Sources of variability in distortion product otoacoustic emissions

Abstract

The goal of this study was to determine the extent to which the variability seen in distortion product otoacoustic emissions (DPOAEs), among ears with normal hearing, could be accounted for. Several factors were selected for investigation, including behavioral threshold, differences in middle-ear transmission characteristics either in the forward or the reverse direction, and differences in contributions from the distortion and reflection sources. These variables were assessed after optimizing stimulus parameters for individual ears at each frequency. A multiple-linear regression was performed to identify whether the selected variables, either individually or in combination, explained significant portions of variability in DPOAE responses. Behavioral threshold at the f(2) frequency and behavioral threshold squared at that same frequency explained the largest amount of variability in DPOAE level, compared to the other variables. The combined model explained a small, but significant, amount of variance in DPOAE level at five frequencies. A large amount of residual variability remained, even at frequencies where the model accounted for significant amounts of variance.

Figure 1

Mean DPOAE (circles) and noise (triangles) levels as a function of L₂ for each of the nine f₂ frequencies. Error bars represent±1 SD.

Figure 2

L_d corrected plotted as a function of L₂ at each of nine f₂ frequencies. Each panel includes estimates of L_d from 40 subjects at each of 13 L₂ levels, for a total of 520 points per panel. The dashed line at L_d corrected=0 is the mean residual variance and provides a point of reference.

Figure 3

L_d corrected as a function of behavioral threshold, with data at nine f₂ frequencies shown separately in each panel. The line in each panel represents the quadratic function that was fitted to the data. The slope values represent the unstandardized coefficients for behavioral threshold and behavioral threshold squared, respectively. An asterisk indicates a significant relationship.

Figure 4

L_d corrected as a function of energy reflectance at the f₂ frequency, with data at nine f₂ frequencies shown in each panel. The line in each panel represents a linear fit to the data. The slope values represent the unstandardized coefficients for energy reflectance at the f₂ frequency. The unstandardized coefficients were examined for all variables because not all of the independent variables share the same variance and they are based on the original, not the standardized scores of the predictor. An asterisk indicates a significant relationship.

Figure 5

L_d corrected as a function of admittance magnitude at the f₂ frequency, with data at nine f₂ frequencies shown separately in each panel. The line in each panel represents a linear fit to the data and the slope values represent the unstandardized coefficients for admittance magnitude at the f₂ frequency. An asterisk indicates a significant relationship.

Figure 6

L_d corrected as a function of energy reflectance at the f_d frequency, with data at nine f₂ frequencies shown separately in each panel. The line in each panel represents a linear fit to the data and the slope values represent the unstandardized coefficients for energy reflectance at the f_d frequency. An asterisk indicates a significant relationship.

Figure 7

L_d corrected as a function of equivalent volume at the f_d frequency, with data at nine f₂ frequencies shown separately in each panel. Following the same conventions as in Fig. 5, the line in each panel represents a linear fit to the data and the slope values represent the unstandardized coefficients for equivalent volume at the f_d frequency. An asterisk indicates a significant relationship.

Figure 8

DPOAE level as a function of L₂ for 1000, 2000, and 4000 Hz plotted separately in each panel. The solid line represents the DPOAE collected without a suppressor and the dashed line represents the DPOAE collected with a suppressor tone present.

Figure 9

Normalized slope plotted as a function of f₂ frequency. The top panel represents the status of the cochlea and plots behavioral threshold at the f₂ frequency and behavioral threshold squared at that same frequency. The second panel represents middle-ear transmission in the forward direction by plotting energy reflectance and admittance magnitude at the f₂ frequency. The third panel represents middle-ear transmission in the reverse direction by plotting energy reflectance and equivalent volume at f_d. The bottom panel represents the reflection-source contribution. Significant values are represented by filled symbols and nonsignificant values are represented by open symbols.