Auditory Flashcards
(93 cards)
1
Q
What is the range of frequencies the ear responds to?
A
20Hz to 20kHz
2
Q
What is the equation for amplitude of a sound?
A
dB SPL = 20 log 10 P/Po
P=root mean squared pressure of sound in microPa
Po = minimum audible sound pressure detectable by normal human listeners under ideal test conditions
3
Q
What is the range of human speech?
A
2000-4000Hz
4
Q
What is the limit of human hearing?
A
0dB
5
Q
What is the limit of hearing being painful?
A
120dB
6
Q
What are the effects of the external ear?
A
Pinna: casts acoustic shadows to help you localise sound
Pinna + EAM: creates resonance patterns that make hearing louder for frequencies around 4kHz (human hearing)
7
Q
What is the normal level in decibels of a conversation?
A
70dB
8
Q
What is the shape of a curve of frequency against auditory threshold?
A
Parabola - at 2kHz minimum threshold (0dB), at 20kHz or 20Hz higher thresholds (highest threshold 20Hz)
9
Q
What is the shape of a curve of frequency against pain threshold?
A
Straight line at 120dB
10
Q
What is the function of the middle ear?
A
- Transformer action
Ensures most of the sond energy falling on the eardrum is absorbed and transmitted to the cochlea
Impedence matching of low impedence air to high impedence cochlear fluids
11
Q
How is the impedence matching of the middle ear done?
A
s = stapes d = drum P = pressure A = area L = perpendicular distance from pivot
Transformer ratio
Ps/Pd = (Ad/As)* (Ld/Ls) = 14 * 1.3 = 18.2
18.2 is the perfect match between media whose acoustic impedences are in the ratio of 18.2 ^2
Means that 50% of incident energy is absorbed by cochlear fluids
- Area of eardrum is 14x area of footplate of stapes so pressure 14x higher at oval window
- Ossicular chain acts as a pivoted crank that introduces a lever ratio to the motion so the force at the stapes is 1.3 x that at the drum
12
Q
What is the pivot in the middle ear?
A
The incus
13
Q
What is the effect of the tensor tympani?
A
Inserts into the foot of the malleus
Stiffens the eardrum
Drum moves less at a given pressure
Contracts at sound pressures above 80dB so as to protect the inner ear from over-stimulation
14
Q
What is the effect of stapedius?
A
Inserts into stapes
Pulls side of stapes so it alters the angle at which the footplate of the stapes plunges into the oval window
Causes slippage at the incudo-stapedial joint
Contracts at sound pressures above 80dB so as to protect the inner ear from over-stimulation
15
Q
What is the combined effect of the middle ear muscles?
A
Limited sensitivity loss of 30-40dB
16
Q
How much does the stapes vibrate?
A
0dB 0.1 angstrom
120dB 1 micrometer
17
Q
How do primates reduce resonance in the middle ear?
A
- Air cells act as an acoustic damping layer
- Pressure waves dissipated as it flows through the air cell system (no walls to reflect off of)
18
Q
What is the organisation of the cochlea?
A
Fluid-filled tubes coiled into a spiral Think of as straight tube: Scala vestibule Cochlear partition (scala media with basilar membrane and Reissner's membrane) Scala tympani
19
Q
Why does the fluid displaced by the stapes have to move out of the round window?
A
Walls of cochlea are rigid
Liquid is incompressible
20
Q
How long is the cochlear partition?
A
35mm
21
Q
How wide is the cochlear partition?
A
100micrometers at base to 500micrometers at apex
22
Q
What is the name of the hole at the apex of the cochlear partition? What is the function?
A
Helicotrema
Prevent partition vibrating at sub-auditory frequencies
(Fluid moving below 20Hz moves through helicotrema without displacing the cochlear partition) Uncouples cochlea from changes in atmospheric pressure (Eustachian tube does only infrequently)
23
Q
What is the effect of the width of the cochlear partition changing along its length?
A
Partition is stiffer near to the base than the apex
So vibrates maximally near to the stapes for high frequency tones, and maximally near to the apex for low frequency ones.
24
Q
What is the mapping of frequencies onto the cochlea?
A
Tonotopic logarithmic - equal increments of distance represent equal logarithmic increments in the frequency domain
High freq close to stapes, low freq further from stapes
25
What is the travelling wave?
Partition executes a travelling wave from stapes to apex
Each point on the partition undergoes a sinusoidal vibration at the driving frequency
The amplitude of this vibration varies continuously at the driving frequency
The maximum of the envelope coincides with the characteristic place on the cochlea's frequency map
Phase of vibration also changes continuously along the partition so that more apical points lab more than stapedial ones (distinguishes travelling wave from standing waves)
26
How was the cochlear partition observed?
Excise human temporal bones
Sprinkle silver particles on basilar membrane
Use stroboscopic illumination to follow the movements of the particles
27
Why do high frequencies vibrate closer to the stapes?
Take the path of least resistance
For high frequency sound, minimising the liquid passed through better as attenuated by liquid media, whereas for low frequencies want to minimise stiffness of partition as less attenuated by liquid
28
What is the receptor in hearing?
Hair cells in the Organ of Corti
29
What are the 3 membranes in the cochlea and where are they?
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Scala vestibuli (perilymph)
Reissner's membrane
Scala media (endolymph) within this tectorial membrane on top of hair cells
Basilar membrane
Scala tympani
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30
What are the arrangement of hair cells on the organ of Corti?
Three longitudinal rows of outer hair cells
| 1 longitudinal row of inner hair cells
31
What is the ionic composition of the scala of the cochlea?
Vestibuli and tympani like ECF (sodium 140mM K+ 7mM, Ca2+ 1mM potential 0)
Scala media like intracellular solution (Na+ 1mM K+ 154 mM Ca2+ 10microM potential +100mV)
32
What are the microvilli of hair cells called?
Stereocilia, about 100/cell
33
How do stereocilia contribute to mechanoelectrical transduction?
Stereocilia connected by tip links
As hair bundle moves, cation channels connected to the tip links open and close
Displacement of bundle towards longest cilia opens more (base level 15% open), displacement away from longest cilia closes channels open at rest
Also tip links only run in one direction, parallel to bundle's plane of bilateral symmetry, so ciliary bundle is directionally sensitive
34
What drives K+ movement into hair cells?
Only electrical gradient (similar conc in both) +100 - -65mV = +165 mV driving force
35
What else in the head uses directionally sensitive hair cells?
Utricle and saccule
36
Which way are tip links oriented?
Perpendicular to long axis of cochlea, so sensitive to the shear between the tectorial and basilar membranes that occurs during vibration
37
What are the different types of hearing loss?
Conductive: reduced transmission of sound in the inner ear e.g. wax/otitis media
Sensorineural hearing loss: damage to hari cells, esp outers, or neural elements of the auditory pathway. Presbycusis, noise damage, Meniere's disease (inner ear damage), acoustic neuroma etc.
38
Define presbycusis
Age related hearing loss
39
What makes endolymph?
Stria vascularis
40
What is atmospheric pressure?
101.3kPa
41
What is the centre of the cochlear spiral called?
The modiolus
42
What is the role of inner hair cells?
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Respond to the local motion of the basilar membrane. Depolarise by K+!!!
Release Glu (excitatory) to terminals of auditory nerve, so sensory output of cochlea
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43
What is the role of outer hair cells?
Length regulated by membrane potential - shorten when depolarised. Provides a motor action that feeds back on basilar membrane to amplify its motion and reduce the local damping effect of cochlear fluids. Feedback improves sensitivity of ear and sharpens tuning of cochlear partition.
44
What are the afferent neurons leaving the cochlea?
Bipolar cells with cell bodies in spiral ganglion
45
Where do the auditory nerve fibres arise from?
95% inner hair cell
46
What is the divergence/convergence of inner hair cells?
3000:50000 so 1:20 divergence
47
How does the ear intensity code?
Mean firing rate increases as sound pressure increases
Saturates at 40dB above threshold
Different fibres have thresholds that vary themselves over 40dB
So overall about 80dB range of intensity coded
48
How does the ear frequency code?
- Place code: Each fibre sequentially along encodes a limited range of frequencies, so any activity in the fibre signals those frequencies. Only code at high frequency
- Periodicity code: For freq below 4kHz phase locking as well as place code. May not play a role in perception of pitch (used for sound localisation)
49
What is a tuning curve?
A graph of the frequency a fibre responds to against the fibre's threshold in dB
50
What is the minimum point of the tuning curve called?
Characteristic frequency
A single frequency to which a nerve fibre is most sensitive
Most of these thresholds lie close to the threshold of hearing
51
What is two-tone suppression?
Regions above and below a tuning curve exist where a second tone can suppress the response to a first tone lying within the excitatory area
Has a particularly strong influence on the phase-locked component of an auditory nerve fibre's discharge
Function is to enhance contrast in the frequency domain
Shows that the auditory system is non linear (if it were a second tone could only be additive or have no effect)
52
How do we know two-tone suppression doesn't work by synaptic interactions?
Not synaptic interactions - onset too fast
53
Define phase locking
Timing of APs synchronises with individual cycles of the stimulus
54
Why can't phase locking occur above 4kHz?
Cycles are shorter than the refractory period
| Receptor potentials filtered of their periodic components by the time constant of the hair cell's membrane
55
What determines pitch?
Pure tone: Mainly frequency, small influence of intensity
Complex tone: Brain tries to fit the components present to a regular harmonic series, then assigns the pitch to the fundamental base frequency of the series whether or not there is sound energy at that frequency or not
56
Give an example of a series with a missing fundamental
A complex tone that is assigned a fundamental frequency that is not actually present e.g. complex tone made of 200Hz, 300Hz, 400Hz heard the same as a pure tone of 100Hz
57
Where do the efferent fibres running to the cochlea come from?
Superior olive
58
What are the efferent pathways to the cochlea?
1. Crossed pathway to outer hair cells
| 2. Uncrossed pathway to afferent terminals of CN VIII beneath the inner hair cells
59
Describe the crossed pathway
Detuning: Causes a reduction in sensitivity at the tip of each auditory nerve fibre's tuning curve without much change to the skirts of the curve, thus reducing sensitivity and frequency selectivity
Probably reduce motor feedback from the outer hair cells to the cochlear partition, so affect the responses of afferents arising from the inner hair cells
60
What evokes the crossed pathway?
Spontaneous activity: Low levels of background noise
| Reflex activity: presenting tones at quite moderate sound levels to either ear. Reflex is frequency sensitive
61
What is the function of the crossed pathway?
Suppress responses to lower level background sounds so to emphasise more interesting aspects of a sound
62
How many pathways are there between the ear and the cortex?
4+
63
From what point is auditory information bilaterally represented?
All levels in the pathway above the level of the cochlear nuclei, so superior olive
64
Are there more fibres in the auditory or the optic nerve?
Optic!! >1mil
| Auditory 30K
65
Draw the afferent auditory pathway
1. Enters brainstem and bifurcates
2. One branch to anteroventral cochlear nucleus, other to the posteroventral and dorsal cochlear nuclei
3. Either cross dorsally and join with the output of the superior olivary nucleus to ascend in the lateral lemniscus
4. Pass ventrally in the trapezoid body to join with either superior olive
5. SO via lat lemniscus to IC
6. IC via brachium to MGN, also crosses here
7. MGN to A1
66
What do the medial superior olive (MSO) and lateral superior olive (LSO) do?
MSO: Interaural time differences
LSO: Interaural intensity differences
67
What is the structure of the MSO?
Sheet of bipolar neurones whose lateral dendrites receive input from ipsilateral ear and medial dendrites receive input from the contralateral ear
Respond to small time differences between the auditory stimuli delivered to the two ears
68
What is the structure of the LSO?
Most LSO neurons excited by ipsilateral stimuli and inhibited by contralateral ones so sensitive to interaural intensity differences
69
What is the role of the inferior colliculus output that goes to the deep layers of the superior colliculus?
SC cells also respond to visual stimuli
| Cells may be involved in coordinating head and eye movements, as well as orientating the pinnae towards sound source
70
What is the name of the pathway between the IC and the MGB?
Brachium of the IC
71
Where is the primary auditory cortex?
Upper bank of the superior temporal gyrus
72
What is the cortex required and not required for? How do we know?
Not required for frequency discrimination
Is required for detection of the temporal pattern of auditory stimuli
In the cat, sensitivity to sounds and the ability to react to changes in frequency are unimpaired following bilateral ablation of the auditory cortex. The cortex is however required for the cat to detect the temporal pattern of auditory stimuli.
73
How do we determine the distance away a sound is?
We are bad except for speech where experience allows us to memorise loudness at different distances
74
What are the angles defining the direction of a sound source?
Measured from mid point between the ears
Azimuth = horizontal (straight ahead 0, right 90 continuing round)
Elevation/depression = vertical mid-saggital plane. Zero straight ahead, 90 immediately above.
75
What are the accuracy limitations of sound localisation?
1-2degrees azimuth
| 10 degrees elevation but only achieved for source directly in front with broad spectral composition
76
Explain the logic behind interaural time differences
Sound has to travel further to reach one ear and so arrives later. The path difference, and hence the time difference, varies with angle of azimuth
77
What is the maximum interaural time difference?
660microseconds at 90 or 270 degrees of azimuth
78
What is the minimum interaural time difference?
10 microseconds
79
What are interaural phase differences?
Extra path lengths means that the sound waves reach each ear out of phase
After about 1kHz the phase difference exceeds one cycle, so phase comparisons become ambiguous
Encoding a sound's waveform is no longer preserved in the discharge of the auditory nerve above a few kHz anyway, so interaural phase differences can't be used at high frequencies
80
What are interaural intensity differences?
Above 2kHz the wavelength of sound is comparable to the dimensions of the head
The more distant ear is shadowed by the head and consequently the intensity of the sound is lower
Function of azimuth and frequency
Maximum at 90 and 270 degrees azimuth, and at high frequencies - 20dB
81
What are cones of confusion?
The same set of interaural time and intensity cues may be possessed by a source at many different locations in space. A spherical bodyless head with symmetrical pinnae: sound source could lie anywhere on a conical surface spreading out from its apex at the centre of the ear
82
Why do cones of confusion not actually exist?
Asymmetry of head, complicated shape of pinnae, shadowing effect of body
'Spectral coloration' - operates monaurally
Learn to associate a particular coloration with a particular direction, then use this information to interpret the binaural cues
83
Summarise how we locate a sound
1. Distance = learn volumes associated with distances for speech
2. Angle = interaural sound differences and interaural intensity differences
84
What happens when a source is on the midplane?
Body is mirror symmetrical about midplain so only anatomical feature that introduces spectral coloration is the front-back asymmetrical pinnae
85
How does the MSO use interaural time differences?
Jeffress' model
Coincidence detector cells fire only when epsp occurs simultaneously from both inputs
Input from one cochlea is delayed more the other by a difference in conduction distances (delay line)
Thus sound has to occur sooner on the delayed side to excite at the same time as that from the undelayed one
Thus cells have a preference for a particular interaural delay
It is a bilateral system, each LSO just delays with the contralateral hemifield
Delays due to the travelling wave require the input axons to come from exactly the same place within each cochlea
Layers process different frequencies
86
What is the model for the MSO?
Nucleus laminaris of the barn owl
87
How does the LSO use interaural intensity differences?
Cells selectively excited by inputs from one ear and inhibited by those from the other
Thus detect small intensity differences
88
Where is analysis of directional coloration?
Don't know!! Below midbrain in owls as cells in midbrain of a barn owl respond only to sources lying in a limited RF in directional space.
'Place' cells described in deep layers of the superior colliculus in mammals
89
What determines timbre?
Harmonics
90
Where is the cell body of the hair cell?
Perilymph
91
How many times does the cochlea turn around the modiolus?
2.5x
92
Where is Broca's area and what does it do?
Broca’s area lies in the frontal cortex adjacent to that part of the primary motor cortex which controls the mouth, the tongue and the larynx. It is involved in speech production including aspects of syntax.
93
Where is Wernicke's area and what does it do?
Wernike's area lies on the posterior part of the temporal lobe. It is important for speech comprehension. Both areas are unilaterally represented in the dominant (usually left) hemisphere alone.
