Improving stimulation strategies for epiretinal prostheses

2013-04-26Epiretinal implants for the blind are designed to stimulate surviving retinal neurons, thus bypassing the diseased photoreceptor layer. Single-unit or multielectrode recordings from isolated animal retina are commonly used to inform the design of these implants. However, such electrical recordings provide limited information about the spatial patterns of retinal activation. Calcium imaging overcomes this limitation, as imaging enables high spatial resolution mapping of retinal ganglion cell (RGC) activity as well as simultaneous recording from hundreds of RGCs. ❧ I developed a method for labeling the majority of ganglion cells in adult rat retina with genetically encoded calcium indicators (GECIs). Intravitreal injection of an adeno-associated viral vector targeted roughly 85% of ganglion cells with high specificity. Due to the large fluorescence signals provided by GECIs, I was able to visualize the retina's response to electrical stimulation in real time. Imaging transduced retinas mounted on multielectrode arrays revealed how varying stimulus parameters dramatically affects the spatial extent of RGC activation. ❧ Recent human subject testing indicated that patients often see large, elongated phosphenes due to stimulation of RGC axon bundles. My experiments revealed two potential strategies for avoiding stimulation of axons: Short pulses (≤ 0.1 ms) selectively stimulate ganglion cell somata at thresholds 40–60% lower than their axons, while long pulses (≥ 25 ms) preferentially target bipolar cells. No axonal activation was observed during stimulation with long pulses, regardless of amplitude. Responses were focal and gradually became larger as stimulus amplitude was increased. ❧ Low-frequency sinusoids can be used as an alternative to long pulses, as they require significantly less charge to evoke responses. When stimulating with short pulses, interphase gaps can be used to reduce overall thresholds by 10–25%. Retinal degeneration appears to have no effect on response shape; however, bipolar cell thresholds in degenerate animals were elevated by threefold. ❧ Multielectrode stimulation with long pulses revealed that controlled patterns of ganglion cells can be activated with minimal electrode interactions. Lines and letters were patterned on the retina with high spatial resolution. An array design with 75-µm-diameter electrodes on a 150-µm pitch should permit a resolution of 0.26° of visual field and a Snellen acuity of 20/312