Generation of DPOAEs in the guinea pig.

In humans, distortion product otoacoustic emissions (DPOAEs) at frequencies lower than the f(2) stimulus frequency are a composite of two separate sources, these two sources involving two distinctly different mechanisms for their production: non-linear distortion and linear coherent reflection [Talmadge et al., J. Acoust. Soc. Am. 104 (1998) 1517-1543; Talmadge et al., J. Acoust. Soc. Am. 105 (1999) 275-292; Shera and Guinan, J. Acoust. Soc. Am. 105 (1999) 332-348; Kalluri and Shera, J. Acoust. Soc. Am. 109 (2001) 662-637]. In rodents, DPOAEs are larger, consistent with broader filters; however the evidence for two separate mechanisms of DPOAE production as seen in humans is limited. In this study, we report DPOAE amplitude and phase fine structure from the guinea pig with f(2)/f(1) held constant at 1.2 and f(2) swept over a range of frequencies. Inverse Fast Fourier Transform analysis and time-domain windowing were used to separate the two components. Both the 2f(1)-f(2) DPOAE and the 2f(2)-f(1) DPOAE were examined. It was found that, commensurate with human data, the guinea pig DPOAE is a composite of two components arising from different mechanisms. It would appear that the 2f(1)-f(2) emission measured in the ear canal is usually dominated by non-linear distortion, at least for a stimulus frequency ratio of 1.2. The 2f(2)-f(1) DPOAE exhibits amplitude fine structure that, for the animals examined, is predominantly due to the variation in amplitude of the place-fixed component. Cochlear delay times appear consistent with a linear coherent reflection mechanism from the distortion product place for both the 2f(1)-f(2) and 2f(2)-f(1) place-fixed components.