Abstract
Cochlear implants (CIs) are considered the most successful neuroprosthesis as they enable speech comprehension in the majority of half a million CI users suffering from sensorineural hearing loss. By electrically stimulating the auditory nerve, CIs constitute an interface re‐connecting the brain and the auditory scene, providing the patient with information regarding the latter. However, since electric current is hard to focus in conductive environments such as the cochlea, the precision of electrical sound encoding—and thus quality of artificial hearing—is limited. Recently, optogenetic stimulation of the cochlea has been suggested as an alternative approach for hearing restoration. Cochlear optogenetics promises increased spectral selectivity of artificial sound encoding, hence improved hearing, as light can conveniently be confined in space to activate the auditory nerve within smaller tonotopic ranges. In this review, we discuss the latest experimental and technological developments of cochlear optogenetics and outline the remaining challenges on the way to clinical translation.
Highlights
Cochlear implants (CIs) are considered the most successful neuroprosthesis as they enable speech comprehension in the majority of half a million CI users suffering from sensorineural hearing loss
The cochlea of the inner ear functions as a spectral analyzer of these features: Due to its intrinsic mechanical properties, different frequency components are decomposed along the cochlea and the cochlear traveling waves— waves in the intracochlear fluids which have been relayed from air pressure waves via the ossicles—activate inner and outer hair cells (IHCs and OHCs) at different cochlear locations, establishing a frequency map in the cochlea
Ribbon synapses Specialized synapses in the inner ear and retina which are characterized by electron-dense structures which tether synaptic vesicles to presynaptic active zones
Summary
Adeno-associated virus (AAV) Single-strand DNA virus, considered to be non-disease-causing, often used as vector of choice for expressing transgenes of interest for gene-therapeutic approaches. Rosenthal’s canal Cavity in the modiolus housing the cell bodies of spiral ganglion neurons. Spiral ganglion neurons (SGNs) Bipolar neurons housed in Rosenthal’s canal in the modiolus who innervate hair cells and whose axons form the auditory nerve, projecting to the cochlear nucleus in the auditory brainstem. Massive current spread from each of the active electrodes recruits rather large populations of SGNs, which limits the precision by which eCIs can utilize the place-frequency code of the cochlea (Shannon, 1983; Kral et al, 1998). Optical hearing—a promising alternative for improved hearing restoration Using light as an alternative strategy for artificial sound encoding, it might be possible to overcome the limitations of eCIs. As light can be conveniently confined in space, it enables SGN stimulation with higher spatial selectivity, resulting in improved spectral selectivity
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