Download Auditory perception of amplitude and frequency modulations in sounds

A brief course on …
Auditory Perception
of Amplitude and Frequency
Modulations in Sounds
Christian Lorenzi
Laboratoire des Systèmes Perceptifs
UMR CNRS 8248
Dépt d’Etudes Cognitives
Institut d’Etude de la Cognition
Ecole normale supérieure, Paris
PSL Research University
Overall plan of this presentation
• Short introduction
• Assessment of modulation perception in humans
- Characterizing deficits in modulation perception
- Auditory mechanisms of modulation perception
• Role of temporal modulations in sound recognition &
auditory scene analysis
- Effects of cochlear damage on the perception of
speech modulation cues
• Conclusions
A short introduction
to the study of auditory perception
of temporal modulations in sounds …
The ear is a frequency analyzer
Peripheral auditory system
Outer
ear
Middle
ear
Basilar
membrane
Hair
cells
Central auditory system
Auditory
nerve fibers
Brainstem
Auditory
cortex
A steady sound
The ‘internal power spectrum’
The excitation pattern
The (peripheral) auditory system
(the basilar membrane in the
cochlea) decomposes sounds
– such as this steady sound –
into their audio-frequency
components
Cochear filters
Inner ear
Steady vs modulated sounds
Amplitude (linear units)
An example of steady sound
Time (s)
oboe
Steady vs modulated sounds
However, most natural sounds, such as this speech signal /ababa/,
show pronounced amplitude and frequency modulations (AM and FM components)
Envelopes (ERBx1)
3758
/ababa/ - 3 bands, unprocessed
2656
0.8
Center Frequency (Hz)
Amplitude
(linear
Amplitude (linear
units) units)
1
0.6
0.4
0.2
0
-0.2
-0.4
-0.6
-0.8
-1
1858
1281
863
561
343
184
0
0.2
0.4
0.8
0.6
Time (ms)
Time (s)
1
1.2
1.4
0.2
0.4
0.6
0.8
1
1.2
Time(s)
Natural sounds, e.g. speech sounds, show salient modulations
The ear is not only a frequency
analyzer; it is also a demodulator
Amplitude
Speech sounds as modulated sounds
0.4
Amplitude
0.2
0
-0.2
-0.4
-0.6
0
Time
These modulations can be
studied by taking speech sounds,
passing them through a bank a
bandpass (analysis) filters (as the
cochlea does), and extracting the
AM and FM components of each
narrowband signal
AM
The Temporal Envelope (E)
Amplitude
Speech sounds as modulated sounds
0.4
Amplitude
0.2
0
-0.2
-0.4
-0.6
0
Time
FM
The Temporal Fine Structure (TFS)
Speech sounds as modulated sounds
The narrowband signal at the output of a given analysis (cochlear) filter
• Sk(t) = Ek(t) . cos( φk(t) )
The temporal fine structure
(FM component)
~
• Sk(t) : analytic signal of Sk(t)
~
The envelope
(AM component)
E(t) = | Sk(t) |
~
φk (t) = arg(Sk(t))
There are many ways for decomposing a given signal into an AM and a FM carrier. Here,
narrowband signals are modelled as the product of an AM (the envelope) and a FM carrier (the
temporal fine structure). AM is positive and real. The AM and the FM components are obtained by
using the Hilbert transform. However, alternative AM/FM decompositions have been considered.
Gilbert & Lorenzi (JASA, 2006)
Sheft et al. (JASA, 2008)
Speech sounds as modulated sounds
Modulation spectra
Sheft et al. (JASA, 2008)
Syllabic Rate
(3-4 syllables/sec)
FM
These modulations range
AM
between ~2-500 Hz
These modulation spectra are obtained by taking the Fourier transform of AM and FM patterns extracted from minutes of speech
signals. Comparable modulation spectra are obtained in French, English, German, etc.
Speech sounds as modulated sounds
Other the last decades, several groups of researchers have attempted to test the validity of the following 3 assumptions
Syllabic Rate
(3-4 syllables/sec)
FM
These modulations range
AM
between ~2-500 Hz
H1: These modulations carry useful timbre (phonetic) information
Effects of cochlear damage & ageing
Oral
comprehension
Ageing
Cochlear
damage
Speech identification performance
Sheft et al. (Ear & Hearing, 2012)
NH: normal hearing
HI: hearing impaired
H2: Deficits in modulation perception explain
poor speech perception for hearing-impaired (HI) and elderly persons
Rehabilitation
Oral
comprehension
Speech identification performance
A hearing aid
Cochlear implantees (CI)
A cochlear implant
Gnansia et al. (2014)
70% correct for CI
versus
100 % for NH listeners
H3: Poor transmission of modulations explains
poor speech perception despite rehabilitation via Hearing Aids
How do we assess
modulation perception in humans ?
The ‘listener’
The ‘experimenter’
Assessment of modulation perception
in humans: sine AM and sine FM
Unmodulated sine-tone carrier
AM tone
1
2
0
-0.5
-1
0
-1
0
0.5
1
Time (s)
1.5
-2
2
0
0.5
4
x 10
1
0.5
0.5
0
1
Time (s)
1.5
2
4
x 10
FM tone
1
Amplitude
Amplitude
Unmodulated sine-tone carrier
The modulation detection
threshold can be tracked
by varying systematically
modulation depth.
0
-0.5
-0.5
-1
In a typical modulationdetection
task,
each
listener is presented with
successive
trials,
corresponding
to
two
successive sounds. On
each trial, the two sounds
are presented in random
order: an unmodulated
carrier and a modulated
carrier. The listener is then
asked to indicate which
sound is modulated.
1
Amplitude
Amplitude
0.5
0
0.5
1
Time (s)
1.5
2
4
x 10
-1
0
0.5
1
Time (s)
1.5
2
4
x 10
Assessment of modulation perception
in humans: sine AM and sine FM
50% depth
1.5
AM tone
1
1
0.5
Amplitude
Unmodulated sine-tone carrier
2
0
-0.5
-1
1
Amplitude
Amplitude
0.5
0
-1.5
0
0.5
1
Time (s)
1.5
1
Time (s)
1.5
x 10
1.5
0
2
4
25% depth
1
-0.5
-1
Amplitude
0.5
-1
0
0.5
1
Time (s)
1.5
-2
2
0
-0.5
-1
0
0.5
4
x 10
1
Time (s)
1.5
2
-1.5
4
x 10
0
0.5
12% depth
1.5
Unmodulated sine-tone carrier
FM tone
1
Amplitude
0.5
1
1
2
4
x 10
0
-0.5
-1
0.5
0.5
Amplitude
Amplitude
-1.5
0
-0.5
-0.5
-1
0
0
0.5
1
Time (s)
1.5
2
4
x 10
-1
0
0.5
1
Time (s)
1.5
0
0.5
1
Time (s)
1.5
…
2
4
x 10
Track
modulation depth
at threshold
for a given modulation rate, fm
2
4
x 10
Assessment of modulation perception
in humans: Sensitivity to AM
These ‘temporal
modulation transfer
functions’ show AM
detection thresholds as a
function of AM rate, for a
given carrier
Modulation rate, fm (Hz)
Modulation depth, m (%)
Modulation depth, m (%)
Lorenzi et al. (2001a,b)
fm (Hz)
Modulation
rate, fm (Hz)
Assessment of modulation perception
in humans: Sensitivity to AM
These ‘temporal modulation
transfer functions’ are lowpass
in shape; sensitivity drops
(degrades) when AM rate is
greater than about 50-100 Hz
Modulation rate, fm (Hz)
Modulation depth, m (%)
Modulation depth, m (%)
Lorenzi et al. (2001a,b)
fm (Hz)
Modulation
rate, fm (Hz)
The auditory system operates as a lowpass filter for AM,
smearing out fast (>50-100 Hz) AM fluctuations
Assessment of modulation perception
in humans: Sensitivity to FM
Frequency excursion (Hz)
fm=2 Hz
fm=20 Hz
The auditory system is also
‘sluggish’ for FM detection
Carrier frequency, fc=500 Hz
Number of cycles of modulation
Stimulus duration
Wallaert et al.
Characterizing deficits in modulation sensitivity
Peripheral auditory system
Outer
ear
Middle
ear
Basilar
membrane
Hair
cells
Central auditory system
Auditory
nerve fibers
Brainstem
Auditory
cortex
Inner ear
Cochlear Damage
Ageing effects
The auditory system can be damaged:
• peripherally (lesions of inner and outer hair cells, damage to the auditory nerve)
• centrally (brain lesions, ageing effects)
Peripheral damage causes sensorineural hearing loss (SNHL). Central damage cause central auditory processing disorders.
However, the effects of cochlear lesions are often associated with ageing effects, as in the case of presbycusis, a common form of
sensorineural hearing loss observed in elderly people.
It is important to separate the effects of cochlear damage from ageing effects on the ability to perform a given auditory task.
Characterizing deficits in modulation sensitivity
Cochlear damage is often associated with reduced
audibility – as shown by the pure-tone audiogram
(below).
‘Suprathreshold’ Auditory Deficits
Léger et al. (Hear Res, 2012)
However, cochlear damage is also associated with
suprathresholds auditory deficits, that is deficits in the
ability to discriminate audible sounds. As an example,
speech comprehension can be impaired even in regions
of near-normal hearing (below).
Abnormal sensitivity to AM and FM:
Two suprathreshold auditory deficits
caused by cochlear damage
and/or ageing
Characterizing deficits in AM sensitivity
Individual and mean data from
NHy (yound normal-hearing
listeners), NHe (elderly normalhearing listeners), HIy (yound
hearing-impaired listeners), HIe
(elderly
hearing-impaired
listeners)
fm=5 Hz
Modulation depth (dB)
Ageing effect
Ageing degrades
AM sensitivity
Hearing loss
preserves
AM sensitivity
SNHL effect
Carrier frequency (Hz)
Wallaert et al. (ARO, 2015)
Characterizing deficits in FM sensitivity
Frequency excursion (Hz)
fm=5 Hz
Ageing effect
Ageing
preserves
FM sensitivity
Hearing loss
degrades
FM sensitivity
SNHL effect
Carrier frequency (Hz)
Wallaert et al. (ARO, 2015)
Characterizing deficits in modulation perception
In summary:
Cochlear damage & ageing affect AM and FM sensitivity differently :
- Cochlear lesions degrade FM sensitivity while sparing AM sensitivity
- Ageing degrades AM sensitivity while sparing FM sensitivity
Distinct auditory mechanisms for AM and FM processing ?
Auditory Periphery
AM FM
Auditory Centers
Separate processing paths ?
AM (Envelope)
Cochear filters
FM (Temporal fine structure)
Auditory mechanisms
of modulation perception
Auditory Periphery
Auditory Centers
AM FM
Cochear filters
?
Are AM and FM cues processed
by totally distinct auditory
mechanisms, or do they share a
common code?
Auditory mechanisms of AM perception
Before exposure
After exposure
AM detection thresholds were
measured in young normalhearing
listeners,
at
3
modulation rates. Measures
were conducted before and after
exposure to a 15-min 16-Hz AM
tone (with the same carrier).
Adaptation effects
Bruckert et al. (JASA, 2006)
Auditory mechanisms of AM perception
Before exposure
After exposure
AM detection thresholds are
selectivity degraded (increased) at
the exposure AM rate (16 Hz).
This adaptation effect is often
interpreted as evidence for the
existence of neural units tuned to
specific AM rates.
Adaptation effects
Bruckert et al. (JASA, 2006)
Auditory mechanisms of AM perception
Millman et al. (JASA, 2002)
AM detection thresholds
were measured in young
normal-hearing listeners,
Measures
were
conducted
in
the
presence or in the
absence of a secondary
(masking) AM applied to
the the same carrier as
the target AM.
The rates of target and
masker
AMs
were
systematically varied.
Masking effects
Auditory mechanisms of AM perception
Millman et al. (JASA, 2002)
Detection thresholds for
the target AM were found
to increase (degrade) in
the presence of the
secondary (masking) AM.
The modulation masking
effect was greater when
the rates of target and
masker AMs were close
to each other.
BMF ∈ [2-100 Hz]
BW
1 oct.
Masking effects
The peaked (bandpass)
aspect of modulation
masking
patterns
is
interpreted as evidence
for the existence of tuned
modulation channels in
the auditory system
Auditory mechanisms of AM perception
Target modulation
detection threshold
Peaked
modulationmasking patterns can be
found for AM rates
between about 2 and 100
Hz, suggesting that AM
channels
are
tuned
between 2-100 Hz. These
channels are believed to
be broadly tuned (Q=1)
Target
modulation rate
fixed to 100 Hz
In the presence
of a modulation
masker
Modulation
masking
Masking effects
No modulation
masker
Masker
modulation rate
Lorenzi et al. (JSLHR, 1997)
Auditory mechanisms of AM perception
0
Magnitude (dB)
Carrier effects on AM
detection thresholds can
be predicted by the
characteristics of the
modulation spectrum of
the
carrier.
The
modulation spectrum of a
narrowband
noise
is
triangular (see insert).
This explains why AM
detection thresholds are
selectivity degraded for
low AM rates only when
the
carrier
is
a
narrowband
noise
(instead of a sine tone).
-20
-40
- 60
0
2
4
6
8 10
Modulation rate of inherent fluctuations (Hz)
Masking effects
Lorenzi et al. (JASA, 2001)
Auditory mechanisms of AM perception
Cortical neurons are tuned to
a given ‘best modulation rate’
Two recording sites (PAC)
Primary Auditory Cortex
Neurophysiological evidence supporting the notion of
central AM channels was found in humans.
SEEG and fMRI studies have found that cortical units are
tuned to best modulation rates, below 100 Hz.
The (central) auditory system
is selectively tuned for AM
Giraud et al. (J Neurophysiol, 2000);
Liégeois-Chauvel et al. (Cereb Cortex, 2004)
Auditory mechanisms of AM perception
Cortical neurons are tuned to
a given ‘best modulation rate’
Two recording sites (PAC)
Primary Auditory Cortex
A SEEG study was conducted on epileptic patients wearing
intracranial electrodes in primary and secondary auditory areas. AM
stimuli with varying AM rates were presented in free field to these
patients. Patients listened to them passively. Auditory evoked
responses (local field potentials) were measured on each electrode in
response to these AM sounds, and analyzed to build neural ‘temporal
modulation transfer functions’. These functions were found to be
tuned. These SEEG data are consistent with f-MRI data obtained in
another study.
The (central) auditory system
is selectively tuned for AM
Giraud et al. (J Neurophysiol, 2000);
Liégeois-Chauvel et al. (Cereb Cortex, 2004)
Frequency excursion (Hz)
Auditory mechanisms of FM perception
fm=2 Hz
fm=20 Hz
Carrier frequency
fc=500 Hz
Number of cycles of modulation
FM processing was investigated by measuring FM detection thresholds in young normalhearing listeners. FM detection thresholds were measured in the absence or in the presence
of a superimposed (masking) AM at the same rate as FM.
Wallaert et al.
Frequency excursion (Hz)
Auditory mechanisms of FM perception
fm=2 Hz
fm=20 Hz
Carrier frequency
fc=500 Hz
Number of cycles of modulation
AM interferes with FM processing
→ FM is encoded as (converted into) AM (envelope cues)
Wallaert et al.
AM
time
Auditory mechanisms of FM perception
Cochlear filter
Amplitude
FM can be converted into
AM thanks to cochlear
filtering.
The
differential
attenuation of cochlear filters
transforms
frequency
excursions into changes in
excitation at the output of the
cochlear filter.
∆f
FM
frequency
FM is encoded as (converted into) AM (envelope cues)
This suggests a common code for AM and FM. In other words, FM may not be encoded as such in the auditory system.
Auditory mechanisms of FM perception
The temporal fine structure (TFS, < ~1-2kHz) is encoded via
neural phase locking in auditory-nerve fibers
0.03
0.02
0.01
0
-0.01
-0.02
-0.03
2.2
2.25
2.3
2.35
2.4
2.45
2.5
∆t
2.55
2.6
2.65
4
x 10
spike trains in auditory-nerve fibers
However, auditory neuroscientists have demonstrated the existence of another mechanism able to encode FM accurately,
for carrier frequencies below about 1-2 kHz, and FM rates below about 10 Hz: neural phase locking.
Auditory mechanisms of FM perception
The temporal fine structure (TFS, < ~1-2kHz) is encoded via
neural phase locking in auditory-nerve fibers
0.03
0.02
0.01
0
-0.01
-0.02
-0.03
2.2
2.25
2.3
2.35
2.4
2.45
2.5
∆t
2.55
2.6
2.65
4
x 10
spike trains in auditory-nerve fibers
How can we demonstrate that FM is also encoded via neural phase locking ?
Auditory sensitivity to changes in TFS interaural phase is constrained by
neural phase locking in auditory nerve fibers
Track IPD
at threshold
Auditory mechanisms of FM perception
The temporal fine structure (TFS, < ~1-2kHz) is encoded via
neural phase locking in auditory-nerve fibers
0.03
0.02
0.01
0
-0.01
-0.02
-0.03
2.2
2.25
2.3
2.35
2.4
2.45
2.5
∆t
2.55
2.6
2.65
4
x 10
spike trains in auditory-nerve fibers
Changes in TFS interaural phase elicit a ‘spatial percept’ (changes in perceived sound laterality)
Auditory sensitivity to changes in TFS interaural phase is constrained by
neural phase locking in auditory nerve fibers
Track IPD
at threshold
Auditory mechanisms of FM perception
The temporal fine structure (TFS) is encoded via
neural phase locking in auditory-nerve fibers (iff < ~1-2kHz)
0.03
0.02
0.01
0
-0.01
-0.02
-0.03
2.2
2.25
2.3
2.35
2.4
2.45
2.5
∆t
2.55
2.6
2.65
4
x 10
spike trains in auditory-nerve fibers
Changes in TFS interaural phase elicit ‘spatial percepts’ (changes in perceived sound laterality)
Auditory sensitivity to changes in TFS interaural phase is constrained by
neural phase locking in auditory nerve fibers
Track IPD
at threshold
A change in interaural phase
Auditory mechanisms of FM perception
Auditory sensitivity to AM, FM, and IPD was measured in the same young, normal-hearing listeners
FM vs IPD
AM vs IPD
20
5
r=0.6; p<.01
ns
AM detection threshold (%)
16
16
4
14
14
3.5
12
12
3
10
10
2.5
2
FM sentivity is
the only task
correlated to
IPD thresholds
1.5
ns
18
AM detection threshold (%)
18
4.5
FM detection threshold (Hz)
AM vs FM
20
8
6
4
8
6
4
2
2
1
0
0.2
0.4
0.6
0.8
1
Interaural Phase Difference
IPD (Radians)
0
0.2
0.4
0.6
0.8
1
Interaural Phase Difference
IPD (Radians)
0
1
2
3
4
5
FM detection threshold (Hz)
FM detection is constrained by the phase locking properties
of auditory neurons
Paraouty et al. (ARO, 2015)
Auditory mechanisms of modulation
perception
Proposed architecture for AM and FM processing in the early and central auditory system
Auditory Periphery
Auditory Centers
AM FM
Features
Ek(t)
Cochear filters
TFSk(t)
Phase
Locking
Features
- Pitch
- Timbre
In conclusion:
Partially distinct auditory mechanisms
for AM and FM perception
Shamma & Lorenzi (JASA, 2013)
Role of temporal modulations in sound
recognition & auditory scene analysis
What are the respective roles of AM and FM cues in sound recognition and sound-source separation ?
AM speech
0.5
AM+FM speech
0
-0.5
0
0.1
0.2
0.3
Time (s)
0.4
0.5
FM speech
Vocoders are used to degrade selectively
AM and/or FM components in speech signals
Effects of degrading AM and FM cues
on speech recognition in quiet
AM speech
0.5
0
-0.5
0
0.1
0.2
0.3
Time (s)
0.4
0.5
AM+FM speech
Effects of degrading AM and FM cues
on speech recognition in quiet
Normal-hearing listeners
AM speech
Perfect recognition for
syllables or sentences
Limited training
required
FM speech
Intensive training
often required
Poor recognition for
sentences
AM cues are the most salient cues for speech recognition in quiet
Gilbert et al. (JASA, 2006, 2007); Sheft et al. (JASA, 2008); Ardoint et al. (Hear Res, IJA, 2010)
Lorenzi et al. (PNAS, 2006)
Effects of degrading AM and FM cues
on speech recognition in quiet
It follows that speech processors of
hearing aids and cochlear implants
should
not
restrict
unduly
the
transmission of AM cues relevant to
human hearing
Cochlear implant (CI)
Speech processor
AM
Pulse
Generator
Mapping
Micro
AGC
Pulse
Generator
AM
… as illustrated here by the detrimental
efffects of amplitude compression on
the transmission of AM cues
Mapping
Effects of varying
Compression Ratio (CR)
Won et al. (JARO, 2014)
Electrode outputs
Effects of degrading AM and FM cues
on speech recognition in quiet
This is consistent with studies showing a
significant correlation between AM detection
thresholds and speech recognition in quiet
(measured in free field) in CI patients
Cochlear implantees (CI)
r~0.5; p<.05
Good transmission/reception of AM cues
is often associated with good speech recognition in quiet
Gnansia et al. (IJA, 2014)
Effects of degrading AM and FM cues
on speech recognition in quiet
Cochlear implantees (CI)
r~0.5; p<.05
Good transmission/reception of AM cues
is often associated with good speech recognition in quiet
Gnansia et al. (IJA, 2014)
Effects of degrading AM and FM cues
on speech recognition in quiet
Cochlear implantees (CI)
r~0.5; p<.05
Good transmission/reception of AM cues
is often associated with good speech recognition in quiet
Gnansia et al. (IJA, 2014)
Effects of degrading FM cues
on speech recognition in masking noise
Speech recognition is degraded when speech is presented against a background noise masker.
The masking effect is dependent on the AM content of the background noise masker.
NH: normal hearing HI: hearing impaired
Benefit from noise fluctuations
for NH listeners
Limited benefit
for HI listeners
0.5
0.5
0.5
0
0
0
-0.5
0
0.1
0.2
0.3
0.4
Time (s)
Lorenzi et al. (IJA, 2006)
Clean speech
0.5
-0.5
0
0.1
0.2
0.3
0.4
0.5
Time (s)
+ a notionally
‘steady’ noise masker
-0.5
0
0.1
0.2
0.3
0.4
0.5
Time (s)
+ an AM
noise masker
Gnansia et al. (Hear Res, 2008; JASA, 2009)
Effects of degrading FM cues
on speech recognition in masking noise
Speech recognition is substantially improved when a slow AM is superimposed to the noise masker.
This effects is called ‘speech masking release’.
NH: normal hearing HI: hearing impaired
Benefit from noise fluctuations
for NH listeners
Limited benefit
for HI listeners
0.5
0.5
0.5
0
0
0
-0.5
0
0.1
0.2
0.3
0.4
Time (s)
Lorenzi et al. (IJA, 2006)
Clean speech
0.5
-0.5
0
0.1
0.2
0.3
0.4
0.5
Time (s)
+ a notionally
‘steady’ noise masker
-0.5
0
0.1
0.2
0.3
0.4
0.5
Time (s)
+ an AM
noise masker
Gnansia et al. (Hear Res, 2008; JASA, 2009)
Effects of degrading FM cues
on speech recognition in masking noise
0.5
AM speech
0
-0.5
0
0.1
0.2
0.3
0.4
0.5
Time (s)
AM+FM speech
0.5
0
-0.5
0
0.1
0.2
0.3
0.4
0.5
Time (s)
The speech masking release effect can be studied by passing the speech+noise mixtures [i.e., the (notionally) steady or
AM noise added to speech] though a vocoder degrading selectively FM cues while preserving AM cues.
Effects of degrading FM cues
on speech recognition in masking noise
Speech masking release
is
measured
for
unprocessed
and
processed
(vocoded)
speech+noise mixtures.
Maximum release is
found for a 100-% AM
depth.
Poorer masking release
is found when degrading
FM cues.
40
35
Masking release (%)
Speech masking release
is measured for young
normal-hearing listeners,
as a function of masker
AM depth.
30
Unprocessed - fm = 8Hz
Processed - fm = 8Hz
25
20
15
10
5
Performance
0
-5
12.5
25
50 59.5 70.7
84.1
100
Modulation depth (%)
FM cues help separating speech from noise
Degrading FM cues simulates the limited benefit
from noise fluctuations for HI listeners
Gnansia et al. (Hear Res, 2008; JASA, 2009)
Effects of degrading FM cues
on speech recognition in masking noise
Gnansia et al. (2014)
Speech identification performance
Cochlear implantees (CI)
This is consistent with CI data (knowing that CI processors do not transmit FM cues)
Degrading FM cues simulates the limited benefit
from noise fluctuations for CI listeners
Effects of cochlear damage and ageing
on speech recognition in masking noise
Complex FM patterns
This is consistent with studies showing that FM detection and discrimination
is significantly correlated with speech recognition scores in noise for hearingimpaired listeners
r~0.5 ; p<.05
Deficits in FM sensitivity explain - partly –
poor speech recognition in noise
Sheft et al. (Ear & Hearing, 2012)
Conclusions
• The auditory system is able to extract temporal
modulations in complex sounds such as speech
• AM (‘envelope’) cues convey useful timbre
(phonetic) information
• FM (‘temporal fine structure’) cues convey useful
segregation cues
• AM and FM cues are processed by partially
independent auditory mechanisms
Conclusions
• Cochlear damage and ageing affect AM and FM
processing differently
• Cochlear damage degrades FM processing but
preserves AM processing; Ageing alters AM processing
• Deficits in FM perception explain – at least partially –
the poorer-than-normal speech perception in noise
typically associated with cochlear damage
• Hearing Aids and Cochlear implants should not restrict
the transmission of AM and FM cues relevant for
human hearing