Theory of forward and reverse middle-ear transmission applied to otoacoustic emissions in infant and adult ears

Abstract

The purpose of this study is to understand why otoacoustic emission (OAE) levels are higher in normal-hearing human infants relative to adults. In a previous study, distortion product (DP) OAE input/output (I/O) functions were shown to differ at f2 = 6 kHz in adults compared to infants through 6 months of age. These DPOAE I/0 functions were used to noninvasively assess immaturities in forward/reverse transmission through the ear canal and middle ear [Abdala, C., and Keefe, D. H., (2006). J. Acoust Soc. Am. 120, 3832-3842]. In the present study, ear-canal reflectance and DPOAEs measured in the same ears were analyzed using a scattering-matrix model of forward and reverse transmission in the ear canal, middle ear, and cochlea. Reflectance measurements were sensitive to frequency-dependent effects of ear-canal and middle-ear transmission that differed across OAE type and subject age. Results indicated that DPOAE levels were larger in infants mainly because the reverse middle-ear transmittance level varied with ear-canal area, which differed by more than a factor of 7 between term infants and adults. The forward middle-ear transmittance level was -16 dB less in infants, so that the conductive efficiency was poorer in infants than adults.

FIG. 1

Signal flow diagram in external, middle, and inner ears. On pressure variables and S matrices, a superscript + denotes wave in forward direction, and superscript − denotes wave in reverse direction.

FIG. 2

The median source reflectance is shown for the probe used in infant (black solid line) and adult measurements (gray dashed line), while the distribution of source-reflectance responses is shown by the corresponding box and whisker plots spaced an octave apart. Box and whiskers plots are also shown at each of the f_DP and f₂ frequencies. The magnitude of the source reflectance is shown in the top panel and the phase of the source reflectance is shown in the bottom panel. The distribution of responses was based on N=20 calibrations of the adult probe and N=25 calibrations using the infant probe, with each calibration performed on a separate day.

FIG. 3

The median ear-canal reflectance (average in frequency over each 1/12th octave) is shown for the adult group (gray line) and the full-term infant group (black line). The distribution of ear-canal reflectance responses in each group is also shown by box and whisker plots spaced an octave apart. Box and whiskers plots are shown at each of the f_DP and f₂ frequencies. The ear-canal energy reflectance is shown in the top panel and its unwrapped phase is shown in the bottom panel.

FIG. 4

The median ear-canal reflectance (average in frequency over each 1/3 octave) is shown for the adult group (gray line) and six infant groups (black lines). The magnitude of the ear-canal reflectance is shown in the top panel and its unwrapped phase is shown in the bottom panel. The line style and line thickness for each age group is listed in the legend.

FIG. 5

The absolute level of the median forward ear-canal transfer function is shown in the top panel for the adult group (gray line) and six infant groups (black lines). The line style and line thickness are listed in the legend for each infant age group. The corresponding absolute level of the median reverse ear-canal transfer function is shown in the bottom panel.

FIG. 6

Top panel: a round-trip ear-canal transfer function for a DPOAE test with f₂/f₁ = 1.2 is plotted vs f₂ as the sum of the medians of the forward ear-canal transfer function level L_FE(f₂) and the reverse ear-canal transfer function level L_RE(f_DP). Bottom panel: a round-trip ear-canal transfer function for a SFOAE test is plotted vs its stimulus frequency (denoted f₂) as the sum of the medians of the forward ear-canal transfer-function level L_FE(f₂) and the reverse ear-canal transfer-function level L_RE(f₂). Results are shown for differing age groups with line styles as in Fig. 5.

FIG. 7

Top panel: the level of each infant group relative to the adult group of the median forward ear-canal transfer function is plotted. The line style and line thickness are listed in the legend for each infant age group. Bottom panel: The corresponding level of each infant group relative to the adult group of the median reverse ear-canal transfer function is plotted.

FIG. 8

The median of the acoustic estimate of the cross-sectional area of the ear canal is plotted as a function of age for infants and adults (solid line). Box and whisker plots show the distribution of measured areas in the present study with asterisks denoting an outlier in the term-infant group and in the 5 month group. The right-hand axis expresses this area as the diameter (in mm) of the equivalent circular cross section. The acoustic estimate of the ear-canal area from Keefe et al. (1993) are shown in a dashed line with circles as markers.

FIG. 9

The box plots of the median and IQR of relative area level ΔL_α (in dB) are plotted as a function of age based on the ratio of the median adult ear-canal area to the distribution of ear-canal areas in each infant age group.

FIG. 10

Top panel: the relative forward transfer-function level ΔL_F data of Abdala and Keefe (2006) (dashed line, upwards triangle marker) and the ear-canal component ΔL_FE (solid black line, circle marker) are used to estimate the relative forward middle-ear transmittance level ΔL_FM (solid gray line, downwards triangle marker) at f₂=6 kHz as a function of infant age. Bottom panel: the relative reverse transfer-function level ΔL_R data of Abdala and Keefe (2006) (dashed line, upwards triangle marker) are plotted as a function of infant age at L_DP=4 kHz, as is its ear-canal component ΔL_RE (solid black line, circle marker), and its middle-ear component ΔL_RE (solid gray line, downwards triangle marker). The ΔL_R is compared to the model estimate $\mathrm{\Delta }{L}_R^{\prime }$ plot (dashed-dotted line, × marker). The error bars in each estimate are shown as ±1 SE. Some data are slightly offset horizontally to improve clarity in viewing error bars.