Spontaneous otoacoustic emissions in an active feed-forward model of the cochlea

Numerical simulation of spontaneous otoacoustic emissions (SOAE) was done using a 1-dimensional macromechanical cochlear model with an active 'feed-forward' force in every section of the basilar membrane (BM). When the activity of the force was increased the model showed more stability than a 'feed-back' model and could have excitation curves with larger tips without divergence of the solution. The stability broke up when either (1) the damping of the BM was made slightly irregular throughout the BM or (2) the feed-forward force was switched off at a certain part of the BM, and limit cycle oscillations (LCO) emerged within the cochlea. Critical feed-forward value for the emergence of LCO in the first setting of BM (1), which was intended to simulate physiological variations in the distribution of outer hair cells, was searched utilizing the 'ringing' of delayed evoked otoacoustic emissions (DEOAE). In the course of the search, smooth transition from a DEOAE to a set of SOAEs was found to occur keeping the same spectral fine structure of the emissions when the feed-forward force surpassed a certain value. It was, as a result suggested that the two kinds of emissions may have the same origin. In the second setting of BM (2), which was intended to simulate pathological cases. LCOs tended to be stronger than in the first one and they had similarity not only to real SOAEs but also to tinnitus in the way they showed up very close to or at the "edge' of the switched-off part of the BM.