Effect of Calibration Method on DPOAE Measurements: II. Threshold Prediction

1,3 2,3 3 3 3 3Abigail R. Rogers , Sienna R. Burke , Judy G. Kopun , Hongyang Tan , Stephen T. Neely , & Michael P. Gorga1 2 3Indiana University, Bloomington, IN, University of Maryland, College Park, MD, Boys Town National Research Hospital, Omaha, NE

Introduction

Methods

Conclusion

Acknowledgements

References

Distortion product otoacoustic emissions (DPOAEs) are used as an objective measure of cochlear function. To ensure that intended stimulus levels are presented to the ear, in situ calibrations typically are performed in sound pressure level (SPL) prior to measurements. SPL calibrations, however, are susceptible to influence from standing waves at specific frequencies depending on the ear-canal dimensions (e.g., Siegel, 1994, 2007; Siegel and Hirohata, 1994). This situation can result in over- or under-estimation of stimulus level at the eardrum. Recently, Scheperle et al. (2008) showed that stimuli calibrated in forward pressure level (FPL) provide more consistent DPOAE measurements than stimuli calibrated in SPL. The purpose of the present study is to extend the work of Scheperle et al. by comparing SPL and FPL calibrations using correlations between DPOAE thresholds and behavioral thresholds for normal-hearing and hearing-impaired subjects. Effects of stimulus frequency, calibration method, and method of threshold prediction were examined in order to determine which calibration method and which DPOAE threshold estimate results in the highest correlation with behavioral threshold. In a companion study, Burke et al. (2009) tested the extent to which calibration method impacts test performance, defined as the ability of DPOAEs to accurately classify ears as either normal hearing or hearing impaired. The results from that study are presented in a separate poster.

155 subjects with normal and impaired hearing and ages ranging from 11 to 79 years participated in the study. Tympanometry indicated normal middle-ear function just prior to DPOAE measurements for all subjects. A pure-tone audiogram was obtained for each subject using common clinical procedures.

For DPOAE measurements, five calibrations were obtained, one using SPL and four using FPL. FPL is derived from SPL using The venin-equivalent source characteristics, which were obtained from measurements in five brass cavities of known lengths. The source calibration allows the forward and reverse pressure-wave components of the stimulus to be separated mathematically. FPL represents only the level of the forward pressure wave, thus avoiding standing-wave problems. FPL calibrations included a reference condition derived from 25 repeated measurements prior to any data collection and a daily calibration just prior to each data-collection session. Both reference and daily calibrations were obtained at room and body temperature.

DPOAE input/output (I/O) functions were measured at f 's of 2, 3, 4, 6, 2

and 8 kHz with f /f @ 1.22. The L starting level was 70 dB and decreased 2 1 2

in 5-dB steps. L was set according to the equation L = 0.4L + 39 1 1 2

(Kummer et al.,1998). To increase data-collection efficiency, measurements were terminated once the DPOAE level was less than 3 dB above the noise.

DPOAE thresholds were estimated using two methods: threshold was taken as the lowest L for which the SNR = 3 dB (SNR method) and a 2

method developed by Boege & Janssen (2002), in which DPOAE levels were converted to pressure and fit with a linear function (LR method). The functions were solved for the L that corresponded to 0 Pa, which 2

was defined as DPOAE threshold. Once DPOAE thresholds were estimated using both SNR and LR methods, they were correlated with behavioral thresholds. These correlations were used to compare calibration methods in terms of the accuracy with which they predicted behavioral threshold.

Effects of frequency were similar to those noted by Gorga et al. (2003). Correlations between behavioral and DPOAE thresholds were greatest at 3-6 kHz and lowest at 8 kHz. There were no consistent differences between correlations related to calibration method. Correlations were slightly higher when DPOAE thresholds were estimated using the LR technique, compared to the SNR technique. Differences between behavioral and DPOAE thresholds also were largest when the SNR technique was used to estimate DPOAE threshold. In total, these results suggest that SPL calibrations may be adequate when attempting to predict pure-tone thresholds from DPOAE measurements. These results were not anticipated, given the known effects of standing waves on estimates of ear-canal SPL at the plane of the probe (Siegel, 1994; Siegel and Hirohata, 1994; Siegel, 2007; Driesbach and Siegel, 2001). The results are particularly surprising because frequencies were studied at which standing-wave effects are expected to occur. However, it is important to note that differences between calibrations will only occur at or near frequencies where standing waves are present. It may be that for many of the conditions in this experiment, standing waves did not occur at or near the specific frequency being tested.

This work was supported by several NIH grants: DC T35 8757, DC R01 2251, DC P30 4662, DC R13 6616, DC R01 8318

?Boege & Janssen (2002). J. Acoust. Soc. Am., 111(4), 1810-1818.?Dreisbach & Siegel (2001). J. Acoust. Soc. Am., 110(5), Pt. 1, 2456-2467.?Gorga et al. (2003). J. Acoust. Soc. Am., 113(6), 3275-3283.?Kummer et al. (1998). J. Acoust. Soc. Am., 103(6), 3431-3444.?Scheperle et al. (2008). J. Acoust. Soc. Am., 124(1), 288-300.

rd?Siegel (2007). Otoacoustic Emissions: Clinical Applications 3 Ed. Thieme:

New York. ?Siegel (1994). J. Acoust. Soc. Am., 95(5), Pt. 1, 2589-2597.?Siegel & Hirohata (1994). Hear. Res., 80, 146–152.

Figure 1. Individual DPOAE I/O functions for two normal-hearing and two hearing-impaired subjects. I/O functions obtained using SPL calibrations are sometimes set apart from those obtained using FPL calibrations. However, for some frequencies, all of the I/O functions lie close to one another. The variability in these functions across calibration method, frequency and subjects suggests that differences in calibration cannot be predicted a priori.

Figure 2. Both DPOAE threshold-estimation techniques for all five calibration methods using data from a normal-hearing subject (behavioral threshold = 5 dB HL at 4 kHz). Top panel: DPOAE I/O functions are shown with DPOAE level (dB) as a function of L . Thresholds were 2

defined as the lowest L for which 2

the SNR > 3 dB, and are provided as insets in the top panel for each calibration methods. The threshold for SPL is 10 dB lower than for the FPL calibrations. This finding suggests that, during SPL calibration, a standing wave occurred, resulting in destructive interaction between incident and reflective waves. As a consequence, the level at the plane of the probe was less than the level at the eardrum, resulting in an underestimation of threshold. Bottom panel: DPOAE I/O functions are shown

following transformation of the data into pressure (Pa). Linear regressionswere fit to the data and solved for the L 's resulting in 2

pressures of 0 Pa, which were defined as threshold (Boege & Janssen, 2002). While the differences between SPL and FPL calibrations are less using this approach, it is still the case that the DPOAE threshold based on SPL calibration was lower than any of the thresholds based on FPL calibration.

Figure 3. Behavioral thresholds as a function of DPOAE thresholds for each calibration method collapsed across frequency. The SNR and LR estimates of DPOAE thresholds are shown in left and right columns, respectively. The solid line in each panel represents the best-fit line to the data. Also shown as insets are correlation coefficients, the number of threshold comparisons, and standard errors. To improve the accuracy with which data from DPOAE I/O functions predicted behavioral thresholds, all behavioral thresholds less than 0 dB HL were set to 0 dB HL and all estimated DPOAE thresholds less than 0 dB were set to 0 dB. Another truncation occurred at the upper limits of behavioral and estimated DPOAE thresholds in that any threshold greater than 60 dB was set to 60 dB. The differences between correlation coefficients across calibration methods are less than 0.05 for each of the two threshold-prediction

methods. The differences between correlation coefficients for the threshold-prediction methods are less than 0.03 for each of the calibration methods.

Figure 4. For each calibration method, the difference between DPOAE and behavioral thresholds as a function of behavioral threshold is shown, collapsed across frequency. Open and close symbols repersent differences when DPOAE thresholds were estimated with SNR and LR methods, respectively. The differences between SNR and LR threshold estimations are small. However, almost without exception, there is better agreement between behavioral thresholds and those estimated using the LR method. Differences between DPOAE and behavioral thresholds are near and below zero at behavioral thresholds of 45 to 60 dB HL. This may be due to a smaller number of conditions where thresholds could be estimated from DPOAEs at these HLs, the truncation of thresholds noted above, or more accurate predictions of behavioral thresholds at these levels. There is no apparent effect of calibration

methods. While it appears that DPOAE thresholds for both threshold-estimation techniques overestimate behavioral thresholds, the size of this effect appears to be independent of calibration procedure.

Figure 5. Behavioral threshold as a function of DPOAE thresholds at each frequency (shown separately in each row) and for each calibration method (shown separately in each column) when the SNR threshold estimation method is used. Effects of frequency are similar to those reported by Gorga et al. (2003). Correlations were greatest at 3-6 kHz, lowest at 8 kHz, with intermediate correlations at 2 kHz. There does not appear to be an effect of calibration method at any frequency.

Figure 6. Behavioral threshold as a function of DPOAE threshold when DPOAE threshold was estimated using the LR method, following the convention used in Fig. 5. Just as was the case when DPOAE threshold was estimated using the SNR method, the highest correlations were observed for frequencies between 3 and 6 kHz, with the poorest agreement at 8 kHz. Also similar to the results shown in Fig. 5, there is no effect of calibration method on the correlation between behavioral and DPOAE thresholds.

Results