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Neurophysiology > Audition > Flashcards

Audition Flashcards

1
Q

What actually is sound

A

Vibrations of objects set up pressure waves in the surrounding air

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2
Q

What allows sound to travel

A

The ‘elastic’ property of air allows pressure waves to propagate

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3
Q

What are sine waves

A

Produced by pure tones, (waveform made of a single frequency) wave frequency is directly related to pitch and amplitude is directly related to perceived loudness

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4
Q

What is the frequency range of sine waves

A

20-Hz-20kHz (10 octaves)

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5
Q

What is the pressure range of sine waves

A

20uPa- 10^8 uPa (0-140dB)

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6
Q

What scale do octave and decibels follow

A

Logarithmic scale- every doubling of frequency increases pitch by one octave, every doubling of amplitude increases loudness by 6dB

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7
Q

Are most sounds pure tones?

A

No- most real-world objects vibrate at multiple frequencies

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8
Q

What is Fourier analysis

A

A mathematical procedure that makes it possible to represent waveforms as the sum of sine waves

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9
Q

How does the ear conduct Fourier analysis

A

It decodes sound into its frequency (sine) components

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10
Q

What is the (Fourier) spectrum

A

A frequency domain description of a waveform- states the amplitudes of the cosine components of the waveform rather than describing the waveform as a function of time

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11
Q

What are ‘narrow band’ sounds

A

Sounds in which a relatively small no of components contain most of the energy eg pure tone is an extreme example

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12
Q

Properties of ‘narrow band’ sounds

A

More or less periodic, may evoke an identifiable pitch

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13
Q

What are ‘broad band’ sounds

A

Contain very many components of similar amplitudes eg clicks, noises

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14
Q

Properties of ‘broad band’ sounds

A

Often don’t evoke a strong pitch

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15
Q

When does the Fourier spectrum work as a complete description of a soudn

A

Only if the frequency composition of the sound is constant over time- yet most natural sounds vary with time

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16
Q

What is a spectrogram

A

Produced by dividing sounds into short time segments, and spectra calculated for each time segment in turn

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17
Q

How are spectrograms relevant to the auditory system

A

You could say the job of the ear is producing a spectrogram of incoming sounds like a ‘neurogram’, and the brain performs further spectro-temporal analysis of the ‘neurogram’ to instantly identify sounds

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18
Q

What do auditory nerve fibre discharge rates depend on

A

The amount of soudn energy at or near the neuron’s characteristic frequency

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19
Q

What does the external ear consists of

A

Pinna, external auditory canal

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20
Q

What separates the external and middle ear

A

The eardrum/tympanic membrane

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21
Q

How does the pinna filter sound

A

Its convolutions filter sound according to the direction it enters the ear eg higher frequency sounds can enter the auditory canal more effectively when they come from an elevated source than at a lower level

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22
Q

What is the consequence of the pinna filtering sound based on its direction

A

Subtle differences in amplification between vertically low vs high high-frequency sounds allows vertical localisation of sound

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23
Q

How does the pinna ampify sound

A

Pinna collects sound and acts as a funnel, certain features of sound are amplified while others are attenuated before they enter the ear canal

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24
Q

What frequencies of sound are amplified by the pinna

A

Pinna amplifies frequencies around 2-4kHz, the range most human speech sounds fall into, 30-100 fold (Purves et al,2001)

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25
Q

What is the middle ear

A

Air filled cavity between the tympanic membrane (eardrum) and inner ear, contains 3 ossicles

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26
Q

What are the 3 ossicles of the middle ear

A

Malleus, incus, stapes

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27
Q

What other part of the head is the middle ear connected to

A

Connected to the back of the throat by the Eustachian tube

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28
Q

What is the role of the middle ear

A

Transmit sound vibrations from the tympanic membrane to the oval window of the inner ear in a way that minimises energy loss

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29
Q

Why are sound waves not transmitted through the air of the middle ear

A

A lot of energy would be lost and reflected away from the oval window when the sound waves reached it (99.9%), due to the greater acoustic impendance of the cochlear fluid that exerts pressure at the back of the oval window

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30
Q

What 2 mechanisms does the middle ear use to amplify sound to minimise energy loss and allow impedance matching

A

Ossicles, difference in SA

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31
Q

What 2 muscles are attached to the malleus and stapes

A

Stapes muscle (stapedius) and malleus muscle (tensor tympani)

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32
Q

What do the ossicles do to achieve impedance matching

A

Ossicles act as levers to transform the large movements of the tympanic membrane into smaller but stronger vibrations at the oval window, plus the lever arm formed by the malleus is slighlty longer than that of the incus, increasing pressure by a factor of 1.3

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33
Q

How is impendace matching achieved in the middle ear by SA differences

A

The sound pressure across the relatively large eardrym is concentrated on the much smaller oval window

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34
Q

By how much does our middle ear increase our sensitivity to sound

A

By about 30dB, the same amount that would otherwise be lost- pressure at the oval window is 20x greater than at the tympanic membrane

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35
Q

How can the middle ear muscles have a protective function

A

Attenuation reflex- can protect the ear from damage by very large sounds, protect delicate structures from damage (however, 50-100msec delay means it can’t protect against sudden loud sound eg explosions)

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36
Q

How can the Stapedius reflex allow filtering of sound

A

Suppresses intense, low-frequency sounds or continuous sounds, or when we vocalise, allowing us to discern high-frequency sounds in a noisy environment eg speech, and prevent us hearing our own voice too loudly

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37
Q

What comprises the inner ear

A

Fluid filled chambers- semi circular canalds, and a coiled tube in a hard bony shell aka the cochlea

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38
Q

What 2 fluids does the cochlea contain

A

Perilymph and endolymph

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39
Q

How long would the cochlea be if uncoiled

A

30 mm long

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40
Q

Where is perilymph located

A

Scala vestibuli and scala tympani

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41
Q

Where is endolymph located

A

Scala media

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42
Q

What is the ionic composition of perilymph

A

Main ionic component is Na+

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43
Q

What is the ionic composition of endolymph

A

Contains lots of K+ due to it leaking in from the stria vascularis, much more positively charged than perilymph

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44
Q

What separates the scala media and scala tympani

A

Basila membrane

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45
Q

What is the organ of corti

A

Sits on the basilar membrane, contains hair cells that connect to the auditory nerve leaving the cochlea

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46
Q

What is the result in the cochlea of the stapes pushing the oval window membrane inwards

A

It increases pressure in the fluid-filled space of the cochlea in waves, causing the round window to bulge out and the basilar membrane to move in waves (von Bekesy)

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47
Q

What is the effect of complex sounds on the basilar membrane

A

Cause deflections at several positions along the membrane as they contain multiple frequencies

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48
Q

How many hair cells are in the human ear

A

15-20,000

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49
Q

How does the basilar membrane have a place code for frequency

A

By vibrating in different places depending on the frequency of the sound, the membrane achieves an analysis of the frequency components of sound- tonotopy

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50
Q

How does structure of the basilar membrane change across it

A

Stiffness decreases as you move from base to apex, and gets wider (like a flipper)

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51
Q

At which end of the basilar membrane do high frequency vs low frequency sounds cause vibration

A

Stiff base- high frequency
Less stiff apex- low frequency
The position of the peak of the travelling wave depends on the sound frequency

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52
Q

How does movement of the basilar membrane cause movement of the hair cell bundles

A

Up-down movement of the basilar membrae causes the tectorial membrane to slide sideways over the membrane, causing a sideways displacement of the stereocilia on the cochlear hair cells

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53
Q

What is the tectorial membrane

A

A membrane covering the organ of corti

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54
Q

How does displacement of the hair cell bundle cause the opening of K+ channels in the hair cells

A

Movement of hair cell bundles changes the tension on tip links joining the stereocilia of the cell, opening/closing stretch sensitive K+ channels

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55
Q

What is the result of opening K+ channels in hair cells

A

Influx of K+ from the endolymph, causing a depolarisation of the hair cell membrane, which opens voltage-gated Ca2+ channels and increases probability of glutamate release

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56
Q

What do hair cells release glutamate onto

A

Auditory nerve fibre

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57
Q

What is movement of the basilar membrane directly related to

A

Deflection of stereocilia, amount of K+ influx, amount of membrane potential depolarisation

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58
Q

What do hair cells do instead of firing APs

A

Change their membrane potential to release glutamate upon stereocilia deflection

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59
Q

What is an AC response for hair cells

A

At low frequencies, the hair cell membrane potential follows every cycle of the stimulus

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60
Q

What is a DC response for hair cells

A

At high frequencies, membrane potential is unable to follow individual cycles, so instead remains depolarised throughout duration of stimulus

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61
Q

Why does DC mode response for hair cells come about

A

Due to a slight asymmetry in effects of displacing stereocilia- opening channels can depolarise the membrane more than closing them hyperpolarises it (because only a small proportion of stereocilia channels are open at rest)

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62
Q

How can the relationship between membrane potential and sound input be tested experimentally

A

Sticking an intracellular recording electrode into a hair cell and measuring its membrane potential

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63
Q

How does the outer hair cell change in response to hyperpolarisation and depolarisation

A

It changes its length in resposne to depolarisation as depolarisation activates prestin

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64
Q

What mediates the length change of OHCs

A

A motor protein called prestin in the OHC membrane

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65
Q

What is the result of OHC motility

A

Produced localised amplification of the basilar membrane motion, leading to higher sensitivity and sharper frequency tuning
Also a source of non-linearity as weak stimuli are amplified more effectively than strong ones

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66
Q

The basilar membrane (in the way it uses place coding) acts as a mechanical…

A

Frequency analyser

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67
Q

What do the auditory nerve fibres do after leaving the cochlea

A

The auditory nerve fibres join the 8th cranial nerve and branch, terminating in the anteroventral and dorsal cochlear nuclei

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68
Q

Where do the cochlear nuclei project to

A

The ipsilateral and contralateral superior olivary complex

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69
Q

What does the nuclei of lateral lemniscus receive input from

A

The cochlear nucleui and superiori olivary complex (2 previous structures)

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70
Q

What is the principle aufitory structure of midbrain

A

Inferior colliculus

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71
Q

Where does the inferior colliculus receive input from

A

All brainstem nuclei (previous structures)

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72
Q

Where does the medial geniculate body receive intput from

A

Inferior colliculus

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73
Q

What is the medial geniculate body

A

The auditory part of the thalamus

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74
Q

What does the auditory cortex receive input from

A

The medial geniculate body

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75
Q

Summary of auditory pathway

A

Cochlea -> cochlear nuclei -> superior olivary complex -> nuclei of lateral lemniscus->inferior colliculus ->medial geniculate body-> auditory cortex

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76
Q

How many rows of outer hair and inner hair cells are there

A

3 rows of OHCs, 2 rows of IHCs

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77
Q

What nerves form the axons that travel through the auditory nerve to connect hair cells to cochlear nucleus

A

Spiral ganglion neurons

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78
Q

What are the 2 types of auditory nerve fibres that inner vs outer hair cells connect to

A

IHCs- type 1 fibres

OHCs- type 2 fibres

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79
Q

What are the properties of type 1 AN fibres

A

Connect to IHCs, myelinated, thick, fast signal transmission, form more specific connections

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80
Q

What are the properties of type 2 AN fibres

A

Connect to OHCs, unmyelinated, slow, play a minor role in auditory processing

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81
Q

What is the relative no of type 1 and type 2 AN fibres

A

Type 1 fibres outnumber type 2 fibres by a factor of 10

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82
Q

How many fibres are IHCs typically innervated by

A

10-20 type 1 fibres to one IHC

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83
Q

How many fibres are OHCs typically innervated by

A

6 type 2 fibres connect to each OHC, typically have to share each fibre with 10 other OHCs (so less specific info)

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84
Q

What axons does the superior olivary complex send down to connect with earlier structures

A

SOC sends axons connecting with AN dendrites and OHC- likely function is protection against damage from high intensity noise and improvement of signal-noise ratio

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85
Q

What power does 100dB SPL (sound pressure level) exert

A

10mW/m^2

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86
Q

What is the size of the eardrum in m

A

0.0001m^2

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87
Q

How much power does sound of 100dB exert on each eardrum

A

1uW

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88
Q

What is the effect of a 0dB sound on the eardrum

A

A 0dB sound (20uPa) moves the eardrum by less than the diameter of one H molecule

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89
Q

What code do AN fibres use to encode intensity

A

AN fibres use a rate code- they increase their firing rate as a function of sound intensity
Population code- due to sensitivity differences between low/intermediate/high SR fibres

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90
Q

What different types of AN fibres are there (3 types, not 1 and 2)

A

Low spontaneous rate fibres, intermediate spontaneous rate, high spontaneous rate- differ in their sensitivity and dynamic range

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91
Q

What are the properties of low spontaneous rate fibres

A

Have the highest threshold, saturate only at high sound levels (above 90dB)

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92
Q

What are the properties of intermediate spontaneous rate fibres

A

Intermediate thresholds, saturate by about 60dB

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93
Q

What are the properties of high spontaneous rate fibres

A

Most sensitive fibres aka lowest threshold, may saturate by 40dB

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94
Q

What is the lowest sound level that will elicit AP firing

A

eg 40db for high SR fibres

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95
Q

What is the effect of increasing sound intensity on the range of frequencies a nerve fibre responds to

A

As intensity increases, larger areas of the basilar membrane will vibrate, meaning nerve fibres respond to a greater range of frequencies

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96
Q

What does the tuning band of a auditory nerve fibre show

A

The range of tone frequencies a nerve responds to over a range of dB (as dB increases, the band gets wider)

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97
Q

What is the range at which we have greatest sensitivity to frequency

A

Between 2-5kHz

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98
Q

At what frequencies does sensitivity decrease

A

At particularly high and particularly low frequencies- outside of 2-5kHz range

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99
Q

Through which structures is tonotopic organisation maintained

A

Auditory nerve, cochlear nuclei, medial superior olive, medial nucleus

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100
Q

What type of code does tonotopy create

A

Place code and population code for sound frequency

101
Q

How do auditory nerve fibres encode sound frequency

A

ANs fire APs to low frequency sounds at particular times via phase locking, and the spike time intervals encode temporal stimulus features aka sound frequency

102
Q

What limits the phase locking of spikes of a single neuron

A

Refractory period of at least 1ms sets an upper limit of 1kHz for phase locking

103
Q

Why are individual nerve fibres not particularly informative at high frequencies w phase locking

A

Neurons in practice can’t sustain firing rates over 600Hz, and even at lower rates, they tend not to fire at every stimulus cycle as the nerve fibre response is stochastic

104
Q

How does a population of neurons encode frequency via phase locking

A

If spikes from a no of nerve fibres are combined, it provides info about the temporal structure of the sound from the temporal distribution of the spikes up to frequencies that surpass the phase locking limit of individual nerve fibres

105
Q

What sets the absolute limit for the temporal resolution of frequency that can be encoded by phase locking

A

The AC mode limit of the hair cell membrane potential, no AC response for frequencies above 3kHz

106
Q

How many auditory nerve afferents are in the auditory nerve

A

10,000, 95% are type 1

107
Q

Which parts of the cochlear nucleus do different parts of the basilar membrane project to

A

Base of basilar membrane projects to medial CN (HF), apex of basilar membrane projects to lateral CN (LF)

108
Q

What are the different parts of the cochlear nucleus that fibres project to

A

Dorsal, posteroventral and anteroventral

109
Q

What do auditory nerve fibres do when they enter the cochlear nucleus

A

Bifocate, and send fibres to the different parts of the cochlear nucleus to form synapse with multiple cochlear nucleus neurons

110
Q

What cell types are contained in the anteroventral part of the cochlear nucleus

A

Globular bushy cells and spherical bushy

111
Q

What do globular and spherical bushy cells (AVCN) cells receive input from

A

A small no of very large excitatory synapses from auditory nerve fibres

112
Q

What responses to globular and spherical bushy cells (AVCN) SHOW

A

Primary-like responses, meaning their firing patterns are almost identical to the firing patterns of their auditory nerve input and preserve into contained in phase locking

113
Q

What cell types are contained in the posteroventral cochlear nucleus

A

Octopus cells and multipolar cells

114
Q

What do octopus cells and multipolar cells in the PVCN receive input from

A

Octopus cells receive convergent input from many AFs

Multipolar cells receive input from multiple ANs mostly on the dendrites

115
Q

What responses to octopus and multipolar cells of the PVCN show

A

Octopus cells are very broadly tuned to to their convergent input and fire a single AP in response to a sudden burst
Multipolar cells show chopper responses when stimulated with pure tones

116
Q

What are the chopper responses shown by multipolar PVCN cells

A

Regular, rhythmic bursts, but burst frequency is unrelated to tone stimuli frequency

117
Q

What cells are contained in the dorsal cochlear nucleus

A

Pyramidal cells

118
Q

What response do pyramidal cels in the DCN show

A

Pauser response aka fire strongly to onset of sound, followed by an inhibitory period, then ramping up of firing rate again

119
Q

How do the different cell types in the cochlear nucleus differ

A

Each cell type differs in morphology and response properties- attracts different aspects of the incoming acoustic info, and passes this on to different points in the auditory pathway

120
Q

What are the tuning curves of globular/spherical bushy cells AND octopus cells

A

Tuning curves excited by a range of frequency that becomes wider with increasing sound intensity (look like AN tuning curves)

121
Q

Why do the tuning curves of multipolar and pyramidal cells in the CN differ from those of the others/ANs?

A

More complex tuning curves with inhibitory regions where firing rates are suppressed below spontaneous rates because of the tuning of their input by inhibitory CN neurons

122
Q

What do multipolar cells have that provides lateral inhibition to affect their tuning curve

A

Inhibitory side bands

123
Q

How may pyramidal cell tuning curves have very complex shapes

A

May have large inhibitory regions and only small excitatory pitches due to lateral inhibition

124
Q

Where/what do globular and spherical bushy cells in the ACVN project to

A

Primarylike info is projected to the superior olivary nuclei

125
Q

Where/what do multipolar PVCN cells and pyramidal DCN cells project to

A

Multipolar- chopper info
Pyramidal- pauser info
May use lateral inhibition to extract spectral contrast, project to inferior colliculus

126
Q

What are onset cells (in CN)

A

An inhomogenous class- some are multipolar and some are stellate

127
Q

What do onset cells do in the CN

A

May encode temporal pattern info across many AN fibres or encode sound intensity

128
Q

Where do onset cells project to (from CN)

A

Project to inferior colliculus or lateral lemniscus

129
Q

What is the first stage of binaural convergence of info from the 2 ears

A

The superior olivary nuclei

130
Q

What are the 2 parts of the superior olivary nuclei

A

Lateral superior olive (LSO) and medial superior olive (MSO)

131
Q

What does the MSO receive input from

A

Excitatory input from CN of both sides

132
Q

What does the LSO receive input from

A

Excitatory input from ipsilateral CN, inhibitory input from the contralateral CN via medial nucleus of trapezoid body (MNTB)

133
Q

What is the synapse between the LSO and medial nucleus of trapezoid body

A

Calyx of Held, because it is the biggest synapse in our brain

134
Q

What is the concept of interaural time/level differences

A

Sounds arrive earlier and are louder at the near ear to the sound

135
Q

What is specialised for the processing of interaural level differences

A

Lateral superior olive

136
Q

When will an LSO neuron only fire when processing interaural level differences and why

A

LSO neuron will fire strongly only when a loud sound is received in the ipsilateral ear and a quiet sound in the contralateral ear, because of the inhibitory connection from the contralateral side by the MNTB

137
Q

What is the MNTB

A

Medial nucleus of trapezoid body

138
Q

How does the LSO encode sound source direction using the interaural level difference

A

Rate code- fire much more strongly when I>C

139
Q

How are interaural level differences affected by frequency

A

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

140
Q

Study into effect of frequency on ILD- procedure?

A

Tiny microphones places into the ear canals of humans, different frequency sounds from diferent locations in space all aruond the subject presented

141
Q

Study into effect of frequency on ILD- results

A

LF- ILD only varies over a range of 30db

HF- ILD varies over a range of 80dB, more useful

142
Q

Why do low frequency stimuli create lower ILD than high frequency stimuli?

A

LF sound waves can more easily wrap around the head without being attenuated

143
Q

What is specialised for processing of interaural time differences

A

Medial superior olive, neurons are sensitive to changes in ITD

144
Q

What code was originally thought to be used for sound source direction in the MSO

A

Place code- now disputed for mammals

145
Q

What does the brain need to compare in order to process iTDs

A

Compare the arrival time of sounds at the 2 ears

146
Q

At which ITD is firing of MSO neurons highest

A

Firing peaks when sound is slightly on the contralateral side (accounts for longer contralateral delay)

147
Q

What innervates the MSO

A

AVCN neurons project to the MSO via spherical bushy cells connected to end bulb of Held synapses

148
Q

What are the properties of the end bulb f Held synapses that join the AVCN to th MSO

A

Like the calyx of Held- unusually large, operate with very high temporal precision

149
Q

How does the end bulb of Held synapse between the AVCN and MSO operate with such high temporal precision

A

A single presynaptic AP at an end bulb of Held synapse can trigger an AP in the postsynaptic cell, allowing the MSO to be provided with info that reflects the timing of the sound arriving at both ears

150
Q

What model has been proposed to explain ITD analysis at the MSO

A

Jeffress (1948) Delay Line and Coincidence Detector Model

151
Q

When do MSO neurons fire maximally in detecting ITD

A

Fire maximally ONLY if they receive simultaneous input from both ears

152
Q

What is the result of input from different ears being delayed by different amounts on the way to the MSO which detects ITD

A

Because the input from different ears is delayed by different amounts (eg one side’s afferent axons are longer), the MSO neuron will fire maximally only if an interaural delay in the arrival time at the ears exactly compensates for the transmission delay

153
Q

What different types of MSO neurons are there as the result of the coincience detection property of MSO neurons in detecting ITDs

A

Different MSO neurons become tuned to characteristic interaural delays

154
Q

How does the MSO use a place code for sound source direction

A

Different neurons in different areas of the MSO will be activated by different ITDs that result from sound stimuli from different directions

155
Q

How are ITDs affected by frequency?

A

We are bad at extracting ITDs from high frequency sounds as our ANs can’t phase lock to high frequency stimuli, so the high temporal info needed for processing ITDs can’t be extracted

156
Q

How big are normal ITDs in humans

A

‘Unaided’ ITDs are max 0.7ms in humans

157
Q

What is the smallest detectable difference in ITD

A

0.01ms

158
Q

What scientists have donw recent work that sheds doubt on whether the Jeffress (1948) Coincidece Detector Model is a good description for ITD in the mammalian MSO

A

McAlpine, Palmer, Grothe

159
Q

What does the inferior colliculus do

A

Major (+ big) processing centre- collects and integrates input from all auditory brainstem nuclei, obligatory relay for all ascending auditory info

160
Q

How does the input to the IC affect the excitability of its neurons

A

Most inputs to the IC are from the other hemisphere, so most neurons in the IC and above are most strongly excited by sounds presented in the contralateral ear

161
Q

How is the IC divided

A

Has a number of anatomical subdivisions- central nucleus, dorsal cortex and external nucleus in the shell

162
Q

Which subdivisinos of the IC are tonotopically organised

A

The largest, the central nucleus, is tonotopically organised

The dorsal cortex and external nucleus (in the shell) are NOT

163
Q

How is the central nucleus of the IC tonotopically organised

A

High frequency neurons are in the ventral part, low frequency neurons are in the dorsal part

164
Q

Where does integration of acoustic with contextual info start

A

The inferior colliculus

165
Q

Study showing how neurons in the IC can be impacted by non-acoustic signals eg behaviour

A

Chen and Song (2019)- speed of firing in the IC increased as a mouse increases its running speed

166
Q

Where does the inferior colliculus project to other than the medial geniculate body (aka outside the main auditory pathway)

A

Superior colliculus

167
Q

What is the superior colliculus

A

Multisensory nucleus- mostly concerned with control of eye and head movements

168
Q

How is the superior colliculus involved in audition

A

Only place in the brain we have found a map of auditory space- in the deeper layers of SC, aligned with map of visual space found in upper layers

169
Q

What does the medial geniculate body consist of

A

3 major nuclei, the ventral, medial and dorsal MGB

170
Q

What do the different parts of the medial geniculate body receive input from

A

Ventral MGB receives input from central nucleus of the IC

Dorsal and medial MGB receive input from shell of IC AND non-auditory structures

171
Q

What properties do dorsal and medial medial geniculate bodies have as a result of non-auditory structures

A

MGB neurons may, to a greater extent than the IC, represent non-acoustic features of sensory stimuli and be even more modulated by other sensory/motor systems

172
Q

Evidence of multimodal integration in the medial geniculate body

A

Neurons in the MGB of a mouse showed an attenuated response to acoustic info following somatosensory stimulation (whiskers)

173
Q

What is another name for the medial geniculate body

A

The auditory thalamus

174
Q

What type of integration do we see more and more as we move up through the hierarchy of the auditory pathway

A

More and more multimodal integration eg info from motor system and sensory systems integrated with auditory info

175
Q

Which lobe is the auditory cortex located in

A

Temporal lobe

176
Q

What is the primary auditory cortex also called

A

A1, Brodmann’s area 41

177
Q

Which sections of the auditory cortex are tonotopically organised

A

A1 is strictly tonopically organised

Organisation starts to break down in A2 and higher cortical areas beyond

178
Q

How do primary and higher order cortical areas show increasingly complex response properties

A

They integrate acoustic info with contextual info (other sensory input, motor output, memories, expectations) to make sense of it

179
Q

Example of integration of memories/expectations with acoustic info

A

Eliades and Wang (2008)- monkeys neurons in A1 signal mismatches between the expected sensory feedback from a vocalisation and the actual sensory feedback, a type of error signal crucial for fine tuning our vocal production

180
Q

How is the auditory cortex divided up

A

Divided into 6 different layers with different functions

181
Q

What is the flow of information through the auditory cortex

A

Thalamus -> L4 -> L2/3 ->L5 ->L6-> out

182
Q

What descending projections of the auditory cortex are there

A

Deep layer neurons (L5 and L6) descend to the thalamus, inferior colliculus and brainstem through MASSIVE projections

183
Q

What are the descending projections from the auditory cortex thought to be responible for

A

Controlling and gating the flow of info in subcortical structures

184
Q

From which direction does the thalamus auditory nucleus receive more inputs

A

Receives more inputs from the cortex than it receives ascending input

185
Q

How are cortical circuits highly plastic

A

Can readily change their response properties as a result of new experiences, suggesting the cortex is essential for perceptual learning

186
Q

Which areas of the auditory system are not well understood

A

Auditory midbrain, thalamus and cortex

187
Q

What is the frequency of sound

A

The no of compressed patches of air that pass by our ears per second (Hz)

188
Q

What is the intensity/amplitude of sound

A

The difference in pressure between compressed and rarefied patches of air

189
Q

What is the attenuation reflex

A

A loud noise triggers a neural response that causes the tensor tympani and stapedius muscles to contract, making the ossicle chain more rigid and decreasing sound conduction to the inner ear

190
Q

What 3 chambers is the cochlea divided into

A

Scala vestibuli, scala media, scala tympani

191
Q

What does Reissner’s membrane separate

A

Scala vestibuli from the scala media

192
Q

Where is the round window

A

At the base of the cochlea, the scala tympani meets the round window

193
Q

What is the stria vascularis

A

The endothelium lining one wall of the scala media contacting the endolymph

194
Q

What does the stria vascularis do

A

Absorbs Na+ and secretes K+ into the endolymph

195
Q

Why can low frequency sounds travel further up the basilar membrane

A

They can travel further before all of their energy is dissipated, compared to higher frequencies that dissipate most of their energy at the stiff base

196
Q

Why are the auditory receptors called hair cells

A

They each have 10-300 stereocilia extending from the top

197
Q

What type of cells are hair cells

A

Specialised epithelial cells, not neurons

198
Q

Where are the outer hair cells

A

Further out than the rods of Corti

199
Q

Where are the inner hair cells

A

Between the rods of Corti and the modiolus (central axis of cochlea)

200
Q

What moves the OHC cilia vs IHC cilia

A

OHC cilia moved by the bending of the tectorial membrane over them
IHC cilia probably pushed by moving endolymph

201
Q

What makes it so that stereocilia only bend at their base

A

Aligned actin filaments make stereocilia rigid rods that only bend at their base

202
Q

Who pioneered an approach of studying hair cells in isolation

A

Hudspeth (1980s)- a sound wave causing stereocilia to bend back and forth causes the hair cell to generate a receptor potential that alternately hyperpolarises and depolarises from the resting potential of -70mV

203
Q

How does the opening of K+ cause depolarisatino of the hair cells since for most cells opening K+ channels hyperpolarises cells

A

Because of the unusually high K+ conc in the endolymph

204
Q

How many spiral ganglion neurites does each IHC feed

A

Each IHC feeds about 10 spiral ganglion neurites

205
Q

What is the cochlear amplifier

A

The action of OHCs as tiny motors to amplify the movement of the basilar membrane during low-intensity sound stimuli

206
Q

Evidence for the importance of prestin in OHCs amplification

A

Ruggero and Rich (1991)- Furosemide, a drug that reduces the flexing of the basilar membrane in resposne to sound, is thought to act by deactivating OHC motor proteins like prestin

207
Q

What is the result of amplification of basilar membrane motion by the OHCs

A

Increased transduction process, meaning higher sensitivity and sharper frequency tuning in the inner ear

208
Q

How can descending input from the brain to the cochlea regulate auditory sensitivity

A

Stimulation of efferent fibres projecting to the OHCs causes them to release ACh, changing the shape of OHCs and affecting the responses of IHCs

209
Q

Clinical evidence for the importance of OHCs in amplification

A

Damage to OHCs by drugs toxic to them eg quinine can cause nerve deafness, associated with loss of hair cells in the cochlea, and is treated with a hearing aid that amplifies the sound in the place of missing hair cells

210
Q

Evidence for the amplificayion by OHCS favouring weaker sounds

A

Ruggero et al (1997)- plotted the mechanical gain of the basilar membrane in response to pure tones, fuond OHCS amplify weaker sounds more strongly with a gain of up to 60dB

211
Q

What form of nonlinearity does the cochlear amplifier follow

A

Compressive linearity- allows a wide range of sound pressure amplitude inputs to be mapped onto a more limited range of basilar membrane vibration amplitudes

212
Q

Evidence for the differential amplification of sound across the basilar membrane

A

Two-tone suppresion- OHCs lie side by side so the amplification they create can’t operate entirely independently on each small patch of the basilar membrane.. if two pure tones are close in frequency, the response to one may appear disproportionately small if both are simultaneous (Cooper, 1996)

213
Q

What is the auditory nerve also called

A

The auditory vestibular nerve

214
Q

Evidence for the existence of the dorsal and ventral cochlear nuclei

A

Brain stem damage can only produce deafness in one ear if a cochlear nucleus on one side is destroyed (all other brain stem nuclei are binuaral)

215
Q

What is phase locking

A

The consistent firing of a cell at the same phase of a sound wave eg at the peaks of a wave

216
Q

What is the volley principle

A

The pooled activity of a population of neurons that fire in a phase-locked manner (but each on a different cycle of the input signal) can sum a response to every cycle, so represent frequency

217
Q

How is frequency encoded over 3kHz (upper limit of phaselocking)

A

By tonotopy alone

218
Q

Evidence for specialisation of auditory neurons for timing

A

Oertel et al- some ANs have very low membrane resistance and fast time constants, helping them convey precise timing info

219
Q

Evidence for specilaisation of cochlear nuclei cells

A

Golding Bal and Ferragamo- octopus CN cells have exceptionally large mutually opposing types of voltage-sensitive ion channels that give the cells fast time constants, allowing them to coincidence detect in the submillisecond time range

220
Q

What is the difference between the requirements for horizontal vs vertical sound localisation

A

Good horizontal localisation requires a comparison of the soudns reaching the 2 ears, but good vertical localisation does not

221
Q

How do we detect ITD with continuous tones that are always present at both ears

A

The time at which the SAME PHASE of the sound wave reaches each ear is compared instead eg peak

222
Q

In what circumstances can we not use interaural delay of the same phase of a sound to determine sound location

A

Continuous tones at HIGH FREQUENCIES (>2000Hz) as one cycle of the sound is smaller than the distance between your ears, so many peaks will fit in between your ears and there is not longer a simple relation between sound direction/peak arrival time

223
Q

How does the brain localise continuous tones at high frequencies

A

Uses the interaural intensity difference between the ears, due to the sound shadow cast by your head- neurons sensitive to intensity differences can use this info to locate the sound

224
Q

What is the duplex theory of sound localisation

A

20-2000Hz- ITD is used

2000-20,000Hz- ILD is used for localisation

225
Q

Evidence for possible place coding of ITD

A

Recordings from LSO show each neuron gives its greatest response to a particular interaural delay, meaning each may encode a particular position in the horizontal plane

226
Q

Study sggesting an alternate method for sound localisation by ITD in mammals

A

Studies on gerbils suggest synaptic inhibition rather than axonal delay lines generates the sensitivity of LSO to ITD

227
Q

How can vertical sound localisation be impaired

A

By placing a tube in the auditory canal to bypass the pinna, or simply covering the pinna convolutions

228
Q

How do we assess vertical localisation

A

The bumps and ridges of the pinna reflect the sound, meaning the delays between the direct path and reflected path change as sound moves vertically, so the combined sound is subtly different when it comes from above or below

229
Q

How do axons from the MGN project to A1

A

Via the internal capsule in an array called acoustic radiation

230
Q

How is the structure of A1 similar to the layers of V1

A

Layer I contain few cell bodies, layers II/III contain mostly small pyramidal cells, layer IV where MGN axons terminate, layers V and VI contain mostly large pyramidal

231
Q

Evidence suggesting columnar organsiation in A1

A

In electrode penetrations in monkeys, the cells tend to have similar characteristic frequencies, suggesting a frequency columnar organisation

232
Q

In which way is the tonotopic map on A1

A

Low frequencies- rostrally and laterally
High frequencies- caudally and medially
Isofrequency bands running mediolaterally across A1

233
Q

How do cortical neurons differ

A

Different temporal response patters, some intensity tuned, some respond to clicks/frequency-modulated sounds/animal vocalisation

234
Q

Example of a higher level audio processing region

A

Wernicke’s area, impairs ability to interpret language

235
Q

What are the 2 types of deafness

A

Conduction deafness, nerve deafness

236
Q

What can cause conduction deafness

A

Rupture of tympanic membrane, pathology of ossicles eg many diseases impair transfer of suond by binding the ossiclds to the bone of the middle ear

237
Q

What is conductino deafness

A

Deafness caused by a disturbance in the conduction of sound from the outer ear to the cochlea

238
Q

What is nerve deafness

A

Associated with loss of neurons in the auditory nerve or hair cells in the cochlea

239
Q

What can cause nerve deafness

A

Tumours affecting the inner ear, drugs toxic to hair cells eg quinine, exposure to loud noises

240
Q

How do cochlear implants work

A

Take advantage of the tonotopic arrangment of auditory nerve fibres- electrical stimulation near the base of the cochlea evokes perception of high frequency sounds and vv

241
Q

What supports the idea of isofrequency bands in the cortex

A

Animal studies indicate small lesions in A1 can produce specific localisation defecits for sounds within a limited frequency range

242
Q

Why do high frequency sounds not travel far along the basilar membrane

A

At high frequencies, the fluid filled column pathway is a source of mechanical resistance due to inertia, as it takes a lot of force to push and pull the cochlear fluid at such a high frequency, so choosing to travel along the basilar membrane path offers less resistance

243
Q

What determines whether a wave will pass far or not far along the basilar membrane

A

Its frequency- it will travel along a ‘compromise path’ that is long enough for stiffness to somewhat decrease but not too long inertial resistance is super high

244
Q

How is the basilar membrane a biological Fourier analyser

A

The membrane can be seen as a set of logarithmically spaced mechanical filters each with their own resonance frequency, like a gamma-tone filter bank (Schnupp et al, 2011)

245
Q

What is a gamma tone filter bank

A

Gamma filters suppress frequencies other than those that match their resonance characteristics, so a set each tuned to a characteristic frequency can filter incoming sounds via passing them through the filters in parallel, allowing for frequency analysis and production of a ‘neurogram’

246
Q

What forms the filters that make up the gamma tone filter bank on the basilar membrane

A

Each small piece of the basilar membrane that responds to a characterstic frequency

247
Q

What is basilar membrane filtering later sharpened by

A

Zwislocki and Kletsky (1979)- further neural processes that form a psycholgically vulnerable second filter

248
Q

Evidence of ‘dancing’ hair cell

A

Video by Ashmore (2008), recorded from an OHC in a guinea pig cochlea injected with an electrical current waveform of a song, cell appears to stretch and contract rhythmically to te song

Neurophysiology (23 decks)

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