:21,964
And the probe-- probe
is always exciting it.

700
00:41:23,860 --> 00:41:27,110
So that is a second
explanation for masking.

701
00:41:27,110 --> 00:41:30,130
Now in this case, the
olivocochlear system

702
00:41:30,130 --> 00:41:34,020
can actually help you
with adaptive masking.

703
00:41:35,630 --> 00:41:37,220
How can it help you?

704
00:41:37,220 --> 00:41:40,370
Well, in this
case, if you also--

705
00:41:40,370 --> 00:41:42,660
when you start to
hear this noise,

706
00:41:42,660 --> 00:41:45,905
you call that olivocochlear
system into play.

707
00:41:47,100 --> 00:41:48,260
And it acts.

708
00:41:48,260 --> 00:41:50,410
What does it act to do?

709
00:41:50,410 --> 00:41:52,695
It decreases the firing
of the auditory nerve.

710
00:41:55,030 --> 00:41:57,330
Then when you have
the tone come along,

711
00:41:57,330 --> 00:41:59,130
you have plenty of
neurotransmitter

712
00:41:59,130 --> 00:42:02,870
left in the hair cell because
you haven't released it all.

713
00:42:02,870 --> 00:42:06,130
And you have at least
some to be released

714
00:42:06,130 --> 00:42:08,670
in response to the tone.

715
00:42:08,670 --> 00:42:11,400
So in this case,
the third function

716
00:42:11,400 --> 00:42:14,400
then of the
olivocochlear system is

717
00:42:14,400 --> 00:42:16,850
to reduce the
effects of masking.

718
00:42:16,850 --> 00:42:19,510
And we should have
said back here

719
00:42:19,510 --> 00:42:24,090
when I listed the functions,
reduce the effects of noise

720
00:42:24,090 --> 00:42:27,220
masking, especially
adaptive noise masking.

721
00:42:27,220 --> 00:42:34,120
The kind where the masker
excites the fiber unlike

722
00:42:34,120 --> 00:42:37,790
suppressive masking where the
masker reduces the response.

723
00:42:43,320 --> 00:42:48,370
There are certainly important
and reliable studies

724
00:42:48,370 --> 00:42:51,650
where animals in which the
olivocochlear bundle has

725
00:42:51,650 --> 00:42:55,810
been cut have much more of a
problem with noise masking.

726
00:42:55,810 --> 00:43:01,910
They cannot detect signals that
are buried in a noise masker

727
00:43:01,910 --> 00:43:05,030
as well as animals with an
intact olivocochlear bundle.

728
00:43:06,220 --> 00:43:08,035
So clearly, that is
a viable function.

729
00:43:10,400 --> 00:43:15,670
Now finally, and this is pretty
important for our course here,

730
00:43:15,670 --> 00:43:19,400
there's been this fourth idea
of what the olivocochlear

731
00:43:19,400 --> 00:43:21,680
reflex could do.

732
00:43:21,680 --> 00:43:25,100
And it's always been a
little bit wishy-washy

733
00:43:25,100 --> 00:43:28,720
because there hasn't been really
good experimental evidence.

734
00:43:28,720 --> 00:43:32,010
Until this paper-- this
paper Delano et al,

735
00:43:32,010 --> 00:43:37,215
2007-- is the paper that we
have listed for reading today.

736
00:43:38,920 --> 00:43:43,720
And this paper clearly
shows that another function

737
00:43:43,720 --> 00:43:47,920
of the olivocochlear system is
when you're paying attention

738
00:43:47,920 --> 00:43:49,950
to something that
is not auditory,

739
00:43:49,950 --> 00:43:55,560
like a visual stimulus,
the olivocochlear system

740
00:43:55,560 --> 00:43:58,570
acts and reduces your
sense of hearing.

741
00:43:58,570 --> 00:44:03,170
You're not using hearing
for whatever task,

742
00:44:03,170 --> 00:44:08,090
so you desensitize your
hearing and you pay attention

743
00:44:08,090 --> 00:44:09,330
to the visual system.

744
00:44:09,330 --> 00:44:12,860
OK, so how is this
going to work?

745
00:44:12,860 --> 00:44:16,820
So this paper
trained chinchillas

746
00:44:16,820 --> 00:44:18,050
as the experimental animal.

747
00:44:19,860 --> 00:44:23,160
And the task was to pay
attention to lights.

748
00:44:23,160 --> 00:44:25,010
So let me get my
pointer here so I

749
00:44:25,010 --> 00:44:27,230
can point out a little better.

750
00:44:32,730 --> 00:44:35,890
So this is the task
in part A here.

751
00:44:35,890 --> 00:44:41,090
And this neutral cue is a
light that's straight ahead

752
00:44:41,090 --> 00:44:42,720
from the experimental animal.

753
00:44:42,720 --> 00:44:44,820
And that goes on and
says to the animal

754
00:44:44,820 --> 00:44:46,905
that the trial is starting.

755
00:44:48,010 --> 00:44:53,600
Then, that goes off and
one of two targets appears.

756
00:44:53,600 --> 00:44:56,950
Either a left target
or a right target.

757
00:44:56,950 --> 00:44:59,610
And these are little spots
of light to the animal's left

758
00:44:59,610 --> 00:45:00,450
or to the right.

759
00:45:01,910 --> 00:45:06,380
The task then is if the
left target went on,

760
00:45:06,380 --> 00:45:10,100
the animal goes and
presses the left lever.

761
00:45:10,100 --> 00:45:12,980
And if it does that correctly,
it gets a food reward.

762
00:45:14,430 --> 00:45:16,825
If the light went
on on the right,

763
00:45:16,825 --> 00:45:18,200
the right target
was illuminated,

764
00:45:18,200 --> 00:45:20,200
the right lever is
supposed to be pressed.

765
00:45:20,200 --> 00:45:23,000
And if the animal
does that correctly,

766
00:45:23,000 --> 00:45:24,670
then it gets a food reward.

767
00:45:24,670 --> 00:45:29,260
If it does nothing or if
it presses the wrong lever,

768
00:45:29,260 --> 00:45:30,990
it's punished by a timeout.

769
00:45:30,990 --> 00:45:32,810
And the animals
are food deprived,

770
00:45:32,810 --> 00:45:35,320
so they're motivated
to do this task.

771
00:45:35,320 --> 00:45:36,590
That's the visual task.

772
00:45:37,690 --> 00:45:41,700
Now, on top of that there's
some auditory stimuli presented.

773
00:45:41,700 --> 00:45:44,210
And they're not
relevant to the task.

774
00:45:44,210 --> 00:45:46,790
They're just ongoing
all the time.

775
00:45:46,790 --> 00:45:54,500
And here then in B are the
target and the neutral cue

776
00:45:54,500 --> 00:45:56,310
lights and the response period.

777
00:45:56,310 --> 00:45:58,850
And the auditory stimulus
going on all the time

778
00:45:58,850 --> 00:46:01,411
is a click or a tone burst.

779
00:46:01,411 --> 00:46:02,910
And it's just going
on all the time.

780
00:46:04,050 --> 00:46:06,640
And they're making
some recordings

781
00:46:06,640 --> 00:46:07,850
from the auditory system.

782
00:46:08,880 --> 00:46:12,130
And in this case, they're
making a recording

783
00:46:12,130 --> 00:46:14,840
from the round window
of the cochlea.

784
00:46:14,840 --> 00:46:18,830
And if you had a microscope,
you could see some little blips

785
00:46:18,830 --> 00:46:20,740
right here in response
to the clicks.

786
00:46:21,970 --> 00:46:23,225
And they're pretty big here.

787
00:46:24,270 --> 00:46:26,280
But as time goes on,
they get smaller.

788
00:46:28,970 --> 00:46:30,490
And that's plotted right here.

789
00:46:30,490 --> 00:46:33,320
The response they're
measuring is called the CAP,

790
00:46:33,320 --> 00:46:35,425
and that's the Compound
Action Potential.

791
00:46:52,410 --> 00:46:55,475
And if you are astute
here, the action potential

792
00:46:55,475 --> 00:46:58,330
is another word for
"spikes" or "impulses."

793
00:46:58,330 --> 00:47:01,190
But they're not measuring
from one single fiber,

794
00:47:01,190 --> 00:47:03,930
they're putting a big
electrode on the round window

795
00:47:03,930 --> 00:47:07,820
of the cochlear and they're
measuring the summed

796
00:47:07,820 --> 00:47:11,650
or compound action potential
from the whole auditory nerve.

797
00:47:11,650 --> 00:47:14,080
It's just a convenient
place to do it.

798
00:47:14,080 --> 00:47:15,740
You can do it in
an awake animal.

799
00:47:15,740 --> 00:47:18,550
You can do it in awake behaving
animal like we have here.

800
00:47:19,580 --> 00:47:23,290
When you turn on a click, almost
all the auditory nerve fibers

801
00:47:23,290 --> 00:47:26,540
fire synchronously and
you get a big response.

802
00:47:26,540 --> 00:47:28,440
Hardly have to do
any averaging at all.

803
00:47:30,100 --> 00:47:33,855
OK, so that's the response,
the compound action potential.

804
00:47:36,120 --> 00:47:41,720
And this graph plots the CAP
amplitude-- how big it is.

805
00:47:42,780 --> 00:47:45,820
Upward on the graph
is a big amplitude

806
00:47:45,820 --> 00:47:47,715
and lower is a
diminished amplitude.

807
00:47:48,730 --> 00:47:50,810
And right here at
zero-- they just

808
00:47:50,810 --> 00:47:53,380
call it zero because
that's sort of the baseline

809
00:47:53,380 --> 00:47:54,735
before the trial even starts.

810
00:47:55,740 --> 00:48:00,899
So in this first bar here
is the neutral OK cue.

811
00:48:00,899 --> 00:48:03,190
That's the thing in the middle
that says to the animal,

812
00:48:03,190 --> 00:48:04,135
the task is starting.

813
00:48:05,640 --> 00:48:08,640
And right away during
that neutral cue,

814
00:48:08,640 --> 00:48:12,980
these black dots show you that
the compound action potential

815
00:48:12,980 --> 00:48:13,695
is decreasing.

816
00:48:15,920 --> 00:48:19,270
Then, the neutral cue goes
off and the target-- one

817
00:48:19,270 --> 00:48:20,270
of the targets goes on.

818
00:48:20,270 --> 00:48:21,860
Either the left or right.

819
00:48:21,860 --> 00:48:25,555
And the compound action
potential further decreases.

820
00:48:27,290 --> 00:48:30,640
Then, the animal makes
its response here

821
00:48:30,640 --> 00:48:31,740
in this dashed line.

822
00:48:33,220 --> 00:48:37,820
OK, gets its food reward
and the CAP comes back up.

823
00:48:37,820 --> 00:48:40,780
OK, now how do we explain this?

824
00:48:45,890 --> 00:48:49,340
Well, the CAP is the
summed firing rate

825
00:48:49,340 --> 00:48:51,260
of many auditory nerve fibers.

826
00:48:52,900 --> 00:48:57,700
Instead of stimulation
of the OC neurons

827
00:48:57,700 --> 00:49:00,550
decreasing the firing rate,
essentially the animal

828
00:49:00,550 --> 00:49:04,670
itself has turned
on its OC neurons

829
00:49:04,670 --> 00:49:06,610
and the firing
rate has gone down.

830
00:49:06,610 --> 00:49:11,755
Or the summed response, in
this case, has gone down.

831
00:49:13,730 --> 00:49:17,400
The animal has said to
the olivocochlear system,

832
00:49:17,400 --> 00:49:19,630
I'm starting a visual task.

833
00:49:19,630 --> 00:49:24,970
I don't want to pay attention to
extraneous things, like sounds.

834
00:49:24,970 --> 00:49:28,360
So I'm going to
decrease my sensitivity

835
00:49:28,360 --> 00:49:31,510
in hearing by activating
these olivocochlear neurons.

836
00:49:31,510 --> 00:49:34,582
Now, pay attention to
the important targets,

837
00:49:34,582 --> 00:49:35,290
which are visual.

838
00:49:40,760 --> 00:49:43,935
Now, there are some other
symbols in here that are open.

839
00:49:43,935 --> 00:49:46,000
They're a little
bit harder to see.

840
00:49:47,740 --> 00:49:49,700
But there are some
CAP amplitudes

841
00:49:49,700 --> 00:49:52,505
from another trial of
these same animals.

842
00:49:53,560 --> 00:49:56,880
And for one reason or
another, the chinchillas

843
00:49:56,880 --> 00:49:58,065
didn't always do the task.

844
00:49:59,900 --> 00:50:03,960
Every now and then they would
just not pay attention to it.

845
00:50:03,960 --> 00:50:05,210
They wouldn't press the lever.

846
00:50:06,570 --> 00:50:08,630
They wouldn't respond at all.

847
00:50:08,630 --> 00:50:11,550
As if they were
sort of spaced out,

848
00:50:11,550 --> 00:50:14,060
thinking about something
else if you will.

849
00:50:14,060 --> 00:50:18,464
And in those cases,
which are timeout trials.

850
00:50:18,464 --> 00:50:19,922
Let's see, what
did they call them?

851
00:50:27,794 --> 00:50:28,670
Ah, omissions.

852
00:50:29,950 --> 00:50:32,420
This little symbol
right here is omissions.

853
00:50:32,420 --> 00:50:34,670
So it's an omission trial
where the animal didn't even

854
00:50:34,670 --> 00:50:35,790
do the task.

855
00:50:35,790 --> 00:50:38,510
In that case, the
olivocochlear system

856
00:50:38,510 --> 00:50:40,630
is apparently not
called into play at all.

857
00:50:40,630 --> 00:50:42,765
The animal is just
not doing the task.

858
00:50:46,020 --> 00:50:50,570
Now, the investigators were
very astute and they said,

859
00:50:50,570 --> 00:50:52,630
we're going to make
the task a little more

860
00:50:52,630 --> 00:50:54,930
difficult for the
animals and see

861
00:50:54,930 --> 00:50:59,450
if we get a bigger effect here.

862
00:50:59,450 --> 00:51:01,740
And the way to make
the task difficult

863
00:51:01,740 --> 00:51:03,465
is make the targets brief.

864
00:51:05,270 --> 00:51:08,720
So instead of the target going
on for a couple of seconds,

865
00:51:08,720 --> 00:51:12,130
the target went on for
just a half a second.

866
00:51:12,130 --> 00:51:14,005
And here's a trial with
just a half a second.

867
00:51:15,160 --> 00:51:19,115
And there was a big decrease
in the auditory response.

868
00:51:22,310 --> 00:51:23,940
If the target went
on for a long time,

869
00:51:23,940 --> 00:51:25,200
there was less of a decrease.

870
00:51:25,200 --> 00:51:29,000
And this is the effect plotted
as a function of target

871
00:51:29,000 --> 00:51:29,750
duration.

872
00:51:29,750 --> 00:51:32,897
Brief targets made
this job harder

873
00:51:32,897 --> 00:51:33,980
to do for the chinchillas.

874
00:51:37,700 --> 00:51:40,290
So this is clearly,
at least to me,

875
00:51:40,290 --> 00:51:46,310
some evidence that you call into
play the olivocochlear system

876
00:51:46,310 --> 00:51:48,380
when you're doing a visual task.

877
00:51:48,380 --> 00:51:52,950
So maybe as humans, we do this
when we're trying to read.

878
00:51:52,950 --> 00:51:56,295
We're concentrating on
the book, visual stimulus.

879
00:51:56,295 --> 00:52:00,020
And our neighbors
music is going on.

880
00:52:00,020 --> 00:52:01,820
It's not relevant
to what we're doing.

881
00:52:01,820 --> 00:52:03,900
Maybe we're listening
to it subconsciously,

882
00:52:03,900 --> 00:52:08,490
but we decrease the response
to that auditory stimulus

883
00:52:08,490 --> 00:52:10,455
because it's not
important to the task.

884
00:52:11,560 --> 00:52:14,970
So clearly then,
this is good evidence

885
00:52:14,970 --> 00:52:21,860
from this single study
for the last function

886
00:52:21,860 --> 00:52:23,850
for the olivocochlear
system, which

887
00:52:23,850 --> 00:52:29,420
is that you reduce your hearing
sensitivity when attending

888
00:52:29,420 --> 00:52:33,350
to visual or perhaps
other modality tasks.

889
00:52:33,350 --> 00:52:35,705
So these are the
four basic functions

890
00:52:35,705 --> 00:52:37,510
then for the
olivocochlear system.

891
00:52:39,380 --> 00:52:42,510
Now, before I leave the
olivocochlear system,

892
00:52:42,510 --> 00:52:47,005
let's do a little review here
on exactly how it's acting.

893
00:52:48,030 --> 00:52:53,370
So we said we're activating
these systems going out

894
00:52:53,370 --> 00:52:55,730
to the hair cells
and we're reducing

895
00:52:55,730 --> 00:52:57,830
the responses of the
auditory nerve fibers.

896
00:52:59,460 --> 00:53:04,500
Something I haven't told you
but was observed maybe 20

897
00:53:04,500 --> 00:53:07,030
years after the phenomenon
was first discovered

898
00:53:07,030 --> 00:53:11,190
is the idea of which
of these systems

899
00:53:11,190 --> 00:53:13,470
is being used here
when we electrically

900
00:53:13,470 --> 00:53:17,090
stimulate the system
and activated it.

901
00:53:17,090 --> 00:53:20,770
It turns out that the
medial olivocochlear neurons

902
00:53:20,770 --> 00:53:23,680
have big, fat myelinated axons.

903
00:53:23,680 --> 00:53:25,930
And you can maybe appreciate
that from this drawing

904
00:53:25,930 --> 00:53:27,300
that these are very thick lines.

905
00:53:28,710 --> 00:53:32,590
The lateral olivocochlear
neurons have very thin axons.

906
00:53:34,020 --> 00:53:38,310
And one effect of that
besides how fast they conduct

907
00:53:38,310 --> 00:53:42,050
impulses is when you stimulate,
for example, at this position

908
00:53:42,050 --> 00:53:46,780
here in the brainstem, it takes
a huge amount of stimulating

909
00:53:46,780 --> 00:53:50,130
current to activate
very thin nerve

910
00:53:50,130 --> 00:53:52,900
fibers like the lateral
olivocochlear neurons.

911
00:53:52,900 --> 00:53:55,510
If you apply a huge
stimulating current here,

912
00:53:55,510 --> 00:53:57,950
the current spreads
to everywhere,

913
00:53:57,950 --> 00:54:00,190
including the
facial nerve, which

914
00:54:00,190 --> 00:54:03,730
causes the experimental
animal to twitch.

915
00:54:03,730 --> 00:54:05,400
So that's almost never done.

916
00:54:05,400 --> 00:54:08,110
More moderate levels
of stimulation current

917
00:54:08,110 --> 00:54:09,590
are applied here.

918
00:54:09,590 --> 00:54:13,290
And at those levels, the
predominant system activating

919
00:54:13,290 --> 00:54:16,280
is the medial
olivocochlear neurons.

920
00:54:16,280 --> 00:54:18,680
Those MOC neurons
send their fibers out

921
00:54:18,680 --> 00:54:19,875
to the outer hair cells.

922
00:54:21,140 --> 00:54:23,680
And we say when they
act on the outer hair

923
00:54:23,680 --> 00:54:26,900
cells, the responses of
the nerve fibers diminish.

924
00:54:29,290 --> 00:54:33,790
As we review, we should be
able to account for that.

925
00:54:48,120 --> 00:54:50,470
OK, we are stimulating
nerve fibers

926
00:54:50,470 --> 00:54:55,285
that go out to the
outer hair cells here.

927
00:55:07,830 --> 00:55:09,720
And we're recording
the responses

928
00:55:09,720 --> 00:55:14,176
from the auditory nerve
fibers right here.

929
00:55:14,176 --> 00:55:17,290
Of course, these
olivocochlear neurons

930
00:55:17,290 --> 00:55:20,260
are sending messages
out to the periphery.

931
00:55:20,260 --> 00:55:23,480
And the auditory nerve
fibers are sending messages

932
00:55:23,480 --> 00:55:24,310
into the brain.

933
00:55:28,440 --> 00:55:31,780
Somebody explain to me
what's happening here.

934
00:55:31,780 --> 00:55:37,590
What are we doing to the cochlea
to cause these responses to go

935
00:55:37,590 --> 00:55:40,435
down when we apply
this simulation?

936
00:55:41,830 --> 00:55:43,040
Anybody?

937
00:55:43,040 --> 00:55:45,200
We're affecting the
outer hair cells, right?

938
00:55:45,200 --> 00:55:46,745
What are the outer hair cells?

939
00:55:48,910 --> 00:55:50,510
The cochlea amplifier, right?

940
00:56:00,230 --> 00:56:03,490
It turns out that
releasing the acetylcholine

941
00:56:03,490 --> 00:56:06,184
on to the outer
hair cells decreases

942
00:56:06,184 --> 00:56:07,100
their electromotility.

943
00:56:09,110 --> 00:56:15,470
So in effect, it discusses the
gain of the cochlear amplifier.

944
00:56:18,220 --> 00:56:22,870
If the cochlea is
less amplified,

945
00:56:22,870 --> 00:56:26,080
the inner hair cell stereocilia
are going to be bent less.

946
00:56:27,230 --> 00:56:29,670
They're going to release
less neurotransmitter

947
00:56:29,670 --> 00:56:32,564
to the associated
auditory nerve fibers.

948
00:56:32,564 --> 00:56:34,480
And we're going to measure
less of a response.

949
00:56:36,030 --> 00:56:39,570
That is how the
olivocochlear neurons--

950
00:56:39,570 --> 00:56:42,110
those that go to the
outer hair cells--

951
00:56:42,110 --> 00:56:45,220
affect the responses of
the auditory nerve fiber.

952
00:56:45,220 --> 00:56:51,870
So this decrease
in firing rate then

953
00:56:51,870 --> 00:56:54,630
is a manifestation
of turning down

954
00:56:54,630 --> 00:56:56,270
the gain of the
cochlear amplifier.

955
00:56:58,320 --> 00:57:01,600
It was quite a mystery when
this innervation was first

956
00:57:01,600 --> 00:57:04,220
worked out before the
outer hair cells really

957
00:57:04,220 --> 00:57:05,670
were known to be the amplifier.

958
00:57:06,820 --> 00:57:11,060
How could stimulation of fibers
going to the outer hair cells

959
00:57:11,060 --> 00:57:13,760
give you decreases in
responses of fibers

960
00:57:13,760 --> 00:57:15,820
coming from the
inner hair cells?

961
00:57:15,820 --> 00:57:19,300
And now that the outer
hair cells are clearly

962
00:57:19,300 --> 00:57:21,790
associated with the cochlear
amplifier, it's clear.

963
00:57:23,240 --> 00:57:28,500
Now, what was the
loss of sensitivity

964
00:57:28,500 --> 00:57:30,940
when the outer hair cells
were completely gone

965
00:57:30,940 --> 00:57:35,450
or when their prestin
was knocked out?

966
00:57:35,450 --> 00:57:35,988
Anybody?

967
00:57:35,988 --> 00:57:37,363
What was the loss
of sensitivity?

968
00:57:39,240 --> 00:57:41,970
40 to 60 dB, right?

969
00:57:41,970 --> 00:57:46,120
So if you eliminated
the cochlear amplifier,

970
00:57:46,120 --> 00:57:49,685
we had a loss of 40 to 60 dB.

971
00:57:57,010 --> 00:57:59,790
What kind of effect do
we have from stimulation

972
00:57:59,790 --> 00:58:02,180
of the olivocochlear neurons?

973
00:58:02,180 --> 00:58:08,630
Well, the length of this arrow
when brought down to the x-axis

974
00:58:08,630 --> 00:58:12,100
shows you how much
sensitivity is lost.

975
00:58:13,730 --> 00:58:17,560
You could overcome
this by dialing

976
00:58:17,560 --> 00:58:22,460
in an increase of sound pressure
level to make up for it.

977
00:58:22,460 --> 00:58:25,720
You could titrate the effect
of decreasing the gain

978
00:58:25,720 --> 00:58:27,480
by increasing the
sound stimulus.

979
00:58:28,660 --> 00:58:31,480
And the width of that arrow,
the length of that arrow,

980
00:58:31,480 --> 00:58:33,890
is about 25 dB.

981
00:58:38,670 --> 00:58:42,370
So instead of eliminating
the cochlear amplifier,

982
00:58:42,370 --> 00:58:46,511
you just reduce its gain by the
action of these olivocochlear

983
00:58:46,511 --> 00:58:47,010
neurons.

984
00:58:47,010 --> 00:58:49,020
The effect is about 25 dB.

985
00:58:49,020 --> 00:58:54,450
Which is a fairly strong effect,
but you're not completely

986
00:58:54,450 --> 00:58:57,000
ridding the cochlea of
the cochlear amplifier.

987
00:58:57,000 --> 00:58:58,550
You're decreasing its gain.

988
00:59:00,430 --> 00:59:06,680
OK, so that's kind of a review
on outer hair cell function

989
00:59:06,680 --> 00:59:09,800
and how these olivocochlear
neurons affect the outer hair

990
00:59:09,800 --> 00:59:10,300
cells.

991
00:59:12,420 --> 00:59:16,080
So you really have a
controllable cochlear amplifier

992
00:59:16,080 --> 00:59:19,020
controlled by these
olivocochlear neurons,

993
00:59:19,020 --> 00:59:21,220
or efferent neurons,
or descending neurons

994
00:59:21,220 --> 00:59:24,730
coming from the brain
out to the cochlea.

995
00:59:24,730 --> 00:59:26,182
Any questions about that?

996
00:59:28,971 --> 00:59:29,470
OK.

997
00:59:29,470 --> 00:59:34,470
If there aren't, then I'll go
on to another reflex pathway.

998
00:59:41,120 --> 00:59:44,493
Maybe I actually do have one
more thing to talk about.

999
00:59:48,025 --> 00:59:48,525
Right.

1000
00:59:50,130 --> 00:59:54,710
Let me just mention this
idea that this reflex pathway

1001
00:59:54,710 --> 00:59:57,430
is kind of getting teased
out by current studies.

1002
00:59:58,680 --> 01:00:03,580
And it can be a little
bit complicated,

1003
01:00:03,580 --> 01:00:05,870
so let me show you
what people are working

1004
01:00:05,870 --> 01:00:09,750
on now in the
olivocochlear reflex.

1005
01:00:09,750 --> 01:00:11,830
Here, I think I've
flipped the slide on you

1006
01:00:11,830 --> 01:00:16,220
so that this slide shows
the MOC neurons going

1007
01:00:16,220 --> 01:00:17,940
to the left cochlea.

1008
01:00:17,940 --> 01:00:18,650
OK.

1009
01:00:18,650 --> 01:00:20,250
That's the ipsilateral cochlea.

1010
01:00:22,770 --> 01:00:25,960
How do these MOC neurons
get their inputs?

1011
01:00:25,960 --> 01:00:27,230
Where do they come from?

1012
01:00:29,680 --> 01:00:30,390
OK.

1013
01:00:30,390 --> 01:00:36,300
Well, the red and blue are
the olivocochlear neurons

1014
01:00:36,300 --> 01:00:39,105
coming out to the cochlea and
the purple are their inputs.

1015
01:00:41,910 --> 01:00:45,800
And all of the inputs have to,
of course, use the cochlea.

1016
01:00:45,800 --> 01:00:50,300
To get this olivocochlear
neurons to fire in response

1017
01:00:50,300 --> 01:00:53,320
to sound, you have to
activate the cochlea.

1018
01:00:53,320 --> 01:00:55,490
You activate the auditory nerve.

1019
01:00:55,490 --> 01:00:59,030
The auditory nerve goes into
the cochlea nucleus then.

1020
01:00:59,030 --> 01:01:02,250
And that's where the
limits of our understanding

1021
01:01:02,250 --> 01:01:05,990
are sort of-- we are
approaching the limits

1022
01:01:05,990 --> 01:01:07,250
of our understanding.

1023
01:01:07,250 --> 01:01:11,750
What are the cochlear
nucleus neurons

1024
01:01:11,750 --> 01:01:15,720
that are the so-called MOC
reflex interneurons that

1025
01:01:15,720 --> 01:01:17,570
are drawn in purple here?

1026
01:01:17,570 --> 01:01:19,610
When we talked about
the cochlear nucleus,

1027
01:01:19,610 --> 01:01:23,140
we identified a bunch of
different types of neurons.

1028
01:01:23,140 --> 01:01:27,170
We had spherical
cells, globular cells.

1029
01:01:27,170 --> 01:01:29,110
Both of those are
known as bushy cells.

1030
01:01:29,110 --> 01:01:30,515
We had stellate cells.

1031
01:01:31,860 --> 01:01:33,710
We had pyramidal cells.

1032
01:01:33,710 --> 01:01:35,640
We had octopus cells.

1033
01:01:35,640 --> 01:01:42,080
Which of those provide the
inputs for the MOC neurons?

1034
01:01:44,300 --> 01:01:46,580
And that's assuming
a direct pathway,

1035
01:01:46,580 --> 01:01:49,530
which I'll give you from some
other experimental evidence

1036
01:01:49,530 --> 01:01:50,740
which I won't show you.

1037
01:01:53,060 --> 01:01:55,570
At least one group
of cochlear nucleus

1038
01:01:55,570 --> 01:01:58,210
neurons projects
to the MOC neurons.

1039
01:01:58,210 --> 01:01:59,610
And how do I know that?

1040
01:01:59,610 --> 01:02:03,610
Well, you can record
from the MOC neurons

1041
01:02:03,610 --> 01:02:05,680
and turn a sound on.

1042
01:02:05,680 --> 01:02:08,590
And in less than 5 milliseconds,
you can get a response.

1043
01:02:10,660 --> 01:02:13,680
So there isn't enough time
for the reflex pathway

1044
01:02:13,680 --> 01:02:16,620
to go up to the
auditory cortex, which

1045
01:02:16,620 --> 01:02:18,680
takes about 4 or 5 synapses.

1046
01:02:18,680 --> 01:02:21,760
And to go around there
and start coming down,

1047
01:02:21,760 --> 01:02:24,640
that takes 10 or
20 milliseconds.

1048
01:02:24,640 --> 01:02:26,305
You get a response
here very quickly.

1049
01:02:27,440 --> 01:02:31,570
And so there isn't enough time,
except for cochlear nucleus

1050
01:02:31,570 --> 01:02:32,860
to project directly there.

1051
01:02:35,510 --> 01:02:39,930
So which of those cochlear
nucleus neurons do project?

1052
01:02:39,930 --> 01:02:41,855
Well, we don't know
absolutely for sure.

1053
01:02:43,400 --> 01:02:47,420
But it looks like
from lesion studies

1054
01:02:47,420 --> 01:02:50,960
that probably the stellate
cells in the cochlear nucleus.

1055
01:03:00,170 --> 01:03:01,990
And especially
the stellate cells

1056
01:03:01,990 --> 01:03:06,780
in the part of the cochlear
nucleus called the PVCN.

1057
01:03:06,780 --> 01:03:10,940
And I think we talked
about the ventral cochlear

1058
01:03:10,940 --> 01:03:13,520
nucleus and the dorsal
cochlear nucleus

1059
01:03:13,520 --> 01:03:16,300
as being the big main divisions.

1060
01:03:16,300 --> 01:03:20,470
In the VCN, there's two
subdivisions, AVCN and PVCN.

1061
01:03:22,430 --> 01:03:26,030
And the stellate cells
that seem to be-- the MOC

1062
01:03:26,030 --> 01:03:28,900
reflex interneurons
are in the PVCN.

1063
01:03:28,900 --> 01:03:29,820
How do we know that?

1064
01:03:30,950 --> 01:03:34,130
You can make lesion studies in
various parts of the cochlear

1065
01:03:34,130 --> 01:03:34,700
nucleus.

1066
01:03:34,700 --> 01:03:39,610
If you lesion the DCN, this
reflex goes along fine.

1067
01:03:39,610 --> 01:03:42,330
If you lesion the AVCN,
it goes along fine.

1068
01:03:44,090 --> 01:03:46,267
In the PVCN, if you
make a lesion there,

1069
01:03:46,267 --> 01:03:47,350
the reflex is interrupted.

1070
01:03:51,570 --> 01:03:55,540
In the PVCN, there are a
number of types of neurons.

1071
01:03:55,540 --> 01:03:59,160
The ones that seem to have the
right characteristics in terms

1072
01:03:59,160 --> 01:04:03,750
of latency, sustainability
of response,

1073
01:04:03,750 --> 01:04:06,720
tuning, are the stellate cells.

1074
01:04:06,720 --> 01:04:09,080
So it's a little bit
of a squishy argument.

1075
01:04:09,080 --> 01:04:12,980
Stellate cells seem to
project to the right way.

1076
01:04:12,980 --> 01:04:16,200
So most of the evidence
is behind the idea

1077
01:04:16,200 --> 01:04:19,040
that those particular
cochlear nucleus

1078
01:04:19,040 --> 01:04:22,760
cells are the ones that are
the reflex interneurons.

1079
01:04:23,850 --> 01:04:29,300
Now, another point of this slide
is that the MOC reflex pathway

1080
01:04:29,300 --> 01:04:31,740
is consensually organize.

1081
01:04:31,740 --> 01:04:33,680
So what does consensual mean?

1082
01:04:35,640 --> 01:04:38,790
What does consensual in
terms of reflex mean?

1083
01:04:38,790 --> 01:04:41,020
Not in terms of
consensual sex, in terms

1084
01:04:41,020 --> 01:04:43,930
of reflexes or
brainstem organization.

1085
01:04:45,160 --> 01:04:49,270
How does the detective in the
old 1950s black and white movie

1086
01:04:49,270 --> 01:04:51,440
come to the victim
whose lying down

1087
01:04:51,440 --> 01:04:54,300
and shine a flashlight
in the eye-- how does he

1088
01:04:54,300 --> 01:04:56,130
decide that the
person is really dead?

1089
01:05:00,700 --> 01:05:04,570
The pupil constricts if the
person's still alive, right?

1090
01:05:04,570 --> 01:05:06,270
The brainstem is still working.

1091
01:05:06,270 --> 01:05:07,860
It's a very quick test.

1092
01:05:07,860 --> 01:05:09,450
You don't have to
have a stethoscope

1093
01:05:09,450 --> 01:05:10,616
and listen to the heartbeat.

1094
01:05:11,840 --> 01:05:14,370
You can be brain dead and
still have a heartbeat.

1095
01:05:14,370 --> 01:05:17,250
But if you shine light
in a person's eye,

1096
01:05:17,250 --> 01:05:19,030
the pupil constricts, right?

1097
01:05:19,030 --> 01:05:21,014
So that is a brainstem reflects.

1098
01:05:21,014 --> 01:05:22,430
The pupillary
constriction reflex.

1099
01:05:22,430 --> 01:05:25,640
It turns out if you shine
light in the right eye, what

1100
01:05:25,640 --> 01:05:27,040
happens to your left pupil?

1101
01:05:28,440 --> 01:05:30,150
It constricts, right.

1102
01:05:30,150 --> 01:05:34,610
So both pupils constrict from
just a stimulus in one eye.

1103
01:05:34,610 --> 01:05:36,620
And that is called consensual.

1104
01:05:36,620 --> 01:05:38,300
It means in agreement.

1105
01:05:38,300 --> 01:05:41,620
The left and the right side do
the same thing in agreement.

1106
01:05:43,910 --> 01:05:47,240
So most of these
brainstem reflexes

1107
01:05:47,240 --> 01:05:48,890
are consensually organized.

1108
01:05:48,890 --> 01:05:51,360
And it turns out
that you can look

1109
01:05:51,360 --> 01:05:53,942
at the MOC reflex
in your left ear.

1110
01:05:53,942 --> 01:05:55,650
And obviously, if you
put sound in there,

1111
01:05:55,650 --> 01:05:57,985
that reflex is
going to take place.

1112
01:05:57,985 --> 01:06:00,210
But you can also
elicit that reflex

1113
01:06:00,210 --> 01:06:01,560
by sound in the right ear.

1114
01:06:03,530 --> 01:06:08,990
For some reason, sound in either
ear can activate the reflex.

1115
01:06:08,990 --> 01:06:13,880
Not quite as well,
labeling studies

1116
01:06:13,880 --> 01:06:18,040
have shown that just
some of the MOC neurons

1117
01:06:18,040 --> 01:06:22,530
are responsive to sound in
the ipsilateral cochlea.

1118
01:06:22,530 --> 01:06:25,130
That is, the cochlea
that they project to.

1119
01:06:27,280 --> 01:06:31,590
And about half as many
are responsive to sound

1120
01:06:31,590 --> 01:06:32,860
in the other ear.

1121
01:06:32,860 --> 01:06:36,020
The so-called
contralateral cochlea

1122
01:06:36,020 --> 01:06:39,010
and the so-called
contra response,

1123
01:06:39,010 --> 01:06:40,810
MOC neurons in red here.

1124
01:06:43,840 --> 01:06:46,120
Labeling studies have
shown these contra

1125
01:06:46,120 --> 01:06:49,600
response neurons are sitting
on the side of the brain

1126
01:06:49,600 --> 01:06:52,160
as the cochlea that
they innervate.

1127
01:06:52,160 --> 01:06:54,950
But the ipsi
response neurons are

1128
01:06:54,950 --> 01:06:58,090
located on the opposite side
of the brain [INAUDIBLE].

1129
01:07:00,330 --> 01:07:04,560
And knowing those
response characteristics,

1130
01:07:04,560 --> 01:07:07,440
then you can draw
the purple pathway

1131
01:07:07,440 --> 01:07:10,000
that's necessary to drive them.

1132
01:07:10,000 --> 01:07:14,140
These contra response neurons
respond to sound over here,

1133
01:07:14,140 --> 01:07:15,925
so they must get
this purple arrow.

1134
01:07:19,180 --> 01:07:24,070
These ipsi response neurons
are responsive to sound here,

1135
01:07:24,070 --> 01:07:25,970
so they must get
this purple pathway.

1136
01:07:27,132 --> 01:07:33,100
And it's kind of complicated,
but that's the kind of studies

1137
01:07:33,100 --> 01:07:36,460
that people do now on
these brainstem reflexes.

1138
01:07:38,060 --> 01:07:40,020
And I want to just
say one more thing.

1139
01:07:41,300 --> 01:07:45,345
People are now working on where
the neurons go in the cochlea.

1140
01:07:47,030 --> 01:07:52,230
So these are some tuning
curves from these MOC neurons.

1141
01:07:52,230 --> 01:07:55,290
So they show sharp tuning
just like the auditory nerve.

1142
01:07:57,400 --> 01:07:59,380
And where do they go?

1143
01:07:59,380 --> 01:08:05,130
Well, they have a very nice
so-called tonotopic projection

1144
01:08:05,130 --> 01:08:06,740
into the cochlea.

1145
01:08:06,740 --> 01:08:12,840
That is, an MOC
neuron with a high CF

1146
01:08:12,840 --> 01:08:16,365
goes and innervates at a
very basal cochlear location.

1147
01:08:18,330 --> 01:08:18,830
and.

1148
01:08:18,830 --> 01:08:23,479
An MOC neuron with a low CF
goes and innervates closer

1149
01:08:23,479 --> 01:08:24,195
to the apex.

1150
01:08:27,510 --> 01:08:30,550
And this black line
shows the mapping

1151
01:08:30,550 --> 01:08:32,524
for auditory nerve fibers.

1152
01:08:32,524 --> 01:08:34,399
Those are the ones coming
out of the cochlea.

1153
01:08:35,700 --> 01:08:38,149
And the colored dots
show the mapping

1154
01:08:38,149 --> 01:08:40,450
for the fibers going
back on to the cochlea.

1155
01:08:44,396 --> 01:08:46,229
Everything, remember,
in the auditory system

1156
01:08:46,229 --> 01:08:48,060
is tonotopically organized.

1157
01:08:48,060 --> 01:08:52,560
So it's no surprise
that that would happen.

1158
01:08:52,560 --> 01:08:56,790
But it opens up the idea
maybe that if you're

1159
01:08:56,790 --> 01:09:01,160
interested in turning off
your sense of hearing just

1160
01:09:01,160 --> 01:09:06,880
for low frequencies, you
could activate these fibers,

1161
01:09:06,880 --> 01:09:10,560
go out to the cochlea
in just the apical part,

1162
01:09:10,560 --> 01:09:12,200
and leave the basal part.

1163
01:09:12,200 --> 01:09:14,109
Let's say you're very
interested in listening

1164
01:09:14,109 --> 01:09:16,250
to high-frequency sounds.

1165
01:09:16,250 --> 01:09:18,000
Leave the basal part intact.

1166
01:09:18,000 --> 01:09:19,069
Or vice-versa.

1167
01:09:19,069 --> 01:09:22,640
You could control the
cochlea in a frequency band

1168
01:09:22,640 --> 01:09:23,910
by frequency band manner.

1169
01:09:25,170 --> 01:09:28,700
The anatomical
substrate there is

1170
01:09:28,700 --> 01:09:30,939
laid for that type
of manipulation.

1171
01:09:30,939 --> 01:09:33,456
You don't have to shut down
the whole sense of hearing.

1172
01:09:38,000 --> 01:09:43,689
OK, so let's move on
to the second system

1173
01:09:43,689 --> 01:09:44,830
that I want to talk about.

1174
01:09:44,830 --> 01:09:50,660
That is, the brainstem reflexes
associated with the stapedius

1175
01:09:50,660 --> 01:09:52,830
and tenser tympani
middle-ear muscles.

1176
01:09:57,380 --> 01:10:00,280
These are two muscles that
are in the middle ear.

1177
01:10:00,280 --> 01:10:03,040
Remember the middle ear was
the part of the ear that

1178
01:10:03,040 --> 01:10:06,915
starts at the eardrum
and ends at the cochlea?

1179
01:10:08,506 --> 01:10:09,880
There are two
middle ear muscles.

1180
01:10:09,880 --> 01:10:11,160
There's a tensor tympani.

1181
01:10:11,160 --> 01:10:12,890
This is shaded in brown here.

1182
01:10:14,180 --> 01:10:18,380
And there's the stapedius
shaded in red here.

1183
01:10:18,380 --> 01:10:20,810
The stapedius tugs
on the stapes.

1184
01:10:23,150 --> 01:10:26,650
And the tensor tympani
tugs on the malleus.

1185
01:10:26,650 --> 01:10:29,805
But it gets its name
because when it contracts,

1186
01:10:29,805 --> 01:10:32,380
it looks like-- if you're
looking at the eardrum

1187
01:10:32,380 --> 01:10:35,670
through an otoscope, it looks
like the tympanic membrane gets

1188
01:10:35,670 --> 01:10:36,295
really tense.

1189
01:10:36,295 --> 01:10:39,300
It looks a little flaccid
before it contracts,

1190
01:10:39,300 --> 01:10:40,930
and then it gets real tense.

1191
01:10:40,930 --> 01:10:42,360
So it's a tensor tympani.

1192
01:10:45,030 --> 01:10:48,163
And what happens when
those muscles contract?

1193
01:10:49,410 --> 01:10:51,840
That's shown in this next slide.

1194
01:10:51,840 --> 01:10:55,340
They reduce the
sound transmission

1195
01:10:55,340 --> 01:10:56,690
through the middle ear.

1196
01:10:56,690 --> 01:11:01,700
This is just simply a
graph on contraction

1197
01:11:01,700 --> 01:11:05,290
of the stapedius muscle.

1198
01:11:07,175 --> 01:11:08,466
This is during the contraction.

1199
01:11:09,790 --> 01:11:11,430
This is the magnitude change.

1200
01:11:11,430 --> 01:11:17,760
So it looks like sound doesn't
get through the middle ear

1201
01:11:17,760 --> 01:11:20,200
anywhere near as
well as normally.

1202
01:11:20,200 --> 01:11:23,545
And you have about a 25 dB
decrease in sensitivity.

1203
01:11:25,150 --> 01:11:30,630
Simply what that means is
that usually the eardrum moves

1204
01:11:30,630 --> 01:11:33,680
and these bones move and they
convey efficiently the sound

1205
01:11:33,680 --> 01:11:35,210
into the inner ear.

1206
01:11:35,210 --> 01:11:39,610
But if you tug on the
muscles and contract them,

1207
01:11:39,610 --> 01:11:42,260
then the bones don't
vibrate as easily.

1208
01:11:42,260 --> 01:11:45,730
And the sound doesn't
get into the inner ear.

1209
01:11:45,730 --> 01:11:48,800
So this is again, like
the olivocochlear system,

1210
01:11:48,800 --> 01:11:52,510
an inhibitory system in
that when it's activated

1211
01:11:52,510 --> 01:11:55,327
it reduces your
sensitivity of hearing.

1212
01:12:01,670 --> 01:12:04,000
Now, how are these
muscles controlled?

1213
01:12:04,000 --> 01:12:06,630
Well, they're controlled by
neurons coming from the brain.

1214
01:12:07,810 --> 01:12:10,950
But in the case of
muscles, of course,

1215
01:12:10,950 --> 01:12:13,110
they're always on the
same side of the brain

1216
01:12:13,110 --> 01:12:14,360
as the muscles they innervate.

1217
01:12:14,360 --> 01:12:18,880
So the left stapedius muscle
is controlled by motor neurons

1218
01:12:18,880 --> 01:12:20,300
on the left side of your brain.

1219
01:12:21,400 --> 01:12:25,490
And the stapedius
muscle motor neurons

1220
01:12:25,490 --> 01:12:27,870
run in the seventh
cranial nerve.

1221
01:12:33,050 --> 01:12:34,650
And what's the
seventh cranial nerve?

1222
01:12:34,650 --> 01:12:35,150
Anybody?

1223
01:12:39,010 --> 01:12:47,190
We have the eighth cranial nerve
is auditory and vestibular.

1224
01:12:51,410 --> 01:12:52,975
What's the seventh?

1225
01:12:56,620 --> 01:12:57,292
Anybody?

1226
01:12:57,292 --> 01:12:58,000
AUDIENCE: Facial.

1227
01:12:58,000 --> 01:12:59,041
PROFESSOR: Facial, right.

1228
01:13:04,750 --> 01:13:13,400
So the stapedius axons
go in that cranial nerve.

1229
01:13:15,010 --> 01:13:18,060
The stapedius motor
neuron axons, should say.

1230
01:13:20,060 --> 01:13:28,760
And for the tensor tympani,
it's the fifth cranial nerve.

1231
01:13:35,564 --> 01:13:36,536
Tensor tympani.

1232
01:13:46,290 --> 01:13:49,310
So some very
interesting experiments

1233
01:13:49,310 --> 01:13:52,250
have been done in
people who have

1234
01:13:52,250 --> 01:13:56,380
a compromise of their
seventh cranial nerve.

1235
01:13:56,380 --> 01:13:58,840
Does anybody know
what Bell's palsy is?

1236
01:14:04,280 --> 01:14:06,220
Does anybody know
what a palsy is?

1237
01:14:08,590 --> 01:14:11,934
Palsy means your motor neurons
aren't working so well.

1238
01:14:11,934 --> 01:14:12,600
So what happens?

1239
01:14:14,140 --> 01:14:16,630
In Bell's palsy, the
seventh cranial nerve

1240
01:14:16,630 --> 01:14:18,415
innervates facial musculature.

1241
01:14:20,280 --> 01:14:22,990
So if you have a problem,
a cut seventh nerve

1242
01:14:22,990 --> 01:14:25,780
or a Bell's palsy, which
is a viral infection

1243
01:14:25,780 --> 01:14:28,810
of the seventh cranial
nerve on one side, one

1244
01:14:28,810 --> 01:14:33,180
side of your facial features
droop because the muscles

1245
01:14:33,180 --> 01:14:37,920
there, which ordinary keep
your muscles in good control

1246
01:14:37,920 --> 01:14:40,430
of your face, they're
not working anymore.

1247
01:14:42,140 --> 01:14:46,220
So a person with a Bell's palsy
has a droopy face on that side.

1248
01:14:46,220 --> 01:14:49,060
The other side is fine because
the other seventh cranial

1249
01:14:49,060 --> 01:14:50,360
nerve is working just fine.

1250
01:14:51,630 --> 01:14:52,830
Or it usually is.

1251
01:14:52,830 --> 01:14:55,615
It's very often the case
that it's unilateral.

1252
01:14:57,880 --> 01:15:04,310
In Scandinavia where you can
do experiments on humans,

1253
01:15:04,310 --> 01:15:07,940
or you could at least
do experiments on humans

1254
01:15:07,940 --> 01:15:12,770
to a much greater extent than
in the United States, what

1255
01:15:12,770 --> 01:15:15,430
some enterprising
researchers did was

1256
01:15:15,430 --> 01:15:19,510
they took people who had
Bell's palsy and they said,

1257
01:15:19,510 --> 01:15:23,480
well, you can go and
work in your ordinary job

1258
01:15:23,480 --> 01:15:26,515
in the automobile factory
where it's really loud.

1259
01:15:27,930 --> 01:15:32,140
They tested their hearing
before they went to work

1260
01:15:32,140 --> 01:15:35,680
and they tested their hearing
at the end of the work day.

1261
01:15:35,680 --> 01:15:38,410
They tested it in the left
ear and in the right ear.

1262
01:15:38,410 --> 01:15:40,370
Let's say the right ear
was the palsied ear.

1263
01:15:42,770 --> 01:15:46,030
And then, left ear,
which was normal,

1264
01:15:46,030 --> 01:15:48,100
person came out at
the end of the workday

1265
01:15:48,100 --> 01:15:49,520
and their hearing
was just right.

1266
01:15:50,550 --> 01:15:53,840
In the palsied side,
at the end of the day

1267
01:15:53,840 --> 01:15:56,800
they had a temporary
threshold shift.

1268
01:15:56,800 --> 01:16:00,050
That means that if they're
hearing was down at 0 dB

1269
01:16:00,050 --> 01:16:02,800
at the beginning of the
day, their thresholds

1270
01:16:02,800 --> 01:16:04,890
were elevated at
the end of the day.

1271
01:16:04,890 --> 01:16:06,930
And hopefully, it's temporary.

1272
01:16:06,930 --> 01:16:09,390
That is, if you sleep all
night and things recover,

1273
01:16:09,390 --> 01:16:12,116
you're back to
normal the next day.

1274
01:16:12,116 --> 01:16:14,910
But what had happened-- this
is a beautiful experiment

1275
01:16:14,910 --> 01:16:17,320
because it's
controlled-- left-right

1276
01:16:17,320 --> 01:16:18,680
in the same individual.

1277
01:16:18,680 --> 01:16:23,190
So whatever that person did--
took drugs, or got infections,

1278
01:16:23,190 --> 01:16:25,480
or whatever-- was
hopefully bilateral.

1279
01:16:26,500 --> 01:16:29,700
It's a human, so you
can test very well

1280
01:16:29,700 --> 01:16:31,450
thresholds of hearing.

1281
01:16:31,450 --> 01:16:34,130
Presumably then, the
stapedius muscle,

1282
01:16:34,130 --> 01:16:38,320
which was not working
on the palsied side,

1283
01:16:38,320 --> 01:16:40,620
couldn't protect the
ear from the high levels

1284
01:16:40,620 --> 01:16:42,690
of sound in the
work environment.

1285
01:16:42,690 --> 01:16:44,510
And there was
damage-- hopefully,

1286
01:16:44,510 --> 01:16:47,390
temporary-- to that
person's hearing.

1287
01:16:47,390 --> 01:16:51,630
So clearly, the contraction
of these muscles,

1288
01:16:51,630 --> 01:16:56,100
which makes sound not go
through the middle ear as well,

1289
01:16:56,100 --> 01:16:59,990
is of beneficial effect.

1290
01:16:59,990 --> 01:17:02,970
For example, they protect
the cochlea from damage.

1291
01:17:04,400 --> 01:17:08,890
People who are at the NFL games
are having their middle ear

1292
01:17:08,890 --> 01:17:09,940
muscles contract.

1293
01:17:09,940 --> 01:17:11,097
Certainly, the stapedius.

1294
01:17:11,097 --> 01:17:12,430
And probably the tensor tympani.

1295
01:17:14,450 --> 01:17:18,940
They probably reduce the
effects of noise masking.

1296
01:17:18,940 --> 01:17:23,330
So that is a little
twist here in terms

1297
01:17:23,330 --> 01:17:26,530
of the spectrum of
where these muscles act.

1298
01:17:27,710 --> 01:17:31,170
It turns out when the muscles
contract, both the stapedius

1299
01:17:31,170 --> 01:17:35,310
and the tensor tympani,
they affect low frequencies

1300
01:17:35,310 --> 01:17:37,400
to a much greater
extent than highs.

1301
01:17:38,510 --> 01:17:43,190
For example, here you have
a reduction in transmission

1302
01:17:43,190 --> 01:17:46,330
through the middle ear of 25
dB at the lowest frequency

1303
01:17:46,330 --> 01:17:49,845
and 0, or even actually,
improved transmission

1304
01:17:49,845 --> 01:17:50,845
at the high frequencies.

1305
01:17:52,410 --> 01:17:55,940
And if you've ever been in
a car and you accelerate

1306
01:17:55,940 --> 01:17:58,540
to go on the highway,
you know the low rumbling

1307
01:17:58,540 --> 01:18:03,010
of the sound of the car means
you have to turn the radio up

1308
01:18:03,010 --> 01:18:05,970
when you want to enjoy your
music because the low rumbling

1309
01:18:05,970 --> 01:18:08,390
of the car, these
low frequencies

1310
01:18:08,390 --> 01:18:11,030
tend to mask the
interesting high frequencies

1311
01:18:11,030 --> 01:18:12,985
or mid-frequencies of
the music that you're

1312
01:18:12,985 --> 01:18:14,360
trying to listen
to on the radio.

1313
01:18:15,670 --> 01:18:20,840
So low frequencies tend to mask
mid and highs very effectively.

1314
01:18:20,840 --> 01:18:22,640
And so if you decrease
the transmission

1315
01:18:22,640 --> 01:18:25,430
for the low
frequencies, you reduce

1316
01:18:25,430 --> 01:18:26,690
the effects of noise masking.

1317
01:18:28,170 --> 01:18:30,235
Especially by
low-frequency noises.

1318
01:18:32,770 --> 01:18:34,900
Now finally, an
interesting thing

1319
01:18:34,900 --> 01:18:38,490
that is clearly known for
the middle ear muscles

1320
01:18:38,490 --> 01:18:42,040
is that they
contract just before

1321
01:18:42,040 --> 01:18:44,120
and during when you speak.

1322
01:18:44,120 --> 01:18:45,719
During you speak, OK?

1323
01:18:45,719 --> 01:18:46,885
I don't know who wrote that.

1324
01:18:48,040 --> 01:18:50,770
But anyway, when I speak,
I'm contracting my middle ear

1325
01:18:50,770 --> 01:18:51,280
muscles.

1326
01:18:51,280 --> 01:18:55,870
And the idea there is
perhaps when you're speaking,

1327
01:18:55,870 --> 01:18:57,720
you don't want to
listen to yourself.

1328
01:18:57,720 --> 01:19:01,610
And if you're speaking in
a loud voice, like 80 dB,

1329
01:19:01,610 --> 01:19:03,830
I'm trying to project here.

1330
01:19:03,830 --> 01:19:07,490
Or if I'm yelling at
some family members,

1331
01:19:07,490 --> 01:19:12,890
I want to not decrease my
sense of hearing or damage it.

1332
01:19:12,890 --> 01:19:14,954
So I contract my own
middle ear muscles

1333
01:19:14,954 --> 01:19:16,120
to prevent self-stimulation.

1334
01:19:17,930 --> 01:19:21,940
After I finish my
speaking or vocalization,

1335
01:19:21,940 --> 01:19:26,920
my muscles then stop
their contraction,

1336
01:19:26,920 --> 01:19:28,010
going back to normal.

1337
01:19:28,010 --> 01:19:30,570
And then my sense of
hearing is very acute

1338
01:19:30,570 --> 01:19:33,950
because it hasn't been damaged
by my own vocalization.

1339
01:19:35,460 --> 01:19:37,320
And later on in
the course, when we

1340
01:19:37,320 --> 01:19:40,260
talk about echolocation in bats.

1341
01:19:40,260 --> 01:19:43,490
So bats send out
this pulse of sound,

1342
01:19:43,490 --> 01:19:45,580
which is their vocalization.

1343
01:19:45,580 --> 01:19:47,530
And they listen for an echo.

1344
01:19:47,530 --> 01:19:49,540
The pulse of sound
can be 120 dB.

1345
01:19:51,350 --> 01:19:55,350
They're really screaming because
their targets, the insects

1346
01:19:55,350 --> 01:19:57,910
that they hunt, are
very, very small.

1347
01:19:57,910 --> 01:19:59,940
And there's not much
physical surface

1348
01:19:59,940 --> 01:20:01,490
for that to reflect off.

1349
01:20:01,490 --> 01:20:05,190
So the reflecting
sound is very small.

1350
01:20:05,190 --> 01:20:07,730
And so they maximize
that reflection

1351
01:20:07,730 --> 01:20:12,300
by emitting a very
high-level acoustic pulse.

1352
01:20:12,300 --> 01:20:17,550
And they don't want to damage
or desensitize their hearing

1353
01:20:17,550 --> 01:20:20,370
because they're listening
to very soft reflections

1354
01:20:20,370 --> 01:20:21,670
from the echoes.

1355
01:20:21,670 --> 01:20:25,460
So bats clearly tense their
middle ear muscles right

1356
01:20:25,460 --> 01:20:28,480
before they make this
echo-locating pulse.

1357
01:20:30,270 --> 01:20:33,970
We don't know if that's the case
for the olivocochlear neurons.

1358
01:20:33,970 --> 01:20:36,380
It just hasn't
been investigated.

1359
01:20:36,380 --> 01:20:41,200
It's very hard to measure their
effects during vocalizations.

1360
01:20:41,200 --> 01:20:43,340
But perhaps they do.

1361
01:20:43,340 --> 01:20:45,014
Certainly, the middle
ear muscles do.

1362
01:20:49,280 --> 01:20:54,267
OK, that's all I wanted to say
so we're sort of out of time.

1363
01:20:54,267 --> 01:20:54,850
Any questions?

1364
01:20:56,010 --> 01:20:58,390
And I want to make
one announcement

1365
01:20:58,390 --> 01:21:00,602
about Wednesday's class.

1366
01:21:00,602 --> 01:21:02,060
So in Wednesday's
class we're going

1367
01:21:02,060 --> 01:21:05,510
to be talking about
sound localization,

1368
01:21:05,510 --> 01:21:10,030
and listening to sounds that
differ in intraural timing

1369
01:21:10,030 --> 01:21:11,550
and level difference.

1370
01:21:11,550 --> 01:21:14,640
And so the usual
demonstrations that I play

1371
01:21:14,640 --> 01:21:16,430
aren't going to work
very well because we

1372
01:21:16,430 --> 01:21:19,040
want to manipulate
just one of those cues.

1373
01:21:19,040 --> 01:21:21,160
So we're going to be
listening in headphones.

1374
01:21:21,160 --> 01:21:23,290
So I have some great demos.

1375
01:21:23,290 --> 01:21:28,130
So please, if you can, bring
some headphones or earbuds.

1376
01:21:28,130 --> 01:21:31,370
And maybe download the demos
from the course website.

1377
01:21:32,450 --> 01:21:35,710
I have a few players that I
can also circulate around.

1378
01:21:35,710 --> 01:21:38,860
But if some people have
these demos on their laptops,

1379
01:21:38,860 --> 01:21:40,570
that would be more convenient.

1380
01:21:40,570 --> 01:21:41,170
OK?

1381
01:21:41,170 --> 01:21:43,320
See you on Wednesday.