title: Interphase gap as a means to reduce electrical stimulation thresholds for epiretinal prostheses.
abstract: OBJECTIVE
  Epiretinal prostheses are designed to restore functional vision to the blind by electrically stimulating surviving retinal neurons. These devices have classically employed symmetric biphasic current pulses in order to maintain a balance of charge. Prior electrophysiological and psychophysical studies in peripheral nerve show that adding an interphase gap (IPG) between the two phases makes stimulation more efficient than pulses with no gap. This led us to investigate the effect of IPG duration on retinal stimulation thresholds.
  
  
  METHODS
  We measured retinal ganglion cell (RGC) electrical thresholds in salamander retina and phosphene perceptual thresholds in epiretinal prosthesis patients during stimulation with different IPG lengths. We also built Hodgkin-Huxley-type models of RGCs to further study how IPG affects thresholds.
  
  
  RESULTS
  In general, there was a negative exponential correlation between threshold and IPG duration. Durations greater than or equal to ~0.5 ms reduced salamander RGC thresholds by 20-25%. Psychophysical testing in five retinal prosthesis patients indicated that stimulating with IPGs can decrease perceptual thresholds by 10-15%. Results from computational models of RGCs corroborated the observed behavior.
  
  
  CONCLUSIONS
  Incorporating interphase gaps can reduce the power consumption of epiretinal prostheses and increase the available dynamic range of phosphene size and brightness.
authors.forename: Andrew C
authors.initials: AC
authors.lastname: Weitz
authors.forename: Matthew R
authors.initials: MR
authors.lastname: Behrend
authors.forename: Ashish K
authors.initials: AK
authors.lastname: Ahuja
authors.forename: Punita
authors.initials: P
authors.lastname: Christopher
authors.forename: Jianing
authors.initials: J
authors.lastname: Wei
authors.forename: Varalakshmi
authors.initials: V
authors.lastname: Wuyyuru
authors.forename: Uday
authors.initials: U
authors.lastname: Patel
authors.forename: Robert J
authors.initials: RJ
authors.lastname: Greenberg
authors.forename: Mark S
authors.initials: MS
authors.lastname: Humayun
authors.forename: Robert H
authors.initials: RH
authors.lastname: Chow
authors.forename: James D
authors.initials: JD
authors.lastname: Weiland
mesh_terms: D000465:Algorithms; D000557:Ambystoma; D000818:Animals; D002415:Cats; D003198:Computer Simulation; D004558:Electric Stimulation; D004566:Electrodes; D055724:Electrophysiological Phenomena / Q000502:physiology; D006801:Humans; D008959:Models, Neurological; D011474:Prosthesis Design; D011601:Psychophysics; D012160:Retina / Q000502:physiology*; D012165:Retinal Ganglion Cells / Q000502:physiology; D012684:Sensory Thresholds; D014785:Vision, Ocular / Q000502:physiology; D057488:Visual Prosthesis