Phase delay measurements of distortion product otoacoustic emissions at 2f1-f2 and 2f2-f1 in human ears.

The present study used distortion product otoacoustic emission (DPOAE) latency as a tool to provide information about the generation sites of 2f1-f2 and 2f2-f1 DPOAEs in humans. The DPOAE 2f1-f2 is supposed to be generated near the f2 site, but little is known about the 2f2-f1 DPOAE processing in humans. The present work sought to test several hypotheses as to the possible generation of 2f1-f2 at the f2 site and of 2f2-f1 at the 2f2-f1 site as well as their backward reflection site, by comparing latencies of the two DPOAEs, under appropriate frequency manipulation. The effect of stimulus level was also studied. The latency values were calculated as the phase-lag related to the frequency shift, using the ILO92 software. Amplitudes were lower and latencies shorter for 2f2-f1 than for 2f1-f2. As expected, 2f1-f2 and 2f2-f1 DPOAE latency decreased with increasing stimulus level and frequency. At 70 dB SPL, latencies of DPOAEs with primaries f1 and f2 or f1 and f2, chosen so as to obtain 2f1-f2 = 2f2 - f1, were identical, whereas at 55 dB SPL the similarities were less obvious, suggesting two different generation processes. The present study suggests that the comparison of several DPOAE components may produce useful information about their processing within the cochlea.