Operational Design Considerations for Retinal Prostheses
  Three critical improvements for present day and future retinal vision implants are proposed and discussed: (1) A time profile for the stimulation current that leads predominantly to transverse stimulation of nerve cells; (2) auxiliary electric currents for electric field shaping with a time profile chosen such that these currents have small probability to cause stimulation; and (3) a local area scanning procedure that results in high pixel density for image/percept formation (except for losses at the boundary of an electrode array). 
Erich W. Schmid and Wolfgang Fink
q-bio.NC