The goal for neuroprostheses is to restore neural function to a condition having the fidelity of a healthy system. However, contemporary neural prostheses, including cochlear implants, are not able to achieve this goal. The devices use electrical current to stimulate the neurons, which spreads in the tissue and consequently does not allow stimulation of focused neuron populations. Therefore, high fidelity stimulation is not possible. In our model system, the cochlea, it has been argued that the performance of cochlear implant users could be increased significantly if more discrete non-overlapping locations of neurons situated along the electrode could be stimulated simultaneously. This might be possible with devices that use focal optical radiation to stimulate neurons. Today we know that infrared neural stimulation (INS) is possible, that stimulation rates can be achieved that allow encoding of acoustic information, that the spatial selectivity in the cochlea is more selective than electrical stimulation, and that single channel stimulation in chronic experiments shows no functional damage of the cochlea over at least six weeks. The developments proposed in this R01 are a logical progression of previous experiments.
The aims i nclude the fabrication and testing of hybrid opto-electrical arrays to be surgically inserted into a cat cochlea and showing that each channel of multichannel INS can independently encode information to be perceived by the auditory system. Recordings from the inferior colliculus will be used to construct spatial tuning curves (STCs). Non-overlapping STCs indicate separation of the channels. The stimulation pattern will be changed systematically until an optimum match is achieved between and acoustically evoked response and the response obtained from the coding strategy. Long-term stimulation after chronic implantation of a multi-channel device into a cat cochlea will determine the safety. Strategies such as combined opto-electrical stimulation or shaping the optical pulses can reduce the power required for optical stimulation and will be explored. Results will be confirmed through histology. At the conclusion of this project, a prototype human optical cochlear implant will be available for a clinical study based on the physical and the optical requirements.
The long term objective of the proposed developments and testing is to design and build safe optical neural prostheses with significantly improved spatial selectivity for spiral ganglion cell stimulation. Combined with a novel coding strategy it is expected that cochlear implants will provide significantly more independent perceptual channels to the implant user that can be used in parallel and thus improve speech recognition in noisy listening environments and provide music appreciation.
