A second cochlear-frequency map that correlates distortion product and neural tuning measurements

Acoustic intermodulation distortion products (DPs) are generated by the nonlinear motion of the basilar membrane (BM) in the cochlea, and propagate back to the ear canal where they may be measured. One common method of measuring these distortion products is to hold the higher-primary frequency f₂fixed while varying the lower-primary frequency f₁. When doing this, it is well known that the ear canal distortion product is maximum for a particular value of f₂/f₁, usually between 1.1 and 1.4. In fact all odd order distortion products of the form f_d ^{(n )}=f₁ - n(f₂-f₁), n= 1,2,3,. are maximum at the same f_d ⁽ⁿ⁾, independent of the order n, but dependent on f₂which determines the place of DP generation. In this paper, it is argued that this maximum must result from filtering by micromechanical resonances within the cochlea. In fact the frequency where the neural tuning curve “tip” meets the “tail” is the same as the frequency where the distortion products are maximum. This suggests that each section of the basilar membrane must consist of two resonant impedances. The first is the usual series basilar membrane resonant impedance that gives rise to the characteristic frequency (CF). The second resonant impedance must be tuned to a frequency that is lower than the CF and must act as a shunt across the inner hair-cells, since it acts to reduce the forward transmission to the neuron, while, at the same time, it maximally couples all the distortion products back into the cochlear fluids, giving them a frequency dependent increase at its resonant frequency. Thus the proposed second mechanical resonance concept explains a great deal of complicated and confusing data. For pure tone excitation, the second resonance modifies the traveling wave excitation pattern (EP) basal to its characteristic place (CP). A good candidate for this second mechanical resonance would be a resonance of the tectorial membrane (TM), tuned to the neural tip-tail frequency at each place.