title: Electrically elicited visual evoked potentials in Argus II retinal implant wearers.
abstract: OBJECTIVE
  We characterized electrically elicited visual evoked potentials (eVEPs) in Argus II retinal implant wearers.
  
  
  METHODS
  eVEPs were recorded in four subjects, and analyzed by determining amplitude and latency of the first two positive peaks (P1 and P2). Subjects provided subjective feedback by rating the brightness and size of the phosphenes. We established eVEP input-output relationships, eVEP variability between and within subjects, the effect of stimulating different areas of the retina, and the maximal pulse rate to record eVEPs reliably.
  
  
  RESULTS
  eVEP waveforms had low signal-to-noise ratios, requiring long recording times and substantial signal processing. Waveforms varied between subjects, but showed good reproducibility and consistent parameter dependence within subjects. P2 amplitude overall was the most robust outcome measure and proved an accurate indicator of subjective threshold. Peak latencies showed small within-subject variability, yet their correlation with stimulus level and subjective rating were more variable than that of peak amplitudes. Pulse rates of up to (2)/3 Hz resulted in reliable eVEP recordings. Perceived phosphene brightness declined over time, as reflected in P1 amplitude, but not in P2 amplitude or peak latencies. Stimulating-electrode location significantly affected P1 and P2 amplitude and latency, but not subjective percepts.
  
  
  CONCLUSIONS
  While recording times and signal processing are more demanding than for standard visually evoked potential (VEP) recordings, the eVEP has proven to be a reliable tool to verify retinal implant functionality. eVEPs correlated with various stimulus parameters and with perceptual ratings. In view of these findings, eVEPs may become an important tool in functional investigations of retinal prostheses. (ClinicalTrials.gov number NCT00407602.) Dutch Abstract.
authors.affiliation: Department of Ophthalmology, Johns Hopkins University, Baltimore, MD, USA. Christiaan.Stronks@nicta.com.au
authors.forename: H Christiaan
authors.initials: HC
authors.lastname: Stronks
authors.forename: Michael P
authors.initials: MP
authors.lastname: Barry
authors.forename: Gislin
authors.initials: G
authors.lastname: Dagnelie
mesh_terms: D000328:Adult; D004558:Electric Stimulation; D004567:Electrodes, Implanted; D005074:Evoked Potentials, Visual / Q000502:physiology*; D005260:Female; D006801:Humans; D008027:Light; D008297:Male; D010719:Phosphenes / Q000502:physiology; D010775:Photic Stimulation / Q000379:methods; D015203:Reproducibility of Results; D012160:Retina / Q000502:physiology*; D055351:Retinal Neurons / Q000502:physiology*; D012684:Sensory Thresholds / Q000502:physiology; D059629:Signal-To-Noise Ratio; D014796:Visual Perception / Q000502:physiology*; D057488:Visual Prosthesis
keywords: psychophysics; retinal implant; visual evoked potential; visual prosthesis