deafness-new
Hearing with light
Cochlear implants enable most users to understand speech in quiet environments. However, they suffer from limited quality of sound coding: due to the wide spatial spread of the electrical current from each of the electrode contacts, too many auditory nerve cells in the vicinity of the contact are stimulated simultaneously (Fig. 1). Therefore, the number of independent stimulation channels is low (typically under 10) and the discrimination of pitch and volume when hearing with cochlear implants is limited. This severely restricts the ability to understand speech in background noise, follow musical melodies, or interpret the emotional tone in speech. Since light can be spatially focused better, this fundamental problem could be overcome by using light instead of electrical current to stimulate the neurons. Each light source would then excite a significantly smaller group of cells compared to “electrical hearing” (Fig. 1). In this way, future optical cochlear implants could transmit sound information via 64 stimulation channels with micro-scale light sources. This promises a significant improvement in the discrimination of pitch and volume and thus a breakthrough in the restoration of hearing for profoundly deaf people.
Figure 1: Schematic representation of the function of cochlear implants (Kleinlogel et al., Physiol Rev. 2020, modified)
Left: Current electrical cochlear implant with large lateral spread of electrical stimulation from each of the 12 electrode contacts. Right: Future optical implant with numerous emitters whose light is focused on the auditory nerve.
The goal: From the electrical to the optical cochlear implant through optogenetics
The optical cochlear implant (oCI) currently under development uses optogenetics to restore hearing by introducing light-gated ion channels into the auditory nerve via local gene therapy to enable direct neuronal stimulation by light. Stimulation with light can be spatially much better confined than the stimulation of cells with electrical current, as used in the electrical CIs currently in use.
The milestones towards clinical application
Where we currently stand: The EKFZ OT is working on preclinical oCIs for the fundamental characterization of optical hearing and on the development of the human prototype for clinical trials. In the clinical oCI, some components of the electrical implant can be used, but other components such as the waveguide-based optical stimulator and the sound coding strategy must be newly developed and approved according to the strict testing regulations for active implantable medical devices. In this process, patient safety and long-term stability, which must be ensured even under the most adverse conditions, are at the
forefront.
Further information on the Institute for Auditory Neuroscience (IAN) and Department of Otolaryngology at the University Medical Center Göttingen
Optogenetic stimulation is intended to overcome the aforementioned limitation, as light can be used in a more spatially targeted manner and thus specific parts of the cochlea can be excited more precisely. As a result, optical cochlear implants (oCI) can achieve greater frequency selectivity and thus significantly improve the hearing impression.
This gives hope for another major step towards normalcy for patients: communication in everyday situations and participation in a normal social life.
