The "inverse problem" solved for a three-dimensional model of the cochlea. III. Brushing-up the solution method.

In two earlier papers [de Boer, J. Acoust. Soc. Am. 98, 896-903 and 904-910 (1995)] the inherent problems of the inverse-solution method in cochlear mechanics were described. The present paper shows results obtained with a more universal solution method. With the new method it is possible to construct a three-dimensional model of the cochlea producing a response that accurately simulates a measured mechanical basilar-membrane response. With earlier methods this could not be done. The inverse solution invariably yields that, with low stimulus levels, the model simulating a viable cochlea must be locally active. For the response of a dead animal a passive model is sufficient. Once more the inherent intricacies and problems of the inverse-solution method are discussed. Conservation of fluid volume leads to the concept of the "virtual stapes velocity." For best results, the input signal to the inverse-solution procedure should be acquired in the form of a "composite cross-correlation spectrum." Inverse analysis can, under certain circumstances, produce aberrant results. In this paper it is shown why the resulting impedance function is the most accurate in the region of the response peak. Therefore, it is unlikely that a passive model would exist of which the response simulates the data obtained from a healthy animal.