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 1

What is sound? 2

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How we measure sound (1) Sound intensity = pressure Pressure measured in N/m 2 = Pa Threshold of hearing ~2 x Pa (atmospheric pressure 10 5 Pa, i.e. we can detect pressure change of 1 part in 5 billion!) P 0 = 2 x Pa is used as the reference for hearing measurement 4

How we measure sound (2) A given sound pressure level (SPL) (call it P x ), expressed in decibels (dB), is: SPL = 20 log 10 P x /P 0 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) = times reference SPL 100 dB (Walkman) = times reference SPL 140 dB (gun shot) = x 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 Hz Red = speech 6

Structure of the ear Outer ear Middle ear Inner ear (cochlea) 7

Role of outer ear Sound sensitivity (dB) Frequency (kHz) Sound from 45° in front Sound from ahead...directional sound sensitivity (also front-back) 8

Demonstrating the role of the outer ear 9 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: 9

Middle ear 10

Middle ear 1.3x force increase 11

Middle ear Tympanic membrane: 55 mm 2 area Oval window: 3.2 mm 2 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 log 10 22) 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 18

The cochlea 19

Movement of the basilar membrane (1) 20

Movement of the basilar membrane (2) 21

Movement of the basilar membrane (3) 22

The organ of Corti 23

The organ of Corti 24

Hair cells in the organ of Corti 25

Movement of the organ of Corti 26

Depolarisation No depolarisation Glutamate release No glutamate release Auditory transduction: like vestibular Endolymph Perilymph 27

Endolymph and perilymph Perilymph Endolymph K + pumped in Perilymph: normal extracellular fluid Endolymph: high K +, +80 mV 28

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) 29

Cochlear hair cell innervation The inner hair cells are the major sensory receptors 30

Frequency coding in the cochlea 31

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 33

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 34

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 36

Losing the cochlear amplifier: damage caused by excessive noise Normal 120 dB 1 hour 37

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: 3

Auditory nerve activity 40

Auditory nerve activity (1) 41

Auditory nerve activity (2) 42

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) 43

Into the brainstem 44

Brainstem auditory pathways 45

Cell types in the cochlear nuclei Stellate cells: frequency coding Bushy cells: time coding 46

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 = difference in path length (min 1 cm) b = distance between ears (~20 cm) sin  = a / b sin a = 1/20 sin a = 0.05 sin  = 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 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: 49

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) 50