Virtual electrodes by current steering in retinal prostheses.

PURPOSE Retinal prostheses are an approved treatment for vision restoration in retinal degenerative diseases; however, present implants have limited resolution and simply increasing the number of electrodes is limited by design issues. In cochlear implants, virtual electrodes can be created by simultaneous stimulation of adjacent physical electrodes (current steering). The present study assessed whether this type of current steering can be adapted for retinal implants. METHODS Suprachoroidal electrode arrays were implanted in four normally sighted cat eyes. Electrode pairs were driven simultaneously at different current levels and current ratios. Multiunit spiking activity in the visual cortex was recorded. Spike distribution across channels enabled generation of cortical activation maps and calculation of centroid positions. For each current configuration, centroid shifts between two virtual electrodes were compared to shifts obtained from physical electrode stimulation. RESULTS Using current steering, virtual electrodes with different cortical activation maps could be created. Cortical centroids were found to shift as a function of the current ratio used for virtual electrodes and were similar to the centroid shifts seen when using physical electrodes. In addition, the cortical response to stimulation of a physical electrode could be reproduced by applying current steering to electrodes on either side of the physical electrode. CONCLUSIONS These results suggest that current steering can alter activation patterns in the visual cortex and could enhance visual perception in retinal implants by eliciting phosphene percepts intermediate between those elicited by physical electrodes. These results inform development of new electrode arrays that can take advantage of current steering.