Multipolar Field Shaping in a Suprachoroidal Visual Prosthesis.

Visual prostheses are now an available mobility aid for patients blinded by degenerative retinal diseases. However, the spatial resolution of existing devices is still insufficient to deliver normal levels of mobility vision without stimulation strategies, which enable existing devices to deliver several different percepts per stimulation site. A stimulation strategy, in which field shaping is achieved by incorporating multipolar (bipolar and tripolar) stimulation could convey additional information to a user of a visual prosthesis, as compared with monopolar stimulation, is investigated. Electrical stimulus response thresholds were simulated using morphologically and physiologically accurate cable models of human retinal ganglion cell (RGC) axons. From the population response patterns which could be evoked in simulation, multipolar field-shaping stimulation from one location could convey as much information as monopolar array stimulation. This result is confirmed in vitro by applying a Bayesian classification analysis to multielectrode array recordings of RGC population responses to extracellular stimulation. In vitro recorded population responses to individual stimuli in vitro could be used to train a Baysian classifier, which could correctly identify individual stimuli as predicted by the simulated population responses. In both simulation and in vitro experiments, monopolar thresholds were not significantly different to multipolar thresholds.