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, with 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 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 stimulated inner retinal neurons while avoiding activation of ganglion cell axons, thus confining retinal responses to the site of the electrode. 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.

Fig. 1. Effects of stimulus pulse duration on RGC responses to electrical stimulation

Electrode size (200 μm diameter) is the same as the Argus II. (A) Burst stimulation with 0.4-ms pulses activated a streak of RGCs extending from the transparent stimulating electrode (green circle) to the edge of the retina. (Left) Fluorescence image mosaic of the GCaMP5G-labeled retina before stimulation. (Right) Background-subtracted GCaMP5G responses to electrical stimulation. (B) Spatial threshold maps. The stimulating electrode is drawn as a black circle. Each colored dot represents the average threshold charge density (log₂ scale) needed to stimulate cells at its location. Unfilled gray dots indicate areas containing cells that did not respond to stimulation. Small, medium, and large dots specify 1–2, 3–4, and 5+ cells, respectively. Maps are oriented such that the optic disc lies to the left of the image, with axons running horizontally toward their originating somata on the right [same orientation as (A)]. Each map contains data from 3 or 4 retinas (table S2). (C) Threshold as a function of displacement from electrode center for 100-ms pulses (n = 344 RGCs). (D) Change in RGC stimulation threshold as a function of pulse width in the presence of synaptic blockers (CNQX, D-APV, L-APB, strychnine, and picrotoxin). Pulses longer than 8 ms did not evoke responses in the presence of blockers. Thresholds for all pulse widths except 0.06 ms rose significantly once blockers were applied (** indicates P < 0.001 compared to the no blocker condition with paired t-tests). Error bars indicate SEM. Data were fit with an inverse sigmoid: y = −a ln[b /( x − c) −1] + d .

Fig. 2. Comparison of 20-Hz sine and square wave stimulation

Electrode diameter is 200 μm. Sine wave thresholds are specified in zero-to-peak amplitude. (A) Spatial threshold maps (as in Fig. 1B). The sine wave map (top) contains data from 3 retinas (table S2). Of the 344 cells that we imaged far (≥ 225 μm) to the right of the electrode perimeter, only 5 were stimulated antidromically. Based on the maximum stimulus amplitude delivered, minimum selectivity ratio (see Supplementary Materials) for 20-Hz sine waves was 16.7. The square wave map (bottom) is the same one shown in Fig. 1B for 25-ms pulses, except that the color scale has been changed. (B) Individual RGC thresholds for 20-Hz sine and square waves. Each data point represents a different cell (n = 121). Data were combined from two retinas. Square wave thresholds were 22.8 ± 35.5% higher than sine wave thresholds (P < 0.001, generalized estimating equations comparing means adjusted for repeated measures; percent change calculated from raw means).

Fig. 3. Effects of retinal degeneration (top row) and electrode size (subsequent rows) on RGC responses to electrical stimulation

Spatial threshold maps for three pulse durations that cover the gamut of response types: direct RGC stimulation (0.06 ms, left column), combined ganglion and bipolar cell stimulation (1 ms, middle column), and bipolar cell stimulation (25 ms, right column) in RD and WT rats. Each map contains data from 3 or 4 retinas (table S2). In all cases, 0.06-ms pulses provided good selectivity for local somata over passing axons, 1-ms pulses provided poor selectivity (fig. S1), and 25-ms pulses produced focal responses. The bottom row shows background-subtracted GCaMP5G responses to suprathreshold stimuli for 30-μm-diameter electrodes (red circles). Images from each retina were transformed into the same reference frame and averaged.

Fig. 4. Patterns resulting from multielectrode stimulation of RGCs

(A) Line stimulation with four 30-μm-diameter electrodes (red circles) on a 75-μm pitch. Electrodes are oriented transverse (top and middle rows) and parallel (bottom row) to axon bundles. Leftmost images show local ganglion cell layer anatomy as revealed by Alexa Fluor 594 fluorescence (see Materials and Methods). Subsequent images show background-subtracted GCaMP5G responses to stimulation at different amplitudes (I_min is slightly above threshold). Responses generally become stronger with increasing amplitude. (Top row) 0.1 ms pulse width. (Middle and bottom rows) 25 ms pulse width. (B) (Left) Letters patterned onto a single retina with transparent 75-μm-diameter electrodes (green circles) on a 150-μm pitch. Pulse width is 25 ms. The letters conform to the definition of Snellen optotype, which requires a critical detail size (stroke and gap width) that subtends 1/5 of the total height. (Right) The word LIFT spelled by combining the letters L, F, and T with a line pattern from (A). The image shows the actual size of the word at typical reading distance (40 cm).

Fig. 5. Effect of stimulus pulse duration on phosphene shape in a retinal prosthesis subject

(A) The stimulus waveform, a train of either 0.45-ms/phase (red) or 25-ms/phase (blue) pulses, each applied at 20 Hz for a duration of 500 ms. Twenty-five millisecond pulses were presented as sinusoids in order to increase the likelihood of staying within electrochemical safety limits (see Fig. 2). Pseudo-sinusoids were used because the implant hardware does not permit stimulation with true sine waves. (B) Subject’s drawings of phosphenes elicited from stimulation with electrode C4 (top row) and C3 (bottom row). Plots show the average percept drawn across up to 5 trials (see table S4). Electrode diameter is 520 μm (top row) or 260 μm (bottom row).