A 2ND COCHLEAR-FREQUENCY MAP THAT CORRELATES DISTORTION-PRODUCT AND NEURAL TUNING MEASUREMENTS

Acoustic intermodulation distortion products (DPs) are generated by th e nonlinear motion of the basilar membrane (BM) in the cochlea, and pr opagate back to the ear canal where they may be measured. One common m ethod of measuring these distortion products is to hold the higher-pri mary frequency f2 fixed while varying the lower-primary frequency f1. When doing this, it is well known that the ear canal distortion produc t 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) = f 1-n(f2-f1), n= 1,2,3,... are maximum at the same f(d)(n), independent of the order n, but dependent on f2 which determines the place of DP g eneration. In this paper, it is argued that this maximum must result f rom filtering by micromechanical resonances within the cochlea. In fac t the frequency where the neural tuning curve ''tip'' meets the ''tail '' is the same as the frequency where the distortion products are maxi mum. This suggests that each section of the basilar membrane must cons ist of two resonant impedances. The first is the usual series basilar membrane must impedance that gives rise to the characteristic frequenc y (CF). The second resonant impedance must be tuned to a frequency tha t is lower than the CF and must act as a shunt across the inner hair-c ells, since it acts to reduce the forward transmission to the neuron, while, at the same time, it maximally couples all the distortion produ cts back into the cochlear fluids, giving them a frequency dependent i ncrease 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 trav eling 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.