Download Perception of amplitude modulation by hearing

Perception of amplitude modulation by hearing-impaired
listeners: The audibility of component modulation and detection
of phase change in three-component modulators
Aleksander Seka兲
Institute of Acoustics, Adam Mickiewicz University, 85 Umultowska, 61-614 Poznan, Poland
Brian C. J. Mooreb兲
Department of Experimental Psychology, University of Cambridge, Downing Street,
Cambridge CB2 3EB, England
共Received 4 August 2005; revised 24 October 2005; accepted 25 October 2005兲
Two experiments were conducted to assess whether hearing-impaired listeners have a reduced
ability to process suprathreshold complex patterns of modulation applied to a 4-kHz sinusoidal
carrier. Experiment 1 examined the ability to “hear out” the modulation frequency of the central
component of a three-component modulator, using the method described by Sek and Moore 关J.
Acoust. Soc. Am. 113, 2801–2811 共2003兲兴. Scores were around 70–80% correct when the
components in the three-component modulator were widely spaced and when the frequencies of the
target and comparison different sufficiently, but decreased when the components in the modulator
were closely spaced. Experiment 2 examined the ability to hear a change in the relative phase of the
components in a three-component modulator with harmonically spaced components. The frequency
of the central component, f c, was either 50 or 100 Hz. Scores were about 70% correct when the
component spacing was 艋0.5f c, but decreased markedly for greater spacings. Performance was only
slightly impaired by randomizing the overall modulation depth from one stimulus to the next. For
both experiments, performance was only slightly worse than for normally hearing listeners,
indicating that cochlear hearing loss does not markedly affect the ability to process suprathreshold
complex patterns of modulation. © 2006 Acoustical Society of America.
关DOI: 10.1121/1.2139631兴
PACS number共s兲: 43.66.Mk, 43.66.Nm, 43.66.Sr 关AJO兴
I. INTRODUCTION
The perception of amplitude modulation 共AM兲 plays an
important role in many aspects of auditory perception, including speech perception 共Plomp, 1983; Shannon et al.
1995兲 and the ability to analyze mixtures of sounds arising
from different sources 共Darwin and Carlyon, 1995兲. Hearingimpaired people often have difficulty in understanding
speech in the presence of background sounds 共Plomp, 1978,
1994; Moore, 1998兲 and it is possible that part of this difficulty stems from abnormalities in the processing of amplitude modulation. This possibility is assessed in the present
experiments.
Previous studies of AM perception by hearing-impaired
people have mostly focused on the detection of AM 共Formby,
1982;
Bacon
and
Gleitman,
1992;
Moore
et al., 1992; Moore and Glasberg, 2001兲 or the discrimination of AM rate 共Formby, 1986; Grant, 1998兲. Overall, the
results from these studies suggest that performance is not
usually adversely affected by cochlear hearing loss, except
when a portion of the carrier is inaudible. Here, we focus on
the perception of complex patterns of modulation applied to
a兲
Electronic mail: [email protected]
Author to whom correspondence should be addressed. Electronic mail:
[email protected]
b兲
J. Acoust. Soc. Am. 119 共1兲, January 2006
Pages: 507–514
a single carrier for modulation depths well above threshold.
We start with a review of previous related studies.
Moore et al. 共1996兲 tested listeners with unilateral hearing loss. They were presented with an amplitude-modulated
sinusoid to one ear, and were asked to adjust the AM depth
of a sinusoid presented alternately to the other ear, so as to
match the perceived modulation depth. The results indicated
that the perceived amount of modulation in the impaired ear
was “magnified” compared to that in the normal ear, and it
was argued that this was a manifestation of loudness recruitment 共Steinberg and Gardner, 1937兲, probably resulting from
the loss of fast-acting compression on the basilar membrane
共Rhode and Robles, 1974, Moore and Glasberg, 1997, 2004兲.
Tandetnik et al. 共2001兲 measured second-order modulation detection thresholds in normally hearing and hearingimpaired listeners. Second-order modulation is modulation
applied to the modulation depth of an AM signal, i.e., modulation of the modulator. The second-order modulation thresholds were measured as a function of the frequency of the
modulation applied to the modulation depth 共referred to as
fm
⬘ 兲, using values of f m⬘ from 1 to 11 Hz, and a fixed firstorder modulation frequency of 16 Hz. The audio-frequency
carrier was broadband white noise. The thresholds for the
hearing-impaired listeners were within the normal range for
fm
⬘ = 3 , 5, and 11 Hz, and were higher 共poorer兲 than normal
for f m
⬘ = 1 and 7 Hz. These results suggest that cochlear hearing loss may have some deleterious effect on the ability to
0001-4966/2006/119共1兲/507/8/$22.50
© 2006 Acoustical Society of America
507
process complex temporal envelopes, but the effect is not
large. In a later study using a 2-kHz sinusoidal carrier, detection thresholds for second-order AM were found to be
essentially the same for normally hearing and hearingimpaired listeners 共Füllgrabe et al., 2003兲.
Turner et al. 共1995兲 used “noise-vocoder” processing to
compare the ability of hearing-impaired and normally hearing listeners to identify speech on the basis of temporal envelope cues in a few spectral regions. They found that performance did not differ significantly for the two groups.
However, recent unpublished data obtained in our laboratory
indicate that hearing-impaired listeners often perform more
poorly than normally hearing listeners when trying to understand “noise-vocoded” speech when background sounds are
present.
Although most of the studies described above suggest
that the ability of hearing-impaired listeners to process amplitude modulation is nearly normal, there is at least one
study suggesting that this may not be the case. Lorenzi
et al. 共1997兲 measured the ability to detect 100-Hz sinusoidal
AM applied to a white noise carrier, as a function of the
frequency of a masking sinusoidal AM applied to the same
noise carrier. For listeners with normal hearing, the modulation masking patterns had a broad bandpass characteristic
with a peak at 100 Hz. One listener with a moderate hearing
loss also showed a broadly tuned pattern with a peak at
100 Hz. However, the two other listeners with moderate or
mild-to-severe hearing loss showed broader patterns with
more of a lowpass characteristic. These results suggest that
frequency selectivity in the modulation domain can be reduced in listeners with sensorineural hearing loss.
In the present experiments, we examine the ability of
hearing-impaired listeners to process complex patterns of suprathreshold modulation imposed on a single sinusoidal carrier, i.e., we examine within-channel modulation processing.
The two tasks used 共Sek and Moore, 2003兲 were originally
designed to test the concept that the envelopes of the outputs
of the auditory filters are fed to a second array of overlapping
bandpass filters tuned to different envelope modulation rates.
This set of filters is usually called a “modulation filter bank”
共MFB兲 共Dau et al., 1997a, 1997b兲. The main purpose of the
present experiments was not to test the concept of the MFB,
but rather to use the two tasks as measures of the ability of
hearing-impaired listeners to process complex suprathreshold
patterns of AM imposed on a single sinusoidal carrier. This
allowed us to examine modulation processing while avoiding
possible confounding effects of reduced frequency selectivity
in hearing-impaired listeners. It also allowed us to avoid confounding effects of variations in audibility with frequency,
which can occur when a broadband carrier is used.
II. EXPERIMENT 1: “HEARING OUT” COMPONENT
MODULATION
In our first experiment, we assessed the extent to which
hearing-impaired listeners can “hear out” the component
modulations in a complex modulator, in the same way that
spectral components in a complex sound can be heard out in
the audio-frequency domain 共Plomp, 1964; Plomp and
Mimpen, 1968; Soderquist, 1970; Moore and Ohgushi,
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J. Acoust. Soc. Am., Vol. 119, No. 1, January 2006
TABLE I. For each hearing-impaired listener, the table shows hearing loss
at 4 kHz, test level, gender/age, and likely etiology of the hearing loss.
Listener
HL at 4 kHz
共dB兲
Test level
共dB SPL兲
Gender/Age
共years兲
S1
S2
S3
S4
S5
S6
S7
S8
60
40
50
50
65
65
80
75
85
83
88
85
90
80
96
93
M/62
M/55
F/68
M/47
M/53
F/69
F/73
M/71
Etiology
Noise exposure
Infection
Antibiotics
Head injury
Antibiotics
Unknown
Presbycusis 共?兲
Noise exposure
1993兲. Performance was measured as a function of the frequency separation of the modulator components. The method
was identical to that used by Sek and Moore 共2003兲 with
normally hearing listeners, so it is described only briefly
here.
A. Listeners and test levels
Eight hearing-impaired listeners were tested. All were
paid for their services. The air-bone gap in the audiogram
was always 艋10 dB, indicating that the hearing losses were
sensorineural, presumably cochlear, in origin. Only one ear
was tested for each listener. This was the ear that had a
hearing loss closest to 50 dB at the test frequency of 4 kHz.
The hearing loss at 4 kHz ranged from 40 to 80 dB. The
level of the 4-kHz sinusoidal carrier was chosen to give a
comfortable loudness for each listener. Initially, the carrier
was presented with a level of 80– 85 dB SPL for listeners
with hearing loss at 4 kHz up to 65 dB, and 85– 90 dB SPL
for listeners with hearing loss at 4 kHz greater than 65 dB.
Listeners were asked to indicate whether the carrier was at a
comfortable level. If the carrier was judged too soft, the level
was increased, while if it was judged too loud, the level was
decreased.
Generally, the required adjustments were small
共±2 to 4 dB兲. Table I shows, for each listener, the hearing
loss at 4 kHz, the test level, the gender and age, and the
probable etiology of the hearing loss.
Listeners were trained both on the task for this experiment and the task for experiment 2 共described later兲, on alternate days. Training lasted 4 – 6 days with about 2.5 h of
training per day. Following training, performance appeared
to be stable for all listeners.
B. Method and stimuli
To maximize the likelihood that listeners could perform
the task, we used a modulator containing only three components, with relatively wide frequency spacings between the
components. We assessed whether listeners could hear out
the central component of this complex modulator. The carrier
was a 4-kHz sinusoid. To avoid any influence of resolvable
spectral sidebands 共Sek and Moore, 1994; Kohlrausch et al.,
2000; Moore and Glasberg, 2001兲, the highest modulation
frequency used was 190 Hz, which is less than one-half of
the “normal” bandwidth of the auditory filter, ERBN, at
A. Sek and B. C. J. Moore: Modulation perception by hearing-impaired listeners
FIG. 1. The left panel shows mean results of experiment 1 for the hearing-impaired listeners tested here.
The right panel shows mean results for the same conditions for the normally hearing listeners tested by Sek
and Moore 共2003兲. The percent correct identification of
the target is plotted as a function of the ratio of f target
and f comp, using as numerator whichever of the two was
the larger. Each symbol denotes one pair of frequencies
for the flanking modulator components, as indicated by
the key. Error bars indicate ±1 standard deviation 共SD兲
across listeners.
4 kHz 共Glasberg and Moore, 1990兲. Since hearing-impaired
listeners usually have auditory-filter bandwidths that are
larger than normal 共Pick et al., 1977; Glasberg and Moore,
1986兲, it was very unlikely that the listeners would be able to
“hear out” any spectral sidebands 共Moore and Glasberg,
2001兲.
The task was designed so that listeners could not perform it by judging the mean rate of the complex modulator.
On each trial, three modulated stimuli were presented. The
modulator of the first stimulus contained three components.
Within a run, the frequencies of the outer two components
共called “flanking” components兲 were fixed and the frequency
of the central 共“target”兲 component, f target, was drawn randomly from one of five possible values: 80, 89.4, 100, 111.8,
and 125 Hz. The modulators of the second and third stimuli
contained just a single component. In one of these, selected
at random, the modulation frequency was equal to that of the
target. In the other, the 共“comparison”兲 modulation frequency, f comp, was randomly selected from one of the four
other possible values of the target. Listeners were required to
indicate whether the second or third stimulus contained a
modulation component that was present in the first stimulus.
Feedback was provided by lights on the response box indicating the correct interval. Listeners were seated in a doublewalled sound-attenuating chamber.
The results were analyzed in terms of the ratio between
f target and f comp, taking as the numerator whichever of these
two was the larger. The smallest ratio was equal to 1.118.
This was chosen as it is somewhat larger than the threshold
for detection of a change in modulation frequency of a sinusoidal carrier 共Lee, 1994; Lemanska et al., 2002; Füllgrabe
and Lorenzi, 2003兲. A run started with five trials using the
easiest condition, with the largest ratio of f target and f comp.
Scores for these initial five trials were discarded. Then each
possible value of f target was paired with each possible value
of f comp two times, giving a total of 40 scored trials per run.
Data presented are the result of at least 17 runs per listener
共more usually, 20–23 runs兲.
Stimuli were generated using a Tucker-Davis Technologies 共TDT兲 array processor 共TDT-AP2兲 in a host PC, and a
16-bit digital-to-analog converter 共TDT-DD1兲 operating at a
50-kHz sampling rate. The stimuli were attenuated
共TDT-PA4兲 and sent through an output amplifier 共TDT-HB6兲
to a Sennheiser HD580 earphone. Each modulator compoJ. Acoust. Soc. Am., Vol. 119, No. 1, January 2006
nent had a modulation index of 0.33. This was chosen so as
to avoid overmodulation of the three-component modulator,
while ensuring that the modulation was clearly audible. The
starting phase of each modulator component was chosen randomly for each and every stimulus.
On each trial, the carrier was presented in three bursts
separated by silent intervals of 400 ms. Each burst had a
20-ms raised-cosine rise and fall, and an overall duration
共including rise/fall times兲 of 1000 ms. The modulation was
applied during the whole of the carrier. The flanking component modulation frequencies were 10 and 190 Hz, 30 and
170 Hz, and 52.6 and 190 Hz. These values were also used
by Sek and Moore 共2003兲. The first pair of flanking frequencies was chosen to be widely spaced, so that it would be
possible to hear out the middle component even if selectivity
in the modulation domain was very poor. The components
were equally spaced from the central target component
共100 Hz兲 on a linear frequency scale. The second pair was
chosen to be closer in modulation frequency to the target
component, while keeping the components symmetrically
placed 共on a linear frequency scale兲 around the frequency of
the central target component. This was done to assess the
ability to hear out the central modulation component under
more difficult conditions. For the third pair, the components
were equally spaced from the central target component on a
logarithmic scale.
C. Results and discussion
The pattern of the results was similar across the eight
listeners, and mean results across listeners are shown in the
left panel of Fig. 1; the right panel shows the mean results
for normally hearing listeners obtained by Sek and Moore
共2003兲 for the same stimuli, but using a carrier level of
70 dB SPL. Percent-correct scores are plotted as a function
of the ratio f target / f comp, with the frequencies of the flanking
components as parameter. For the smallest ratio f target / f comp,
scores for the hearing-impaired listeners are close to the
chance level of 50%. However, scores increase as the ratio
increases, and for a ratio of 1.56 the mean scores are all
above 50%. Performance was better for the widest spacing of
the flanking modulator components 共10 and 190 Hz兲 than for
the two smaller spacings.
A within-subjects analysis of variance 共ANOVA兲 was
A. Sek and B. C. J. Moore: Modulation perception by hearing-impaired listeners
509
conducted on the percent correct scores for the hearingimpaired listeners, with factors ratio f target / f comp and frequencies of the flanking components. There was a significant
main effect of the ratio f target / f comp; F共3 , 21兲 = 28.55, p
⬍ 0.001. This suggests an ability to hear out the target modulation in the complex modulator when f target and f comp differ
sufficiently. There was also a significant main effect of the
frequencies of the flanking components; F共2 , 14兲 = 18.74, p
⬍ 0.001. Post hoc tests, based on the least-significant differences test, showed that performance was significantly better
共p ⬍ 0.01兲 for the 10- and 190-Hz flanking components than
for either of the other two combinations of flanking components. There was no significant difference in scores between
the 30- and 170-Hz combination and the 52- and 190-Hz
combination. There was a significant interaction between the
two main factors; F共6 , 42兲 = 2.42, p = 0.042. This reflects the
fact that the improvement in performance with increasing
value of the ratio f target / f comp was greater for the 10- and
190-Hz combination of flanking components than for either
of the other two combinations. However, post hoc tests
showed that the improvement in performance with increasing
value of the ratio f target / f comp was significant 共p ⬍ 0.01兲 for
all three pairs of flanking components.
The pattern of results was very similar for the hearingimpaired listeners tested here and the normally hearing listeners tested by Sek and Moore 共2003兲. For the largest ratio
f target / f comp, the normally hearing listeners performed slightly
better than the hearing-impaired listeners when the flanking
component frequencies were 10 and 190 Hz, and 30 and
170 Hz, but the reverse was true when the flanking component frequencies were 52.6 and 190 Hz. To assess whether
the difference between the two groups was significant, an
ANOVA was conducted on the mean percent correct scores
for each group, with the following factors: ratio f target / f comp,
frequencies of the flanking components, and group membership. The error variance was estimated from the three-way
interaction term. The main effect of group membership was
significant; F共1 , 6兲 = 6.39, p = 0.045. There was also a significant interaction between group membership and the frequencies of the flanking components; F共2 , 6兲 = 11.77, p = 0.008.
This reflects the finding that performance was affected more
by the frequencies of the flanking components for the normal
than for the impaired listeners.
In summary, the results indicate that the hearingimpaired listeners had some ability to hear out the target
modulation in the complex modulator when the flanking
components were widely spaced in frequency from the target, and when f target and f comp differed sufficiently. The ability is comparable to, but slightly worse than, that found for
normally hearing listeners. Performance for both groups was
never perfect even for the widest spacing of the modulator
components, which is consistent with the relatively broad
tuning of the modulation filters inferred from experiments on
modulation masking 共Dau et al., 1997a, 1997b; Ewert and
Dau, 2000兲 and on forward masking of AM 共Wojtczak and
Viemeister, 2005兲.
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III. EXPERIMENT 2: THE DETECTION OF CHANGES
IN MODULATOR COMPONENT PHASE
A. Background
In the audio-frequency domain, listeners are more sensitive to within-channel phase changes than to acrosschannel phase changes 共Zwicker, 1952; Patterson, 1987; Uppenkamp et al., 2001; Moore, 2003兲. Hence, changes in
phase sensitivity with changes in the frequency spacing of
the components probably reflect the influence of the frequency selectivity of the auditory system. Applying this rationale to the modulation domain, one would expect high
sensitivity to component phase for closely spaced modulator
components, and poorer sensitivity for widely spaced components. Such a pattern of results was observed by Sek and
Moore 共2003兲 for normally hearing listeners in a task requiring detection of a change in relative modulator phase of a
three-component modulator.
One complicating factor when interpreting phase effects
in the modulation domain is that the internal representation
of the envelopes of sounds may be distorted by the presence
of nonlinearities in the auditory system, such as basilarmembrane compression 共Rhode and Robles, 1974兲 and neural saturation and adaptation effects 共Shofner et al., 1996兲.
These nonlinearities can result in the introduction of components in the effective modulation spectrum that were not
present in the stimulus itself 共Moore et al., 1999; Verhey et
al., 2003; Sek and Moore, 2004; Füllgrabe et al., 2005兲, and
they can also result in differences in effective “internal” rootmean-square 共RMS兲 value of the modulator when the relative phases of modulator components are changed. Like Sek
and Moore 共2003兲, we included a condition in which the
modulation depth of all components was fixed, and a condition in which the effective modulation depth was made unreliable as a cue by randomizing the overall modulation
depth from one stimulus to the next. Comparison of the results for the two conditions provides an indication of the
extent to which effective modulation depth was used as a cue
when there was no randomization of modulation depth.
B. Listeners
Eight hearing-impaired listeners were tested; they were
the same as for experiment 1. All listeners were paid for their
services.
C. Stimuli
The equipment and earphone were the same as for experiment 1. The carrier was again a 4-kHz sinusoid, and its
level for each listener was the same as for experiment 1.
Listeners were required to discriminate two three-component
complex modulators differing only in the relative phases of
their components. Each modulator component was of equal
amplitude. The center component of the modulator had a
frequency, f c, of 50 or 100 Hz. The spacing of the components was 5, 15, 25, 35, or 45 Hz for the 50-Hz f c, and 10,
30, 50, 70, or 90 Hz for the 100-Hz f c. For one modulator,
the starting phase was 0° 共sine phase兲 for all components. We
refer to this as 0-phase. The waveforms for the 0-phase
A. Sek and B. C. J. Moore: Modulation perception by hearing-impaired listeners
modulator had relatively high peak factors; see Sek and
Moore 共2003兲 for details. For the other modulator, the lowest
component was phase-shifted by 180° or ␲ radians 共the other
two components starting at 0°兲. We refer to this as ␲-phase.
The waveforms for this modulator had lower peak factors
than for the 0-phase modulator.
On each trial, three stimuli were presented. Durations
and levels were the same as for experiment 1. In one set of
conditions, the modulation index m of each component was
fixed at 0.2. In another set of conditions, each modulator
component had the same value of m, but the value of m was
randomly varied from one stimulus to the next, over a range
of ±3 dB in terms of 20 log m 共on a uniform scale in terms of
log m兲; the value of m varied between 0.1416 and 0.2825.
D. Procedure
A three-interval three-alternative forced-choice task was
used. Two intervals contained the 0-phase stimulus. The
other interval, selected at random from the three intervals,
contained the ␲-phase stimulus. The task of the listener was
to select the interval that was different from the other two.
Feedback was given via lights on the response box. A run
started with five trials using the smallest frequency spacing
of the modulator components; pilot trials indicated that performance was relatively good for this spacing. Then, in successive trials, stimuli with each frequency spacing were presented once, in ascending order. This sequence was repeated
ten times to give a total of 55 trials per run. Results from the
first five trials of each run were discarded. For each listener,
each modulator center frequency, and each modulation-depth
condition 共fixed or randomized兲, 20 runs were obtained.
E. Results
The pattern of results was reasonably similar across listeners, and mean results across listeners are shown in the top
two panels of Fig. 2. Performance worsened with increasing
frequency separation of the modulator components, for all
listeners. However, even for the largest frequency separation,
scores for all listeners remained above the score that would
be achieved by guessing 共33.3%兲. For f c = 100 Hz, performance for listeners 1 and 2 was somewhat poorer when the
modulation depth was randomized 共squares兲 than when it
was fixed, suggesting that, when the modulation depth was
fixed, these listeners may have made some use of a cue related to the change in “internal” modulation depth. For the
other listeners, and for all listeners when f c = 50 Hz, there
was no consistent effect of randomizing the modulation
depth.
The data for the hearing-impaired listeners were subjected to a within-subjects ANOVA with factors modulator
center frequency 共50 or 100 Hz兲, spacing of the components
relative to the center frequency 共five values兲, and randomization of modulation depth 共absent or present兲. The main effect
of center frequency was significant; F共1 , 7兲 = 10.49, p
= 0.014. Overall performance was a little better for f c
= 100 Hz 共64%兲 than for f c = 50 Hz 共59%兲. The main effect
of frequency spacing of the components was significant;
F共4 , 28兲 = 85.88, p ⬍ 0.001. The main effect of randomization
J. Acoust. Soc. Am., Vol. 119, No. 1, January 2006
FIG. 2. The top two panels show the mean results for the hearing-impaired
listeners tested here, for modulator center frequencies of 50 Hz 共left兲 and
100 Hz 共right兲. The lower two panels show corresponding mean results for
the same conditions for the normally hearing listeners tested by Sek and
Moore 共2003兲. Error bars indicate ±1 SD across listeners.
was also significant; F共1 , 7兲 = 13.21, p = 0.008. The mean
score was 63.5% with no randomization and 59.5% with
randomization. Finally, the interaction of randomization and
frequency spacing of the components was significant, reflecting the fact that randomization of the modulation depth had a
larger effect for small spacings than for large spacings;
F共4 , 28兲 = 4.47, p = 0.006. No other interactions were significant.
The lower two panels of Fig. 2 show the mean results
for the normally hearing listeners tested by Sek and Moore
共2003兲 using a carrier level of 70 dB SPL. The results are
similar for the two groups, except that the hearing-impaired
listeners performed slightly more poorly overall than the normally hearing listeners. To assess whether the difference between the two groups was significant, an ANOVA was conducted on the mean percent correct scores for each group,
with such factors as modulator center frequency 共50 or
100 Hz兲, spacing of the components relative to the center
frequency 共five values兲, randomization of modulation depth
共absent or present兲, and group membership. The error variance was estimated from the three-way and four-way interaction terms. The main effect of group membership was significant; F共1 , 17兲 = 85.81, p ⬍ 0.001. There was also a
significant interaction between group membership and spacing of the components; F共4 , 17兲 = 11.47, p ⬍ 0.001. This reflects the finding that performance changed more with spacing of the components for the normal than for the impaired
listeners.
F. Discussion
Sek and Moore 共2003兲 considered various possible cues
that might be used in the phase-discrimination task. Given
A. Sek and B. C. J. Moore: Modulation perception by hearing-impaired listeners
511
the small size of the effect of randomization of modulation
depth, they argued that listeners did not perform the task
using a cue based on the effective internal depth of the
modulation. Another possible cue introduced by nonlinearities in the auditory system is a distortion component at the
envelope or “beat” rate of the modulation, which is determined by the spacing between modulator components 共Sek
and Moore, 2004; Füllgrabe et al., 2005兲. The strength of this
component might vary with the phase of the modulator components, and this could be used as a cue to discriminate the
phase changes 共possibly mediated via analysis with an
MFB兲. However, in several of the conditions of our experiment, the distortion component would have coincided in frequency with a component that was already present. Given
that the distortion component appears to be weak in amplitude relative to the primaries, it is unlikely that the change in
effective modulation depth of the distortion component
would provide a useful cue in the phase discrimination task,
especially in the condition where the modulation depth was
randomized.
Sek and Moore 共2003兲 also considered and rejected as
possible cues the ratio of the maximum value to the minimum value of the modulator 共max-min兲 共Forrest and Green,
1987; Strickland and Viemeister, 1996兲 and the crest factor
or
the
skewness
of
the
modulator
共Lorenzi
et al., 1999兲.
It seems likely, as argued by Sek and Moore 共2003兲 and
Ewert et al. 共2002兲, that listeners are sensitive to changes in
the shape of the modulator waveform at the output of the
modulation filter centered on 共or close to兲 the central component of the modulator. This shape would be influenced by
the relative phase of the components. When the modulator
components are widely spaced 共spacing greater than 0.5f c兲,
they interact less at the output of the modulation filter, so the
sensitivity to modulator phase is reduced. However, because
of the broad tuning of the modulation filters, some interaction occurs even for wide spacings. This can explain why
performance remained above chance for the largest spacing
used. It was not possible to use much larger spacings, since if
the spacing exceeds f c, one of the modulator components has
a negative frequency.
IV. GENERAL DISCUSSION
In both experiments, the pattern of results obtained for
the hearing-impaired listeners was similar to that obtained
previously for normally hearing listeners. This suggests that
the processing of suprathreshold complex patterns of modulation is not greatly affected by cochlear hearing loss. However, for both experiments, the hearing-impaired listeners
tested here did perform slightly and significantly more
poorly overall than the normally hearing listeners tested by
Sek and Moore 共2003兲. Also, for both experiments, performance of the hearing-impaired listeners was affected less by
the frequencies of the flanking components than performance
of the normally hearing listeners. This is consistent with the
idea that the hearing-impaired listeners had slightly reduced
frequency selectivity in the modulation domain, consistent
with the modulation-masking results of Lorenzi et al. 共1997兲.
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It is not obvious why this should be the case, since the MFB
is usually assumed to occur relatively centrally in the auditory system 共Rees and Møller, 1983; Liegeois-Chauvel et al.,
2004兲, and there is no obvious reason why the operation of
central mechanisms should be affected by cochlear hearing
loss 共Füllgrabe et al., 2003兲. While poorer selectivity in the
modulation domain for hearing-impaired listeners might explain the reduced effect of the spacing of the flanking components in experiment 2, it does not account for the poorer
overall performance; reduced selectivity would lead to more
interaction of components, which would be expected to lead
to better discrimination of the relative phase of the components.
It is of interest to consider other factors that might have
affected the performance of the impaired listeners tested
here. As described in the Introduction, cochlear hearing loss
usually leads to a loss of fast-acting compression on the basilar membrane 共Ruggero et al., 1997兲. For suprathreshold
modulation depths, this can lead to a perceived amount of
modulation that is greater than normal 共Moore et al., 1996兲.
One might argue that in a person with cochlear hearing loss,
the effective modulation depth at the input to central mechanisms is greater than normal. This might lead to an advantage in some modulation-processing tasks, and may account
for why detection of AM is sometimes better than normal
when the comparison is made at equal 共low兲 SLs 共Bacon and
Gleitman, 1992; Moore et al., 1992兲. On the other hand, for
normally hearing listeners, the high-frequency side of the
excitation pattern is processed almost linearly, at least for
high characteristic frequencies 共Robles and Ruggero, 2001兲.
If a large effective modulation depth is beneficial in a specific task, a normally hearing listener can obtain that benefit
by using information from the high-frequency side of the
excitation pattern 共Zwicker, 1956兲, at least when the carrier
is at a relatively high level. The normally hearing listeners of
Sek and Moore 共2003兲 were tested using a carrier level of
70 dB SPL, which would have been sufficient to allow them
to use information from the high-frequency side of the excitation pattern. One might therefore argue that differences in
basilar-membrane compression between normal and hearingimpaired listeners should have little effect on performance.
Another factor that may have affected the relative performance of the normally hearing listeners tested by Sek and
Moore 共2003兲 and the hearing-impaired listeners tested here
is the relatively low SL of the stimuli for the latter, which
ranged from 15 to 43 dB SL 共mean= 27 dB SL兲. Performance on many tasks, including the detection of AM, worsens at very low SLs 共Kohlrausch et al., 2000兲. However,
there is little information on the influence of SL on the processing of suprathreshold amounts of AM. The relatively low
SL of the stimuli in our experiments might have led to
slightly poorer-than-normal performance simply because
relatively few neurons are excited at low SLs, so the neural
code is relatively sparse. Put another way, neural “noise”
may have a greater influence at low SLs than at high SLs.
A final possible reason for the slightly poorer-thannormal performance of the hearing-impaired listeners is connected with the fact that they were older than the normally
hearing listeners tested by Sek and Moore 共2003兲. It has been
A. Sek and B. C. J. Moore: Modulation perception by hearing-impaired listeners
suggested that age can have adverse effects on temporal processing 共Wingfield et al., 1985兲. However, Takahashi and
Bacon 共1992兲 found that there were no significant effects of
age on performance in a variety of modulation processing
tasks once the effect of hearing loss was taken into account
共except for a very modest correlation between age and modulation detection sensitivity at low modulation frequencies兲.
Furthermore, in our data there was no clear trend for the
older listeners 共S7 and S8, aged 73 and 71, respectively兲 to
perform more poorly than the younger listeners 共S4 and S5,
aged 47 and 53, respectively兲. Thus it seems unlikely that
age had a major influence on the results.
Overall, the results of the present experiments indicate
that hearing impairment has relatively little effect on the
ability to process complex suprathreshold patterns of modulation, although the impaired listeners may have slightly reduced frequency selectivity in the modulation domain.
V. CONCLUSIONS
We have described two experiments assessing the ability
of listeners with cochlear hearing loss to process suprathreshold amounts of AM applied to a 4-kHz sinusoidal carrier
presented at a comfortable listening level. Experiment 1 examined the ability to “hear out” the modulation frequency of
the central component of a three-component modulator. All
listeners showed some ability to perform the task when the
components in the modulator were widely spaced and when
the frequencies of the target and comparison differed sufficiently. Scores were poorer when the two flanking components were closer to the central target component frequency
of 100 Hz. This is consistent with the relatively broad tuning
of the modulation filters inferred from experiments on modulation masking. The pattern of the results was very similar to
that found earlier for normally hearing listeners, although
performance overall was slightly poorer than for normally
hearing listeners 共Sek and Moore, 2003兲.
Experiment 2 examined the ability to hear a change in
the relative phase of the components in a three-component
modulator with harmonically spaced components. The frequency of the central component, f c, was either 50 or
100 Hz. Performance was good 共70–80% correct兲 when the
component spacing was 艋0.5f c, but worsened markedly for
frequency spacings greater than that. This is broadly the pattern of results predicted from the concept of the MFB. Performance was only slightly impaired by randomizing the
overall modulation depth from one stimulus to the next. This
suggests that listeners did not use the overall effective internal depth of the modulation as a cue. Again, the pattern of
the results was very similar to that found earlier for normally
hearing listeners, although performance overall was slightly
poorer than for normally hearing listeners 共Sek and Moore,
2003兲.
It may be concluded that the ability to process complex
suprathreshold patterns of AM is affected only slightly by
cochlear hearing loss.
J. Acoust. Soc. Am., Vol. 119, No. 1, January 2006
ACKNOWLEDGMENTS
This work was supported by the Wellcome Trust and by
the MRC. We thank Christian Füllgrabe for helpful discussions and Brian Glasberg for help with statistical analyses.
We also thank Associate Editor Andrew Oxenham, Christian
Lorenzi, and an anonymous reviewer for helpful comments
on an earlier version of this paper.
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