Hearing threshold estimation using concurrent measurement of distortion product otoacoustic emissions and auditory steady-state responses.

Both distortion product otoacoustic emissions (DPOAEs) and auditory steady-state responses (ASSRs) provide frequency-specific assessment of hearing. However, each method suffers from some restrictions. Hearing losses above 50 dB HL are not quantifiable using DPOAEs and their performance at frequencies below 1 kHz is limited, but their recording time is short. In contrast, ASSRs are a time-consuming method but have the ability to determine hearing thresholds in a wider range of frequencies and hearing losses. Thus, recording DPOAEs and ASSRs simultaneously at their adequate frequencies and levels could decrease the overall test time considerably. The goal of the present study was to develop a parameter-setting and test-protocol to measure DPOAEs and ASSRs binaurally and simultaneously at multiple frequencies. Ten normal-hearing and 23 hearing-impaired subjects participated in the study. The interaction of both responses when stimulated simultaneously at frequencies between 0.25 and 6 kHz was examined. Two limiting factors need to be kept. Frequency distance between ASSR carrier frequency f(c) and DPOAE primary tone f(2) needs to be at least 1.5 octaves, and DPOAEs may not be measured if the ASSR stimulus level is 70 dB SPL or above. There was a significant correlation between pure-tone and DPOAE/ASSR-thresholds in sensorineural hearing loss ears.

Hybrid measurement of auditory steady-state responses and distortion product otoacoustic emissions using an amplitude-modulated primary tone.

A maximum auditory steady-state response (ASSR) amplitude is yielded when the ASSR is elicited by an amplitude-modulated tone (f(c)) with a fixed modulation frequency (f(m) = 40 Hz), whereas the maximum distortion product otoacoustic emission (DPOAE) level is yielded when the DPOAE is elicited using a fixed frequency ratio of the primary tones (f2/f1 = 1.2). When eliciting the DPOAE and ASSR by the same tone pair, optimal stimulation is present for either DPOAE or ASSR and thus adequate simultaneous DPOAE/ASSR measurement is not possible across test frequency f2 or f(c), respectively. The purpose of the present study was to determine whether the ASSR and DPOAE can be measured simultaneously without notable restrictions using a DPOAE stimulus setting in which one primary tone is amplitude modulated. A DPOAE of frequency 2f1-f2 and ASSR of modulation frequency 41 Hz were measured in ten normal hearing subjects at a test frequency between 0.5 and 8 kHz (f2 = f(c)). The decrease in the DPOAE level and the loss in ASSR amplitude during hybrid mode stimulation amounted, on average, to only 2.60 dB [standard deviation (SD) = 1.38 dB] and 1.83 dB (SD = 2.38 dB), respectively. These findings suggest simultaneous DPOAE and ASSR measurements to be feasible across all test frequencies when using a DPOAE stimulus setting where the primary tone f2 is amplitude modulated.

Weighted DPOAE input/output-functions: a tool for automatic assessment of hearing loss in clinical application.

2f1-f2 distortion product otoacoustic emission (DPOAE) input/output functions were recorded in 796 ears with sensorineural hearing loss at up to 50 frequencies between 500 Hz and 8 kHz in a wide level range from L2 = 20 to L2 = 65 dB SPL at up to 10 levels ("scissor paradigm", L1 = 0.4 L2 + 39, f2/f1 = 1.2). DPOAEs were accepted as valid for signal-to-noise ratios (SNR) only if they exceeded 6 dB. To assess the DPOAE threshold, DPOAE input/output functions (L2--DPOAE sound pressure diagram) were subject to linear regression and extrapolation; the point of intersection with the L2 coordinate was then interpreted as DPOAE threshold [1]. The linear fit of the function pDP(L2) = a + bL2 (whereby a and b represent the threshold and the slope of the DPOAE growth, respectively), reflects compression and sensitivity of the cochlear amplifier. Using a weighted least mean square error (LMSE) regression, more than 70% of the DPOAE I/O functions of the data set can be used for the threshold estimation. High correlation and relatively small differences between objectively estimated DPOAE threshold and subjectively determined hearing threshold of pure-tone audiograms prove that the weighted extrapolation of DPOAE I/O functions can be a valuable clinical tool for the objective assessment of cochlear hearing loss.