Improving the spatial resolution of epiretinal implants by increasing stimulus pulse duration

Abstract

Retinal prosthetic implants are the only approved treatment for retinitis pigmentosa, a disease of the eye that causes blindness through gradual degeneration of photoreceptors. An array of microelectrodes triggered by input from a camera stimulates surviving retinal neurons, each electrode acting as a pixel. Unintended stimulation of retinal ganglion cell axons causes patients to see large, oblong shapes of light, rather than focal spots, making it difficult for them to perceive forms. To address this problem, we performed calcium imaging in isolated retinas and mapped the patterns of cells activated by different electrical stimulation protocols. We found that pulse durations two orders of magnitude longer than those typically used in existing implants stimulate inner retinal neurons while avoiding activation of ganglion cell axons, thus confining retinal responses to the site of the electrode. We show that multielectrode stimulation with 25-ms pulses can pattern letters on the retina corresponding to a Snellen acuity of 20/312. We validated our findings in a patient with an implanted epiretinal prosthesis by demonstrating that 25-ms pulses evoke focal spots of light.

Sinusoidal Electrical Pulse More Efficiently Evokes Retinal Excitation than Rectangular Electrical Pulse in Retinal Prostheses

“…In a study of rat retina in vitro, Weitz et al reported that a sinusoidal pulse has a lower threshold charge than does a rectangular one. (13) However, this result was obtained using epiretinal stimulation; therefore, the effect of a sinusoidal pulse in STS has not been established. In addition, these in vitro results may not hold true in vivo.…”

“…Weitz et al reported that the threshold charge at which retinal ganglion cells fired was 22.8 ± 35.5% greater for a rectangular pulse than for a sinusoidal pulse. (13) In their in vitro study, they applied direct stimulation to the ganglion cell side of the retina. Although this methodology differed from ours, the results are similar.…”

“…(12) The optimum pulse duration and frequency for retinal ganglion cell activation (with respect to stimulation effectiveness) has been studied in vitro. (13)(14)(15) However, the optimal stimulation waveform shape remains unknown. Accordingly, a rectangular pulse may not be optimal as the stimulation waveform.…”

A Treatise of the Physical Aspects of Phosphenes and Single-Cell Selectivity in Retinal Stimulation

“…These specific-shaped phosphenes can also be generated by the activation of ganglion cell axons [30]. Retinal ganglion cell axons pass within short distances from each other along their path to the brain.…”

“…To evoke a narrow spot of light that resembles a natural signal to the brain and can serve as a building block for the pattern of phosphenes, each electrode should activate a nearby cell [30].…”