Changes in apical cochlear mechanics after cochlear implantation

Project Summary Sound entering the cochlea induces a longitudinally propagating travelling wave along the cochlear partition which includes the organ of Corti. The organ of Corti amplifies travelling waves via force production by outer hair cells. Where this amplification is lost, an array of electrodes called a cochlear implant replaces sound stimulation with electrical stimulation of the auditory nerve. Improved cochlear implants combine electrical and sound stimulation in patients with some intact hearing. These combined implants lead to improved performance. However, approximately half of combined cochlear implant recipients experience a loss of their remaining hearing months after implantation. This implantation-induced hearing loss reduces speech recognition and musicality. Implantation-induced hearing loss may have multiple interacting causes; immune, metabolic, and mechanical. We hypothesize that cochlear scarring (fibrosis/ossification) induced by implantation disrupts travelling wave propagation to the site of low frequency hearing. Links between hearing loss and implant-induced scarring are seen in rodent models, reflecting clinical findings. However, there are no direct measurements of the mechanical consequences of cochlear implantation for low frequency hearing. We will combine our expertise with rodent models of cochlear implantation and the use of the latest generation of imaging interferometry – optical coherence tomography (OCT). In a bid to produce the first data of its kind, we will use OCT vibrometry to characterize low frequency mechanical function in the cochlear apex of chronically implanted animals. We will then produce a 3D map of the scarring inside each cochlea using OCT imaging. Coupled with histology and machine learning powered image analysis, we will compare the extent, location and type of scarring with organ of Corti gain, tuning, distortion, phase and group delay in each cochlea. The results of our OCT vibrometry experiments will be interpreted by computer models of cochlear function. Experiments will also be conducted in acutely implanted models to assess the effect of the cochlear implant upon apical mechanics prior to scarring. Additionally, we will use a model with noise induced hearing loss prior to implantation to test the contribution of high frequency outer hair cells to low frequency hearing performance. Our multidisciplinary team will offer a direct insight into cochlear implant-induced hearing loss and will allow us to test the scarring hypothesis. This project will guide avenues of research geared towards minimizing or preventing cochlear implant-induced hearing loss, and lead to improved quality of life for the recipients of cochlear implants.