Modulated otoacoustic emissions reveal features of the human cochlear impedance at low frequencies

Abstract

Basilar membrane (BM) displacement by loud low-frequency (LF) tones periodically alters the level of distortion-product otoacoustic emissions (DPOAEs) as a function of the phase of the modulating LF tone. For three human subjects the level of a modulating LF tone (15–320 Hz) was adjusted to maintain a constant DPOAE-modulation depth, indicating a constant periodic LF BM displacement. The resulting modulation LF-tone levels match the iso-loudness contours (ISO/R226) except for an antiresonance centered at about 55 Hz. Here, an increase in level of approximately 5 dB above the iso-loudness contour is required to maintain constant periodic LF BM displacement. The antiresonance separates two distinct regions of the cochlear impedance: a slope of 12 dB/octave at frequencies below 55 Hz suggests a mass-controlled impedance resulting from perilymph flow through the helicotrema; a slope of 6 dB/octave above 55 Hz suggests the existence of a traveling wave with resistive impedance. Psychophysical threshold experiments did not show the antiresonance. Antiresonances have been observed previously in animal experiments using cochlear microphonics or perilymph pressure measurements to estimate BM displacement. Modulated DPOAEs are a noninvasive method to investigate the BM movement at low frequencies.