Spontaneous and Transient Evoked Otoacoustic Emissions:
A Racial Comparison
(synopsis from the article published in JAM,2001; (10) pp 20-32)
By Jenny C Y Chan.
Audiological Services Section, Education Department, Hong Kong
Bradley McPherson
Department of Speech and Hearing Sciences, University of Hong Kong, Hong Kong
jennychan@edu.gov.hk
Phone Int + , Fax Int +
Introduction There are two basic types of otoacoustic emissions: spontaneous otoacoustic emissions (SOAEs) and evoked otoacoustic emissions. The former occur in the absence of external stimulation, whereas the latter occur during or after external acoustic stimulation. Evoked emissions are further distinguished by the particular type of stimulation that elicits them. Transient evoked OAEs (TEOAEs), which were first described by Kemp (1978), are evoked by clicks or other brief stimuli. SOAEs’ studies reveal that they can only be detected in about 40 - 50% of normally hearing humans (Penner, Glotzbach, & Huang, 1993). The biological basis of their detection only in relatively few subjects is unknown. A higher prevalence of SOAEs was noted in females and right ears (Khalfa et al., 1997). Studies of gender differences and twins point to the possibility of genetic determinants to the tendency to exhibit SOAEs (Bilger, Matthies, & Hammel, 1990; McFadden & Loehlin, 1995).
Unlike SOAEs, TEOAEs can be detected in nearly all ears from normally hearing humans, regardless of age and gender (Probst, 1990). They provide broad band, cochlea-wide information in a short clinical session.
The objective of this paper is to compare SOAE and TEOAE occurrence and characteristics between Chinese and Caucasian racial groups. We aim at investigating if there is any genetic variation for the capacity to generate OAEs, the possible role of the reverse-transmission model of the middle-ear conduction apparatus in determining OAE characteristics, and any possible cochlear differences between the two racial groups. Materials and Methods Spontaneous otoacoustic emissions and transiently evoked otoacoustic emissions were recorded in both ears of 40 Chinese (20 males, 20 females) and 20 Caucasians (10 males, 10 females) whose ages range from 20 to 35. Subjects were categorized in the corresponding racial group according to their self-reports of racial origin, and their observed skin color, eye color and hair color.
A background information questionnaire was completed by each subject prior to assessment. All subjects included in this study had ages ranging from 20 to 35 years (mean age = 26.2 years). There was no statistically significant difference for age between the two racial groups (t = 1.979, df = 58, p > 0.05). All subjects had a negative history of hearing problems, and did not take any drugs that were thought to affect hearing or OAE amplitude.
Each subject was seated in a sound-proof booth for routine pure-tone audiometry and tympanometry. Pure tone audiometry was tested in both ears of all subjects at octave intervals from 250 Hz to 8 kHz using a Madsen GSI 16 or Madsen OB 822 audiometers. Tympanometry and acoustic reflex thresholds at 1 kHz were tested using a Madsen GSI 33 impedance bridge. All ears included in this study had pure-tone thresholds of 15 dB HL or less at all frequencies tested, normal type A tympanograms, and normal ipsilateral and contralateral acoustic reflex thresholds at 1 kHz.
An ILO Otodynamic Analyzer (ILO 88) was used for OAE recordings, which were made in a sound-proof booth over a 30-minute period. For recording of SOAEs, no external stimulus was required and the microphone output was subjected to Fast Fourier Transform (FFT) with an analysis bandwidth of 12 Hz. At least two separate average spectra of the signal from each ear were collected and stored in the computer system for later analysis.
The presence of a SOAE response was determined by three criteria. Firstly, the response should be any narrowband signal that presents in two consecutive average spectra of the recording from the same test ear. Secondly, the absolute amplitude of the signal should exceed -25 dB SPL. Finally, the amplitude of the signal should exceed the background noise level by 3 dB or more (Penner, Glotzbach, & Huang, 1993).
After recording the SOAE data, the probe was retained in the subject's ear and TEOAE measures were made. The evoked stimuli used were 80 us rectangular clicks presented at 80 ± 2 dB peSPL. Nonlinear click stimuli were used. The response time window was set at 2.5 - 20 ms and the band-pass filter was set in the range from 500 Hz to 6 kHz.
The presence of a TEOAE response was primarily determined by analyzing the reproducibility of the resultant waveform. Frequency-specific maxima in the FFT analyses were also examined
Materials and Methods SOAE racial findings
For the Chinese subjects tested, 16 out of 40 (40%) subjects and 26 out of 80 (32.5%) ears presented SOAEs. Similar data were obtained with the Caucasian subjects: 9 out of 20 (45%) subjects and 14 out of 40 (35%) ears possessed SOAEs. Two-way chi-square analysis was used to compare the percent occurrence of SOAEs between the two racial groups by subject and by ear. Neither comparison reached statistical significance. Figure 1 shows the frequency distribution of SOAEs measured in Chinese and Caucasian subjects. For the Chinese subjects, the data ranged from 585 Hz to 5810 Hz with mean and median frequencies at 2135 Hz and 1904 Hz respectively. For the Caucasian subjects, the data ranged from 585 Hz to 7617 Hz with mean and median frequencies at 1927 Hz and 1562 Hz respectively. There is a trend for SOAEs from Chinese subjects to be more skewed to higher frequencies than those from Caucasians. Mann-Whitney U test also revealed a significant difference for mean SOAE frequency between the two groups of data (U = 1389, NA = 42, NB = 85, p < 0.05).
 Figure 1: Total number of observed SOAEs at each frequency. © JAM 2001
Amplitudes of SOAEs can be expressed in terms of absolute amplitude or signal-to-noise ratio (SNR). For the former, the Chinese data ranged from -24.3 dB SPL to 4.5 dB SPL (mean = -14.9 dB SPL; median = -16.7 dB SPL) and the Caucasian data ranged from -24.7 dB SPL to 2.4 dB SPL (mean = -14.5 dB SPL; median = -16.1 dB SPL). For the latter form of expressing SOAE amplitude, the Chinese data ranged from 3.4 dB to 37.5 dB (mean = 18.4 dB; median = 18.2 dB) and the Caucasian data ranged from 5.2 dB to 42.4 dB (mean = 17.9 dB; median = 15.9 dB). Figure 2 shows the absolute amplitude distributions of the SOAEs measured in Chinese and Caucasian subjects. No statistical significant difference for SOAE amplitude was found between the two groups using the Mann-Whitney U test (for absolute amplitude: U = 1784, NA = 42, NB = 85, p > 0.05; for signal-to-noise ratio: U = 1668, NA = 42, NB = 85, p > 0.05).
 Figure 2: Absolute amplitude distribution of subject SOAEs. © JAM 2001
TEOAE racial findings
In analyzing the TEOAEs, the ILO 88 software broke down all the recordings into their component frequencies in five equal frequency bands centered on 1, 2, 3, 4 and 5 kHz.A band reproducibility of > 65% or a signal-to-noise ratio of +3 dB was considered indications of true TEOAE responses in any frequency band. Using these criteria, TEOAEs were recordable in 95% and 97.5 % of the Chinese and Caucasian ears respectively. No significant difference was found in the whole wave reproducibility between the two groups using an independent t-test (t = 1.842, df = 113, p > 0.05), with mean values of 88.3% and 84.4% for Chinese and Caucasian ears respectively.
To study the effect of race on the TEOAE responses at different frequency bands, SNR at frequency bands centered around 1, 2, 3, 4, 5 kHz were recorded. Figure 3 shows the means at each frequency band in the Chinese and Caucasian groups. There is a trend for Chinese to have higher SNRs in high frequency components of the TEOAEs than Caucasians. An analysis of variance further indicated statistically significant differences in the SNRs across races (F(1, 546) = 16.16, p < .0001). There were also significant differences in SNR across frequencies (F(4, 546) = 29.15, p < .0001). No race X frequency interaction occurred. The results of a Newman-Keuls test revealed a significant difference between the two racial groups for the 5 kHz data (p < .05). For lower frequency data (1 - 4 kHz), although the p values decreased with increasing frequency, pairwise comparisons between the two racial groups did not reveal significant differences.
 Figure 3: Mean signal-to-noise ratio of subject TEOAEs. © JAM 2001
For the overall response of the TEOAEs recorded from both races, data was expressed in terms of SNR in dB. The Chinese data ranged from -0.2 dB to 19.4 dB (mean = 10.9 dB; median = 11.2 dB) and the Caucasian data ranged from 0.5 dB to 20.5 dB (mean = 10.0 dB; median = 8.9 dB). No statistically significant difference was found between the two groups using an independent t-test (t = 1.010, df = 113, p > 0.05).
Discussion The present SOAE prevalence findings do not provide support to the hypothesis of a racial predisposition in the determination of SOAE expression. Similar figures were found both in terms of subjects and ears for the two races. Most SOAEs in the large pooled sample were found between 0.5 kHz and 2.5 kHz (Figure 1). However, subtle differences were found in the SOAE frequency distributions between the two races. Significantly more SOAEs from Chinese ears were found in the higher frequencies than those from Caucasian ears. An examination of SOAE amplitudes in the two racial groups showed no differences in their distributions.
TEOAEs were recordable in 95.8% of all the ears examined in the large pooled sample. No difference in the TEOAE prevalence was noted between the two racial groups. Similar to the SOAE findings, it was found that the TEOAE responses from Chinese had slightly higher SNRs in the higher frequency components than Caucasians. No difference was found in the TEOAE response distributions between the two races.
The demonstrated racial difference in the frequency distributions for both SOAEs and TEOAEs was an interesting finding. It has been suggested in previous study that similar results in SOAEs maybe related to middle-ear conduction properties (Whitehead et al., 1993). According to this model, any difference in the physical properties of the middle ear ossicles, tympanic membrane and external auditory meatus can affect the OAEs recorded.
Since tympanometric results represent preliminary data in considering middle ear functions, statistical comparisons of the estimated ear canal volume, the static admittance and the tympanometric peak pressure between the two racial groups were carried out. Statistically significant differences were found in the former two parameters between the two racial groups using the independent two-tailed t-test (for canal volume, t = 3.807, df = 118, p < .005; for static admittance, t = 6.403, df = 118, p < .0001). Accordingly, both canal volume and static admittance were smaller in Chinese than in Caucasians. It is possible that the racial differences in the canal volume and static admittance are at least two of the underlying reasons for the different frequency distributions of OAEs found.
Another possible source of racial differences in SOAE characteristics is cochlear differences. Different levels of melanin affect human’s pigmentation, and have been postulated to affect hearing because the specialized melanocytes form an important component of the stria vascularis in the cochlea (Garber et al., 1982). Therefore, physiological variations inside the cochlea due to different levels of melanin in different racial groups are suspected. In the study by Whitehead et al. (1993), the greatest difference in SOAE expressions (although statistically not significant) was found between Negroes and Caucasians. Since the difference of SOAE prevalence or characteristics in people with different levels of melanin is not dramatic, Negroes and Caucasians would be the two most suitable racial groups for any further study.
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