The sensation of hearing arises from the interplay of micromechanical motion within the organ of Corti (OC). Optical coherence tomography (OCT) vibrometry has been used to examine the simultaneous motions of OC structures, including the basilar membrane (BM), outer hair cells (OHCs) and reticular lamina (RL), with gerbil and guinea pig (GP) as common animal models. Boosting (motion responses larger than passive motion), and nonlinearity at frequencies below the best frequency (sub-BF) are a robust observation within the gerbil OHC-region. Fallah et al. (2021) compared basal OC mechanics between gerbil and GP and found that GP OC motions at sub-BF frequencies were generally less boosted and showed less nonlinearity than those in gerbil. In the present work, we expand upon the GP measurements by measuring intra-OC motion through both the round window (RW) and a basal cochleostomy in the same experiment. We found substantial differences in the motions when measured through the RW versus the cochleostomy. In the OHC/Deiters Cell(DC) region, sub-BF nonlinearity and boosting were present in measurements through a cochleostomy, but not through the RW. A mild degree of sub-BF nonlinearity was present in the RL region in RW measurements, but not through the cochleostomy. The discussion proposes hypotheses for these differences, including optical angle dependency and mechanical perturbation. Finally, a complex difference analysis was used to extract the internal motion of OC structures riding on the BM motion. The stimulus-level-dependence of the internal OC motions was reduced in the phase responses and increased in the amplitude responses compared to the directly measured OC motion, and irregularities such as nonmonotonic scaling in the measured motion were not present in the extracted internal motion.
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