| Abstract #403, Date 2/15/99, Session K6, Poster (B150) |
| The temporal envelope of the 2f1-f2 distortion product otoacoustic emission |
| *E.M. Relkin, B.A. Prieve, A. Sterns (Syracuse University) |
We have developed a technique for measuring the temporal envelope of the 2f1-f2 distortion product otoacoustic emission (DPOAE). The method is based on lock-in detection of the in-phase and quadrature components of the DPOAE followed by signal averaging of each component. Primary tones were produced by synchronized digital oscillators that shared a common clock. Outputs of the oscillators were gated synchronously. Onset and offsets were shaped by cosine-squared functions 20 ms in duration and total tone duration was 520 ms. Tone bursts were repeated every 2.53 seconds. The magnitude and phase of the DPOAE were computed as functions of time from the averaged in-phase and quadrature components The resolution with which the temporal envelope was measured was limited by the output filter of the lock-in detector, which was set for a time constant of 3 ms. Onset latencies were calculated from the rising portion of the magnitude versus time function referenced to the sound pressure of the primary tones in the ear canal. We have completed a normative study in 10 young adult listeners. The envelope of the DPOAE was measured as a function of the primary levels with f2 level fixed at 10 dB below the level of f1. The f2 level was incremented in 5 dB steps from 45 to 75 dB SPL and f2/f1 ratio was 1.25. Input/output functions were obtained for f2 frequencies of 2.0, 4.0, 6.25, and 8.0 kHz. Input/output functions demonstrated well-established characteristics and onset latencies decreased with increasing primary levels. However, onset latencies did not vary systematically as a function of the DPOAE frequency within subjects, casting doubt on the interpretation of these latencies as representative of basilar membrane travel times. We have begun studies of the time course of ipsilateral and contralateral effects that have been attributed by others to the medial olivocochlear efferent system. Preliminary results of those studies will also be presented. |