title: Current steering in retinal stimulation via a quasimonopolar stimulation paradigm.
abstract: OBJECTIVE
  Research to restore some degree of vision to patients suffering from retinal degeneration is becoming increasingly more promising. Several groups have chosen electrical stimulation of the remaining network of a degenerate retina as a means to generate discrete light percepts (phosphenes). Approaches vary significantly, with the greatest difference being the location of the stimulating electrode itself.
  
  
  METHODS
  Suprachoroidal positioning offers excellent mechanical stability and surgical simplicity; however, at the cost of activation thresholds and focused stimulation due to the distance from the electrodes to the target neurons. Past studies proposed a hexapolar electrode configuration to focus the cortical activation and minimize cross-talk between electrodes during concurrent stimulation. The high impedance nature of the choroid and pigment epithelium, however, cause current to shunt between the stimulating and return electrodes, resulting in even higher activation thresholds. In our study, we analyzed the effect of stimulating the feline retina using a quasimonopolar stimulation by simultaneously stimulating a hexapolar and distant monopolar return configurations.
  
  
  RESULTS
  Results of in vivo studies showed that quasimonopolar stimulation can be used to maintain the activation containment properties of hexapolar stimulation, while lowering the activation threshold to values almost equivalent to those of monopolar stimulation.
  
  
  CONCLUSIONS
  The optimal stimulus was found to be composed of a subthreshold monopolar stimulus combined with a suprathreshold hexapolar stimulation. This resulted in a decrease of activation threshold of 60% with respect to hexapolar alone, but with no discernible deleterious effect on the charge containment of a pure hexapolar stimulation.
authors: Graduate School of Biomedical Engineering, The University of New South Wales, Kensington, Australia. p.matteucci@unsw.edu.au
authors: Paul B
authors: PB
authors: Matteucci
authors: Spencer C
authors: SC
authors: Chen
authors: David
authors: D
authors: Tsai
authors: Christopher W D
authors: CW
authors: Dodds
authors: Socrates
authors: S
authors: Dokos
authors: John W
authors: JW
authors: Morley
authors: Nigel H
authors: NH
authors: Lovell
authors: Gregg J
authors: GJ
authors: Suaning
mesh_terms: D000818:Animals; D002415:Cats; D002829:Choroid / Q000601:surgery; D004558:Electric Stimulation / Q000379:methods*; D004567:Electrodes, Implanted; D005074:Evoked Potentials, Visual; D010719:Phosphenes; D019736:Prostheses and Implants; D012165:Retinal Ganglion Cells / Q000502:physiology*; D014796:Visual Perception / Q000502:physiology; D057488:Visual Prosthesis
keywords: current steering; stimulation; suprachoroidal; vision prosthesis