The effect of the helicotrema on low-frequency cochlear mechanics and hearing

At very low frequencies, when the travelling wave reaches the apical end of the cochlea, perilymph is forced through the helicotrema. Iso-modulation curves of distortion product otoacoustic emissions (DPIMCs) reveal non-invasively the frequency-dependence of low-frequency sound transmission onto the basilar membrane, and demonstrate that this shunting contributes in humans to the steep decline in hearing sensitivity below 40 Hz. Just above, an irregularity often indicates a resonant interaction between perilymph inertia in the helicotrema and apical basilar membrane compliance. Anticipating an effect of this resonance on sound perception, we measured DPIMCs and equal-loudness contour (ELC) between 20 and 160 Hz in 14 subjects. A pronounced (non-monotonic) resonance was visible in 16 DPIMCs of 26 measurable ears, and nine of the 14 ELCs. Eight subjects of the 12 bilaterally measurable subjects had similar DPIMC in both ears. DPIMCs and ELC of approximately half the subjects were clearly related.