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PROFESSOR: Last time we
talked about processing

00:00:29.600 --> 00:00:33.780
of cues that are used for
binaural localization of sound.

00:00:33.780 --> 00:00:37.936
Those being the interaural
time and interaural level

00:00:37.936 --> 00:00:38.435
differences.

00:00:41.400 --> 00:00:45.550
And we have those cues because
the ears are physically

00:00:45.550 --> 00:00:48.160
separated on the
sides of our head.

00:00:49.380 --> 00:00:52.780
And because of the physical
characteristics of sound,

00:00:52.780 --> 00:00:55.340
for example, the
velocity of sound in air.

00:00:57.420 --> 00:01:02.230
And we talked toward the
end of last time's lecture

00:01:02.230 --> 00:01:06.370
about the neural processing
of interaural time differences

00:01:06.370 --> 00:01:09.995
in the Medial Superior
Olive, or MSO.

00:01:11.560 --> 00:01:14.140
And we talked about
the Jeffress model

00:01:14.140 --> 00:01:20.880
and how it can help recreate
a neural mapping that

00:01:20.880 --> 00:01:22.360
has ITD sensitivity.

00:01:22.360 --> 00:01:27.850
And that's the subject of
the assignment for this year.

00:01:27.850 --> 00:01:33.040
So I just put the text
for the assignment

00:01:33.040 --> 00:01:36.430
up here, just to
mention that there

00:01:36.430 --> 00:01:39.420
was something added
on at the end.

00:01:39.420 --> 00:01:43.350
But the first paragraph
of the assignment

00:01:43.350 --> 00:01:49.005
is basically the lecture and the
sketch of the Jeffress model.

00:01:50.930 --> 00:01:53.990
Except that these
days, people are

00:01:53.990 --> 00:01:58.940
thinking that the axonal
delay lines are not

00:01:58.940 --> 00:02:03.190
how the delay is created in
the medial superior olive.

00:02:03.190 --> 00:02:09.530
And so I ask you for two other
possible neural mechanisms

00:02:09.530 --> 00:02:11.880
that weren't originally
postulated by Jeffress

00:02:11.880 --> 00:02:13.365
that could create delays.

00:02:15.650 --> 00:02:18.470
So that's sort of the
heart of the assignment.

00:02:19.800 --> 00:02:24.940
The second paragraph talks
about some further updates

00:02:24.940 --> 00:02:26.040
to the Jeffress model.

00:02:27.320 --> 00:02:31.790
And there is a paper
that is attached

00:02:31.790 --> 00:02:33.660
to last time's lecture.

00:02:33.660 --> 00:02:39.470
The paper by [? Brandt ?] et al
where they discuss extensively

00:02:39.470 --> 00:02:42.880
some challenges, they call
them, to the Jeffress model.

00:02:42.880 --> 00:02:45.420
And I might call them amendments
to the Jeffress model,

00:02:45.420 --> 00:02:47.470
but things that
weren't originally

00:02:47.470 --> 00:02:50.890
postulated by Jeffress that
have come to light because

00:02:50.890 --> 00:02:56.800
of more recent experimental
studies that don't fit

00:02:56.800 --> 00:02:58.890
with the original version
and their updates.

00:03:00.800 --> 00:03:02.830
And finally, we've been
talking quite a lot

00:03:02.830 --> 00:03:04.710
about cochlear implants.

00:03:04.710 --> 00:03:07.190
And there's a very nice
passage on cochlear implants

00:03:07.190 --> 00:03:10.120
in the textbook that I
encourage you to read.

00:03:10.120 --> 00:03:12.860
And the final paragraph
of the assignment

00:03:12.860 --> 00:03:16.860
is, what problems would
cochlear implant users, even

00:03:16.860 --> 00:03:22.120
those with a left and a
right cochlear implant, have

00:03:22.120 --> 00:03:26.865
if they use the Jeffress
model to localize sounds?

00:03:28.330 --> 00:03:32.760
So that's a little
bit of thinking

00:03:32.760 --> 00:03:34.520
to solve that last problem, too.

00:03:34.520 --> 00:03:37.080
But based on what
we have talked about

00:03:37.080 --> 00:03:39.950
and what the textbook talks
about for cochlear implants,

00:03:39.950 --> 00:03:43.440
you should be able to come
up with an answer for that.

00:03:43.440 --> 00:03:45.950
So any questions
on the assignment?

00:03:45.950 --> 00:03:49.160
It relates very heavily
to last time's lecture

00:03:49.160 --> 00:03:51.140
on the Jeffress
model and the MSO.

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And I guess-- it
doesn't say here,

00:03:56.470 --> 00:03:59.730
but three to five pages would
be appropriate, I think.

00:04:02.380 --> 00:04:05.720
And it's due on
December 4, which

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is the day of the lab tour.

00:04:08.600 --> 00:04:12.440
So on that class day, which
is a week from Wednesday,

00:04:12.440 --> 00:04:14.750
we'll meet at the
Massachusetts Eye and Ear

00:04:14.750 --> 00:04:17.399
Infirmary instead
of meeting here.

00:04:17.399 --> 00:04:19.010
So we'll send you a reminder.

00:04:19.010 --> 00:04:20.760
And I think now on
the website there

00:04:20.760 --> 00:04:23.825
are directions to get to
Mass Eye and Ear Infirmary.

00:04:25.080 --> 00:04:28.780
It's just a simple one-stop
on the Red Line subway

00:04:28.780 --> 00:04:29.440
to get there.

00:04:30.810 --> 00:04:32.570
So the assignment
will be due then.

00:04:32.570 --> 00:04:34.080
And then I'll be
able to grade it

00:04:34.080 --> 00:04:36.200
before we have a review session.

00:04:36.200 --> 00:04:41.100
And we'll talk about the correct
answers for this assignment

00:04:41.100 --> 00:04:43.460
at the time of the
review session, which

00:04:43.460 --> 00:04:46.030
is the next class
after December 4.

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So today, I want to
march into new things.

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Now, we're going to talk
about neural processing

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of interaural level differences.

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Remember if a sound is off
to one side of my head,

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it's going to appear at a higher
sound level in the ear facing

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that sound source than
it will appear in the ear

00:05:12.190 --> 00:05:13.860
away from the sound source.

00:05:13.860 --> 00:05:17.450
So we talked about how big
these cues were last time.

00:05:17.450 --> 00:05:21.800
And they can, for high
frequencies, be maximally 20 dB

00:05:21.800 --> 00:05:24.450
in Interaural Level
Difference, or ILD.

00:05:27.020 --> 00:05:30.820
So those are processed,
at least starting

00:05:30.820 --> 00:05:33.410
in the lateral
superior olive, which

00:05:33.410 --> 00:05:37.080
is another nucleus in the
superior olivary complex, which

00:05:37.080 --> 00:05:41.220
is close to the medial superior
olive we've been talking about.

00:05:41.220 --> 00:05:43.985
So we'll talk about
the process in the LSO.

00:05:45.650 --> 00:05:47.820
Then, we'll talk
about projections

00:05:47.820 --> 00:05:51.080
of that nucleus and the MSO.

00:05:51.080 --> 00:05:52.990
And other parts of
the superior olive

00:05:52.990 --> 00:05:56.270
to the next higher
center in the brainstem,

00:05:56.270 --> 00:06:00.830
which in the auditory pathway is
the Inferior Colliculus, or IC.

00:06:00.830 --> 00:06:04.880
So we'll be talking extensively
about the inferior colliculus,

00:06:04.880 --> 00:06:07.730
which is a large
nucleus in the brainstem

00:06:07.730 --> 00:06:10.100
just caudal to the
superior colliculus

00:06:10.100 --> 00:06:12.800
that you folks talked
about extensively

00:06:12.800 --> 00:06:14.405
in the visual part
of the course.

00:06:16.690 --> 00:06:20.200
In the IC, you have some
interesting neural responses

00:06:20.200 --> 00:06:23.630
that relate to something
called time/intensity trading.

00:06:25.145 --> 00:06:26.530
I misspelled it.

00:06:26.530 --> 00:06:28.540
Not a very good speller, sorry.

00:06:36.770 --> 00:06:38.590
Which we'll talk
about and define.

00:06:38.590 --> 00:06:41.450
We'll talk about some
interesting characteristics

00:06:41.450 --> 00:06:46.280
of room acoustics, like
reflections off the wall,

00:06:46.280 --> 00:06:50.680
and how those don't completely
upset the apple cart of knowing

00:06:50.680 --> 00:06:53.190
where a sound is coming
from because of things

00:06:53.190 --> 00:06:54.670
like the precedence effect.

00:06:57.690 --> 00:07:01.450
And finally, we'll end up with
auditory pathways in the barn

00:07:01.450 --> 00:07:01.950
owl.

00:07:01.950 --> 00:07:05.900
So this species of birds
has very well-developed

00:07:05.900 --> 00:07:07.880
auditory systems.

00:07:07.880 --> 00:07:11.460
And some work at
Caltech has shown

00:07:11.460 --> 00:07:14.440
that these animals
have so-called "space"

00:07:14.440 --> 00:07:17.870
maps in a certain
part of their brain.

00:07:17.870 --> 00:07:19.590
That is, there's
a mapping of where

00:07:19.590 --> 00:07:24.730
the sound is in external
space into space

00:07:24.730 --> 00:07:29.620
in part of their brain
called the optic tectum.

00:07:29.620 --> 00:07:30.700
And we'll go over that.

00:07:30.700 --> 00:07:35.980
And the reading for today is on
how that space map is plastic

00:07:35.980 --> 00:07:37.845
and can be changed
by experience.

00:07:39.200 --> 00:07:42.430
So we'll talk about neuronal
plasticity of the space map.

00:07:44.270 --> 00:07:45.660
OK, so let's get started.

00:07:54.380 --> 00:07:58.040
So the neural processing of the
interaural level differences

00:07:58.040 --> 00:08:01.210
in the lateral superior olive.

00:08:01.210 --> 00:08:05.500
By contrast to what Jeffress
cooked up for the MSO,

00:08:05.500 --> 00:08:07.890
this neural circuit
is very simple.

00:08:09.520 --> 00:08:11.220
And here's how it runs.

00:08:11.220 --> 00:08:15.470
The Lateral Superior Olive on
the left side is here, the LSO.

00:08:15.470 --> 00:08:18.200
There's an LSO on the
right side as well,

00:08:18.200 --> 00:08:20.620
but the circuit is shown for
the one on the left side.

00:08:21.920 --> 00:08:24.450
This LSO on the
left side gets input

00:08:24.450 --> 00:08:27.065
from the cochlear
nucleus on the left side.

00:08:29.240 --> 00:08:30.780
That's excitatory input.

00:08:32.530 --> 00:08:35.765
And it gets input from the
right side that's inhibitory.

00:08:37.429 --> 00:08:40.659
And the way the
inhibitory input works

00:08:40.659 --> 00:08:45.560
is the cochlear nucleus
neurons on the right side

00:08:45.560 --> 00:08:50.140
project across the midline
and into another sub-nucleus

00:08:50.140 --> 00:08:53.830
of the superior olivary
complex designated here

00:08:53.830 --> 00:08:58.410
as the MNTB, which I think
is spelled out here--

00:08:58.410 --> 00:09:03.240
the Medial Nucleus of
the Trapezoid Body.

00:09:03.240 --> 00:09:06.610
So if you look at these
sections under the microscope,

00:09:06.610 --> 00:09:09.260
there's a lot of
crossing fibers here.

00:09:09.260 --> 00:09:11.957
And they sort of look, if you
have a lot of imagination,

00:09:11.957 --> 00:09:12.665
like a trapezoid.

00:09:14.000 --> 00:09:17.520
And this nucleus is within
those crossing fibers.

00:09:17.520 --> 00:09:20.290
So it's within the
trapezoid body.

00:09:20.290 --> 00:09:22.470
That's how it gets its name.

00:09:22.470 --> 00:09:27.270
And the MNTB neurons have
inhibitory neurotransmitter.

00:09:29.360 --> 00:09:33.400
So the cochlear nucleus
excites these MNTB neurons,

00:09:33.400 --> 00:09:36.520
but then they're inhibitory
and they send their axons

00:09:36.520 --> 00:09:37.320
to the LSO.

00:09:38.620 --> 00:09:42.820
And they spill out or
release their inhibitory

00:09:42.820 --> 00:09:45.170
neurotransmitter on
to the LSO neurons.

00:09:48.590 --> 00:09:56.280
So how does this
circuit work then

00:09:56.280 --> 00:10:00.500
if the sound is off to
this side of the slide,

00:10:00.500 --> 00:10:01.800
off to the left side?

00:10:03.460 --> 00:10:06.430
The sound will be of higher
level in the left ear

00:10:06.430 --> 00:10:09.160
and it will exert a
high excitatory effect

00:10:09.160 --> 00:10:10.850
on the cochlear nucleus here.

00:10:10.850 --> 00:10:14.260
And the LSO will be
excited in a big way.

00:10:14.260 --> 00:10:16.180
Of course, a little
bit of that sound

00:10:16.180 --> 00:10:19.910
is going to come over
here to the right side,

00:10:19.910 --> 00:10:23.490
but it won't excite the right
auditory nerve quite as much.

00:10:23.490 --> 00:10:27.130
And this pathway that
eventually becomes inhibitory

00:10:27.130 --> 00:10:30.470
won't be as strongly activated.

00:10:30.470 --> 00:10:33.240
And so the inhibition
will be less.

00:10:33.240 --> 00:10:38.210
So there's an interplay between
excitation here and inhibition.

00:10:38.210 --> 00:10:41.210
And in this case with the
sound off to the left side,

00:10:41.210 --> 00:10:43.420
the excitation will rule.

00:10:43.420 --> 00:10:49.180
And so here's a plot of the
firing of an individual LSO

00:10:49.180 --> 00:10:50.775
neuron that gets
all these inputs.

00:10:52.440 --> 00:10:54.730
And if the sound is off
to the left side, which

00:10:54.730 --> 00:10:58.520
is supposed to be this axis--
this is an axis of Interaural

00:10:58.520 --> 00:11:02.420
Level Difference,
or ILD, where here

00:11:02.420 --> 00:11:05.410
on the left side of the
graph the ipsilateral level

00:11:05.410 --> 00:11:08.280
is greater than the
contralateral level.

00:11:08.280 --> 00:11:11.390
That case, the LSO
neuron will be excited

00:11:11.390 --> 00:11:13.520
and it'll have a
lot of response,

00:11:13.520 --> 00:11:14.535
a high amount of firing.

00:11:17.600 --> 00:11:22.530
On the other hand, if the sound
is over here on the right side,

00:11:22.530 --> 00:11:25.970
it will activate in a big
way the right pathway,

00:11:25.970 --> 00:11:29.345
which will result in a big
inhibitory input to the LSO

00:11:29.345 --> 00:11:32.750
neuron on that left
side of the brain.

00:11:32.750 --> 00:11:35.570
Sure, some of the sound
is going to come over here

00:11:35.570 --> 00:11:38.460
and activate the left pathway.

00:11:38.460 --> 00:11:41.360
But that excitation
won't be as strong.

00:11:41.360 --> 00:11:43.925
So in that case, the
inhibition will rule.

00:11:45.350 --> 00:11:49.000
The LSO neuron gets a
large amount of inhibition

00:11:49.000 --> 00:11:51.130
and its firing rate will be low.

00:11:51.130 --> 00:11:53.490
Its response will be low
because it's inhibited.

00:11:56.000 --> 00:11:59.885
Now, what happens if the sound
is right in the middle, 0 ILD?

00:11:59.885 --> 00:12:02.280
Sound is coming straight ahead.

00:12:02.280 --> 00:12:04.510
The sound is the
same at the two ears.

00:12:04.510 --> 00:12:07.720
And thus, the ILD is 0.

00:12:07.720 --> 00:12:11.950
Well, it sort of depends
on how you wire this up.

00:12:13.490 --> 00:12:16.750
Whether the balance
is perfectly balanced.

00:12:16.750 --> 00:12:19.409
Then, maybe it would
be a 50% response.

00:12:19.409 --> 00:12:21.700
In the case of this graph,
it looks like the inhibition

00:12:21.700 --> 00:12:26.490
is a little bit stronger for
equal sound on the two sides.

00:12:26.490 --> 00:12:30.740
And so the inhibition
dominates at 0 ILD.

00:12:30.740 --> 00:12:34.470
But in actuality, if you
record from the LSO neurons,

00:12:34.470 --> 00:12:37.630
you find all sorts
of combinations.

00:12:37.630 --> 00:12:40.540
Those that have 50%
response rate at 0,

00:12:40.540 --> 00:12:44.100
those that have 90% response,
and those that have 10%

00:12:44.100 --> 00:12:44.810
like this guy.

00:12:46.800 --> 00:12:48.735
So this is a very
simple circuit.

00:12:50.740 --> 00:12:53.770
No coincidence detection.

00:12:53.770 --> 00:12:55.010
No delay lines.

00:12:56.390 --> 00:12:58.280
Well, you should
kind of do a heads

00:12:58.280 --> 00:13:01.940
up here when I talk
about delay and timing

00:13:01.940 --> 00:13:06.280
because this pathway coming
from this contralateral side

00:13:06.280 --> 00:13:07.065
is a lot longer.

00:13:08.950 --> 00:13:11.670
The axons have to
cross the midline.

00:13:11.670 --> 00:13:14.620
And then there's a
delay here because

00:13:14.620 --> 00:13:18.860
at the synapse between
one axon's terminal

00:13:18.860 --> 00:13:21.740
and the cell bodies
of the MNTB neurons,

00:13:21.740 --> 00:13:23.320
there's a little bit of delay.

00:13:23.320 --> 00:13:25.790
This neurotransmitter
has to be released.

00:13:27.310 --> 00:13:30.245
The MNTB neurons have to get
excited and, finally, fire.

00:13:31.480 --> 00:13:35.350
So all that can take a
half a millisecond or so.

00:13:35.350 --> 00:13:40.770
It turns out this axon
is a very thick one.

00:13:40.770 --> 00:13:42.790
This cochlear
nucleus neuron here

00:13:42.790 --> 00:13:46.590
that provides this crossing axon
is called the globular bushy

00:13:46.590 --> 00:13:47.740
cell.

00:13:47.740 --> 00:13:49.825
And we talked about
that a little bit

00:13:49.825 --> 00:13:51.575
when we talked about
the cochlear nucleus.

00:13:53.380 --> 00:13:55.590
It's not important
exactly what type it is,

00:13:55.590 --> 00:13:58.480
but this has the
thickest axon, really,

00:13:58.480 --> 00:14:01.290
of all the axons in
the auditory pathway.

00:14:01.290 --> 00:14:04.515
So it gets across the
midline very quickly.

00:14:06.030 --> 00:14:07.840
And there is a
synaptic delay here.

00:14:07.840 --> 00:14:10.920
So the contralateral input is
going to come in a little bit

00:14:10.920 --> 00:14:12.190
later.

00:14:12.190 --> 00:14:15.480
So sometimes in recording
some LSO neurons,

00:14:15.480 --> 00:14:20.140
you find a little bit of
excitation from this side.

00:14:20.140 --> 00:14:22.175
And then right after,
an instant later,

00:14:22.175 --> 00:14:25.060
a half a millisecond
or a millisecond later,

00:14:25.060 --> 00:14:26.330
you find a big inhibition.

00:14:27.670 --> 00:14:28.700
And so that can happen.

00:14:28.700 --> 00:14:32.070
The sum total though,
in the case where

00:14:32.070 --> 00:14:35.090
the sound is off to
the contralateral side,

00:14:35.090 --> 00:14:36.450
is a dominant inhibition.

00:14:41.460 --> 00:14:44.000
Now, we haven't talked
about neural inhibition

00:14:44.000 --> 00:14:48.520
much in our class, so maybe
we should just mention it.

00:14:48.520 --> 00:14:50.400
And I should definitely
mention the type

00:14:50.400 --> 00:14:53.000
of inhibitory transmitter
that's used here.

00:14:54.095 --> 00:15:15.910
So we have the MNTB neurons
are coming like this

00:15:15.910 --> 00:15:19.130
and sending their axons
to the LSO neurons.

00:15:21.750 --> 00:15:23.570
And they're inhibiting them.

00:15:23.570 --> 00:15:25.780
That's what I mean by
this minus sign here.

00:15:26.880 --> 00:15:31.230
And so this is an
inhibitory synapse

00:15:31.230 --> 00:15:33.275
which inhibits the LSO neurons.

00:15:34.800 --> 00:15:36.560
One can ask the
question is, what

00:15:36.560 --> 00:15:38.730
is the inhibitory
neurotransmitter?

00:15:38.730 --> 00:15:41.410
And in this case,
it's called glycine.

00:15:45.890 --> 00:15:52.210
So glycine is released from
the MNTB neuron terminals

00:15:52.210 --> 00:15:54.810
onto the LSO neurons.

00:15:54.810 --> 00:15:57.630
And so how is that known?

00:15:58.890 --> 00:16:03.080
Well, it's sort of a lot of
little pieces in a puzzle here.

00:16:03.080 --> 00:16:09.320
The MNTB neurons themselves have
the metabolic machinery that

00:16:09.320 --> 00:16:11.270
can make the glycine.

00:16:11.270 --> 00:16:13.270
They transport it down
their nerve axons.

00:16:14.570 --> 00:16:17.290
Glycine is actually a
fairly common chemical

00:16:17.290 --> 00:16:20.610
in all cells of the body,
but these MNTB neurons

00:16:20.610 --> 00:16:22.320
are packed with it.

00:16:22.320 --> 00:16:26.080
If you use antibodies
to glycine,

00:16:26.080 --> 00:16:29.760
they stain these
neurons much darker

00:16:29.760 --> 00:16:31.807
than most neurons in the brain.

00:16:31.807 --> 00:16:33.765
There are other glycinergic
neurons, of course.

00:16:35.430 --> 00:16:38.055
Their axons and their
terminals are darkly stained.

00:16:39.870 --> 00:16:42.720
The lateral superior
olive neurons

00:16:42.720 --> 00:16:45.530
have glycine receptors on them.

00:16:46.950 --> 00:16:50.400
When you put little
puffs of glycine,

00:16:50.400 --> 00:16:52.540
which you can from a pipette.

00:16:52.540 --> 00:16:55.590
You can release
glycine from a pipette

00:16:55.590 --> 00:16:58.810
in an artificial
recording situation.

00:16:58.810 --> 00:17:01.580
When you puff glycine
on to LSO neurons,

00:17:01.580 --> 00:17:03.140
they are inhibited greatly.

00:17:03.140 --> 00:17:05.610
So they certainly have
the receptors for glycine.

00:17:07.270 --> 00:17:11.910
There are uptake systems to
take up the glycine after it's

00:17:11.910 --> 00:17:14.455
been released so that the
inhibition doesn't stay on

00:17:14.455 --> 00:17:14.955
forever.

00:17:16.880 --> 00:17:21.569
When you stimulate, if you go
in and stimulate electrically

00:17:21.569 --> 00:17:26.560
these MNTB neurons, you find
the LSO neurons are inhibited.

00:17:26.560 --> 00:17:31.170
So a lot of little pieces of
the puzzle go into the idea

00:17:31.170 --> 00:17:32.700
that these are
glycinergic neurons.

00:17:34.270 --> 00:17:37.160
And this glycinergic
input is very

00:17:37.160 --> 00:17:38.566
important in ILD sensitivity.

00:17:40.120 --> 00:17:46.740
Now, a lot of textbooks will say
the ITD sensitivity is created

00:17:46.740 --> 00:17:49.780
in the MSO and the ILD
sensitivity is created here

00:17:49.780 --> 00:17:51.040
in the LSO.

00:17:51.040 --> 00:17:52.810
And we're done with it.

00:17:52.810 --> 00:17:55.330
But that's probably not true.

00:17:55.330 --> 00:17:58.210
This is such a simple
circuit, you probably

00:17:58.210 --> 00:18:01.450
have other places in the
auditory pathway where

00:18:01.450 --> 00:18:03.595
ILD sensitivity is also created.

00:18:04.950 --> 00:18:10.785
For example, you have nuclei
in the lateral lemniscus--

00:18:10.785 --> 00:18:14.070
the pathway going up to
the inferior colliculus.

00:18:14.070 --> 00:18:15.810
You have such circuits
probably right

00:18:15.810 --> 00:18:18.149
in the inferior
colliculus and maybe

00:18:18.149 --> 00:18:19.440
at other levels of the pathway.

00:18:20.450 --> 00:18:23.240
So this is not the
only place where

00:18:23.240 --> 00:18:25.280
we find circuits
for ILD sensitivity.

00:18:32.060 --> 00:18:38.440
Now, we talked about last
time how these ILD cues

00:18:38.440 --> 00:18:41.970
were prominent at
high frequencies

00:18:41.970 --> 00:18:47.710
and very-- almost nonexistent
at low frequencies.

00:18:47.710 --> 00:18:50.740
Because low-frequency sound
can bend around the head

00:18:50.740 --> 00:18:52.120
very easily.

00:18:52.120 --> 00:18:54.170
So we had, I think,
for 200 Hertz.

00:18:55.230 --> 00:18:59.260
Even for a sound source located
directly off to the side

00:18:59.260 --> 00:19:00.465
absolutely 0 ILD.

00:19:02.260 --> 00:19:05.510
For 6,000 Hertz,
we have a huge ILD.

00:19:06.900 --> 00:19:10.220
So ILDs are not very
important at low frequencies.

00:19:10.220 --> 00:19:16.530
If you go in to the LSO
and record the frequency

00:19:16.530 --> 00:19:21.440
responsivity of the neurons
there from measurements

00:19:21.440 --> 00:19:22.760
of their tuning curve.

00:19:22.760 --> 00:19:25.510
Now, remember what
a tuning curve was?

00:19:25.510 --> 00:19:28.590
Tuning curve, we had
sort of over and over.

00:19:28.590 --> 00:19:31.875
It was a plot of
sound frequency.

00:19:33.194 --> 00:19:36.960
On the y-axis is sound
pressure level for a response.

00:19:39.780 --> 00:19:42.520
And we have the
v-shaped functions

00:19:42.520 --> 00:19:45.290
and we picked off the
most sensitive frequency.

00:19:45.290 --> 00:19:48.710
And that's the CF of the neuron.

00:19:48.710 --> 00:19:51.420
Neurons in the superior olivary
complex have beautiful CFs.

00:19:52.440 --> 00:19:54.270
And you could do a
CF mapping, which

00:19:54.270 --> 00:19:55.820
is what's done in this study.

00:19:56.970 --> 00:20:02.550
So in the LSO-- right here, you
find CFs from 0 to 1 kilohertz.

00:20:05.430 --> 00:20:08.380
Right in this part
from 4 to 10 kilohertz.

00:20:10.730 --> 00:20:15.030
In this part here,
20 kilohertz and up.

00:20:15.030 --> 00:20:20.600
And this is the typical kind
of funny s-shape of the LSO

00:20:20.600 --> 00:20:25.360
that you see in coronal
sections in this case of the cat

00:20:25.360 --> 00:20:26.530
superior olivary complex.

00:20:29.799 --> 00:20:32.090
If you were to do this mapping
in the cochlear nucleus,

00:20:32.090 --> 00:20:36.040
you'd find a lot of
cochlear nucleus devoted

00:20:36.040 --> 00:20:40.670
to the low CFs, some to the
middle, and some to high.

00:20:40.670 --> 00:20:45.540
In the LSO, you have a
lot of the LSO devoted

00:20:45.540 --> 00:20:50.765
to the high CFs, which is where
ILD cues are very prominent.

00:20:52.000 --> 00:20:55.610
So it makes sense that where
you're processing ILDs,

00:20:55.610 --> 00:21:00.560
you devote a lot of neurons to
responding to the frequencies

00:21:00.560 --> 00:21:02.310
where the cue is very salient.

00:21:04.810 --> 00:21:07.540
The MNTB, which
projects into the LSO,

00:21:07.540 --> 00:21:11.090
has a similar disproportionately
large representation

00:21:11.090 --> 00:21:11.830
of high CFs.

00:21:15.390 --> 00:21:17.250
The MSO is just the opposite.

00:21:18.430 --> 00:21:22.620
There's hardly any MSO
devoted to the very highest

00:21:22.620 --> 00:21:23.370
frequencies.

00:21:23.370 --> 00:21:25.740
And remember, last
time we talked

00:21:25.740 --> 00:21:30.180
about ITDs being ambiguous
at high frequencies

00:21:30.180 --> 00:21:34.970
because the interaural time
difference is still the same,

00:21:34.970 --> 00:21:39.050
but the sound can go through
one or even more complete

00:21:39.050 --> 00:21:42.110
cycles by the time it
gets to the other ear.

00:21:42.110 --> 00:21:46.430
And so you can't
tell what ITD you're

00:21:46.430 --> 00:21:48.055
working with at these
high frequencies.

00:21:49.280 --> 00:21:52.240
On the other hand,
where these ILD cues

00:21:52.240 --> 00:21:54.460
were weak at low
frequencies, the ITDs

00:21:54.460 --> 00:21:56.550
are strong and salient.

00:21:56.550 --> 00:22:02.130
And there's a lot of MSO devoted
to the low characteristic

00:22:02.130 --> 00:22:04.265
frequencies where the
ITDs are prominent.

00:22:07.400 --> 00:22:10.350
So that's what this
text means, that there's

00:22:10.350 --> 00:22:14.390
sort of a disproportionate
amount of territory

00:22:14.390 --> 00:22:18.450
in these nuclei based
on where the cue is

00:22:18.450 --> 00:22:19.900
important in the
frequency domain.

00:22:22.870 --> 00:22:26.390
Now, we have a little bit of
an issue here with the LSO.

00:22:28.440 --> 00:22:35.900
This LSO on this side is going
to respond to sound sources

00:22:35.900 --> 00:22:37.865
on this side of the body.

00:22:39.060 --> 00:22:41.450
And remember in most
sensory systems,

00:22:41.450 --> 00:22:45.530
there's a crossing
such that stimuli

00:22:45.530 --> 00:22:50.540
on the right side of the
body evoke neural responses

00:22:50.540 --> 00:22:52.465
on the left side of the brain.

00:22:52.465 --> 00:22:54.590
This LSO has sort
of got it backwards.

00:22:54.590 --> 00:22:59.070
It's responding with excitation
to sound sources on the right.

00:22:59.070 --> 00:23:01.890
Well, that's taken
care of by virtue

00:23:01.890 --> 00:23:04.700
of the ascending
projections from the LSO

00:23:04.700 --> 00:23:07.350
to the next higher
center, which is

00:23:07.350 --> 00:23:09.220
the Inferior Colliculus, or IC.

00:23:11.780 --> 00:23:14.395
And that's diagrammed
here in this next slide.

00:23:20.430 --> 00:23:23.650
And this shows the
projections of the LSO.

00:23:25.190 --> 00:23:31.200
Again, the LSO on the left side
projecting across the midline

00:23:31.200 --> 00:23:34.210
to the inferior colliculus
on the right side.

00:23:35.900 --> 00:23:38.010
And that projection
would then predict

00:23:38.010 --> 00:23:42.630
that if a sound source was
over on this right side

00:23:42.630 --> 00:23:48.640
and exciting the LSO neuron,
that message would then

00:23:48.640 --> 00:23:53.010
get in a big way to the inferior
colliculus on the left side.

00:23:53.010 --> 00:23:54.840
So that inferior
colliculus would then

00:23:54.840 --> 00:23:57.120
respond to sounds on the
right side of the body.

00:23:58.930 --> 00:24:01.660
For a while, this field
was a little mystified

00:24:01.660 --> 00:24:03.100
because there's
also a projection

00:24:03.100 --> 00:24:05.980
from the left LSO
to the left IC.

00:24:07.180 --> 00:24:09.435
It ended up being an
inhibitory projection.

00:24:11.660 --> 00:24:14.120
So this projection here
that stays on the same side

00:24:14.120 --> 00:24:15.155
is mostly inhibitory.

00:24:16.420 --> 00:24:21.420
It's not exactly clear what
that does, but it's there.

00:24:21.420 --> 00:24:25.955
You can sort of discount it in
terms of the mapping of stimuli

00:24:25.955 --> 00:24:28.850
on one side of the
body to responses

00:24:28.850 --> 00:24:30.125
on the other side of the body.

00:24:32.920 --> 00:24:35.234
The MSO doesn't
have such a problem,

00:24:35.234 --> 00:24:36.275
I'll just say in passing.

00:24:37.442 --> 00:24:40.870
The MSO, just because
of its ITD map--

00:24:40.870 --> 00:24:44.910
if you go back and review
last week's lecture,

00:24:44.910 --> 00:24:46.890
you'll see that
the MSO is already

00:24:46.890 --> 00:24:50.440
mapping ITDs for sound
sources on the opposite side

00:24:50.440 --> 00:24:50.960
of the body.

00:24:53.400 --> 00:24:57.670
Now, there's been a lot of work,
especially in the early days

00:24:57.670 --> 00:25:00.890
of auditory neural
science, on looking

00:25:00.890 --> 00:25:01.953
at the effect of lesions.

00:25:08.220 --> 00:25:11.440
And lesions are a
little bit hard to do

00:25:11.440 --> 00:25:15.130
in a complex as hard as the
superior olivary complex.

00:25:15.130 --> 00:25:18.620
Because if you go in and
try to destroy the LSO,

00:25:18.620 --> 00:25:23.390
invariably right next door
is the MSO on one side.

00:25:23.390 --> 00:25:26.320
And right next door is the MNTB.

00:25:26.320 --> 00:25:30.750
It's very hard to make selective
lesions in the superior olive.

00:25:30.750 --> 00:25:32.670
When you get to the
inferior colliculus-- oh,

00:25:32.670 --> 00:25:34.400
there's a big nucleus.

00:25:34.400 --> 00:25:37.210
And you can go in and
you can destroy it

00:25:37.210 --> 00:25:40.190
in an experimental
animal on just one side.

00:25:41.950 --> 00:25:47.490
When that is done, and the
animal is trained in a task

00:25:47.490 --> 00:25:50.910
to localize where the
sound is coming from,

00:25:50.910 --> 00:25:54.880
it's very clear that a lesion
of the inferior colliculus

00:25:54.880 --> 00:26:00.290
on the right side makes the
animal unable to localize

00:26:00.290 --> 00:26:02.540
sounds on the opposite
side of the body.

00:26:04.000 --> 00:26:08.300
So lesion in the right
inferior colliculus,

00:26:08.300 --> 00:26:11.460
the animal can't localize
sounds on the opposite side.

00:26:13.160 --> 00:26:17.340
The animal still can localize
sounds on the ipsilateral hemi

00:26:17.340 --> 00:26:23.450
field, in the side that
you didn't lesion on

00:26:23.450 --> 00:26:27.070
because the other colliculus
that's still there that's

00:26:27.070 --> 00:26:29.150
not lesioned can
perform the task.

00:26:29.150 --> 00:26:33.190
If you lesion both
inferior colliculi,

00:26:33.190 --> 00:26:35.540
the animal can't
localize sounds anywhere.

00:26:36.810 --> 00:26:38.470
But it's very
clear that a lesion

00:26:38.470 --> 00:26:42.940
on one side of the
auditory pathway

00:26:42.940 --> 00:26:46.100
here makes the animal
unable to localize sounds

00:26:46.100 --> 00:26:47.550
in the opposite hemi field.

00:26:47.550 --> 00:26:50.660
So that's a theme for
sensory processing

00:26:50.660 --> 00:26:53.450
that stimuli on one
side of the body

00:26:53.450 --> 00:26:58.674
are mapped to neural nuclei on
the opposite side of the brain.

00:26:58.674 --> 00:27:00.590
And that's very clear
in the auditory pathway.

00:27:05.450 --> 00:27:12.140
Now, I think last time when we
had the demonstrations of ITDs

00:27:12.140 --> 00:27:16.480
and ILDs in headphones where
we could present one and not

00:27:16.480 --> 00:27:20.680
the other, someone said,
well, it sounded a little bit

00:27:20.680 --> 00:27:24.670
like when we had an ITD when
the sound was on the left side.

00:27:24.670 --> 00:27:28.210
And then later, we had an ILD
with sound on the left side.

00:27:28.210 --> 00:27:31.140
They sounded a little bit
the same, as if the sound

00:27:31.140 --> 00:27:34.090
could be put on one side
by either ITD and ILD.

00:27:34.090 --> 00:27:38.620
And that was an
interesting comment

00:27:38.620 --> 00:27:43.520
because of the phenomenon called
time intensity trading, which

00:27:43.520 --> 00:27:44.940
you can do with headphones.

00:27:44.940 --> 00:27:49.110
I won't demonstrate it because
it's so clear to explain it.

00:27:49.110 --> 00:27:52.180
You can have someone
listen in headphones

00:27:52.180 --> 00:27:54.800
and you can make-- let's see.

00:27:54.800 --> 00:27:59.560
An ITD such that when
that's presented alone,

00:27:59.560 --> 00:28:03.180
the sound is perceived as if
it's coming from the left side.

00:28:05.280 --> 00:28:09.460
You can also present with those
headphones the same sound,

00:28:09.460 --> 00:28:12.020
but make now in
this case an ILD.

00:28:13.290 --> 00:28:16.880
So the sound is higher
on the right side

00:28:16.880 --> 00:28:19.350
so that it sounds like it's
coming from the right side.

00:28:20.510 --> 00:28:22.730
So now we have the
time cues making

00:28:22.730 --> 00:28:24.590
you think it's
coming from the left.

00:28:24.590 --> 00:28:27.020
The intensity cues
making you think

00:28:27.020 --> 00:28:28.970
it's coming from the right.

00:28:28.970 --> 00:28:31.510
When you put those
two things together,

00:28:31.510 --> 00:28:35.730
which you can do
artificially with headphones,

00:28:35.730 --> 00:28:38.410
you find in some
cases that the sound

00:28:38.410 --> 00:28:40.480
sounds like it's coming
from straight ahead.

00:28:42.530 --> 00:28:46.800
And this is called trading
for time and intensity,

00:28:46.800 --> 00:28:49.090
or time/intensity trading.

00:28:49.090 --> 00:28:51.840
And you can balance
one with the other.

00:28:51.840 --> 00:28:54.512
If you do a lot of
sound level on one side

00:28:54.512 --> 00:28:56.220
and just a little time
on the other side,

00:28:56.220 --> 00:28:58.420
it sounds like it's
off to the right.

00:28:59.570 --> 00:29:02.360
If you do them equal, it sounds
like it's straight ahead.

00:29:02.360 --> 00:29:06.360
If you do a lot of time, a
big ITD and a little ILD,

00:29:06.360 --> 00:29:08.880
it sounds like it's a
little off to the left.

00:29:08.880 --> 00:29:12.220
It's a very clear
psychophysical phenomena.

00:29:12.220 --> 00:29:16.700
Where do you find neural
responses that correlate with

00:29:16.700 --> 00:29:18.560
time/ intensity trading?

00:29:18.560 --> 00:29:23.640
Well, you find it in places like
the inferior colliculus, where

00:29:23.640 --> 00:29:27.815
inputs from the LSO and
the MSO first converge.

00:29:29.540 --> 00:29:34.420
So both LSO and MSO project up
into the inferior colliculus.

00:29:35.480 --> 00:29:39.010
And here, you have
the kind of responses

00:29:39.010 --> 00:29:40.250
that are shown on this graph.

00:29:42.740 --> 00:29:48.300
This x-axis is now a time
axis, except it's ITD.

00:29:50.390 --> 00:29:54.270
One side is the inhibitory lead.

00:29:56.780 --> 00:30:03.610
The sound is delayed so that the
ITD is from the opposite ear.

00:30:03.610 --> 00:30:06.650
This is the side
so that the time

00:30:06.650 --> 00:30:08.950
is delayed from the
ear on the same side

00:30:08.950 --> 00:30:12.070
as the inferior colliculus
you're recording from.

00:30:12.070 --> 00:30:16.820
And clearly, this neuron
is sensitive to that ITD

00:30:16.820 --> 00:30:18.720
if you vary it.

00:30:18.720 --> 00:30:21.310
This is the percent
neural response.

00:30:21.310 --> 00:30:25.710
100% would be up here, 0
response would be up here.

00:30:25.710 --> 00:30:29.590
Now, what's varied
as the parameter here

00:30:29.590 --> 00:30:31.483
is the interaural
level difference.

00:30:32.760 --> 00:30:35.110
So in this case, the
contralateral ear

00:30:35.110 --> 00:30:38.390
is-- it looks like 35.

00:30:38.390 --> 00:30:40.273
And the ipsilateral ear is 45.

00:30:41.510 --> 00:30:45.490
They've kept the contralateral
ear the same for the most part.

00:30:45.490 --> 00:30:48.170
And the ipsilateral
ear level has changed.

00:30:48.170 --> 00:30:51.840
And clearly, changing
the level also

00:30:51.840 --> 00:30:54.780
has a big effect on
the neural response.

00:30:54.780 --> 00:30:58.130
So here, for the first time
in the auditory pathway, where

00:30:58.130 --> 00:31:04.490
we're finding ITD and
ILD responses together

00:31:04.490 --> 00:31:07.590
in single neurons--
in a big way.

00:31:07.590 --> 00:31:11.100
You find them in a small
way in the MSO and the LSO.

00:31:11.100 --> 00:31:14.100
But this is a huge effect here.

00:31:14.100 --> 00:31:17.600
And probably, there
is the first place

00:31:17.600 --> 00:31:21.900
where you might have neural
correlates of your perception

00:31:21.900 --> 00:31:24.100
for time/intensity trading.

00:31:28.070 --> 00:31:32.550
So that's one phenomenon I
want to cover that probably

00:31:32.550 --> 00:31:35.420
has a neural
correlate at the level

00:31:35.420 --> 00:31:37.030
of the inferior colliculus.

00:31:37.030 --> 00:31:38.200
And here's another one.

00:31:39.900 --> 00:31:44.180
We've been dealing with
very simple stimuli that

00:31:44.180 --> 00:31:47.770
have just, say,
one ITD or one ILD.

00:31:49.660 --> 00:31:53.570
When you get into a room, it
becomes much more complicated

00:31:53.570 --> 00:31:58.250
because of echoes off
the walls and ceiling

00:31:58.250 --> 00:32:00.110
and floor of the room.

00:32:00.110 --> 00:32:02.750
And there's some very
interesting experiments

00:32:02.750 --> 00:32:06.990
that are done with more natural
stimuli like you find in rooms.

00:32:06.990 --> 00:32:08.940
And we'll go over
just a few of them.

00:32:08.940 --> 00:32:13.930
So here-- and this
is an observer

00:32:13.930 --> 00:32:17.220
listening to a sound source
off to the observer's left.

00:32:18.240 --> 00:32:20.900
And the direct sound
is indicated by A here.

00:32:22.850 --> 00:32:28.820
And most of these data deal with
the interaural time difference

00:32:28.820 --> 00:32:30.870
because this is a
fairly low frequency.

00:32:32.060 --> 00:32:35.390
This interaural time difference
of the direct sound indicated

00:32:35.390 --> 00:32:40.090
by this big arrow in the air
here favors the left ear.

00:32:40.090 --> 00:32:44.380
It's going to be arriving
at the left ear first

00:32:44.380 --> 00:32:46.420
and the right ear
a little bit later.

00:32:46.420 --> 00:32:50.310
So if you just had that
sound, the subject,

00:32:50.310 --> 00:32:52.340
obviously, would
report that the sound

00:32:52.340 --> 00:32:53.705
is coming from the left side.

00:32:55.060 --> 00:33:04.260
And this is a plot of the sound
for the left and right ear

00:33:04.260 --> 00:33:08.590
if you just had this
arrow in A here.

00:33:08.590 --> 00:33:12.630
And in green, it's the
interaural time difference

00:33:12.630 --> 00:33:14.590
for just that direct sound.

00:33:14.590 --> 00:33:19.080
So you can see it's
a fairly stable ITD

00:33:19.080 --> 00:33:22.330
and it's a negative
ITD just by convention.

00:33:23.430 --> 00:33:26.400
Left ear is going to
be negative here, ITD.

00:33:26.400 --> 00:33:28.680
And this is an ITD
in milliseconds

00:33:28.680 --> 00:33:30.215
of 0.4 milliseconds.

00:33:31.500 --> 00:33:34.220
And remember, we said if it was
directly off to the left side,

00:33:34.220 --> 00:33:37.247
it would be about 0.6.

00:33:37.247 --> 00:33:38.955
If it were straight
ahead, it would be 0.

00:33:40.120 --> 00:33:42.480
So what's the sound
stimulus here?

00:33:42.480 --> 00:33:45.720
Well, this is, again,
changing things a little bit.

00:33:45.720 --> 00:33:48.650
We've been talking about very
simple pure tone stimuli,

00:33:48.650 --> 00:33:49.185
or clicks.

00:33:51.080 --> 00:33:54.805
This paper is from
a German group.

00:33:56.090 --> 00:34:02.380
And French or German speakers
have this lovely speech sound,

00:34:02.380 --> 00:34:08.199
which is called like a
trill or rolled R. And I,

00:34:08.199 --> 00:34:11.120
for the life of me,
cannot do this stimulus.

00:34:11.120 --> 00:34:12.020
But I'll try.

00:34:12.020 --> 00:34:14.260
It's something like [ROLLING R].

00:34:14.260 --> 00:34:19.790
As if you were to pronounce
the word in German "reich."

00:34:19.790 --> 00:34:21.540
Or in French, [INAUDIBLE].

00:34:23.780 --> 00:34:26.469
It's impossible for me
to do that because I

00:34:26.469 --> 00:34:28.040
don't speak those languages.

00:34:28.040 --> 00:34:30.860
But anyway, here's
the sound stimulus

00:34:30.860 --> 00:34:32.929
with all the echoes added in.

00:34:34.850 --> 00:34:37.679
And one of these traits just
shows the left ear input

00:34:37.679 --> 00:34:38.694
and the right ear input.

00:34:39.780 --> 00:34:43.449
And these peaks here
are the trills of the R.

00:34:43.449 --> 00:34:47.169
You can see them of just the
left ear input as considered.

00:34:47.169 --> 00:34:50.620
So it's a trill
of the R. And you

00:34:50.620 --> 00:34:53.639
can see how many
milliseconds happened

00:34:53.639 --> 00:34:56.270
between each of those
parts of the trill.

00:34:56.270 --> 00:34:59.030
Maybe like 40
milliseconds or so.

00:35:01.740 --> 00:35:05.750
Now, when the subject
is in a normal room,

00:35:05.750 --> 00:35:10.060
a lot more happens than
just the direct sound.

00:35:10.060 --> 00:35:13.380
Some of the sound comes
from the source, the R here,

00:35:13.380 --> 00:35:16.880
and bounces off the wall
to the subject's right.

00:35:18.070 --> 00:35:20.700
Some of the sound goes
beyond the subject's head

00:35:20.700 --> 00:35:23.320
and bounces off the
wall to the right,

00:35:23.320 --> 00:35:26.880
and then the wall behind the
subject and comes back there.

00:35:28.730 --> 00:35:34.760
And this drawing
here is when you

00:35:34.760 --> 00:35:40.080
take into account several of
these reflections in addition

00:35:40.080 --> 00:35:41.115
to the direct sound.

00:35:43.020 --> 00:35:47.825
And the overall ITD is
still plotted in green.

00:35:49.530 --> 00:35:51.330
And look what a mess it is.

00:35:51.330 --> 00:35:53.120
The ITD is all over the place.

00:35:54.743 --> 00:35:59.440
It starts out like it should
from just the direct sound.

00:35:59.440 --> 00:36:00.930
It's negative here.

00:36:00.930 --> 00:36:03.140
But then real quickly,
it goes past 0.

00:36:03.140 --> 00:36:06.120
And it goes way up here
and it bounces around,

00:36:06.120 --> 00:36:07.485
then it goes back down again.

00:36:09.350 --> 00:36:10.670
It's all over the map.

00:36:12.130 --> 00:36:15.080
So where do you
think that subject

00:36:15.080 --> 00:36:17.140
says the sound is coming from?

00:36:18.430 --> 00:36:21.390
Do you think the subject
says, I can't tell,

00:36:21.390 --> 00:36:22.390
it's all over the place?

00:36:25.810 --> 00:36:28.120
Well, you've been
in rooms and you've

00:36:28.120 --> 00:36:30.760
listened to speakers in rooms.

00:36:30.760 --> 00:36:32.680
You're hearing me
right now and there's

00:36:32.680 --> 00:36:38.330
a lot of sound coming off the
walls to get into your ears

00:36:38.330 --> 00:36:39.890
on both sides.

00:36:39.890 --> 00:36:44.330
If you add up the
reflections from the walls

00:36:44.330 --> 00:36:47.390
to the side and beyond,
it's a lot greater

00:36:47.390 --> 00:36:50.790
than the direct sound in
terms of the total energy.

00:36:50.790 --> 00:36:53.100
But you can close
your eyes and you

00:36:53.100 --> 00:36:56.170
can know that I'm standing
up here to this side of you.

00:36:56.170 --> 00:36:59.670
You don't have any
trouble with that.

00:36:59.670 --> 00:37:03.430
And if you just do a
very careful experiment

00:37:03.430 --> 00:37:05.190
with this subject,
the direct sound,

00:37:05.190 --> 00:37:07.200
and a couple of
reflections like here.

00:37:07.200 --> 00:37:10.080
And you have the
subject with a pointer.

00:37:10.080 --> 00:37:12.650
That subject will say the
speaker is over there.

00:37:12.650 --> 00:37:13.980
It's on the left side.

00:37:13.980 --> 00:37:17.520
It's not behind or
to the wrong side.

00:37:18.750 --> 00:37:20.000
So how do we do that?

00:37:21.460 --> 00:37:25.510
Well, there is something
called the precedence effect,

00:37:25.510 --> 00:37:27.108
which is very important here.

00:37:29.740 --> 00:37:31.780
Which helps you in
a situation where

00:37:31.780 --> 00:37:33.085
you have lots of reflections.

00:37:36.420 --> 00:37:37.950
And what does precedence mean?

00:37:37.950 --> 00:37:40.555
Well, precedence means
something is dominating

00:37:40.555 --> 00:37:42.335
or something is
the most important.

00:37:44.840 --> 00:37:50.710
And in these cases, if
you look very carefully

00:37:50.710 --> 00:37:52.960
at the complex sound,
when you add up

00:37:52.960 --> 00:37:57.000
all the reflections
with the direct sound,

00:37:57.000 --> 00:37:59.900
you can see, if you have
really good eyesight,

00:37:59.900 --> 00:38:05.260
that right at the beginning of
the sort of burst of energy,

00:38:05.260 --> 00:38:08.910
the first thing to get
to the subject ears

00:38:08.910 --> 00:38:10.615
is coming from the left side.

00:38:12.310 --> 00:38:15.270
So the left ear input here
right at the beginning

00:38:15.270 --> 00:38:18.480
of these trills starts out.

00:38:20.010 --> 00:38:21.850
There's a bigger left ear input.

00:38:21.850 --> 00:38:25.820
And that's why you
have a negative ITD

00:38:25.820 --> 00:38:28.030
right at the
beginning of the trill

00:38:28.030 --> 00:38:29.375
before it starts going crazy.

00:38:31.250 --> 00:38:34.160
It turns out that the
most important part

00:38:34.160 --> 00:38:38.110
of this signal for localizing
where the sound comes from

00:38:38.110 --> 00:38:40.910
is in the initial
few milliseconds.

00:38:40.910 --> 00:38:44.750
So subjects bias
their impression

00:38:44.750 --> 00:38:46.990
of where the sound
is coming from right

00:38:46.990 --> 00:38:48.690
at the beginning of the sound.

00:38:48.690 --> 00:38:53.250
And they tend to suppress or
ignore all the remaining parts

00:38:53.250 --> 00:38:56.125
of the sound, at least
for localization purposes.

00:38:57.320 --> 00:39:02.350
So what takes precedence
here is the very initial part

00:39:02.350 --> 00:39:04.410
of the sound signal.

00:39:04.410 --> 00:39:06.420
And that's what the
precedence effect is.

00:39:07.610 --> 00:39:12.460
It's sometimes called by a
different name when you're

00:39:12.460 --> 00:39:15.720
dealing with speech--
the Haas effect.

00:39:18.950 --> 00:39:20.470
But the precedence
effect is more

00:39:20.470 --> 00:39:22.650
general for any type of sound.

00:39:22.650 --> 00:39:24.590
And it really means
that when you're

00:39:24.590 --> 00:39:28.290
in a complex environment with
all sorts of reflections,

00:39:28.290 --> 00:39:32.490
you pay attention to the
very first, or initial, sound

00:39:32.490 --> 00:39:33.590
and ignore the others.

00:39:34.770 --> 00:39:42.030
So the precedence effect
can be studied carefully

00:39:42.030 --> 00:39:47.930
by just narrowing it down to a
direct sound and one reflection

00:39:47.930 --> 00:39:50.120
by this kind of setup here.

00:39:50.120 --> 00:39:55.690
So here's a subject
listening to two speakers

00:39:55.690 --> 00:39:57.590
in an anechoic room.

00:39:57.590 --> 00:40:00.710
And I think we've talked
about anechoic rooms before.

00:40:00.710 --> 00:40:02.870
Anechoic rooms
have these baffles

00:40:02.870 --> 00:40:05.780
on the walls and the floor.

00:40:05.780 --> 00:40:10.090
This person is seated
in a chair in a mesh,

00:40:10.090 --> 00:40:12.240
so he doesn't fall
into the baffles

00:40:12.240 --> 00:40:15.690
below him, which are
also absorbing sound

00:40:15.690 --> 00:40:17.570
on the floor of the room.

00:40:17.570 --> 00:40:19.400
There are baffles in
the ceiling as well.

00:40:19.400 --> 00:40:21.410
So whatever sound
this is presented

00:40:21.410 --> 00:40:22.655
goes right to the subject.

00:40:23.800 --> 00:40:28.200
If it goes beyond him, it
goes to this wall over here

00:40:28.200 --> 00:40:30.160
and is completely absorbed.

00:40:30.160 --> 00:40:31.480
So there are no reflections.

00:40:31.480 --> 00:40:33.040
There's just the direct sound.

00:40:34.800 --> 00:40:37.970
So you can say, well, one of
these is the direct sound.

00:40:37.970 --> 00:40:39.940
And a little bit
later, I'm going

00:40:39.940 --> 00:40:41.740
to introduce a
second sound coming

00:40:41.740 --> 00:40:45.700
from somewhere else,
which is the echo.

00:40:46.740 --> 00:40:51.250
So this might simulate
a wall on this side

00:40:51.250 --> 00:40:53.320
of the subject that reflects.

00:40:53.320 --> 00:40:54.940
In this case, there's
no reflection.

00:40:54.940 --> 00:40:58.840
But you can say this is an
artificial echo presented

00:40:58.840 --> 00:41:00.735
by this second loudspeaker.

00:41:02.190 --> 00:41:06.216
What happens if we change
the timing between those two

00:41:06.216 --> 00:41:06.715
sounds?

00:41:07.900 --> 00:41:09.375
And that's what's plotted here.

00:41:10.940 --> 00:41:14.100
This is the delay
between the first sound

00:41:14.100 --> 00:41:15.730
and the second sound
in milliseconds.

00:41:17.110 --> 00:41:20.470
Now, for ITDS and
sound localization,

00:41:20.470 --> 00:41:27.030
remember we've been talking
about ITDs way down here from 0

00:41:27.030 --> 00:41:30.000
to about 0.6 milliseconds.

00:41:30.000 --> 00:41:35.510
That's where you have sound
localization way down here.

00:41:35.510 --> 00:41:40.570
And when you do a delay
that's that short,

00:41:40.570 --> 00:41:43.345
it sounds to a subject like
there's just one sound.

00:41:45.580 --> 00:41:48.900
If the ITD is at
exactly 0, this subject

00:41:48.900 --> 00:41:50.710
will perceive the
sound source being

00:41:50.710 --> 00:41:52.705
directly in between
these two speakers.

00:41:55.510 --> 00:41:59.660
As the lagging sound
gets greater and greater,

00:41:59.660 --> 00:42:04.540
the subject will start to
perceive that the source goes

00:42:04.540 --> 00:42:07.180
to the original or
first sound emitted.

00:42:09.930 --> 00:42:14.210
And when the delay comes
to the maximal delay

00:42:14.210 --> 00:42:16.040
for the size of the
human head, which

00:42:16.040 --> 00:42:19.080
is about 0.6
milliseconds right here,

00:42:19.080 --> 00:42:21.260
the subject will
perceive that it's only

00:42:21.260 --> 00:42:23.405
coming from that
initial loudspeaker.

00:42:25.780 --> 00:42:30.530
Then, as delays get
further and further on,

00:42:30.530 --> 00:42:33.090
up between about
0.6 milliseconds

00:42:33.090 --> 00:42:35.850
and a little over
10 milliseconds,

00:42:35.850 --> 00:42:39.490
the subject will still say,
I still hear one sound.

00:42:39.490 --> 00:42:41.860
And it's still coming
from that speaker.

00:42:41.860 --> 00:42:45.040
But now it's starting to
sound a little bit different.

00:42:45.040 --> 00:42:48.080
In fact, it doesn't
sound as dead anymore.

00:42:48.080 --> 00:42:51.540
It sounds like I'm in
a pretty live room.

00:42:51.540 --> 00:42:52.770
So what's a live room?

00:42:52.770 --> 00:42:57.015
A room that's reverberant,
like a church or a cathedral.

00:42:57.015 --> 00:42:58.345
It sounds roomy.

00:42:58.345 --> 00:42:59.595
It sounds like there's volume.

00:43:01.650 --> 00:43:06.130
Over here, this is the region
called the precedence effect,

00:43:06.130 --> 00:43:09.170
where you ignore
that lagging sound.

00:43:09.170 --> 00:43:11.420
And it still sounds like
there's just one sound,

00:43:11.420 --> 00:43:13.160
but it sounds different.

00:43:13.160 --> 00:43:17.002
This is the region of the
precedence effect from 0.6

00:43:17.002 --> 00:43:19.570
to maybe 10 milliseconds or so.

00:43:19.570 --> 00:43:23.570
Then, as the delay
becomes longer and longer,

00:43:23.570 --> 00:43:26.520
you start to
perceive two sounds.

00:43:26.520 --> 00:43:29.680
You hear, let's say, a
click from the first speaker

00:43:29.680 --> 00:43:33.760
and a second click a little bit
later from the second speaker.

00:43:33.760 --> 00:43:36.320
And now, the delay
is long enough

00:43:36.320 --> 00:43:37.870
so you actually hear an echo.

00:43:37.870 --> 00:43:41.210
You hear two sounds, an
initial sound and an echo.

00:43:43.310 --> 00:43:45.550
So that's the perception.

00:43:45.550 --> 00:43:48.195
And there's this big region here
called the precedence effect.

00:43:49.390 --> 00:43:51.640
So I have a
demonstration, if you

00:43:51.640 --> 00:43:54.520
don't believe me, about echoes.

00:43:54.520 --> 00:43:57.770
And the demonstration is
really vivid, I think.

00:43:59.260 --> 00:44:04.160
The echoes become more-- you
don't hear the echoes here

00:44:04.160 --> 00:44:06.520
because it's a
precedence effect.

00:44:06.520 --> 00:44:08.540
Maybe it sounds a
little bit more roomy,

00:44:08.540 --> 00:44:10.140
but you don't hear an echo.

00:44:10.140 --> 00:44:12.719
But there definitely
are echoes there.

00:44:12.719 --> 00:44:14.260
You may not be able
to perceive them.

00:44:15.440 --> 00:44:18.110
So let me play
this demonstration.

00:44:18.110 --> 00:44:20.480
The demonstration is--
I think, the best part

00:44:20.480 --> 00:44:23.590
of the demonstration is
someone taking a brick

00:44:23.590 --> 00:44:25.800
and hitting it with a hammer.

00:44:25.800 --> 00:44:28.160
And that makes a
big click, right?

00:44:28.160 --> 00:44:32.447
Well, after that big
sound, that impact sound,

00:44:32.447 --> 00:44:33.280
there's some echoes.

00:44:35.750 --> 00:44:39.100
You can't hear them
very well except when

00:44:39.100 --> 00:44:42.130
they do this neat trick on
this demonstration, which

00:44:42.130 --> 00:44:44.445
is to play the sound
recording backwards.

00:44:45.900 --> 00:44:48.540
And then, the
echoes start first,

00:44:48.540 --> 00:44:50.900
and then you hear the
hammer hitting the brick.

00:44:52.820 --> 00:44:55.120
They also have
some text on here.

00:44:55.120 --> 00:44:58.660
And they read the text, and then
they play the text backward.

00:44:58.660 --> 00:45:00.480
To me, that's not so obvious.

00:45:00.480 --> 00:45:03.080
So they do this demonstration
of hitting the brick.

00:45:03.080 --> 00:45:07.110
They do it in an anechoic
room right here first.

00:45:07.110 --> 00:45:07.900
There's no echoes.

00:45:09.140 --> 00:45:11.620
Second, they do it
in a normal room,

00:45:11.620 --> 00:45:15.170
like this room where
there's some reverberation.

00:45:15.170 --> 00:45:17.020
But a lot of the
reverberation is

00:45:17.020 --> 00:45:19.410
stopped by the
carpet on the floor

00:45:19.410 --> 00:45:21.670
and the clothes I'm wearing.

00:45:21.670 --> 00:45:25.470
And the seat cushions
absorb some of the echoes.

00:45:25.470 --> 00:45:27.890
Then finally, they do this
demonstration a third time.

00:45:27.890 --> 00:45:29.290
And they do that backwards, too.

00:45:29.290 --> 00:45:32.960
Then they do it a third time
in a very reverberant room,

00:45:32.960 --> 00:45:36.610
like a church or a cathedral,
where you hit the brick

00:45:36.610 --> 00:45:38.940
and it just sort of
rings for quite a ways.

00:45:40.350 --> 00:45:42.320
So let's play this
demonstration and see

00:45:42.320 --> 00:45:45.324
if it lives up to
my description.

00:45:45.324 --> 00:45:45.990
[AUDIO PLAYBACK]

00:45:45.990 --> 00:45:47.982
-[INAUDIBLE] echoes.

00:45:47.982 --> 00:45:50.970
First, in an anechoic room.

00:45:50.970 --> 00:45:52.962
Then, in a conference room.

00:45:52.962 --> 00:45:55.950
Then finally, in a
very reverberant space.

00:45:55.950 --> 00:45:59.436
You will hear a hammer
striking a brick followed

00:45:59.436 --> 00:46:02.424
by an old Scottish [INAUDIBLE].

00:46:02.424 --> 00:46:04.914
Playing these sounds
backwards focuses

00:46:04.914 --> 00:46:07.902
our attention on the
echoes that occur.

00:46:12.890 --> 00:46:16.005
From ghoulies and ghosties,
and long-leggedy beasties

00:46:16.005 --> 00:46:19.900
and things that go bump in the
night, good lord deliver us.

00:46:19.900 --> 00:46:21.742
PROFESSOR: OK, that's forward.

00:46:21.742 --> 00:46:22.450
This is backward.

00:46:24.414 --> 00:46:31.775
-[SPEAKING BACKWARD]

00:46:31.775 --> 00:46:33.400
PROFESSOR: OK, now
the conference room.

00:46:40.750 --> 00:46:44.670
-From ghoulies and ghosties,
and long-leggedy beasties

00:46:44.670 --> 00:46:48.590
and things that go bump in the
night, good lord deliver us.

00:46:53.000 --> 00:46:59.537
[SPEAKING BACKWARD]

00:46:59.537 --> 00:47:01.620
PROFESSOR: All right, now
in the reverberant room.

00:47:10.380 --> 00:47:14.104
-From ghoulies and ghosties,
and long-leggedy beasties

00:47:14.104 --> 00:47:17.527
and things that go bump in the
night, good lord deliver us.

00:47:23.395 --> 00:47:31.010
[SPEAKING BACKWARD]

00:47:31.010 --> 00:47:34.445
[END AUDIO PLAYBACK]

00:47:34.445 --> 00:47:39.040
PROFESSOR: All right, so I
like especially the sound

00:47:39.040 --> 00:47:42.370
of the hammer hitting the
brick played backwards

00:47:42.370 --> 00:47:46.070
in the reverberant room
because it's going to pssew.

00:47:46.070 --> 00:47:49.460
And all that pssew
leading up to the impact

00:47:49.460 --> 00:47:52.650
is the echo that you
just completely discount

00:47:52.650 --> 00:47:57.030
because of the precedence
effect in the normal hearing.

00:47:59.490 --> 00:48:04.805
OK, so that brings me
up to the special part

00:48:04.805 --> 00:48:06.305
of my lecture, which
is the reading.

00:48:07.660 --> 00:48:12.480
And this little quotation
is by a musician.

00:48:13.660 --> 00:48:17.540
And of course, musicians
love reverberant rooms,

00:48:17.540 --> 00:48:21.140
like churches or cathedrals,
or concert halls, or whatever.

00:48:21.140 --> 00:48:25.730
So E. Power Biggs, who was
the organist at Harvard

00:48:25.730 --> 00:48:28.480
for a long time, made
many famous recordings

00:48:28.480 --> 00:48:31.800
of organ music said,
"An organist will

00:48:31.800 --> 00:48:35.190
take all the reverberation
time he is given,

00:48:35.190 --> 00:48:40.220
and then ask for a bit more,
for ample reverberation is

00:48:40.220 --> 00:48:42.640
part of organ music itself.

00:48:42.640 --> 00:48:45.780
Many of Bach's organ works
are designed actually

00:48:45.780 --> 00:48:47.143
to exploit reverberation.

00:48:48.520 --> 00:48:52.150
Consider the pause that follows
the ornamented proclamation

00:48:52.150 --> 00:48:56.260
that opens the famous 'Toccata
in D Minor.' Obviously,

00:48:56.260 --> 00:48:59.970
this is for the enjoyment of the
notes as they remain suspended

00:48:59.970 --> 00:49:02.710
in the air."

00:49:02.710 --> 00:49:06.286
So musicians love
reverberations.

00:49:07.410 --> 00:49:13.620
And that's the reason that
halls where we appreciate music,

00:49:13.620 --> 00:49:17.980
like Boston Symphony Hall--
and they have some measurements

00:49:17.980 --> 00:49:22.300
of reverberation time
here for opera houses.

00:49:22.300 --> 00:49:25.256
1.3 seconds.

00:49:26.260 --> 00:49:30.410
OK, that's the time the
echoes take to decay.

00:49:31.700 --> 00:49:34.625
Symphony Hall in
Birmingham, 2.4 seconds.

00:49:35.760 --> 00:49:42.090
St. Paul's Cathedral in London,
13 seconds reverberation time.

00:49:42.090 --> 00:49:46.110
Now in contrast, when you
have theaters for speech,

00:49:46.110 --> 00:49:48.580
like Shakespeare
drama theaters, you

00:49:48.580 --> 00:49:50.480
don't want all those
reverberations.

00:49:50.480 --> 00:49:53.360
You want, for example,
a theater for speech

00:49:53.360 --> 00:49:57.470
here is quoted at having
a reverb time of 0.9

00:49:57.470 --> 00:50:00.300
seconds because you don't
want all these echoes

00:50:00.300 --> 00:50:04.135
to interfere with your
interpretation of the speech.

00:50:06.370 --> 00:50:10.800
And average living
room, 0.4 seconds.

00:50:10.800 --> 00:50:13.830
Reverberation time is given
for the great outdoors.

00:50:13.830 --> 00:50:14.668
Anybody guess?

00:50:17.108 --> 00:50:18.506
0.0.

00:50:18.506 --> 00:50:19.960
All right.

00:50:19.960 --> 00:50:23.185
The outdoors has no
walls, ceiling, or floor.

00:50:25.770 --> 00:50:27.650
Now, why are we talking
about this here?

00:50:27.650 --> 00:50:31.840
Because in the
inferior colliculus,

00:50:31.840 --> 00:50:36.163
you find some neurons that
show precedence-like responses.

00:50:37.700 --> 00:50:42.475
And so here is a recording from
an inferior colliculus neuron.

00:50:43.520 --> 00:50:47.400
And this was a study where
they used infants and adults,

00:50:47.400 --> 00:50:50.405
but these are just the
data for the adult.

00:50:51.580 --> 00:50:55.500
And the stimuli are two sounds.

00:50:55.500 --> 00:50:56.560
I think they are clicks.

00:50:57.950 --> 00:51:01.532
The first sound starts
at time 10 milliseconds.

00:51:01.532 --> 00:51:03.680
So this is the time axis.

00:51:03.680 --> 00:51:05.490
And this is the
dot raster display.

00:51:05.490 --> 00:51:07.320
So each little
dot here-- they're

00:51:07.320 --> 00:51:09.710
a little bit hard to
see-- is a neural spike.

00:51:10.880 --> 00:51:12.870
And there are many, many trials.

00:51:12.870 --> 00:51:15.120
OK, perhaps 50 or 100 trials.

00:51:15.120 --> 00:51:18.670
And you can see reliably on
all the trials, the neuron

00:51:18.670 --> 00:51:20.260
responded to the first stimulus.

00:51:22.090 --> 00:51:24.010
When the second
stimulus occurred

00:51:24.010 --> 00:51:28.370
at 101 millisecond
delayed, the neuron

00:51:28.370 --> 00:51:31.050
also faithfully responded
to the second stimulus.

00:51:32.500 --> 00:51:36.100
But as the delay was
shortened less and less,

00:51:36.100 --> 00:51:40.130
the neuron eventually stopped
responding for a delay

00:51:40.130 --> 00:51:42.840
that's-- in this case,
it looks like about 20

00:51:42.840 --> 00:51:43.903
or so milliseconds.

00:51:48.680 --> 00:51:50.930
Well, this is certainly
precedence-like behavior.

00:51:52.270 --> 00:51:56.655
That the neural response to the
second stimulus is attenuated.

00:51:58.280 --> 00:52:00.630
This delay, where
this particular neuron

00:52:00.630 --> 00:52:03.770
starts cutting out,
is not exactly where

00:52:03.770 --> 00:52:07.447
we stop losing the
precedence effect in humans.

00:52:07.447 --> 00:52:08.405
This is from an animal.

00:52:09.580 --> 00:52:14.380
And this animal is anesthetized,
so many of the processes

00:52:14.380 --> 00:52:16.920
are slowed down by anesthesia.

00:52:16.920 --> 00:52:19.360
And perhaps the animal
is a little cool,

00:52:19.360 --> 00:52:22.600
which might make these
things abnormally long.

00:52:22.600 --> 00:52:27.060
But certainly, this kind of
precedence-like responses

00:52:27.060 --> 00:52:30.270
are on the way toward
explaining the precedence effect

00:52:30.270 --> 00:52:32.207
at the level of the
inferior colliculus.

00:52:35.340 --> 00:52:37.570
And this is the work
of Ruth Litovsky

00:52:37.570 --> 00:52:39.675
from University of Wisconsin.

00:52:42.650 --> 00:52:46.300
OK, so now I want to
shift gears a little bit

00:52:46.300 --> 00:52:53.830
and go onto a different
species, which is the barn owl.

00:52:53.830 --> 00:52:56.880
And this is mostly the
work of-- originally,

00:52:56.880 --> 00:52:58.345
Mark Konishi at Caltech.

00:52:59.480 --> 00:53:03.645
And now, Eric Knudsen at
Stanford in California.

00:53:05.150 --> 00:53:08.430
And why did they choose
to study the barn owl?

00:53:08.430 --> 00:53:12.350
So many of us study
mammalian models

00:53:12.350 --> 00:53:16.930
because we want to know what
human hearing is all about.

00:53:16.930 --> 00:53:21.650
And it's hard to record
from human brains,

00:53:21.650 --> 00:53:24.890
but we want to choose an
animal that's like the human.

00:53:24.890 --> 00:53:25.780
So we use a mammal.

00:53:25.780 --> 00:53:28.030
Why did they choose
the barn owl?

00:53:29.130 --> 00:53:31.570
A lot of successes
in neuroscience

00:53:31.570 --> 00:53:33.410
have been choosing
an animal that's

00:53:33.410 --> 00:53:35.560
specialized for a certain task.

00:53:35.560 --> 00:53:37.905
And barn owls are very
specialized for hearing.

00:53:39.310 --> 00:53:44.730
So if you take a barn
owl and blindfold the owl

00:53:44.730 --> 00:53:49.200
and turn a mouse
loose in a room.

00:53:49.200 --> 00:53:51.840
As long as the mouse is
making a little bit of sound,

00:53:51.840 --> 00:53:54.370
the barn owl can quickly
fly over to the mouse

00:53:54.370 --> 00:53:56.350
and catch it and eat it.

00:53:56.350 --> 00:54:01.290
So the prey of barn owls
are mice and insects,

00:54:01.290 --> 00:54:04.465
like grasshoppers that
are down on the floor.

00:54:05.490 --> 00:54:09.850
And if you've ever seen or
watched a barn owl hunting,

00:54:09.850 --> 00:54:12.850
you can clearly
see them at night.

00:54:12.850 --> 00:54:14.710
They don't come
out during the day.

00:54:14.710 --> 00:54:16.080
But at night they come out.

00:54:16.080 --> 00:54:21.340
I watched one a lot of evenings
when I was in California.

00:54:21.340 --> 00:54:24.140
The owl would come and
sit on a basketball hoop.

00:54:24.140 --> 00:54:26.050
And it would just perch there.

00:54:26.050 --> 00:54:28.095
And it would move
its head all around.

00:54:28.919 --> 00:54:31.210
And I didn't really know what
it was doing at the time,

00:54:31.210 --> 00:54:36.530
until later, when I read some
of this work and it said that

00:54:36.530 --> 00:54:39.890
owls' eyes cannot
move in their head.

00:54:39.890 --> 00:54:41.210
The owl's eyes are fixed.

00:54:42.660 --> 00:54:46.950
So this beautiful control
of the eyeball position

00:54:46.950 --> 00:54:50.175
that we have in
mammalian eye control

00:54:50.175 --> 00:54:52.670
is not present in most birds.

00:54:52.670 --> 00:54:54.965
And certainly is not
present in the owl.

00:54:54.965 --> 00:54:58.000
So to move eyes, you
have to move the head.

00:54:58.000 --> 00:55:01.280
And of course, you're
moving the ears as well.

00:55:01.280 --> 00:55:03.450
In the barn owl,
and in other words,

00:55:03.450 --> 00:55:06.990
you don't have an external
pinna, which we said

00:55:06.990 --> 00:55:10.120
introduces a lot of help
to localizing sounds.

00:55:10.120 --> 00:55:12.200
But the barn owl
is a unique bird

00:55:12.200 --> 00:55:16.900
in that there is some
external stuff going on,

00:55:16.900 --> 00:55:19.630
which is called
the facial rough.

00:55:19.630 --> 00:55:25.240
And that's formed by feathers
on the face of the owl.

00:55:25.240 --> 00:55:29.250
You can see they're sort of
like fan-shaped down here.

00:55:29.250 --> 00:55:34.350
And over here, they
go over the ear canal.

00:55:34.350 --> 00:55:36.215
There's an ear canal, certainly.

00:55:36.215 --> 00:55:41.490
They go over the ear canal
in this perioral flap.

00:55:41.490 --> 00:55:44.420
And the opening for sound
to get in is below that.

00:55:44.420 --> 00:55:46.170
And also, above it
there's an opening.

00:55:47.229 --> 00:55:48.770
So the barn owl
doesn't have a pinna,

00:55:48.770 --> 00:55:51.360
but it has some
modified feathers

00:55:51.360 --> 00:55:52.475
on the front of its face.

00:55:54.320 --> 00:55:56.955
Barn owl feathers are
also interesting that they

00:55:56.955 --> 00:55:57.720
are modified.

00:55:57.720 --> 00:56:00.430
The feathers on the
wings are modified.

00:56:00.430 --> 00:56:03.100
When this owl took off, when I
watched it on that basketball

00:56:03.100 --> 00:56:05.890
hoop, every now and then it
would take off and go down

00:56:05.890 --> 00:56:08.120
to the basketball court below.

00:56:08.120 --> 00:56:09.960
And you couldn't hear anything.

00:56:09.960 --> 00:56:14.430
Owl feathers are
specifically designed so even

00:56:14.430 --> 00:56:19.270
the air going over them
when the owl flaps its wings

00:56:19.270 --> 00:56:23.780
is completely silent because the
owls don't want whatever it's

00:56:23.780 --> 00:56:26.700
hunting to hear
them approaching it.

00:56:26.700 --> 00:56:28.930
So barn owl wing
feathers are specifically

00:56:28.930 --> 00:56:31.250
designed to be
acoustically silent.

00:56:35.150 --> 00:56:39.590
OK, so a lot of work has been
done on the barn owl pathway.

00:56:39.590 --> 00:56:41.630
Now, this is a
little bit different

00:56:41.630 --> 00:56:47.640
because if you've looked at
the brains of avian species,

00:56:47.640 --> 00:56:49.050
they're a little bit different.

00:56:49.050 --> 00:56:50.765
They evolve differently
than mammals.

00:56:51.780 --> 00:56:54.040
But they have some
analogous nuclei.

00:56:55.830 --> 00:56:58.100
Here are the cochlear nuclei.

00:56:58.100 --> 00:57:00.350
That's with this
little text here.

00:57:00.350 --> 00:57:03.030
This is supposed to be the
owl pathway on the left side

00:57:03.030 --> 00:57:04.430
and the right side of the brain.

00:57:04.430 --> 00:57:07.260
This is the midline
in dashed lines.

00:57:07.260 --> 00:57:12.940
The owl cochlear nuclei are
split up into two parts.

00:57:12.940 --> 00:57:15.740
And the one that's featured
here is labeled NM.

00:57:18.700 --> 00:57:27.790
So NM stands for
Nucleus Magnocellularis.

00:57:36.240 --> 00:57:39.030
And so we can all figure
out what this means.

00:57:39.030 --> 00:57:42.850
Cellularlaris means the
cell or the nerve cells.

00:57:42.850 --> 00:57:44.640
Magno means big.

00:57:44.640 --> 00:57:46.580
These are the big cells.

00:57:46.580 --> 00:57:50.050
So there's some other parts
of the bird cochlear nuclei

00:57:50.050 --> 00:57:51.620
where the cells are smaller.

00:57:51.620 --> 00:57:54.955
But this is the
big-- big cell part.

00:57:56.180 --> 00:57:58.930
And in nucleus
magnocellularis, you

00:57:58.930 --> 00:58:01.405
have beautiful phase locking.

00:58:03.450 --> 00:58:06.350
We talked about
that being typical

00:58:06.350 --> 00:58:08.770
of the mammalian pathway.

00:58:08.770 --> 00:58:11.120
In the cochlear
nucleus, the bushy cells

00:58:11.120 --> 00:58:13.260
have good phase locking.

00:58:13.260 --> 00:58:15.310
Maybe even better than
the auditory nerve.

00:58:16.930 --> 00:58:20.960
In the owl, the
nucleus magnocellularis

00:58:20.960 --> 00:58:23.140
neurons have excellent
phase locking.

00:58:23.140 --> 00:58:26.430
And so they're keeping track
of the stimulus waveform.

00:58:26.430 --> 00:58:28.210
The timing is important to them.

00:58:30.070 --> 00:58:32.420
They project centrally.

00:58:35.360 --> 00:58:37.960
The one on the left side and
the one on the right side

00:58:37.960 --> 00:58:42.330
converge onto a nucleus
that's sensitive to interaural

00:58:42.330 --> 00:58:43.256
time differences.

00:58:44.480 --> 00:58:47.250
That's the avian
equivalent of the MSO.

00:58:49.930 --> 00:59:02.031
And it's called NL,
Nucleus Laminaris.

00:59:05.260 --> 00:59:06.910
OK, lamina means sheet.

00:59:07.816 --> 00:59:08.815
OK, and this is a sheet.

00:59:10.194 --> 00:59:14.160
It looks like that
anyway in the anatomy.

00:59:14.160 --> 00:59:16.840
And there, the neurons
are sensitive to ITDs.

00:59:18.160 --> 00:59:21.065
And there's a beautiful
Jeffress model there.

00:59:22.570 --> 00:59:25.870
Most of the papers
on the mammalian MSO

00:59:25.870 --> 00:59:28.500
say, we know there's
a beautiful Jeffress

00:59:28.500 --> 00:59:32.960
model in the avian
nucleus laminaris.

00:59:32.960 --> 00:59:35.540
But in the mammal, we're
starting to rethink it.

00:59:36.820 --> 00:59:38.650
This is a beautiful
Jeffress model

00:59:38.650 --> 00:59:46.416
where you find
neural responses that

00:59:46.416 --> 00:59:50.010
are very strongly peaked to ITD.

00:59:50.010 --> 00:59:51.840
So they fire for a certain ITD.

00:59:53.100 --> 00:59:54.360
This is the firing rate.

00:59:58.260 --> 01:00:01.150
But don't fire much
at all to other ITDs.

01:00:01.150 --> 01:00:03.260
They're strongly tuned to ITD.

01:00:04.830 --> 01:00:09.620
The nucleus laminaris in turn
projects across the midline

01:00:09.620 --> 01:00:12.376
here to the inferior colliculus.

01:00:14.230 --> 01:00:17.120
And we haven't talked
about it, but there

01:00:17.120 --> 01:00:20.250
are several subregions of
the inferior colliculus.

01:00:21.610 --> 01:00:23.160
The big one is called the ICC.

01:00:24.530 --> 01:00:28.600
And that's called the
Inferior Colliculus Central

01:00:28.600 --> 01:00:30.840
part, or central nucleus.

01:00:30.840 --> 01:00:32.250
That's true in mammals as well.

01:00:32.250 --> 01:00:34.180
That's the big part.

01:00:34.180 --> 01:00:35.445
Some would call it the core.

01:00:36.700 --> 01:00:39.440
That's what's indicated
here, the core

01:00:39.440 --> 01:00:40.565
of the inferior colliculus.

01:00:42.720 --> 01:00:46.790
And it, in turn,
projects to other places,

01:00:46.790 --> 01:00:48.155
like the lateral part.

01:00:50.310 --> 01:00:56.230
And it finally projects from
the lateral part to the ICX.

01:01:00.910 --> 01:01:04.800
And the ICX is the
Inferior Colliculus.

01:01:06.050 --> 01:01:07.823
And X stands for External.

01:01:11.170 --> 01:01:13.875
So the external part of
the inferior colliculus.

01:01:15.100 --> 01:01:18.450
And there is where some
very interesting responses

01:01:18.450 --> 01:01:20.147
take place in the barn owl.

01:01:21.520 --> 01:01:23.500
And we'll look at those
responses right now.

01:01:30.760 --> 01:01:33.270
So these experiments
were first done

01:01:33.270 --> 01:01:43.640
by Mark Konishi in, I
believe, the 1970s at Caltech.

01:01:43.640 --> 01:01:45.590
And the experimental
subject is seen.

01:01:47.060 --> 01:01:48.740
Here is the barn owl right here.

01:01:50.280 --> 01:01:51.430
And where are his wings?

01:01:51.430 --> 01:01:54.040
Well, his wings are folded down.

01:01:54.040 --> 01:01:58.830
And he's in a little, sort of
like a tube sock, if you will.

01:01:58.830 --> 01:01:59.940
His wings are constrained.

01:01:59.940 --> 01:02:05.350
And the tube sock is mounted
on a pole or a pedestal.

01:02:05.350 --> 01:02:06.710
And he's just sitting there.

01:02:06.710 --> 01:02:07.740
He can move his head.

01:02:09.320 --> 01:02:11.090
Actually, in many of
these experiments,

01:02:11.090 --> 01:02:12.692
the head is clamped.

01:02:12.692 --> 01:02:13.650
But he's sitting there.

01:02:13.650 --> 01:02:14.150
He's awake.

01:02:14.917 --> 01:02:16.125
He's listening to the sounds.

01:02:18.300 --> 01:02:20.375
And the sound is
presented by a speaker.

01:02:21.860 --> 01:02:23.465
The speaker is on a big hoop.

01:02:24.740 --> 01:02:25.810
You see that hoop?

01:02:27.040 --> 01:02:29.320
And the speaker is
sitting on the hoop.

01:02:29.320 --> 01:02:31.900
And there's a little
motor in the speaker.

01:02:31.900 --> 01:02:35.540
And the speaker can be moved
by driving the motor over here.

01:02:36.634 --> 01:02:39.070
Or you can move the motor
the other direction.

01:02:39.070 --> 01:02:42.970
You can move the speaker over
here or down here or up here,

01:02:42.970 --> 01:02:45.820
wherever you want to on
that hoop the speaker can

01:02:45.820 --> 01:02:46.930
be driven by the motor.

01:02:47.990 --> 01:02:53.120
And because the hoop is mounted
on two posts on the side,

01:02:53.120 --> 01:02:57.000
the whole hoop can be swung
up or it can be swung down.

01:02:58.580 --> 01:03:03.380
OK, so you can put that speaker
anywhere this way and anywhere

01:03:03.380 --> 01:03:05.370
up or down that you want to.

01:03:05.370 --> 01:03:08.840
And so you can put that speaker
in the entire-- any position

01:03:08.840 --> 01:03:14.080
you want to in the entire
frontal hemi field of the owl.

01:03:14.080 --> 01:03:16.120
And I suppose they
didn't do that.

01:03:16.120 --> 01:03:20.570
I suppose you could put it in
the rear hemi field as well.

01:03:20.570 --> 01:03:25.260
But these data are just from the
frontal hemi field of the owl.

01:03:27.550 --> 01:03:33.230
And since the owl's head is
mounted and is not moving,

01:03:33.230 --> 01:03:35.350
you can apply a little
local anesthetic

01:03:35.350 --> 01:03:36.990
and open up the skull.

01:03:36.990 --> 01:03:42.440
And you can advance an electrode
into the ICX, the External

01:03:42.440 --> 01:03:46.380
Nucleus of the
Inferior Colliculus,

01:03:46.380 --> 01:03:48.900
and make recordings from
single neurons there.

01:03:51.170 --> 01:03:54.120
And in this case, the
recordings are made from the ICX

01:03:54.120 --> 01:03:55.050
on the right side.

01:03:58.800 --> 01:04:02.200
And what's found
in this response

01:04:02.200 --> 01:04:07.980
plot here is these neurons have
restricted receptive fields

01:04:07.980 --> 01:04:08.480
in space.

01:04:10.090 --> 01:04:13.960
So what's plotted here is
the neuro-responsive field

01:04:13.960 --> 01:04:17.940
in dashed lines and
the most vigorous part

01:04:17.940 --> 01:04:19.920
in the shaded area there.

01:04:21.180 --> 01:04:24.750
And this axis, the
x-axis, is azimuth.

01:04:26.202 --> 01:04:28.740
And the y-axis is elevation.

01:04:31.560 --> 01:04:36.850
And this is a fairly restrictive
part of the whole hemi field.

01:04:36.850 --> 01:04:40.810
And one of these-- this is one
response area for one neuron.

01:04:41.910 --> 01:04:45.410
And a whole bunch
of these rectangles

01:04:45.410 --> 01:04:48.810
are plotted down here with
a little diagram of the owl

01:04:48.810 --> 01:04:50.955
right in the center
of this globe.

01:04:53.420 --> 01:04:57.130
So there's 1, 2, 3,
4, 5, 6, 7, 8, 9,

01:04:57.130 --> 01:05:02.090
10-- about a dozen neural
receptive fields for ICX

01:05:02.090 --> 01:05:04.150
neurons in that owl.

01:05:05.240 --> 01:05:08.710
Notice that they're
all fairly discrete.

01:05:08.710 --> 01:05:11.520
That is, it only
responds when the speaker

01:05:11.520 --> 01:05:17.630
is in a certain part of the
frontal field of the owl.

01:05:19.790 --> 01:05:22.360
Notice also that we're
recording from the right side.

01:05:23.390 --> 01:05:29.630
And most of the receptive fields
are off to the left of the owl.

01:05:29.630 --> 01:05:31.850
They're on the opposite side.

01:05:31.850 --> 01:05:35.770
So clearly, sound
stimuli on one side

01:05:35.770 --> 01:05:39.880
are mapped to the
opposite brain side.

01:05:39.880 --> 01:05:42.630
That's not true for
these three, but they're

01:05:42.630 --> 01:05:43.660
close to the midline.

01:05:45.930 --> 01:05:49.930
Most of the receptive fields
are not straight ahead.

01:05:49.930 --> 01:05:51.890
They're actually
down below the owl.

01:05:52.900 --> 01:05:55.427
There's one or two that are
just a little bit above,

01:05:55.427 --> 01:05:57.010
but there are none
that are way above.

01:05:57.010 --> 01:05:58.710
Most of them are
down below the owl.

01:05:59.720 --> 01:06:03.360
Remember, the owl is sitting
on the basketball hoop

01:06:03.360 --> 01:06:05.990
and he's listening for
targets down below.

01:06:07.240 --> 01:06:11.030
This makes a lot of sense that
most of the receptive fields

01:06:11.030 --> 01:06:15.790
are down below the owl, not
at the same level of the owl.

01:06:15.790 --> 01:06:18.750
It doesn't care about
a mouse making a slam

01:06:18.750 --> 01:06:20.270
dunk at the other hoop.

01:06:20.270 --> 01:06:25.320
It cares about the mouse
down on the basketball court.

01:06:28.540 --> 01:06:31.590
And finally, maybe the most
important and most interesting

01:06:31.590 --> 01:06:36.980
part of these responses
is the progression

01:06:36.980 --> 01:06:38.870
of where the
receptive fields are

01:06:38.870 --> 01:06:43.010
in space versus where they
are along the dimensions

01:06:43.010 --> 01:06:43.760
of the ICX.

01:06:44.990 --> 01:06:50.820
So that this receptive field was
located over here in the ICX.

01:06:51.940 --> 01:06:55.000
And as you move this way and
encountered a different neuron,

01:06:55.000 --> 01:06:57.330
it was located over here
and its receptive field

01:06:57.330 --> 01:07:00.230
was moved over
this way a little.

01:07:00.230 --> 01:07:02.850
As you move further
in that direction,

01:07:02.850 --> 01:07:05.680
you encountered another
receptive field even further

01:07:05.680 --> 01:07:07.200
in this dimension.

01:07:07.200 --> 01:07:10.930
And then finally, way
over here laterally,

01:07:10.930 --> 01:07:13.180
you encountered these
receptive fields

01:07:13.180 --> 01:07:14.505
that were way off to the side.

01:07:15.780 --> 01:07:20.460
So along this dimension
of auditory space,

01:07:20.460 --> 01:07:23.260
receptive fields were
found along this dimension

01:07:23.260 --> 01:07:24.920
of the ICX.

01:07:24.920 --> 01:07:27.740
There was also a
mapping going this way.

01:07:29.220 --> 01:07:33.120
This is clearly then what
some people call a space map.

01:07:33.120 --> 01:07:37.410
A mapping of auditory
space into position

01:07:37.410 --> 01:07:41.830
within a certain part, the
ICX, of the owl's brain.

01:07:43.160 --> 01:07:45.040
OK, it's a beautiful mapping.

01:07:45.040 --> 01:07:47.750
Neuroscientists love mappings.

01:07:47.750 --> 01:07:51.480
You've heard me talk
about tonotopic mappings

01:07:51.480 --> 01:07:52.590
out the wazoo.

01:07:52.590 --> 01:07:55.790
We love tonotopic mappings
because CF is then

01:07:55.790 --> 01:07:56.430
very important.

01:07:58.520 --> 01:08:03.134
This clearly is important
to have organization

01:08:03.134 --> 01:08:05.300
of the receptive fields on
a dimension in the brain.

01:08:06.470 --> 01:08:11.530
People have spent many,
many years looking

01:08:11.530 --> 01:08:16.960
for these kind of mappings in
the mammalian auditory pathway

01:08:16.960 --> 01:08:18.529
and not found them.

01:08:20.109 --> 01:08:22.800
People have looked in
the inferior colliculus,

01:08:22.800 --> 01:08:26.600
in the analogous part,
the central part,

01:08:26.600 --> 01:08:28.180
the external part.

01:08:28.180 --> 01:08:31.660
It is hard to record
from the external part

01:08:31.660 --> 01:08:33.930
of the mammalian
inferior colliculus

01:08:33.930 --> 01:08:35.899
because it's a small
edge structure,

01:08:35.899 --> 01:08:37.900
but it has not been found.

01:08:38.930 --> 01:08:42.460
People have looked in
the medial geniculate,

01:08:42.460 --> 01:08:47.100
in the auditory cortex,
and looked for organization

01:08:47.100 --> 01:08:51.520
of spatial receptive
fields and not found them.

01:08:51.520 --> 01:08:56.170
So on nuclei in the main
part of the auditory pathway,

01:08:56.170 --> 01:09:00.149
you do not find space maps
in the mammalian system.

01:09:00.149 --> 01:09:04.139
So the one place you
find spatial organization

01:09:04.139 --> 01:09:11.100
in the mammal is in the
mammalian superior colliculus.

01:09:19.688 --> 01:09:20.979
And you're probably going, huh?

01:09:20.979 --> 01:09:23.990
I thought you just said the
superior colliculus is visual?

01:09:25.010 --> 01:09:25.700
Well, it is.

01:09:25.700 --> 01:09:29.609
But it's a layered structure
that, if I'm not mistaken,

01:09:29.609 --> 01:09:32.040
the top three layers
are exclusively visual.

01:09:32.040 --> 01:09:35.520
But if you go down to lower
layers, the bottom layers

01:09:35.520 --> 01:09:38.670
of the superior colliculus,
you start to encounter neurons

01:09:38.670 --> 01:09:42.710
that respond to visual as
well as auditory stimuli.

01:09:43.910 --> 01:09:47.730
And you may have talked
about the visual mapping

01:09:47.730 --> 01:09:48.855
of the superior colliculus.

01:09:50.160 --> 01:09:52.840
And those neurons
in the deep layers

01:09:52.840 --> 01:09:56.740
that are also responsive
to auditory stimuli--

01:09:56.740 --> 01:09:59.270
they're mostly ILD sensitive.

01:09:59.270 --> 01:10:03.770
They're mapped in line with
the visual receptive fields.

01:10:03.770 --> 01:10:07.860
They're also space mapped
to a certain extent.

01:10:07.860 --> 01:10:11.730
Now, that nucleus clearly
is not on the main drag

01:10:11.730 --> 01:10:13.110
of the auditory pathway.

01:10:13.110 --> 01:10:16.910
The auditory pathway is cochlear
nucleus, superior olive,

01:10:16.910 --> 01:10:20.950
inferior colliculus, medial
geniculate and cortex.

01:10:20.950 --> 01:10:26.740
So you do have a space map
in the mammalian deep layers

01:10:26.740 --> 01:10:28.350
of the superior
colliculus, but not

01:10:28.350 --> 01:10:32.600
on the main parts of the
mammalian auditory pathway.

01:10:32.600 --> 01:10:34.210
So that's been the finding.

01:10:34.210 --> 01:10:36.700
Now, that's negative evidence.

01:10:36.700 --> 01:10:39.620
It's not clear that I
won't be back here teaching

01:10:39.620 --> 01:10:41.660
the course next year.

01:10:41.660 --> 01:10:43.245
And we'll read a
paper that says,

01:10:43.245 --> 01:10:45.210
ah, space map in wherever.

01:10:45.210 --> 01:10:48.770
But it hasn't been found so
far with this one exception.

01:10:51.650 --> 01:10:58.010
Now, the paper that we
read for today's class

01:10:58.010 --> 01:11:04.040
talks about-- back
to the barn owl.

01:11:05.810 --> 01:11:08.580
A place in the barn
owl, which is-- they

01:11:08.580 --> 01:11:11.300
call it the optic
tectum in the birds.

01:11:11.300 --> 01:11:15.000
But it's analogous to the
superior colliculus in mammals.

01:11:15.000 --> 01:11:20.930
A place where, as I said, you
find auditory spacing maps that

01:11:20.930 --> 01:11:23.375
are in line with
visual space maps.

01:11:24.450 --> 01:11:29.470
And they do a very interesting
and elegant experiment

01:11:29.470 --> 01:11:31.445
where one of those
maps is distorted.

01:11:33.220 --> 01:11:37.390
And you study the resulting
effect on the other map.

01:11:37.390 --> 01:11:38.810
OK, so how did they do that?

01:11:44.380 --> 01:11:48.400
So this is n owl, but
it's a juvenile owl.

01:11:48.400 --> 01:11:49.430
An owl chick.

01:11:51.320 --> 01:11:55.460
And it's wearing some
interesting things on its eyes.

01:11:55.460 --> 01:11:56.470
Those aren't its eyes.

01:11:56.470 --> 01:11:59.820
Those are some prisms
that the investigators

01:11:59.820 --> 01:12:03.530
have put on the owl's eyes.

01:12:03.530 --> 01:12:07.580
And they deflect the visual
field a certain amount

01:12:07.580 --> 01:12:09.520
depending on the
size of the prism.

01:12:10.620 --> 01:12:17.130
And I can't remember
what the deflection was.

01:12:17.130 --> 01:12:18.770
I seem to remember 30 degrees.

01:12:20.730 --> 01:12:25.960
So the visual field is
deflected 30 degrees.

01:12:25.960 --> 01:12:30.800
And as I said before, the owl's
eyes are fixed in the head.

01:12:30.800 --> 01:12:33.530
So putting on these
goggles, no matter

01:12:33.530 --> 01:12:37.540
what-- the goggles are going to
move if the owl moves its head.

01:12:37.540 --> 01:12:41.370
So no matter what the owl does,
the visual receptive fields

01:12:41.370 --> 01:12:43.420
of all these neurons,
everything in vision

01:12:43.420 --> 01:12:44.905
is shifted 30 degrees.

01:12:46.740 --> 01:12:47.945
This is normal.

01:12:49.700 --> 01:12:52.151
These might be receptive fields
from neurons in the brain

01:12:52.151 --> 01:12:52.650
somewhere.

01:12:53.720 --> 01:12:55.450
This is when the
prisms are added.

01:12:56.560 --> 01:13:01.470
Here, you've shifted the
visual receptive field.

01:13:01.470 --> 01:13:03.010
The auditory
receptive field-- you

01:13:03.010 --> 01:13:05.400
haven't changed the ears at all.

01:13:05.400 --> 01:13:07.225
The auditory receptive
field is the same.

01:13:08.820 --> 01:13:13.920
What's found when you do
that in juvenile owls,

01:13:13.920 --> 01:13:15.730
you come back eight weeks later.

01:13:15.730 --> 01:13:19.040
You find, oh my gosh, the
auditory receptive field

01:13:19.040 --> 01:13:20.820
has actually moved.

01:13:20.820 --> 01:13:22.230
It shifted.

01:13:22.230 --> 01:13:25.720
You knew where you ere
recording from in the brain.

01:13:25.720 --> 01:13:28.726
In this case, the recordings
are made in the optic tectum,

01:13:28.726 --> 01:13:29.850
in the superior colliculus.

01:13:31.460 --> 01:13:33.760
You know the dimensions
and you come back

01:13:33.760 --> 01:13:36.540
and you're expecting to see
auditory receptive fields

01:13:36.540 --> 01:13:39.670
like that, but
they've been shifted.

01:13:39.670 --> 01:13:43.480
So juvenile owls with
prism experience,

01:13:43.480 --> 01:13:46.170
given a number of
weeks to compensate,

01:13:46.170 --> 01:13:49.340
shift the auditory
receptive fields

01:13:49.340 --> 01:13:50.985
so that the two are
back in alignment.

01:13:52.910 --> 01:13:55.030
This group has also
shown that if you

01:13:55.030 --> 01:13:58.702
do this experiment
with adult owls,

01:13:58.702 --> 01:14:00.195
you don't get such shifts.

01:14:01.420 --> 01:14:05.860
You come back to an adult
owl with these same prisms,

01:14:05.860 --> 01:14:08.230
eight weeks later you
have still a mismatch.

01:14:09.260 --> 01:14:13.630
So plasticity clearly takes
place in the juveniles

01:14:13.630 --> 01:14:15.740
and not in the adults.

01:14:15.740 --> 01:14:20.230
And they've then likened
it to those old folks of us

01:14:20.230 --> 01:14:22.120
trying to learn a
foreign language.

01:14:22.120 --> 01:14:24.570
It's really tough because
we didn't have experience

01:14:24.570 --> 01:14:25.973
with it while we were juveniles.

01:14:27.860 --> 01:14:31.920
So neural plasticity and
these learning new things--

01:14:31.920 --> 01:14:37.130
if this is learning something,
it's a bit of a stretch.

01:14:37.130 --> 01:14:40.910
But it's more difficult to
learn things as an adult

01:14:40.910 --> 01:14:42.015
than it is juveniles.

01:14:43.720 --> 01:14:49.710
Now, the experiment-- an even
further twist in this paper

01:14:49.710 --> 01:14:55.525
that we read for today, which is
now the subject is adult owls.

01:14:55.525 --> 01:14:57.475
The recordings were
made from adult owls.

01:14:58.840 --> 01:15:00.300
But there are two groups.

01:15:00.300 --> 01:15:02.560
One is a plain, old
control adult owl.

01:15:03.840 --> 01:15:08.300
The other is an adult owl
that when it was a juvenile

01:15:08.300 --> 01:15:10.080
had experience with the prisms.

01:15:12.290 --> 01:15:15.415
That experience was long
ago, six months before.

01:15:17.240 --> 01:15:19.820
The prisms were on
for eight weeks or so.

01:15:20.840 --> 01:15:22.910
These shifts took place.

01:15:22.910 --> 01:15:25.000
The prisms were removed.

01:15:25.000 --> 01:15:27.630
The owl is allowed
to grow up and become

01:15:27.630 --> 01:15:29.550
an adult with normal vision.

01:15:31.310 --> 01:15:36.040
Then, take these two groups of
owls, put the prisms on again.

01:15:36.040 --> 01:15:39.910
We've already said that
adults don't have the capacity

01:15:39.910 --> 01:15:40.930
to remap.

01:15:42.270 --> 01:15:46.390
So many of them just
stayed the same.

01:15:46.390 --> 01:15:48.640
These are the open circles here.

01:15:48.640 --> 01:15:50.810
The auditory receptive
field is the same.

01:15:50.810 --> 01:15:52.740
This is before the prisms.

01:15:52.740 --> 01:15:54.180
This is eight weeks after.

01:15:54.180 --> 01:15:57.295
The open circles are
from the adult owls

01:15:57.295 --> 01:15:59.770
that didn't have any
juvenile prism experience.

01:16:01.050 --> 01:16:04.960
But the adult owls-- they're
now adults with the prisms on.

01:16:06.310 --> 01:16:09.480
The adult owls with the
juvenile prism experiences,

01:16:09.480 --> 01:16:11.790
one of the neurons
is recorded here.

01:16:11.790 --> 01:16:13.850
It has an auditory
receptive field

01:16:13.850 --> 01:16:17.110
that's shifted to
make it in line

01:16:17.110 --> 01:16:21.600
with the shifted visual
receptive fields.

01:16:21.600 --> 01:16:24.500
This is now showing
then that adults

01:16:24.500 --> 01:16:28.390
that have the
juvenile experience

01:16:28.390 --> 01:16:33.210
have some plastic ability to
re-map their auditory receptive

01:16:33.210 --> 01:16:36.870
field so that you have alignment
with the visual receptive

01:16:36.870 --> 01:16:37.370
fields.

01:16:38.550 --> 01:16:41.130
Clearly, a very
interesting experiment.

01:16:41.130 --> 01:16:43.720
Showing, if you will,
a type of maybe memory

01:16:43.720 --> 01:16:48.530
trace that these owls
have retained something

01:16:48.530 --> 01:16:51.580
that was altered by the
juvenile experience.

01:16:51.580 --> 01:16:53.170
So what could that be?

01:16:59.440 --> 01:17:02.480
The group has in
other studies gone on

01:17:02.480 --> 01:17:07.300
and looked at the projections
between these two boxes

01:17:07.300 --> 01:17:10.590
here from the central
nucleus of the colliculus

01:17:10.590 --> 01:17:12.160
to the external nucleus.

01:17:13.470 --> 01:17:17.030
And clearly, shown in juvenile
animals that have the prism

01:17:17.030 --> 01:17:19.650
experiences that the
axons that were headed

01:17:19.650 --> 01:17:24.880
for direct innervation
take a little change

01:17:24.880 --> 01:17:28.375
and they regrow some
axonal projections.

01:17:29.690 --> 01:17:31.570
They've studied these.

01:17:31.570 --> 01:17:38.070
And it looks like this regrowth
of axon between those two

01:17:38.070 --> 01:17:38.640
areas.

01:17:38.640 --> 01:17:41.410
And that's what was meant
by this little circle that

01:17:41.410 --> 01:17:46.610
says locus or site of
plasticity is manifested

01:17:46.610 --> 01:17:49.820
by a change in growth
of axons right there.

01:17:49.820 --> 01:17:52.560
And maybe those
adult animals that

01:17:52.560 --> 01:17:54.910
have juvenile
experience retain some

01:17:54.910 --> 01:18:00.490
of those axonal projections
that have been changed

01:18:00.490 --> 01:18:02.330
as a result of the experience.

01:18:02.330 --> 01:18:04.600
And clearly, it takes
many weeks for axons

01:18:04.600 --> 01:18:07.120
to grow or change
their connections.

01:18:07.120 --> 01:18:09.340
Maybe that's the thing
that's much easier

01:18:09.340 --> 01:18:12.620
to do if you're
a juvenile animal

01:18:12.620 --> 01:18:14.890
and you're reacting to
these changed stimuli.

01:18:17.550 --> 01:18:21.120
Another way to change
responses there

01:18:21.120 --> 01:18:24.330
is to have everything
connected to everything,

01:18:24.330 --> 01:18:28.780
but certain things emphasize
certain synapses very strong

01:18:28.780 --> 01:18:30.850
and the weak ones not emphasize.

01:18:30.850 --> 01:18:33.760
Maybe when you have
the prism experience,

01:18:33.760 --> 01:18:37.260
the previously
de-emphasized synapses

01:18:37.260 --> 01:18:41.670
become upregulated without
any change of axons.

01:18:41.670 --> 01:18:43.240
But clearly, this
group has shown

01:18:43.240 --> 01:18:46.440
that the axons have changed
their growth patterns.

01:18:46.440 --> 01:18:49.664
So that could be a mechanism
for the plasticity.

01:18:55.940 --> 01:18:59.990
And I think that's all
I wanted to say today.

01:18:59.990 --> 01:19:02.580
So I have five minutes
if you guys want

01:19:02.580 --> 01:19:05.960
to ask questions about anything.

01:19:05.960 --> 01:19:06.794
Yeah.

01:19:06.794 --> 01:19:09.214
AUDIENCE: Can you go
over how prisms actually

01:19:09.214 --> 01:19:11.150
change auditory
perception again?

01:19:11.150 --> 01:19:12.605
Like, how are the prisms--

01:19:12.605 --> 01:19:13.855
PROFESSOR: Back to precedence?

01:19:15.125 --> 01:19:16.000
Is that what you're--

01:19:16.000 --> 01:19:18.090
AUDIENCE: How are
prisms changing--

01:19:18.090 --> 01:19:19.610
PROFESSOR: How are prisms?

01:19:19.610 --> 01:19:20.340
AUDIENCE: Yeah.

01:19:20.340 --> 01:19:22.220
PROFESSOR: That's not clear.

01:19:22.220 --> 01:19:24.316
What's clear is that the
fields are mismatched.

01:19:27.396 --> 01:19:30.330
AUDIENCE: So we don't
know why that happens?

01:19:30.330 --> 01:19:31.580
PROFESSOR: That's correct.

01:19:31.580 --> 01:19:32.170
Yeah.

01:19:32.170 --> 01:19:37.940
But I presume what is
happening to the owl

01:19:37.940 --> 01:19:41.670
during these eight weeks is
the owl is seeing an object

01:19:41.670 --> 01:19:43.150
and hearing it as well.

01:19:43.150 --> 01:19:45.980
For example, a mouse down here.

01:19:45.980 --> 01:19:48.390
The owl sees it
and it goes for it.

01:19:48.390 --> 01:19:51.620
But actually, because its
visual fields are off,

01:19:51.620 --> 01:19:54.210
it goes over here and
the object is over here.

01:19:55.350 --> 01:19:59.060
But the auditory cues, if
it paid attention to them,

01:19:59.060 --> 01:20:00.870
it would go here.

01:20:00.870 --> 01:20:05.760
So it's sensing a
misalignment in experience.

01:20:05.760 --> 01:20:07.220
There's no training
involved here,

01:20:07.220 --> 01:20:11.530
but the owls are allowed
to hunt prey and experience

01:20:11.530 --> 01:20:13.450
environmental sounds.

01:20:13.450 --> 01:20:16.830
So they clearly then have
a mismatch between vision

01:20:16.830 --> 01:20:19.160
and audition in
these eight weeks.

01:20:20.457 --> 01:20:20.957
Yeah.

01:20:20.957 --> 01:20:22.332
AUDIENCE: Does
this sort of then,

01:20:22.332 --> 01:20:25.448
I guess suggest that the
auditory input is somehow

01:20:25.448 --> 01:20:26.945
more important?

01:20:26.945 --> 01:20:31.436
Because rather than say a visual
input shifting [INAUDIBLE]?

01:20:34.450 --> 01:20:35.250
PROFESSOR: Yes.

01:20:35.250 --> 01:20:36.695
Yes, you could say that.

01:20:38.210 --> 01:20:41.180
I mean, it would be interesting
to do the converse experiment.

01:20:41.180 --> 01:20:45.670
Keep the eyeballs normal
and somehow distort

01:20:45.670 --> 01:20:49.040
the auditory receptive fields.

01:20:49.040 --> 01:20:53.470
So you could do that with
ITDs by putting a tube

01:20:53.470 --> 01:20:56.050
and lengthening the
ITD on one side.

01:20:57.170 --> 01:21:01.550
That would be an interesting
sort of counter experiment

01:21:01.550 --> 01:21:04.260
to get at what
you're asking about.

01:21:04.260 --> 01:21:06.063
It would distort the other cue.

01:21:13.170 --> 01:21:14.090
OK, great.

01:21:14.090 --> 01:21:17.420
We'll see you back
on Wednesday then.