title: Brightness as a function of current amplitude in human retinal electrical stimulation.
abstract: OBJECTIVE
  With the goal of eventually restoring functional vision in patients with retinal degenerative diseases, USC/Second Sight Medical Products, Inc. chronically implanted blind human subjects with a prototype epiretinal prosthesis consisting of a 4 x 4 array of 16 stimulating electrodes. To accurately represent a visual scene, a visual prosthesis must convey luminance information across a range of brightness levels. To achieve this, the brightness of phosphenes produced by an individual electrode should scale appropriately with luminance, and the same luminance should produce equivalently bright phosphenes across the entire electrode array. The goal was to examine how apparent brightness changes as a function of stimulation intensity across electrodes.
  
  
  METHODS
  As described in previous studies, electrical stimulation of intact cells of the neural retina using this prosthetic device reliably elicits visual percepts in human subjects blinded by retinitis pigmentosa. Here, apparent brightness for a range of electrical amplitudes was measured using both subjective magnitude rating and brightness-matching procedures in chronically implanted human subjects.
  
  
  RESULTS
  It was found that apparent brightness can be described as a power function of stimulation intensity. The same model can also predict brightness matching across electrodes.
  
  
  CONCLUSIONS
  These results suggest that a relatively simple model for scaling current across electrodes may be capable of producing equivalently bright phosphenes across an entire array. (ClinicalTrials.gov number, NCT00279500.).
authors.affiliation: Second Sight Medical Products, Inc., Sylmar, California, USA.
authors.forename: Scott H
authors.initials: SH
authors.lastname: Greenwald
authors.forename: Alan
authors.initials: A
authors.lastname: Horsager
authors.forename: Mark S
authors.initials: MS
authors.lastname: Humayun
authors.forename: Robert J
authors.initials: RJ
authors.lastname: Greenberg
authors.forename: Matthew J
authors.initials: MJ
authors.lastname: McMahon
authors.forename: Ione
authors.initials: I
authors.lastname: Fine
mesh_terms: D001766:Blindness / Q000534:rehabilitation; D004558:Electric Stimulation; D004567:Electrodes, Implanted; D006801:Humans; D008027:Light; D008839:Microelectrodes; D008954:Models, Biological; D010719:Phosphenes / Q000502:physiology*; D019919:Prosthesis Implantation; D012160:Retina / Q000502:physiology*; D012162:Retinal Degeneration / Q000503:physiopathology*; D012684:Sensory Thresholds / Q000502:physiology; D014796:Visual Perception / Q000502:physiology*