Evidence for multiple DPOAE components based upon group delay of the 2f(1)-f(2) distortion in the gerbil

The cochlear delay of the 2f(1)-f(2) distortion product otoacoustic emission (DPOAE) was measured using the phase gradient method. With a constant f(2) and swept f(1), the resulting phase change of 2f(1)-f(2) was used to calculate the group delay for f(2) frequencies from 1 to 60 kHz. For f(2) frequencies between 2 and 60 kHz, the group delays were between 2.2 and 0.11 ms and continuously decreased for increasing f(2) and for increasing primary stimulus levels. For f(2) frequencies below 2 kHz, the group delay decreased to around 1 ms and was largely independent of stimulus level. The ratio curves resulting from the f(1) sweeps for high frequencies (f(2)16 kHz) displayed the typical mammalian shape with a peak in the level of 2f(1)-f(2) for a larger primary frequency separation (f(2)/f(1)1.15) and decreasing 2f(1)-f(2) level for smaller primary separation. In addition to this typical level maximum, for f(2) frequencies from about 1.8 to 16 kHz, the ratio curves displayed a second component in the form of an increase in the level of 2f(1)-f(2) for small primary separation at higher primary levels (level of f(2)30 dB SPL). For f(2) frequencies below 1.8 kHz, only the second component and no typical ratio peak as for higher f(2) could be observed and the associated group delay was always close to 0.8 ms. Several possible causes for this behavior are discussed, including different modes of DPOAE generation and modulation as well as changes in the nature of mechanical processing from base to apex in the gerbil cochlea. To evaluate the relative sensitivity of non-linear cochlear mechanics, an iso-distortion threshold curve was constructed from acoustical growth functions of the 2f(1)-f(2) DPOAE at optimum primary separation, by plotting the level of f(2) sufficient to evoke a distortion of -10 dB SPL as a function of f(2)2.5 kHz but failed to reflect the sensitivity for lower frequencies. This may be a consequence of more linear frequency processing in the apex.