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PROFESSOR: Last time we were
talking about the hair cells.

9
00:00:28,440 --> 00:00:30,710
And there's a picture
of a hair cell here.

10
00:00:32,340 --> 00:00:35,710
And what did we do
about the hair cells?

11
00:00:35,710 --> 00:00:39,980
We talked about the two types,
inner hair cells-- plain old,

12
00:00:39,980 --> 00:00:42,630
ordinary receptor cells.

13
00:00:42,630 --> 00:00:44,310
And we talked about
outer hair cells,

14
00:00:44,310 --> 00:00:46,100
which have this
wonderful characteristic

15
00:00:46,100 --> 00:00:47,360
of electromotility.

16
00:00:48,550 --> 00:00:52,320
When their hair bundle
is bent back and forth,

17
00:00:52,320 --> 00:00:54,165
their internal
potential changes.

18
00:00:55,200 --> 00:00:57,845
When they're depolarized,
the cell shortens.

19
00:00:59,330 --> 00:01:02,280
And somehow this
mechanical shortening

20
00:01:02,280 --> 00:01:06,455
adds to the vibrations
of the organ of Corti

21
00:01:06,455 --> 00:01:08,860
and basilar membrane that
were set up by sound.

22
00:01:09,950 --> 00:01:12,930
And it amplifies
those vibrations

23
00:01:12,930 --> 00:01:14,790
so that the inner
hair cells then

24
00:01:14,790 --> 00:01:16,625
are responding to an
amplified vibration.

25
00:01:18,450 --> 00:01:20,100
And the outer hair
cells are then

26
00:01:20,100 --> 00:01:23,680
dubbed by the name of
the cochlear amplifier.

27
00:01:23,680 --> 00:01:29,290
Without the amplification,
you lose 40 to 60 dB

28
00:01:29,290 --> 00:01:31,920
of hearing, which
is a big amount.

29
00:01:31,920 --> 00:01:33,790
A large hearing
loss would result

30
00:01:33,790 --> 00:01:36,050
if you didn't have
outer hair cells

31
00:01:36,050 --> 00:01:38,480
or without their
electromotility, which

32
00:01:38,480 --> 00:01:42,850
were shown by deleting
the gene for Preston

33
00:01:42,850 --> 00:01:47,730
and testing the knockout
mouse, which had a 40 to 60

34
00:01:47,730 --> 00:01:48,725
dB hearing loss.

35
00:01:51,690 --> 00:01:54,120
So questions about that?

36
00:01:54,120 --> 00:01:56,620
Before we talked about
the two hair cells,

37
00:01:56,620 --> 00:02:00,130
we talked about vibration in the
cochlear, what of tuning curve

38
00:02:00,130 --> 00:02:01,330
is.

39
00:02:01,330 --> 00:02:06,520
In that case for tuning
of the basilar membrane,

40
00:02:06,520 --> 00:02:09,100
a particular point
along the cochlear--

41
00:02:09,100 --> 00:02:11,460
so you could bring your
measurement device to one

42
00:02:11,460 --> 00:02:15,890
particular place and measure
its tuning in response to sounds

43
00:02:15,890 --> 00:02:18,520
of different
frequencies, and show

44
00:02:18,520 --> 00:02:21,660
that a single
place vibrates very

45
00:02:21,660 --> 00:02:24,720
nicely to a
particular frequency.

46
00:02:24,720 --> 00:02:27,190
That is you don't have
to send in very much

47
00:02:27,190 --> 00:02:29,240
sound into the ear.

48
00:02:29,240 --> 00:02:31,920
But if you go off that
particular frequency,

49
00:02:31,920 --> 00:02:33,700
you have to boost
the sound a lot

50
00:02:33,700 --> 00:02:36,015
to get that one
place to vibrate.

51
00:02:39,090 --> 00:02:42,170
And we had the example
of the vibration patterns

52
00:02:42,170 --> 00:02:48,500
that low frequencies stimulated
the cochlear apex the most, way

53
00:02:48,500 --> 00:02:51,020
up near the top of
the snail shell.

54
00:02:51,020 --> 00:02:53,800
And middle frequencies
stimulated the middle.

55
00:02:53,800 --> 00:02:57,480
And high frequencies
stimulated the basal part.

56
00:02:59,040 --> 00:03:01,480
We'll be talking a lot
more about that frequency

57
00:03:01,480 --> 00:03:04,640
organization along
the cochlear today,

58
00:03:04,640 --> 00:03:07,860
when we talk about
auditory nerve fibers.

59
00:03:09,340 --> 00:03:11,005
So here's a roadmap for today.

60
00:03:12,060 --> 00:03:14,320
We're going to concentrate
on the auditory nerve.

61
00:03:15,960 --> 00:03:17,936
And I just put down
some numbers so I

62
00:03:17,936 --> 00:03:20,060
wouldn't forget to tell
you how many auditory nerve

63
00:03:20,060 --> 00:03:21,304
fibers there are.

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00:03:21,304 --> 00:03:25,690
There are approximately
30,000 auditory nerve fibers

65
00:03:25,690 --> 00:03:26,750
in humans.

66
00:03:26,750 --> 00:03:29,270
So that means in your
left ear you have 30,000.

67
00:03:29,270 --> 00:03:32,880
And in you're right ear you
have 30,000 sending messages

68
00:03:32,880 --> 00:03:34,950
from the ear to the brain.

69
00:03:34,950 --> 00:03:36,640
So that's a pretty
hefty number, right?

70
00:03:37,710 --> 00:03:41,110
How many optic
nerve fibers do you

71
00:03:41,110 --> 00:03:43,390
have or does a primate have?

72
00:03:43,390 --> 00:03:46,970
I'm sure Dr. Schiller
went over that number.

73
00:03:46,970 --> 00:03:49,880
We're pretty visual on animals.

74
00:03:51,470 --> 00:03:54,470
So our sense of vision
is well developed.

75
00:03:54,470 --> 00:03:58,310
So how many nerve fibers
go from the retina

76
00:03:58,310 --> 00:04:00,960
into the brain compared
to this number?

77
00:04:02,892 --> 00:04:03,600
Anybody remember?

78
00:04:05,100 --> 00:04:06,750
Well, that's a good
number to remember.

79
00:04:06,750 --> 00:04:11,610
It turns out there about 1
million optic nerve fibers

80
00:04:11,610 --> 00:04:13,200
from the retina into the brain.

81
00:04:13,200 --> 00:04:15,510
And here we have 30,000.

82
00:04:15,510 --> 00:04:19,470
So which is the most important
sense, vision or audition?

83
00:04:19,470 --> 00:04:23,390
Or which sense conveys
messages more efficiently,

84
00:04:23,390 --> 00:04:24,150
should we say?

85
00:04:26,310 --> 00:04:29,920
Well, obviously, primates
are very visual animals.

86
00:04:29,920 --> 00:04:33,831
So we have a lot more nerve
fibers sending messages

87
00:04:33,831 --> 00:04:35,830
into the brain about
vision than we do audition.

88
00:04:37,860 --> 00:04:41,440
So I may not have given you
numbers for the hair cells.

89
00:04:41,440 --> 00:04:46,250
In humans we have about
3,500 inner hair cells

90
00:04:46,250 --> 00:04:51,122
and about 12,000 outer
hair cells per cochlea.

91
00:04:51,122 --> 00:04:52,330
OK, so those are the numbers.

92
00:04:54,840 --> 00:04:59,110
So today, we'll talk about
the two types of nerve fibers.

93
00:04:59,110 --> 00:05:00,800
As we have two
types of hair cells,

94
00:05:00,800 --> 00:05:03,100
we have two types
of nerve fibers.

95
00:05:03,100 --> 00:05:05,250
We'll talk about
tuning curves now

96
00:05:05,250 --> 00:05:09,310
for the responses of
auditory nerve fibers.

97
00:05:09,310 --> 00:05:13,320
And we'll talk about
tonotopic organization.

98
00:05:13,320 --> 00:05:15,930
That is organization
of frequency

99
00:05:15,930 --> 00:05:18,960
to place within
the cochlea, which

100
00:05:18,960 --> 00:05:21,790
is one of the codes
for sound frequency.

101
00:05:21,790 --> 00:05:25,600
How do we know we're listening
to 1,000 Hertz and not 2,000

102
00:05:25,600 --> 00:05:28,660
Hertz by which place
along the cochlea

103
00:05:28,660 --> 00:05:31,380
and which group of auditory
nerve fibers is responding.

104
00:05:33,190 --> 00:05:35,190
Then we'll get away
from auditory nerve

105
00:05:35,190 --> 00:05:37,490
and have some listening
demonstrations.

106
00:05:37,490 --> 00:05:40,970
We'll see how good we
are at discriminating

107
00:05:40,970 --> 00:05:42,875
two frequencies that
are very close together.

108
00:05:44,170 --> 00:05:46,590
And we'll talk about
some tuning curves

109
00:05:46,590 --> 00:05:49,095
that are based on
psychophysical measure.

110
00:05:50,120 --> 00:05:50,970
That is listening.

111
00:05:52,190 --> 00:05:54,630
You can take some tuning curves
by just a human listener.

112
00:05:55,920 --> 00:05:57,970
Then we'll get back
to auditory nerve

113
00:05:57,970 --> 00:06:01,530
and talk about a different
code for sound frequency.

114
00:06:01,530 --> 00:06:05,780
That is the temporal code
for sound frequency, which

115
00:06:05,780 --> 00:06:08,580
involves a phenomenon
called phase

116
00:06:08,580 --> 00:06:10,340
locking of the auditory nerve.

117
00:06:11,430 --> 00:06:13,010
Then we'll talk
about how that's very

118
00:06:13,010 --> 00:06:17,070
important in your listening
to musical intervals.

119
00:06:17,070 --> 00:06:19,370
And the most important
musical interval

120
00:06:19,370 --> 00:06:22,380
is the octave, so we'll have
a demonstration of an octave.

121
00:06:25,280 --> 00:06:31,460
OK, so one of my problems as I
pass by the MIT Coop on the way

122
00:06:31,460 --> 00:06:34,360
to class, and I
always buy something.

123
00:06:34,360 --> 00:06:37,060
So I did a reading last week.

124
00:06:37,060 --> 00:06:39,800
And so we'll have a little
reading from this book,

125
00:06:39,800 --> 00:06:40,720
I Am Malala.

126
00:06:41,940 --> 00:06:46,460
She was the girl who was
shot right and recovered

127
00:06:46,460 --> 00:06:50,220
and was a candidate for
the Nobel Peace Prize.

128
00:06:50,220 --> 00:06:52,700
Maybe next year she'll
get the Peace Prize.

129
00:06:54,077 --> 00:06:54,910
I haven't read this.

130
00:06:54,910 --> 00:06:56,770
I just picked it up
a few minutes ago.

131
00:06:56,770 --> 00:06:59,186
But I went straight to the
section about her surgery.

132
00:07:00,290 --> 00:07:03,210
So she was shot in
the head on one side.

133
00:07:03,210 --> 00:07:05,390
And she said, "While
I was in surgery--"

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00:07:05,390 --> 00:07:07,670
this is after recovery.

135
00:07:07,670 --> 00:07:11,140
This is a further
surgery she underwent.

136
00:07:11,140 --> 00:07:14,050
"While I was in surgery,
Mr. Irving, the surgeon who

137
00:07:14,050 --> 00:07:18,330
had repaired my nerve--"
that's her facial nerve--

138
00:07:18,330 --> 00:07:22,120
"also had a solution for
my damaged left ear drum.

139
00:07:22,120 --> 00:07:26,200
He put a small electronic
device called a cochlear implant

140
00:07:26,200 --> 00:07:31,800
inside my head near the ear,
and told me that in a month

141
00:07:31,800 --> 00:07:34,610
they would fit the
external part on my head.

142
00:07:34,610 --> 00:07:37,570
And then I should be able
to hear from that ear."

143
00:07:37,570 --> 00:07:41,990
OK, so the cochlear
implant is a device

144
00:07:41,990 --> 00:07:45,000
that stimulates the
auditory nerve fibers.

145
00:07:45,000 --> 00:07:49,480
And in a person who's
had a gunshot wound--

146
00:07:49,480 --> 00:07:53,570
either because of the loud
sound or the mechanical trauma

147
00:07:53,570 --> 00:07:56,450
to the ear or temporal
bone-- possibly

148
00:07:56,450 --> 00:08:00,090
the hair cells are damaged
or are completely missing.

149
00:08:00,090 --> 00:08:02,020
And the auditory
nerve fibers remain.

150
00:08:03,450 --> 00:08:05,870
The person is deaf
without the hair cells.

151
00:08:05,870 --> 00:08:09,190
But the device called
the cochlear implant

152
00:08:09,190 --> 00:08:12,830
can be inserted inside
this person's cochlear

153
00:08:12,830 --> 00:08:14,675
to stimulate the auditory nerve.

154
00:08:15,990 --> 00:08:19,760
And we'll have a discussion of
the cochlear implant next week

155
00:08:19,760 --> 00:08:23,980
when we have a demonstrator
come to class who's deaf.

156
00:08:23,980 --> 00:08:25,960
And she'll show you
about her implant.

157
00:08:25,960 --> 00:08:29,810
But we do need to know a
lot about the auditory nerve

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00:08:29,810 --> 00:08:33,230
response before we can
really think about what

159
00:08:33,230 --> 00:08:36,500
is the good coding strategy
for cochlear implant.

160
00:08:36,500 --> 00:08:40,280
That is how do we take
the sound information

161
00:08:40,280 --> 00:08:43,039
and translate it
into the shocks that

162
00:08:43,039 --> 00:08:45,250
are provided by the cochlear
implant electrodes that

163
00:08:45,250 --> 00:08:47,300
stimulate the nerve fibers.

164
00:08:47,300 --> 00:08:50,630
Because little electric
currents in the cochlear implant

165
00:08:50,630 --> 00:08:53,610
are made to stimulate
the auditory nerve fibers

166
00:08:53,610 --> 00:08:55,745
that can then send
messages to the brain.

167
00:08:56,820 --> 00:08:59,130
So it's just a little
motivator for what's

168
00:08:59,130 --> 00:09:01,230
important about
auditory nerve code.

169
00:09:03,980 --> 00:09:08,780
So we'll start out today
with the hair cells.

170
00:09:08,780 --> 00:09:10,940
And these are the
auditory nerves here.

171
00:09:12,850 --> 00:09:16,150
One thing that's interesting
about vision and audition

172
00:09:16,150 --> 00:09:19,500
is the look of the
synapse between the hair

173
00:09:19,500 --> 00:09:24,820
cell and the nerve fiber, and
between the photoreceptor--

174
00:09:24,820 --> 00:09:27,520
you have rods and
cones in the retina.

175
00:09:27,520 --> 00:09:30,620
And they have associated
nerve terminals here.

176
00:09:30,620 --> 00:09:33,020
And these are
electron micrographs,

177
00:09:33,020 --> 00:09:36,630
taken with a very high powered
electron microscope that

178
00:09:36,630 --> 00:09:41,840
looks at the synapse between
the photoreceptor up here

179
00:09:41,840 --> 00:09:46,350
or the hair cell up here and the
associated nerve terminal down

180
00:09:46,350 --> 00:09:50,970
here, or the associated
either horizontal cell

181
00:09:50,970 --> 00:09:52,725
or bipolar cell down here.

182
00:09:53,990 --> 00:09:56,460
So in each case,
you have obviously

183
00:09:56,460 --> 00:09:58,290
the synapse here
is a little gap.

184
00:09:59,880 --> 00:10:03,200
And you have synaptic
vesicles that

185
00:10:03,200 --> 00:10:04,985
contain the neurotransmitter.

186
00:10:04,985 --> 00:10:07,500
And they're indicated
here in the photoreceptor.

187
00:10:07,500 --> 00:10:08,880
SV is the vesicle.

188
00:10:08,880 --> 00:10:11,075
And inside that vesicle
is the neurotransmitter.

189
00:10:12,330 --> 00:10:16,730
When the receptor
cell depolarizes,

190
00:10:16,730 --> 00:10:20,120
these synaptic vesicles
fuse and release

191
00:10:20,120 --> 00:10:24,920
their neurotransmitter
into the cleft and fire

192
00:10:24,920 --> 00:10:28,390
or activate their
post-synaptic element.

193
00:10:28,390 --> 00:10:32,060
In the case of the hair cell,
the auditory nerve fiber.

194
00:10:32,060 --> 00:10:37,270
This structure here is
called the synaptic ribbon.

195
00:10:37,270 --> 00:10:41,440
And it's supposed to coordinate
the release of the vesicles.

196
00:10:41,440 --> 00:10:44,380
And they call it a
ribbon in the hair cell

197
00:10:44,380 --> 00:10:47,460
here, even though it looks
like a big which ball.

198
00:10:47,460 --> 00:10:49,730
It doesn't look like
a ribbon at all.

199
00:10:49,730 --> 00:10:51,680
But it's called a
ribbon, because it

200
00:10:51,680 --> 00:10:53,230
has the same molecular basis.

201
00:10:53,230 --> 00:10:59,380
It has a lot of interesting
proteins and mechanisms

202
00:10:59,380 --> 00:11:02,390
to coordinate the release
of these neurotransmitter

203
00:11:02,390 --> 00:11:04,790
vesicles, which presumably
are synthesize up here

204
00:11:04,790 --> 00:11:08,450
in the cytoplasm and are
brought down to the ribbon

205
00:11:08,450 --> 00:11:13,290
and coordinated and released
at the hair cell to nerve fiber

206
00:11:13,290 --> 00:11:13,790
synapse.

207
00:11:13,790 --> 00:11:16,580
So I just wanted to
show you the look

208
00:11:16,580 --> 00:11:19,950
of the synapse in the
electron microscope.

209
00:11:19,950 --> 00:11:21,230
So that's what it looks like.

210
00:11:23,730 --> 00:11:29,440
And the next slide
here is this schematic

211
00:11:29,440 --> 00:11:33,360
of the two types of hair cells,
inner hair cells and the three

212
00:11:33,360 --> 00:11:36,185
rows of outer hair cells, and
their associated nerve fibers.

213
00:11:37,650 --> 00:11:41,310
And I think I mentioned
last time that almost all

214
00:11:41,310 --> 00:11:44,290
of the nerve fibers, the ones
that are sending messages

215
00:11:44,290 --> 00:11:47,350
to the brain at least, are
associated with the inner hair

216
00:11:47,350 --> 00:11:47,850
cells.

217
00:11:47,850 --> 00:11:50,430
So you can see how many
individual terminals there

218
00:11:50,430 --> 00:11:56,900
are-- as many as 20 on a
single inner hair cell.

219
00:11:56,900 --> 00:12:00,300
By contrast, the outer
hair cells-- you can see,

220
00:12:00,300 --> 00:12:02,370
well, this one
has three of them.

221
00:12:02,370 --> 00:12:04,520
But they're all coming
from the same fiber, which

222
00:12:04,520 --> 00:12:08,190
also innervates the
neighboring hair cells.

223
00:12:08,190 --> 00:12:11,030
So there are very few
of these so-called type

224
00:12:11,030 --> 00:12:13,686
two auditory nerve fibers.

225
00:12:13,686 --> 00:12:14,560
Here are the numbers.

226
00:12:14,560 --> 00:12:16,780
So this total is in cats.

227
00:12:16,780 --> 00:12:19,140
Cats have more nerve
fibers than humans,

228
00:12:19,140 --> 00:12:22,250
a total of maybe 50,000.

229
00:12:22,250 --> 00:12:28,110
About 45,000 of them are
the type ones, associated

230
00:12:28,110 --> 00:12:30,970
with inner hair
cells, and only 5,000

231
00:12:30,970 --> 00:12:33,645
or the type twos associated
with outer hair cells.

232
00:12:37,530 --> 00:12:39,870
So you can see by
this ratio then

233
00:12:39,870 --> 00:12:43,720
that most of the information
is being sent into the brain

234
00:12:43,720 --> 00:12:47,670
by the type one fibers, sending
messages from the inner hair

235
00:12:47,670 --> 00:12:48,170
cells.

236
00:12:49,740 --> 00:12:52,960
Those axons of the type
one fibers are thick.

237
00:12:54,410 --> 00:12:59,250
They have a myelin
covering, compared

238
00:12:59,250 --> 00:13:01,505
to the type two fibers,
which are very thin

239
00:13:01,505 --> 00:13:02,546
and they're unmyelinated.

240
00:13:04,370 --> 00:13:08,450
And actually, one of the very
interesting unknown facts

241
00:13:08,450 --> 00:13:13,860
about the auditory system
is that as far as we know,

242
00:13:13,860 --> 00:13:18,180
no recordings have ever been
made to sample the type two

243
00:13:18,180 --> 00:13:19,340
responses to sound.

244
00:13:20,501 --> 00:13:24,060
Do they respond to
different frequencies?

245
00:13:24,060 --> 00:13:25,147
Are they widely tuned?

246
00:13:25,147 --> 00:13:25,730
Narrowly tune?

247
00:13:25,730 --> 00:13:27,700
We don't know that at all.

248
00:13:27,700 --> 00:13:31,350
And it turns out that it's
just very difficult to sample

249
00:13:31,350 --> 00:13:36,745
from such thin axons as you
find in the type two fibers.

250
00:13:39,550 --> 00:13:43,610
So I actually have a grant
submitted to the National

251
00:13:43,610 --> 00:13:46,710
Institute of Health to use
a special type of electrodes

252
00:13:46,710 --> 00:13:49,050
to record from the type twos.

253
00:13:49,050 --> 00:13:51,550
I think it's being
reviewed next week.

254
00:13:51,550 --> 00:13:54,260
And I hope it gets funded
because then maybe I'll

255
00:13:54,260 --> 00:13:55,990
figure out this mystery.

256
00:13:57,540 --> 00:13:59,960
But it will be challenging
not only because they're thin,

257
00:13:59,960 --> 00:14:02,250
but because there
are fewer of them.

258
00:14:05,260 --> 00:14:09,160
So when I talk about auditory
nerve fiber recordings

259
00:14:09,160 --> 00:14:12,950
for this class, I'm going to
be talking about the type ones.

260
00:14:12,950 --> 00:14:14,700
That's the only kind we know of.

261
00:14:17,370 --> 00:14:22,220
And here is an example tuning
curve or receptive field

262
00:14:22,220 --> 00:14:24,330
for a type one
auditory nerve fiber.

263
00:14:26,180 --> 00:14:28,650
Now, I think Peter
Schiller probably

264
00:14:28,650 --> 00:14:32,960
talked about single
unit recordings

265
00:14:32,960 --> 00:14:36,275
with micro electrodes.

266
00:14:43,470 --> 00:14:44,600
So you have your nerve.

267
00:14:51,160 --> 00:14:52,700
It could be the optic nerve.

268
00:14:52,700 --> 00:14:56,270
It could be the
auditory nerve, which

269
00:14:56,270 --> 00:14:57,530
is what we're talking about.

270
00:14:59,482 --> 00:15:17,040
You have a microelectrode,
which is put into the nerve.

271
00:15:17,040 --> 00:15:18,820
And the tip of
the microelectrode

272
00:15:18,820 --> 00:15:20,230
is very, very tiny.

273
00:15:20,230 --> 00:15:23,440
It could be less than 1
micrometer in diameter.

274
00:15:25,790 --> 00:15:29,920
And usually the electrode
is filled with a conducting

275
00:15:29,920 --> 00:15:32,920
solution like
potassium chloride.

276
00:15:34,270 --> 00:15:37,830
And the pipette that's
filled with a KCL

277
00:15:37,830 --> 00:15:39,260
comes out to a big open end.

278
00:15:39,260 --> 00:15:45,450
And you can stick a wire in here
and run it to your amplifier

279
00:15:45,450 --> 00:15:48,520
and record the so-called spikes.

280
00:15:48,520 --> 00:15:51,410
You guys talked
about spikes, right?

281
00:15:51,410 --> 00:15:59,810
So you're recording the
spikes, AKA action potentials,

282
00:15:59,810 --> 00:16:01,186
AKA impulses.

283
00:16:03,330 --> 00:16:07,560
And if you want to do
this in a dramatic way,

284
00:16:07,560 --> 00:16:10,540
you send this signal
also to a loud speaker

285
00:16:10,540 --> 00:16:11,750
and you listen to them.

286
00:16:14,800 --> 00:16:16,560
And maybe we'll
have a demonstration

287
00:16:16,560 --> 00:16:18,610
at the end of the year on these.

288
00:16:18,610 --> 00:16:20,860
It's pretty nice
to listen to that.

289
00:16:20,860 --> 00:16:22,850
So you put your
electrode in there.

290
00:16:22,850 --> 00:16:26,230
And you move it
around until you have

291
00:16:26,230 --> 00:16:28,075
what's called a single unit.

292
00:16:34,320 --> 00:16:36,190
And why is it called
a single unit?

293
00:16:36,190 --> 00:16:38,490
Well, in the old
days, people didn't

294
00:16:38,490 --> 00:16:40,090
know what was being recorded.

295
00:16:40,090 --> 00:16:41,340
Is it a cell body?

296
00:16:43,440 --> 00:16:44,912
Is it a nerve axon?

297
00:16:44,912 --> 00:16:45,703
Is it the dendrite?

298
00:16:48,190 --> 00:16:50,200
What is it ?

299
00:16:50,200 --> 00:16:52,900
All they knew is that
coming out of the amplifier,

300
00:16:52,900 --> 00:16:54,735
they saw this spike.

301
00:17:00,430 --> 00:17:01,790
And that's what's plotted here.

302
00:17:01,790 --> 00:17:02,956
These are a bunch of spikes.

303
00:17:03,940 --> 00:17:06,890
And it's called a single
unit, because most

304
00:17:06,890 --> 00:17:09,260
of the time when you get
one of these recordings,

305
00:17:09,260 --> 00:17:11,109
the spikes look all the same.

306
00:17:11,109 --> 00:17:13,775
But every now and then you get a
recording that looks like this.

307
00:17:21,660 --> 00:17:26,340
And this is interpreted as
being fiber or axon number one.

308
00:17:27,369 --> 00:17:29,400
Here's another number one.

309
00:17:29,400 --> 00:17:32,510
And this is a second
fiber that's nearby.

310
00:17:32,510 --> 00:17:34,230
But it's a different one.

311
00:17:34,230 --> 00:17:37,429
Maybe there were
actually two fibers right

312
00:17:37,429 --> 00:17:38,220
next to each other.

313
00:17:38,220 --> 00:17:39,660
And you could
record both of them.

314
00:17:39,660 --> 00:17:40,695
That's very unusual.

315
00:17:41,730 --> 00:17:47,160
More commonly, you just have a
recording from one single unit.

316
00:17:47,160 --> 00:17:49,190
And the interpretation
is you are sampling

317
00:17:49,190 --> 00:17:56,840
from just one auditory nerve
fiber out of a total of 40,000.

318
00:17:56,840 --> 00:17:57,750
Is that clear?

319
00:18:00,580 --> 00:18:03,160
So such experiments are
done in the auditory nerve.

320
00:18:03,160 --> 00:18:08,410
In this case, I think
the experimental animal

321
00:18:08,410 --> 00:18:09,440
was a Guinea pig.

322
00:18:11,100 --> 00:18:13,750
And in this case,
it's recordings

323
00:18:13,750 --> 00:18:16,090
from a chinchilla
auditory nerve.

324
00:18:19,690 --> 00:18:21,370
So what's the stimulus?

325
00:18:21,370 --> 00:18:24,150
Well, this is a plot
of sound frequency,

326
00:18:24,150 --> 00:18:25,525
sound frequency in kilohertz.

327
00:18:28,470 --> 00:18:33,120
And this axis, on the y-axis,
is sound pressure level.

328
00:18:33,120 --> 00:18:35,470
So this is how loud
it is, if you will.

329
00:18:37,610 --> 00:18:41,030
And at a very low
or soft tone level,

330
00:18:41,030 --> 00:18:45,080
if this frequency is swept
from low to high frequencies,

331
00:18:45,080 --> 00:18:49,130
there was hardly any spikes
coming from that single unit.

332
00:18:50,150 --> 00:18:52,580
But if you boosted the
level up a little bit.

333
00:18:53,720 --> 00:18:58,210
And you came to a frequency
that it was about 10 kilohertz,

334
00:18:58,210 --> 00:19:00,840
there were a bunch of spikes
produced by that single unit.

335
00:19:03,030 --> 00:19:06,430
Then if you boosted the level
up so it was a moderate level,

336
00:19:06,430 --> 00:19:11,780
there were spikes anywhere from
8 kilohertz up to 11 kilohertz.

337
00:19:11,780 --> 00:19:15,260
All that band of frequencies
caused a response.

338
00:19:16,780 --> 00:19:21,180
Then at the highest level,
everything caused a response,

339
00:19:21,180 --> 00:19:25,440
from the lowest frequencies
up to about 12 kilohertz,

340
00:19:25,440 --> 00:19:26,450
and nothing above.

341
00:19:29,740 --> 00:19:32,150
What's this activity out here?

342
00:19:32,150 --> 00:19:34,490
I said nothing above
and nothing over here.

343
00:19:34,490 --> 00:19:36,850
Well, there's some
spontaneous firing.

344
00:19:43,970 --> 00:19:47,880
So even if you turn the
sound completely off,

345
00:19:47,880 --> 00:19:50,530
these nerve fibers have
a little bit of activity.

346
00:19:50,530 --> 00:19:51,765
They fire some impulses.

347
00:19:53,040 --> 00:19:54,260
There's an ongoing thing.

348
00:19:59,300 --> 00:20:02,960
If you outlined this
response area with a line--

349
00:20:02,960 --> 00:20:06,960
that line is the border, say,
between spontaneous firing

350
00:20:06,960 --> 00:20:11,400
or no firing and a response.

351
00:20:11,400 --> 00:20:14,654
So inside of the receptive
area there's a response.

352
00:20:14,654 --> 00:20:15,820
And outside there's nothing.

353
00:20:16,970 --> 00:20:19,605
Those lines are
called tuning curves.

354
00:20:20,690 --> 00:20:24,280
And here are a bunch of tuning
curves from a chinchilla.

355
00:20:24,280 --> 00:20:28,710
And there are one,
two, three, four, five,

356
00:20:28,710 --> 00:20:31,280
six different tuning curves.

357
00:20:31,280 --> 00:20:35,340
So what the experiment did was
they moved the electrode in

358
00:20:35,340 --> 00:20:36,930
and got one single unit.

359
00:20:39,290 --> 00:20:42,700
And then they moved
the electrode,

360
00:20:42,700 --> 00:20:44,350
let's say deeper into the nerve.

361
00:20:48,890 --> 00:20:51,150
And now they sampled
a different neuron,

362
00:20:51,150 --> 00:20:52,315
a different single unit.

363
00:20:53,960 --> 00:20:56,030
OK, maybe got this tuning curve.

364
00:20:56,030 --> 00:20:57,980
Then they went
deeper and sampled

365
00:20:57,980 --> 00:21:00,920
from this one and this one
and this one and this one.

366
00:21:00,920 --> 00:21:03,150
And the idea that it's
a different one-- well,

367
00:21:03,150 --> 00:21:04,940
the response is different.

368
00:21:04,940 --> 00:21:07,060
But also, as you
move the electrode,

369
00:21:07,060 --> 00:21:10,460
you lost the single
unit, number one.

370
00:21:10,460 --> 00:21:13,900
And you've maybe put it
deeper, a millimeter or so.

371
00:21:13,900 --> 00:21:15,642
It's a huge distance.

372
00:21:15,642 --> 00:21:16,725
And you've got a new unit.

373
00:21:17,960 --> 00:21:20,580
The action potentials
probably look different.

374
00:21:20,580 --> 00:21:21,295
That's a second.

375
00:21:24,320 --> 00:21:27,250
OK, so these are tuning
curves there then

376
00:21:27,250 --> 00:21:29,990
from six different single units.

377
00:21:29,990 --> 00:21:33,520
And each of them comes
down to a pretty nice tip.

378
00:21:34,970 --> 00:21:39,990
And if you take that
tip and the very lowest

379
00:21:39,990 --> 00:21:42,710
sound level they're
caused a response

380
00:21:42,710 --> 00:21:46,340
and extrapolate that to the
x-axis, you get a frequency.

381
00:21:47,470 --> 00:21:57,491
And that frequency is called
the CF, or characteristic

382
00:21:57,491 --> 00:21:57,990
frequency.

383
00:22:02,100 --> 00:22:04,720
OK, so CF is a very
important term.

384
00:22:04,720 --> 00:22:08,500
You should know that the CF
is the very tip of the tuning

385
00:22:08,500 --> 00:22:09,000
curve.

386
00:22:10,710 --> 00:22:13,610
And the CF is different
from frequency.

387
00:22:13,610 --> 00:22:16,430
Frequency is whatever
you want to dial in

388
00:22:16,430 --> 00:22:18,715
with your sound oscillator.

389
00:22:19,760 --> 00:22:24,490
But CF is a particular
characteristic of a neuron,

390
00:22:24,490 --> 00:22:26,705
in this case an
auditory nerve fiber,

391
00:22:26,705 --> 00:22:29,890
that you're recording from.

392
00:22:29,890 --> 00:22:31,920
And it's a
characteristic that it

393
00:22:31,920 --> 00:22:36,815
has that you measured from it.

394
00:22:38,170 --> 00:22:41,320
Many of these tuning
curves, in addition

395
00:22:41,320 --> 00:22:46,270
to having a CF and a so-called
tip region, also have a tail.

396
00:22:47,390 --> 00:22:52,340
And in this very high CF
neuron, the tail goes like this.

397
00:22:52,340 --> 00:22:53,890
And then there's
actually, I think,

398
00:22:53,890 --> 00:22:57,200
something that I dashed in
here, a dashed line here.

399
00:22:57,200 --> 00:22:59,730
And the tail continues
way down here.

400
00:23:01,350 --> 00:23:04,130
These experiments didn't
want to boost the sound level

401
00:23:04,130 --> 00:23:08,600
to get all the tail above 80
dBs because of possible damage.

402
00:23:08,600 --> 00:23:11,260
If you crank up too much
sound-- just like you

403
00:23:11,260 --> 00:23:13,510
get a gunshot to the head
is a very loud sound--

404
00:23:13,510 --> 00:23:15,694
it can cause damage
to the hair cells.

405
00:23:15,694 --> 00:23:16,860
They didn't want to do that.

406
00:23:16,860 --> 00:23:20,120
But you could see the tail
of this response area.

407
00:23:20,120 --> 00:23:22,985
It's a nice tip and a nice tail.

408
00:23:28,310 --> 00:23:32,290
OK, now, right away we have
a beautiful potential code

409
00:23:32,290 --> 00:23:33,790
for sound frequency.

410
00:23:33,790 --> 00:23:36,510
How do I know I'm
listening to 8 kilohertz?

411
00:23:37,690 --> 00:23:41,380
Well, this nerve fiber
responds very nicely,

412
00:23:41,380 --> 00:23:42,705
lots of action potentials.

413
00:23:44,270 --> 00:23:47,420
How do I know I'm
listening to 1 kilohertz?

414
00:23:47,420 --> 00:23:49,590
Well, that same nerve
fiber might respond.

415
00:23:49,590 --> 00:23:52,720
But I have to get the sound
level to very loud level,

416
00:23:52,720 --> 00:23:55,070
like 80 dBs ATL.

417
00:23:55,070 --> 00:23:58,290
But these other guys over
here with CFs of 1 kilohertz

418
00:23:58,290 --> 00:24:00,200
would respond at
a very low sound.

419
00:24:02,850 --> 00:24:05,620
So then we have a
code of which fiber

420
00:24:05,620 --> 00:24:09,050
you're listening
to tells you which

421
00:24:09,050 --> 00:24:10,320
frequency you're listening to.

422
00:24:10,320 --> 00:24:12,030
It's very important.

423
00:24:12,030 --> 00:24:15,160
You judge an instrument,
like a violin,

424
00:24:15,160 --> 00:24:17,920
by its combination
of frequencies.

425
00:24:17,920 --> 00:24:20,200
A guitar has a different
recombination frequencies.

426
00:24:21,260 --> 00:24:23,510
Male speakers generally
have deeper voices

427
00:24:23,510 --> 00:24:27,310
than female speakers, deeper
meaning more low frequencies.

428
00:24:28,500 --> 00:24:32,640
Female and children's voices
are higher in frequency.

429
00:24:32,640 --> 00:24:34,420
So frequency is
essential for you

430
00:24:34,420 --> 00:24:38,690
to identify what sound
stimulus you are listening.

431
00:24:41,930 --> 00:24:45,460
Why do we call this a place
code for sound frequency?

432
00:24:45,460 --> 00:24:50,400
Well, as we talked about before,
different parts of the cochlea

433
00:24:50,400 --> 00:24:52,390
respond to different
frequencies.

434
00:24:52,390 --> 00:24:57,340
Here is a beautiful
example of the place

435
00:24:57,340 --> 00:24:59,210
map for auditory nerve fibers.

436
00:25:00,290 --> 00:25:05,090
And in this case,
microelectrode recordings

437
00:25:05,090 --> 00:25:08,280
are done as we described before.

438
00:25:12,240 --> 00:25:16,700
But instead of just a plain
old potassium chloride solution

439
00:25:16,700 --> 00:25:23,280
in the microelectrode,
it's filled

440
00:25:23,280 --> 00:25:26,053
with a substance
called a neural tracer.

441
00:25:30,670 --> 00:25:33,080
What are examples
of neural tracers?

442
00:25:33,080 --> 00:25:36,950
Has anybody played around
with neural tracers before?

443
00:25:36,950 --> 00:25:39,000
Give me some
examples of chemicals

444
00:25:39,000 --> 00:25:40,000
that are neural tracers.

445
00:25:42,853 --> 00:25:43,353
Anybody?

446
00:25:47,720 --> 00:25:58,750
This one is a funny name
horseradish peroxidose,

447
00:25:58,750 --> 00:26:00,060
abbreviated HRP.

448
00:26:03,080 --> 00:26:10,900
Another one is biocytin,
OK, biotinylated

449
00:26:10,900 --> 00:26:13,050
dextran amine, PDA.

450
00:26:13,050 --> 00:26:14,965
There's millions of
them, Lucifer yellow.

451
00:26:20,320 --> 00:26:22,970
You can tell that I'm
a tracer kind of guy.

452
00:26:22,970 --> 00:26:25,740
I use tracers all the
time in my experiments.

453
00:26:25,740 --> 00:26:28,180
So what you do with
these neural tracers,

454
00:26:28,180 --> 00:26:32,070
it's convenient if
they are charged.

455
00:26:32,070 --> 00:26:33,820
For example,
horseradish peroxidose,

456
00:26:33,820 --> 00:26:35,040
this is a positive charge.

457
00:26:38,830 --> 00:26:45,580
And you can apply positive
current to the pipette up here.

458
00:26:45,580 --> 00:26:48,990
That's going to tend to
force positive charge

459
00:26:48,990 --> 00:26:50,710
out the tip of the electrode.

460
00:26:50,710 --> 00:26:55,520
You can expel a
positive ion out the tip

461
00:26:55,520 --> 00:26:57,750
by this technique, which
is called iontophoresis.

462
00:27:05,040 --> 00:27:07,010
And if it happens
that your tip is

463
00:27:07,010 --> 00:27:16,040
close to or ideally inside
an axon, some of that HRP

464
00:27:16,040 --> 00:27:18,940
is going to come out
the tip of the electrode

465
00:27:18,940 --> 00:27:20,250
and go into the axon.

466
00:27:23,730 --> 00:27:25,180
And why did we pick HRP?

467
00:27:26,440 --> 00:27:30,750
Because it's picked up by
chemical transport systems

468
00:27:30,750 --> 00:27:33,085
that transport
things along axons.

469
00:27:34,059 --> 00:27:35,350
And there are several of these.

470
00:27:35,350 --> 00:27:38,100
There's fast axonal transport.

471
00:27:38,100 --> 00:27:38,720
There's slow.

472
00:27:38,720 --> 00:27:39,575
There's medium.

473
00:27:39,575 --> 00:27:40,950
There's a whole
bunch of systems,

474
00:27:40,950 --> 00:27:45,765
because this axon is
coming from a cell here.

475
00:27:47,536 --> 00:27:49,260
It's connected to the cell body.

476
00:27:49,260 --> 00:27:53,750
In the cell body you make
things like neurotransmitter,

477
00:27:53,750 --> 00:27:55,510
because that's where
you can make protein.

478
00:27:55,510 --> 00:27:58,840
And that neurotransmitter
has to get down

479
00:27:58,840 --> 00:28:02,410
to the tip of the axon, which in
the case of the auditory nerve

480
00:28:02,410 --> 00:28:04,460
is in the cochlear
nucleus of the brain.

481
00:28:05,660 --> 00:28:08,370
So there are all these transport
systems transporting things.

482
00:28:10,010 --> 00:28:12,394
And it just turns out
that some chemicals

483
00:28:12,394 --> 00:28:13,310
are picked up by them.

484
00:28:13,310 --> 00:28:15,190
HRP is one of them.

485
00:28:15,190 --> 00:28:22,070
When you iontophorese
HRP into a nerve fiber,

486
00:28:22,070 --> 00:28:25,000
it's transported to all
parts of the nerve fiber,

487
00:28:25,000 --> 00:28:27,990
including to the cell
and including out

488
00:28:27,990 --> 00:28:31,360
to the tip of the nerve
fiber on the hair cell.

489
00:28:32,760 --> 00:28:37,480
So here is an example
of iontophoretically

490
00:28:37,480 --> 00:28:39,090
labeled nerve fibers.

491
00:28:39,090 --> 00:28:42,810
And there's five or six of them.

492
00:28:42,810 --> 00:28:45,440
The recording site was
here in the auditory nerve.

493
00:28:46,730 --> 00:28:48,494
This is a diagram
of the cochlea.

494
00:28:50,500 --> 00:28:53,020
This is the so-called
Schwann glial border,

495
00:28:53,020 --> 00:28:57,740
which defines the
periphery and the brain.

496
00:28:57,740 --> 00:28:58,950
So this would be the brain.

497
00:28:58,950 --> 00:29:01,074
So these are the nerve
fibers going into the brain.

498
00:29:02,440 --> 00:29:04,160
They were recorded in
the auditory nerve.

499
00:29:04,160 --> 00:29:08,190
And you can trace them
out into the periphery.

500
00:29:08,190 --> 00:29:13,320
Right here is the cell body
of the auditory nerve fiber.

501
00:29:13,320 --> 00:29:15,190
Every neuron has a cell body.

502
00:29:16,260 --> 00:29:18,110
Most neurons have axons.

503
00:29:18,110 --> 00:29:19,855
The axon was what was recorded.

504
00:29:21,380 --> 00:29:24,340
And the auditory nerve
neuron has a cell body.

505
00:29:24,340 --> 00:29:26,410
And it also has
a peripheral axon

506
00:29:26,410 --> 00:29:30,240
that goes out to the periphery
and contacts an inner hair

507
00:29:30,240 --> 00:29:30,740
cell.

508
00:29:32,710 --> 00:29:35,170
As we saw before, these
are type one auditory nerve

509
00:29:35,170 --> 00:29:36,950
fibers going to
inner hair cells.

510
00:29:36,950 --> 00:29:39,510
And it contacts usually
one inner hair cell.

511
00:29:41,310 --> 00:29:45,040
Now, you can know exactly
where that auditory nerve

512
00:29:45,040 --> 00:29:50,220
fiber started out by
tracing it and by tracing

513
00:29:50,220 --> 00:29:53,160
the base of the cochlea
through the spiral

514
00:29:53,160 --> 00:29:54,540
and all the way up to the apex.

515
00:29:55,660 --> 00:30:00,750
So starting at the base to the
apex-- so that's 100% distance,

516
00:30:00,750 --> 00:30:01,460
let's say.

517
00:30:03,110 --> 00:30:06,080
And if this were halfway
between the base and the apex,

518
00:30:06,080 --> 00:30:08,915
that would be the
50% distance place.

519
00:30:11,060 --> 00:30:14,700
This guy ending up near the
apex might be 80% distance

520
00:30:14,700 --> 00:30:16,635
from the base to the apex.

521
00:30:16,635 --> 00:30:18,760
OK, does everybody see how
I can make that mapping?

522
00:30:20,520 --> 00:30:24,160
These sausages here
are the outlines

523
00:30:24,160 --> 00:30:27,090
of the ganglion, the spiral
ganglion, where the cell

524
00:30:27,090 --> 00:30:28,810
bodies are of the
auditory nerve fibers.

525
00:30:30,850 --> 00:30:32,120
So what good is that mapping?

526
00:30:32,120 --> 00:30:39,300
Well, before we put the HRP in,
we measured the tuning curve.

527
00:30:40,940 --> 00:30:45,700
And we got the CF
from the tuning curve.

528
00:30:45,700 --> 00:30:48,442
So we measured the CF.

529
00:30:48,442 --> 00:30:53,390
We injected the neurotransmitter
to label the auditory nerve

530
00:30:53,390 --> 00:30:54,570
fiber.

531
00:30:54,570 --> 00:30:58,010
And we reconstructed
where the labeled ending

532
00:30:58,010 --> 00:31:01,250
of the auditory nerve fiber
contacted its inner hair cell.

533
00:31:03,180 --> 00:31:04,940
Why did we do this
for five of these?

534
00:31:04,940 --> 00:31:06,670
Well, in the
ultimate experiment,

535
00:31:06,670 --> 00:31:07,800
you just do it for one.

536
00:31:09,370 --> 00:31:12,450
But if you're getting good at
reconstructing the mapping,

537
00:31:12,450 --> 00:31:16,110
you can tell it should
be about the 50% place.

538
00:31:16,110 --> 00:31:17,460
And you go and find it's 51%.

539
00:31:17,460 --> 00:31:20,870
You know that fiber was
different than the one

540
00:31:20,870 --> 00:31:21,370
up there.

541
00:31:24,340 --> 00:31:30,810
Then you make your mapping--
characteristic frequency

542
00:31:30,810 --> 00:31:34,210
to position of enervation
along the cochlea.

543
00:31:34,210 --> 00:31:35,370
And here is the mapping.

544
00:31:35,370 --> 00:31:36,120
These are the CFs.

545
00:31:38,780 --> 00:31:42,630
And this is the percent
distance along the cochlea

546
00:31:42,630 --> 00:31:43,895
from the base.

547
00:31:45,570 --> 00:31:50,230
So 0% distance from the base
would be the extreme base.

548
00:31:50,230 --> 00:31:52,980
100% distance would
be the extreme apex.

549
00:31:54,370 --> 00:31:57,230
And you can see this
beautiful mapping of CF

550
00:31:57,230 --> 00:32:01,290
to position, almost
a straight line,

551
00:32:01,290 --> 00:32:02,920
until you get to the lowest CFs.

552
00:32:04,840 --> 00:32:07,230
And this, as usual, in
the auditory system,

553
00:32:07,230 --> 00:32:10,950
this frequency axis,
this is the CF axis now.

554
00:32:10,950 --> 00:32:12,185
It's on a log scale.

555
00:32:14,790 --> 00:32:19,560
So log frequency maps to linear
distance along the cochlea.

556
00:32:22,010 --> 00:32:30,210
Now, if the brain hears that
the 50% distance auditory nerve

557
00:32:30,210 --> 00:32:33,050
fiber is responding and
no other auditory nerve

558
00:32:33,050 --> 00:32:36,410
fiber is responding,
it knows it's

559
00:32:36,410 --> 00:32:38,835
listening to a 3
kilohertz frequency.

560
00:32:41,220 --> 00:32:46,130
Place to frequency
mapping is tonotopic.

561
00:32:47,700 --> 00:32:49,720
I said that opposite.

562
00:32:49,720 --> 00:32:52,460
Frequency to place is tonotopic.

563
00:32:52,460 --> 00:33:14,040
So this is a tonotopic mapping--
frequency to place, tonotopic.

564
00:33:14,040 --> 00:33:15,300
And why is that important?

565
00:33:15,300 --> 00:33:16,940
Well, it happens in the cochlea.

566
00:33:18,810 --> 00:33:21,090
It happens in the
auditory nerve.

567
00:33:21,090 --> 00:33:24,140
It happens in the cochlear
nucleus of the brain.

568
00:33:24,140 --> 00:33:26,780
It happens in almost
all the auditory

569
00:33:26,780 --> 00:33:30,290
centers in the entire brain,
all the way up to the cortex.

570
00:33:31,660 --> 00:33:35,400
You have neurons or
fibers responding

571
00:33:35,400 --> 00:33:38,220
to low CFs over
here in the brain.

572
00:33:38,220 --> 00:33:40,420
And if you move your
electrode over here,

573
00:33:40,420 --> 00:33:43,270
you find they're responding
to mid frequencies.

574
00:33:43,270 --> 00:33:47,750
And if you move them over here,
they're responding to high CFs.

575
00:33:49,790 --> 00:33:52,180
So this organization
is fundamental.

576
00:33:52,180 --> 00:33:56,030
It starts at the receptor
level in the cochlea.

577
00:33:56,030 --> 00:33:59,510
It's conveyed by the nerve
into the cochlear nucleus.

578
00:33:59,510 --> 00:34:01,770
And you have these
beautiful frequency--

579
00:34:01,770 --> 00:34:04,740
they're actually CF
organizations in the brain.

580
00:34:06,620 --> 00:34:09,000
So the place code
for sound frequency

581
00:34:09,000 --> 00:34:10,960
presumes that each
frequency stimulates

582
00:34:10,960 --> 00:34:12,695
a certain place along cochlea.

583
00:34:18,699 --> 00:34:21,219
And I guess, if
you generalize this

584
00:34:21,219 --> 00:34:25,290
from the auditory system
to the visual system--

585
00:34:25,290 --> 00:34:27,719
if you have a
particular light source,

586
00:34:27,719 --> 00:34:31,429
like that light over there, and
my eyes are looking this way,

587
00:34:31,429 --> 00:34:34,810
that light is going to
stimulate a particular place

588
00:34:34,810 --> 00:34:37,530
in my left retina and
in my right retina.

589
00:34:37,530 --> 00:34:42,179
So you have a coding for
where that light is along

590
00:34:42,179 --> 00:34:43,949
the place in the retina.

591
00:34:43,949 --> 00:34:45,540
In the auditory
system, you don't

592
00:34:45,540 --> 00:34:46,969
have that kind of a place code.

593
00:34:46,969 --> 00:34:49,902
You have a place code
for sound frequency.

594
00:34:49,902 --> 00:34:50,735
It's very different.

595
00:34:52,370 --> 00:34:54,349
The cochlea maps frequency.

596
00:35:01,400 --> 00:35:03,060
How can we use this code?

597
00:35:04,450 --> 00:35:07,810
We're actually very good at
distinguishing closely spaced

598
00:35:07,810 --> 00:35:09,030
frequencies.

599
00:35:09,030 --> 00:35:11,900
And here is now some
psychophyscial data

600
00:35:11,900 --> 00:35:13,630
from human listeners.

601
00:35:13,630 --> 00:35:17,630
We're going to get away from
the auditory nerve for awhile

602
00:35:17,630 --> 00:35:19,135
and talk about
listening studies.

603
00:35:20,610 --> 00:35:22,366
Here is to graph of frequency.

604
00:35:24,700 --> 00:35:27,525
And on the y-axis is delta f.

605
00:35:29,290 --> 00:35:30,215
What's delta f?

606
00:35:35,180 --> 00:35:42,765
Delta f is the just noticeable
difference for frequency.

607
00:35:45,732 --> 00:35:47,565
Of course, we're talk
about sound frequency.

608
00:35:53,430 --> 00:35:54,930
And how is the
experiment conducted?

609
00:35:54,930 --> 00:35:56,138
Well, you have your listener.

610
00:35:56,960 --> 00:35:59,110
Your listener is
listening to sound.

611
00:35:59,110 --> 00:36:01,115
And you give them a
1 kilohertz sound.

612
00:36:02,460 --> 00:36:05,470
And then you give them
a 2 kilohertz sound.

613
00:36:05,470 --> 00:36:08,560
The experimenter says, does it
sound the same or different?

614
00:36:08,560 --> 00:36:10,150
Ah, completely different.

615
00:36:10,150 --> 00:36:14,820
OK, 1 kilohertz sound
and a 1,100 kilohertz

616
00:36:14,820 --> 00:36:16,080
sound, same or different?

617
00:36:16,080 --> 00:36:17,490
Ah completely different.

618
00:36:17,490 --> 00:36:21,570
OK, 1,000 hertz sound
and 1,010 hertz sound.

619
00:36:21,570 --> 00:36:22,910
Ah, it's different.

620
00:36:22,910 --> 00:36:29,590
1,000 hertz and a
1,002 hertz sound?

621
00:36:29,590 --> 00:36:30,480
I'm not so sure.

622
00:36:30,480 --> 00:36:31,480
Give it to me again.

623
00:36:31,480 --> 00:36:34,920
OK, 1,000 hertz
sound, 1,002 hertz?

624
00:36:34,920 --> 00:36:36,770
Eh, it's just a
little bit different.

625
00:36:36,770 --> 00:36:39,350
1,000 hertz sound and
a 1,001 hertz sound?

626
00:36:39,350 --> 00:36:39,850
Same.

627
00:36:41,040 --> 00:36:43,160
OK, so that's the experiment.

628
00:36:43,160 --> 00:36:49,840
So we have the graph here for
the just noticeable difference

629
00:36:49,840 --> 00:36:52,450
in frequency, as a
function of frequency.

630
00:36:52,450 --> 00:36:54,270
And at 1,000
hertz-- that's right

631
00:36:54,270 --> 00:36:58,760
in the middle of your hearing
range-- the delta f-- well,

632
00:36:58,760 --> 00:37:01,700
it's hard to read that
axis-- the delta f

633
00:37:01,700 --> 00:37:05,285
is about 1 or 2 hertz.

634
00:37:06,840 --> 00:37:12,260
So 1,000 vs 1,002
hertz is just barely

635
00:37:12,260 --> 00:37:15,900
distinguishable for
human listeners.

636
00:37:18,280 --> 00:37:21,010
You can do that experimental
a little bit differently.

637
00:37:21,010 --> 00:37:25,140
Instead of giving two tones,
you can give one tone and vary

638
00:37:25,140 --> 00:37:26,570
its frequency a little.

639
00:37:28,920 --> 00:37:32,030
And that's kind of
a pleasing sound.

640
00:37:32,030 --> 00:37:34,890
Does everybody
know what a vibrato

641
00:37:34,890 --> 00:37:37,075
is on a stringed instrument?

642
00:37:40,420 --> 00:37:42,920
That's a plain A.
But if you vibrate

643
00:37:42,920 --> 00:37:49,130
it a little-- that's
the frequencies

644
00:37:49,130 --> 00:37:51,140
going back and forth.

645
00:37:51,140 --> 00:37:53,760
Everybody could hear
that vibrato right?

646
00:37:53,760 --> 00:37:57,140
Even though I'm changing the
frequency just a tiny bit.

647
00:37:57,140 --> 00:38:00,230
You could do the experiment
by vibrating the frequency

648
00:38:00,230 --> 00:38:01,520
just one single frequency.

649
00:38:02,750 --> 00:38:03,850
Is it vibrating?

650
00:38:03,850 --> 00:38:04,955
Or is it not vibrating?

651
00:38:06,010 --> 00:38:08,330
And you get about
the same result.

652
00:38:08,330 --> 00:38:10,400
That's what the second graph is.

653
00:38:11,780 --> 00:38:17,040
People who are tone deaf,
not proficient music,

654
00:38:17,040 --> 00:38:20,590
don't have any hearing
problems are almost always

655
00:38:20,590 --> 00:38:23,860
able to distinguish frequencies
with a little bit of training.

656
00:38:23,860 --> 00:38:27,810
The training is now here's the
task, that type of training.

657
00:38:27,810 --> 00:38:30,720
OK, so I have a
demonstration here.

658
00:38:30,720 --> 00:38:34,500
And we can listen to this and
see how good you guys are--

659
00:38:34,500 --> 00:38:39,290
you know, naive,
untrained listeners--

660
00:38:39,290 --> 00:38:41,970
and see if we're good at
distinguishing frequency.

661
00:38:41,970 --> 00:38:46,200
So the demonstration is
a little bit complicated.

662
00:38:46,200 --> 00:38:47,940
So I'll go through it.

663
00:38:47,940 --> 00:38:51,520
It's going to give you
1,000 hertz, standard,

664
00:38:51,520 --> 00:38:53,825
middle of your range
hearing frequency.

665
00:38:55,680 --> 00:38:58,249
And it's going to give you
a bunch of different groups.

666
00:38:58,249 --> 00:38:59,790
I'm going to go
through these slowly.

667
00:39:01,060 --> 00:39:05,340
And in each group, we
have 1,000 one hertz,

668
00:39:05,340 --> 00:39:08,670
and 1,000 hertz plus delta f.

669
00:39:08,670 --> 00:39:13,440
OK, delta f for group one is
10 hertz, big frequency space.

670
00:39:14,680 --> 00:39:17,630
And what you're
going to listen to

671
00:39:17,630 --> 00:39:27,120
is A, B, A, A, where is f--
1,000 hertz-- and f plus delta

672
00:39:27,120 --> 00:39:31,010
f-- 1,010 hertz.

673
00:39:31,010 --> 00:39:36,130
And B will be first, 1,010
hertz and then 1,000 hertz.

674
00:39:36,130 --> 00:39:40,190
Then A, 1,000
hertz, 1,010 hertz;

675
00:39:40,190 --> 00:39:43,690
and another A, 1,000
hertz, 1,010 hertz,

676
00:39:43,690 --> 00:39:45,970
just to give you a bunch
of different examples.

677
00:39:47,340 --> 00:39:53,370
Then group two, delta f will be
a little bit harder, 9 hertz,

678
00:39:53,370 --> 00:39:57,350
OK, so on and so forth,
down to group 10, which

679
00:39:57,350 --> 00:40:00,160
will be delta f of 1 hertz.

680
00:40:00,160 --> 00:40:08,490
Or seeing if we can distinguish
1,000 and 1,001 hertz.

681
00:40:08,490 --> 00:40:10,370
OK, so let's listen to this.

682
00:40:14,300 --> 00:40:18,360
MAN IN AUDIO: Frequency
difference file for J and D.

683
00:40:18,360 --> 00:40:21,276
You will hear 10 groups
of four tone pairs.

684
00:40:21,276 --> 00:40:24,358
In each group, there is a
small frequency difference

685
00:40:24,358 --> 00:40:26,850
between the tones
of the pairs, which

686
00:40:26,850 --> 00:40:29,190
decreases in each
successive group.

687
00:40:31,532 --> 00:40:35,004
[BEEPING OF TONE PAIRS]

688
00:40:39,834 --> 00:40:41,000
PROFESSOR: That's group one.

689
00:40:42,170 --> 00:40:43,620
Here's group two.

690
00:40:43,620 --> 00:40:47,120
[BEEPING OF TONE PAIRS]

691
00:42:35,000 --> 00:42:39,260
PROFESSOR: OK, could
everybody do the big interval,

692
00:42:39,260 --> 00:42:40,940
delta f equals 10?

693
00:42:40,940 --> 00:42:43,350
Raise your hand if
you could do that.

694
00:42:43,350 --> 00:42:47,770
Most-- some people can.

695
00:42:47,770 --> 00:42:49,570
I-- it's not problem.

696
00:42:49,570 --> 00:42:52,857
OK, how about your limits
for people who could do it.

697
00:42:52,857 --> 00:42:53,440
When did you--

698
00:42:53,440 --> 00:42:54,660
AUDIENCE: I heard eight.

699
00:42:54,660 --> 00:42:56,030
PROFESSOR: About eight, OK.

700
00:42:58,420 --> 00:43:00,693
And what was your
limit going down?

701
00:43:00,693 --> 00:43:01,318
AUDIENCE: Nine.

702
00:43:02,990 --> 00:43:04,920
PROFESSOR: Group
nine or delta f?

703
00:43:04,920 --> 00:43:06,515
OK, so delta f of 2.

704
00:43:08,070 --> 00:43:10,490
I cut out about
between two and three.

705
00:43:12,890 --> 00:43:16,110
Well, for those of us who could
do it, without any training

706
00:43:16,110 --> 00:43:22,350
at all, you get to what the
best results are-- people

707
00:43:22,350 --> 00:43:25,095
who have done this for
days and days and practice.

708
00:43:26,190 --> 00:43:28,340
And this is not an
ideal listening room.

709
00:43:28,340 --> 00:43:30,090
There's a lot of fan noise.

710
00:43:30,090 --> 00:43:33,310
There's some distractions too.

711
00:43:33,310 --> 00:43:36,470
Ideally, you'd be in a
completely quiet environment,

712
00:43:36,470 --> 00:43:37,640
perhaps wearing headphones.

713
00:43:39,180 --> 00:43:40,660
But it works pretty well.

714
00:43:40,660 --> 00:43:43,450
I'm not sure what it says
about people can't do it.

715
00:43:44,560 --> 00:43:48,074
And there certainly are.

716
00:43:48,074 --> 00:43:49,490
So I don't know
if you should have

717
00:43:49,490 --> 00:43:52,770
your hearing tested or whatever.

718
00:43:52,770 --> 00:43:55,220
But for those of
us who could do it,

719
00:43:55,220 --> 00:43:58,910
you get quickly to the
best possible results.

720
00:43:58,910 --> 00:44:02,770
So you could do a
calculation then on these.

721
00:44:02,770 --> 00:44:04,500
We know what delta f is.

722
00:44:04,500 --> 00:44:08,190
Let's say it's 2
hertz at 1,000 hertz.

723
00:44:08,190 --> 00:44:10,300
And let's go back to
our mapping experiment.

724
00:44:12,400 --> 00:44:18,600
So here's 1,000 hertz CF.

725
00:44:18,600 --> 00:44:20,740
Let's say we're
listening at the CF.

726
00:44:23,140 --> 00:44:28,640
And we're moving from 1,000 to
1,002 hertz, the best possible

727
00:44:28,640 --> 00:44:30,770
psychophysical performance.

728
00:44:30,770 --> 00:44:34,490
We can go up along this
cochlear frequency map

729
00:44:34,490 --> 00:44:37,340
and say, well, what
percent distance did

730
00:44:37,340 --> 00:44:44,250
we move from the 1,000 hertz
point to the 1,002 hertz point?

731
00:44:44,250 --> 00:44:46,120
And I don't know where it is.

732
00:44:46,120 --> 00:44:51,550
Well, it's about the 70%
distance place in this animal.

733
00:44:51,550 --> 00:44:53,130
This is a cat, of course.

734
00:44:53,130 --> 00:44:55,150
You can't do these kinds
of studies in humans.

735
00:44:55,150 --> 00:44:56,950
You could map it out in human.

736
00:44:58,300 --> 00:45:02,135
It turns out that if you know
how many inner hair cells there

737
00:45:02,135 --> 00:45:07,670
are-- we had that number before
along the base to apex spiral--

738
00:45:07,670 --> 00:45:09,760
and you know the
distance you're moving,

739
00:45:09,760 --> 00:45:12,920
it turns out you can make
the calculation, the best

740
00:45:12,920 --> 00:45:15,090
possible performance.

741
00:45:15,090 --> 00:45:18,990
You're moving from one inner
hair cell to its neighbor.

742
00:45:18,990 --> 00:45:22,450
So it's a very,
very small increment

743
00:45:22,450 --> 00:45:24,625
along the cochlear
spiral you're moving.

744
00:45:26,070 --> 00:45:29,110
That increment is
associated with the best

745
00:45:29,110 --> 00:45:32,570
possible psychophysical
performance

746
00:45:32,570 --> 00:45:35,180
in terms of frequency
distinction.

747
00:45:35,180 --> 00:45:38,380
OK, that's the
cochlear frequency map.

748
00:45:44,030 --> 00:45:45,778
OK, any questions about that?

749
00:45:48,050 --> 00:45:50,200
Now let's go back to
the auditory nerve

750
00:45:50,200 --> 00:45:54,430
and talk more about coding
for sound frequency.

751
00:45:56,670 --> 00:46:03,300
So far, we've just been
exploring single tone response

752
00:46:03,300 --> 00:46:03,800
areas.

753
00:46:03,800 --> 00:46:07,800
So now let's make the stimulus
a little bit more advanced

754
00:46:07,800 --> 00:46:10,325
and talk about
coding for two tones.

755
00:46:12,290 --> 00:46:14,615
What happens when
you have two tones?

756
00:46:14,615 --> 00:46:18,820
Well, here is a tuning curve,
plotted with open symbols

757
00:46:18,820 --> 00:46:22,270
here, for the kind
of tuning curve

758
00:46:22,270 --> 00:46:23,765
with one tone we had before.

759
00:46:25,830 --> 00:46:29,640
So everything within this
white area is excitatory.

760
00:46:29,640 --> 00:46:34,891
You put a frequency of 7
kilohertz in at 40 dB SPL.

761
00:46:34,891 --> 00:46:37,515
And the neuron is going to fire
all kinds of action potentials.

762
00:46:39,590 --> 00:46:43,390
Now, let's put in a tone
right at this triangle,

763
00:46:43,390 --> 00:46:45,160
called the probe tone.

764
00:46:45,160 --> 00:46:47,052
It's usually right at the CF.

765
00:46:47,052 --> 00:46:49,256
And it's above the threshold.

766
00:46:51,100 --> 00:46:53,770
In this case, it looks
like it's about 25 dB.

767
00:46:53,770 --> 00:46:55,855
And it gets the
neuron responding.

768
00:46:56,880 --> 00:47:01,260
You put that probe
tone in the neuron

769
00:47:01,260 --> 00:47:03,850
is going to fire some
action potentials.

770
00:47:03,850 --> 00:47:05,940
And keep that probe
tone in so the neuron

771
00:47:05,940 --> 00:47:07,570
is firing action potentials.

772
00:47:07,570 --> 00:47:09,400
And put a second tone in.

773
00:47:10,570 --> 00:47:14,645
And the second tone is often
outside the response areas.

774
00:47:16,680 --> 00:47:20,560
And it turns out that
anywhere in this shaded area

775
00:47:20,560 --> 00:47:25,540
above the CF or below
the CF, a second tone,

776
00:47:25,540 --> 00:47:28,990
as is illustrated here,
will decrease the response

777
00:47:28,990 --> 00:47:31,150
to the probe tone in
a dramatic fashion.

778
00:47:32,560 --> 00:47:35,480
Then, when you turn off
this second tone-- sometimes

779
00:47:35,480 --> 00:47:40,395
called a suppressing tone-- the
original activity comes back.

780
00:47:42,350 --> 00:47:46,245
And this phenomenon is
called two-tone suppression.

781
00:47:47,750 --> 00:47:50,170
At first, it was called
two-tone inhibition.

782
00:47:50,170 --> 00:47:55,330
People thought, oh, OK, there's
another nearby neighbor nerve

783
00:47:55,330 --> 00:47:57,440
fiber that's inhibiting
this first one.

784
00:47:57,440 --> 00:47:58,600
And they looked in
the cochlea and there

785
00:47:58,600 --> 00:47:59,933
weren't any inhibitory synapses.

786
00:48:01,880 --> 00:48:03,990
OK, so that was a problem.

787
00:48:03,990 --> 00:48:05,560
They started calling
it suppression.

788
00:48:05,560 --> 00:48:07,690
And they actually
ended up finding it

789
00:48:07,690 --> 00:48:10,730
in the movement of
the basilar membrane.

790
00:48:10,730 --> 00:48:13,210
So it's just something
about the vibration pattern

791
00:48:13,210 --> 00:48:17,770
of the cochlea that causes
the movement of the membranes

792
00:48:17,770 --> 00:48:20,200
to be diminished
by a second tone

793
00:48:20,200 --> 00:48:21,450
on either side of the first.

794
00:48:24,930 --> 00:48:26,810
Now, why do I bring this up?

795
00:48:26,810 --> 00:48:29,465
Well, it's kind of interesting
in a number of contexts.

796
00:48:30,520 --> 00:48:33,365
Two-tone suppression might
be a form of gain control.

797
00:48:34,660 --> 00:48:37,440
If you just had the
excitatory tuning curve,

798
00:48:37,440 --> 00:48:40,760
and you started listening in a
restaurant where everybody was

799
00:48:40,760 --> 00:48:43,450
talking and there
was a lot of sound,

800
00:48:43,450 --> 00:48:46,040
all your auditory
nerve fibers might

801
00:48:46,040 --> 00:48:49,020
be discharging at
their maximal rates.

802
00:48:49,020 --> 00:48:52,619
And you wouldn't be able to tell
the interesting conversation

803
00:48:52,619 --> 00:48:53,910
your two neighbors were having.

804
00:48:53,910 --> 00:48:55,330
You wouldn't be
able to eavesdrop.

805
00:48:55,330 --> 00:48:57,595
You wouldn't be having
a conversation yourself.

806
00:48:58,850 --> 00:49:01,770
So two-tone suppression
is a form of gain control,

807
00:49:01,770 --> 00:49:07,990
where the side bands reduce
the response to the main band,

808
00:49:07,990 --> 00:49:10,405
so that not everything's
being driven into saturation.

809
00:49:11,970 --> 00:49:12,850
That's one reason.

810
00:49:14,060 --> 00:49:16,270
And a second reason
is you can actually

811
00:49:16,270 --> 00:49:21,490
use this in a sort of a
tricky psychophysical paradigm

812
00:49:21,490 --> 00:49:26,060
to measure the tuning
of human listeners.

813
00:49:26,060 --> 00:49:29,950
We obviously can't go into
a human's auditory nerve

814
00:49:29,950 --> 00:49:31,910
with a microelectrode,
although it's

815
00:49:31,910 --> 00:49:33,580
been done a couple
times in surgery.

816
00:49:33,580 --> 00:49:34,260
But it's rare.

817
00:49:36,610 --> 00:49:41,680
It's easy to do a so-called
two-tone suppression paradigm,

818
00:49:41,680 --> 00:49:44,520
where you have the person
listen to the probe tone.

819
00:49:44,520 --> 00:49:46,490
You say to the
listener, here's a tone.

820
00:49:46,490 --> 00:49:48,650
I want you to listen to that.

821
00:49:48,650 --> 00:49:50,340
I'm going to put
in a second tone.

822
00:49:50,340 --> 00:49:51,266
I ignore that.

823
00:49:51,266 --> 00:49:52,140
Don't worry about it.

824
00:49:52,140 --> 00:49:54,170
Just listen to that
first probe tone.

825
00:49:56,650 --> 00:49:59,060
Tell me if you can hear
that original probe tone.

826
00:49:59,060 --> 00:50:00,220
Ah, yeah, sure I can hear.

827
00:50:00,220 --> 00:50:01,511
I'm going to put a second tone.

828
00:50:01,511 --> 00:50:03,395
Oh, I can't hear the
probe tone anymore.

829
00:50:04,720 --> 00:50:08,760
OK the second or side tone
has suppressed the response

830
00:50:08,760 --> 00:50:10,070
to the probe tone.

831
00:50:10,070 --> 00:50:12,200
And you can use that
as a measure of tuning,

832
00:50:12,200 --> 00:50:16,390
because these suppression areas
flank the excitatory area.

833
00:50:16,390 --> 00:50:20,140
And so here are some
results from humans

834
00:50:20,140 --> 00:50:24,480
in a so-called psychophysical
tuning curve paradigm.

835
00:50:25,660 --> 00:50:29,540
And these are a half a
dozen or so tuning curves.

836
00:50:29,540 --> 00:50:35,175
Each one has associated with
it a probe tone or a test tone.

837
00:50:36,360 --> 00:50:39,910
The task is listen
to that test tone

838
00:50:39,910 --> 00:50:43,965
and tell me if you still
hear it or if it's gone away.

839
00:50:45,070 --> 00:50:47,960
The experimenter
introduce a second tone,

840
00:50:47,960 --> 00:50:51,965
a so-called masker, at those
frequencies and levels.

841
00:50:53,490 --> 00:50:57,700
And at where the line
is drawn, the person

842
00:50:57,700 --> 00:50:59,850
who's listening
to the probe tone

843
00:50:59,850 --> 00:51:02,800
says, I can't hear that
probe tone anymore.

844
00:51:02,800 --> 00:51:04,620
Something happened to it.

845
00:51:04,620 --> 00:51:06,990
Well, two-tone suppression
happened to it.

846
00:51:06,990 --> 00:51:11,530
The masker masked the response
to the probe tone or test tone.

847
00:51:13,554 --> 00:51:15,470
And look at the shapes
of those tuning curves.

848
00:51:15,470 --> 00:51:18,050
They look like good old
auditory nerve fibers.

849
00:51:18,050 --> 00:51:19,350
They have a CF.

850
00:51:19,350 --> 00:51:21,940
The CF is right at the probe.

851
00:51:21,940 --> 00:51:23,125
They have a tip region.

852
00:51:24,240 --> 00:51:25,690
They have a tail region.

853
00:51:25,690 --> 00:51:30,120
If you measure the
sharpness, how wide they are,

854
00:51:30,120 --> 00:51:31,960
they're really
sharp at high CFs.

855
00:51:31,960 --> 00:51:35,445
And they get a little bit
broader as a CFs goes down.

856
00:51:36,830 --> 00:51:39,230
At high CFs they have
a tip and a tail.

857
00:51:39,230 --> 00:51:41,530
At low CFs they look
more like v-shape.

858
00:51:44,510 --> 00:51:48,430
We can go back to the
auditory nerve tuning curve

859
00:51:48,430 --> 00:51:49,530
with those in mind.

860
00:51:54,580 --> 00:51:55,970
And look how similar they are.

861
00:51:55,970 --> 00:52:00,690
Here's high CF,
tip and the tail.

862
00:52:00,690 --> 00:52:03,620
Low CF, just sort of a
plain v ad they're wider.

863
00:52:05,680 --> 00:52:08,560
Human psychophysical
tuning curves

864
00:52:08,560 --> 00:52:10,753
have that same general look.

865
00:52:14,750 --> 00:52:17,400
Now, remember, this is a
very different paradigm.

866
00:52:17,400 --> 00:52:19,770
Here there are two tones.

867
00:52:19,770 --> 00:52:21,670
The probe tone is one of them.

868
00:52:21,670 --> 00:52:24,870
And the masker or the
second suppressor tone

869
00:52:24,870 --> 00:52:26,200
is the second one.

870
00:52:26,200 --> 00:52:29,110
Whereas in good old fashioned
auditory nerve fiber

871
00:52:29,110 --> 00:52:34,160
tuning curve there was just
one, the excitatory tone.

872
00:52:36,550 --> 00:52:38,205
OK, so psychophysical
tuning curves

873
00:52:38,205 --> 00:52:40,980
are obtained from humans
in the following paradigm.

874
00:52:40,980 --> 00:52:42,860
We went over that.

875
00:52:42,860 --> 00:52:44,860
These tuning curves and
the neural tuning curves

876
00:52:44,860 --> 00:52:46,330
from animals are
roughly similar.

877
00:52:49,950 --> 00:52:54,240
Now, what would you expect
to happen to these tuning

878
00:52:54,240 --> 00:52:59,370
curves and the
neural tuning curves

879
00:52:59,370 --> 00:53:01,175
if you had an outer
hair cell problem?

880
00:53:15,630 --> 00:53:17,430
And this is kind of
the classic-- oh, yeah,

881
00:53:17,430 --> 00:53:20,490
you can sort of pass that
around-- a classic exam

882
00:53:20,490 --> 00:53:21,430
question.

883
00:53:21,430 --> 00:53:23,040
Draw a tuning curve.

884
00:53:23,040 --> 00:53:24,840
So you label this
with frequency.

885
00:53:27,560 --> 00:53:35,290
This is the sound pressure level
for a response-- I don't know.

886
00:53:35,290 --> 00:53:40,806
We can say whatever response you
want to-- 10 spikes per second.

887
00:53:45,490 --> 00:53:46,950
Label the CF.

888
00:53:46,950 --> 00:53:47,510
Here it is.

889
00:53:49,300 --> 00:53:50,010
This is a normal.

890
00:53:53,310 --> 00:53:54,390
What's the axis here?

891
00:53:54,390 --> 00:53:58,150
Well, the CF might be-- the
threshold might be at 0 dB.

892
00:53:59,400 --> 00:54:02,290
The tail comes in-- let's go
to our animal tuning curve

893
00:54:02,290 --> 00:54:03,420
just so we get this right.

894
00:54:05,344 --> 00:54:07,270
Oops, pressed the wrong button.

895
00:54:08,990 --> 00:54:13,630
OK, so the tip on this
one it's about 20.

896
00:54:13,630 --> 00:54:16,340
The tail is coming in about 60.

897
00:54:16,340 --> 00:54:19,330
So we are starting down--
well, let's say it's 20.

898
00:54:19,330 --> 00:54:25,055
This is going to be
60 dBs SPL-- normal.

899
00:54:26,760 --> 00:54:32,210
Draw the tuning curve in an
animal where the outer hair

900
00:54:32,210 --> 00:54:33,075
cells are damaged.

901
00:54:35,807 --> 00:54:37,515
Well, you could say,
there's no response.

902
00:54:37,515 --> 00:54:39,060
That wouldn't be quite right.

903
00:54:42,480 --> 00:54:51,300
OK, remember we're-- this is
the nerve fiber we're recording

904
00:54:51,300 --> 00:54:53,280
from, a type one.

905
00:54:53,280 --> 00:54:55,920
This is the inner hair cell.

906
00:54:55,920 --> 00:54:57,320
These are the outer hair cells.

907
00:55:05,450 --> 00:55:08,230
And we're saying damage
them, lesion them.

908
00:55:09,370 --> 00:55:11,810
You could have it
in a knockout animal

909
00:55:11,810 --> 00:55:13,745
where they had
lost their Preston.

910
00:55:18,690 --> 00:55:21,395
OK, so the cochlear
amplifier is lost.

911
00:55:23,070 --> 00:55:24,549
What sort of a
hearing loss do have

912
00:55:24,549 --> 00:55:26,090
when you lose the
cochlear amplifier?

913
00:55:29,670 --> 00:55:31,340
40 to 60 dB, right?

914
00:55:32,670 --> 00:55:34,510
Well, what's this interval?

915
00:55:34,510 --> 00:55:35,750
40 dB, right?

916
00:55:36,980 --> 00:55:40,180
And it turns out,
when you record

917
00:55:40,180 --> 00:55:47,760
from a preparation in which the
outer hair cells are lesioned,

918
00:55:47,760 --> 00:55:55,090
this is the kind of tuning curve
you find when the outer hair

919
00:55:55,090 --> 00:55:59,720
cells are killed or lesioned--
a tip-less tuning curve.

920
00:55:59,720 --> 00:56:03,842
At least from these high
frequencies that have a tip.

921
00:56:03,842 --> 00:56:07,260
And the lows they
look more bowl shaped.

922
00:56:07,260 --> 00:56:09,495
But there's a 40 to
60 dB hearing loss.

923
00:56:09,495 --> 00:56:13,180
You're not deaf, but you have
a greatly altered function.

924
00:56:15,610 --> 00:56:20,400
How good would this
function be for telling

925
00:56:20,400 --> 00:56:28,230
the difference between
1,000 hertz and 1,002 hertz?

926
00:56:28,230 --> 00:56:30,040
Not so good, right?

927
00:56:30,040 --> 00:56:34,020
You need a very
sharply tuned function

928
00:56:34,020 --> 00:56:39,350
to tell or discriminate between
two closely spaced frequencies.

929
00:56:39,350 --> 00:56:41,680
If you have an outer
hair cell problem,

930
00:56:41,680 --> 00:56:45,090
not only are your going
to be much less sensitive,

931
00:56:45,090 --> 00:56:47,612
but you're not going to be
so good at distinguishing

932
00:56:47,612 --> 00:56:48,445
between frequencies.

933
00:56:50,730 --> 00:56:53,100
Another way to think
about it is that if there

934
00:56:53,100 --> 00:56:59,190
were a whole bunch of
frequencies down here

935
00:56:59,190 --> 00:57:01,680
and your hearing
aid boosted them,

936
00:57:01,680 --> 00:57:04,290
you wouldn't be able to
listen to your characteristic

937
00:57:04,290 --> 00:57:08,250
frequency anymore, because
these side frequencies were

938
00:57:08,250 --> 00:57:10,570
getting into your response area.

939
00:57:10,570 --> 00:57:19,020
So these are non-selective
response areas,

940
00:57:19,020 --> 00:57:22,800
where the normal or sharply
tuned are very selective.

941
00:57:24,917 --> 00:57:26,250
And what are they selective for?

942
00:57:26,250 --> 00:57:28,795
For sound frequency.

943
00:57:35,520 --> 00:57:37,980
OK, so the outer
hair cells give you

944
00:57:37,980 --> 00:57:42,490
this big boost in sensitivity
and sharp tuning of the tip.

945
00:57:43,530 --> 00:57:46,175
That's the cochlear amplifier
part of the function.

946
00:57:48,540 --> 00:57:50,900
OK, now, how could we do this?

947
00:57:50,900 --> 00:57:52,624
Well, recently, within
the last 10 years,

948
00:57:52,624 --> 00:57:54,290
you can have a
[? knocked ?] out animal.

949
00:57:55,540 --> 00:57:58,180
But in the old days, you
could lesion outer hair cells

950
00:57:58,180 --> 00:57:59,840
by many means.

951
00:57:59,840 --> 00:58:02,950
You could lesion
them by loud sounds.

952
00:58:02,950 --> 00:58:04,580
Well, loud sounds
actually end up

953
00:58:04,580 --> 00:58:07,780
affecting inner hair cells
a little bit as well.

954
00:58:07,780 --> 00:58:10,800
So the preferred method of
lesioning outer hair cells

955
00:58:10,800 --> 00:58:13,070
was with drugs.

956
00:58:13,070 --> 00:58:18,646
For example, kanamycin is
a very good antibiotic.

957
00:58:20,410 --> 00:58:21,650
It kills bacteria.

958
00:58:21,650 --> 00:58:22,900
Unfortunately, it's audatoxic.

959
00:58:22,900 --> 00:58:25,440
It kills hair cells.

960
00:58:25,440 --> 00:58:28,380
And if you give it to animals
in just the right dose,

961
00:58:28,380 --> 00:58:31,620
you can kill the outer hair
cells, which for some reason--

962
00:58:31,620 --> 00:58:34,710
it's not known-- are
more sensitive to them.

963
00:58:34,710 --> 00:58:36,140
If you give them
a higher dose, it

964
00:58:36,140 --> 00:58:38,320
will also kill the
inner hair cell.

965
00:58:38,320 --> 00:58:40,680
But you can create
animal preparations

966
00:58:40,680 --> 00:58:43,490
in which the outer
hair cells are gone

967
00:58:43,490 --> 00:58:45,350
and the inner hassles
are remaining,

968
00:58:45,350 --> 00:58:48,560
at least over a particular
part of the cochlea.

969
00:58:48,560 --> 00:58:51,181
And from that part, you can
record these tip-less tuning

970
00:58:51,181 --> 00:58:51,680
curves.

971
00:58:58,730 --> 00:59:02,770
OK, so that is
mostly what I want

972
00:59:02,770 --> 00:59:05,680
to say about place coding
for sound frequency.

973
00:59:07,320 --> 00:59:11,230
And now, I want to get into the
second code for sound frequency

974
00:59:11,230 --> 00:59:15,190
that we have, which is
a temporal code that's

975
00:59:15,190 --> 00:59:20,320
based on the finding
of temporal synchrony

976
00:59:20,320 --> 00:59:21,540
in the auditory nerve.

977
00:59:23,110 --> 00:59:24,890
This is the so-called
phase-locking.

978
00:59:26,600 --> 00:59:30,270
Again, we're doing the same kind
of experimental preparation.

979
00:59:30,270 --> 00:59:33,630
We stick are recording
electrode in the auditory nerve.

980
00:59:33,630 --> 00:59:37,650
And we record from one
single auditory nerve fiber.

981
00:59:37,650 --> 00:59:39,670
And we measure it's spikes.

982
00:59:39,670 --> 00:59:41,490
Each one of these
little blips is a spike.

983
00:59:44,350 --> 00:59:48,120
The very top trace is
the sound wave form.

984
00:59:48,120 --> 00:59:51,210
The next trace is the response
of the auditory nerve fiber.

985
00:59:51,210 --> 00:59:55,070
And these are supra-imposed
multiple traces.

986
00:59:55,070 --> 00:59:57,780
And that trace is
with no stimulus.

987
00:59:57,780 --> 01:00:02,330
So this auditory nerve fiber
is obviously very happy firing

988
01:00:02,330 --> 01:00:03,850
long, spontaneous activity.

989
01:00:05,940 --> 01:00:07,630
Then let's turn the sound on.

990
01:00:07,630 --> 01:00:09,990
The top trace is on now.

991
01:00:09,990 --> 01:00:11,540
This is with the stimulus.

992
01:00:13,480 --> 01:00:18,440
And look at how these auditory
nerve fiber impulses tend

993
01:00:18,440 --> 01:00:23,540
to line up at a particular
phase of the sound stimulus.

994
01:00:25,170 --> 01:00:26,190
What's phase?

995
01:00:26,190 --> 01:00:39,230
Well, it's just the
degrees, the sine wave,

996
01:00:39,230 --> 01:00:44,860
as a function of time,
the sound pressure--

997
01:00:44,860 --> 01:00:51,590
this is sound
pressure-- and it's

998
01:00:51,590 --> 01:01:00,220
going through 360 degrees of
phase here-- 180 degrees here.

999
01:01:00,220 --> 01:01:03,490
And it looks like
many of the spikes

1000
01:01:03,490 --> 01:01:10,580
are lining up around
80 degree point.

1001
01:01:10,580 --> 01:01:13,280
So a lot of the
firing is right here.

1002
01:01:13,280 --> 01:01:15,290
Not so much firing here.

1003
01:01:15,290 --> 01:01:16,660
Not so much firing here.

1004
01:01:17,670 --> 01:01:19,275
And then another
waveform comes along

1005
01:01:19,275 --> 01:01:22,290
and you get some more
firing about the same time.

1006
01:01:22,290 --> 01:01:25,740
Now, one very
common misconception

1007
01:01:25,740 --> 01:01:29,930
about phase-locking is that
every time the sound wave form

1008
01:01:29,930 --> 01:01:33,680
goes through-- in
this case 80 degrees--

1009
01:01:33,680 --> 01:01:35,230
the fiber fires an impulse.

1010
01:01:35,230 --> 01:01:36,830
That's not true at all.

1011
01:01:36,830 --> 01:01:40,040
Here is a single trace, showing
excellent phase-locking.

1012
01:01:41,920 --> 01:01:45,780
And there's a response
to the first wave form.

1013
01:01:45,780 --> 01:01:48,650
But then the fiber takes
a break and doesn't

1014
01:01:48,650 --> 01:01:49,830
respond during the second.

1015
01:01:51,420 --> 01:01:54,680
And it looks like it responds
on the third and the fourth.

1016
01:01:55,720 --> 01:01:57,750
But then it takes a
longer break and doesn't

1017
01:01:57,750 --> 01:02:02,640
respond at the fifth or sixth,
but it responds at the seventh,

1018
01:02:02,640 --> 01:02:06,190
and not at the eighth or ninth,
then on the 10th and 11th.

1019
01:02:06,190 --> 01:02:07,450
So it doesn't matter.

1020
01:02:07,450 --> 01:02:10,710
You don't have to respond
in every single waveform.

1021
01:02:10,710 --> 01:02:13,570
You can respond in
one wave form and take

1022
01:02:13,570 --> 01:02:19,080
a break for 100 waveforms,
as long as when you respond,

1023
01:02:19,080 --> 01:02:22,100
the next time it's
on the same point

1024
01:02:22,100 --> 01:02:25,285
or in the same phase
in the sound wave.

1025
01:02:27,820 --> 01:02:31,070
So typically, to
get these data, you

1026
01:02:31,070 --> 01:02:33,260
average over many
hundreds or even

1027
01:02:33,260 --> 01:02:37,660
thousands of stimulus cycles,
where one complete cycle

1028
01:02:37,660 --> 01:02:39,865
is 0 to 360 degrees.

1029
01:02:41,720 --> 01:02:47,570
These are plots of
auditory nerve firing.

1030
01:02:47,570 --> 01:02:51,590
So this is a firing rate access
percent of total impulses.

1031
01:02:53,120 --> 01:02:55,030
This is now a time axis.

1032
01:02:57,060 --> 01:02:59,810
So we're just saying when
does it fire along the time.

1033
01:03:01,200 --> 01:03:03,290
And the stimulus,
I believe, here

1034
01:03:03,290 --> 01:03:07,370
is 1,000 hertz, so it's the
middle of the hearing range.

1035
01:03:07,370 --> 01:03:08,870
And this is excellent
phase-locking.

1036
01:03:12,770 --> 01:03:15,110
If you were to
quantify this-- there

1037
01:03:15,110 --> 01:03:16,860
are many ways to
quantify this-- but you

1038
01:03:16,860 --> 01:03:22,140
could fit, for example, a
Fourier series, to that.

1039
01:03:22,140 --> 01:03:24,530
And you could plot
just the fundamental

1040
01:03:24,530 --> 01:03:26,340
of the Fourier series.

1041
01:03:26,340 --> 01:03:29,030
And that's what's known as the
synchronization coefficient.

1042
01:03:31,530 --> 01:03:33,480
And plot it as a
function of frequency.

1043
01:03:34,520 --> 01:03:36,800
You could make your
measurements at 1,000 hertz,

1044
01:03:36,800 --> 01:03:38,175
which is this
point on the graph.

1045
01:03:39,280 --> 01:03:41,260
You could make them
at 5,000 hertz.

1046
01:03:41,260 --> 01:03:43,025
You could make
them at 500 hertz.

1047
01:03:45,320 --> 01:03:47,630
This synchronization
coefficient ends up

1048
01:03:47,630 --> 01:03:50,740
being between 0.8 and
0.9 for low frequencies.

1049
01:03:50,740 --> 01:03:53,370
And then it rolls
off essentially

1050
01:03:53,370 --> 01:03:57,560
to be random firing at
around 3,000 or 4,000,

1051
01:03:57,560 --> 01:04:00,100
certainly by 5,000 hertz.

1052
01:04:00,100 --> 01:04:03,970
So this behavior,
this phase-locking

1053
01:04:03,970 --> 01:04:07,205
goes away toward the high
end of our hearing range.

1054
01:04:08,910 --> 01:04:14,360
It just means that the auditory
nerve can no longer synchronize

1055
01:04:14,360 --> 01:04:15,910
at very high frequency.

1056
01:04:15,910 --> 01:04:17,300
So what's going on here?

1057
01:04:24,570 --> 01:04:28,400
The auditory nerve fiber
is getting its messages

1058
01:04:28,400 --> 01:04:29,760
from the hair cell, right?

1059
01:04:31,890 --> 01:04:34,160
Here's the auditory nerve fiber.

1060
01:04:34,160 --> 01:04:36,172
And it's hooked up to
an inner hair cell.

1061
01:04:41,260 --> 01:04:42,380
And it's sending messages.

1062
01:04:43,700 --> 01:04:44,628
What are the messages?

1063
01:04:44,628 --> 01:04:45,336
Neurotransmitter.

1064
01:04:48,750 --> 01:04:51,600
When the wave form
goes like this,

1065
01:04:51,600 --> 01:04:53,640
the auditory nerve
fiber is responding.

1066
01:04:53,640 --> 01:04:55,480
Ah, it's getting lots
of neurotransmitter.

1067
01:04:57,950 --> 01:05:00,870
Well, that was when
the stereocilia

1068
01:05:00,870 --> 01:05:02,180
were bent one direction.

1069
01:05:03,910 --> 01:05:06,040
Ions flowed in.

1070
01:05:07,400 --> 01:05:09,069
The inner hair cell
was depolarized.

1071
01:05:09,069 --> 01:05:10,610
It released lots of
neurotransmitter.

1072
01:05:12,930 --> 01:05:16,550
Let's go a little
bit longer in time

1073
01:05:16,550 --> 01:05:18,580
to this bottom part
of the phase curve.

1074
01:05:19,770 --> 01:05:23,850
The stereocilia were bent
the opposite direction.

1075
01:05:23,850 --> 01:05:25,645
The ion channels closed off.

1076
01:05:26,760 --> 01:05:30,340
The inner hair cell went back to
its rest-- minus 80 millivolts,

1077
01:05:30,340 --> 01:05:30,870
let's say.

1078
01:05:31,960 --> 01:05:34,090
And it said, I'm
not excited anymore.

1079
01:05:34,090 --> 01:05:38,440
I'm going to shut off the
flow of neurotransmitter.

1080
01:05:38,440 --> 01:05:42,739
The auditory nerve fiber
goes, oh, we're quiet.

1081
01:05:42,739 --> 01:05:43,780
We don't need to respond.

1082
01:05:46,150 --> 01:05:50,680
Go back to the other direction,
then the stereocilia back

1083
01:05:50,680 --> 01:05:51,850
the other way.

1084
01:05:51,850 --> 01:05:53,340
Ah, I'm depolarized.

1085
01:05:53,340 --> 01:05:55,492
I'm going to go to
minus 30 millivolts.

1086
01:05:55,492 --> 01:05:57,200
Ah, well, let's release
neurotransmitter.

1087
01:05:59,150 --> 01:06:01,100
Oh, wow, there's
something going on.

1088
01:06:01,100 --> 01:06:02,600
I'm going to fire.

1089
01:06:02,600 --> 01:06:05,060
I'm going to fire all
these action potentials.

1090
01:06:05,060 --> 01:06:06,870
It's going back and
forth, back and forth.

1091
01:06:08,420 --> 01:06:12,120
At some point though, this
is going back and forth

1092
01:06:12,120 --> 01:06:16,710
so fast that this just
gets to be a blur.

1093
01:06:18,330 --> 01:06:19,700
There is a sound there.

1094
01:06:19,700 --> 01:06:22,040
It's depolarizing the hair cell.

1095
01:06:22,040 --> 01:06:25,250
But it can't do this
push pull kind of thing.

1096
01:06:25,250 --> 01:06:27,140
It's not fast enough.

1097
01:06:27,140 --> 01:06:29,050
Even though there's a
nice synaptic ribbon

1098
01:06:29,050 --> 01:06:32,070
there to coordinate the
release of the vesicles,

1099
01:06:32,070 --> 01:06:33,790
it gets overwhelmed.

1100
01:06:33,790 --> 01:06:38,730
Remember, at 1,000 hertz,
this is going back and forth

1101
01:06:38,730 --> 01:06:39,480
in 1 millisecond.

1102
01:06:41,050 --> 01:06:43,670
And 5,000 hertz, it's
going back and forth

1103
01:06:43,670 --> 01:06:46,050
five times in 1 millisecond.

1104
01:06:46,050 --> 01:06:47,381
That's pretty fast.

1105
01:06:47,381 --> 01:06:48,380
And it gets overwhelmed.

1106
01:06:49,900 --> 01:06:50,960
There's a response.

1107
01:06:50,960 --> 01:06:54,428
There's more action potentials
with the stimulus than without.

1108
01:06:54,428 --> 01:06:55,886
But they're no
longer synchronized.

1109
01:06:57,130 --> 01:06:57,970
It gets overwhelmed.

1110
01:06:57,970 --> 01:06:59,615
And phase-lacking goes away.

1111
01:07:01,330 --> 01:07:05,960
We can distinguish
5,000 from 6,000 hertz

1112
01:07:05,960 --> 01:07:07,465
very nicely when we listen.

1113
01:07:08,690 --> 01:07:11,260
We're not using this
code, because there's

1114
01:07:11,260 --> 01:07:15,440
no temporal synchrony in
the auditory nerve at very

1115
01:07:15,440 --> 01:07:16,950
high frequencies.

1116
01:07:16,950 --> 01:07:22,560
This is a kind of an interesting
code for sound frequency,

1117
01:07:22,560 --> 01:07:25,060
because the timing
is going to be

1118
01:07:25,060 --> 01:07:26,710
different for
different frequencies.

1119
01:07:28,190 --> 01:07:38,220
Imagine at low frequencies--
and imagine just

1120
01:07:38,220 --> 01:07:41,130
for the sake of argument--
that the auditory nerve

1121
01:07:41,130 --> 01:07:46,280
fiber is going to respond on
every single stimulus peak.

1122
01:07:46,280 --> 01:07:52,700
Let's say this is 1,000 hertz.

1123
01:07:52,700 --> 01:08:00,040
And now let's say we dial
in 2,000 hertz, which

1124
01:08:00,040 --> 01:08:04,830
is going to end up
going twice as fast.

1125
01:08:04,830 --> 01:08:07,020
I'm not a very
good artists here.

1126
01:08:07,020 --> 01:08:09,900
But you can imagine
that the firing

1127
01:08:09,900 --> 01:08:15,020
is going to be twice as often,
if for the sake of argument

1128
01:08:15,020 --> 01:08:20,410
we're firing in every stimulus
frequency, which may not

1129
01:08:20,410 --> 01:08:21,340
happen.

1130
01:08:21,340 --> 01:08:23,200
But this is kind of
an interesting code.

1131
01:08:23,200 --> 01:08:25,370
Because if you're
sitting in the brain

1132
01:08:25,370 --> 01:08:28,600
and you're getting
firing very far apart,

1133
01:08:28,600 --> 01:08:31,094
you're going to say, OK,
that's a low frequency.

1134
01:08:32,600 --> 01:08:34,699
But if you're getting
firing very close together,

1135
01:08:34,699 --> 01:08:38,490
you're going to say, oh,
that's a higher frequency.

1136
01:08:38,490 --> 01:08:42,210
So is there some little
detector in the brain

1137
01:08:42,210 --> 01:08:44,370
that's detecting
these intervals?

1138
01:08:44,370 --> 01:08:46,470
How fast the firing?

1139
01:08:46,470 --> 01:08:48,180
Well, we don't know that.

1140
01:08:48,180 --> 01:08:49,920
But we certainly
know that a code

1141
01:08:49,920 --> 01:08:56,240
is available in the auditory
nerve at low frequencies,

1142
01:08:56,240 --> 01:08:59,319
but not at high frequencies
like 5 kilohertz.

1143
01:08:59,319 --> 01:09:02,020
So what's the
evidence that we're

1144
01:09:02,020 --> 01:09:04,010
using one code or the other?

1145
01:09:04,010 --> 01:09:08,660
Clearly, the place
code has to provide us

1146
01:09:08,660 --> 01:09:10,899
with frequency information
at a higher frequency.

1147
01:09:10,899 --> 01:09:14,890
There is no temporal code
at those high frequencies.

1148
01:09:14,890 --> 01:09:17,130
Down low, which code do we use?

1149
01:09:17,130 --> 01:09:18,610
Well, we probably use both.

1150
01:09:20,790 --> 01:09:22,919
That's another way of
saying, I'm not really sure.

1151
01:09:25,890 --> 01:09:30,810
But let me give you some data
from musical intervals that

1152
01:09:30,810 --> 01:09:33,616
might suggest that
this time code is used.

1153
01:09:35,000 --> 01:09:37,290
What are the data?

1154
01:09:37,290 --> 01:09:40,380
We have to talk a little
bit about perception

1155
01:09:40,380 --> 01:09:41,290
of musical intervals.

1156
01:09:44,960 --> 01:09:47,800
And we might as well start
out with the most important

1157
01:09:47,800 --> 01:09:50,983
musical interval,
which is the octave.

1158
01:09:52,210 --> 01:09:55,200
Does everybody know
what an octave is?

1159
01:09:55,200 --> 01:09:57,704
Yeah, what it is an octave?

1160
01:09:59,950 --> 01:10:01,830
I can't explain
it, but I know it.

1161
01:10:04,000 --> 01:10:04,960
What about on a piano?

1162
01:10:04,960 --> 01:10:09,045
You go down and hit middle
C, where is the octave?

1163
01:10:09,045 --> 01:10:10,382
AUDIENCE: The next C.

1164
01:10:10,382 --> 01:10:11,590
PROFESSOR: The next C, right.

1165
01:10:11,590 --> 01:10:13,920
You've even called
it the same letter,

1166
01:10:13,920 --> 01:10:15,675
because it sounds so similar.

1167
01:10:16,740 --> 01:10:20,200
But in precise physical
terms, an octave

1168
01:10:20,200 --> 01:10:21,435
is a doubling of frequency.

1169
01:10:23,330 --> 01:10:27,720
Whatever frequency middle C was,
if you double that frequency,

1170
01:10:27,720 --> 01:10:31,310
you get an octave
above middle C.

1171
01:10:31,310 --> 01:10:36,170
So we have some data
here for two intervals,

1172
01:10:36,170 --> 01:10:40,310
one 440 hertz-- two
frequencies, one 440 hertz

1173
01:10:40,310 --> 01:10:42,330
and another an
octave above, 880.

1174
01:10:42,330 --> 01:10:43,760
So double it.

1175
01:10:43,760 --> 01:10:48,750
And why did we pick 440 hertz?

1176
01:10:48,750 --> 01:10:54,539
So that corresponds to a
note-- just-- yeah, right,

1177
01:10:54,539 --> 01:10:56,080
it corresponds to
A. And I was trying

1178
01:10:56,080 --> 01:10:59,460
to think if the A is
below or above middle C. I

1179
01:10:59,460 --> 01:11:00,670
think it's above.

1180
01:11:00,670 --> 01:11:03,600
So what's important-- you
guys knew that right away.

1181
01:11:03,600 --> 01:11:04,770
What's important about that?

1182
01:11:04,770 --> 01:11:06,805
AUDIENCE: Orchestras tune to it.

1183
01:11:06,805 --> 01:11:08,180
PROFESSOR: Orchestras
tune to it.

1184
01:11:08,180 --> 01:11:10,540
So can you give it to me?

1185
01:11:11,880 --> 01:11:17,900
OK, I'll give it to you.
[WHISTLES] Sorry, that's A 440.

1186
01:11:17,900 --> 01:11:20,240
And so here's A
440 on the violin.

1187
01:11:20,240 --> 01:11:24,150
[PLUCKS A NOTE] OK,
now, how do I know that?

1188
01:11:25,831 --> 01:11:27,080
Because orchestras tune to it.

1189
01:11:28,710 --> 01:11:32,340
So for about 20 years,
I sat in an orchestra.

1190
01:11:32,340 --> 01:11:36,800
And the first thing you did--
[LAUGHS] OK, tune, you guys.

1191
01:11:36,800 --> 01:11:38,790
And what instrument
gives the tuning note?

1192
01:11:38,790 --> 01:11:41,310
If you're in junior high, it's
this little electronic thing.

1193
01:11:41,310 --> 01:11:44,760
But if you're in the BSO, what
instrument gives the tuning

1194
01:11:44,760 --> 01:11:45,468
note?

1195
01:11:45,468 --> 01:11:46,472
AUDIENCE: The violin.

1196
01:11:46,472 --> 01:11:48,513
PROFESSOR: No, violins go
out of tune like crazy.

1197
01:11:48,513 --> 01:11:49,400
AUDIENCE: Oboe.

1198
01:11:49,400 --> 01:11:51,150
PROFESSOR: Oboe, right,
because the oboe's

1199
01:11:51,150 --> 01:11:52,990
a very stable instrument.

1200
01:11:52,990 --> 01:11:55,030
And if the barometric
pressure goes up

1201
01:11:55,030 --> 01:11:56,870
and the humidity
goes down, the oboe's

1202
01:11:56,870 --> 01:11:59,550
still going to give you A 440.

1203
01:11:59,550 --> 01:12:04,840
So the A 440 is a very
important musical note.

1204
01:12:04,840 --> 01:12:07,040
And all these
instruments, of course,

1205
01:12:07,040 --> 01:12:08,519
have a whole bunch of harmonics.

1206
01:12:08,519 --> 01:12:11,060
This string is vibrating in a
whole bunch of different modes.

1207
01:12:12,380 --> 01:12:17,140
But the fundamental, the length
where the whole string vibrates

1208
01:12:17,140 --> 01:12:18,490
is A 440.

1209
01:12:18,490 --> 01:12:22,020
OK, so here's A 440
or approximately.

1210
01:12:26,300 --> 01:12:29,350
Now, an octave above that
is a very nice sounds.

1211
01:12:29,350 --> 01:12:36,500
It's another A. That's
A 880, the fundamental.

1212
01:12:36,500 --> 01:12:42,235
And if I sound them together,
they sound very beautiful.

1213
01:12:43,720 --> 01:12:46,570
And in any musical
culture, an octave

1214
01:12:46,570 --> 01:12:49,240
is a very predominant
interval, because it

1215
01:12:49,240 --> 01:12:51,620
sounds so wonderful to your ear.

1216
01:12:53,030 --> 01:12:56,520
And violinists, I can tell you
from experience, practice a lot

1217
01:12:56,520 --> 01:13:00,140
of time trying to tune
their octaves perfectly.

1218
01:13:00,140 --> 01:13:02,710
And if you've ever listened to
a professionals go like this,

1219
01:13:02,710 --> 01:13:04,510
and every time they
go up and down,

1220
01:13:04,510 --> 01:13:06,690
the octave is just beautiful.

1221
01:13:06,690 --> 01:13:10,395
But if you've been to middle
school or elementary school,

1222
01:13:10,395 --> 01:13:11,395
it's a little different.

1223
01:13:13,010 --> 01:13:16,710
Because sometimes when those
students play an octave,

1224
01:13:16,710 --> 01:13:20,090
it doesn't really hit
to be exactly an octave.

1225
01:13:20,090 --> 01:13:22,850
And now I'm going to give
you a demonstration that's

1226
01:13:22,850 --> 01:13:26,320
440 and not quite 880.

1227
01:13:26,320 --> 01:13:29,200
OK And it's not going to
sound exactly the same.

1228
01:13:29,200 --> 01:13:30,162
So here it is.

1229
01:13:34,900 --> 01:13:38,190
And that's an interval I've
listened to many times.

1230
01:13:39,240 --> 01:13:40,910
But it's not a desired interval.

1231
01:13:40,910 --> 01:13:42,530
It's a very dissonant interval.

1232
01:13:43,790 --> 01:13:49,070
And what is terribly
displeasing about something

1233
01:13:49,070 --> 01:13:52,760
that's not quite an
octave versus an octave.

1234
01:13:53,850 --> 01:13:56,830
That is a question
that the place code

1235
01:13:56,830 --> 01:13:59,350
has a lot of problems with.

1236
01:13:59,350 --> 01:14:05,520
Because, for example, along
the cochlea there is a place--

1237
01:14:05,520 --> 01:14:10,220
it's quite near the
apex-- for the 440.

1238
01:14:10,220 --> 01:14:12,680
And then if you go
more basally, there's

1239
01:14:12,680 --> 01:14:15,170
another place for the 880.

1240
01:14:15,170 --> 01:14:18,684
And there's a place
for the 879 and 878.

1241
01:14:18,684 --> 01:14:20,100
And those would
be very dissonant.

1242
01:14:21,190 --> 01:14:26,980
But there's no reason that
those two things have any links

1243
01:14:26,980 --> 01:14:29,240
to one another in
the place code.

1244
01:14:29,240 --> 01:14:32,420
There's a place for 1,000
and a place for 2,000.

1245
01:14:32,420 --> 01:14:34,440
Why do they sound so
wonderful together?

1246
01:14:35,740 --> 01:14:38,940
The timing code though
has an answer for that.

1247
01:14:38,940 --> 01:14:43,410
And here is some
data to show you

1248
01:14:43,410 --> 01:14:46,060
why those two intervals
[? meld ?] very good together.

1249
01:14:48,050 --> 01:14:50,270
If you look at
the spike pattern,

1250
01:14:50,270 --> 01:14:54,560
in response to either
one of these frequencies,

1251
01:14:54,560 --> 01:14:58,010
and compute what are
called the intervals

1252
01:14:58,010 --> 01:15:02,840
between the spike-- so-called
interspike intervals--

1253
01:15:02,840 --> 01:15:05,715
every time you get a spike,
you start your clock ticking.

1254
01:15:07,320 --> 01:15:11,210
And that interval is timed
until the next spike fires.

1255
01:15:11,210 --> 01:15:13,160
That's an interspike interval.

1256
01:15:14,570 --> 01:15:17,530
And obviously, if
this is phase-locked,

1257
01:15:17,530 --> 01:15:20,740
these intervals are going
to have a close relationship

1258
01:15:20,740 --> 01:15:23,760
to the stimulus period.

1259
01:15:23,760 --> 01:15:28,150
So here's a spike and here's
an approximately two-cycle

1260
01:15:28,150 --> 01:15:29,710
interspike interval.

1261
01:15:29,710 --> 01:15:34,470
Here's a short interval,
but it's one complete phase.

1262
01:15:34,470 --> 01:15:37,660
Here's a long interval, but
it's now three complete phases.

1263
01:15:38,670 --> 01:15:40,610
Here's another
three-phase interval.

1264
01:15:40,610 --> 01:15:42,705
Here's a one-phase interval.

1265
01:15:43,720 --> 01:15:48,450
You could make a very nice
plot of the interspike interval

1266
01:15:48,450 --> 01:15:50,900
in milliseconds, the
time between the spikes.

1267
01:15:52,430 --> 01:15:55,430
And these are the number of
occurrences on the y-axis.

1268
01:15:55,430 --> 01:15:59,060
So for 440 hertz it's
the dashed curve here.

1269
01:15:59,060 --> 01:16:03,180
And you get a big peak
here that's a multiple

1270
01:16:03,180 --> 01:16:05,040
of the period.

1271
01:16:05,040 --> 01:16:08,170
So at 440 hertz,
the sound wave form

1272
01:16:08,170 --> 01:16:11,690
is taking about 2
and 1/2 milliseconds

1273
01:16:11,690 --> 01:16:14,610
to go through one
complete cycle.

1274
01:16:14,610 --> 01:16:18,830
And these intervals would be
firing on successive periods,

1275
01:16:18,830 --> 01:16:21,050
which, obviously, the
nerve fiber can do.

1276
01:16:22,170 --> 01:16:24,500
But sometimes they
take a break, and they

1277
01:16:24,500 --> 01:16:27,290
fire only every other period.

1278
01:16:27,290 --> 01:16:30,060
And that's a double
of this interval.

1279
01:16:30,060 --> 01:16:33,120
And so you have a lot
of firing at about 4

1280
01:16:33,120 --> 01:16:37,450
and 1/2 milliseconds, a lot of
firing at about 7 milliseconds,

1281
01:16:37,450 --> 01:16:39,290
and so on and so forth.

1282
01:16:39,290 --> 01:16:40,880
So this is an
interspike interval

1283
01:16:40,880 --> 01:16:45,317
from auditory nerve firing in
response to this low frequency.

1284
01:16:45,317 --> 01:16:46,650
Now, let's double the frequency.

1285
01:16:48,830 --> 01:16:51,520
Now, the sound wave form
is going back and forth

1286
01:16:51,520 --> 01:16:52,340
twice as fast.

1287
01:16:54,180 --> 01:16:57,920
And you have-- no surprise--
firing, in some cases,

1288
01:16:57,920 --> 01:17:00,500
twice as short intervals.

1289
01:17:00,500 --> 01:17:03,930
So here's an interval
for the 880 hertz

1290
01:17:03,930 --> 01:17:06,420
that's about 1 and
1/2 milliseconds.

1291
01:17:08,520 --> 01:17:10,860
But here we have
a firing pattern

1292
01:17:10,860 --> 01:17:15,290
that's exactly-- within the
limits of experimental error--

1293
01:17:15,290 --> 01:17:18,320
exactly the same as
for the 440 hertz.

1294
01:17:19,660 --> 01:17:22,520
Here we have an interval
that's representative

1295
01:17:22,520 --> 01:17:26,870
of skipping a stimulus waveform
or two stimulus waveforms

1296
01:17:26,870 --> 01:17:27,460
for 880.

1297
01:17:27,460 --> 01:17:32,840
But here we have a peek at
exactly the same as 440 hertz,

1298
01:17:32,840 --> 01:17:35,240
because these
intervals are lining up

1299
01:17:35,240 --> 01:17:39,685
every other one for the
presentation of the octave.

1300
01:17:40,760 --> 01:17:44,030
When you put those
two sounds together,

1301
01:17:44,030 --> 01:17:46,670
you're going to get the
combination pattern.

1302
01:17:46,670 --> 01:17:48,130
And many of the
intervals are going

1303
01:17:48,130 --> 01:17:51,260
to be precisely on one another.

1304
01:17:51,260 --> 01:17:55,290
And that is a very pleasing
sensation for you to listen to.

1305
01:17:56,570 --> 01:17:59,880
If you look at other very
common musical intervals,

1306
01:17:59,880 --> 01:18:03,890
like the fifth or
the fourth, you

1307
01:18:03,890 --> 01:18:07,390
will have many
overlapping periods

1308
01:18:07,390 --> 01:18:10,370
of interspike intervals
in auditory nerve firing.

1309
01:18:10,370 --> 01:18:13,650
And those are very
common musical intervals.

1310
01:18:13,650 --> 01:18:19,830
If you look at dissonant
interval, like 440 and 870,

1311
01:18:19,830 --> 01:18:25,050
there will be no overlap
amongst those two frequencies

1312
01:18:25,050 --> 01:18:26,722
in the auditory nerve firing.

1313
01:18:30,020 --> 01:18:33,250
Now, let's go back
to psychophysics

1314
01:18:33,250 --> 01:18:37,270
and give you one more
interesting piece of the puzzle

1315
01:18:37,270 --> 01:18:42,120
here for why temporal codes
might be important is active

1316
01:18:42,120 --> 01:18:45,740
matches become more difficult--
and actually impossible--

1317
01:18:45,740 --> 01:18:47,015
above 5 kilohertz.

1318
01:18:48,840 --> 01:18:50,230
OK, well, how does that fit in?

1319
01:18:50,230 --> 01:18:54,370
Well, we just said
that phase-locking--

1320
01:18:54,370 --> 01:18:57,110
because the hair cell and
auditory nerve can't keep up

1321
01:18:57,110 --> 01:18:59,260
with one another,
the phase-locking

1322
01:18:59,260 --> 01:19:01,060
diminishes for these
high frequencies.

1323
01:19:01,060 --> 01:19:04,030
And there it becomes
impossible to match

1324
01:19:04,030 --> 01:19:05,660
because you don't
have this timing

1325
01:19:05,660 --> 01:19:07,166
code in the auditory nerve.

1326
01:19:08,760 --> 01:19:13,310
So most of musical sounds
are confined to the spectrum

1327
01:19:13,310 --> 01:19:14,635
below 3 kilohertz.

1328
01:19:16,000 --> 01:19:19,690
If you look even at the upper
limit of the piano keyboard,

1329
01:19:19,690 --> 01:19:21,710
you're sort of right
at 3 kilohertz or so.

1330
01:19:23,380 --> 01:19:24,990
And that's probably the reason.

1331
01:19:24,990 --> 01:19:26,120
It's a very likely reason.

1332
01:19:28,390 --> 01:19:31,510
Now, we have a research
paper for today.

1333
01:19:31,510 --> 01:19:33,850
And I'll give you the
bottom line or the take home

1334
01:19:33,850 --> 01:19:36,930
message for that.

1335
01:19:36,930 --> 01:19:40,120
And this is an interesting
neuralphysiological study

1336
01:19:40,120 --> 01:19:45,260
based on a psychophysical
phenomenon called

1337
01:19:45,260 --> 01:19:48,080
the octave enlargement effect.

1338
01:19:49,750 --> 01:19:53,860
So I wasn't quite
truthful when I told you

1339
01:19:53,860 --> 01:19:58,110
that octaves are the
most perfect interval

1340
01:19:58,110 --> 01:20:02,130
to listen to, because it turns
out if you have people-- if you

1341
01:20:02,130 --> 01:20:05,830
give people a low tone and
give them an oscillator.

1342
01:20:05,830 --> 01:20:08,540
And say, dial in an
octave above that,

1343
01:20:08,540 --> 01:20:11,390
they'll dial it in and say,
ah, that sounds so great.

1344
01:20:11,390 --> 01:20:13,300
But if you look
really carefully,

1345
01:20:13,300 --> 01:20:15,960
it's not exactly an octave.

1346
01:20:15,960 --> 01:20:18,220
It's a small deviation.

1347
01:20:18,220 --> 01:20:22,510
What they dial in-- especially
at high frequencies-- remember,

1348
01:20:22,510 --> 01:20:24,590
you can't do this at
really high frequencies,

1349
01:20:24,590 --> 01:20:28,450
but at 2,500 or 1,500
hertz, toward the high end

1350
01:20:28,450 --> 01:20:32,060
of where you can do it-- they
dial in actually a little bit

1351
01:20:32,060 --> 01:20:33,145
more than an octave.

1352
01:20:33,145 --> 01:20:35,480
And they say, ah,
that sounds great.

1353
01:20:35,480 --> 01:20:37,510
But it's not exactly an octave.

1354
01:20:37,510 --> 01:20:42,000
So this paper looked and said,
what about auditory nerve fiber

1355
01:20:42,000 --> 01:20:46,440
firing can explain this
octave enlargement effect?

1356
01:20:46,440 --> 01:20:49,580
The fact that people
dial in a little bit more

1357
01:20:49,580 --> 01:20:52,720
than an octave for
the upper tone.

1358
01:20:52,720 --> 01:20:54,595
So these are the
psychophysical measurements.

1359
01:20:55,600 --> 01:20:57,540
And they just give
you previous studies.

1360
01:20:57,540 --> 01:21:00,305
What they did was they recorded
from the auditory nerve.

1361
01:21:01,390 --> 01:21:03,920
And they looked at interspike
interval histograms,

1362
01:21:03,920 --> 01:21:05,660
like we've just
been talking about.

1363
01:21:06,970 --> 01:21:10,250
And they saw something
really interesting

1364
01:21:10,250 --> 01:21:11,765
at the very high frequencies.

1365
01:21:12,830 --> 01:21:15,380
So they're going to especially
concentrate in here.

1366
01:21:17,400 --> 01:21:20,970
They found that the
very first interval

1367
01:21:20,970 --> 01:21:24,710
didn't match exactly
what was predicted.

1368
01:21:24,710 --> 01:21:27,730
So this is a stimulus
of 1,750 hertz,

1369
01:21:27,730 --> 01:21:30,400
so toward the right
end of the graph here.

1370
01:21:30,400 --> 01:21:34,570
And where you'd predict
the intervals to happen

1371
01:21:34,570 --> 01:21:38,360
is shown by the
vertical dashed lines.

1372
01:21:38,360 --> 01:21:41,130
And they didn't fire right
on those predictions,

1373
01:21:41,130 --> 01:21:44,055
except for the very
shortest intervals.

1374
01:21:45,530 --> 01:21:47,220
And they said,
what's going on here?

1375
01:21:47,220 --> 01:21:52,160
Well, when you get to
very high frequencies,

1376
01:21:52,160 --> 01:21:54,710
what are we talking about
for these intervals?

1377
01:21:54,710 --> 01:21:57,880
Even at 1,000 hertz,
what's the time scale here?

1378
01:22:01,390 --> 01:22:02,490
This is 1 millisecond.

1379
01:22:05,430 --> 01:22:10,510
What problems do you get when
you ask a nerve fiber to fire

1380
01:22:10,510 --> 01:22:13,780
and then you ask it to fire
again a millisecond later?

1381
01:22:13,780 --> 01:22:15,030
That's a very brief interval.

1382
01:22:15,030 --> 01:22:15,765
Anybody know?

1383
01:22:21,820 --> 01:22:24,010
What is the limit of firing?

1384
01:22:24,010 --> 01:22:28,514
Can nerve fibers fire closely
spaced action potentials,

1385
01:22:28,514 --> 01:22:29,930
you know, less
than a millisecond?

1386
01:22:29,930 --> 01:22:31,346
What's the problem
that they have?

1387
01:22:31,346 --> 01:22:33,110
AUDIENCE: Is it that
they are polarized?

1388
01:22:33,110 --> 01:22:35,740
PROFESSOR: They're
hyperpolarized, right.

1389
01:22:35,740 --> 01:22:36,355
What else?

1390
01:22:36,355 --> 01:22:38,770
AUDIENCE: Because of
the refractory period.

1391
01:22:38,770 --> 01:22:39,770
PROFESSOR: That's right.

1392
01:22:39,770 --> 01:22:47,580
There's something called
the refractory period, which

1393
01:22:47,580 --> 01:22:49,230
can cause them to hyperpolarize.

1394
01:22:49,230 --> 01:22:54,320
So what's happening-- if this
were a nerve cell membrane,

1395
01:22:54,320 --> 01:22:58,770
is what happens is sodium
channels can open up

1396
01:22:58,770 --> 01:23:01,420
to allow sodium to come in
and depolarize the neuron

1397
01:23:01,420 --> 01:23:02,765
and fire an action potential.

1398
01:23:04,550 --> 01:23:06,490
And then those channels
are turned off.

1399
01:23:06,490 --> 01:23:09,600
And potassium channels
open up and allow potassium

1400
01:23:09,600 --> 01:23:11,233
to go back and
even hyperpolarize.

1401
01:23:12,370 --> 01:23:16,640
But these channels take a
little bit of time to recover.

1402
01:23:16,640 --> 01:23:19,180
It takes a little bit
of time for the sodium

1403
01:23:19,180 --> 01:23:23,250
to turn off and get ready to
fire another action potential.

1404
01:23:23,250 --> 01:23:26,020
It takes a lot longer time
for the potassium channel

1405
01:23:26,020 --> 01:23:29,650
to close and get ready to
fire another action potential.

1406
01:23:29,650 --> 01:23:32,580
And the refractory
period is the time

1407
01:23:32,580 --> 01:23:35,370
it takes for everything
to recover fully

1408
01:23:35,370 --> 01:23:36,845
so we're ready to fire again.

1409
01:23:38,570 --> 01:23:41,351
And in the limit, that's
supposed to be about 1

1410
01:23:41,351 --> 01:23:41,850
millisecond.

1411
01:23:44,700 --> 01:23:48,320
That's the absolute
real refractory period.

1412
01:23:48,320 --> 01:23:51,040
So when I was drawing here
things a millisecond and less,

1413
01:23:51,040 --> 01:23:53,280
I wasn't really being truthful.

1414
01:23:53,280 --> 01:23:56,180
There's something called the
relative refractory period,

1415
01:23:56,180 --> 01:23:57,737
which is a couple
of milliseconds.

1416
01:24:01,640 --> 01:24:04,880
And the nerve fiber can
respond, but it's not

1417
01:24:04,880 --> 01:24:07,955
going to respond quite
as quickly as before.

1418
01:24:08,970 --> 01:24:11,510
All these channels
aren't completely reset.

1419
01:24:11,510 --> 01:24:16,520
It's going to take a little
bit longer time to respond.

1420
01:24:16,520 --> 01:24:20,100
That's what's going on
in this very first peak.

1421
01:24:20,100 --> 01:24:22,990
Remember, this first
peak indicates firing

1422
01:24:22,990 --> 01:24:30,130
at successive cycles of the
sound waveform at 1,750 hertz.

1423
01:24:30,130 --> 01:24:32,780
It's a very brief interval.

1424
01:24:32,780 --> 01:24:34,890
And what happened is you fired.

1425
01:24:34,890 --> 01:24:36,470
And then the next
action potential

1426
01:24:36,470 --> 01:24:39,120
you fired, but you
delayed a little bit.

1427
01:24:39,120 --> 01:24:41,140
So the interval is
a little bit longer.

1428
01:24:43,210 --> 01:24:46,270
That pushed this one
up to the next peak,

1429
01:24:46,270 --> 01:24:50,030
so that this interval
was actually too short.

1430
01:24:50,030 --> 01:24:52,380
What the brain is
getting is an interval

1431
01:24:52,380 --> 01:24:54,560
that's a little bit too short.

1432
01:24:54,560 --> 01:24:57,110
When it hears that,
it says, I want

1433
01:24:57,110 --> 01:24:59,900
to recreate the higher frequency
to be an interval that's

1434
01:24:59,900 --> 01:25:02,920
a little too short, I'm going
to dial in a little bit too high

1435
01:25:02,920 --> 01:25:03,490
in frequency.

1436
01:25:05,310 --> 01:25:09,190
OK, because that
frequency sounds better

1437
01:25:09,190 --> 01:25:10,890
with this too short
of an interval.

1438
01:25:12,590 --> 01:25:14,090
So go ahead and read that paper.

1439
01:25:14,090 --> 01:25:19,170
It's a very interesting study
of how neuronal firing can

1440
01:25:19,170 --> 01:25:21,390
give you a
psychophysical phenomena.

1441
01:25:22,610 --> 01:25:23,790
That's quite interesting.

1442
01:25:23,790 --> 01:25:27,230
And it happens especially
at the high frequencies.

1443
01:25:27,230 --> 01:25:28,970
Octave matches the
low frequencies

1444
01:25:28,970 --> 01:25:32,746
are sort of as you would predict
as is auditory nerve firing.

1445
01:25:37,610 --> 01:25:39,310
OK, any questions?

1446
01:25:39,310 --> 01:25:41,430
If not, we'll meet
back on Wednesday.

1447
01:25:41,430 --> 01:25:43,560
And we'll talk about the
cochlear nucleus, which

1448
01:25:43,560 --> 01:25:47,310
is the beginning of the
central auditory pathway.