Download The physiology of hearing impairment 1

The physiology of hearing impairment 1
Young ED (2011) Neural coding of sound with cochlear damage. In C.G. Le Prell, D. Henderson, R.R. Fay,
and A.N. Popper (Eds.) Noise Induced Hearing Loss: Scientific Advances. New York, Springer. (in press
2011). Moore BCJ (2007) Cochlear Hearing Loss. John Wiley and Sons.
Liberman MC (1984) Single-neuron labeling and chronic cochlear pathology. I. Threshold shift and
characteristic-frequency shift. Hearing Res 16:33-41.
Liberman MC, Dodds LW (1984) Single-neuron labeling and chronic cochlear pathology. III. Stereocilia
damage and alterations of threshold tuning curves. Hearing Res 16:55-74.
Liberman MC, Kiang NY (1984) Single-neuron labeling and chronic cochlear pathology. IV. Stereocilia
damage and alterations in rate- and phase-level functions. Hearing Res 16:75-90.
Liberman MC, Dodds LW (1984) Single-neuron labeling and chronic cochlear pathology. II. Stereocilia
damage and alterations of spontaneous discharge rates. Hearing Res 16:43-53.
Moore BCJ (2004) Testing the concept of softness imperception: Loudness near threshold for hearingimpaired ears. J Acoust Soc Am 115:3103-3111.
Moore BC et al. A test for the diagnosis of dead regions in the cochlea. Br J Audiol. 34:205 (2000).
Hearing impairment can
affect each of the steps
in auditory transduction
Conductive - damage (eardrum puncture), pathological processes (bone growth), or blockage of
external and middle ear (not shown above).
Metabolic - damage or degeneration of the stria vascularis (SV) or other components of cochlear
homeostasis, indirectly affecting hair-cell transduction
Sensorineural - damage to, degeneration of, or decreased gain of hair cells (IHC, OHC),
degeneration of auditory nerve fibers (SGN, AN), or central auditory neurons and circuits.
These lecture focuses on sensorineural hearing impairment (SNHL) and the
functional consequences of damage to hair cells or auditory nerve fibers. SNHL has four well-studied causes:
Acoustic trauma - exposure to very loud sounds for a long enough time to damage hair cells
or auditory-nerve fibers.
Ototoxic substances - some antibiotics (gentamycin), cancer drugs (cisplatin), and various
other toxins specifically damage hair cells or other components of the cochlea.
Genetic defects - any genetic defect that affects an essential functional element of cochlear
physiology can cause deafness (e.g. Usher’s syndrome, caused by a defect in a myosin or in one
of four other genes).
Aging - which appears to operate indirectly via degeneration of the stria vascularis, leading
to a reduction in the endolymphatic potential (EP) and a decrease in gain of hair cells.
The perceptual deficits of SNHL:
Loss of audibility
Loudness recruitment
Degraded frequency tuning
Auditory neuropathy, degraded temporal precision
Tinnitus
To illustrate two of the primary behavioral effects of hearing impairment, look at equalloudness contours, along which tones of different frequencies sound equally loud.
Equal-loudness
contours
Braida et al.
Two problems: Threshold shift and recruitment.
2. Loudness
growth is faster
than normal
(recruitment)
1. Soft
sounds can't
be heard
Braida et al.
The largest problem in hearing impairment is loss of audibility, sounds cannot
be heard. The extent to which audibility alone can account for the degraded
performance of hearing impaired listeners is evaluated with the articulation
index. f1
AI = P ∫ I( f ) W ( f ) df
f0
P - proficiency (fudge factor)
I(f) - relative importance of
different frequencies for
understanding speech, empirically
determined in normal listeners
W(f) - fraction of the speech that is
audible above noise masking or
elevated threshold (roughly the red
region at right) AI analysis shows that, with parameters P and I(f) set for normal listeners, the AI
accurately predicts loss of performance in mild to moderately impaired listeners
(<50 dB), but not in moderate-to-severe group (>55 dB). Apparently there are
additional problems in such subjects.
The difference is
usually worse in
older persons.
normal
thr. shift < 50 dB
Note
The task was recognition of
spoken word lists with 9
different degrees of LP, HP,
and BP filtering.
thr. shift > 55 dB
Pavlovic, 1984
The second problem, recruitment: loudness grows more rapidly than normal over
some range, so that sounds of ≈80-100 dB are equally loud in a normal and an
impaired listener.
Note the narrowing of the
dynamic range from the
green to the red arrow.
The behavior of impaired loudness
near threshold is controversial. Some
(Buus and Florentine, 2001) claim
that loudness is elevated right at
threshold, but the preponderance of
evidence indicates that this is not so
and that loudness growth is more like
the blue curve (Moore, 2004).
Recruitment is important in hearing-aid design, because the reduced dynamic range requires
compressor circuits in aids. These circuits are problematic in that they are nonlinear and distort
the sound.
Buus and Florentine, 2001
The third problem: psychophysical auditory filters
are wider in hearing-impaired ears. This means the
ability to analyze sound by its frequency content is
impaired.
Note broadening
(same arrow)
Glasberg and Moore, 1986
The implications of
broadened auditory filters:
A vowel spectrum
The same spectrum on the cochlea’s
frequency scale (in units of auditory
filter bandwidths)
. . . the same with
moderate hearing
impairment
(auditory filters 2x
and 4x wider) The theoretical excitation pattern on
the basilar membrane for this
stimulus in a normal ear (the
spectrum smoothed by auditory
filters)
Note the loss of spectral detail, e.g.
the locations of the formant peaks,
with decreased cochlear filtering.
This is one likely explanation of
below-AI performance in speech
perception and suggests problems
with background noise. Number of ERBs, E!
Moore, 1995
The fourth problem: auditory
neuropathy in which the thresholds
are normal or show mild
impairment, but subjects have
considerable difficulty with
complex perception, like speech.
Thresholds of a person at two ages
(19 and 32) show mild impairment
(<60 dB) in both ears.
However, the speech perception
performance (% of words on an
open set test) is much worse than
expected from the hearing loss
(green and orange symbols).
The other data are from a
population of similar subjects.
Starr et al. 1996
Auditory neuropathy subjects show little or no ABR, meaning a lack of
synchronous activity in the CNS. However, their DPOAEs (distortion product
otoacoustic emissions) are normal, showing no deficit in outer hair cell function.
Note inversion of
waveform, this is CM
OAE
noise
Starr et al. 1996
The physiological correlates of perceptual problems in hearing impairment.
Inner and outer hair cells: outer hair cells amplify BM displacement; inner hair
cells are the transducers for ANFs
C. Smith
Hair cell damage has been studied
experimentally with acoustic trauma.
These data show auditory nerve tuning
and thresholds after such trauma
(temporary threshold shift has been
allowed to resolve).
Note the 1. threshold shifts and
2. broadening of tuning
when compared to normal tuning
curves (gray).
Substantial
trauma was
observed
here in the
absence of
much hair
cell loss.
(The exposure was a 50 Hz noise
band centered at 2 kHz, 115 dB for
2 hr, 54 day recovery)
Liberman and Dodds, 1984
Another example of hair cell loss from
acoustic trauma.
In this case there is a dead region marked
by the red band. At these frequencies, no
auditory nerve fibers were found and
there was substantial hair cell damage.
normal
threshold
Dead regions have been described in psychophysical
tests. Sophisticated masking methods can detect such
regions (see Moore, 2007).
Importantly, amplifying sounds at frequencies in a
dead region does not help speech perception, even
though it should increase audibility (from the AI).
The reason is that sounds in a dead region are being
heard through the spread of tuning curves from
adjacent regions with existing IHCs and ANFs.
!
No auditory nerve fibers were
recorded in the region where IHCs
were mostly missing.
Liberman and Mulroy, 1982
What about the regions where hair
cells remain, but substantial
threshold shifts are still observed
(the blue rectangle)?
Liberman and Mulroy, 1982
A view of normal hair
cells from above,
showing the orderly
rows of cilia.
Kandel et al., 2000
Pickles, 1988
Cilia in the normal guinea pig cochlea,
showing tip lengths (arrows) between
the tips of shorter cilia and the sides of
their longer neighbors.
Pickles 1988
But intact hair cells may have damaged cilia after acoustic trauma
Inner hair cell after acoustic trauma;
damage is most severe to tallest cilia.
Arrows show intact tip links, arrowheads
show broken ones
Outer hair cell after acoustic trauma; cilia are
fused and missing tip links.
Pickles, 1988
Normal
View used to
evaluate cilia in
Liberman’s
experiments
Acoustic trauma
Liberman and Dodds, 1984
Generally there was a good correlation between threshold shift and ciliary damage, in ears
with surviving hair cells but substantial threshold shifts.
Liberman and Mulroy, 1982
Acoustic trauma produces a complex
and variable mixture of IHC and
OHC damage.
Ototoxic antibiotics (kanamycin)
produce a simpler lesion in which
regions with only OHC damage can
be found.
Unit 50 was localized using HRP
injection to the middle of such a
region (dashed line). This example
shows the effects of a severe outer
hair cell lesion, with little inner hair
cell damage, including minimal cilia
damage.
Liberman and Dodds, 1984
Summary of the effects of different types of hair-cell damage
IHC damage produces
threshold shift with no
loss of tuning.
Liberman and Dodds, 1984
OHC damage produces
threshold shift and broadened
tuning. Auditory nerve fibers are apparently driven by two opposite-phase processes, called
C1 and C2; examples are shown below for two fibers from normal animals.
C1
C2
C1
C2
C2 tuning
C1 tuning
C1
C2
Apparently
no C2 here
Liberman and Kiang, 1984
Acoustic trauma seems to specifically affect C1 without modifying C2, shown below
in two fibers from traumatized ears.
reduced normal
C1
C2
reduced normal
C1
C2
C2
only
Liberman and Kiang, 1984
Hypersensitive tails in W-shaped tuning curves occur when there is no sign of a C1C2 transition (no phase shift), presumably in the absence of C1.
Liberman and Kiang correlated hypersensitive tails with cochlear regions where the
IHCs are missing the tall row of stereocilia. Maybe C1 is the IHC driven through
its tall stereocilia by sharply-tuned (due to the OHC) BM motion and C2 is the IHC
driven through its short stereocilia. hypersensitive
tail
Normal fiber, same BF
Liberman MC, Kiang, NY-S. Single-neuron labeling and chronic cochlear pathology. IV.
Stereocilia damage and alterations in rate and phase-level functions. Hearing Res. 16:75-90
(1984).
Fiber in traumatized ear
No C1-C2 transition
Liberman and Kiang, 1984