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

Area of Science:

• Auditory Neuroscience
• Otoacoustic Emissions Research
• Bioacoustics

Background:

• Intracochlear distortion products (iDPs) are crucial for hearing but challenging to measure directly.
• Distortion product otoacoustic emissions (DPOAEs) are commonly used to infer iDPs, but their accuracy is debated.

Purpose of the Study:

• To investigate the relationship between ear-canal DPOAEs and actual intracochlear distortion products (iDPs).
• To compare the behavior of DPOAEs and inferred iDPs using a novel secondary DPOAE (DPOAE(2ry)) measurement technique.
• To assess the applicability of these findings in humans.

Main Methods:

• Generated secondary DPOAEs (DPOAE(2ry)) by probing intracochlear distortion products (iDPs) with a third tone.
• Compared DPOAE and DPOAE(2ry) measures, including f2/f1 ratio functions, level/phase maps, and interference-response areas (IRAs), in rabbits, gerbils, and a human volunteer.
• Validated the inferred iDP behavior against direct intracochlear pressure measurements in gerbils.

Main Results:

• DPOAEs and inferred iDPs exhibited distinct behaviors, particularly in ratio functions and phase patterns.
• DPOAE ratio functions peaked at higher f2/f1 ratios (1.21-1.25) than inferred iDPs (~1).
• DPOAE(2ry) measures showed more stable phases and lacked common IRA features seen in DPOAEs, aligning with direct intracochlear measurements.

Conclusions:

• Ear-canal DPOAEs may not always accurately represent intracochlear distortion products.
• Findings support a 'beamforming' hypothesis where intracochlear components can cancel in the ear canal but add in phase towards the measurement point.
• The DPOAE(2ry) method offers a promising approach to better understand iDPs and auditory function in humans.