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

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 range500 - 5000 Hz

Red = speech

6

Structure of the ear

Outer ear

Middle ear

Inner ear(cochlea)

7

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)

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:http://www.youtube.com/watch?v=FsyE9omc20k

9

Middle ear

10

Middle ear

1.3xforce

increase

11

Middle ear

Tympanic membrane:

55 mm2 area

Oval window:3.2 mm2 area

i.e. 17-fold decreasein 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

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 depolarisationGlutamate release No glutamate release

Auditory transduction: like vestibular

Endolymph

Perilymph

27

Endolymph and perilymph

Perilymph

Perilymph

Endolymph

K+ pumped in

Perilymph: normal extracellular fluidEndolymph: 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: http://en.wikipedia.org/wiki/The_Mosquito

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 codingBushy 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

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

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