Audition 1 and 2

Question 1

What is sound?

Answer 1

a pressure wave

when an object vibrates it pushes against the air near it and compresses it

this compression propagates away from the object as a compression wave

As the object returns to its original position it draws air back and creates a rarefaction wave

Pressure range: 20µPa - 20^8µPa

able to be transmitted through the air because air has a mass and a stiffness

Question 2

What happens when the pressure wave arrives at the eardrum?

Answer 2

travels at the speed of sound: 1235km/h
displaces the eardrum setting in motion the series of events than enable us to register, perceive and interpret the sound.

Question 3

What are pure tones?

Answer 3

sinusoidal pressure waves of a single frequency

their frequency is directly related to the pitch of the sound

Question 4

what is frequency perceived as

Answer 4

pitch
high frequency corresponds to high pitch
can perceive frequencies between 20Hz-20kHz
large range so use logarithmic scale
hence octives: every doubling of frequency increases pitch by one octane
most real world objects vibrate at multiple frequencies

Question 5

what is amplitude perceived as

Answer 5

loudness
high amplitude corresponds to loud sounds
10-130dB
large range so use logarithmic scale
hence decibel scale: every doubling of amplitude increases the loudness by 6dB

Question 6

What is Fourier analysis

Answer 6

Mathematical procedure that makes it possible to approximate vibrations as a sum of sine wave vibrations.

Every sound can be decomposed into sine components and represented in the frequency domain.

Instead of describing waveform as a function of time, states the amplitude and phases of the cosine components of the sounds.

The ear also decomposes the incoming sound into its frequency components and so does something very similar to Fourier analysis

Question 7

What are narrow band sounds?

Answer 7

Pure tones only contain one frequency so they are narrow band

In narrow band sounds a relatively small number of components contain most of the energy.

Narrow band sounds are more or less periodic and may evoke an identifiable pitch

Question 8

What are broad band sounds?

Answer 8

Broad band sounds contain very many components of similar amplitude and often do not evoke a strong pitch, although there are exceptions.

Noises and clicks are typical examples of natural broadband sounds.

Question 9

What is a spectogram?

Answer 9

The spectrum is a complete description of a sound but only if the frequency composition of the sound is constant over time.

However, natural sounds usually do vary with time.
To deal with this, sounds can be divided into short time segments and spectra calculated for each time segment in turn.

The result of this spectral temporal analysis is a spectrogram.

Each column indicates a segment of time and colour indicates the energy at a particular frequency.

Question 10

what is a neurogram?

Answer 10

As a crude approximation one might say that it is the job of the ear to produce spectrograms of the incoming ounces, and that the brain interprets the spectrogram to identify sound.

Nerve fibres are ordered according to their preferred frequency with fibres that prefer low frequency sounds at the bottom.

Activity of auditory nerve fibres depends on the amount of sound energy near the neurons preferred frequency.

Question 11

How do we go from a spectrogram to auditory scene analysis?

Answer 11

the job of the auditory system is to perform a sort of spectro-temporal analysis to identify objects and although the neurogram idea may be appealing, it is clear that this is not the end of the story.

More steps of analysis are required.

Our auditory system is very good at performing these additional steps and easily identifies sounds even when to us there appears to be very little structure in the spectrogram.

Can perform further analyses from what your brain has already learned about sound.

Question 12

What are the three divisions of the ear

Answer 12

the external ear
the middle ear
the inner ear

Question 13

What does the external ear consist of?

Answer 13

the pinna
the external auditory canal
separated from the middle ear by the tympanic membrane

Question 14

What does the pinna do?

Answer 14

collects sound

acts like a funnel

complex shape with several folds and ridges

certain features of sounds reaching the ear are attenuated and others are amplified before entering the auditory canal

Sound frequencies between 2-4kHz are generally boosted

How much particular frequencies of the sound are amplified or attenuated depends on the direction from which they enter the ear

In sounds presented from 45’ from above, the low frequencies are amplified more and the largest attenuation occurs at slightly higher frequencies than in sounds presented from angles

enables the localisation of sound in terms of elevation

Question 15

How is sound localised along the horizontal dimension?

Answer 15

use both ears and sounds that arrive earlier in one than the other and are louder in one than the other

Question 16

what does the middle ear consist of?

Answer 16

the air filled cavity between the tympanic membrane and inner ear

connects to the throat via the eustachian tube

the oscilles: incus, malleus and stapes.

also contains two muscles connected to the malleus (tensor tympanic muscle) and stapes (stapedus muscles)

Question 17

why is impedance matching important

Answer 17

if sounds were transmitted through air directly to the cochlear much of the energy in the sound would be lost because the acoustic impedance of water is higher than air.

instead the vibrations of the tympanic membrane are transmitted to the inner ear by a set of small bones.

Question 18

How does the middle ear achieve impedance matching?

Answer 18

takes sound pressure over the relatively large area of the tympanic membrane (60mm^2) and concentrates the sound pressure onto the much smaller surface area of the stapes (3mm^2)

the lever arm formed by the malleus is slightly longer than the incus resulting in a further increase in pressure by a factor of 1.3 dB

Overall the middle ear increases the sensitivity of out ears by about 30dB
(about equal to how much would have been lost without impedance matching)

Question 19

How do muscles in the middle ear protect from loud sounds?

Answer 19

acoustic/stapedius/middle ear reflex

the tensor tympanic muscle connected to the malleus stiffens the movement

the stapedius contracts to pull the stapes away from the tympanic membrane

reduces the motion of the middle ear bones and protect delicate inner ear structures from damage

slow so doesn’t protect from sudden loud sounds

activated when talking

most effective in reducing the intensity of low frequency sounds.

Helps us understand speech sounds in the presence of loud, low frequency background noise

Question 20

what are the components of the inner ear?

Answer 20

The inner comprises several fluid filled chambers.

The semicircular canals are part of the vestibular system involved in maintenance of balance.

The cochlea
for hearing
coiled tube enclosed in a hard, bony shell
filled with perilymph and endolymph fluids
contains hair cells.

The scala vestibuli and scala tympani are filled with perilymph
similar in composition to cerebral spinal fluid
contains sodium as its major cation.

The scala media is filled by endolymph
much more positively charged than the perilymph because it contains a lot of potassium.

high [K+] is due to the stria vascularis from which potassium leaks into the endolymph

Between the scala tympani and the scala media is the basilar membrane

on the basilar membrane is the organ of corti
contains the hair cells which connect to the auditory nerve which leaves the cochlea.

The only two openings in the cochlea are the oval window and the round window

Question 21

How do the ossicles transfer sound waves to the cochlea?

Answer 21

When the eardrum is moved by a sound wave, that movement is transmitted via the bones of the middle ear into the membrane of the oval window.

Every time the stapes pushes in the oval window membrane this increases the pressure in the fluid filled spaces of the cochlea and causes the fluids to move and the round window to bulge out.

This change in pressure also causes movement of the basilar membrane

Question 22

What are the mechanical properties of the basilar membrane?

Answer 22

Narrow and stiff at the base end of the cochlea but wide and floppy at the far apical end.

When the sound wave is transmitted onto the oval window it causes waves in the fluids of the cochlea which in turn causes waves in the basilar membrane.

Because of the difference in thickness along the length of the basiliar membrane these travelling waves reach their peaks at different positions.

The position of the peak depends on the frequency of the sound.

Question 23

How is frequency tuning achieved along the basilar membrane?

Answer 23

High frequency sounds move the basilar membrane most at its basal end.

By vibrating in different places depending on the frequency of the sound the basilar membrane achieves an analysis of the frequency components of sound and establishes a place code for frequency.

This property of the basilar membrane is the basis of tonotopy.

Question 24

What is tonotopy?

Answer 24

sounds with similar frequency content are processed in topographically organised region of the brain

Question 25

how do different types of sounds peak in the basilar membrane?

Answer 25

Different pure tones cause different maximal deflection at different positions depending on frequency.

Complex sounds cause multiple peaks of deflection at many different positions as they contain multiple frequencies

Question 26

What is the significance of the movement of the organ of corti?

Answer 26

When the basilar membrane vibrates from acoustic stimulation, the organ of corti will move up and down with it as it sits on top.

In the lower part of the membrane are hair cells

in the inner part is a single row of inner hair cells
in the outer part are three rows of outer hair cells.

Ontop of the hair cells is the tectorial membrane

Up-down movement of the basilar membrane causes the tectorial membrane to slide sideways over the membrane causing a sideways displacement of the hair cell bundles in the cochlear hair cells.

Question 27

how is transduction mediated by the hair cells?

Answer 27

Sterocilia are 3-5mm long and each bundle are connected by tip links

Movement of the bundle is thought to change the tension on the tip links thereby closing or opening stretch sensitive K+ channels.

K+ channels opening causes an influx of K+ from the endolymph, depolarisation of the hair cell membrane, opening of V-gated Ca2+ channels and an increased probability of transmitter release.

Question 28

What is meant by ‘receptor potentials’?

Answer 28

Hair cells do not have axons or dendrites and don’t fire action potentials.
They do form glutamatergic excitatory synaptic contacts with the neurons of the spiral ganglion along the lower end.

Nerveless they change the membrane potential and release glutamate upon deflection of the stereocilia

membrane potential closely reflects the movement of the basilar membrane because this movement is directly related to the movement of the stereocilia which is directly related to the amount of potassium influx which is directly related to the amount of membrane potential depolarisation

At low frequencies the membrane potential of the hair cell follows every cycle of the stimulus (AC response)
At high frequencies membrane potential is unable to follow individual cycles instead it remains depolarised through duration of stimulus (DC response)

Question 29

What active mechanisms contribute to the cochlear mechanisms?

Answer 29

Outer hair cells contain a unique membrane protein called prestin which causes them to shorten every time they are depolarised and lengthen every time they are hyperpolarised.
This shortening causes a localised amplification of the movement of the basilar membrane which in turn causes the stereocilia to be deflected more, which causes more depolarisation which causes more contraction and so on in a positive feedback loops
The result of this is greater sensitivity in particular for low intensity sounds and sharper frequency and better frequency resolution

Question 30

What is the auditory pathway?

Answer 30

Upon leaving the cochlear, the auditory fibres join the eighth cranial nerve and enter the cochlear nucleus (CN) in the brainstem
There they branch, and the branches terminate in the dorsal cochlear nucleus and the anteroventral cochlear nucleus.
The CN projects to the ipsilateral and contralateral auditory superior complex where signals from the two ears are first combined.
The nuclei of the lateral meniscus receive input from the superior olive as well as the CN.
The principal auditory structure of the midbrain, is the inferior colliculus (IC), which receives input from all the brainstem nuclei and projects to the auditory part of the thalamus - the medial geniculate body (MGB).
The MGB projects to the auditory cortex.

Question 31

what is the afferent innervation of the hair cells?

Answer 31

Hair cells form glutamatergic excitatory synaptic contacts with the neurons of the spiral ganglion along the lower end.
Spiral ganglion neurons form the long axons that travel through the auditory nerve (AN) and connect hair cells to the cochlear nucleus (first auditory relay station in the brain).
Spiral ganglion axons are therefore known as auditory nerve fibres.

Question 32

What are the two types of auditory nerve fibres?

Answer 32

Type 1 are thick, myelinated and capable of fast signal transmission.
Type 2 are unmyelinated and slow.

Inner hair cells connect to type one fibres
Type 1 form more specific connections so each hair cell is innervated by 10-20 type 1 fibres.
Type one fibres outnumber type two 10 fold.

Outer hair cells connect to type two.
Appears type 2 play a minor role in auditory processing.
Outer hair cells connect to about 6 nerve fibres each.
Typically they have to share each type 2 fibre with about 10 other hair cells.
Information sent by the outer hair cells must therefore be slow, less plentiful and less specific.

Question 33

What is the smallest sound pressure the average person can detect?

Answer 33

about 20 µPa which can be defined as zero dB.
20µPa is equivalent to a pressure variation of 2/1010 particles such that a 0dB sound moves the eardrum by less than the diameter of one hydrogen molecule.

Question 34

How do auditory nerve fibres code sound intensity?

Answer 34

AN fibres increase their firing rate as a function of sound intensity (Rate coding).
AN fibres differ in sensitivity and dynamic range.
Low spontaneous rate fibres have the highest thresholds and saturate only at high sound levels (above 90dB)
Intermediate SR fibres have intermediate thresholds and saturate by about 60dB.
High SR fibres are the most sensitive and may saturate by 40dB.
In order to process sound intensity the brain has to rely on the differences in sensitivity which allow for population code for intensity.

Question 35

How do auditory nerve fibres code frequency?

Answer 35

Whether a given nerve fibre responds to a sound of a particular frequency depends on where along the cochlear it connects to the hair cells.
If it connects at the basal end of the cochlear it responds most to high frequencies.
If it connects at the apical end it responds most to low frequencies.
No action potentials fired below a certain tone level (dB) - threshold of about 40dB
AN responds most to frequencies 5-10 kHz
Range of frequencies it responds to depends on the sound level
Low sound levels, range is small. High sound levels, range is large.
Due to properties of the basilar membrane as more intense sounds cause larger and larger areas of the basilar membrane to vibrate,

each AN fibre has a specific frequency tuning curve that is usually measured at a specific level relative to threshold to control for sound level

Question 36

How is tonotopy preserved in the auditory nerve?

Answer 36

The basis of tonotopy are the mechanical properties of the basilar membrane because different sound frequencies cause different parts of the basilar membrane to vibrate.
This tonotopic arrangement is preserved in the AN and most other parts of the auditory system and is the major organising principle of the auditory system.
Nerve fibres are tonotopically arranged the fibres lie anatomically next to nerves with increasing tuning curves
Sensitivity decrease at particularly low and particularly high frequencies.
Difference in sensitivity as a function of frequency is due to the mechanical properties of the outer, middle and inner ear.

Question 37

What is phase locking?

Answer 37

encode frequency and temporal patterns.

the spike pattern of auditory nerve fibres at low frequency sounds occur at specific times as opposed to being random

consistent firing of a cell at the same phase of a sound wave

At low frequencies some neurons fire action potentials every time the sound has a particular phase, for example each time it peaks. Therefore frequency can be easily identified as it is equal to the neuron’s action potentials

Intermediate sound frequencies are represented by many neurons collectively in combination with tonotopy.

frequencies above 4kHz cannot be coded using phase locking as the sound waves cycle too quickly for the action potentials to accurately represent their timing so tonotopy is relied on exclusively

Question 38

Where are the first binaural interactions?

Answer 38

at the level of the superior olive

important for sound localisation

Question 39

What is the cochlear nucleus?

Answer 39

First structure of the auditory pathway after the AN
It has a dorsal CN, posteroventral CN and anteroventral CN
as the AN enter, they bifurcate and send branches to the different parts of the CN where they form synapses.
AN connecting to hair cells at the base of the basilar membrane which is tuned to high frequencies, project to medial parts of the CN
AN connecting to the apex of the basilar membrane connect to lateral parts of the CN

Question 40

What are the different cell types in the cochlear nucleus?

Answer 40

Pyramidal cells
octopus cells
globular bushy cells
multipolar cells
spherical bushy cells

range of different types of neurons with different anatomical location, morphology, cellular physiology, synaptic inputs and temporal and spectral response properties.
Each cell extracts different aspects of the incoming acoustic information and passess this on to different points in the auditory pathway
Also differ in their frequency tuning

Question 41

What are pyramidal cells?

Answer 41

in the DCN
Exhibit pauser response - they fire strongly to the onset of the sound followed by an inhibitory period and then another increase in firing rate
Very complex tuning curves with complex shapes with large inhibitory regions and only small excitatory patches. Lateral inhibition is used to extract spectral contrast.
Project to the inferior colliculus

Question 42

What are octopus cells?

Answer 42

cells in the PVCN
Receive convergent input from many AN fibres and therefore vary broadly in frequency field
Onset response - They tend to fire just a single action potential in response to a sudden burst
High frequency tuning curves - become excited by a range of frequencies that becomes wider with increasing sound intensity look like tuning curve of AN fibres
May encode temporal pattern information across many AN fibres or encode sound intensity amongst other things.
They project to the inferior colliculus or lateral lemniscus.

Question 43

What are globular bush cells?

Answer 43

in the AVCN
Primary like response with notch P
Preserve temporal information contained in phase locking and project to the superior olivary nuclei
High frequency tuning curves

Question 44

What are multipolar cells?

Answer 44

in the PVCN
Synaptic input from multiple AN and receive those mostly on the dendrites
Show chopper response when stimulated - there is a point with regular rhythmic bursts, but the burst frequency is unrelated to that of the tone stimuli.
Complex tuning curves with inhibitory regions where firing rates are suppressed to below spontaneous rates because of the input they receive from inhibitory CN neurons. With lateral inhibition side bands used to extract spectral contrast.
Project to the inferior colliculus

Question 45

What are spherical bushy cells?

Answer 45

in the AVCN.
Receive a small number of very large excitatory synapses from the AN fibres.
Exhibit primary like responses - their firing patterns are almost identical to the firing patterns of the AN inputs.
High frequency tuning curve

Question 46

What are the superior olivary nuclei?

Answer 46

Two major parts:
medial superior olive (MSO) - excitatory input from each ear
Lateral superior olive (LSO) - inhibitory input from the contralateral side via medial nucleus of trapezoid body (MNTB).
The contralateral cochlear nucleus sends an excitatory projection to the medial nucleus of the trapezoid body
The synapse between the CN axon terminals and the MNTB is called the calyx of Held, named because it is the largest synapse in our brain.

Question 47

What are binaural localisation cues?

Answer 47

Important the input from both of our ears converge because comparing the signals arriving enables us to derive information about the location of sound sources.
These localisation cues depend on the signals from the two ears so are referred to as binaural localisation cues.
When there is a sound source to one side of our head, sound waves arrive at both ears however the sound waves arriving at the cloer ear travel a shorter distance
This means that any sound source that is off to one side produces interaural time difference cues (ITD)
The more distance the ear lies in the acoustic shadow of the head, ear waves are also attenuated and will be louder in the near ear providing a second cue called interaural level difference cues (ILD)

Question 48

How are interaural level differences processed?

Answer 48

The LSO is specialised for the processing of ILD and neurons in the LSO are sensitive to changes in interaural difference
CN neurons on each side increase their firing rate when the intensity of the sound increases.
LSO neurons will fire strongly only when a loud sound is received in the ipsilateral ear and a quiet sound in the contralateral ear. It will not fire in the opposite situation due to the inhibitory connection on the contralateral side via the MNTD

Question 49

How are ILDs effected by frequency?

Answer 49

ILDs are highly frequency dependent
At higher sound frequencies ILDs tend to become larger, more complex and potentially more informative.

Low frequency sound waves can more easily wrap around the head and so are attenuated less.
Most useful for sound location with high frequency stimuli

Question 50

How are interaural time differences processed?

Answer 50

MSO is specialised for the processing of interaural time differences

Neurons in the MSO are sensitive in ITD
In order to process ITD the brain must compare the arrival time of sounds at the two ears

In order to ensure an efficient transmission of information to the MSO, AN are linked to the PVCN and project to the MSO via a relay of three synapses.

These synapses operate with extremely high temporal precision.

A single presynaptic action potential at an endbulb of the synapse is enough to trigger an action potential in the postsynaptic bushy cell.

Provides the MSO information that precisely reflects the timing of information arriving at the two ears

MSO neurons only fire when there coincident excitation from both ears

Because different MSO neurons receive ipsilateral and contralateral input via delay line of different lengths, different MSO neurons can encode different ITD

Question 51

How does the MSO detect ITDs?

Answer 51

MSO neurons are thought to fire maximally only if they receive simultaneous input from both ears - place coding
If the input from one or the other ear is delayed by the same amount, because the afferent axons are longer or shorter, then the MSO neuron will fire maximally only if an interaural delay in the arrival time at the ears exactly compensated for the transmission delay.
In this way the MSO neurons become tuned to characteristic interaural delays

Question 52

How long are interaural time differences usually?

Answer 52

Unaided ITDs are at mos 0.7ms long in humans
The smallest detectable difference in ITD is about 0.01ms
Not frequency dependent but we are not good at detecting ITDs from high frequency sound because our auditory nerve fibres cannot phase lock high frequency stimuli which means that the fine temporal information necessary for processing ITD cannot be extracted for those stimuli.

Question 53

What is the structure and function of the auditory midbrain?

Answer 53

From the the brainstem, auditory information is passed on to the inferior colliculus (IT) in the midbrain
The IC is very large and considered a major processing centre
It collects and integrates input from all auditory brainstem nuclei and it is an obligatory relay for all ascending auditory information.
Most inputs to the IC are from the other hemisphere so most neurons in the IC and above are more strongly excited by sounds presented in the contralateral ear
The ic has a number of anatomical subdivisions
The largest one is the central nucleus which is tonotopically organised, while the others in the shell of the IC (dorsal cortex and external nucleus) are not.

Question 54

How is tonotopy preserved in the IC?

Answer 54

Neurons tuned to higher frequencies are found in the central part
Neurons tuned to lower frequencies are found in the dorsal part of he CN

Question 55

Where does the integration of acoustic and contextual information start?

Answer 55

More and more evidence emerging that IC is one of the first stations in the auditory pathway where non-acoustic signal have strong impact on neural activity
E.g activity is also modulated by behaviour; engagement of motor systems (locomotion speed) can impact neural firing in IC.

Question 56

What is the relevance of the SC?

Answer 56

One of the connections from the IC targets the superior colliculus (SC)
Not part of the main auditory pathway but it is the only part of the brain where we have found a map of auditory space.
Mostly concerned with the control of eye and head movements
The map is found in the deeper layers of the SC and is aligned with a map of visual space found in the upper layers of the SC
Not considered part of the classical auditory pathway

Question 57

What is the structure and function of the auditory cortex?

Answer 57

Located in the temporal lobe
Like the preceding stages of the auditory system, the primary auditory cortex is also tonotopically ordered.
Although this tonotopic organisation starts to break down in the secondary auditory cortex and higher auditory cortex areas, beyond the secondary auditory cortex.
The primary and higher order cortical areas integrate acoustic information with contextual information like other sensory input, motor output, memories and expectations to make sense of the acoustic input.
As a result we see increasingly complex response properties in auditory cortical neurons.
Some neurons in auditory cortex can signal mismatches between the expected sensory feedback from a vocalisation and the actual sensory feedback, and the actual sensory feedback, a type of error signal that may be crucial for fine tuning our vocal production

Question 58

What are the differences between cortical layers?

Answer 58

Appear to fulfil different functions
Layer 4 is mostly responsible for receiving thalamic input and it projects itself mostly to layers 2 and 3.
Neurons in layer 2 and 3 integrate thalamic input with input from other auditory and non-auditory cortical areas and project to layer 5.
Layer 5 and 6 are the output layers of the auditory cortex and these deep layer neurons project to other cortical areas as well as other subcortical structures.

Question 59

What are the descending projections of the auditory cortex?

Answer 59

Originate in layer 5 and layer 6
Send axons to the thalamus, inferior colliculus and brainstem.
Massive projections.
Function still largely unclear but the cortex may with its descending projections be able to control and gate the flow of information in subcortical structures.