Recently, Boege and Janssen [J. Acoust. Soc. Am. 111, 1810-1818 (2002)] fit linear equations to distortion product otoacoustic emission (DPOAE) input/output (UO) functions after the DPOAE level (in dB SPL) was converted into pressure (in microPa). Significant correlations were observed between these DPOAE thresholds and audiometric thresholds. The present study extends their work by (1) evaluating the effect of frequency, (2) determining the behavioral thresholds in those conditions that did not meet inclusion criteria, and (3) including a wider range of stimulus levels. DPOAE I/O functions were measured in as many as 278 ears of subjects with normal and impaired hearing. Nine f2 frequencies (500 to 8000 Hz in 1/2-octave steps) were used, L2 ranged from 10 to 85 dB SPL (5-dB steps), and L1 was set according to the equation L1 = 0.4L2 + 39 dB [Kummer et al., J. Acoust. Soc. Am. 103, 3431-3444 (1998)] for L2 levels up to 65 dB SPL, beyond which L1 = L2. For the same conditions as those used by Boege and Janssen, we observed a frequency effect such that correlations were higher for mid-frequency threshold comparisons. In addition, a larger proportion of conditions not meeting inclusion criteria at mid and high frequencies had hearing losses exceeding 30 dB HL, compared to lower frequencies. These results suggest that DPOAE I/O functions can be used to predict audiometric thresholds with greater accuracy at mid and high frequencies, but only when certain inclusion criteria are met. When the SNR inclusion criterion is not met, the expected amount of hearing loss increases. Increasing the range of input levels from 20-65 dB SPL to 10-85 dB SPL increased the number of functions meeting inclusion criteria and increased the overall correlation between DPOAE and behavioral thresholds.

DPOAE input/output (I/O) functions were measured at 7f2 frequencies (1 to 8 kHz; f2/f1 = 1.22) over a range of levels (-5 to 95 dB SPL) in normal-hearing and hearing-impaired human ears. L1-L2 was level dependent in order to produce the largest 2f1-f2 responses in normal ears. System distortion was determined by collecting DP data in six different acoustic cavities. These data were used to derive a multiple linear regression model to predict system distortion levels. The model was tested on cochlear-implant users and used to estimate system distortion in all other ears. At most but not all f2's, measurements in cochlear implant ears were consistent with model predictions. At all f2 frequencies, the ears with normal auditory thresholds produced I/O functions characterized by compressive nonlinear regions at moderate levels, with more rapid growth at low and high stimulus levels. As auditory threshold increased, DPOAE threshold increased, accompanied by DPOAE amplitude reductions, notably over the range of levels where normal ears showed compression. The slope of the I/O function was steeper in impaired ears. The data from normal-hearing ears resembled direct measurements of basilar membrane displacement in lower animals. Data from ears with hearing loss showed that the compressive region was affected by cochlear damage; however, responses at high levels of stimulation resembled those observed in normal ears.

ABSTRACTresults should help advance the basic knowledge of human cochlear mechanics operating under the control of the MOC feedback system and contribute to Previous studies of animals observed a phenomenon of the development of practical applications such as idenadaptation of distortion product otoacoustic emission tifying people at high risk of acoustical injury and a (DPOAE) and found that the phenomenon was mediclinical test of the functional status of the MOC system. ated to a large extent by the medial olivocochlear

Evidence of the compressive growth of basilar-membrane displacement can be seen in distortion-product otoacoustic emission (DPOAE) levels measured as a function of stimulus level. When the levels of the two stimulus tones (f1 and f2) are related by the formula L1 = 39 dB + 0.4 x L2 [Kummer et al., J. Acoust. Soc. Am. 103, 3431-3444 (1998)] the shape of the function relating DPOAE level to L2 is similar (up to an L2 of 70 dB SPL) to the classic Fletcher and Munson [J. Acoust. Soc. Am. 9, 1-10 (1933)] loudness function when plotted on a logarithmic scale. Explicit estimates of compression have been derived based on recent DPOAE measurements from the laboratory. If DPOAE growth rate is defined as the slope of the DPOAE I/O function (in dB/dB), then a cogent definition of compression is the reciprocal of the growth rate. In humans with normal hearing, compression varies from about 1 at threshold to about 4 at 70 dB SPL. With hearing loss, compression is still about 1 at threshold, but grows more slowly above threshold. Median DPOAE I/O data from ears with normal hearing, mild loss, and moderate loss are each well fit by log functions. When the I/O function is logarithmic, then the corresponding compression is a linear function of stimulus level. Evidence of cochlear compression also exists in DPOAE suppression tuning curves, which indicate the level of a third stimulus tone (f3) that reduces DPOAE level by 3 dB. All three stimulus tones generate compressive growth within the cochlea; however, only the relative compression (RC) of the primary and suppressor responses is observable in DPOAE suppression data. An RC value of 1 indicates that the cochlear responses to the primary and suppressor components grow at the same rate. In normal ears, RC rises to 4, when f3 is an octave below f2. The similarities between DPOAE and loudness compression estimates suggest the possibility of predicting loudness growth from DPOAEs; however, intersubject variability makes such predictions difficult at this time.

A more accurate determination of auditory status at a specific frequency can be obtained by combining multiple predictor variables. Although the DF and LF multivariate approaches resulted in the greatest separation between normal and impaired distributions, overlap still exists, which suggests that there would be value in continued efforts to improve DPOAE test performance.