Optimal voltage stimulation parameters for network-mediated responses in wild type andrd10mouse retinal ganglion cells

Abstract

To further improve the quality of visual percepts elicited by microelectronic retinal prosthetics, substantial efforts have been made to understand how retinal neurons respond to electrical stimulation. It is generally assumed that a sufficiently strong stimulus will recruit most retinal neurons. However, recent evidence has shown that the responses of some retinal neurons decrease with excessively strong stimuli (a non-monotonic response function). Therefore, it is necessary to identify stimuli that can be used to activate the majority of retinal neurons even when such non-monotonic cells are part of the neuronal population. Taking these non-monotonic responses into consideration, we establish the optimal voltage stimulation parameters (amplitude, duration, and polarity) for epiretinal stimulation of network-mediated (indirect) ganglion cell responses. We recorded responses from 3958 mouse retinal ganglion cells (RGCs) in both healthy (wild type, WT) and a degenerating (rd10) mouse model of retinitis pigmentosa-using flat-mounted retina on a microelectrode array. Rectangular monophasic voltage-controlled pulses were presented with varying voltage, duration, and polarity. We found that in 4-5 weeks old rd10 mice the RGC thresholds were comparable to those of WT. There was a marked response variability among mouse RGCs. To account for this variability, we interpolated the percentage of RGCs activated at each point in the voltage-polarity-duration stimulus space, thus identifying the optimal voltage-controlled pulse (-2.4 V, 0.88 ms). The identified optimal voltage pulse can activate at least 65% of potentially responsive RGCs in both mouse strains. Furthermore, this pulse is well within the range of stimuli demonstrated to be safe and effective for retinal implant patients. Such optimized stimuli and the underlying method used to identify them support a high yield of responsive RGCs and will serve as an effective guideline for future in vitro investigations of retinal electrostimulation by establishing standard stimuli for each unique experimental condition.

The Spatial Extent of Epiretinal Electrical Stimulation in the Healthy Mouse Retina

“…From Figure 2b it is clear that while -2000 and -2500 mV stimuli elicited robust responses, weaker voltages did not. This result was consistent across our population, in contrast to previous reports [13]. After careful consideration, we judge the difference to result from the higher impedance of the electrodes used here as well as higher thresholds for such small electrodes.…”

“…Similar distances for direct RGC stimulation were reported by Sekirnjak et al who confirmed that RGCs as far as 160 µm away can be stimulated in the rat retina [12]. Recently, our group used a MEA with 30 µm electrode diameter and 200 µm interelectrode distance to show that RGC responsiveness in wild type mice declines swiftly from 200 µm to 283 µm [13]. Importantly, however, we found reliable RGC responses in a small subset of RGCs even for distances up to 1000 µm.…”

“…The vast majority were only responsive to -2000 and -2500 mV. In light of previous experiments [13], we interpret this to reflect less current for a given voltage due to smaller electrodes and higher impedance relative to previous experiments. A cell was considered ‘responsive’ if the average response to one of the voltages was above threshold (3 standard deviations above the spontaneous rate) and with a minimum average response rate of 60% - at least 3 spikes for every 5 stimulus repetitions.…”

“…According to our previous report, the fraction of responsive RGCs in wild type mice on a ‘standard’ MEA (30 µm electrode diameter and 200 µm interelectrode distance) diminished speedily from about 50% at 200 µm to 20% at 283 µm with little additional drop up to 1000 µm [13]. Nevertheless, we find inconsistencies in the literature regarding the choice of how to define a threshold response or the reporting of RGC recruitment versus the average RGC firing rate.…”

“…Nevertheless, in the course of a previous study, we observed evidence that even such long intervals might not be sufficient to return the retina to baseline for electrical stimulation [13]. Therefore, in addition to investigating the spatial spread of electrical stimulation, we designed the present experiments in and up/down sequence to test whether electrical stimulus history at such long intervals influences responses.…”