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Hearing
•Sound and the limits to hearing
•Structure of the ear:
Outer, middle, inner
•Outer ear and middle ear functions
•Inner ear: the cochlea
- Frequency tuning and the cochlear amplifier
- Hair cell damage
•Neural coding of sound: frequency and timing
•Sound localisation: brainstem processing
•Auditory cortex
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What is sound?
2
What is sound?
3
How we measure sound (1)
•Sound intensity = pressure
•Pressure measured in N/m2 = Pa
•Threshold of hearing ~2 x 10-5 Pa
(atmospheric pressure 105 Pa, i.e. we can
detect pressure change of 1 part in
5 billion!)
•P0 = 2 x 10-5 Pa is used as the reference for
hearing measurement
4
How we measure sound (2)
•A given sound pressure level (SPL)
(call it Px), expressed in decibels (dB), is:
SPL = 20 log10 Px/P0
•e.g.
•20 dB (whisper) = 10 times reference SPL
•40 dB (rainfall) = 100 times reference SPL
•60 dB (speech) = 1000 times reference SPL
•80 dB (traffic) = 10000 times reference SPL
•100 dB (Walkman) = 100000 times reference SPL
•140 dB (gun shot) = 10000000x reference SPL
5
How we measure sound (3)
Each line joins
points with the
same subjective
loudness
Our hearing is
most sensitive in
the range
500 - 5000 Hz
Red = speech
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Structure of the ear
Outer ear
Middle ear
Inner ear
(cochlea)
7
Sound sensitivity (dB)
Role of outer ear
Sound from 45°
in front
Sound from ahead
0.1
1
Frequency (kHz)
10
...directional sound sensitivity
(also front-back)
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Demonstrating the role of the outer ear
•Front/back sound
localisation: sound is
changed as it reflects off
the complex shape of
the outer ear
•Try out some sample
recordings…
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Demonstrating the role of the outer ear
Binaural recording using microphones in an artificial head:
http://www.youtube.com/watch?v=FsyE9omc20k
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Middle ear
10
Middle ear
1.3x
force
increase
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Middle ear
Tympanic
membrane:
55 mm2 area
Oval window:
3.2 mm2 area
i.e. 17-fold
decrease
in area
12
Middle ear
•1.3-fold increase in force
•17-fold decrease in surface area
•gives a 22-fold increase in pressure
•i.e. ~26 dB amplification
(20 x log1022)
•More pressure needed to move fluid
than air (higher impedance): this is
supplied by the middle ear bones
13
Middle ear
Size of middle ear
bones
14
Adjusting middle ear transmission
Contraction of
stapedius &
tensor tympani

reduced sound
transmission
15
Loss of middle ear amplification
(middle ear conduction deafness)
Bone
Air
16
The cochlea
17
The cochlea
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The cochlea
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Movement of the basilar membrane (1)
20
Movement of the basilar membrane (2)
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Movement of the basilar membrane (3)
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The organ of Corti
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The organ of Corti
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Hair cells in the organ of Corti
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Movement of the organ of Corti
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Auditory transduction: like vestibular
Endolymph
Perilymph
Depolarisation
Glutamate release
No depolarisation
No glutamate release
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Endolymph and perilymph
K+ pumped in
Perilymph
Endolymph
Perilymph
Perilymph: normal extracellular fluid
Endolymph: high K+, +80 mV
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Cochlear hair cell innervation
~10 afferent nerve
fibres per inner hair
cell
One afferent per
several outer hair
cells
Outer hair cells are
not primarily sensory!
(we’ll see soon what
they are for)
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Cochlear hair cell innervation
The inner hair cells
are the major
sensory receptors
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Frequency coding in
the cochlea
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Movement of the basilar membrane (1)
...how does this work exactly?
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Movement of the basilar membrane (2)
von Bekesy 1960
(in cadavers)
Suggests “place coding”
but not tight enough to
explain human pitch
discrimination
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Movement of the basilar membrane (3)
Basilar membrane
movement in live cats:
Can account for
frequency tuning
But why does it move
more in live cochlea?
Auditory nerve activity
Basilar membrane movement
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Movement of the basilar membrane (4):
the cochlear amplifier
Answer: outer hair cells
are contractile
Depolarisation makes
them contract
(fast motor protein:
prestin)
35
The cochlear amplifier
If a simple depolarisation causes an outer hair cell
to contract, what would an OHC do with an auditory
stimulus?
It will do what your OHCs are doing all the time...
(watch the film clip)
This active contraction of OHCs increases basilar
membrane movement locally:
stronger stimulus for inner hair cells
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Losing the cochlear amplifier:
damage caused by excessive noise
Normal
120 dB 1 hour
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More examples
Normal
Noise damaged
Hearing loss without functional OHCs
can be up to ~60 dB!
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Inner ear hearing loss
(usual in old age)
This underlies the Mosquito teenager deterrent:
http://en.wikipedia.org/wiki/The_Mosquito
3
Auditory nerve activity
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Auditory nerve activity (1)
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Auditory nerve activity (2)
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Auditory nerve activity
•Loudness is coded as number of spikes
•Frequency is coded based on which nerve
fibres are active
•Action potential activity is time-locked to the
stimulus (always at the same part of the
waveform)
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Into the brainstem
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Brainstem auditory pathways
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Cell types in the cochlear nuclei
Stellate cells:
frequency coding
Bushy cells: time
coding
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Processing in the medial/lateral superior
olive: sound localisation
•Intensity differences: lateral superior olive
•Inter-aural time delay: medial superior olive
•How do we work out location from time delay?
47
Straight ahead
Sound
coming
from this
angle
a
a = difference
in path length
(min 1 cm)
sin a = a / b
sin a = 1/20
sin a = 0.05
sin a = 2.8 °
a
a
b
b = distance
between ears
(~20 cm)
•We can tell within <3 ° where a sound is coming from
•The 1 cm path length difference corresponds to a time
difference of about 30 microseconds
48
Sound localisation in the medial superior
olive
•1 cm path length difference = about 30 μs
•How can we detect this?
•Bilateral input already at the second synapse:
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Auditory cortex
•Little studied in humans
•Tonotopic organisation
•Most cells bilateral,
stimulated by one side,
inhibited by the other
•Several areas involved
(possibly up to nine
separate tonotopic maps)
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