When people start losing their hearing, a conventional hearing aid can usually help them. If the hearing loss is profound, however, these devices often do not provide a satisfactory solution or any solution at all. In these cases, one uses cochlear implants that bypass the dysfunctional parts of the inner ear. The technique involves an electrode array being implanted in the cochlea, a spiral-shaped cavity in the inner labyrinth. The electrodes stimulate the auditory nerve fibres with electrical impulses, and the brain interprets this information as acoustic signals. By comparison, a hearing aid picks up an acoustic signal with a microphone and transmits it acoustically to the auditory canal via a speaker.
Individual adjustments are complex
A patient-specific adaptation of a cochlear implant is particularly important for understanding spoken language, because even weak background noise can cause major problems, or hinder being able to enjoy listening to music. There are many parameters that can be adjusted to suit the respective individual, as well as countless variations of how to regulate the flow of current between the electrodes. Although most people fitted with cochlear implants can understand spoken language again, there are individual differences in how well they perform, since the limited number of stimulation electrodes in cochlear implants makes it impossible to reproduce perfectly all individual characteristics. “A human auditory nerve consists of 30,000 nerve fibres that process signals in a variety of pulse patterns,” explains Anneliese Schrott-Fischer, who heads the Laboratory for Inner Ear Research at the Medical Innsbruck University Department of Otorhinolaryngology. “This is all the more reason to try and generate good artificial nerve patterns that are as well adapted as possible to the different status of a patient's remaining nerve fibres in all frequency ranges.”
Precise computer models of the inner ear
Schrott-Fischer launched her trial together with postdoctoral researcher Amirreza Heshmat and very renowned experts in medical and simulation technology – Frank Rattay from the Vienna University of Technology and Werner Hemmert from the Technical University of Munich. Their joint project, which was funded by the Austrian Science Fund FWF, aimed at creating precisely segmented computer models of the human cochlea and reconstructing the course of the remaining nerve fibres. Computer tomography scans of human temporal bones from the laboratory's own collection served as the basis. With the help of a computer simulation the researchers calculated what effects the respective deficiency of an ear had on the electric current used to stimulate the nerve fibres in the cochlea. The results enabled the team to explore the exact mechanisms involved in the electrical excitation of the auditory nerve and to understand existing limitations.
“Our findings show that the propagation of the current in the inner ear is extremely far-reaching – an electrode that is actually located deep in the inner ear can still excite neurons that sit much further towards the entrance of the cochlea. This channel crosstalk, as we call it, is the main limitation found in current cochlear implants,” notes Werner Hemmert, Professor of Bio-Inspired Information Processing. The model calculations also reveal that the excitation patterns of the auditory nerve fibres are a great deal more irregular than previously assumed. “This is because of the complicated path of the nerve fibres, which start out running individually through the central bone in the inner ear and then form bundles as they are approaching the brain.” As the inner ear has a spiral-shaped structure, individual fibres are also stimulated in different coils, which makes it difficult for the brain to interpret the excitation patterns correctly.
The findings not only reveal limitations, but also harbour great potential for vital further developments of current cochlear implants, since the systematic investigation of the anatomical variations of a cochlea and the concomitant effects open the door for an improved diagnosis of the state of health of the auditory nerve fibres. “This in turn forms the basis for patient-specific coding strategies for the implants,” says Frank Rattay, Professor of Computational Neuroscience. “With the appropriate measurements, implants could be evaluated before they are built, approved and implanted.” Once this is achieved, it is only up to the human hearing system to get used to the artificially generated activity patterns of the auditory nerve, which translates into a significantly higher quality of life for the patients. Schrott-Fischer highlights the interdisciplinary aspect: “The success of this project depended essentially on the interdisciplinary cooperation of all groups with their different research focuses.” Follow-up projects in order to complete the computer models are already in the pipeline. “The results will be of great benefit to manufacturers of cochlear implants,” Schrott-Fischer concludes.
Anneliese Schrott-Fischer heads the Laboratory for Inner Ear Research at the Department of Otorhinolaryngology at Medical University of Innsbruck. Her teacher and mentor Heinrich Spoendlin, a pioneer of inner ear research, played a major role in ensuring that her research focus remained on the ear even after she completed her studies in biology. In the project “Electrical Stimulation of the Human Auditory Nerve” (2019-2022), which received EUR 228,000 in funding from the Austrian Science Fund FWF, Schrott-Fischer conducted research with Amirreza Heshmat, Frank Rattay (TU Vienna) and Werner Hemmert (TU Munich) on computer models relating to the complex processes involved in the electrical stimulation of neurons by cochlear implants.
Heshmat A., Sajedi S., Schrott-Fischer A., & Rattay F.: Polarity Sensitivity of Human Auditory Nerve Fibers Based on Pulse Shape, Cochlear Implant Stimulation Strategy and Array, in: Frontiers in Neuroscience 2021
Heshmat A., Sajedi S., Johnson Chacko L., Fischer N., Schrott-Fischer A. & Rattay F.: Dendritic degeneration of human auditory nerve fibers and its impact on the spiking pattern under regular conditions and during cochlear implant stimulation, in: Frontiers in Neuroscience 2020
Potrusil T., Heshmat A., Sajedi S., Wenger C., Chacko L.J., Glueckert R., Schrott-Fischer A. & Rattay F.: Finite element analysis and three-dimensional reconstruction of tonotopically aligned human auditory fiber pathways: a computational environment for modeling electrical stimulation by a cochlear implant based on micro-CT, in: Hearing Research, Vol. 393, 2020