Human efferent adaptation of DPOAEs in the L1, L2 space

Abstract

The adaptive properties of distortion product otoacoustic emissions (DPOAEs) at 2 f 1 − f 2 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 f 2 of 1.55 kHz ( f 2/ f 1 = 1.21) was evaluated as a function of the levels of the primary tones in a matrix of L 1, L 2 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 L 1, L 2 space. The 2 f 1 − f 2 DPOAE revealed consistent regions of suppression (⩽−0.5 dB difference) or enhancement (⩾+0.5 dB difference) with respect to baseline measures within the L 1, L 2 matrix for both acoustic-stimulation conditions. Specifically, 2 f 1 − f 2 DPOAE suppressions of 1–2 dB occurred for both monaural and binaural presentations, typically at level combinations in which L 1 > L 2. In contrast, larger 2 f 1 − f 2 DPOAE enhancements of 3–4 dB occurred for only the binaural condition, at primary-tone level combinations where L 1 < L 2. Although adaptation activity was also evaluated for the DPOAEs at f 2 − f 1, 2 f 2 − f 1, and 3 f 1 − 2 f 2, these emissions were either immeasurable (e.g., f 2 − f 1) 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 2 f 1 − f 2 results suggest that a potentially important area for adaptation measures exists in the L 1, L 2 space, when L 1 is lower than L 2. 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.