Download Distortion-Product Otoacoustic Emissions in Schoolchildren: Effects

J Am Acad Audiol 12 : 506-513 (2001)
Distortion-Product Otoacoustic Emissions in
Schoolchildren : Effects of Ear Asymmetry,
Handedness, and Gender
Tegan Keogh*
Joseph Kei*
Carlie Driscoll*
Veronica Smyth*
Abstract
The present study examined effects of ear asymmetry, handedness, and gender on distortionproduct otoacoustic emissions (DPOAEs) obtained from schoolchildren . A total of 1003
children (528 boys and 475 girls), with a mean age of 6.2 years (SD = 0.4, range = 5.2-7 .9
years), were tested in a quiet room at their schools using the GSI-60 DPOAE system . A
distortion-product (DP)-gram was obtained for each ear, with f2 varying from 1 .1 to 6 .0 kHz
and the ratio of f2/f1 at 1 .21 . The signal-to-noise ratios (SNRs) (DPOAE amplitude minus the
mean noise floor) at the tested frequencies 1 .1, 1 .5, 1 .9, 2.4, 3 .0, 3.8, 4 .8, and 6.0 kHz were
measured . The results revealed a small but significant difference in SNR between ears, with
right ears showing a higher mean SNR than left ears at 1 .9, 3.0, 3.8, and 6.0 kHz. At these
frequencies, the difference in mean SNR between ears was less than 1 dB . A significant
gender effect was also found. Girls exhibited a higher SNR than boys at 3.8, 4.8, and 6.0
kHz. The difference in mean SNR, as a result of the gender effect, was about 1 to 2 dB at
these frequencies . There was no significant difference in mean SNR between left-handed
and right-handed children for all tested frequencies .
Key Words : Children, distortion-product otoacoustic emissions, ear asymmetry, gender,
handedness, hearing screening
Abbreviations : DPOAE = distortion-product otoacoustic emission, OAE = otoacoustic emission, SNR = signal-to-noise ratio, SOAE = spontaneous otoacoustic emission, TEOAE _
transient evoked otoacoustic emission
tudies of auditory function in the general
population revealed an ear asymmetry
effect . Right ears consistently demonstrate better responses than left ears . For example, Chung et al (1983) found that the mean
pure-tone threshold for right ears was significantly better than that for left ears . Investigations have also shown that objective tests of
hearing, such as auditory brainstem responses,
reveal larger amplitudes and shorter latencies
for the right ear in comparison with the left ear
(Levine et al, 1988). In speech perception or
dichotic listening tasks, right-handed subjects
S
*Department of Speech Pathology and Audiology, The
University of Queensland, St . Lucia, Queensland, Australia
Reprint requests : Joseph Kei, Department of Speech
Pathology and Audiology, The University of Queensland,
St . Lucia, Queensland 4072, Australia
506
report more accurately words presented to the
right than to the left ear (Kimura, 1967 ; Bryden,
1986). The ear asymmetry effect is also evident
in the objective measurement of spontaneous
otoacoustic emissions (SOAEs), with the right ear
producing stronger emissions than the left ear
(Bilger et al, 1990 ; Burns et al, 1992 ; Khalfa et
al, 1997). Studies of transient evoked otoacoustic
emissions (TEOAEs) have also shown significantly larger emissions in the right in comparison with the left ear (Khalfa and Collett, 1996 ;
Aidan et al, 1997 ; Kei et al, 1997 ; Khalfa et al,
1997 ; Newmark et al, 1997 ; Driscoll et al, 1999).
Although the presence of an ear asymmetry
effect in SOAEs and TEOAEs is widely accepted,
its existence in distortion-product otoacoustic
emissions (DPOAEs) has not been confirmed. To
date, studies have revealed no asymmetry in
DPOAEs between the ears (e .g ., LonsburyMartin et al, 1997, p. 96). However, these studies
DPOAEs: Ear Asymmetry, Handedness, and Gender Effects/Keogh et al
involved small sample sizes . Given the small
effect size and inadequate power of statistical
tests used in the analyses, the results may not
have been conclusive .
The exact cause of ear asymmetry in otoacoustic emissions (OAEs) is as yet undetermined. At present, there are only speculations
about the phenomenon . For example, Previc
(1991) suggested that the asymmetry was a
result of a prenatal asymmetry occurring in the
auditory system during the first trimester of
pregnancy. McFadden (1993) speculated that
the influence of efferent activity, specifically
concerning efferent auditory inhibition, was a
cause of ear asymmetry, with the efferent system exerting a stronger influence on the left
than the right ear . This theory has been evaluated in a few studies that have focused on
whether the medial olivocochlear bundle, which
is an efferent system affecting the motility of
outer hair cells, is responsible for the greater
OAEs in the right ear (Morlet et al, 1993 ; Khalfa
and Collet, 1996 ; Khalfa et al, 1997, 1998 ; Maison et al, 1997) . In summary, these studies suggest that ear asymmetry might be a product of
the efferent system and/or a result of a prena-
tal asymmetry.
The possibility that ear asymmetry is a
genetically determined function, similar to that
of handedness, cannot be excluded . Handedness is the most easily observed expression of
cerebral lateralization (Peters, 1995) . The estimate for people in the United States writing
with the left hand is about 12 percent (10 .5% for
females and 13% for males) (Gilbert and Wisocki,
1992) . Handedness is related to other behavioral
asymmetries such as footedness, eyedness, and
ear dominance . In general, lateral preference for
the eyes and the ears is less clearly expressed
than hand preference . Studies on footedness
(Searleman, 1980 ; Peters, 1988) and eyedness
(Porac and Coren, 1976) have shown that righthanded persons tend to have a right foot preference and a right-sided visual preference . In
contrast, left-handed persons have either a left
foot or eye preference, or a right preference that
is far less pronounced than that evidenced in
right-handers . Investigations of the effect of
handedness on auditory tasks (e .g., a dichotic listening task and pure-tone audiometry) show
that the right ear advantage is reversed or less
pronounced for left-handed persons (Demarest
and Demarest, 1980 ; Emmerich et al, 1988 ; Pirila et al, 1991) . However, there have been no
reports in the literature that compare DPOAEs
between left-handed and right-handed persons .
Gender differences are found for almost all
measures of auditory function, including OAEs .
The studies by Driscoll et al (1999), Kei et al
(1997), and various other investigators (eg,
Aidan et al, 1997 ; Newmark et al, 1997 ; Driscoll
et al, 2000) have revealed that females have
stronger TEOAEs than males across many age
groups from infancy to adulthood . Gender differences are also observed for DPOAE measures obtained from adults (e .g., Gaskill and
Brown, 1990 ; Lonsbury-Martin et al, 1990 ;
Cacace et al, 1996) . For instance, the study by
Lonsbury-Martin et al (1990), based on the data
from 79 female ears and 70 male ears, found that
DPOAE amplitudes (especially in the high frequencies) were greater for women than for men .
Supporting evidence for this effect in children,
however, has not been established .
The present study aimed to examine the
effects of ear asymmetry, handedness, and gender on DPOAEs obtained from schoolchildren .
METHOD
Subjects
A total of 1003 schoolchildren (528 boys and
475 girls), with a mean age of 6 .2 years (range
= 4 .1-7 .9 years, SD = 0 .4), participated in the present study. They were recruited from Grade 1
children who were enrolled in 23 primary schools
in Brisbane, Australia . No particular selection
criteria were required for participation in this
study, apart from full written consent from the
children's parents .
Although the data from all 1003 children
were included for the initial statistical analysis,
the data for 819 (426 boys and 393 girls) of the
1003 children, who had passed tympanometry
and the pure-tone screening test (pass/fail criteria to be described later), were used for a subsequent analysis .
From among the 1003 children, 159 lefthanded children (94 boys and 65 girls) were
identified . Children who wrote with their left
hand were classified as left-handers . This classification was adopted because in Australia,
where pressures against the use of the left hand
are restricted to religious and ethnic minorities, the choice of writing hand should provide
the single most stable indicator of a bias toward
the left hand (Peters, 1995). The percentage of
left-handers (15.9%) in this cohort of schoolchildren is comparable to the figure reported by
Gilbert and Wisocki (1992) . Of these 159 lefthanded children, only 128 (69 boys and 59 girls)
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Journal of the American Academy of Audiology/Volume 12, Number 10, November/December 2001
Table 1
Mean DPOAE SNR (in dB) and SD Values for the Left and Right Ears for 1003 Children
(528 Boys and 475 Girls)
Mean DPOAE SNR (SD)
Left Ear
f2 Frequency (kHz)
1 .1
1 .5
1 .9
2.4
Boys
Girls
1 .7 (10 .9)
16 .4 (7 .0)
9.9 (8 .7)
2 .5 (11 .8)
16 .5 (7 .0)
10 .9 (8 .4)
12 .5
13 .3
14 .6
11 .6
13 .4
14 .8
15 .8
13 .3
14 .3
3 .0
3 .8
4 .8
6 .0
Right Ear
(6 .7)
(5 .6)
(5 .5)
(6 .4)
(6 .9)
14 .9
Boys
1 .7 (11 .1)
16 .6
(7 .1)
14 .6
(7 .3)
11 .2
14 .9
13 .0
13 .9
(6 .8)
(5 .1)
(5 .8)
(7 .1)
(7 .0)
12 .5
Procedures
Subjects were tested individually, in a seated
position, in non-sound-treated rooms within
each school . Ambient noise levels, measured
with a CEL-254 sound level meter, ranged from
34 to 51 dBA. An otoscopic examination was
conducted by an audiologist prior to all testing
to ensure that all tests were not affected by conditions of the external auditory meatus such as
collapsing ear canals or excessive cerumen.
Pure-tone screening, tympanometry, and DPOAE
test were randomly administered in the six possible permutations .
Tympanometry was performed using a Madsen Zodiac 901 Middle Ear Analyzer . Tympanometry is essential to detect middle ear
dysfunction, which reduces or obliterates OAEs
(Owens et al, 1993 ; Nozza et al, 1997). Failure
in tympanometry was defined as any result that
could be classified as a type B or C2 tympanogram based on the modified version of
Jerger's (1970) classification system (refer to
the Appendix for further details) .
Pure-tone screening was performed using
the Madsen Micromate 304 Screening Audiometer, fitted with ME70 noise-excluding audiocups .
508
3 .2 (11 .5)
16 .5
(7 .3)
15 .8
(7 .4)
(8 .0)
(6 .6)
(5 .0)
(5 .6)
11 .2
15 .1
13 .0
15 .1
(6 .5)
13 .5
DPOAE = distortion-product otoacoustic emission ; SNR = signal-to-noise ratio,
passed the pure-tone screening and tympanometry. Their mean age was 6.2 years (range
= 5.4-7 .3 years, SD = 0.39) . The data from these
128 left-handed children were compared with
those for 128 right-handed children who were
selected from among the same pool of 1003 children to match the left-handed children for age
(within 1 month) and gender. These right-handed
children also passed the pure-tone screening
and tympanometry.
Girls
(9 .0)
(6 .6)
(6 .0)
(6 .1)
(7 .0)
Pure tones of frequencies 0 .5, 1, 2, and 4 kHz
were presented to each ear at 20 dB HL. If a child
failed to respond twice to three consecutive presentations at any frequency at 20 dB HL, the
threshold for that frequency was determined .
Failure in pure-tone screening was indicated if
the hearing threshold at any frequency exceeded
25 dB HL .
DPOAEs were obtained from all 1003 children using the GSI-60 DPOAE System . Adequacy of probe fit was inspected prior to the
commencement of data acquisition . A series of
simultaneous pure-tone pairs, of frequencies fl
and f2, at intensities of 65 dB SPL (L1) and 55
dB SPL (L2), respectively, were delivered to the
test ear and produced a DP-gram. These stimulus intensity levels were chosen based on recommendations concerning optimal results in
humans (Harris et al, 1989 ; Whitehead et al,
1995 ; Stover et al, 1996). The test frequency
ratio (f2/fl) was set at 1.21 to ensure optimal
DPOAE results (Harris et al, 1989). All other
aspects of the test protocol were set in default
mode . In the event of high ambient noise (levels exceeding 50 dBA), testing was temporarily
paused . Measurement parameters included
amplitude of the distortion product (DP-amp),
noise floor, and SNR (defined as DP-amp minus
noise floor) at f2 frequencies of 1.1, 1.5, 1.9, 2.4,
3.0, 3 .8, 4.8, and 6 .0 kHz.
RESULTS
T
able 1 shows the mean DPOAE SNR and
standard deviation values for the ear and
gender conditions for all 1003 children . As
shown in Table 1, the mean SNR attains its maximum value at 1.5 kHz, with a dip at 1.9 kHz.
DPOAEs: Ear Asymmetry, Handedness, and Gender Effects/Keogb et al
Table 2
Summary of ANOVA Results for
DPOAE SNR at Various f2 Frequencies for 1003
Children (528 Boys and 475 Girls), Showing the
Main Effects and Interactions
Ear Effect
Gender
Effect
Ear x Gender
11
1 .5
1,9
2,4
NS
NS
r
NS
NS
NS
NS
NS
NS
NS
NS
NS
3 .8
4 .8
6,0
*
NS
*
*
*
NS
NS
NS
f2 Frequency
(kHz)
3.0
NS
NS
DPOAE = distortion-product otoacoustic emission, SNR =
signal-to-noise ratio : NS = not significant at 95% con fid ence
level .
*Significant at p < .05 .
Table 3 Summary of ANOVA Results for
DPOAE SNR at Various f2 Frequencies for 819
Children (426 Boys and 393 Girls) Who Passed
Pure-Tone Screening (<_ 25 dB HL from 0 .5-4
kHz) and Tympanometry (Only Type A and C1
Tympanograms)
Ear Effect
Gender
Effect
Ear x Gender
2 .4
3 .0
3 .8
NS
NS
*
NS
NS
*
NS
NS
NS
NS
NS
*
NS
NS
NS
NS
NS
NS
6 .0
*
f2 Frequency
(kHz)
1 .1
1 .5
1 .9
4.8
NS
*
NS
NS
DPOAE = distortion-product otoacoustic emission ; SNR =
signal-to-noise ratio ; NS = not significant at 95% confidence
level
*Significant at p < .05
The mean SNR ranges from 11 .6 to 15 .8 dB
between 2 .4 and 6 .0 kHz . The standard deviation values below 2 kHz are generally greater
than those above 2 kHz .
A factorial model, which included two factors (ear and gender) and all interactions, was
fitted to the data with SNR as the dependent
variable . The significance of any term was
assessed using the analysis of variance (ANOVA)
for each of the f2 frequencies tested (1 .1, 1 .5, 1 .9,
2 .4, 3 .0, 3 .6, 4 .8, and 6 .0 kHz) . Table 2 shows a
summary of the ANOVA results for DPOAE SNR
at these frequencies for all 1003 children .
The main effect for ear was significant at 1 .9,
3 .0, 3 .8, and 6 .0 kHz (with respective values of
F = 8 .11, df = 1, 1002, p = .004 ; F = 7 .14, df = 1,
1002, p = .008 ; F = 7 .21, df = 1, 1002, p = .007 ;
and F = 5 .85, df = 1, 1002, p = .016), with right
ears showing a higher SNR value than left ears .
The results also revealed a gender effect at 3 .8,
4 .8, and 6 .0 kHz (with values of F = 7 .21, df =
1, 1002, p = .007 ; F = 9 .98, df = 1, 1002, p = .002 ;
and F =11 .52, df =1, 1002, p = .001, respectively) .
The difference in mean SNR was about 1 to 2 dB
at these frequencies . Girls displayed a higher
SNR value than boys . The ear x gender interaction did not reach significance for any of the
test frequencies .
One of the possible confounding factors in
the above analysis is children's hearing loss . To
minimize the effect of hearing loss on the DPOAE
analysis, the above statistical analysis was
repeated only for children who passed pure-tone
screening and tympanometry . The results of the
analysis are shown in Table 3. When compared
with Table 2, the results from Table 3 show
essentially the same pattern of results, except
for the disappearance of the significant ear effect
at 3 kHz .
Table 4 shows the mean DPOAE SNR and
standard deviation values for 128 left-handed
and 128 right-handed children matched to the
left-handers for age and gender. All children
passed pure-tone screening and tympanometry.
As shown in Table 4, the mean SNR exceeds 11
dB for all ears irrespective of handedness at all
frequencies except for 1.1 kHz .
To investigate whether left-handed and
right-handed children display different patterns
of results for DPOAE SNRs, a three-factor
ANOVA (with ear as within-subject factor and
gender and handedness as between-subject factors) was performed on SNR data . The results
showed a significant main effect for ear at 1.9
(F = 8 .23, df = 1, 252, p = .004) and at 6.0 kHz
(F = 5.27, df = 1, 252, p = .022), with the right
ears showing a higher SNR value than the left
ears . However, the main effect for handedness
and handedness x ear interaction for all tested
frequencies did not reach significance .
A significant handedness x gender interaction was present at 1 .1 (F = 5 .15, df = 1, 252,
p = .023) and at 1 .9 kHz (F = 4 .66, df = 1, 252,
p = .032) . These significant interactions are
graphically illustrated in Figure 1 . In essence,
Figure 1 indicates that the mean DPOAE SNR
for right-handed girls is greater than that for
right-handed boys . However, this phenomenon
is reversed for the left-handed children . As noise
contamination might be a confounding factor
509
Journal of the American Academy of Audiology/Volume 12, Number 10,
November/December 2001
Table 4 Mean DPOAE SNR (in dB) and SD Values (Italics) for the Left and Right
Ears for 128 Lefthanded Children (69 Boys and 59 Girls) and 128 Right-handed Children Matched
to the Left-handers
for Age and Gender
Left-handers
Boys
f2 Frequency
1 .1
1 .5
1 .9
2 .4
3 .0
3.8
4 .8
6 .0
Mean
SD
Mean
SD
Mean
SD
Mean
SD
Girls
LE
RE
LE
RE
LE
RE
3 .8
9 .9
18 .1
6 .1
5 .4
10.2
18 .5
5.7
3 .1
11 .3
16 .2
7.1
4 .1
9.8
16 .7
7.0
12 .6
6.8
16 .0
1 .8
10.5
17 .9
7.1
3 .2
10.2
18 .0
6.3
6 .4
9 .0
17 .6
7.1
6 .2
10 . 1
18 . 4
5.2
16 .2
17 .5
16 .6
16 .7
12 .8
7.4
15 .8
4.5
SD
Mean
4 .2
16 .7
Mean
SD
Boys
RE
14 .0
4 .8
SD
Girls
LE
Mean
SD
Mean
Right-handers
15 .0
4.5
12 .9
5.1
15 .1
5.2
11 .4
7.2
4 .9
8.4
16 .7
15 .6
14 .9
3.8
13 .4
4 .5
3.9
17 .2
14 .3
5 .0
15 .9
4.2
7.2
11 .5
8.7
5.7
14 .1
13 .1
7.0
6.2
14 .3
4.8
16 .8
14 .4
6 .6
15 .8
6.4
17 .2
4.0
15 .1
3.9
17 .4
3.7
15 .9
4 .6
17 .4
13 .7
5 .9
14 .7
5.4
13 .5
6 .5
14 .3
5.4
15 .2
5.7
6.0
4.7
5.5
13 .4
7.1
13 .8
7.6
5.9
14 .9
5.9
14 .6
5.0
6. 2
14 .9
4 .5
16 .6
5 .2
17 .9
14 .9
4.8
5 .3
16 .1
5 .0
17.5
5.5
All children passed pure-tone screening (<_25 dB HL from 0.5-4 kHz) and tympanometry
(only type A and C1 tympanograms) .
DPOAE = distortion-product otoacoustic emission ; SNR = signal-to-noise ratio; LE = left
ear ; RE = right ear,
in DPOAE recordings at low frequencies, further
analysis of the noise floor was performed using
a three-factor (ear, handedness, and gender)
ANOVA with noise (noise floor recorded on the
DP-gram) as the dependent variable. The results
showed no significant main effect for all independent variables and their interactions (p > .05)
at both 1.1 and 1.9 kHz.
DISCUSSION
T
he findings from the present study revealed
an ear asymmetry effect on DPOAEs in
schoolchildren . Right ears were found to produce
stronger DPOAEs than left ears mainly in the
higher frequencies (1 .9, 3.0, 3 .8, and 6.0 kHz) .
This pattern of results remained practically
unchanged (except for the results at 3 .0 kHz
only) even when children who failed pure-tone
screening and tympanometry were excluded
from the statistical analysis . However, despite
the significance of this ear asymmetry effect, the
mean difference in DPOAE SNR between ears
was small. The mean differences in SNR between
the ears were less than 1 dB . Given these small
differences, it is not surprising to find that previous studies based on a small sample size (e .g .,
Lonsbury-Martin et al, 1997) did not report an
ear asymmetry effect in DPOAEs . In compari-
510
son, the ear asymmetry effect that exists in
DPOAEs is not as large an effect as that seen
in studies of TEOAEs in which the mean difference between ears was about 1 to 2 dB (Kbalfa
and Collet, 1996 ; Aidan et al, 1997 ; Kei et al,
1997 ; Khalfa et al, 1997; Newmark et al, 1997 ;
Driscoll et al, 1999). The contribution of the
SOAEs, which are more frequently found in
females and in right ears, to the TEOAE recordings may account for the greater difference in
TEOAEs between ears than in DPOAEs (Kulawiec and Orlando, 1995). However, the contribution of SOAEs to DPOAE recordings is negligible
unless the frequency of SOAE coincides with
the DPOAE frequency (2f1-f2) .
The reasons for the ear asymmetry effect in
DPOAEs are, as yet, unclear. It is possible that
the slightly poorer hearing sensitivity in the
left than in the right ear could result in an ear
asymmetry effect . However, the pure-tone screening test in the present study did not provide
hearing threshold data for each ear. The speculations that ear asymmetry in DPOAEs could
be a product of the efferent system (McFadden,
1993), a result of a prenatal asymmetry before
birth (Previc, 1991), and/or cerebral lateralization (Peters, 1995) still stand . But more evidence is needed to confirm such possible causes
of ear asymmetry.
DPOAEs : Ear Asymmetry, Handedness, and Gender Effects/Keogh et al
16U
N
0
r
z
rn
W
a
O
a
1412
10
8
J
6
42
0
LH/Boy
~~ i- I
LH/Girl
RH/Boy
RH/Girl
Handed ness/Gender
01 .1 kHz E1 .9 kHZ
LH = left handed, RH = right handed
Figure l Mean DPOAE SNR at 1.1 and 1.9 kHz for 69
left-handed boys and 59 left-handed girls and 128 righthanded children matched to the left-handers for age and
gender. The mean age for the left-handed children is 6.2
years (SD = 0.39) . All children passed pure-tone screening (25 dB HL from 0.5-4 kHz) and tympanometry (only
type A and C1 tympanograms). LH and RH represent lefthanded and right-handed, respectively. Vertical bars
indicate 1 SD from the mean .
The present study also revealed a significant
gender effect in DPOAEs in schoolchildren, predominantly in the high frequencies . Again, this
effect remained unchanged when children who
failed pure-tone screening and tympanometry
were excluded from the present study. The magnitude of this gender effect was greater than that
of the ear asymmetry effect and ranged from 1
to 2 dB between the two genders (see Table 1) .
This gender effect in schoolchildren is in keeping with the findings of Gaskill and Brown
(1990), Moulin et al (1993), and Cacace et al
(1996) for adults . The possible confounding factor that SOAEs, which occur more frequently in
females (Kulawiec and Orlando, 1995), might
have contributed to DPOAEs is regarded as
minimal in view of the rare coincidence of the
SOAE and DPOAE frequencies .
In contrast, this gender effect was not
observed below 3 .8 kHz . This finding is consistent with that of Lonsbury-Martin et al (1990),
who found that the difference in DPOAEs
between males and females in the low frequencies was small . The other reason for the
absence of the gender effect in the low frequencies is the possibility of the interference of
environmental and subject noise (e .g ., from
breathing and jaw movements) with the DPOAE
recording process . Such noise interference is
reflected in the higher standard deviation values of the SNR in the lower-frequency region
(see Table 1) . These noises, being more intense
in the lower-frequency region, raised the noise
floor and decreased the SNR (e .g ., at 1 .1 kHz)
at these frequencies .
Possible reasons for the gender effect on
DPOAEs are numerous . It is possible that the
hearing sensitivity of the boys was poorer than
that of the girls . However, since the pure-tone
screening test in the present study did not provide hearing threshold data for each child, this
claim is only tentative . The gender effect might
be attributed to the anatomic and physiologic dif-
ferences at the cochlear level (Don et al, 1993)
between the boys and girls. It is also possible that
different body temperature between boys and
girls may give rise to gender differences in
DPOAEs because the length and electromotility
of outer hair cells have been found to be temperature dependent (Ashmore and Holley, 1988 ;
Gitter, 1992) . The difference in efferent inhibition to individual cochleas may be relatively
less in females than in males (McFadden, 1993) .
An interesting finding of the present study
is the absence of the effect of handedness on
DPOAE SNR since the main effect for handedness and handedness x ear interaction for all
tested frequencies did not reach significance .
There was no difference in the ear asymmetry
effect on SNR between left-handed and right-
handed children . Although the literature shows
that left-handers have either less pronounced or
reversed lateralizations in footedness (Searleman, 1980 ; Peters, 1988), eyedness (Porac and
Coren, 1976), and speech perception (Demarest
and Demarest, 1980 ; Emmerich et al, 1988),
the case for DPOAEs obtained from schoolchildren cannot be established . However, the
generalization of this result should be made
with caution in view of the small effect size and
relatively small sample size (256 children) .
With this small sample size, the pattern of
results from the three-factor (ear, handedness,
and gender) ANOVA is different from the pattern of results obtained previously (see Tables
2 and 3) in that the gender effect has disappeared altogether for all frequencies . The main
effect for ear was significant only at 1.9 and 6.0
kHz, with right ears showing more robust
DPOAEs than left ears . The handedness x
gender interaction was significant for DPOAE
SNR at 1.1 and 1 .9 kHz. The graphic representation of these results, shown in Figure 1,
indicates a greater mean DPOAE SNR for righthanded girls than for right-handed boys at
both 1 .1 and 1 .9 kHz. However, this phenomenon is reversed for the left-handed children .
511
Journal of the American Academy of Audiology/Volume 12, Number 10, November/December 2001
It is not clear why such a reversal has occurred
at these frequencies . Further analysis of the
noise at these frequencies did not show any significant difference in noise floor across the two
genders and handedness .
In summary, the present study revealed an
ear asymmetry effect in DPOAEs obtained from
1003 schoolchildren . Right ears exhibited
stronger DPOAEs at 1.9, 3.0, 3.8, and 6.0 kHz
than left ears, but the difference in SNR was
small. A gender effect was observed only in the
high frequencies (3 .8, 4.8, and 6 .0 kHz), with girls
consistently displaying stronger DPOAEs compared with boys . However, no difference in
DPOAE SNR between 128 left-handed and 128
right-handed children (matched for age and gender) was found . This means that although
reversed lateralization of functions such as eyedness, footedness, and speech perception is seen
in left-handers, they are not evidence in DPOAE
SNR measures in this cohort of schoolchildren .
Acknowledgment. This article was presented at the
17th Biennial Symposium of the International Evoked
Response Audiometry Study Group, July 22-27, 2001,
University of British Columbia, Vancouver, Canada . The
current investigation was supported by a very generous
bequest from Mrs. Julie Luttrell in memory of her daughter, Suzanne Luttrell, and supplemented by a research
grant from the University of Queensland . The authors
would like to thank Queensland Health, Queensland
Education, Queensland Catholic Education, and participating schools for their cooperation and assistance during
the course of this study. We also thank the Starkey
Laboratories Australia Pty Ltd for supplying the GSI-60
DPOAE system for use in this study.
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Appendix
M, Lonsbury-Martin
of distortion-product
levels in normal and
Ll, L2 space. JAcoust
Classification of Tympanogram
Results for Children
Classification
Static
Compliance (mL)
Middle Ear
Pressure (daPa)
Type A
Type C 1
Type C2
0.2-1 .5
0.2-1 .5
0.2-1 .5
-101 to -200
<--201
Type B
Flat
+50 to -100
No peak