Properties of amplifying elements in the cochlea

We derive the basilar-membrane (BM) impedance Z(x) - where x denotes place - from response data of the basilar membrane (BM). The impedance contains an "active"'. component Z(act) (x) that can cause the fluid wave in the model to be amplified over a certain frequency-dependent region. In the present study various properties of Z(act)(x) are analyzed. Converted to the time domain Z(act)(x) becomes a function Z(act)(t) of time t. Z(act)(t) is found to be noncausal. This points to a 'non-classical' model, and the most appropriate cochlear model should include "feed-forward". Associated with Z(act) (x) is the "active" sound pressure pact (x). We consider the hypothesis that p(act) (x) is a weighted average of the BM velocity v(x) over space (x), with REAL weighting factors h(x') (negative x' corresponds to "feed-forward", positive x' to "feed-backward"). Real h(x') implies that no form of frequency-space filtering is involved. The weighting factors h(x') were estimated using a modified minimum-square-error method. We found that in 28 out of 36 experiments model responses computed on the basis of the reconstructed p(act)(x) are compatible with experimental data. In all cases both feed-forward and feed-backward components arc to be included.