Harmonic distortion in intracochlear pressure and its analysis to explore the cochlear amplifier.

Intracochlear pressure was measured close to the basal basilar membrane in gerbil with pure-tone stimulation. This report describes harmonic distortion in the pressure. The harmonic components were tuned in frequency and physiologically vulnerable, implying that they were related to the cell-derived force that sharpens tuning at low levels in healthy cochleae. For stimulus frequencies in the vicinity of the best frequency the harmonic distortion appeared to be produced locally, at the place of measurement. Therefore, it could be explored with a local nonlinear model. The combined model and observations demonstrate two specific points: First, the harmonics in the cell-based force were likely similar in size to the harmonics in pressure (multiplied by area) close to the basilar membrane. This is distinctly different than the situation for the fundamental component, where the cell-based force is apparently much smaller than the pressure (times area). Second, although the fundamental component of the measured pressure was much larger than its harmonic components, the harmonic and fundamental components of the active force were likely much more similar in size. This allows the harmonic components in the pressure to be used as an indirect measure of the active force.