Field Shaping in Retinal Prosthesis: Current Focusing, Current Steering, and Simultaneous Stimulation

Abstract

Microstimulation of nervous tissue is a proven technique for the delivery of therapies, and perceptual sensations, such as deep brain implants, spinal cord stimulators, or cochlear implants. The field of visual prosthetics is presently at the cutting edge of this discipline, as experience in cochlear implants is turned to the problem of blindness. However, neurostimulators of all sorts are subject to many limitations on the quality of therapy which may be delivered. Working within the field of visual prosthesis, and specifically within one surgical approach to visual prosthesis (the suprachoroidal retinal prosthesis), we set out to explore a family of techniques which has been used to improve the quality of therapy delivered by other neurostimulators — field shaping. Initially, our investigation of field shaping was limited to an investigation of the virtual channel effect, but later was expanded to include other manipulations of the electrical fields generated by neurostimulators. In the course of this investigation, we developed mathematical models of retinal stimulation, experimental models of suprachoroidal stimulation for measuring the spatial dispersion of responses in isolated retina to shaped electrical fields, and multichannel signal processing techniques for analysing the data so generated. Having developed these approaches and techniques, we present results – simulated, derived from recordings in isolated retina, and derived from recordings in visual cortex – which shed light on the properties and relative efficacies of several different field-shaping techniques. We found that field shaping techniques are indeed promising for improving the performance of suprachoroidal visual prostheses. The virtual channel effect, while weak, was found consistently in our data, in agreement with our model. Manipulating patterns of activation through variation of the local return electrodes was found to be more robust. Finally, the behaviour of concurrent stimulation of various types applied in-vivo was found to match closely with predictions about the effects of concurrent stimulation generated by our model.