September 2016
Volume 57, Issue 12
Open Access
ARVO Annual Meeting Abstract  |   September 2016
Focal electrical stimulation of the retina
Author Affiliations & Notes
  • Andrew C Weitz
    Ophthalmology, University of Southern California, Los Angeles, California, United States
    Biomedical Engineering, University of Southern California, Los Angeles, California, United States
  • Devyani Nanduri
    Biomedical Engineering, University of Southern California, Los Angeles, California, United States
  • Robert Jay Greenberg
    Second Sight Medical Products, Inc., Sylmar, California, United States
  • Mark S Humayun
    Ophthalmology, University of Southern California, Los Angeles, California, United States
    Biomedical Engineering, University of Southern California, Los Angeles, California, United States
  • Robert H Chow
    Physiology & Biophysics, University of Southern California, Los Angeles, California, United States
    Biomedical Engineering, University of Southern California, Los Angeles, California, United States
  • James D Weiland
    Ophthalmology, University of Southern California, Los Angeles, California, United States
    Biomedical Engineering, University of Southern California, Los Angeles, California, United States
  • Footnotes
    Commercial Relationships   Andrew Weitz, None; Devyani Nanduri, None; Robert Greenberg, Second Sight Medical Products, Inc. (E); Mark Humayun, Second Sight Medical Products, Inc. (I), Second Sight Medical Products, Inc. (C), Second Sight Medical Products, Inc. (P); Robert Chow, None; James Weiland, None
  • Footnotes
    Support  NSF Grant EEC-0310723, NIH Grant 1R01EY022931, NIH Grant R01 GM85791, Research to Prevent Blindness
Investigative Ophthalmology & Visual Science September 2016, Vol.57, 3714. doi:
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    • Get Citation

      Andrew C Weitz, Devyani Nanduri, Robert Jay Greenberg, Mark S Humayun, Robert H Chow, James D Weiland; Focal electrical stimulation of the retina. Invest. Ophthalmol. Vis. Sci. 2016;57(12):3714.

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      © ARVO (1962-2015); The Authors (2016-present)

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Abstract

Purpose : Epiretinal implants have restored partial vision to the blind by electrically stimulating surviving retinal neurons. Unintended stimulation of retinal ganglion cell (RGC) axons causes patients to see large, elongated visual phosphenes, making it difficult for them to perceive forms. This study tested whether alternative electrical stimulus waveforms could produce focal retinal activation.

Methods : The GCaMP5G calcium indicator was virally expressed in RGCs of wild-type (Long Evans) and retinal degenerate (S334ter-line-3) rats. Transduced retinas were isolated and mounted RGC-side-down on a multielectrode array. Calcium imaging was used to map the patterns of cells activated by different electrical stimulation protocols. Electrode diameter (30–200 µm) and pulse duration (0.06–100 ms/phase) were varied. Synaptic blockers were used in some experiments to pharmacologically isolate RGCs. The effect of pulse width on phosphene shape was tested in an epiretinal implant (Argus I) subject, blind with retinitis pigmentosa.

Results : In rats, pulse durations 16 ms/phase and shorter stimulated axons, activating a streak of RGCs that extended to the retina’s edge. Pulses 25 ms/phase and longer stimulated inner retinal neurons while avoiding RGC axons, producing focal responses. Threshold charge density became higher as pulse width increased (e.g., for 200-µm electrodes, stimulus thresholds were 1.8 and 22.4 µC/cm2 for 0.1- and 25-ms/phase pulses, respectively). Response shapes in wild-type and degenerate retinas were similar, but stimulus thresholds of inner retinal cells were elevated by 3× in retinal degenerate rats. Multielectrode stimulation with 25-ms/phase pulses was used to pattern letters on the retina. The letter V, for example, was generated with seven 30-µm electrodes (see figure). The height of the V on the retina corresponds to 0.78° of human visual angle and is equivalent to viewing 5-mm-tall text from typical reading distance (40 cm). Psychophysical testing in an epiretinal implant patient demonstrated that short pulses (0.46 ms/phase) produce elongated phosphenes, while long pulses (25 ms/phase) evoke focal spots of light.

Conclusions : Our results demonstrate that stimulus pulse durations two orders of magnitude longer than those typically used in existing retinal implants avoid RGC axons and activate the retina focally. If validated through further human testing, these findings may lead to improved visual acuity for epiretinal implant users.

This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.

 

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