Edward V. Porsov

It is desirous to preserve residual low-frequency hearing in cochlear implant recipients, as dual stimulation using electrical and acoustic signals results in superior speech-in-noise processing and musicality. However, cochlear implant recipients commonly lose their residual acoustic hearing gradually following surgical placement of the array. The governing mechanism is not understood and is likely due to multiple factors including surgical trauma, inflammation, and the foreign body induced immune response to the array, including formation of intracochlear fibrosis. Normal mechanical function of the cochlea is expected to underpin residual hearing but has not been directly studied. This function includes efficient transmission of sound to the cochlea via the middle ear, sound propagation via the basilar membrane travelling wave, mechanical tuning characteristics, and the non-linear cochlear amplification of responses to low-frequency stimulation. Here, we examined these features in cochleae and middle ears of unimplanted, acutely and chronically implanted guinea pigs using our minimally invasive preparation and optical coherence tomography vibrometry. We observed that mechanical tuning and amplification is not significantly altered by either acute or chronic implant. However, the response threshold was raised and proportional to middle ear responses. Additionally, the organ of Corti phase, and its sound intensity dependent changes, are unaltered by CI. Finally, a cluster analysis revealed sensitivity loss in chronically implanted animals is caused by middle ear dysfunction. Hence, in the guinea pig model, the dominant etiology of residual hearing loss is conductive. More research is required to establish whether residual hearing loss in humans similarly shares conductive elements.

Upon acoustic stimulation of the mammalian cochlea, a travelling wave will move in a base-to-apex direction, peaking close to the resonant place of the basilar membrane. A gradient of different resonances along the basilar membrane gives rise to frequency-place coding, or tonotopicity, throughout the majority of the cochlea. Per our previous findings, the organ of Corti in the guinea pig cochlear apex does not display tonotopicity. The behavior of the basilar membrane has not been explored throughout the cochlear apex, and so hypothetically tonotopicity is conserved at the level of this structure. Outer hair cells, which influence organ of Corti and basilar membrane mechanics, provide amplification throughout the cochlea. The mechanism for this process in the region close to the helicotrema, which is important for speech and music perception, is not fully described. Here, we describe the acoustically evoked vibrations measured using optical coherence tomography (OCT) at three locations within the guinea pig cochlear apex. We note a decline in the ratio of organ of Corti and basilar membrane gain with proximity to the helicotrema and show that distinct nonlinear behaviors are associated with each location. Phase differences between the cellular portion of the organ of Corti and basilar membrane were present and remained after furosemide treatment. We report that the frequency tuning of the apical basilar membrane is inconsistent with standard models. The analysis of nonlinearity indicates that the mechanisms governing sensory transduction, while still not elucidated, change rapidly within the apical turn—findings that are important for understanding how communication-relevant sounds are encoded by the cochlea.