Human efferent adaptation of DPOAEs in the L₁,L₂ space

The adaptive properties of distortion product otoacoustic emissions (DPOAEs) at 2f₁ - f₂ were investigated in 12 ears of normally hearing adults aged 18-30 years using long-lasting 1-s primary-tone on-times. In this manner, DPOAE adaptation at a single f₂ of 1.55 kHz (f₂/f₁ = 1.21) was evaluated as a function of the levels of the primary tones in a matrix of L₁,L₂ settings, which varied from 45 to 80 dB SPL, in 5-dB steps. DPOAEs were elicited under both monaural and binaural stimulus-presentation conditions. Adaptation was defined as the difference in DPOAE levels between the initial 92-ms baseline measure using a standard protocol and one obtained during the final 92 ms of the prolonged 1-s primary-tones. These differences were averaged across subjects to create contour plots of mean adaptation in the L₁,L₂ space. The 2f₁ - f₂ DPOAE revealed consistent regions of suppression (≤-0.5 dB difference) or enhancement (≥+0.5 dB difference) with respect to baseline measures within the L₁,L₂ matrix for both acoustic-stimulation conditions. Specifically, 2f₁ - f ₂ DPOAE suppressions of 1-2 dB occurred for both monaural and binaural presentations, typically at level combinations in which L₁ > L₂. In contrast, larger 2f₁ - f₂ DPOAE enhancements of 3-4 dB occurred for only the binaural condition, at primary-tone level combinations where L₁ < L₂. Although adaptation activity was also evaluated for the DPOAEs at f₂ - f₁, 2f₂ - f₁, and 3f₁ - 2f₂, these emissions were either immeasurable (e.g., f₂ - f₁) or only present in a subset of subjects over a narrow range of primary-tone frequencies and levels that did not support a systematic analysis. In summary, the 2f ₁ - f₂ results suggest that a potentially important area for adaptation measures exists in the L₁,L₂ space, when L₁ is lower than L₂. This combination of primary-tone levels can lead to large DPOAE adaptation effects that may be related to a notch in the DPOAE response/growth or input/output (I/O) function.