Article
Towards the development of a micro-coil based cortical visual prosthesis
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Published: | December 10, 2019 |
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Objective: We are developing a cortical visual prosthesis that uses magnetic stimulation (instead of electrical) to drive cortical neurons. This approach is attractive because the induced fields from coils are spatially asymmetric and therefore, can activate local pyramidal neurons while avoiding passing axons. This confines activation to a focal region and promises higher acuity levels. Also, because there is no direct contact between coils and cortical tissue, many of the stability concerns associated with implantable electrodes can be avoided.
Materials and methods: In vitro experiments are used for proof of principle and to compare different coil designs. In vivo experiments measure cortical surface activity (ECoG) in response to electric or magnetic stimulation. A custom-designed 128-channel recording array on V1 capturee the spread of activation and allowed comparison between micro-coils and electrodes.
Results: In vitro experiments confirmed the more selective activation from coils. Electric and magnetic stimulation both elicited ECoG responses, although electrodes activate a spatially expansive area, often more than 1 mm from the stimulation site, while activation from coils was confined to a focal area around the stimulation site (approximately 300 microns in diameter). In vitro testing of human cortical tissue (resected during medically-necessary neurosurgical procedures) revealed comparable sensitivity of individual neurons in both species.
Discussion: Our experiments to date continue to support the viability of micro-coils as an effective alternative to conventional micro-electrodes. The focal activation from coils supports the creation of independent phosphenes that can be summated to produce more spatially-complex percepts. Future efforts include psychophysical testing in NHPs and long-term stability testing in rats will be discussed.