Physiologically based three dimensional cochlear models: An interspecies comparison

In this study, cochlear mechanics of four different species (gerbil, chinchilla, cat, and human) are studied through physiologically-based cochlear model. The present work serves as an extension of previous cochlear modeling research by improving on the older models and incorporating new imaging technologies.Cochlear physiologies for four species gerbil, chinchilla, cat, and human, are studied using two separate refinements to the three-dimensional hydrodynamic cochlear models presented in Yoon et al., 2007. One refinement is the application of the time-averaged Lagrangian density. The other refinement is the incorporation of a "push-pull mechanism to examine the active case of hair cell motility. The objective of this work is to create a computational model of cochlear dynamics that could more accurately reflect existing cochlear physiological measurements for the following parameters: (i) basilar membrane (BM) velocity (VBM) (ii) intracochlear pressure in the scala tympani (PST) (iii) cochlear input impedance (ZC) and (iv) BM vibration and its effect on neural threshold regulation. Good agreement in both magnitude and phase is found between simulation results and physiological measurements for VBM and PST for the gerbil and chinchilla. The large phase excursion observed in the previous models (Lim and Steele, 2002 Yoon et al., 2007) is rectified. Similarly, good agreement is found between the calculated and measured results for ZC in gerbil, chinchilla, cat and human. Interspecies BM vibration thresholds are estimated by matching the simulation results to the neural threshold measurements. The VBM threshold curve shows a better fit to the neural threshold curves than the BM displacement threshold curve suggesting that inner hair cells respond more strongly to BM velocity. This gives support to our suggestion that the present model is close to the actual behavior of mammalian cochleae including gerbil, chinchilla, cat, and human cochlea.By virtue of the three-dimensional cochlear model, intracochlear pressure in the ST is obtained by adding the fast wave to the traveling pressure slow wave. From the intracochlear pressure simulation, derived quantities: (1) BM velocity, (2) pressure difference across OC, and (3) OC impedance in the base, are calculated by following Olsons estimation (1998). These quantities are compared with animal measurements and showed excellent agreement. From the validated gerbil cochlear model, the exact theoretical OC impedance is obtained and compared with the estimated theoretical OC impedance. By comparing exact and estimated theoretical OC impedances, a fast wave component in the estimated theoretical OC impedance is founded and it causes phase fluctuation out of the reasonable range (negative real part of impedance in the passive response) and notches in the estimated theoretical OC impedance. The exact theoretical OC impedance from the active model shows negative real components which represents active process from the OHCs motility. Measurements of the spatial distribution of the response of the BM at a fixed frequency (Ren, 2002) and the frequency distribution at a fixed point (Ren and Nuttall, 2001) offer an unusual opportunity for validation of model calculations. The comparison of results from the model and experiment is promising. Using a single set of anatomically based parameters, the model predicts several significant features of cochlear response. The CF-to-place map in the passive model, frequency and spatial responses of BM velocity were in close agreement with those observed in animal measurement.