Optogenetic stimulation of the cochlea—A review of mechanisms, measurements, and first models

Abstract

This review evaluates the potential of optogenetic methods for the stimulation of the auditory nerve and assesses the feasability of optogenetic cochlear implants (CIs). It provides an overview of all critical steps like opsin targeting strategies, how opsins work, how their function can be modeled and included in neuronal models and the properties of light sources available for optical stimulation. From these foundations, quantitative estimates for the number of independent stimulation channels and the temporal precision of optogenetic stimulation of the auditory nerve are derived and compared with state-of-the-art electrical CIs. We conclude that optogenetic CIs have the potential to increase the number of independent stimulation channels by up to one order of magnitude to about 100, but only if light sources are able to deliver confined illumination patterns independently and parallelly. Already now, opsin variants like ChETA and Chronos enable driving of the auditory nerve up to rates of 200 spikes/s, close to the physiological value of their maximum sustained firing rate. Apart from requiring 10 times more energy than electrical stimulation, optical CIs still face major hurdles concerning the safety of gene transfection and optrode array implantation, for example, before becoming an option to replace electrical CIs.

Primate optogenetics: Progress and prognosis

“…An ideal implant would modulate each spiral ganglion neuron independently, but conventional cochlear implants usually contain <10 useful channels because the electrical currents they produce spread widely and affect large neuronal populations. An optogenetic cochlear implant might support as many as 100 independent channels because light can be directed with greater precision than electrical currents (121,122).…”

Cochlear Implant Strategies and Biomaterials from Past to Future

“…Hearing pathways can be stimulated by acoustic signal (hearing aids), by electrical (C.I.) signal, optical or by heat (infrared laser) [50,51].…”

“…Light can be directed, or even more, it can be focused on a precise target. By focusing a better spatial framing is achieved, resulting in increased frequency of resolutions and a resolution of better sound intensity [51].…”

“…The results, as of 2000, obtained in children implanted for congenital deafness made lower the minimum implantation age from 18 to 12 months. Thereafter, it is recommended to implant at 9 months and even at a younger age, in the idea that, according to the studies , performed, the implantation was performed at a younger age, the better the performance in terms of speech development and social integration are better [1,12,51,52].…”

Hearing restoration by a low-weight power-efficient multichannel optogenetic cochlear implant system

“…Light offers an alternative mode of SGN stimulation with the potential to overcome this bottleneck (reviewed in ref. [14][15][16][17]. Considering the ability to confine light in space, future optical cochlear implants (oCIs) could activate smaller fractions of SGNs and, hence, enable a higher number of perceptually independent stimulation channels.…”