hearing and deafness 1. anatomy & physiology€¦ · • gives ear a logarithmic (non-linear)...
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Hearing and Deafness 1. Anatomy & physiology
Chris DarwinWeb site for lectures, lecture notes and filtering lab:http://www.lfesci.sussex.ac.uk/home/Chris_Darwin/
look under: "Teaching material for students" "Perception & Attention"
Protection
Impedance match
Capture; Amplify mid-freqs
Vertical direction coding
Frequency analysis
Transduction
Outer, middle & inner ear
Middle ear structure
Conductive hearing loss
• Sounds don’t get into cochlea
• Middle ear problems
• Helped by surgery and by amplification
Protection
Impedance match
Capture; Amplify mid-freqs
Vertical direction coding
Frequency analysis
Transduction
Outer, middle & inner ear
Cochlea
Cochlea cross-section
Travelling wave on basilarmembrane sorts sounds by frequency
distance along basilar membrane
amp
litu
de
of
vib
rati
on
base apex
high frequencies low frequencies
0
Reponse of basilar membraneto sine waves
Each point on the membrane responds best to a different frequency:high freq at base, low at apex.
amadeus praat
Organ of Corti
Innerhair cell
Hair CellStereocilia
Auditory nerve innervation
OHC (2)spiral afferent (green)medial efferent (red)
IHC (1)
radial afferent (blue)lateral efferent (pink)
Auditory nerverate-intensity functions
log amplitude (dB SPL)
100
80
60
40
20
sp
ikes
/ s
eco
nd
30 60 90
high spontaneous rate
low spontaneous rate
saturation
many
few
Phase Locking of Inner Hair Cells
Auditory nerve connected to inner hair cell tends to fire
at the same phase of the stimulating waveform.
Phase-locking
pres
sure
-1
-0.5
0
0.5
1
0 0.2 0.4 0.6 0.8 1
Inter-spike IntervalsResponse to Low Frequency tones
time (t)
volta
ge
Response to High Frequency tones > 5kHz
Random intervals
time (t)
2 periods 1 periodnerve spike
volta
ge
Innervs
OuterHair Cells
Inner vs Outer Hair Cells
Inner Hair Cells Outer Hair Cells
Sensory Motor
Afferent nerves Efferent nerves
Single row c.3 rows
OHC movement
PassiveNo OHC movement
ActiveWith OHC movement
OHC activity
• Increases sensitivity (lowers thresholds)
• Increases selectivity (reduces bandwidth of auditory filter)
• Gives ear a logarithmic (non-linear) amplitude response
• Produce Oto-acoustic emissions
OHCs are relatively more active for quiet sounds than forloud sounds.
They only amplify sounds that have the characteristicfrequency of their place.
Conductive vs Sensori-neural deafness
Conductive Sensori-neural Sensori-neural
Origin Middle-ear Cochlea (IHCs) Cochlea (OHCs)
Thresholds Raised Raised Raised
Filter bandwidths Normal Normal Increased
Loudness growth Normal Normal Increased (Recruitment
Becomes linear, soNo combination tonesOr two-tone suppression
Mostly a combination ofOHC and IHC damage
Auditory nerve frequency-threshold curves
log frequency
log
am
pli
tud
e (d
B S
PL
)
100
80
60
40
20
Characteristic Frequency
Normal Threshold
Normal bandwidth
Auditory tuning curves
log frequency
log
am
pli
tud
e (d
B S
PL
) 100
80
60
40
20Characteristic Frequency
Normal Threshold
Abnormal Threshold
Normal bandwidth
Normal bandwidth
Healthy ear
Inner hair-celldamage
Outer-hair cell damage
log frequency
log
am
pli
tud
e (d
B S
PL
) 100
80
60
40
20
Characteristic Frequency
Normal Threshold
Abnormal Threshold
Normal bandwidth
Abnormal bandwidth
BM becomes linear without OHCs(furosemide injection)
Amplification greater and tuningmore selective at low levels
Robles, L. and Ruggero, M. A. (2001). "Mechanics of the mammalian cochlea," Physiological Review 81, 1305-1352.
Normal auditory non-linearities
• Normal loudness growth (follows Weber’s Law)
• Combination tones
• Two-tone suppression
• Oto-acoustic emissions