title: Virtual electrodes by current steering in retinal prostheses.
abstract: OBJECTIVE
  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.
authors.affiliation: Bionics Institute, East Melbourne, Victoria, Australia Lübeck University of Applied Sciences, Lübeck, Germany.
authors.forename: Gerald
authors.initials: G
authors.lastname: Dumm
authors.affiliation: Bionics Institute, East Melbourne, Victoria, Australia Medical Bionics Department, University of Melbourne, East Melbourne, Victoria, Australia.
authors.forename: James B
authors.initials: JB
authors.lastname: Fallon
authors.affiliation: Bionics Institute, East Melbourne, Victoria, Australia Medical Bionics Department, University of Melbourne, East Melbourne, Victoria, Australia.
authors.forename: Chris E
authors.initials: CE
authors.lastname: Williams
authors.affiliation: Bionics Institute, East Melbourne, Victoria, Australia Medical Bionics Department, University of Melbourne, East Melbourne, Victoria, Australia.
authors.forename: Mohit N
authors.initials: MN
authors.lastname: Shivdasani
mesh_terms: D000818:Animals; D002415:Cats; D004558:Electric Stimulation / Q000379:methods*; D004567:Electrodes, Implanted; D005074:Evoked Potentials, Visual / Q000502:physiology; D023421:Models, Animal; D012160:Retina / Q000502:physiology / Q000601:surgery; D012162:Retinal Degeneration / Q000534:rehabilitation; D014793:Visual Cortex / Q000502:physiology*; D014796:Visual Perception / Q000502:physiology*; D057488:Visual Prosthesis
keywords: current steering; electrophysiology; resolution; retinal implant; visual cortex