doi:10.1093/brain/awn308
Brain 2009: 132; 524–536
| 524
BRAIN
A JOURNAL OF NEUROLOGY
Enhanced discrimination of low-frequency sounds
for subjects with high-frequency dead regions
Brian C. J. Moore1 and S. N. Vinay2
1 Department of Experimental Psychology, Cambridge University, Downing Street, Cambridge CB2 3EB, UK
2 Department of Audiology, All India Institute of Speech and Hearing, Manasagangothri, Mysore 570 006, India
Research using animals suggests that a lesion in the basal portion of the cochlea, causing a high-frequency ‘dead region’, leads
to cortical reorganization, such that frequencies just below the edge frequency of the dead region, fe, become over-represented.
We set out to determine if this reorganization has functional benefits. Two groups of subjects were tested, with and without
acquired high-frequency dead regions, as assessed using the TEN(HL) test. For the ears with dead regions, the value of fe was
close to 1000 or 1500 Hz. The two groups were matched in terms of audiometric thresholds for frequencies below fe and
in terms of age. Three subjects with unilateral dead regions (with matched low-frequency audiometric thresholds across ears)
were also tested. Three tasks were used: (i) frequency discrimination of sinusoidal tones. The level of every stimulus was roved
over a 12-dB range to reduce the salience of loudness cues. The center frequencies used ranged from 250 Hz to just below fe;
(ii) detection of sinusoidal amplitude modulation of a sinusoidal carrier. Carrier frequencies of 500 and 800 Hz were used with
all subjects, and an additional carrier frequency of 1200 Hz was used for ears with fe close to 1500 Hz and their matched
counterparts. Modulation frequencies were 4, 50 and 100 Hz; (iii) identification of consonants in nonsense syllables. The
syllables were lowpass filtered at 1000 or 1500 Hz (depending on the value of fe) and complementary highpass-filtered
noise was presented to prevent use of information from neurons tuned above fe. For the frequency-discrimination task, the
ears with dead regions showed a significant local improvement (‘enhanced’ thresholds) for frequencies just below fe, as has
been reported previously. For the subjects with unilateral dead regions, the enhancement occurred only for the ears with dead
regions. For the amplitude-modulation detection task, thresholds were generally lower for the ears with dead regions than
for the ears without, and this effect was statistically significant. For the subjects with unilateral dead regions, thresholds were
lower for the ears with dead regions than for the ears without. Consonant identification was significantly better for the ears
with than without dead regions, and this was true for the subjects with unilateral dead regions. We conclude that a dead region
at high frequencies is associated with a better ability to process information at low frequencies. These effects may reflect
cortical plasticity induced by the dead regions.
Keywords: cortical plasticity; cochlear dead region; frequency discrimination; amplitude-modulation detection; consonant
identification
Abbreviations: CD = compact disk; CF = characteristic frequency; F = step used in frequency discrimination task; DLF = difference
limen for frequency; ERBN = equivalent rectangular bandwidth of the auditory filter for normal-hearing listeners; fcut-off = cut-off
frequency of hearing loss; fe = edge frequency of a dead region; IHC = inner hair cell; LE = left ear; m = modulation index; RE = right
ear; SD = standard deviation; SL = sensation level; SII = speech intelligibility index; TEN = threshold-equalizing noise
Received August 1, 2008. Revised September 23, 2008. Accepted October 22, 2008. Advance Access publication November 26, 2008
ß The Author (2008). Published by Oxford University Press on behalf of the Guarantors of Brain. All rights reserved.
For Permissions, please email: journals.permissions@oxfordjournals.org
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Correspondence to: Brian C. J. Moore,
Department of Experimental Psychology,
University of Cambridge, Downing Street,
Cambridge CB2 3EB, UK
Enhanced discrimination with dead regions
Introduction
| 525
et al., 2000). Here, the term high-frequency dead region is used
to refer to a dead region which starts at fe and extends upwards
from there.
Studies using animals have shown that damage to the basal
region of the cochlea, effectively producing a high-frequency
dead region, can lead to the region of the auditory cortex that
previously responded to the damaged region of the cochlea
becoming ‘tuned’ to frequencies corresponding to the adjacent
undamaged region (Robertson and Irvine, 1989; Harrison et al.,
1991; Kakigi et al., 2000). The possible functional consequences
of this have been explored in several studies. McDermott et al.
(1998) measured frequency difference limens (DLFs) for pure
tones using five subjects with steeply sloping high-frequency
sensorineural hearing loss. It is likely that the subjects had highfrequency dead regions, but no specific test for this was performed. DLFs were measured using sinusoids presented at a
level that followed each subject’s equal-loudness contour. In addition, the level was varied (roved) in each observation interval by
a random value between –3 dB and +3 dB, to reduce the salience
of loudness cues. The cut-off frequency of the hearing loss, fcut-off,
was defined as the lowest frequency for which the absolute
threshold was greater than 15 dB HL and for which the slope
of the threshold curve at higher frequencies was greater than or
equal to 50 dB per octave. Four out of five subjects showed lower
DLFs near fcut-off than at surrounding frequencies. McDermott
et al. (1998) suggested that this effect could be a result of functional reorganization of the auditory cortex. However, the lower
DLF near fcut-off might have reflected the use of loudness cues.
The 6-dB rove range may not have been enough to eliminate
completely the loudness differences across frequencies, especially
over the frequency range where the hearing loss changed rapidly.
Thai-Van et al. (2003) conducted an experiment similar to that
of McDermott et al. (1998), but they measured DLFs for stimuli
roved over a 12-dB range. They tested five subjects with highfrequency sensorineural hearing loss. They defined fcut-off as ‘the
highest test frequency above the audiogram edge (identified by
visual inspection) at which the measured absolute threshold was
within 5 dB of the best absolute threshold’. All subjects were
diagnosed as having high-frequency dead regions, using the TEN
test (Moore et al., 2000). They found enhanced DLFs at or near
fcut-off.
Kluk and Moore (2006b) measured DLFs using subjects with
dead regions for whom the values of fe had been determined
precisely using psychophysical tuning curves. To prevent the use
of loudness cues, stimuli for the measurement of DLFs had a mean
level falling along an equal-loudness contour and levels were
roved over a 12-dB range. DLFs were measured for 13 subjects
with a dead region in at least one ear. Almost all subjects with
bilateral hearing loss exhibited enhanced DLFs near fe, consistent
with cortical reorganization.
One purpose of this study was to replicate the existence of
enhanced DLFs near fe for subjects with bilateral dead regions.
To reduce the likelihood that the results would be influenced by
the use of loudness cues, subjects were selected to have relatively
flat hearing losses for frequencies below fe. We also aimed to
provide more information about whether or not enhanced DLFs
occur in cases of unilateral dead regions. Studies of animals with
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It is now well established that damage to a sensory organ can
lead to changes in the organization of the cortical region responding to that organ, even in adults (Harrison et al., 1991; Irvine
and Rajan, 1995; Kakigi et al., 2000; Irvine and Wright, 2005;
Thai-Van et al., 2007). This is sometimes described as brain
‘plasticity’. Often, the results can be explained by rerouting of
neural inputs between adjacent cortical regions (Kakigi et al.,
2000; Thai-Van et al., 2003). Although such reorganization
has been extensively studied for many sensory systems
(Ramachandran, 2005), the extent to which cortical reorganization
is of functional benefit remains somewhat unclear; the reorganization may reflect largely a response to injury with limited functional
benefits.
In this article we are especially concerned with the possible
benefits of cortical reorganization in response to a ‘dead region’
in the cochlea. A dead region is a region where the inner hair cells
(IHCs) and/or neurons are functioning very poorly, if at all
(Moore, 2001, 2004). The IHCs act as transducers, converting
vibration on the basilar membrane into activity in the auditory
nerve. Hence, little or no information is transmitted to the brain
about basilar-membrane vibration in a dead region. However, a
tone or other signal producing peak vibration in that region may
be detected by off-place (off-frequency) listening; the signal may
be detected in the ‘wrong’ place in the cochlea. This is the basis
for psychophysical tests for detecting a dead region, namely psychophysical tuning curves (Thornton and Abbas, 1980; Florentine
and Houtsma, 1983; Moore and Alcántara, 2001; Kluk and
Moore, 2005; Sek et al., 2005; Kluk and Moore, 2006a) and
the threshold-equalizing noise (TEN) test (Moore et al., 2000,
2004).
The threshold-equalizing noise (TEN) test is described here, as it
was used in this study for diagnosing dead regions. The test
involves measuring the threshold for detecting a pure tone presented in a background noise called ‘threshold-equalizing noise’.
The noise spectrum (its intensity as a function of frequency) is
shaped in such a way that the threshold for detecting a tone in
the noise is approximately the same over a wide range of tone
frequencies, for people with normal hearing. The masked threshold is approximately equal to the nominal level of the noise, specified as the level in a 132-Hz wide band centered at 1000 Hz. The
value of 132 Hz corresponds to the equivalent rectangular bandwidth of the auditory filter, as determined using young, normally
hearing listeners, which is denoted ERBN (Moore, 2003). When
the pure-tone signal frequency falls in a dead region, the signal
will only be detected when it produces sufficient basilar-membrane vibration at a remote region in the cochlea where there
are surviving IHCs and neurons. The amount of vibration at this
remote region will be less than in the dead region, and so the
noise will be very effective in masking it. Thus, the signal threshold
is expected to be markedly higher than normal. A masked threshold that is 10 dB higher than normal is taken as indicating a dead
region (Moore et al., 2000, 2004). The extent of a dead region is
defined in terms of its edge frequency (or frequencies), fe, which
corresponds to the characteristic frequency (CF) of the IHCs
and/or neurons immediately adjacent to the dead region (Moore
Brain 2009: 132; 524–536
526
| Brain 2009: 132; 524–536
Methods
Subjects
Twelve subjects diagnosed with sensorineural hearing loss participated
in the study. The purpose of the study was explained to the subjects,
and their consent was obtained for participation in the study. They are
denoted S1–S12. Subjects were selected after screening a much larger
number of subjects. Details of the subjects are given in Table 1.
Audiometric thresholds were measured using a Madsen OB922 dualchannel clinical audiometer with Telephonics TDH39 headphones. The
modified Hughson-Westlake procedure described by Carhart and
Jerger (1959) was used. Middle-ear function was checked with a
Grason-Stadler Tympstar Immittance meter and found to be normal
for all subjects. Transient evoked otoacoustic emissions were assessed
using click stimuli and an Otodynamics ILO 292 analyzer and were
found to be absent for all subjects. The ages of the subjects ranged
from 22 to 74 years. There were nine males and four females. All
subjects had post-lingually acquired hearing loss and all had a hearing
loss for five years or more at the time of testing. None of the subjects
had non-auditory neural conditions and none had any history of ear
discharge or fullness in the ear.
Each ear of each subject was considered separately; the right ear is
denoted RE and the left ear is denoted LE. S7 and S8 were tested
using one ear only as they were available only for a limited time. The
diagnosis of dead regions was based on the results of the TEN(HL) test
(Moore et al., 2004); see below for details. When dead regions were
identified, subjects/ears were selected to have dead regions with edge
frequency, fe, close to 1000 or 1500 Hz. S1RE, S2RE and LE, S5LE,
S6LE, S8LE, S9RE, S11RE and LE, S12RE and LE were diagnosed to
have dead regions. Thus, there were 11 ears with dead regions. The
remaining subjects/ears did not have dead regions, but were selected
to match the severity of hearing loss of the ears with dead regions for
frequencies below 1000 Hz. For each ear with a dead region, a matching ear without a dead region was selected, either within the same
subject or in a different subject. For example, for S1 the RE (with a
dead region) was matched to the LE of the same subject (without a
dead region). For S2, the RE (with a dead region) was matched to the
LE of S3 (without a dead region), while the LE of S2 (with a dead
region) was matched to the RE of S3. There were 11 ears without
dead regions. The mean audiometric thresholds (with SDs in parentheses) at 250, 500 and 750 Hz were 40 (20), 50 (16) and 58 (16) dB
HL, respectively, for the ears with dead regions and 41 (17), 48 (16)
and 53 (15) dB HL, respectively for the ears without dead regions. The
mean (SD) age was 47 (21) years for the ears with dead regions and
49 (21) years for the ears without dead regions. The ears with dead
regions usually had more severe hearing losses than the ears without
dead regions for frequencies of 1000 Hz and above, although this was
not the case for the two ears of S4.
TEN test
For the version of the TEN test used here, all levels are specified in dB
Hearing Level (HL); this version of the test is called the TEN(HL) test,
and it is available on a compact disk (CD) (Moore et al., 2004). The
TEN(HL)-test CD was replayed via a Philips 729 K CD player connected
to a Madsen OB922 audiometer equipped with Telephonics TDH39
earphones. The level of the signal and the TEN(HL) were controlled
using the attenuators in the audiometer. The TEN(HL) level was 70 dB
HL/ERBN. The signal level was varied in 2-dB steps to determine
the masked thresholds, as recommended by Moore et al. (2004).
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unilateral dead regions suggest that cortical over-representation
is unlikely to be observed for responses to the normally hearing
ears. For example, Rajan et al. (1993) induced unilateral dead
regions in cats, and measured cortical responses on the contralateral side to the lesioned ear. They observed no reorganization
when the normally hearing ears were stimulated. However, in the
study of Kluk and Moore (2006b), one subject with a high-frequency dead region in one ear and good hearing in the other ear
showed an enhanced DLF in her better ear. It is not known
whether enhanced DLFs occur in subjects with bilateral hearing
loss but with a dead region in one ear only. We tested three
such subjects.
Although there have been several studies measuring frequency
discrimination for subjects with dead regions, we are not aware of
any studies of amplitude discrimination or modulation detection
for such subjects. In this study we measured thresholds for detecting amplitude modulation using carrier frequencies below fe, and
using three modulation frequencies, 4, 50 and 100 Hz. An advantage of measuring the ability to detect amplitude changes rather
than frequency changes is that the results are unlikely to be
affected by differences in overall loudness from one stimulus to
the next.
Another issue is whether cortical plasticity leads to subjects with
high-frequency dead regions making more effective use of lowfrequency information in the recognition of speech than subjects
without high-frequency dead regions. This possibility is suggested
by the work of Vestergaard (2003). He tested 22 hearing-impaired
subjects of whom 11 showed evidence for high-frequency dead
regions based on the TEN test. Intelligibility was measured for
lowpass-filtered speech presented via the subjects’ own hearing
aids. For a given level of speech audibility, as calculated using
the speech intelligibility index (SII; ANSI, 1997), the subjects
with dead regions performed better than the subjects without
dead regions, especially when the speech audibility was low (SII
value below 0.4). In other words, the subjects with dead regions
appeared to make more effective use of low-frequency speech
information. A limitation of this experiment is that subjects with
and without dead regions were not matched in terms of their
audiometric thresholds at low frequencies. Thus, differences in
the ability to make use of low-frequency speech information
might have been related to differences in the degree of low-frequency hearing loss. Also, the type of hearing aid varied across
subjects and was not matched for subjects with and without dead
regions.
In this study, we measured speech intelligibility for lowpass-filtered speech for two groups of subjects, with and without highfrequency dead regions. The two groups were matched in terms
of audiometric thresholds at low frequencies and had a similar
distribution of ages. In contrast to the study of Vestergaard
(2003), all subjects were tested using speech presented via the
same headphones, at a level sufficient to make the speech comfortably loud. To ensure that the speech was not being processed
via neurons tuned above the cut-off frequency of the speech for
subjects without dead regions (Horwitz et al., 2002; Warren et al.,
2004), the lowpass-filtered speech was presented together with
complementary highpass-filtered noise.
B. C. J. Moore and S. N.Vinay
Enhanced discrimination with dead regions
Brain 2009: 132; 524–536
| 527
Table 1 Details of the subjects
Subject/ear
Age (years)
Gender
Duration (years)
Frequency (Hz)
250
S1RE
28
M
10
LE
S2RE
5
24
M
8
LE
S3RE
58
F
5
74
F
15
60
60
M
10
10
20
70
F
5
LE
25
20
S7LE
50
M
8
10
S8RE
53
M
10
30
S9RE
33
M
6
45
LE
S10RE
45
28
M
7
LE
S11RE
55
70
M
15
LE
S12RE
LE
50
50
60
22
F
5
55
50
10
68
10
70
50
70
50
70
60
76
55
74
60
74
60
74
40
72
50
70
35
70
35
72
40
70
40
70
50
70
55
70
60
70
65
76
65
74
60
72
65
74
55
74
750
15
70
15
70
60
74
70
74
65
76
55
72
70
76
65
74
45
72
55
76
45
70
40
70
45
70
55
70
60
70
65
70
65
74
70
76
70
74
65
74
60
74
55
74
1000
15
70
20
70
80a
92
90a
NR
60
74
55
74
75
78
70
78
50
70
60
74
60
74
45
70
50
74
75a
86
65
74
65
70
80a
94
75a
86
70
74
70
76
60
76
60
74
1500
a
75
NR
70
72
115
NR
120
NR
60
76
55
72
75
78
75a
86
55
72
80a
92
70
76
55
72
80a
96
110
NR
60
72
65
74
85
98
75
86
75a
86
75a
86
60
74
60
72
2000
3000
4000
8000
90
NR
80
80
120
NR
120
NR
60
74
60
74
75
76
75
NR
55
72
80
92
70
74
75
76
95
NR
NR
NR
65
74
65
74
85
96
80
NR
80
NR
85
NR
60
72
60
74
100
NR
90
88
NR
NR
115
NR
65
72
65
74
75
80
80
NR
60
72
90
NR
65
72
75
76
105
NR
NR
NR
70
76
70
76
90
NR
80
NR
90
NR
80
NR
65
74
60
76
105
NR
90
88
NR
NR
115
NR
65
78
70
74
80
80
80
NR
65
74
95
NR
65
72
75
78
115
NR
NR
NR
75
78
75
78
90
NR
80
NR
95
NR
75
NR
70
78
65
76
105
95
NR
NR
90
70
100
100
60
95
60
70
NR
NR
65
80
90
90
105
NR
60
60
Columns 2–4 show age, gender and duration of deafness. For each cell in Columns 5–13, the upper number shows the audiometric threshold and (for Columns 6–12)
the lower number shows the masked threshold in TEN(HL) with a level of 70 dB/ERBN. Bold numbers indicate frequencies within the dead region. NR indicates no
response at the output limit of the audiometer.
a Edge frequency (fe) of a dead region.
A ‘no response (NR)’ was recorded when the subject did not indicate
hearing the signal at the maximum output level of the audiometer,
which was 100 dB HL for the signals derived from the TEN(HL) CD.
The diagnosis of the presence or absence of a dead region at a specific
frequency was based on the criteria suggested by Moore et al. (2004).
If the masked threshold in the TEN(HL) was 10 dB or more above the
TEN(HL) level/ERBN (i.e. was 80 dB HL or more), and the TEN(HL)
elevated the absolute threshold by 10 dB or more, then a dead region
was assumed to be present at the signal frequency. If the masked
threshold in the TEN(HL) was less than 10 dB above the TEN(HL)
level/ERBN, and the TEN(HL) elevated the absolute threshold by 10
dB or more, then a dead region was assumed to be absent. Results
were considered inconclusive if the masked threshold in the TEN(HL)
was 10 dB or more above the TEN(HL) level/ERBN, but the TEN(HL)
did not elevate the absolute threshold by 10 dB or more, or the
threshold in the TEN(HL) was unmeasurable because the output
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65
LE
S6RE
55
50
LE
S5RE
50
50
LE
S4RE
5
500
528
| Brain 2009: 132; 524–536
limitation of the audiometer was reached and the highest possible level
was not 10 dB or more above the absolute threshold. This did not
happen in any case for the frequency corresponding to fe. However, it
often happened for higher frequencies, e.g. for S2RE and S2LE. In
such cases, it was assumed that the dead region extended upwards
from fe, including the frequencies for which thresholds in the TEN(HL)
were too high to be measured.
Measurement of absolute thresholds
Procedure for measuring frequency
discrimination
A variation on a two-interval two-alternative forced-choice method
was used to measure frequency-discrimination thresholds for pure
tones, using the same equipment as for the measurement of absolute
thresholds in dB SPL. The task was designed to be easy to learn and
not to require naming of the direction of a pitch change, which is
difficult for some subjects (Semal and Demany, 2006). In one interval
of a trial (selected randomly), there were four successive 500-ms
bursts (including 20-ms raised-cosine ramps) of a tone A, with a
fixed frequency. The bursts were separated by 100 ms. In the other
interval, tones A and B alternated, with the same 100-ms inter-burst
interval, giving the pattern ABAB. Tone B had a frequency that was
higher than that of tone A by F Hz. The task of the subject was to
choose the interval in which the sound changed across the four tone
bursts within an interval. The intervals were indicated by boxes on the
computer screen (labelled 1 and 2), each of which was lit up in blue
during the appropriate interval. A response could be made either by
clicking on one of the two boxes using the computer mouse or by
using keys ‘1’ and ‘2’ on the numeric keypad of the computer keyboard. Feedback was provided after the response by ‘flashing’
the box that had been selected, with green for a correct response
and red for an incorrect response. To make it difficult for subjects to
use loudness cues to detect the frequency changes, the level of each
and every tone was varied randomly from one presentation to the
next, with a level range of 12 dB (6 dB) around the nominal level.
The amount of shift, F, was varied using an adaptive procedure. A
run was started with a relatively large value of F, chosen to make the
task easy at the start of a run. Following two correct responses in a
row, the value of F was decreased, while following one incorrect
response it was increased. The procedure continued until eight turnpoints had occurred. The value of F was changed by a factor of
1.953 (1.253) until one turnpoint had occurred, then by a factor of
1.5625 (1.252) until the second turnpoint had occurred, and then by a
factor of 1.25. The threshold was estimated as the geometric mean of
the values of F at the last six turnpoints. In what follows, thresholds
are expressed as a percentage of the center frequency.
For ears with fe close to 1000 Hz, and for the matched ears,
the frequencies used for tone A were 250, 300, 400, 500, 600,
750, 800 and 900. Additional frequencies of 1000 and 1200 Hz
were tested for ears with fe close to 1500 Hz and also for the matched
ears. The order of testing the different frequencies was randomized for
each subject. The task was administered twice for each frequency and
the final threshold was taken as the mean of the two estimates.
For most ears the test was administered with the stimuli at 20 dB SL,
based on the absolute threshold measured with the forced-choice procedure described above. However, for a few ears, stimuli at 20 dB SL
became uncomfortably loud at some frequencies. In such cases, the
level was reduced to ensure that all stimuli were of comfortable loudness. S11 was tested using a level of 15 dB SL for both ears. S4 was
tested using a level of 15 dB SL for the LE and 10 dB SL for the RE.
Procedure for measuring amplitudemodulation detection thresholds
Thresholds for detecting amplitude modulation were determined using
an adaptive two-interval two-alternative forced-choice method, similar
to that described above and using the same equipment as for the
measurement of absolute thresholds in dB SPL. One interval contained
an unmodulated carrier and the other interval contained a carrier that
was amplitude modulated with modulation index m. The subject had
to indicate the interval with the modulation. The total power was
equated across the two intervals. The duration of each carrier was
1000 ms, including 20-ms raised-cosine rise-fall times. The value of
m was adjusted using a two-down one-up adaptive procedure. The
initial value was chosen to make the modulation clearly audible. The
value of m was adjusted by a factor of 1.253 until two turnpoints had
occurred. Then m was adjusted by a factor of 1.252 until two
more turnpoints had occurred. Finally, m was adjusted by a factor
of 1.25 until eight further turnpoints had occurred. The threshold
was taken as the geometric mean of the values of m at the last
eight turnpoints. At least two threshold estimates were obtained for
each condition; where time permitted, an additional estimate was
obtained. The final threshold for a given condition was taken as the
mean across all estimates. In what follows, thresholds are expressed as
20log10(m). The carrier level was 20 dB SL, i.e. 20 dB above the
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In addition to the measurement of audiometric thresholds as described
above, absolute thresholds for pure-tone signals were measured more
precisely in dB SPL using an adaptive two-alternative forced-choice
method. These precisely measured absolute thresholds were used to
set the sensations levels (SLs) of the stimuli used in the measurement
of frequency discrimination and amplitude-modulation detection (see
below for details). Stimuli were digitally generated using a personal
computer equipped with a SoundMAX sound card. The output of the
sound card was fed to Sennheiser HD265 earphones. These are
‘closed’ type earphones which give good isolation between the two
ears. They are diffuse-field equalized. The sampling frequency was
25 kHz. The signal level was started above the threshold level as estimated from the audiogram. The signal could occur either in interval
one or interval two, selected at random. The signal lasted 200 ms,
including 20-ms raised-cosine rise/fall times, and the intervals were
separated by 500 ms. The intervals were indicated by boxes on the
computer screen (labelled 1 and 2), each of which was lit up in blue
during the appropriate interval. The subject responded by clicking on
the appropriate box with a computer mouse, or by pressing button 1
or 2 on the computer numeric keypad. Feedback was provided by
flashing the box in green for a correct answer and red for an incorrect
answer. A two-down, one-up procedure was used (Levitt, 1971). Six
turnpoints were obtained. The step size was initially 6 dB. It was
changed to 4 dB after one turnpoint, and to 2 dB after the second
turnpoint. The threshold was taken as the mean signal level at the last
four turnpoints.
The calibration of the system was checked by comparing absolute
thresholds at 1000 Hz measured in dB HL using the audiometer and in
dB SPL using the system described above. In theory, the normal monaural absolute threshold (minimum audible pressure) in dB SPL is about
6.5 dB, so the threshold measured using the PC-based system should,
on average, be 6.5 dB higher than the threshold using the audiometer.
This was checked using each ear in turn of eleven subjects, and was
confirmed to be the case.
B. C. J. Moore and S. N.Vinay
Enhanced discrimination with dead regions
absolute threshold for a pure tone at the carrier frequency, as measured using the adaptive procedure described above, except when this
led to uncomfortable loudness. The exceptions are the same as
described for frequency discrimination.
The carrier frequencies were 500 and 800 Hz for ears with values of
fe equal to 1000 Hz and for the matched ears. An additional carrier
frequency of 1200 Hz was tested for ears with fe equal to 1500 Hz and
for corresponding matched ears. The modulation frequencies were 4,
50 and 100 Hz. The order of testing the different carrier and modulation frequencies was randomized for each subject.
Brain 2009: 132; 524–536
| 529
similar for the two ears. Despite this, masked thresholds in the
TEN(HL) were much higher for the ear with a dead region than
for the ear without a dead region. It should be noted that the
‘true’ value of fe lies between the highest frequency for which the
TEN(HL)-test criteria are not met and the lowest frequency for
which the criteria are met (Kluk and Moore, 2005, 2006a, b).
On this basis, there were five ears with fe between 750 and
1000 Hz and six ears with fe between 1000 and 1500 Hz.
Frequency discrimination
Consonant identification task
Results
TEN(HL) test
The results of the TEN(HL) test are summarized in Table 1, by the
lower entries in the cells in columns 6–12. Recall that the TEN(HL)
level was 70 dB/ERBN, so a masked threshold of 80 dB HL or
higher is taken as indicating a dead region, provided that the
masked threshold is 10 dB or more above the absolute threshold.
For three subjects, S1, S4 and S5, the results indicated a dead
region in one ear only. For S1 and S5, the absolute thresholds
at high frequencies were higher for the ear with a dead region
than for the other ear. However, for S4 absolute thresholds were
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Consonant identification was measured at the most comfortable
level for vowel-consonant-vowel nonsense syllables, spoken by a
man. The vowels /a/, /i/ and /u/ were used (same vowel at start
and end of a syllable) in combination with each of 21 consonants:
This gave
63 tokens per list. Ten lists were recorded, each with the stimuli in
a different order. Each ear was tested separately. Two or more lists
were administered for each ear depending upon the availability of
the subject.
We wished to assess the ability to identify speech based on the use
of low-frequency information alone. Recall that the ears with and
without dead regions were matched in terms of audiometric thresholds
at low frequencies. For the ears with fe close to 1000 Hz, and their
matched counterparts, the speech was lowpass filtered at 1000 Hz,
while for ears with fe close to 1500 Hz, and their matched counterparts, the speech was lowpass filtered at 1500 Hz. To prevent the use
of information from neural channels tuned above the cut-off frequency of the speech, often referred to as off-frequency listening
(Van Tasell and Turner, 1984; Dubno and Ahlstrom, 1995; Lorenzi
et al., 2008), the stimuli were presented with a complementary highpass-filtered white noise. The highpass cut-off frequency of the noise
was equal to the lowpass cut-off frequency of the speech. The spectrum level of the noise within its passband was set 10 dB below the
mean spectrum level of the speech over the frequency range 250–
500 Hz. This was done to avoid any strong effects of downward
spread of masking from the noise. Filtering of the speech and noise
was done using Adobe Audition version 2 software. The filters were
designed so that the response dropped by 50 dB over a frequency
range of 85 Hz and then declined more gradually. The filtered speech
and noise stimuli were transferred to CD. During the experiment, the
stimuli were replayed via the same computer and sound card as
described earlier, and presented via Sennheiser HD265 earphones.
For data analysis, the ears were subdivided into four groups: (A)
ears with dead regions with fe close to 1000 Hz; (B) ears with no
dead region matched to those with fe close to 1000 Hz; (C) ears
with dead regions with fe close to 1500 Hz; (D) ears with no dead
region matched to those with fe close to 1500 Hz. The DLFs for
each group, expressed as a percentage of the center frequency,
are shown in Tables 2—5. The tables also show geometric mean
DLFs for each group. Generally, the DLFs did not vary markedly
with frequency for the ears without dead regions (Tables 3 and 5).
However, as found in previous studies (McDermott et al., 1998;
Thai-Van et al., 2003; Kluk and Moore, 2006b), there was often a
dip, a local DLF enhancement, for the ears with dead regions, the
dip occurring at a value somewhat below fe (Tables 2 and 4). Two
analyses of variance (ANOVAs) were conducted. The first was for
Groups A and B, and had frequency as a within-subjects factor
and presence/absence of a dead region as a between-subjects
factor. Only frequencies up to 900 Hz were included. The results
showed no significant effect of frequency, F(6, 24) = 2.12,
P = 0.088, but a significant effect of presence/absence of a dead
region, F(1,4) = 13.32, P = 0.022, and a significant interaction,
F(6,24) = 3.25, P = 0.017. The interaction reflects the dip in the
DLFs that occurred for Group A for frequencies of 600 and
800 Hz. The difference between DLFs for Groups A and B was
significant (P50.05) for the frequencies of 600 and 800 Hz,
using Fisher’s protected least-significant differences (LSD) test
(Keppel, 1991), but was not significant for any other frequency.
The second ANOVA was similar, except that it was based on the
data for Groups C and D and included frequencies up to 1200 Hz.
For this ANOVA, the effect of frequency was significant,
F(8,40) = 2.43, P = 0.03 and the effect of presence/absence of a
dead region was also significant, F(1,5) = 38.3, P = 0.002. Again,
the interaction was significant, F(8,40) = 2.49, P = 0.027. The interaction reflects the dip in the DLFs that occurred for Group C for
frequencies of 900 and 1000 Hz. The difference between DLFs for
Groups C and D was significant (P50.05) for the frequencies of 900
and 1000 Hz, using Fisher’s protected LSD test, but was not significant for any other frequencies. Overall, the results confirm the
results of previous studies in showing enhanced DLFs for ears with
dead regions for frequencies somewhat below fe.
Of particular interest here are the results for the three subjects,
S1, S4 and S5, with a dead region in one ear only. The results
for S1 are shown in Fig. 1. Absolute thresholds in dB SPL, measured with the forced-choice procedure, are shown in the bottom
panels. Stars not connected by a line indicate frequencies falling
within the dead region. The DLFs (top panel) for the RE of S1
(with dead region) showed a very distinct dip with a minimum
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B. C. J. Moore and S. N.Vinay
Table 2 DLF values (%) for ears with dead regions with fe close to 1000 Hz
Subject
250
300
400
500
600
800
900
1000
1200
Fe
S2RE
S2LE
S8RE
S10RE
S10LE
Mean
5.2
3.3
3.1
3.5
3.2
3.6
2.8
3.7
4.2
2.3
4.5
3.4
5.6
3.1
2.4
4.1
1.7
3.1
4.7
1.2
5.2
2.2
4.8
3.1
1.1
1.7
0.5
0.7
0.6
0.8
0.5
4.0
1.3
3.7
1.2
1.6
5.5
4.9
1.8
4.6
3.6
3.8
–
–
–
–
–
–
–
–
–
–
1000
1000
1000
1000
1000
Table 3 DLF values for ears with no dead region matched to those with fe close to 1000 Hz
250
300
400
500
600
800
900
1000
1200
fe
S3RE
S3LE
S6RE
S9RE
S9LE
Mean
5.1
5.2
4.1
2.2
4.2
4.0
6.7
6.1
3.5
4.8
3.1
4.6
4.8
4.5
2.3
3.6
2.6
3.4
6.3
3.6
2.6
4.2
4.3
4.0
5.3
4.7
5.1
2.7
4.5
4.3
4.5
4.5
4.6
3.2
3.4
4.0
5.6
5.8
2.8
2.3
2.7
3.6
5.2
6.9
3.3
–
–
3.9
5.2
4.4
–
–
–
–
–
–
–
Table 4 DLF values for ears with dead regions with fe close to 1500 Hz
Subject
250
300
400
500
600
800
900
1000
1200
fe
S1RE
S4LE
S5LE
S7LE
S11RE
S11LE
Mean
4.1
4.6
3.1
5.3
4.5
4.2
4.2
3.6
5.4
2.2
3.2
3.2
3.5
3.4
4.5
4.1
3.2
2.5
3.4
3.8
3.5
5.3
2.8
1.6
3.1
3.6
4.2
3.2
4.2
1.7
4.5
1.4
1.7
3.9
2.6
1.8
3.9
4.1
2.7
2.8
0.7
2.3
0.7
4.2
1.5
0.8
4.6
0.6
1.5
0.3
1.2
1.6
0.7
0.5
3.2
0.9
3.7
3.6
3.8
3.2
2.8
4.1
3.5
1500
1500
1500
1500
1500
1500
Table 5 DLF values for ears with no dead region matched to those with fe close to 1500 Hz
Subject
250
300
400
500
600
800
900
1000
1200
fe
S1LE
S4RE
S5RE
S6LE
S12RE
S12LE
Mean
4.2
3.6
5.2
2.6
5.1
3.8
4.0
3.9
2.7
4.6
3.8
4.2
4.2
3.8
3.5
3.9
4.2
3.4
5.6
2.9
3.8
5.1
4.7
3.8
4.1
3.8
3.6
4.2
4.8
6.2
5.6
2.6
4.7
4.3
4.5
4.6
5.3
2.8
2.3
4.5
2.5
3.5
5.7
4.4
3.9
2.8
3.9
5.2
4.2
5.4
5.1
4.4
3.2
2.1
4.6
3.9
4.7
4.2
6.1
3.5
2.7
3.5
4.0
–
–
–
–
–
–
at 1000 Hz. The DLFs for the LE (no dead region) were relatively
constant as a function of frequency. The results for S4 are shown
in Fig. 2. The DLFs for the LE (with dead region) showed two dips,
the largest one falling at 1000 Hz. The DLFs for the RE (no dead
region) did not show any distinct dips but tended to be better for
frequencies below 500 Hz. The results for S5 are shown in Fig. 3.
The DLFs for the LE (with dead region) again showed two dips,
the largest one falling at 900–1000 Hz. The DLFs for the RE
(no dead region) showed a small dip at 800 Hz, but were otherwise relatively constant. In summary, the results for the three
subjects with unilateral dead regions indicate that local DLF
enhancement occurred for the ears with dead regions, at a
frequency somewhat below fe, but the enhancement did not
transfer to the opposite ears.
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Subject
Enhanced discrimination with dead regions
Brain 2009: 132; 524–536
| 531
region in the left ear (right panels). Absolute thresholds for frequencies where a dead region was diagnosed to be present are indicated
by star symbols, not connected by a line. For the DLFs, error bars show 1 SD across repetitions.
Amplitude-modulation detection
thresholds
Table 6 shows the means and SDs of the amplitude-modulation
detection thresholds, expressed as 20log10(m), for the ears with
dead regions and the ears without dead regions. For every combination of carrier frequency and modulation frequency, the mean
was lower (better) for the ears with dead regions than for the ears
without dead regions. An ANOVA was conducted on the data for
the lower two carrier frequencies only (as not all ears were tested
using the 1200 Hz carrier), with presence/absence of a dead
region as an across-subjects factor and carrier frequency and modulation frequency as within-subjects factors. The main effect of
presence/absence of a dead region was significant, F(1,20) =
15.29, P50.001, confirming that the ears with dead regions performed better than the ears without dead regions. The main effect
of carrier frequency was not significant: F(1, 20) = 0.54, P = 0.47.
The main effect of modulation frequency was significant: F(2,40)
= 10.03, P50.001. This reflects the fact that performance was
slightly poorer for the modulation frequency of 100 Hz (mean =
14.3 dB) than for the modulation frequencies of 4 and 50 Hz
(means of 15.6 and 16.2 dB, respectively). There were no
significant interactions. Thus, in the case of amplitude-modulation
detection, the ears with dead regions showed better performance
than the ears without dead regions even for a carrier frequency
(500 Hz) that was well below fe, and this effect was roughly
independent of modulation frequency.
Of particular interest here are the results for the three subjects,
S1, S4 and S5, with a dead region in one ear only. These results
are shown in Fig. 4. In each case, the amplitude-modulation
detection thresholds for the ear with a dead region are plotted
on the left while those for the ear without a dead region are
plotted on the right. For all three subjects, the thresholds
were generally (but not always) lower for the ear with dead
region than for the ear without. The thresholds averaged
across carrier frequencies, for S1, S4 and S5, respectively, were
22.0, 17.3 and 16.7 dB for the ears with dead regions and
14.9, 14.2 and 13.9 dB for the ears without dead regions.
Notice that, for each subject, the absolute thresholds were
similar across ears for the two lower carrier frequencies. Thus,
the differences across ears cannot be attributed to differences in
absolute thresholds. In the case of S1, the absolute thresholds for
the two lower carrier frequencies were close to normal, so loudness recruitment would be expected to be minimal. Thus, at
least for S1, the better performance for the ear with a dead
region cannot be attributed to differences in loudness recruitment across ears. It appears that amplitude-modulation detection
for frequencies below fe is generally better for ears with dead
regions than for (audiometrically matched) ears without dead
regions.
Consonant identification
The means (expressed as a percentage) and SDs of the consonant
identification scores for each ear are shown in Table 7. The table also
indicates whether a given ear had a dead region. Generally, the
scores were higher for the ears with dead regions (mean = 57.2%,
SD = 4.3) than for the ears without dead regions (mean = 47.3%,
Downloaded from http://brain.oxfordjournals.org/ by guest on October 1, 2016
Fig. 1 Absolute thresholds (bottom) and DLFs (top) for subject S1, who had a dead region in the right ear (left panels) and no dead
532
| Brain 2009: 132; 524–536
B. C. J. Moore and S. N.Vinay
Fig. 3 As Figure 1, but for S5, who had a dead region in the left ear, but not in the right.
SD = 4.4). An unrelated-samples t-test showed that the difference
was significant: t(20) = 5.38, P50.001. For each subject with a unilateral dead region, scores were higher for the ear with dead region
(63.5%, 56.5% and 57.7% for S1, S4 and S5, respectively) than for
the ear without dead region (56.5%, 50% and 49.4% for S1, S4
and S5, respectively). A related-samples t-test showed that the
difference in the means (59.6% versus 52.0%) was significant:
t(2) = 8.81, P = 0.013. These results indicate that consonant identification in quiet is better for ears with high-frequency dead regions
than for (audiometrically matched) ears without dead regions when
the stimuli are filtered so as to limit the speech spectrum to frequencies below fe.
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Fig. 2 As Figure 1, but for S4, who had a dead region in the left ear, but not in the right.
Enhanced discrimination with dead regions
Brain 2009: 132; 524–536
| 533
Table 6 Means and SDs of amplitude-modulation detection thresholds, expressed as 20log10(m), for the ears with dead
regions and the ears without dead regions
Carrier frequency, Hz
Modulation frequency, Hz
Mean, ears with dead region
SD, ears with dead region
Mean, ears without dead region
SD, ears without dead region
500
4
17.4
3.2
13.9
2.2
50
18.1
4.0
14.1
2.3
800
100
16.7
3.8
12.9
2.0
4
16.4
3.4
14.9
1.7
50
17.1
2.9
15.0
1.9
1200
100
15.9
1.8
11.6
3.4
5
20.9
2.4
16.3
1.8
50
19.1
2.0
14.7
2.9
100
18.8
1.4
14.8
2.6
For the carrier of 1200 Hz, the means and SDs are only for ears with fe close to 1500 Hz, and their matched counterparts.
Discussion
The present work confirms earlier work, reviewed in the
Introduction, showing that high-frequency dead regions are associated with enhanced DLFs for frequencies just below fe. For ears
with fe between 750 and 1000 Hz, the maximum enhancement
was observed for frequencies ranging from 500 to 800 Hz. For
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Fig. 4 Amplitude-modulation detection thresholds for the
subjects with a dead region in one ear (left column) but not in
the other (right column). Each row shows results for one subject. Thresholds, expressed as 20log10(m), are plotted as a
function of modulation frequency, with carrier frequency as
parameter. Error bars show 1 SD across repetitions.
ears with fe between 1000 and 1500 Hz, the maximum enhancement was observed for frequencies ranging from 900 to 1000 Hz.
For the subjects with unilateral dead regions, the enhancement did
not appear to transfer across ears. The ‘best’ DLFs for the ears
with dead regions were 0.3%, 1.2% and 1.5% for S1, S4 and S5,
respectively, while the ‘best’ DLFs for the ears without dead
regions were 3.5%, 2.7% and 2.8% for S1, S4 and S5,
respectively.
Previous studies of this effect have presented stimuli whose
mean levels fell along an equal-loudness contour, to minimize
the use of loudness cues (McDermott et al., 1998; Thai-Van
et al., 2003; Kluk and Moore, 2006b). That was not considered
necessary here, since the subjects were chosen to have reasonably
uniform (‘flat’) or smoothly varying absolute thresholds over the
frequency range where the DLFs were measured and stimuli were
presented at a constant SL. For example, for the RE of S1 (with
dead region), the absolute thresholds in dB SPL varied by only
3 dB over the range 800–1000 Hz. Given that the level of each
stimulus was roved over a range of 6 dB, it seems very unlikely
that the use of loudness cues could account for the very small DLF
of 0.3% found for the RE of S1 at 900 Hz.
Our results for the subjects with unilateral dead regions differ
from the single case reported by Kluk and Moore (2006b); they
found local DLF enhancement in an ear contralateral to an ear
with a dead region. However, in their case, the contralateral ear
had normal hearing. For our subjects, both ears had hearing loss,
and the ears were matched for the amount of hearing loss at low
frequencies. Further research is needed to clarify the conditions
under which there is transfer across ears. Our results suggest
that a dead region in one ear is not associated with enhanced
DLFs in the contralateral ear when that ear has a hearing loss
but does not have a dead region.
Our results for amplitude-modulation detection show, for the
first time to our knowledge, that a high-frequency dead region
is associated with an improved ability to detect amplitude modulation at low frequencies; amplitude-modulation detection thresholds for low-frequency carriers were lower for the ears with
dead regions than for their matched counterparts. Unlike the
DLFs, this effect was not restricted to a small frequency range
just below fe, but occurred for all carrier frequencies tested,
including 500 Hz. For the carrier frequency of 500 Hz, it is possible
that the amplitude modulation was detected via resolution of the
spectral sidebands when the modulation frequency was 100 Hz
(Kohlrausch et al., 2000); the sidebands in this case fell at 400
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| Brain 2009: 132; 524–536
B. C. J. Moore and S. N.Vinay
Table 7 The fourth column shows means and SDs of
speech intelligibility scores for each tested ear
Ear
DR?
Mean; SD
S1
RE
LE
RE
LE
RE
LE
RE
LE
RE
LE
RE
LE
LE
RE
RE
LE
RE
LE
RE
LE
RE
LE
Yes
63.5;
56.5;
59.7;
54.9;
45.6;
43.2;
50.0;
56.5;
49.4;
58.7;
52.0;
46.0;
61.2;
48.1;
41.5;
44.5;
55.0;
54.0;
56.5;
61.5;
47.0;
44.5;
S2
S3
S4
S5
S6
S7
S8
S9
S10
S11
S12
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
2.1
3.5
4.2
3.5
2.1
3.5
2.8
3.5
3.5
1.4
5.6
2.8
2.1
2.1
2.1
2.1
5.6
2.8
3.5
2.1
1.4
2.1
and 600 Hz. However, detection of spectral sidebands usually
leads to improved modulation-detection thresholds, whereas our
data showed a small worsening of modulation-detection thresholds for the 100-Hz modulation frequency, for ears both with and
without dead regions. Also, spectral sidebands are difficult to
detect at low sensation levels, as used here, especially for listeners
with hearing loss (Moore and Glasberg, 2001). Hence, we think it
is unlikely that our results were influenced by the detection of
spectral sidebands.
A potential difficulty in interpreting improved amplitude-modulation detection thresholds is that cochlear hearing loss and the
associated loudness recruitment can lead to improved amplitudemodulation detection at low SLs; for a review, see Moore (2007).
Indeed, this is the basis for the SISI test (Jerger, 1962; Buus et al.,
1982a, b). However, we believe that this is unlikely to have influenced the difference between results for the ears with and without
dead regions, since the ears were matched in terms of low-frequency audiometric thresholds, and the amount of loudness
recruitment caused by cochlear hearing loss is closely related to
the amount of hearing loss (Miskolczy-Fodor, 1960; Moore et al.,
1999; Moore and Glasberg, 2004). Also, even for subjects like S1,
who had normal hearing for frequencies of 1000 Hz and below,
amplitude-modulation detection thresholds were lower for the ear
with a dead region than for the ear without a dead region. Thus,
the most likely explanation for our finding of better amplitudemodulation detection for ears with than without dead regions is
that it is a consequence of cortical reorganization for the ears with
dead regions. It is, of course, possible that cortical reorganization
might itself lead to a form of loudness recruitment. The cortical
over-representation of the low-frequency region of the cochlea
that follows a high-frequency dead region might lead to a more
Downloaded from http://brain.oxfordjournals.org/ by guest on October 1, 2016
Subject
rapid than normal growth of loudness with increasing sound level,
and that in itself might lead to improved amplitude-modulation
detection.
Our results for consonant identification indicated that performance was better for ears with than without dead regions when
the stimuli were lowpass filtered so as to restrict the spectrum to
the region below fe, and were presented with a complementary
highpass noise to prevent the use of information from neurons
tuned above fe (in ears without dead regions). In other words,
the ears with dead regions used low-frequency information more
effectively than the matched ears without dead regions. This is
consistent with the results of Vestergard (2003), which were
described in the Introduction, although he did not compare
groups of subjects who were matched for their low-frequency
hearing loss. The effect occurred for the three subjects with unilateral dead regions, all of whom showed better identification of
lowpass filtered speech in the ears with dead regions. Assuming
that the effect reflects cortical reorganization, this indicates that
the reorganization is ear-specific, which is consistent with the findings for frequency-discrimination and amplitude-modulation
detection.
The finding of better use of low-frequency speech cues by ears
with dead regions may have implications for rehabilitation. The
finding might indicate that, for people with long-standing highfrequency dead regions, parts of the brain that normally respond
to high-frequency sounds are ‘taken over’ by low-frequency
sounds, becoming devoted to the analysis of such sounds. A possible treatment option for people with extensive high-frequency
dead regions is a cochlear implant. Such an implant potentially
provides information about high-frequency sounds. However, if
the parts of the brain that normally respond to high-frequency
sounds have become devoted to the analysis of low-frequency
sounds, this may limit the effectiveness of a cochlear implant
(Sharma et al., 2007). An alternative treatment option for
people with extensive high-frequency dead regions is frequency
transposition, whereby the high-frequency speech energy associated with sounds like fricatives (e.g. ‘s’ or ‘sh’) is transposed to
lower frequencies, making information about the fricatives audible
(Simpson et al., 2005; Robinson et al., 2007). Our finding of
enhanced discrimination of low-frequency sounds by people with
high-frequency dead regions suggests that such people may be
able to make effective use of the transposed speech sounds.
We have discussed our results in terms of cortical reorganization. However, since this was a behavioural study, we have no
way of knowing whether the measured effects arose from cortical
plasticity or from plasticity at some other level in the auditory
system, for example in the brainstem. Indeed, there might be
plasticity at many levels in the auditory system. Imaging studies
might be used to shed some light on this issue. However, at present, knowledge of the function of higher levels of the human
auditory system is limited. For example, the human auditory
cortex comprises multiple areas, but the precise number and configuration of these areas has not been clearly established, and their
functional significance remains unclear (Hall et al., 2003).
Therefore, the interpretation of imaging studies and their relationship to auditory plasticity remain somewhat speculative.
Enhanced discrimination with dead regions
Conclusions
We compared performance on three tasks for two groups of subjects, with and without acquired high-frequency dead regions, as
assessed using the TEN(HL) test. For the ears with dead regions,
the value of fe was close to 1000 or 1500 Hz. The two groups
were matched in terms of audiometric thresholds for frequencies
below fe and in terms of age. Three subjects with unilateral dead
regions (with matched low-frequency audiometric thresholds
across ears) were also tested. The results showed:
We conclude that an acquired dead region at high frequencies
is associated with a better ability to process information at low
frequencies for frequency discrimination, amplitude-modulation
detection and consonant identification. The effects are earspecific. These effects may reflect cortical plasticity induced by
the dead regions.
Acknowledgements
We thank the Director, All India Institute of Speech and Hearing,
for providing the facilities to carry out this work. We thank
Aleksander Sek for writing the software for conducting the frequency-discrimination and amplitude-modulation detection tasks,
Brian Glasberg for help with statistical analyses, and Karolina Kluk
and Christian Füllgrabe for helpful comments on an earlier version
of this article. We also thank two anonymous reviewers for helpful
comments.
Funding
Medical Research Council (UK to B.C.J.M.).
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