hearing sound and the limits to hearing structure of the ear: outer, middle, inner outer ear and...
TRANSCRIPT
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?
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What is sound?
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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
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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
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How we measure sound (3)
Each line joins points with the
same subjective loudness
Our hearing is most sensitive in
the range500 - 5000 Hz
Red = speech
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Structure of the ear
Outer ear
Middle ear
Inner ear(cochlea)
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Role of outer earS
ound
sen
sitiv
ity (
dB)
0.1 1 10Frequency (kHz)
Sound from 45°in front
Sound from ahead
...directional sound sensitivity(also front-back)
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Demonstrating the role of the outer ear
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•Front/back sound localisation: sound is changed as it reflects off the complex shape of the outer ear•Try out some sample recordings…
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
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Middle ear
1.3xforce
increase
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Middle ear
Tympanic membrane:
55 mm2 area
Oval window:3.2 mm2 area
i.e. 17-fold decreasein area
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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
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Middle ear
Size of middle ear bones
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Adjusting middle ear transmission
Contraction of stapedius &
tensor tympani
reduced sound transmission
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Loss of middle ear amplification (middle ear conduction deafness)
Bone
Air
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The cochlea
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The cochlea
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The cochlea
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Movement of the basilar membrane (1)
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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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Depolarisation No depolarisationGlutamate release No glutamate release
Auditory transduction: like vestibular
Endolymph
Perilymph
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Endolymph and perilymph
Perilymph
Perilymph
Endolymph
K+ pumped in
Perilymph: normal extracellular fluidEndolymph: 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 30
Frequency coding in the cochlea
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Movement of the basilar membrane (1)
...how does this work exactly?32
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)
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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! 38
Inner ear hearing loss(usual in old age)
This underlies the Mosquito teenager deterrent: http://en.wikipedia.org/wiki/The_Mosquito
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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 codingBushy 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?
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Straight ahead
Soundcomingfrom thisangle
a = differencein path length(min 1 cm)
b = distancebetween ears(~20 cm)
sin = a / bsin a = 1/20sin a = 0.05sin = 2.8 °
a
b
•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
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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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