Comparing Distortion Product Otoacoustic Emissions to Intracochlear Distortion Products Inferred from a Noninvasive Assay

The behavior of intracochlear distortion products (iDPs) was inferred by interacting a probe tone (f₃) with the iDP of interest to produce a “secondary” distortion product otoacoustic emission termed DPOAE^2ry. Measures of the DPOAE^2ry were then used to deduce the properties of the iDP. This approach was used in alert rabbits and anesthetized gerbils to compare ear-canal 2f₁-f₂ and 2f₂-f₁ DPOAE f₂/f₁ ratio functions, level/phase (L/P) maps, and interference-response areas (IRAs) to their simultaneously collected DPOAE^2ry counterparts. These same measures were also collected in a human volunteer to demonstrate similarities with their laboratory animal counterparts and their potential applicability to humans. Results showed that DPOAEs and inferred iDPs evidenced distinct behaviors and properties. That is, DPOAE ratio functions elicited by low-level primaries peaked around an f₂/f₁ = 1.21 or 1.25, depending on species, while the corresponding inferred iDP ratio functions peaked at f₂/f₁ ratios of ~1. Additionally, L/P maps showed rapid phase variation with DPOAE frequency (f_dp) for the narrow-ratio 2f₁-f₂ and all 2f₂-f₁ DPOAEs, while the corresponding DPOAE^2ry measures evidenced relatively constant phases. Common features of narrow-ratio DPOAE IRAs, such as large enhancements for interference tones (ITs) presented above f₂, were not present in DPOAE^2ry IRAs. Finally, based on prior experiments in gerbils, the behavior of the iDP directly measured in intracochlear pressure was compared to the iDP inferred from the DPOAE^2ry and found to be similar. Together, these findings are consistent with the notion that under certain conditions, ear-canal DPOAEs provide poor representations of iDPs and thus support a “beamforming” hypothesis. According to this concept, distributed emission components directed toward the ear canal from the f₂ and basal to f₂ regions can be of differing phases and thus cancel, while these same components directed toward f_dp add in phase.