Dependence of distortion-product otoacoustic emissions on primary levels in normal and impaired ears. I. Effects of decreasing L₂ below L₁

The 2f₁-f₂ distortion-produoctto acoustiecm ission(D POAE) is evokedb y two primaryt oneso frequencis f₁ < f₂, and level L₁ and L₂. Previousr eportsi ndicatet hat decreasin L₂ below L₁ =L₂ Can; (1) increase DPOAE amplitudein normalears, and (2) increase the degree to which DPOAE amplitude in normal ears, and (2) increase the degree to which DPOAE amplitudes are reduced by cochlear trauma. Although both of these factors could be advantageous for clinical applications of DPOAEs, neither has been explored in detail. In the present study, 2f₁-f₂ DPOAE-amplitude frequency functions were collected from normal and impaired ears of rabbits and humans, with L₁ - = L₂, and with L₂ < L₁, at each of three values of L₁. In rabbits, controlled tonal or noise overexposures were used to produce permanent reductions of DPOAE amplitude. Comparison of pre- and postexposure DPOAE-amplitude frequency function demonstrated that the frequency-specific reductions of DPOAEs were enhanced by decreasing L₂ below L₁. In humans, DPOAE-amplitude frequency functions obtained with the various L₁ and L₂ combinations were collected from 16 normal ears to provide preliminary normative data for each stimulus-level condition. The L₁ = L₂ that produced the maximum DPOAE amplitude in normal ears was systematically dependent on L₁. Thus at most frequencies, decreasing L₂ below L₁ = L₂ substantially reduced mean DPOAE amplitude when L₁ 75 dB SPL, but increased mean DPOAE amplitudes at L₁ =65 dB SPL. However, the increase of mean DPOAE amplitude obtained by decreasing L₂ below L₁ =65 dB SPL was small, beingless than 3.5 dB at most frequencies. More importantly, at L₁ =65 dB SPL, L₂ could be decreased considerably below L₁ =L₂ without reducing mean DPOAE amplitude relative to that at L₁ = L₂. Inspection of DPOAE-amplitud frequency functions obtained from subjects with mild or moderate sensorineural hearing losse indicated that, in frequency regions of hearing impairment, decreasing L₂ below L₁ can enhance the degree of reduction of DPOAEs below the corresponding normative amplitudes, without reducing the normative amplitudes. It is concluded that decreasing L₂ below L₁ = L₂ has the potential to enhance the performance of DPOAEs in clinical application.