Distortion product otoacoustic emission suppression tuning curves in normal-hearing and hearing-impaired human ears
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Distortion product otoacoustic emission suppression tuning
curves in normal-hearing and hearing-impaired human ears
Michael P. Gorga,a) Stephen T. Neely, Darcia M. Dierking, Patricia A. Dorn,
Brenda M. Hoover, and Denis F. Fitzpatrick
Boys Town National Research Hospital, Omaha, Nebraska 68131
共Received 17 November 2002; revised 20 March 2003; accepted 21 March 2003兲
Distortion product otoacoustic emission 共DPOAE兲 suppression measurements were made in 20
subjects with normal hearing and 21 subjects with mild-to-moderate hearing loss. The probe
consisted of two primary tones ( f 2 , f 1 ), with f 2 held constant at 4 kHz and f 2 / f 1 ⫽1.22. Primary
levels (L 1 ,L 2 ) were set according to the equation L 1 ⫽0.4L 2 ⫹39 dB 关Kummer et al., J. Acoust.
Soc. Am. 103, 3431–3444 共1998兲兴, with L 2 ranging from 20 to 70 dB SPL 共normal-hearing subjects兲
and 50–70 dB SPL 共subjects with hearing loss兲. Responses elicited by the probe were suppressed by
a third tone ( f 3 ), varying in frequency from 1 octave below to 21 octave above f 2 . Suppressor level
(L 3 ) varied from 5 to 85 dB SPL. Responses in the presence of the suppressor were subtracted from
the unsuppressed condition in order to convert the data into decrements 共amount of suppression兲.
The slopes of the decrement versus L 3 functions were less steep for lower frequency suppressors
and more steep for higher frequency suppressors in impaired ears. Suppression tuning curves,
constructed by selecting the L 3 that resulted in 3 dB of suppression as a function of f 3 , resulted in
tuning curves that were similar in appearance for normal and impaired ears. Although variable, Q 10
and Q ERB were slightly larger in impaired ears regardless of whether the comparisons were made at
equivalent SPL or equivalent sensation levels 共SL兲. Larger tip-to-tail differences were observed in
ears with normal hearing when compared at either the same SPL or the same SL, with a much larger
effect at similar SL. These results are consistent with the view that subjects with normal hearing and
mild-to-moderate hearing loss have similar tuning around a frequency for which the hearing loss
exists, but reduced cochlear-amplifier gain. © 2003 Acoustical Society of America.
关DOI: 10.1121/1.1575751兴
PACS numbers: 43.64.Ha, 43.64.Jb 关BLM兴
I. INTRODUCTION auditory status at mid- and high frequencies, performing less
accurately at lower frequencies. These efforts have focused
Distortion-product otoacoustic emissions 共DPOAE兲 are primarily on the ability of DPOAEs to identify auditory sta-
a byproduct of normal nonlinear cochlear processes. Their tus, as defined by the pure-tone audiogram. As such, they
generation is linked to the status of the outer hair cells have focused on dichotomous decisions in which DPOAE
共OHC兲. Recent findings, in which DPOAE data were col- measurements were used to classify an ear as having either
lected in mice for which the putative ‘‘motor’’ molecule normal hearing or hearing impairment. To a lesser extent,
共prestin兲 was knocked out, are consistent with the view that others have described the relation between behavioral thresh-
DPOAEs are generated by OHC motility 共Liberman et al., olds and DPOAEs, by relating DPOAE threshold and audi-
2002兲. Under appropriate stimulus conditions, the normal- tory thresholds 共Martin et al., 1990; Gorga et al., 1996;
hearing ear 共having normal OHC function兲 produces these Boege and Janssen, 2002; Gorga et al., 2003兲, or by relating
distortion products in response to pairs of primary tones. DPOAE level and auditory threshold 共Martin et al., 1990;
Because OHC damage results in a reduction or loss of nor- Dorn et al., 2001; Gorga et al., 2002a兲.
mal nonlinear behavior 共e.g., Dallos et al., 1980; Kim, 1980兲, Threshold elevation, however, is only one of several
one manifestation of damage to the normal nonlinear system consequences of OHC damage. There are both mechanical
共i.e., the OHC system兲 is the reduction or loss of DPOAEs. It and neural data from lower animals, indicating that fre-
also is known that damage to OHCs results in auditory quency selectivity may be reduced and response growth may
threshold elevation. become more rapid when OHC damage exists 共e.g., Evans,
The relationships among OHC status, auditory function, 1974; Kiang et al., 1976; Dallos and Harris, 1978; Liberman
and DPOAE levels have led to the application of DPOAE and Dodds, 1984; Sewell, 1984; Gorga and Abbas, 1981b;
measurements to the task of identifying hearing loss. Several Ruggero and Rich, 1991兲. Frequency selectivity in these
studies have described DPOAE measurements in normal- studies typically is defined by the parametrization of tuning
hearing and hearing-impaired ears 共e.g., Martin et al., 1990; curves, using measures such as Q 10 and differences in thresh-
Gorga et al., 1993, 1996, 1997, 2000; Kim et al., 1996兲. old at the tip and on the tail of tuning curves. Response
These studies have shown that DPOAEs accurately identify growth refers to the rate at which either discharge rate 共for
single-unit studies兲, displacement or velocity 共for basilar-
a兲
Electronic mail: gorga@boystown.org membrane studies兲, or masking 共for whole-nerve action po-
J. Acoust. Soc. Am. 114 (1), July 2003 0001-4966/2003/114(1)/263/16/$19.00 © 2003 Acoustical Society of America 263tential studies兲 grows as stimulus level is increased. These abnormal growth of loudness兲 so that eventually the percep-
‘‘suprathreshold’’ effects may be related to perceptual phe- tual consequences can be predicted from the objective mea-
nomena such as the reduced speech-perception abilities or sures. This long-term goal has clinical importance related to
loudness recruitment that sometimes are associated with pe- the identification and the rehabilitation of hearing loss 共such
ripheral hearing loss. For example, compensating for abnor- as the selection of hearing-aid characteristics兲 in infants and
mally rapid growth of loudness is the motivation for includ- young children who may be unable to provide behavioral
ing compression in many hearing aids. In one sense, the responses to sound, especially subjective judgments such as
compression circuit of the hearing aid is attempting to com- loudness. While a great deal is already known about the re-
pensate for the loss of the compressive behavior of the nor- lationship between DPOAE measurements and sensitivity
mal cochlea. loss 共e.g., Martin et al., 1990; Gorga et al., 1993, 1997,
While direct measurements of frequency selectivity or 2000; Stover et al., 1996; Kim et al., 1996; Boege and Jan-
response growth are not possible in humans, indirect mea- ssen, 2002兲, the present work is viewed as an initial step
sures may provide insight into underlying properties related towards understanding the relationship between DPOAE
to cochlear processing. Psychoacoustic and electrophysi- measurements and suprathreshold consequences of cochlear
ologic masking studies are motivated by a desire to indirectly damage. The primary aim is to determine whether DPOAE
estimate representations of cochlear properties that have measurements of frequency selectivity and response growth
been identified from more direct, invasive measurements in are possible in subjects having hearing loss. This initial step
lower animals. Recently, for example, Oxenham and Plack is needed prior to efforts to correlate DPOAE data with su-
共1997兲 related the growth of forward masking 共measured be- prathreshold perceptual phenomena because of the tendency
haviorally兲 to nonlinear cochlear processing, based on a for DPOAEs to be reduced or absent when hearing loss ex-
comparison of on-frequency and low-frequency masking ef- ists. If DPOAE suppression studies can be performed in hu-
fects. mans with hearing loss, future work will determine the ex-
DPOAE measurements in a suppression paradigm have tent to which these measurements correlate with behavioral
also been used to describe peripheral response properties in findings.
humans. Several studies have shown that DPOAE suppres-
sion measurements provide information related to the site of II. METHODS
generation for DPOAEs and the tuning properties of the co-
chlea at the place共s兲 of DPOAE generation, at least for ears A. Subjects
with normal cochleae 共e.g., Brown and Kemp, 1984; Martin Twenty-two ears of 20 subjects with normal hearing and
et al., 1987兲. Furthermore, studies in humans have shown 29 ears of 25 subjects with hearing loss participated in these
that the reductions in DPOAE levels in the presence of a studies. The two subject groups differed in age, with the
suppressor 共Abdala, 1998, 2001; Abdala et al., 1996; Kum- normal-hearing subjects being younger 共mean⫽24.9 years,
mer et al., 1995; Gorga et al., 2002b兲 share a dependence on s.d.⫽13.4 years兲 than the group of subjects with hearing loss
frequency that is similar to other measures of response 共mean⫽57.3 years, s.d.⫽14.6 years兲. It was difficult to re-
growth such as single-unit rate-level functions 共Sachs and cruit older normal-hearing subjects or younger hearing-
Abbas, 1974; Schmiedt and Zwislocki, 1980兲 and measure- impaired subjects, due to factors associated with subject
ments of basilar-membrane motion 共Ruggero and Rich, availability. Thus, there is concern that some of the results
1991; Ruggero et al., 1997兲, that were based on more direct reviewed below might have been influenced by age rather
measurements in lower animals. These similarities have led than hearing loss. As will be seen, however, it is not obvious
us to pursue DPOAE suppression measurements in patients how age could have affected the data in such a way as to
with cochlear hearing loss in order to determine whether result in the outcomes that were observed.
changes in response growth and tuning, evident in direct Normal hearing was defined as audiometric thresholds
measurements from lower animals, can be indirectly ob- less than or equal to 15 dB HL 共re: ANSI, 1996兲 at 4 kHz.
served in humans using the same noninvasive DPOAE tech- The mean threshold for subjects in the normal-hearing group
niques that have been applied with humans having normal at 4 kHz 共the probe frequency; see below兲 was 5.7 dB HL
auditory thresholds. 共s.d.⫽4.4 dB兲. Subjects with thresholds of 20 dB HL or
The present study describes our initial efforts to measure greater at 4 kHz were classified as hearing impaired. It was
DPOAE suppression in human subjects with mild-to- assumed that the hearing losses in these subjects were of
moderate hearing loss. Subjects with normal hearing are in- cochlear origin, based upon their case histories, pure-tone
cluded for comparison purposes. Of primary interest was de- audiometry, acoustic immittance measures, and prior results
termining whether reduced frequency selectivity and/or more of any additional diagnostic tests. However, a precise diag-
rapid response growth, predicted from single-unit and nosis of etiology was not available for the majority of the
basilar-membrane studies in lower animals, can be observed subjects with hearing loss. This ‘‘diagnostic ambiguity’’
in indirect DPOAE measurements in humans. A long-term highlights one of the issues that is confronted whenever stud-
goal of our research program has always been to determine ies are conducted in humans. Whereas in animal studies, co-
whether there are relationships between objective measure- chlear damage is typically induced and, therefore, controlled,
ments of cochlear function 共such as DPOAE and/or evoked such control is frequently impossible in studies that involve
potential 共EP兲 measurements兲 and perceptual consequences humans with hearing loss. In fact, it is difficult to find a
of damage to the cochlea 共such as threshold elevation or homogeneous group of human subjects with hearing loss.
264 J. Acoust. Soc. Am., Vol. 114, No. 1, July 2003 Gorga et al.: DPOAE suppression tuning curvesSubjects with hearing loss were selected according to these cavities may not provide a good model for individual
their thresholds at 4 kHz, which was the f 2 frequency that ears. The best solution might be one in which acoustic inten-
was used as the ‘‘probe frequency’’ during the DPOAE sup- sity is measured 共Neely and Gorga, 1998兲. That approach,
pression experiments. Special efforts were made to include however, has not been implemented in any widely available
subjects for whom the magnitude of the loss was between 20 device or software system. Thus, while it is recognized that
and 55 dB HL, on the assumption that it would be more real-ear calibration measurements may introduce some er-
likely that DPOAE level would be too reduced or the re- rors, it is viewed as an acceptable compromise, given the
sponse would be completely absent for ears with hearing current state of calibration methods.
losses exceeding the upper limit of this range. Even so,
DPOAE suppression experiments were not possible in four C. Procedures
of the 25 subjects with hearing loss because they did not Both DPOAE and noise levels were estimated from the
produce responses of sufficient level to permit reliable esti- energy in the 2 f 1 – f 2 frequency bin. During data collection,
mates of the suppression of that response. Thus, the data
each 2-s sample was alternately stored in one of two buffers.
reported below are based on measurements from 23 ears of
In order to estimate DPOAE level, the contents of the two
21 subjects with mild-to-moderate hearing loss, who repre-
buffers were summed. Noise level was estimated by subtract-
sent a subset of the potential subjects with hearing loss in
ing the contents of one buffer from the contents of the other
this range. The mean thresholds at 4 kHz for the group of
buffer. This approach is attractive in that signal and noise are
subjects with hearing loss was 36.3 dB HL 共s.d.⫽9.1 dB兲.
estimated within the same frequency bin. However, it has the
Thus, on average, the subjects with hearing loss had thresh-
disadvantage of providing a more variable estimate of noise
olds that were 30.6 dB higher than thresholds for the normal-
level, compared to the case when noise levels are estimated
hearing group.
from the contents of several bins adjacent to the 2 f 1 – f 2
共signal兲 frequency bin.
B. Stimuli
Measurement-based stopping rules were used during
DPOAE data were collected with custom-designed soft- DPOAE measurements. For each condition, averaging con-
ware 共EMAV, Neely and Liu, 1994兲. This system controlled a tinued 共1兲 until the noise floor was ⫺25 dB SPL or less, or
sound card 共CardDeluxe, Digital Audio Labs兲 that was 共2兲 for 32 s of artifact-free averaging, whichever occurred
housed in a PC. Separate channels of the sound card were first. The noise-stopping rule allowed us to measure
used to output the two primary tones ( f 1 and f 2 ) that were DPOAEs 共and, therefore, suppression兲 over a wide dynamic
mixed acoustically in the ear canal. The channel that was range and still be confident that measured responses were
used to generate f 2 was also used to generate the suppressor above the level at which system distortion occurred 共see
tone ( f 3 ). The output of the sound card was delivered to a Dorn et al., 2001 for a more complete description of system
probe–microphone system 共Etymotic, ER 10C兲. This system distortion for the present measurement system兲. The time
was modified in order to remove 20 dB of attenuation from limit prevented data collection from continuing indefinitely
each channel, thus permitting the presentation of suppressors for any single stimulus condition.
at higher levels than would be otherwise possible. Separate In each subject, a DPOAE input/output 共I/O兲 function
loudspeakers in the probe were used to transduce the outputs was measured when f 2 ⫽4 kHz. These initial measurements
from the two separate channels of the sound card. The were needed in order to select the range of L 2 levels for each
probe’s microphone was used to measure levels at the plane subject over which the suppression experiments could be
of the probe. conducted. Figure 1 shows mean DPOAE I/O functions for
All DPOAE data were collected for the condition in both normal-hearing and hearing-impaired subjects. The
which f 2 was fixed at 4 kHz, f 2 / f 1 ⫽1.22, the level of mean noise floor is also shown. Note that DPOAE levels
f 2 (L 2 ) was varied from 20 to 70 dB SPL 共in 10-dB steps兲, were less in impaired ears at the same L 2 levels. The re-
and, for each L 2 , L 1 was set according to the equation, L 1 sponse levels at 50–70 dB SPL in impaired ears were more
⫽0.4L 2 ⫹39 dB 共Kummer et al., 1998兲. A third tone, f 3 , was like the levels observed at 20– 40 dB SPL in normal ears.
used to suppress the response elicited by the primary tones. Significant differences in DPOAE level were observed when
Seventeen suppressor frequencies were used, ranging from 1 normal and impaired responses were compared at the same
octave below 共2 kHz兲 to 21 octave above 共5.6 kHz兲 f 2 . Sup- SPL. Differences were not significant when the levels in im-
pressor level (L 3 ) was varied in 5-dB steps from 5 dB SPL paired ears at 50–70 dB SPL were compared to the levels
up to a maximum level of 85 dB SPL. observed in normal ears at 20– 40 dB SPL. That is, if the
Prior to data collection, real-ear measurements at the results are shifted in impaired ears by 30 dB 共an amount that
plane of the probe were used to calibrate the stimuli. This is nearly equivalent to the mean behavioral threshold differ-
calibration procedure may introduce errors, especially for ence between groups兲, the levels produced by both groups
frequencies close to the probe frequency used in the present were similar. Another way of thinking about these findings is
experiment ( f 2 ⫽4 kHz). These errors are a consequence of that normal and impaired ears produced different DPOAE
standing-wave problems introduced by an interaction be- levels when comparisons were made at similar SPL, but not
tween the quarter wavelength of the probe frequency and the when comparisons were made at levels that were approxi-
dimensions of adult ear canals with a probe system in place mately equivalent in terms of sensation level 共SL兲. We will
共Siegel, 1994, 2002兲. However, the use of standard cavities return to this SPL/SL comparison when considering the pri-
for calibration also introduces errors related to the fact that mary findings of the present study.
J. Acoust. Soc. Am., Vol. 114, No. 1, July 2003 Gorga et al.: DPOAE suppression tuning curves 265DPOAE data into amount of suppression 共in dB兲, and they
partially account for variance in absolute DPOAE level
across subjects. Decrement-versus-L 3 functions for each f 3
were fit with a linear equation in order to provide an estimate
of response growth to the suppressor at the place where 共pre-
sumably兲 the 2 f 1 – f 2 distortion product was initially gener-
ated. The fits were restricted to conditions in which the
SNR⭓3 dB, at least one decrement was required to be in the
range from 3 to 15 dB, and, outside of this range, decrements
were required to increase monotonically with respect to L 3 .
This combination of inclusion criteria prevented the inclu-
sion of any conditions in the fits in which no suppression
occurred or in which the response was completely sup-
pressed. In an effort to include data points in the linear fits
for which the SNR was high 共i.e., points for which the dec-
rement was as little as 1 dB兲, these data were transformed by
the equation
FIG. 1. Mean DPOAE and noise level 共dB SPL兲 as a function of L 2 共dB D⫽10 log共 10decr/10⫺1 兲 . 共1兲
SPL兲. Circles represent data from normal ears, while triangles represent data
from ears with hearing loss. Open and filled symbols represent DPOAE and
noise levels, respectively. Error bars represent 1 s.d. Data points were offset
slightly in the L 2 dimension to help visualization. When D⫽0, the decrement⫽3 dB. This transformation had
the effect of linearizing the decrement functions. Finally, in
order to reduce the influence of points with low SNRs, data
The influence of the stopping rule is evident in the con- points were weighted by the following equation:
stancy of noise levels across L 2 and between the two subjects
groups. For each subject, suppression experiments were per-
formed at those L 2 levels for which at least a 10-dB signal- SNR weight⫽ 共 10signal/10兲 / 共 10signal/10⫹10noise/10兲 . 共2兲
to-noise ratio 共SNR兲 was evident in the previously measured
I/O functions. As can be seen in Fig. 1, sufficient DPOAE
level was measured over the range of L 2 levels from 20 to 70 It should be noted, however, that there was little difference
dB SPL in subjects with normal hearing. The range of levels between weighted and unweighted fits to the data. In fact, the
was more restricted in subjects with hearing loss, being lim- two fits superimposed for the majority of conditions. Figure
ited to L 2 levels of 50–70 dB SPL. 2 summarizes the effects of the above treatments to the data
In each set of suppression measurements, L 2 was fixed for one subject with normal hearing. Circles represent the
at one of six levels in normal ears 共20–70 dB SPL, 10-dB unprocessed decrements with signal-to-noise ratios
steps兲 or one of three levels in impaired ears 共50–70 dB 共SNR兲⭓3 dB, triangles represent the same data after trans-
SPL兲. Experimental conditions were then selected such that formation by the above equation, and the lines represent the
f 3 was fixed and its level was varied from 5 to 85 dB SPL weighted fits to the transformed data. The small filled circles
共5-dB steps兲. DPOAE levels were measured for each of these represent conditions not meeting the SNR criterion 共SNR⭐3
17 suppressor levels at each f 3 frequency. However, the ini- dB兲. Thus, they represent conditions in which the DPOAE
tial condition prior to the presentation of each f 3 was a con- was at a level that was nearly or completely suppressed. The
trol condition, in which no suppressor was presented. Once top, middle, and bottom panels show the data for the cases in
an L 3 series had been completed for one f 3 , a different f 3 which the suppressor ( f 3 ) was below 共2.2 kHz兲, close to 共4.1
was selected and the entire process was repeated. This ap- kHz兲, and above 共4.8 kHz兲 f 2 . As can be seen, the transfor-
proach continued until measurements were completed for mation’s only effect was to extend the usable range of data,
each of 17 suppressor frequencies. For each subject, no more and the weighted functions provide good fits to the data.
than 12 hours of data-collection time was required, depend- These trends were evident for decrement functions in both
ing mainly on the range of L 2 levels for which reliable normal-hearing and hearing-impaired ears. The linear equa-
DPOAEs could be measured. This means that data-collection tions were solved for the suppressor level that resulted in a
time was greater for subjects with normal hearing 共for whom decrement of 3 dB 共i.e., D⫽0, or 3 dB of suppression兲.
measurements were possible at up to six L 2 levels兲, but less These ‘‘threshold’’ decrements were plotted as a function of
time was required for subjects with hearing loss. f 3 in order to generate DPOAE suppression tuning curves
Following data collection, DPOAE levels were con- 共STC兲. Because of the differences in L 2 conditions between
verted to decrements 共or amount of suppression兲 by subtract- subject groups, these tuning curves were constructed for L 2
ing the level measured in each suppressor condition from the levels from 20–70 dB SPL in normal-hearing subjects, but
level measured in the preceding control condition. There were restricted to probe levels of 50–70 dB SPL in subjects
were several reasons why the data were converted into dec- with hearing loss, due to the smaller responses for the control
rements. Decrements have the advantage of converting the condition in hearing-impaired ears.
266 J. Acoust. Soc. Am., Vol. 114, No. 1, July 2003 Gorga et al.: DPOAE suppression tuning curvesdata for the opposite case ( f 3 ⬎ f 2 ). Each row represents data
for a different probe level (L 2 ). The heavy line represents
data for the f 3 frequency closest to f 2 for the set of f 3 fre-
quencies shown in each panel. Thinning lines represent data
for f 3 frequencies that are increasingly distant from f 2 . Con-
sistent with previous data describing DPOAE suppression
共e.g., Martin et al., 1987; Kummer et al., 1995; Abdala et al.,
1996; Gorga et al., 2002b兲, suppression occurred at the low-
est levels when f 3 was close to f 2 , with increasing suppres-
sion ‘‘thresholds’’ as f 3 moved away from f 2 in either direc-
tion. In addition, the functions for lower f 3 frequencies
relative to f 2 were steeper, compared to the functions when
f 3 ⬎ f 2 . These trends were evident at all L 2 levels, although
it became progressively more difficult to suppress the
DPOAE as L 2 increased. An estimate of the dynamic range
of the measurements can be obtained by examining the level
at which the decrement functions ‘‘saturate.’’ When this oc-
curs, the DPOAE was completely suppressed into the noise
floor. Examination of these decrement functions indicate that
the dynamic range of the present measurements varied from
about 20 dB (L 2 ⫽20 dB SPL) up to more than 30 dB (L 2
⫽50, 60, and 70 dB SPL兲.
B. Decrement versus suppressor-level functions in
hearing-impaired ears
In similar fashion, Fig. 4 presents mean decrements as a
function of L 3 for subjects with hearing loss. In all respects,
the conventions followed in this figure are identical to those
used in Fig. 3. However, data collection was restricted to L 2
levels of 50 dB SPL or higher. Decrement functions from
impaired ears tended to saturate at lower levels, compared to
similar conditions in subjects with normal hearing. Thus, the
dynamic range of these measurements was reduced, which
probably is due to the same mechanisms that resulted in the
reduced L 2 range 共of unsuppressed conditions兲 over which
measurable DPOAEs were observed. With the exception of
differences in terms of the probe levels (L 2 ’s兲 for which the
experiments could be performed and the reduced dynamic
range, there were no apparent differences between these dec-
rement functions and those observed in subjects with normal
hearing. That is, thresholds were lowest for f 3 frequencies
close to f 2 , and the decrement functions appear steeper
FIG. 2. Decrement 共in dB兲 as a function of suppressor level (L 3 ) for a when f 3 ⬍ f 2 , compared to the case when f 3 ⬎ f 2 .
low-frequency 共2.2 kHz, top兲, on-frequency 共4.1 kHz, middle兲, and high-
frequency suppressor 共4.8 kHz, bottom兲, relative to f 2 for one normal-
hearing subject. L 2 ⫽30 dB SPL. Open circles represent the decrements for
C. Slopes of decrement versus suppressor-level
which the SNR was at least 3 dB, small filled circles represent data for cases functions
in which the SNR criterion was not met, triangles represent the transformed
decrement data meeting the SNR criterion 共see the text兲, and the lines rep-
Each decrement versus L 3 function was fit with a linear
resent linear fits after weighting data according to the SNR. Line fits were regression in order to provide an estimate of the slope of
restricted according to the rules described in the text. these functions, as described previously. Figure 5 plots the
mean slopes from these linear regressions as a function of
III. RESULTS f 3 . In all cases, solid lines are used to depict the estimated
slopes from ears with normal hearing and dotted lines are
A. Decrement versus suppressor-level functions in used to represent the data from ears with hearing loss. Each
normal-hearing ears
panel shows data for a different L 2 . For the reasons de-
Figure 3 shows mean decrement-versus-L 3 functions for scribed above, slope could be estimated for L 2 levels ranging
subjects with normal hearing. The left column represents from 20 to 70 dB SPL in subjects with normal hearing, but
data for the case when the suppressor was lower in frequency could be estimated only for levels from 50 to 70 dB SPL in
than the probe ( f 3 ⬍ f 2 ), while the right column represents subjects with hearing loss.
J. Acoust. Soc. Am., Vol. 114, No. 1, July 2003 Gorga et al.: DPOAE suppression tuning curves 267FIG. 3. Mean decrements 共in dB兲 as a
function of suppressor level (L 3 ) for
subjects with normal hearing. The left
column shows data for the case when
the suppressor frequency ( f 3 ) was less
than the probe frequency ( f 2 ), while
the right column represents the results
when f 3 was greater than f 2 . Each
row presents data for a different probe
level (L 2 ), ranging from 20 to 70 dB
SPL. Within each panel, the heavy
lines represent decrements for the f 3
closest to f 2 of the set of f 3 frequen-
cies represented in each panel. Thin-
ning lines represent data for f 3 fre-
quencies increasingly more distant
from f 2 .
These slope estimates were variable across f 3 , but In order to analyze these frequency effects on slope of
tended to decrease as f 3 increased both for normal and im- the decrement functions, frequency was divided into two
paired ears. For subjects with normal hearing, the slopes groups, f 3 ⬍ f 2 and f 3 ⬎ f 2 . Within these groups, the slopes
were generally greater than 1 for f 3 frequencies below f 2 , for individual f 3 frequencies were averaged to provide single
decreasing rapidly as f 3 moved towards frequencies just estimates for low- and high-frequency suppressors. In addi-
above f 2 . A similar pattern was observed in subjects with tion, stimulus level was treated in two different ways. In the
hearing loss, although the apparent differences in slope as a first analysis, average slopes for normal and impaired ears
function of frequency were less. Shallower slopes were ob- were compared when stimulus level (L 2 ) was constant for
served for the highest f 3 frequencies in both subject groups; the two groups. This means that data were compared when
however, the functions were not always monotonic with fre- L 2 was 50–70 dB SPL for both groups of subjects. In a
quency. The slopes of decrement functions in subjects with second analysis, data for normal ears at L 2 levels of 20– 40
hearing loss appeared to be slightly shallower on the low- dB SPL were compared to data from impaired ears when
frequency side and slightly steeper on the high-frequency L 2 ⫽50– 70 dB SPL. Two observations from the present
side, compared to similar data from ears with normal hear- study provide support for applying this 30-dB shift in the
ing. range of levels over which comparisons between the two
268 J. Acoust. Soc. Am., Vol. 114, No. 1, July 2003 Gorga et al.: DPOAE suppression tuning curvesFIG. 4. Same as Fig. 3, only here data are shown for subjects with hearing loss. Note that data are shown only for L 2 levels of 50, 60, and 70 dB SPL, for
the reasons described in the text.
groups were made. Recall that the mean normal audiometric D. DPOAE suppression tuning curves „STC…
threshold was 5.7 dB HL while the mean thresholds for the
For each L 2 , the linear regressions were solved for the
subjects with hearing loss was 36.3 dB HL, a 30.6-dB dif-
L 3 that resulted in 3 dB of suppression for each f 3 frequency.
ference. In addition, the DPOAE levels differed between the
These levels were then plotted as a function of f 3 to produce
two groups when these levels were compared at the same
DPOAE STCs, which are shown in Fig. 6 for one subject
SPL, but did not differ when the levels produced by impaired
with normal hearing and three subjects with hearing loss.
ears at 50–70 dB SPL were compared to the levels produced
The parameter in each panel is L 2 . The symbols represent
by the normal-hearing group at 20– 40 dB SPL, a 30-dB shift
the L 3 necessary for 3 dB of suppression for each f 3 . Also
for the normal ears 共see Fig. 1兲. Thus, we consider the 30-dB
shift as something akin to making the comparison at equiva- shown are the values of Q 10 , Q ERB , and tip-to-tail differ-
lent SL, at least on average. ences for each STC. Qs were estimated from spline fits to the
Analysis of variance 共ANOVA兲 was used to evaluate the data, while tip-to-tail differences represent the dB differences
effects of frequency 共either above or below f 2 ), level, and between suppression thresholds at f 3 ⫽2.2 kHz and f 3
hearing-status group on estimates of slope. For both equiva- ⫽4.1 kHz. In the normal-hearing subject, sharp tuning was
lent SPL and equivalent SL, the outcomes of the ANOVA evident around the tip, and the difference between thresholds
were the same. Significant effects were observed for fre- at the tip and on the low-frequency tail was 40 dB when
quency and for a frequency⫻hearing-status group interac- L 2 ⫽20 dB SPL. For this subject, there was a systematic de-
tion. That is, the slopes were steeper for suppressors below crease in Q 10 , Q ERB , and tip-to-tail differences as L 2 in-
f 2 compared to the slopes for suppressors above f 2 in both creased, primarily as a result of changes in suppression
groups, but the differences depended on hearing status. Sub- threshold around the tip. Less change was evident on the tail.
jects with normal hearing had greater differences between Data from three subjects with hearing loss are shown in the
slopes above and below f 2 , compared to subjects with hear- next three panels. While the data from impaired ears were
ing loss. This occurred because there was less difference in less orderly compared to the data from the subject with nor-
slope for low- and high-frequency suppressors in subjects mal hearing, the general patterns were similar. At the same
with hearing loss. This observation indicates that, at the SPL, the STCs from the impaired ears appeared similar, and
place where the DPOAE was generated, the growth of re- Q- and tip-to-tail values were grossly in the range observed
sponse for off-frequency and on-frequency stimuli was more for the subject with normal hearing.
similar in ears with hearing loss. Figure 7 shows mean STCs for both normal-hearing and
J. Acoust. Soc. Am., Vol. 114, No. 1, July 2003 Gorga et al.: DPOAE suppression tuning curves 269FIG. 6. Suppressor level (L 3 ) necessary for 3 dB of suppression as a func-
tion of f 3 for a subject with normal hearing 共top兲 and for three subjects with
hearing loss 共bottom three panels兲. Within each panel, the parameter is L 2 .
Q 10 , Q ERB , and tip-to-tail differences for each tuning curve are given
within each panel.
in ears with hearing loss 共primarily because there was a
smaller range of L 2 levels to consider兲, the best frequency
FIG. 5. Mean slopes of decrement versus suppressor level (L 3 ) functions as also shifted towards lower frequencies as L 2 increased. There
a function of suppressor frequency ( f 3 ). Each panel represents data for a was a tendency for the suppression thresholds on the tails of
different L 2 . Within each panel, data from normal-hearing subjects are the STCs to occur at a lower level for ears with hearing loss,
shown as a solid line, and data from subjects with hearing loss are shown as
compared to thresholds for similar frequencies in normal
a dotted line.
ears.
hearing-impaired subjects. Each panel represents data for a
E. Q 10 and Q ERB
different L 2 , with solid lines representing data for normal
ears, and dotted lines representing data for ears with hearing Figure 8 provides a scatter plot of the individual values
loss. As expected from the previous discussion, DPOAE of Q 10 and Q ERB 共top and bottom rows, respectively兲 as a
STCs could be constructed for L 2 levels ranging from 20 to function of audiometric threshold. Q 10 is defined as the best
70 dB SPL in ears with normal hearing and from 50 to 70 dB frequency ( f 3 frequency with the lowest suppression thresh-
SPL in ears with hearing loss. old兲 divided by the bandwidth at a level 10 dB above the
STCs in normal and impaired ears appeared to be similar suppression threshold at best frequency 共BF兲. Q ERB is de-
at the three absolute levels for which comparisons could be fined as the BF divided by the equivalent rectangular band-
made. In the STCs from ears with normal hearing, there was width 共ERB兲. For any filter, the corresponding ERB is the
a tendency for the best suppressor frequency to shift towards bandwidth of the rectangular filter with the same BF re-
lower frequencies as level increased. Although less obvious sponse that passes the same total power. The DPOAE STCs
270 J. Acoust. Soc. Am., Vol. 114, No. 1, July 2003 Gorga et al.: DPOAE suppression tuning curvesFIG. 8. Scatter plots of Q 10 共top row兲 and Q ERB 共bottom row兲 as a function
of audiometric threshold 共dB HL兲 for the case when stimuli were presented
at a constant SPL 共left column兲 or constant SL 共right column兲.
right column similarly collapses data across three L 2 levels,
but includes data when L 2 was more similar in SL. Thus, it
includes data from normal ears when L 2 ⫽20– 40 dB SPL,
but includes data from impaired ears when L 2
⫽50– 70 dB SPL. As can be seen in Fig. 8, there was con-
siderable overlap between normal and impaired Qs, and the
relation between these values and audiometric thresholds
was not obvious. This was true, whether the relation was
evaluated at equivalent SPL or equivalent SL. Correlation
analyses were consistent with this observation at equivalent
SL 共right column, Fig. 8兲. However, correlations (r⫽0.34 or
0.31兲 were significant for both Q 10 and Q ERB at equivalent
SPL 共left column, Fig. 8兲. In spite of these correlations, an
examination of the actual data suggests that a strong relation-
ship between tuning around the tip of the STC and audiomet-
ric threshold did not exist, at least for the data from the
present group of subjects.
Figure 9 shows mean Q 10 and Q ERB in the top and bot-
tom panels, respectively, as a function of L 2 for both normal
and impaired ears. Regardless of which estimate was used,
FIG. 7. Mean DPOAE STCs for normal-hearing and hearing-impaired ears
plotted as the suppressor level in dB SPL for 3 dB of suppression as a the sharpness of tuning decreased as L 2 increased for sub-
function of f 3 . Each panel shows the STC for a different L 2 . STCs for jects with normal hearing. Mean Q 10 values of about 4 were
subjects with normal hearing are shown as solid lines, while STCs for sub- observed when L 2 was 20 or 30 dB SPL, decreasing to a
jects with hearing loss are shown as dotted lines. value of 2.6 at the highest probe levels. Similarly, Q ERB de-
creased from average values of 6.5 at low L 2 levels to about
were inverted for the purpose of computing the ERB. While 4.4 for L 2 levels of 60 dB SPL or greater. For ears with
Q 10 is the more common way of quantifying the sharpness of hearing loss, Q 10 was slightly larger, compared to values
both neural and DPOAE tuning curves, Q ERB has been used observed in ears with normal hearing, when the estimates
to estimate the tuning in behavioral estimates of frequency were derived at the same absolute L 2 levels. Q ERB estimates
resolution 共e.g., Shera, Guinan, and Oxenham, 2002兲. also were slightly larger in ears with hearing loss. For both
The left column shows data from the two groups at con- Q 10 and Q ERB , the differences between groups were small;
stant SPL, but lumps together the data for the three L 2 levels however, an ANOVA for constant SPL conditions revealed
summarized in each panel. Thus, it includes data from nor- that these differences, although small, were significant. No
mal and impaired ears when L 2 ⫽50, 60, and 70 dB SPL. The significant differences were noted when the data for normal
J. Acoust. Soc. Am., Vol. 114, No. 1, July 2003 Gorga et al.: DPOAE suppression tuning curves 271FIG. 10. Scatter plots of tip-to-tail difference 共dB兲 as a function for audio-
metric threshold 共dB HL兲. Left panel: data for stimuli presented at a constant
SPL. Right panel: data for stimuli presented at a constant SL.
Figure 10 provides scatter plots of individual tip-to-tail
differences as a function of audiometric threshold, following
the convention that was used in Fig. 8. Thus, data are pre-
sented for equivalent SPL conditions 共left panel兲 and equiva-
lent SL conditions 共right panel兲. Data are collapsed across
the three L 2 levels represented in each panel, just as they
were for the Q values represented in Fig. 8. A significant
relationship was observed between tip-to-tail differences and
FIG. 9. Mean Q 10 and Q ERB as a function of L 2 in the top and bottom
audiometric thresholds, in that these differences decreased as
panels, respectively. Error bars represent ⫾1 s.d. Data for subjects with audiometric thresholds increased, even though the data were
normal hearing are shown as circles, while data for ears with hearing loss variable at each audiometric threshold. Correlations ranged
are shown as triangles. from 0.38 for the equivalent SPL condition to 0.77 for the
equivalent SL condition. This represents a main observation
ears at 20– 40 dB SPL were compared to data from impaired in the present study. While tuning around the tip of the STCs
ears at 50–70 dB SPL, which is the equivalent SL condition. did not depend strongly on audiometric threshold, the rela-
Whether comparisons were made at constant SPL or SL, and tionship between threshold at the tip and on the tail of the
in spite of the statistical observations, these estimates were STC decreased systematically as threshold increased.
variable in both normal and impaired ears, and the distribu- The results shown in Fig. 10 are summarized in Fig. 11,
tions of normal and impaired Qs overlapped, as can be seen where the top panel plots the mean suppressor level (L 3 )
in plots of individual 共Fig. 8兲 and mean 共Fig. 9兲 data. necessary for 3 dB of suppression as a function of L 2 for
subjects with normal hearing. The parameter is f 3 , with
filled circles showing data for the case when f 3 ⬇ f 2 and open
F. Tip-to-tail differences „dB…
circles showing the results when f 3 ⬍ f 2 . Note that when f 3
Following an approach initially proposed by Mills ⬇ f 2 , the suppressor level necessary to achieve 3 dB of sup-
共1998兲, based on data from animal studies, and since used by pression was nearly equal to probe level (L 2 ), which also
Pienkowski and Kunov 共2001兲 and Gorga et al. 共2002b兲 to means that criterion suppressor level increased linearly with
describe similar human DPOAE data, suppression thresholds L 2 . In contrast, a higher L 3 was required for 3 dB of sup-
were compared for a suppressor at the STC tip 共i.e., close to pression when the suppressor was 2.2 kHz, as expected from
f 2 ) and for a suppressor on the low-frequency tail of the the individual STCs shown in Fig. 6 and the scatter plots in
STC. The differences between these suppression thresholds Fig. 10. For example, when L 2 ⫽20 dB SPL, an L 3 of about
were estimated for the range of L 2 levels at which the mea- 65 dB SPL was needed in order for the criterion suppression
surements were made in each subject group. This quantity, to occur. However, L 3 increased at a slower rate, compared
which is specified in dB, is sometimes referred to as the to L 2 , for this low-frequency suppressor.
tip-to-tail difference or the tip-to-tail ratio. The tails of the In similar fashion, the middle panel plots mean suppres-
STCs were not completely flat, meaning that suppression sion thresholds as a function of L 2 for the same on-frequency
threshold continued to slowly increase as f 3 decreased. As a and low-frequency suppressors in subjects with hearing loss.
consequence, the ‘‘tail’’ threshold was defined as the thresh- In this case, filled triangles represent results when f 3 ⬇ f 2 ,
old for the 2.2–kHz suppressor because it was close to the and open triangles represent data when f 3 ⫽2.2 kHz. As ex-
lowest f 3 used in the present experiment and it was a sup- pected from previous results, suppression was measurable
pressor frequency for which criterion suppression was mea- over a restricted range of L 2 levels in impaired ears, com-
surable in the majority of cases. The tip-to-tail differences pared to ears with normal hearing, and data from impaired
were derived for normal and impaired ears at all L 2 levels at ears were characterized by greater variability. However, for
which STCs could be measured. those L 2 levels at which suppression could be measured, the
272 J. Acoust. Soc. Am., Vol. 114, No. 1, July 2003 Gorga et al.: DPOAE suppression tuning curvessuppression thresholds when f 3 ⫽4.1 kHz were subtracted
from the thresholds when f 3 ⫽2.2 kHz, and this quantity is
plotted as a function of L 2 . Thus, the bottom panel plots the
mean tip-to-tail difference in dB for both subject groups. In
normal-hearing ears, the tip-to-tail difference decreased as
L 2 increased, going from a maximum of about 42 dB (L 2
⫽20 dB SPL) to a minimum of about 12 dB (L 2
⫽70 dB SPL). At the three L 2 levels at which data were
obtained in impaired ears, tip-to-tail differences also system-
atically decreased with L 2 . Ears with hearing loss, however,
produced tip-to-tail differences that were 5– 6 dB smaller
than those produced by subjects with normal hearing at the
same L 2 levels, although variability in these measurements
was large, especially among ears with hearing loss. An
ANOVA revealed significant effects for both L 2 and group,
but no interactions between L 2 and group. This was true for
both the constant SPL and the constant SL conditions.
IV. DISCUSSION
To summarize the results from the present experiment,
threshold and growth of suppression 共measured in the form
of DPOAE decrement vs L 3 functions兲 depended on the re-
lationship between suppressor frequency ( f 3 ) and probe fre-
quency ( f 2 ) both in normal-hearing and hearing-impaired
subjects 共Figs. 3, 4, and 5兲. When f 3 ⬍ f 2 , suppression
threshold occurred at higher levels and the slope of the dec-
rement function was steeper, compared to cases when f 3
⬇ f 2 . While suppression threshold increased as f 3 increased
above f 2 , the slopes of the decrement functions decreased in
both subject groups. There were differences, however, be-
tween normal and impaired ears in that the differences in
slope between low-frequency ( f 3 ⬍ f 2 ) and high-frequency
suppressors ( f 3 ⬎ f 2 ) were less in impaired ears. The
decrement-vs-L 3 data were used to generate DPOAE STCs,
in which the suppressor level necessary for 3 dB of suppres-
sion was plotted as a function of f 3 共Figs. 6 and 7兲. While
STCs were measurable over a more limited range of absolute
probe levels (L 2 ) in ears with hearing loss, at the same ab-
solute probe levels, mean STCs in normal and impaired ears
were similar in appearance. These similarities were noted
around the tips of the STCs, in that there was overlap in the
FIG. 11. Mean suppressor level (L 3 ) necessary for 3 dB of suppression 共in estimates of Q 10 and Q ERB for the two groups 共Figs. 8 and
dB SPL兲 as a function of probe level (L 2 ) for an on-frequency suppressor 9兲. There was a tendency towards sharper tuning around the
( f 3 ⫽4.1 kHz, filled symbols兲 and for a low-frequency suppressor ( f 3 tip of the STC in ears with hearing loss, although this effect
⫽2.2 kHz, open symbols兲. Error bars represent ⫾1 s.d. Top panel presents
data from normal-hearing subjects; middle panel shows results for subjects
was significant only for conditions of equivalent absolute L 2
with hearing loss. Bottom panel: The tip-to-tail difference 共in dB as a func- levels in normal and impaired ears. In contrast, ears with
tion of L 2 ; data from subjects with normal hearing are shown as circles, hearing loss produced smaller tip-to-tail differences, com-
while data from subjects with hearing loss are shown as triangles. pared to subjects with normal hearing, with a larger effect for
comparisons in which L 2 was presented at roughly equiva-
overall pattern of response for low- and on-frequency sup- lent SL 共Figs. 10 and 11兲.
pressors appears to be similar for the two groups. That is, the The relationship between the slopes of decrement func-
‘‘threshold’’ suppressor level approximated L 2 and, thus, tions as a function of suppressor frequency in normal ears
grew linearly with L 2 when f 3 ⬇ f 2 . Higher suppressor levels has been observed in several previous DPOAE studies 共e.g.,
were needed when f 3 ⫽2.2 kHz, while L 3 increased more Kemp and Brown, 1983; Kummer et al., 1995; Abdala, 1998,
slowly for this condition, compared to the case when f 3 2001; Gorga et al., 2002b兲. In turn, these data are consistent
⬇f2. with previous animal studies in which it was shown that the
The data shown in the top two panels are summarized in response at any point along the cochlea grows more rapidly
the bottom panel of Fig. 11, in which the mean levels for when that place is driven by a stimulus whose frequency is
J. Acoust. Soc. Am., Vol. 114, No. 1, July 2003 Gorga et al.: DPOAE suppression tuning curves 273lower than the best frequency for that place 共Rhode, 1971; istered, there was little change at that same place when it was
Sachs and Abbas, 1974; Schmiedt and Zwislocki, 1980; Rug- stimulated with a low-frequency tone.
gero and Rich, 1991; Ruggero et al., 1997兲. It may be reasonable to assume, therefore, that the ef-
The observation that the slopes of these functions in ears fects of the low-frequency suppressors were the same in nor-
with mild-moderate hearing loss were, in general, shallower mal and impaired ears. However, the response to suppressors
for low-frequency suppressors, compared to data in normal must be inferred from the changes their presentation causes
hearing ears was not expected. Previous studies in animals to the response to the probe. Perhaps the slope patterns ob-
suggest that response growth changes as a consequence of served in the present study are a consequence of the fact that
cochlear damage 共Evans, 1974; Sewell, 1984; Gorga and Ab- our measures reflect the relative growth of response between
bas, 1981a,b; Ruggero and Rich, 1991兲. A recent study in suppressor and probe. If the growth of response was more
animals with noise-induced hearing loss described best- alike in impaired ears for low- and on-frequency conditions,
frequency rate-level functions that sometimes were shallower then differences in slope across frequency might be reduced,
than normal, normal, or steeper than normal 共Heinz et al., not because the low-frequency slope has become more shal-
2003兲. They attributed the variation in slopes to differences low, but because the on-frequency slope has become more
in cochlear damage 共IHC damage, OHC damage, or some steep.
combination of IHC and OHC damage兲, although cochlear Measures of tuning, based on the present measurements,
status was not assessed directly. Likewise, the status of the also revealed some interesting trends. DPOAE STCs in nor-
OHCs and the IHCs cannot be known in the present study. In mal and mildly impaired ears appeared to be similar when
those studies that examined response growth for on- and off- they were compared at the same probe levels (L 2 ). In spite
frequency stimuli, however, little or no change in response of the statistical outcome indicating that impaired ears pro-
growth was observed following cochlear damage for a stimu- duced larger Q 10 and Q ERB than normal-hearing ears when
lus that was lower than the best or probe frequency. Stated the probes were presented at equivalent SPL, it is difficult to
differently, the response to a low-frequency stimulus was consider the individual data presented in Fig. 8 and conclude
relatively insensitive to the status of the cochlea at a high- that these differences are meaningful. That is, there was
frequency place. However, the slope of response-growth overlap between Q 10 and Q ERB , regardless of audiometric
functions tended to increase 共relative to the normal case兲 for threshold. These results, at one level, are similar to those
observed in two recent papers in which either loop diuretics
stimuli close to the best frequency when cochlear damage
共Martin et al., 1998兲 or noise exposure 共Howard et al., 2002兲
existed. The present results appear, at least at face value, to
was used to induce temporary, reversible effects in the co-
differ from these previous findings. There was more similar-
chleae of rabbits. This group recently observed similar ef-
ity in the slope of these decrement functions for low- and
fects when more permanent noise-exposure damage was in-
high-frequency suppressors in impaired ears. This occurred
duced in rabbits 共Howard et al., 2003兲. In all three studies,
because there was a small decrease in slope for suppressors
there was little difference in the DPOAE STCs prior to and
below f 2 and perhaps an even smaller increase in slope for
soon after cochlear insult 共although some STCs in impaired
suppressors above f 2 .
ears appeared more variable compared to the pre-exposure or
The differences between the present findings and predic-
pre-treatment cases兲. In fact, there may actually have been a
tions from studies in lower animals may be due to the differ-
tendency for some STCs reported in these previous studies to
ences in the measurement paradigms used to collect the data. become more sharply tuned after insult. In any case, Martin
Unlike previous animal experiments, in which response et al. and Howard et al. observed little or no change in either
growth can be examined directly by presenting a single tone, Q or tip-to-tail differences following cochlear insult. As a
such measurements are not possible in humans. In studies result of these findings, they concluded that DPOAE sup-
involving humans, a probe 共the primaries in the present case兲 pression measurements do not provide the same information
must be presented and response growth to another tone 共the about cochlear tuning as do other measurements, such as
suppressor兲 must be inferred from the changes it causes to single-unit frequency-threshold curves or direct measure-
the probe response. The integrity of the OHCs presumably ments of basilar-membrane tuning. The present observation
should not influence the response to a low frequency at a of little or no difference between the DPOAE STC sharpness
high-frequency place because the high-frequency place only around the tip 共Q兲 in normal and impaired ears would be
responds nonlinearly when it is excited by frequencies close consistent with the observations made by Martin et al. and
to its best frequency. Evidence in support of this view comes Howard et al. There are other physiological data in the form
from single-unit studies in animals where OHC damage ex- of single-unit frequency-threshold curves 共Dallos and Harris,
ists. Tail thresholds remain relatively constant 共or perhaps 1978; Liberman and Dodds, 1984兲 or forward-masking ac-
even hypersensitive兲 so long as the damage is restricted to tion potential 共AP兲 tuning curves 共Gorga and Abbas, 1981b兲,
the OHCs 共Dallos and Harris, 1978; Liberman and Dodds, suggesting that tuning around the best frequency 共single-unit
1984兲. Further support for this view comes from studies in studies兲 or probe frequency 共AP studies兲 may be similar in
which basilar-membrane motion was measured before and ears with normal and impaired hearing, at least for ears with
after treatment with furosemide, an agent known to revers- mild-to-moderate threshold elevations. Stated differently,
ibly impair cochlear function 共Ruggero and Rich, 1991兲. hearing loss 共by definition兲 causes an elevation of threshold
While the response from a high-frequency cochlear place to at best frequency, but not necessarily a decrease in tuning
best-frequency tones changed when furosemide was admin- around best frequency. Thus, the present observations in re-
274 J. Acoust. Soc. Am., Vol. 114, No. 1, July 2003 Gorga et al.: DPOAE suppression tuning curvesgard to Q are consistent with previous DPOAE data and with tatory representation to the probe and not the wider
the results of single-unit and AP masking studies in animals representation due to suppression 共see Sachs and Kiang,
with induced lesions. While the present findings in relation to 1968, for a classic description of excitatory and suppressive
tip-to-tail differences are also in agreement with previous regions in the responses of auditory neurons兲. This would be
single-unit and AP data, they differ from DPOAE data in the case because suppression is memoryless, occurring only
animals with induced cochlear damage that showed no when the probe and suppressor 共or masker兲 are on at the
change in tip-to-tail differences 共Martin et al., 1998; Howard same time. In contrast to the paradigm used by Shera et al.,
et al., 2002, 2003兲. the present measurements were derived from suppression
Conclusions about tuning at the tip in normal and im- measurements, and thus would be expected to result in wider
paired ears might be modified by examining level effects. In estimated bandwidths. In addition, the probe in the present
the present data, Q 10 and Q ERB 共in addition to tip-to-tail dif- study consisted of two tones, slightly different in frequency.
ferences兲 decreased as level increased, an observation that Both tones contribute to the generation of the DPOAE, and,
was more pronounced in ears with normal hearing. Larger Qs because of this, it is possible that a wider range of suppres-
were seen at low probe levels (L 2 levels of 20– 40 dB SPL兲, sors is effective in reducing the response. While both factors
compared to L 2 levels at and above 50 dB SPL. Less sys- may account for the differences in Q estimates between the
tematic changes with level were observed in impaired ears. present study and the data reported by Shera et al., it is un-
However, this may have occurred because of the restricted certain how this distinction would impact the present com-
range of possible measurements in impaired ears. Hearing parisons between normal and impaired ears. Perhaps an ef-
loss in these subjects increased DPOAE threshold and/or re- fect of hearing status would have been observed 共in Q 10 and
duced DPOAE level 共see Fig. 1兲. Because suppression ex- Q ERB) if the data were derived from a paradigm that did not
periments require an unsuppressed response of some level include the effects of suppression. A paradigm in which only
that subsequently will be reduced by the suppressor, it was the excitatory regions were evaluated might have resulted in
necessary to perform the experiments in impaired ears only sharper tuning 共higher Q兲 in normal ears because the sup-
at the higher L 2 levels for which some reduction in tuning pression region would not have been outlined, but might
occurs in normal ears. This suggests that the lack of differ- have had no effect on the tuning in impaired ears. Unfortu-
ences between Qs in normal and impaired ears might repre- nately, isolating the excitatory region in normal ears is not
sent a level effect. Thus, the observation of differences be- possible for measurements like those used in the present
tween normal and impaired ears might depend on the way study.
the level is chosen at which comparisons will be made. This In normal ears, it is assumed that nonlinear processing
argument, however, is not supported by the observation that occurs at a specific place when that place is driven by its best
the overlap in Qs was similar, regardless of whether normal or characteristic frequency. In suppression experiments like
and impaired data were compared at equivalent SPL or the present study or psychophysical masking studies 共Oxen-
equivalent SL 共see Fig. 8兲. The present data, therefore, sug- ham and Plack, 1997兲, the growth of suppression 共or mask-
gest that Q is relatively insensitive to probe level (L 2 ) or ing兲 with probe level is linear when probe frequency ap-
audiometric threshold, at least among subjects with no worse proximates suppressor frequency because both the
than a moderate hearing loss, statistical results for Q not- suppressor 共or masker兲 and the probe are being processed
withstanding. through the same nonlinear mechanism. In contrast, low-
The present estimates of Q ERB in human ears with nor- frequency suppressors 共or maskers兲 are processed more lin-
mal hearing are less than the values recently reported by early at the place where the probe is primarily represented.
Shera et al. 共2002兲, whose estimates were based on stimulus- Thus, a high-frequency probe is processed nonlinearly 共com-
frequency otoacoustic emission 共SFOAE兲 and behavioral pressively兲 at its characteristic place, while a low-frequency
forward-masking measurements in humans. For example, suppressor is processed linearly at the same place. As a con-
their SFOAE estimates of Q ERB were 15–20 at 4 kHz, while sequence, functions relating suppressor level to probe level
their behavioral Q ERB estimates were close to 15 at 4 kHz. grow at a slow rate when suppressor frequency is well below
Their measurements were made with a 40-dB SPL probe. At probe frequency, reflecting the compressive processing for
this level, the current estimates of Q ERB , on average, were the probe, but not for the low-frequency suppressor 共see the
about 6.5. This value is closer to the values 共about 8 or 9 at top two panels of Fig. 11兲.
4 kHz兲 observed in a number of other papers 共as summarized These effects were explored further in the present study
by Glasberg and Moore, 1990兲, in which simultaneous mask- by estimating tip-to-tail differences 共Figs. 10 and 11兲. Tip-to-
ing techniques were used. Thus, the differences between our tail differences decreased as either level or audiometric
estimates and the recent estimates by Shera et al. might re- threshold increased. When grouped dichotomously according
late to the fact that the data reported by Shera et al. represent to hearing status, normal and impaired results differed sig-
the excitatory response area, while the present measurements nificantly, regardless of whether comparisons were made at
共as well as those summarized by Glasberg and Moore兲 in- the same SPL or the same SL. These data suggest that one
clude suppressive, as well as excitatory, areas in the re- effect of hearing loss might be to cause more upward spread
sponse. The SFOAE measurements of Shera et al., although of excitation in ears with hearing loss, even if the tuning
derived in a suppression paradigm, represent the unsup- close to the best frequency ( f 2 in the present experiment兲
pressed response to a single tone. Similarly, their forward- does not differ between normal and impaired ears. These
masking behavioral data presumably describe only the exci- results are consistent with single-unit data 共e.g., Dallos and
J. Acoust. Soc. Am., Vol. 114, No. 1, July 2003 Gorga et al.: DPOAE suppression tuning curves 275You can also read
