May 2020 boiRxiv
According to the World Health Organisation, in 2018 there were 466 million people in the world with hearing loss. It is forecasted that in 2030 this number will reach 630 million and will further grow to 900 million in 2050. If profound, hearing loss makes these people potential candidates for hearing rehabilitation by CIs. Currently there are approximately 700,000 CI users worldwide, most of which achieve good open-set speech understanding in the quiet. CIs are associated with low risk for implantation and device failure. CI systems are composed of an external speech processor and implanted stimulator. They convert sound into electrical currents delivered from an intracochlear electrode array (eCI) to stimulate the spiral ganglion neurons (SGNs), which are ordered according to their characteristic frequency along the tonotopic axis (place-frequency map) that follows the spiral anatomy of the cochlea. Real-time sound processing involves decomposition into frequency bands and extraction of the intensity within each band. These intensities are then used to scale the amplitude of electrical pulses delivered to the electrode at the tonotopic position corresponding to the respective frequency band.
However, due to the wide spread of the electrical current from each of the 12–24 eCI contacts (depending on manufacturer), signals containing information of a given frequency band activate a large fraction of the tonotopically ordered SGNs. This results in limited spectral resolution of sound coding with typically less than ten perceptually independently stimulation channels. Efforts to improve the performance of the electrical CI include current steering using multipolar stimulation as well as intraneural stimulation, but the potential for reducing the spread of electrical excitation seems rather limited. Poor spectral resolution is commonly considered the bottleneck of the eCI that makes more complex listening tasks like communication in noisy or reverberant environments difficult and limits music appreciation. Light offers an alternative mode of SGN stimulation with the potential to overcome this bottleneck. 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. Two approaches toward optical SGN stimulation have been employed: i) infrared direct neural stimulation (INS)and ii) optogenetics. While the INS concept has remained controversial for the cochlea, optogenetics offers a defined molecular mechanism, restores auditory function in various animal models of deafness and has been successfully implemented in preclinical animal studies by several laboratories. The spectral selectivity of optogenetic SGN stimulation has been shown to be greater than that of electrical stimulation and near physiological SGN firing rates can be achieved with fast opsins such as Chronos and f-Chrimson.
In parallel, major advances have been achieved towards the technological implementation of the oCI. Since the proof of concept study on flexible oCIs based on microscale gallium nitride (GaN) light emitting diodes (pLEDs) their optimisation (light extraction and focusing) and technical characterisation has been progressed. In addition, studies with larger emitters and waveguides have been undertaken. However, to the best of our knowledge, implementation and characterisation of a full oCI system has not yet been presented, except for a brief proof of principle demonstration. Such oCI system should employ real-time sound processing and coding strategies employing more stimulation channels than current eCI. Here, we report the development and functional demonstration of a low-weight, wireless, battery-powered oCI sound processor and driver circuitry to be head-mounted for experiments on freely moving animals. We demonstrate the function of the oCI, and its sister eCI, sound processor and driver. In summary, this preclinical oCI system will help paving the way for developing the future clinical oCI for improved hearing restoration in human deafness.