title: Frequency and amplitude modulation have different effects on the percepts elicited by retinal stimulation.
abstract: OBJECTIVE
  In an effort to restore functional form vision, epiretinal prostheses that elicit percepts by directly stimulating remaining retinal circuitry were implanted in human subjects with advanced retinitis pigmentosa RP). In this study, manipulating pulse train frequency and amplitude had different effects on the size and brightness of phosphene appearance.
  
  
  METHODS
  Experiments were performed on a single subject with severe RP (implanted with a 16-channel epiretinal prosthesis in 2004) on nine individual electrodes. Psychophysical techniques were used to measure both the brightness and size of phosphenes when the biphasic pulse train was varied by either modulating the current amplitude (with constant frequency) or the stimulating frequency (with constant current amplitude).
  
  
  RESULTS
  Increasing stimulation frequency always increased brightness, while having a smaller effect on the size of elicited phosphenes. In contrast, increasing stimulation amplitude generally increased both the size and brightness of phosphenes. These experimental findings can be explained by using a simple computational model based on previous psychophysical work and the expected spatial spread of current from a disc electrode.
  
  
  CONCLUSIONS
  Given that amplitude and frequency have separable effects on percept size, these findings suggest that frequency modulation improves the encoding of a wide range of brightness levels without a loss of spatial resolution. Future retinal prosthesis designs could benefit from having the flexibility to manipulate pulse train amplitude and frequency independently (clinicaltrials.gov number, NCT00279500).
authors: Department of Biomedical Engineering, University of Southern California, Los Angeles, California, USA. nanduri@usc.edu
authors: Devyani
authors: D
authors: Nanduri
authors: Ione
authors: I
authors: Fine
authors: Alan
authors: A
authors: Horsager
authors: Geoffrey M
authors: GM
authors: Boynton
authors: Mark S
authors: MS
authors: Humayun
authors: Robert J
authors: RJ
authors: Greenberg
authors: James D
authors: JD
authors: Weiland
mesh_terms: D001766:Blindness / Q000209:etiology / Q000503:physiopathology / Q000534:rehabilitation*; D004599:Electric Stimulation Therapy / Q000295:instrumentation*; D004567:Electrodes, Implanted; D005500:Follow-Up Studies; D006801:Humans; D008839:Microelectrodes; D008875:Middle Aged; D012160:Retina / Q000503:physiopathology*; D012174:Retinitis Pigmentosa / Q000150:complications / Q000503:physiopathology / Q000534:rehabilitation*; D012684:Sensory Thresholds / Q000502:physiology; D016896:Treatment Outcome; D014796:Visual Perception / Q000502:physiology*