Sound Perception

Question 1

What frequencies can we hear sounds ranging between?

Answer 1

20 and 20,000 Hz
20 times per second to 20,000 times per second
We are good at detecting soft sounds, perceiving very small differences

Question 2

How many pure tones can we differentiate up too?

Answer 2

1500 pure tones

Question 3

What is the main function of the outer ear?

Answer 3

Captures sound signals
Amplifies mid frequencies
Vertical direction coding

Question 4

What is the main function of the middle ear?

Answer 4

Protection

Impedance matching

Question 5

What is the main function of the inner ear?

Answer 5

Frequency analysis

Transduction

Question 6

What are the 2 sections of the outer ear?

Answer 6

Pinna

Meatus

Question 7

What does the pinna do?

Answer 7

Affects high frequency sounds by interference between the echoes reflected off its different structures - different frequencies are amplified by different amounts depending on the origin of the sound in the vertical place - brain interprets these changes as direction (if sound is coming from the floor or air)

Question 8

What does the meatus (ear canal) do?

Answer 8

Links the pinna to the ear drum
Resonates at around 2kHz so that frequencies in that region are transmitted better to the cochlea - corresponds to human speech frequency region

Question 9

What does the middle ear consist of?

Answer 9

Three ossicles (tiny bones)
Malleus - hammer
Incus - anvil
Stapes - stirrup

Question 10

What do the three ossicles do?

Answer 10

Transmit vibrations from ear drum to the smaller oval window - the ear drum is much larger than the window so ossicles serve as amplifiers

Question 11

What is impedance matching?

Answer 11

It enables air vibrations (sound) to be efficiently transformed into fluid vibration - turns a large amplitude vibration in air into a small amplitude vibration into fluid

Question 12

What would happen without a middle ear?

Answer 12

Most of the sound would bounce off the cochlea (inner ear)

Question 13

What is the function of the inner ear: the cochlea?

Answer 13

To transform a mechanical signal into neural responses in the 8th cranial nerve
Snail shaped, three chambered tube

Question 14

What are the chambers in the cochlea separated by?

Answer 14

The basilar membrane, on which sits the organ of corti

Question 15

What does the arch in the middle of the organ of corti do?

Answer 15

Separates the inner hair cells from the outer hair cells - on top of these sits the tectorial membrane which is attached along its inner edge

Question 16

What are the stereocillia?

Answer 16

Outer hair cells

Question 17

Where are the stereocillia embedded?

Answer 17

In the tectorial membrane

Question 18

Where are the inner hair cells embedded?

Answer 18

No where, they are free and moved by movement of the fluid (endolymph) that fills the space between the hair cells and tectorial membrane

Question 19

What is endolymph?

Answer 19

Fluid which fills the space between the hair cells and tectorial membrane - bends the hairs

Question 20

What happens when inner hair cells are bend towards the stereocilium?

Answer 20

The cells voltage is increased
more neurotransmitter is released
auditory nerves connected to the hair cell increase firing rate

Question 21

What happens when inner hair cells are bend away from the stereocilium?

Answer 21

the cells voyage is decreased
less neurotransmitter is released
auditory nerves connected decrease their firing rate

Question 22

What is the function of inner hair cells?

Answer 22

Turn mechanical movement of the basilar membrane into neural firing of the auditory nerve (transduction)

Question 23

What do the inner hair cells excite?

Answer 23

The synapses of fibres of the afferent (ear to brain) auditory nerve - each of the 3,000 inner hair cells have 10 auditory nerves attached to it with different diameters, thresholds and rates

Question 24

What is the role of the outer hair cells?

Answer 24

Sharpening
the outer hair cells have synapses from the efferent (brain to ear) fibres, so the brain controls the mechanical properties of the ear
they exhibit motility - act as tiny motors that amplify the mechanical movement of the basilar membrane
responsible for high sensitivity (low thresholds) and sharp turning (high frequency resolution) or normal hearing

Question 25

What does motility mean?

Answer 25

The outer hair cells act as tiny motors that amplify the mechanical movement of the basilar membrane

Question 26

Where does hearing loss come from?

Answer 26

Damage to the outer hair cells

Question 27

What does sound produce?

Answer 27

A travelling wave down the basilar membrane

Question 28

What do different frequencies of sound give?

Answer 28

Maximum vibration at different places along the basilar membrane

Question 29

The cochlea - base and apex

Answer 29

Near the basis it is wider and narrower at the apex, but inside the basilar membrane the basis is narrow and stiff while towards its apex it is wider and more flaccid - opposite

Question 30

Where does the vibration to high frequency tones peak?

Answer 30

At the base of the membrane

Question 31

Where does the vibration to low frequency tones peak?

Answer 31

At the apex

Question 32

What does the place of maximum vibration indicate?

Answer 32

The frequency of the sound wave

Question 33

The place theory of frequency analysis

Answer 33

Neurons have different characteristics depending on their position along the basilar membrane - high frequencies only excite neurons near base, low excite frequencies near the apex
Place code - tonotopic organisation of the cochlea
if 2 complex sounds, the ear can separate these by reporting where there is maximum reflection at basilar membrane

Question 34

The temporal - phase locking theory of pitch perception for pure tones

Answer 34

Competing theory
Auditory nerves connected to inner hair cells fire in phase with the stimulating wave form - independent of position. When amplitude increases, it fires. Intervals between spikes tend to be numbers of cycles - closely spaced when the frequency is high. The brain times these intervals to estimate the frequency of the tone

Question 35

Problem with the phase locking theory

Answer 35

Only works for low frequencies - less than 5kHz in humans

Question 36

Coding of intensity

Answer 36

The louder a sound is, the more frequently the auditory nerve fires
Each inner hair cell has about 10 auditory nerves attached to it - different nerves have different thresholds
most have low - saturate at normal sounds
the minority have higher thresholds - don’t saturate un till sounds are loud