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PHOL0005 - Structure & Function Of The Nervous System > Hearing (Theme C) > Flashcards

Hearing (Theme C) Flashcards

1
Q

What are the 3 primary cues for sound localisation?

A
  1. Interaural time differences (ITDs)
  2. Interaural level differences (ILDs)
  3. Spectral cues
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2
Q

How are Interaural time differences (ITDs) created by the head & ears?

A

A sound on one side of the head will arrive at the nearer ear first, producing a time difference between the 2 ears

(due to differences in path length between the sound source and each ear)

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3
Q

How are Interaural level differences (ILDs) created by the head & ears?

A

A sound on one side of the head will be louder in the nearer ear, producing a difference in sound level between the 2 ears

(Because the head blocks the sound by casting an acoustic shadow)

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4
Q

How are spectral cues created by the pinna?

A

When sound hits the pinna, it bounces around inside in a way that amplifies some frequencies and attenuates others.

This changes the spectrum of the sound (i.e., the amount of energy at different frequencies).

The spectral notch occurs when sound energy at one frequency is much lower than at all other frequencies.

The frequency at which this notch occurs depends on the location of the sound.

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5
Q

Which sound localisation cues are used by the brain for horizontal sound localisation - i.e., to determine if a sound is on the left or the right?

A

ITD & ILDs

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6
Q

Which sound localisation cues do the brain use for vertical sound localisation - i.e., to tell the elevation of sound?

A

Spectral cues

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7
Q

Which sound localisation cues do the brain use to determine whether a sound is front or behind?

A

Spectral cues

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8
Q

Which sound localisation cues does the brain rely on at low frequencies?

Why?

A

ITDs

Because ILDs & spectral cues are very small at low frequencies.

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9
Q

Why are ILDs small at low frequencies?

A

Because low frequency sounds diffract (bend) around the head - therefore the head doesn’t block the sound

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10
Q

What sound localisation cues does the brain relies on at high frequencies?

Why?

A

ILDs & spectral cues

Because at high frequencies, phase-locking in the auditory nerve fibres fails - therefore reducing the brain’s sensitivity to ITDs.

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11
Q

What does the 50% point of a psychometric function correspond to?

A

The ITD that causes participants to guess randomly

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12
Q

What does the 75% point of a psychometric function correspond to?

A

The ITS that causes participants to give a consistent, reliable rightward response - i.e., no longer randomly guessing

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13
Q

What is the ITD threshold?

A

The smallest change in ITD that can be reliably detected

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14
Q

How do you calculate an ITD threshold from a psychometric function?

A

ITD threshold = 75% point - 50% point

I.e., how much you have to change the ITD to go from random guessing (50% point) to a consistent, reliable rightward response (75% point)

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15
Q

How do you convert an ITD threshold into a spatial threshold?

(I.e., what is the smallest change in location that the subject would be able to detect using ITDs?)

A
  1. Record ITDs associated with different directions
  2. Plot ITD as a function of direction
  3. Identify the angle that corresponds to the ITD threshold. This is the spatial threshold (in degrees)
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16
Q

What is the maximum ITD?

A

Maximum ITD = time taken taken for sound to travel from one ear to the other

(This will be experienced when a sound is very close to one ear)

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17
Q

How do you calculate the maximum ITD experienced by someone?

A

Maximum ITD = distance travelled / speed of sound (343 m/s)

*Distance travelled = half the circumference of a circle (head) - 2pir/2

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18
Q

If an ITD of 0 is presented, what % of responses would you expect to be made to the right? What might affect this?

A

~ 50% because the sound is typically perceived in the middle of the head, which forces the participant to guess randomly.

This may not happen if the participant:
- Hears the sound more on one side than the other (e.g., because of problems with headphones or hearing loss)
- Likes to guess that the sound came from a particular side
- Completes very few trials (<10 considered very few)

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19
Q

If you want to locate sounds, is it better to have a small or large ITD threshold?

A

Smaller thresholds associated with better sound localisation

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20
Q

If someone were perfect at the ITD task, what would their data look like?

A

Data would look like a step function - flat and then straight up at 0 then flat again

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21
Q

If someone couldn’t do the ITD task at all, what would their data look like?

A

Data would look like a flat line (threshold = infinite)

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22
Q

What are smallest human ITD thresholds?

A

10 microseconds

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23
Q

Could the 75% point sometimes reflect random guessing?

A

With very few trials, 75% point could reflect random guessing

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24
Q

What impact does head size have on the magnitude of ITDs experienced?

A

Bigger heads produce bigger ITDs which make it easier to locate sounds

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25
Q

What impact does head size have if you compare sound localisation abilities in adult humans with either children or animals?

A

Small animals and children may do worse at sound localisation simply because of their small head size

Also, as head size increases during childhood, children need to learn that their sound localisation cues are changing. If they don’t, they will make consistent errors.

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26
Q

Why is it harder to locate sounds using ITDs at higher frequencies?

A

Phase-locking declines at higher frequencies, which makes it harder to use ITDs

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27
Q

Overall, is it easier to locate sounds using ITDs or ILDs or does it depend on the frequency of the sound?

A

At low frequencies, ITDs are primarily used because the head doesn’t produce large enough ILDs

At high frequencies, ILDs are primarily used because people are not very sensitive to high-frequencies ITDs (as phase-locking in the auditory nerve fails)

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28
Q

What is phase locking, and what happens to it at high frequencies?

A

In auditory nerve fibres, APs are ‘locked’ to a particular phase (the peak) of the waveform
I.e., neurones fire whenever there is a peak of the waveform

This fails at high frequencies, as it is difficult to lock to peaks of the waveform as they are happening so quickly. (This is where the envelope of a sound then becomes important)

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29
Q

What is the Envelope of a sound?

A

Refers to flow fluctuations in the overall intensity of a sound

Auditory nerve fibre APs are locked to the peak of the envelope (and therefore can be used to compare timing of input to ears at high frequencies, when phase-locking fails - this is evidence against the Duplex theory)

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30
Q

How much larger is the tympanic membrane (eardrum) compared to the foot of the stapes?

A

~ 20x larger

(Pressure increases as force is acting on a smaller area)

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31
Q

How fast do pressure waves travel through air (what is the speed of sound)?

A

340 m/s

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32
Q

What makes it difficult for sound to pass between air & water?

A

Acoustic impedance

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33
Q

What is acoustic impedance?

A

The ratio of pressure (potential energy) to movement (KE)

The acoustic impedance of something depends on the physical properties that it is made from

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34
Q

The speed of the travelling wave along the basilar membrane is determined by what (2) factors?

A
  1. Density of the fluid in the basilar membrane
  2. Compliance of the basilar membrane
35
Q

How many OHCs does a human cochlea have?

A

~ 12,000 (around 3x as many as IHCs)

36
Q

Approximately how many nerve cells is each IHC connected to?

A

10-20

37
Q

Compare the arrangement of IHCs & OHCs

A

1 row of IHCs
3 rows of OHCs

38
Q

What is sound?

A

The varying distribution of air molecules, about a fixed point in space, over time

39
Q

How is sound created?

A

By a variation in pressure over time

40
Q

How can sound be represented?

A

By a sine wave

41
Q

Give the equation for frequency (sound)

A

Frequency (Hz) = 1/period

42
Q

What is the human hearing range (frequency)?

A

20-20,000 Hz

43
Q

What is the most sensitive frequency of human hearing range?

A

1000-4000 Hz

44
Q

What is the auditory threshold in humans?

A

20 micro Pa

45
Q

Above what sound pressure is a potential noise exposure issue?

A

> 90dB

46
Q

Give the equation for sound pressure level (dB SPL)

A

Sound pressure level (dB SPL) = 20log(10) x (Pt/Pr)

Pt = test pressure
Pr = reference pressure (20 micropascal)

47
Q

What does the frequency of sound waves correlate to?

A

Pitch

48
Q

What does the amplitude of sound waves correlate to?

A

Loudness

49
Q

What are the structures of the outer ear?

A

Pinna
Auditory canal (external auditory meatus)
Concha
Helix
Lobule

50
Q

What are the structures of the middle ear?

A

Tympanic membrane (eardrum)
Ossicles (3)
Eustachian tube
Middle ear muscles

51
Q

What are the name of the 3 (connected) ossicles in the middle ear?

A

Malleus
Incus
Stapes

52
Q

What is the acoustic reflex?

A

Stapedius muscle (attached to the stapes bone) provides the acoustic reflex - dampens excessive vibrations of the stapes in response to loud sounds

Tensor tympani muscle (attached to the malleus) contributes to the acoustic reflex - stiffens the eardrum (tympanic membrane) to reduce its vibrations in response to loud sounds

53
Q

How do the middle ear muscles serve an important protective function?

A

Reflex contraction can reduce ossicle movement (e.g., in the acoustic reflex)

54
Q

What are the structures of the inner ear?

A

Vestibular labyrinth
Oval window
Auditory-vestibular canal
Cochlea

55
Q

How does the cochlea connect to the stapes?

A

At the oval window

56
Q

What type of deafness is caused by damage to the middle ear?

A

CONDUCTIVE deafness

57
Q

What type of deafness is caused by damage to the inner ear?

A

NERVE deafness

58
Q

What are the functions of the outer ear?

A
  1. Gathers sound & transforms sound pressure at the tympanic membrane
  2. Amplifies sound (10-15 dB for 1/5-7kHz)
  3. Filtering action aids in sound localisation (i.e., pinna, spectral cues)
59
Q

What is the function of the middle ear?

A

Transfers sound energy from external environment to the cochlear

Impedance matching

60
Q

What is impedance matching (done by the middle ear)?

A

Lever action & size difference between the tympanic membrane & oval window provides a 20-30dB pressure gain

This compensates for the impedance mismatch of sound waves in air to cochlear fluid

61
Q

The cochlea is a coiled structure - how many turns in humans?

A

2.5

62
Q

The cochlea has 3 scalae (compartments). What are they?

A
  1. Scala vestibuli (upper chamber)
  2. Scala media / cochlea duct (middle chamber)
  3. Scala tympani (lower chamber)
63
Q

What fluid do the scala vestibuli & scala tympani of the cochlea contain?

A

Perilymph

64
Q

What fluid does the scala media of the cochlea contain?

A

Endolymph

65
Q

What is the helicotrema of the cochlea?

A

The apex of the cochlea

Here the Scala vestibuli (upper chamber) & scala tympani (lower chamber) connect - at this point they are continuous, allowing the perilymph fluid to circulate freely

66
Q

What is the function of the round window of the cochlea?

A

Acts as a pressure relief valve for the cochlea fluid set into motion by the movement of the stapes in the oval window - when the oval window pushes in, the round window pushes out.

This prevents excessive pressure build-up & protects the structures in the inner ear.

67
Q

What is the function of the oval window of the cochlea?

A

The eardrum connects to the stapes, and the footplate of the stapes is attached to the oval window.

As the eardrum & stapes vibrates, it pushes & pulls on the oval window, creating pressure waves in the cochlea - setting the cochlear fluid into motion.

68
Q

What are the functions of the cochlea? (3)

A
  1. Separates frequencies - splits complex sounds (i.e., speech) into single components
  2. Amplifies signal
  3. Sensory transduction - transduces mechanical vibrations into APs
69
Q

What is meant by tonotopy?

A

Frequency-place coding

70
Q

Describe the variations in thickness & flexibility of the BM along the length of the cochlea

A

Systematic increase in width, decrease in thickness & stiffness from base to apex

  • At the base - narrow, thick, stiff - high frequency sounds detected
  • At the apex - wide, thin, flexible - low frequency sounds detected
71
Q

What is meant by the place principle?

A

There is a place on the BM at which each frequency is processed → BM displacement is frequency tuned (frequency dependent)

72
Q

How does the cochlea amplify the sound signal?

A

Through the action of OUTER HAIR CELLS
These amplify basilar membrane vibrations - enhancing selectivity & frequency selectivity

73
Q

How does the cochlea transduce sounds?

A

Through the action of INNER HAIR CELLS

74
Q

What is meant by population-intensity coding of sounds?
And what cells are involved?

A
  1. Louder sounds cause broader displacements of the BM
  2. Activating more IHCs & SGNs along the frequency axis of the cochlea
  3. Providing population-intensity coding
75
Q

How can individual IHCs report over a large intensity range for a given frequency?

A

Auditory nerve fibres from a single IHC have different thresholds and spontaneous firing rates
This means that they have a different dynamic range of intensities to which they are sensitive
Allowing a single IHC to report over a large intensity range for a given frequency

76
Q

What is meant by rate-intensity coding of IHCs & SGNs?

A

A single IHC will be more depolarised as sound intensity increases
Increasing glutamate release and resultant SGN depolarisation
This increases SGN firing rate
Providing ‘rate-intensity’ coding

77
Q

Approximately how many auditory nerve fibres are there in humans?

A

~ 30,000

78
Q

Describe the frequency-place coding by auditory nerve fibres and IHCs

A
  1. IHC receptor potentials are driven by BM movements
  2. Type I auditory nerve fibres are connected to the IHCs, so have the same frequency tuning as the part of the BM they connect to
  3. This tonotopic map (frequency-place coding) is carried on to the CNS
79
Q

Compare type I and type II auditory nerve fibres

A

Type I - (95%) - myelinated - synapse with IHCs - up to 20 afferents per IHC

Type II - 5% - unmyelinated - synapse with OHCs - 1 per 5-100 OHCs - cell bodies located in the spiral ganglion

80
Q

What is the rate code of auditory features?

A
  • A single IHC will be more depolarised as sound intensity increases → increased IHC glutamate release & SGN depolarisation
  • SGN firing rate increases as amplitude (dB SPL) increases → providing ‘rate-intensity coding’
81
Q

What is the place principle coding of auditory features provided by?

A
  • Tonotopic map
  • Labelled lines for frequency (from afferent ANFs)
82
Q

What is the temporal code for auditory features?

A

Phase-locking of auditory nerve fibres at low frequencies (up to ~3000 Hz)

83
Q

What is the ensemble coding of auditory features?

A

Pool information across many auditory nerve fibres (ANFs)

PHOL0005 - Structure & Function Of The Nervous System (19 decks)

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