Abstract
The reported research introduces a noninvasive approach to estimate round-trip outer-middle ear pressure gain using distortion product otoacoustic emissions (DPOAEs). Our ability to hear depends primarily on sound waves traveling through the outer and middle ear toward the inner ear. The role of the outer and middle ear in sound transmission is particularly important for otoacoustic emissions (OAEs), which are sound signals generated in a healthy cochlea and recorded by a sensitive microphone placed in the ear canal. OAEs are used to evaluate the health and function of the cochlea; however, they are also affected by outer and middle ear characteristics. To better assess cochlear health using OAEs, it is critical to quantify the effect of the outer and middle ear on sound transmission. DPOAEs were obtained in two conditions: (i) two-tone and (ii) three-tone. In the two-tone condition, DPOAEs were generated by presenting two primary tones in the ear canal. In the three-tone condition, DPOAEs at the same frequencies (as in the two-tone condition) were generated by the interaction of the lower frequency primary tone in the two-tone condition with a distortion product generated by the interaction of two other external tones. Considering how the primary tones and DPOAEs of the aforementioned conditions were affected by the forward and reverse outer-middle ear transmission, an estimate of the round-trip outer-middle ear pressure gain was obtained. The round-trip outer-middle ear gain estimates ranged from −39 to −17 dB between 1 and 3.3 kHz.
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Abbreviations
- OAE:
-
Otoacoustic emission
- DPOAE:
-
Distortion product otoacoustic emission
- f1 :
-
Primary tone with frequency f1
- f2 :
-
Primary tone with frequency f2
- I/O:
-
Input/output
- DP:
-
Distortion product
- fa :
-
Primary tone with frequency fa
- fb :
-
Primary tone with frequency fb
- OMEG:
-
Outer-middle ear gain
- DPOAE2T :
-
DPOAE generated by the interaction of primary tones f1 and f2 in the two-tone condition
- L2 :
-
Level of the primary tone with frequency f2
- Gft :
-
Forward outer-middle ear transmission
- L2C :
-
L2 Level in the cochlea
- GfC :
-
Forward cochlear gain from base to the f2 place
- L1 :
-
Level of the primary tone with frequency f1
- La :
-
Level of the primary tone with frequency fa
- Lb :
-
Level of the primary tone with frequency fb
- DPOAE ′3T :
-
DPOAE generated by the interaction of primary tones fa and fb in the three-tone condition
- LabC :
-
Distortion product generated by La and Lb
- L ′2C :
-
Best place wave for the distortion product LabC
- DPOAE3T :
-
DPOAE generated by the interaction of f1 and L ′2C in the three-tone condition
- G ′fC :
-
Forward cochlear gain from LabC place to 2fa–fb place
- Gen ′3T :
-
DPOAE3T generator component
- GrC :
-
Cochlear gain from the f2 place to the base
- Grt :
-
Reverse outer-middle ear transmission
- LSF:
-
Least squares fit
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Acknowledgments
The authors would like to thank Dr. Simon Henin and Joshua Hajicek for the helpful discussions. We would also like to acknowledge Dr. Sunil Puria and Kevin O’Connor for providing us with their data.
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Appendix
Appendix
OMEG Estimates and Generator Components Levels
Since the OMEG estimates were computed from the generator components levels, they were affected by the cochlear function. This section explains how the generator components levels for the two- and the three-tone conditions affect the OMEG estimates. The OMEG estimates are shown next to the two- and three-tone generator components levels in Figure 11. Each row of Figure 11 belongs to a different participant (participants’ identifiers are denoted in the left panels). The frequencies corresponding to the local maxima and minima of the OMEG estimates are marked by the vertical red and black dashed lines, respectively (Figure 11). Since the OMEG estimates are negative, minima in OMEG estimates are associated with more attenuation and maxima are associated with less attenuation. As can be observed in Figure 11, frequencies of minima and maxima in OMEG estimates (see left panels) mainly occur in close proximities of frequencies of maxima and minima in the generator components levels (solid lines) of the three-tone condition (see right panels) for all participants. The pattern of the maxima and minima in the generator components levels can affect the OMEG estimates (see “Discussion” section). As seen in Eq. 6, the OMEG estimates are affected by the basal cochlear gain. Removing such effects from the OMEG estimates should be further investigated in future.
Middle Ear Muscle Activation and Ear Canal Calibration
Middle ear muscle activation during data collection can affect the OMEG estimates. The activation of the middle ear muscle can change the levels and phases of the primary tones measured in the ear canal and therefore, the estimates of the OMEG. To check for middle ear muscle activation, the levels and phases of the primary tones were estimated. It should be noted that the probe slippage can also result in changes of the estimated primary tones in the ear canal. Using the LSF technique, explained in “Data Analysis” section, the levels and phases of the primary tones f1 and fa (in the three-tone condition) were estimated at each Lb at the microphone in the ear canal. Since L1 and La primary tone levels presented in the ear canal were the same across different Lbs, the estimated levels and phases of f1 and fa in the ear canal should stay the same at different Lbs unless the middle ear muscle was activated or the probe position changed during the data collection. The difference between the level of f1 primary tone at Lb = 55 dB SPL and the levels of f1 primary tone at Lb = 35, 40, 45, and 50 dB SPL are displayed in Figure 12a for subject NDP12; same differences were calculated for fa primary tone levels and are shown in Figure 12c. The differences between the phases of f1 primary tone, and also the phases of fa primary tone are shown in panels b and d (Fig. 12), respectively. The corresponding levels of Lb for calculating the level and phase differences are shown in the legend of Figure 12. As shown in panels b and d, the phase differences for both f1 and fa for all Lb levels are smaller than 0.08 Rad for subject NDP12; the phase differences for other participants did not exceed 0.1 and 0.3 Rad for f1 and fa, respectively, which are negligible and within the measurement error. L1 and La differences (Fig. 12a, c) have similar patterns across frequency and they do not exceed 5 dB for NDP12; the level difference was smaller than 5 dB for other participants as well.
As explained in the previous paragraph, the estimated primary tone levels f1 and fa can change at different Lbs due to the change in the probe position in the ear canal. The f1 and fa primary level changes are shown in Figure 12a, c). As explained in “Data Collection” section, changes in the position of the probe were checked by recording the ear canal responses to broadband noise (i.e., ear canal calibrations) before and after DPOAE recordings at each Lb level. If the probe remained stable during the data collection, the L1 and La ear canal calibration would remain the same at different recordings. Otherwise, the ear canal calibration would change as a result of probe slippage in the ear canal. In order to relate the primary level changes to the changes in the ear canal calibrations, the difference between the L1 ear canal calibrations along with the L1 change between Lb = 55 and 35 dB SPL are shown in Figure 13a; the comparison when Lb = 55 and 40 dB SPL can be seen in Figure 13b (same graphs for La are shown in Figure 14a, b). If the primary level changes were due to probe slippage, then high correlations between the primary level changes and the ear canal calibration changes were expected. The difference between the mean of the ear canal calibrations obtained before and after DPOAE recording at Lb = 55 dB SPL and the mean of the calibrations at Lb = 35 dB SPL (or at Lb = 40 dB SPL) is shown in gray lines in Figures 13 and 14. The difference between the mean ear canal calibrations at Lb = 55 dB SPL and the calibrations done before the DPOAE recordings at Lb = 35 dB SPL (or Lb = 40 dB SPL) are shown in light green. The difference between the mean ear canal calibrations at Lb = 55 dB SPL and the calibrations collected after the DPOAE recording at Lb = 35 dB SPL (or Lb = 40 dB SPL) are shown in dark green curves. As seen in panel a in Figure 13, the L1 change (between Lb = 35 and 55 dB SPL), shown by blue line, is highly correlated with the difference in the mean ear canal calibration at Lb = 55 and the calibrations after Lb = 35 dB SPL, shown by the dark green line. The difference between the mean L1 ear canal calibration at Lb = 55 dB SPL and the calibration before 35 dB SPL is spiky (Fig. 13a), shown by the light green curve, which is due to the presence of noise during the calibrations. As seen in Figure 13b, the difference between mean L1 ear canal calibrations at Lb = 55 dB SPL and the calibration before and after Lb = 40 dB SPL (shown by the light and dark green lines, respectively), and the L1 and La differences (shown by the blue lines) follow the same pattern and are highly correlated. The difference between the mean L1 ear canal calibrations at Lb = 55 dB SPL and the mean ear canal calibrations at Lb = 40 dB SPL (shown by the gray line) is very similar to the L1 change (see Fig. 13b). The amount of La ear canal calibration changes and L1 differences (Fig. 14a, b) are correlated and consistent with the findings for L1 in Figure 13a, b. Therefore, due to high correlations between the primary level changes and the ear canal calibration changes and also phase changes that were within the measurement error, the probe slippage was the main source for changes in the primary tone levels and phases. However, to determine how much of the change was due to middle ear muscle activation should be further investigated.
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Naghibolhosseini, M., Long, G.R. Estimation of Round-Trip Outer-Middle Ear Gain Using DPOAEs. JARO 18, 121–138 (2017). https://doi.org/10.1007/s10162-016-0592-6
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DOI: https://doi.org/10.1007/s10162-016-0592-6