Download Psychoacoustic and physiological reflections of hearing loss: C315/A6

Psychoacoustic and
physiological reflections of
hearing loss: C315/A6
Andrew Faulkner:
Department of Phonetics & Linguistics
University College London
Where can hearing loss arise?
Conductive hearing loss in the outer ear
• Outer and middle ears
– conductive hearing loss
– a problem in the conduction of sound from air
to the inner ear
• Cochlea plus auditory nerve
– sensori-neural or cochlear hearing loss
• Auditory nerve and beyond
– retro-cochlear hearing loss (beyond the ear)
– not discussed here
Conductive
losses in
the middle
ear
An audiogram completely captures the
attenuation of sound in conductive losses
Eardrum
damage
Otitis
Media
Otosclerosis
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Conductive hearing loss: Summary
• Outer ear (blockages)
– foreign bodies, infections, congenital
malformations
• Middle ear problems
– ruptured eardrums
– otitis media
– otosclerosis
• Equivalent to an attenuation of sound
– helped by surgery and/or amplification
Age related hearing loss
presbyacousis
Sensori-neural hearing loss
• A problem in the inner ear (cochlea),
usually involving the hair cells
– can be caused by
• genetic factors
• mumps or meningitis
• a bang on the head
• age — ‘presbyacusis’
• noise, even ‘leisure noise’
Psychoacoustic concomitants of
sensori-neural (cochlear) hearing loss
• Loss of sensitivity
• Abnormal loudness growth (recruitment)
– Intense sounds appear to have normal loudness
while less intense ones are abnormally quiet or
inaudible.
• Reduction of dynamic range (related to
recruitment)
• Impaired frequency selectivity
threshold →
← dynamic range
→
ULL
-------------------
An auditory area in sensori-neural loss
ULL
-------------------
←—-—→
A normal auditory area
threshold →
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Auditory filtering in normal hearing
Impaired frequency selectivity
Auditory filtering in impaired hearing
35 dB loss at 2 kHz
Impaired frequency selectivity
• Broadened auditory filters effectively smooth
the spectrum of speech leading to …
– a reduced ability to resolve spectral detail
– greater disruption from background noise
Physiological background
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Normal peripheral hearing depends
upon ...
• Inner Hair Cells for sensory transduction of
basilar membrane movements to spikes on the
auditory nerve.
and ...
• Outer Hair Cells which feed energy back into
the basilar membrane amplifying its movement
at relatively low sound levels.
OHC activity is responsible for ...
• Low absolute thresholds.
• Level-dependent amplification of basilar
membrane motion that increases dynamic
range of hearing
• Fine frequency selectivity of the basilar
membrane (place coding).
Consider
the
frequency
response of
a single
place on the
basilar
membrane
What goes wrong in sensori-neural
hearing loss?
Functional loss results from hair cell
damage
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Damage to haircells can cover
extensive areas of
the cochlear
spiral
Outer Hair Cells are relatively
vulnerable to damage, leading to ...
• Decreases in basilar membrane movement and
hence increased thresholds to sound
– hearing loss
• Loss of level-dependent amplification
– reduced dynamic range
– loudness recruitment
Relation of
Hair Cell
loss to
audiogram
Inner Hair Cell (IHC) damage ...
• Leads to a more sparse representation of all
auditory information passed on to higher
auditory centres.
• There are possibly even regions of the cochlea
without any IHCs — so-called dead regions.
• Loss of frequency tuning - widened filters
degraded frequency selectivity
Sensori-neural losses: Summary
Effects
of Organ
of Corti
damage
on
tuning in
the
auditory
nerve
• Three main consequences of cochlear hearing
loss
– Reduced audibility: unable to hear the quieter
parts of speech.
– Loudness recruitment: abnormally rapid growth
of loudness and reduced dynamic range.
– Reduced frequency selectivity: spectral
smearing of speech signal.
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Summary
• All three appear to arise from damage to normal OHC
function.
• Loss of sensitivity and reduced dynamic range can be
helped by amplification and compression in hearing
aids
• Loss of frequency selectivity cannot be corrected
The End
• Caveat
– haven’t discussed effects on temporal processing
abilities …
– but these appear to be relatively robust in cochlear
hearing loss.
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