Subretinal Stimulation Research Articles

A novel GCaMP6f-RCS rat model for studying electrical stimulation in the degenerated retina.

Background: Retinal prostheses aim to restore vision by electrically stimulating the remaining viable retinal cells in Retinal Degeneration (RD) cases. Research in this field necessitates a comprehensive analysis of retinal ganglion cells' (RGCs) responses to assess the obtained visual acuity and quality. Here we present a novel animal model which facilitates the optical recording of RGCs activity in an RD rat. This model can significantly enhance the functional evaluation of vision restoration treatments. Methods: The development of the novel rat model is based on crossbreeding a retinal degenerated Royal College of Surgeons (RCS) rat with a transgenic line expressing the genetic calcium indicator GCaMP6f in the RGCs. Characterization of the model was achieved using Optical Coherence Tomography (OCT) imaging, histology, and electroretinography (ERG) at the ages of 4, 8, and 12weeks. Additionally, optical recordings of RGCs function in response to ex-vivo subretinal electrical stimulations were performed. Results: Histological investigations confirmed the high expression of GCaMP6f in the RGCs and minimal expression in the inner nuclear layer (INL). OCT imaging and histological studies revealed the expected gradual retinal degeneration, as evident by the decrease in retinal thickness with age and the formation of subretinal debris. This degeneration was further confirmed by ERG recordings, which demonstrated a significant decrease in the b-wave amplitude throughout the degeneration process, culminating in its absence at 12weeks in the GCaMP6f-RCS rat. Importantly, the feasibility of investigating subretinal stimulation was demonstrated, revealing a consistent increase in activation threshold throughout degeneration. Furthermore, an increase in the diameter of the activated area with increasing currents was observed. The spatial spread of the activation area in the GCaMP6f-RCS rat was found to be smaller and exhibited faster activation dynamics compared with the GCaMP6f-LE strain. Conclusion: This novel animal model offers an opportunity to deepen our understanding of prosthetically induced retinal responses, potentially leading to significant advancements in prosthetic interventions in visual impairments.

Double‐Sided, Thin‐Film Microelectrode Array with Hemispheric Electrodes for Subretinal Stimulation

AbstractComponents in neural implants, such as the electrode array and stimulator circuit, are often fabricated discretely. This modular fabrication scheme offers flexibility during development but poses difficulties during assembly, as components must be compactly integrated for implantation. It is particularly difficult in cases where the electrode array is required to have a high number of channels, such as in retinal prostheses. This paper presents the development of a parylene C‐based, double‐sided microelectrode array with 294 hemispheric electrodes for subretinal stimulation. The bonding pads on the bottom side of the double‐sided array are connected with electrodes through vias, eliminating the interconnection lines. The array can be integrated with a stimulator circuit through pad‐to‐pad bonding, resulting in a compact implant. The hemispheric electrodes are fabricated using thermally reflowed photoresist infillings, through which the height and width of the hemispheres can be easily controlled. The long‐term stability and biocompatibility of the materials and methods used to fabricate and package the electrodes are demonstrated in in vitro and in vivo environments over months. Finally, subretinal stimulation by the developed electrodes is successfully demonstrated using in vitro retinal patches from mice and monkeys.

Minimizing Iridium Oxide Electrodes for High Visual Acuity Subretinal Stimulation.

Vision loss from diseases of the outer retina, such as age-related macular degeneration, is among the leading causes of irreversible blindness in the world today. The goal of retinal prosthetics is to replace the photo-sensing function of photoreceptors lost in these diseases with optoelectronic hardware to electrically stimulate patterns of retinal activity corresponding to vision. To enable high-resolution retinal prosthetics, the scale of stimulating electrodes must be significantly decreased from current designs; however, this reduces the amount of stimulating current that can be delivered. The efficacy of subretinal stimulation at electrode sizes suitable for high visual acuity retinal prosthesis are not well understood, particularly within the safe charge injection limits of electrode materials. Here, we measure retinal ganglion cell (RGC) responses in a mouse model of blindness to evaluate the stimulation efficacy of 10, 20, and 30 μm diameter iridium oxide electrodes within the electrode charge injection limits, focusing on measures of charge threshold and dynamic range. Stimulation thresholds were lower for smaller electrodes, but larger electrodes could elicit a greater dynamic range of spikes and recruited more ganglion cells within charge injection limits. These findings suggest a practical lower limit for planar electrode size and indicate strategies for maximizing stimulation thresholds and dynamic range.

Vertical-junction photodiodes for smaller pixels in retinal prostheses

Objective. To restore central vision in patients with atrophic age-related macular degeneration, we replace the lost photoreceptors with photovoltaic pixels, which convert light into current and stimulate the secondary retinal neurons. Clinical trials demonstrated prosthetic acuity closely matching the sampling limit of the 100 μm pixels, and hence smaller pixels are required for improving visual acuity. However, with smaller flat bipolar pixels, the electric field penetration depth and the photodiode responsivity significantly decrease, making the device inefficient. Smaller pixels may be enabled by (a) increasing the diode responsivity using vertical p–n junctions and (b) directing the electric field in tissue vertically. Here, we demonstrate such novel photodiodes and test the retinal stimulation in a vertical electric field. Approach. Arrays of silicon photodiodes of 55, 40, 30, and 20 μm in width, with vertical p–n junctions, were fabricated. The electric field in the retina was directed vertically using a common return electrode at the edge of the device. Optical and electronic performance of the diodes was characterized in-vitro, and retinal stimulation threshold measured by recording the visually evoked potentials in rats with retinal degeneration. Main results. The photodiodes exhibited sufficiently low dark current (<10 pA) and responsivity at 880 nm wavelength as high as 0.51 A W−1, with 85% internal quantum efficiency, independent of pixel size. Field mapping in saline demonstrated uniformity of the pixel performance in the array. The full-field stimulation threshold was as low as 0.057 mW mm−2 with 10 ms pulses, independent of pixel size. Significance. Photodiodes with vertical p–n junctions demonstrated excellent charge collection efficiency independent of pixel size, down to 20 μm. Vertically oriented electric field provides a stimulation threshold that is independent of pixel size. These results are the first steps in validation of scaling down the photovoltaic pixels for subretinal stimulation.

Sputtered Iridium Oxide as Electrode Material for Subretinal Stimulation

Sputtered Iridium Oxide as Electrode Material for Subretinal Stimulation

Subretinal Stimulation Chip Set with 3025 Electrodes, Spatial Peaking Filter, Illumination Adaptation and Implant Lifetime Optimization.

A subretinal stimulator chip has been designed and tested, which combines high pixel number with highest simulation voltages, lowest power consumption, spatial peaking and illumination adaptation. A supporting ASIC completes the implantable device electronics. Blind mouse retina has successfully been stimulated in vitro.

Fabrication of Subretinal 3D Microelectrodes with Hexagonal Arrangement.

This study presents the fabrication of three-dimensional (3D) microelectrodes for subretinal stimulation, to accommodate adjacent return electrodes surrounding a stimulating electrode. For retinal prosthetic devices, the arrangement of return electrodes, the electrode size and spacing should be considered together, to reduce the undesired dissipation of electric currents. Here, we applied the hexagonal arrangement to the microelectrode array for the localized activation of retinal cells and better visual acuity. To provide stimuli more efficiently to non-spiking neurons, a 3D structure was created through a customized pressing process, utilizing the elastic property of the materials used in the fabrication processes. The diameter and pitch of the Pt-coated electrodes were 150 μm and 350 μm, respectively, and the height of the protruded electrodes was around 20 μm. The array consisted of 98 hexagonally arranged electrodes, supported by a flexible and transparent polydimethylsiloxane (PDMS) base, with a thickness of 140 μm. Also, the array was coated with 2 μm-thick parylene-C, except the active electrode sites, for more focused stimulation. Finally, the electrochemical properties of the fabricated microelectrodes were characterized, resulting in the mean impedance of 384.87 kΩ at 1 kHz and the charge storage capacity (CSC) of 2.83 mC·cm−2. The fabricated microelectrodes are to be combined with an integrated circuit (IC) for additional in vitro and in vivo experiments.

Improved visual acuity using a retinal implant and an optimized stimulation strategy

Objective. Retinal prosthetic devices hold great promise for the treatment of retinal degenerative diseases such as retinitis pigmentosa and age-related macular degeneration. Through electrical stimulation of the surviving retinal neurons, these devices evoke visual signals that are then relayed to the brain. Currently, the visual prostheses used in clinical trials have few electrodes, thus limiting visual acuity. Electrode arrays with high electrode densities have been developed using novel technologies, including diamond growth and laser machining, and these may provide a more promising route to achieve high visual acuity in blind patients. Approach. Here, we studied the potential spatial resolution of electrical stimulation using diamond electrodes. We did this by labeling retinal ganglion cells in whole mount retina with a calcium indicator in wild-type rats and those with retinal degeneration. We imaged the ganglion cell responses to a range of stimulation parameters, including pulse duration and return electrode configuration. Main results. With sub-retinal stimulation, in which electrodes were in contact with the intact or degenerated photoreceptor layer, we found that biphasic pulses of 0.1 ms phase duration and a local return configuration was the most effective in confining the retinal ganglion cell activation patterns, while also remaining within the safety limits of the materials and providing the best power efficiency. Significance. These results provide an optimized stimulation strategy for retinal implants, which if implemented in a retinal prosthetic is expected to improve the achievable visual acuity.

Design and characterisation of graphene‐based nano‐photodiode array device for photo‐stimulation of subretinal implant

Electrically stimulating the retina of the patients suffering due to extreme retinal disorders such as age related macula degeneration, retinitis pigmentosa is quickly turning into a feasible accomplishment through retinal implants. The objective of this work is to optimise the design, develop and test an organic photovoltaic based Nano Photo Diode Array (NPDA) device for retinal prosthesis application to restoring vision to patients blinded by degenerative retinal diseases. NPDA for subretinal stimulation has been fabricated based on Bulk heterojunction (BHJ) solar cell technology. Each pixel is composed by a graphene alone bottom anode, a semiconducting single wall carbon nanotube (S-SWCNT):C60 blend, and a top cathode in aluminum (Al) of 100 µm or 150 diameter. This NPDA device photovoltaically converts incident light into electric current to stimulate nearby retinal neurons without wired power connections. The fabricated device is able to provide a photo current density of 245 µA/cm 2 and photovoltage of 0.31 V. These results suggest that BHJ cell employed photodiode array used in this work make it better than POLYRETINA and SCNC. The significant improvement in photocurrent density paves the way for scaling up density of NPDA array in implant. Increased number of electrodes will indeed enhance visual acuity to greater extent.

Protuberant Electrode Structures for Subretinal Electrical Stimulation: Modeling, Fabrication and in vivo Evaluation.

Many neural interfaces used for therapeutic applications are based on extracellular electrical stimulation to control cell polarization and thus functional activity. Amongst them, retinal implants have been designed to restore visual perception in blind patients affected by photoreceptor degeneration diseases, such as age-related macular degeneration (AMD) or retinitis pigmentosa (RP). While designing such a neural interface, several aspects must be taken into account, like the stimulation efficiency related to the current distribution within the tissue, the bio-interface optimization to improve resolution and tissue integration, and the material biocompatibility associated with long-term aging. In this study, we investigate the use of original microelectrode geometries for subretinal stimulation. The proposed structures combine the use of 3D wells with protuberant mushroom shaped electrode structures in the bottom, implemented on a flexible substrate that allows the in vivo implantation of the devices. These 3D microelectrode structures were first modeled using finite element analysis. Then, a specific microfabrication process compatible with flexible implants was developed to create the 3D microelectrode structures. These structures were tested in vivo to check the adaptation of the retinal tissue to them. Finally, preliminary in vivo stimulation experiments were performed.

The Effects of Phase Durations on the Spatial Responses of Retinal Ganglion Cells to Epi- and Sub-Retinal Electrical Stimulation.

Retinal prostheses have the potential to restore vision to blind patients that have retinitis pigmentosa or similar hereditary degenerative disorders, by electrically stimulating surviving retinal neurons through implanted electrode arrays. Current retinal prostheses provide limited visual acuity and one challenge is to spatially control neural activation following electrical stimulation. Most of the retinal prostheses are either epi-retinal - in front of the retinal ganglion cell layer, or sub-retinal - behind photoreceptor layer. In this study, we performed calcium imaging of ganglion cells from whole mounted retinas and compared the spread of neural activation between epi-retinal stimulation with a fiber electrode and sub-retinal stimulation with a disk electrode. We investigated the effects of phase durations on the spatial resolution of biphasic stimulation. Our results suggest that with fiber electrode epi-retinal stimulation, the axon bundles activation can lead to significant spread of stimulation, and cannot be avoided simply by changing the phase durations. However, sub-retinal stimulation with very short pulses (phase duration 0.033ms) can effectively confine the activation of retinal ganglion cells.

Investigation of Injection Depth for Subretinal Delivery of Exogenous Glutamate to Restore Vision via Biomimetic Chemical Neuromodulation.

Chemical neuromodulation of the retina using native neurotransmitters to biomimetically activate target retinal neurons through chemical synapses is a promising biomimetic alternative to electrical stimulation for restoring vision in blindness caused by photoreceptor degenerative diseases. Recent research has shown that subretinal chemical stimulation could be advantageous for treating photoreceptor degenerative diseases but many of the parameters for achieving efficacious chemical neuromodulation are yet to be explored. In this paper, we investigated how the depth at which neurotransmitter is injected subretinally affects the success rate, spike rate characteristics (i.e., amplitude, response latency, and time width), and spatial resolution of chemical stimulation in wild-type Long Evans and photoreceptor degenerated S334ter-3 transgenic rat retinas in vitro. We compared the responses to injections of glutamate at the subretinal surface and two subsurface depths near the outer and inner plexiform layers and found that while injections at all depths elicited robust retinal ganglion cell responses, they differed significantly in terms of the spike rate characteristics and spatial resolutions across injection depths. Shallow subsurface injections near the outer plexiform layer evoked the highest spike rate amplitudes and had the highest spatial resolution and success rates, while deep subsurface injections near the inner plexiform layer elicited the shortest latencies and narrowest time widths. Our results suggest that surface injections are suboptimal for subretinal chemical neuromodulation, while shallow subsurface and deep subsurface injections may optimize high spatial and high temporal resolution, respectively. These findings have great significance for the design and development of a potential neurotransmitter-based subretinal prosthesis.

Feasibility study for a glutamate driven subretinal prosthesis: local subretinal application of glutamate on blind retina evoke network-mediated responses in different types of ganglion cells

Objective. A feasibility study for a transmitter based subretinal prosthesis, generating visual responses in blind mouse retina is presented. Approach. Degenerated rd1 mouse retina were stimulated in subretinal configuration by local glutamate (Glu) or NMDA application via micropipettes (~1.5 μm) and thereby the outer retinal activity was recorded by calcium-imaging or the ganglion cell (GC) activity was recorded by the multi-electrode array system. The network mediated activation of GC via bipolar cells was approved by the administration of Glu receptor blockers. Main results. Data of the degenerated and blind rd1 mouse retina reveals that the outer retina is Glu sensitive and that the subretinal Glu stimulation promotes network mediated GC responses. Analysis of the spatial activity-spread indicates that the Glu induced cell activation radius in the outer retina (~12.5 μm) and postsynaptically activated GC (~40 μm) is focal to the stimulation pipette tip. Moreover, the application of NMDA in subretinal space also evoked network mediated GC responses. The Glu-activated GC were identified as ON–OFF, OFF and two ON cells types. Significance. This study evaluates the prerequisite for the function of a transmitter based implant, that after the loss of the photoreceptors, the remnant blind retinal network is Glu sensitive and functional, positively. The differential activation of ON (hyperpolarisation) and OFF (depolarisation) bipolar cells by transmitter Glu is a unique feature and of high interest for retinal implants. Therefore, the respective bipolar cell types could only be driven by glutamatergic stimulation accurately and not by electrical stimulation. The preserved functionality of the blind retina at the onset of complete blindness is motivating to continue research on a transmitter-based prosthesis. Since the artificial Glu stimulation mimics the natural retinal input, early implantation of a Glu-prosthesis might delay the devastating retinal remodelling positively, due to the neuronal-plasticity.

Electrical receptive fields of retinal ganglion cells: Influence of presynaptic neurons.

Implantable retinal stimulators activate surviving neurons to restore a sense of vision in people who have lost their photoreceptors through degenerative diseases. Complex spatial and temporal interactions occur in the retina during multi-electrode stimulation. Due to these complexities, most existing implants activate only a few electrodes at a time, limiting the repertoire of available stimulation patterns. Measuring the spatiotemporal interactions between electrodes and retinal cells, and incorporating them into a model may lead to improved stimulation algorithms that exploit the interactions. Here, we present a computational model that accurately predicts both the spatial and temporal nonlinear interactions of multi-electrode stimulation of rat retinal ganglion cells (RGCs). The model was verified using in vitro recordings of ON, OFF, and ON-OFF RGCs in response to subretinal multi-electrode stimulation with biphasic pulses at three stimulation frequencies (10, 20, 30 Hz). The model gives an estimate of each cell’s spatiotemporal electrical receptive fields (ERFs); i.e., the pattern of stimulation leading to excitation or suppression in the neuron. All cells had excitatory ERFs and many also had suppressive sub-regions of their ERFs. We show that the nonlinearities in observed responses arise largely from activation of presynaptic interneurons. When synaptic transmission was blocked, the number of sub-regions of the ERF was reduced, usually to a single excitatory ERF. This suggests that direct cell activation can be modeled accurately by a one-dimensional model with linear interactions between electrodes, whereas indirect stimulation due to summated presynaptic responses is nonlinear.

Spatiotemporal characteristics of retinal response to network-mediated photovoltaic stimulation.

Subretinal prostheses aim at restoring sight to patients blinded by photoreceptor degeneration using electrical activation of the surviving inner retinal neurons. Today, such implants deliver visual information with low-frequency stimulation, resulting in discontinuous visual percepts. We measured retinal responses to complex visual stimuli delivered at video rate via a photovoltaic subretinal implant and by visible light. Using a multielectrode array to record from retinal ganglion cells (RGCs) in the healthy and degenerated rat retina ex vivo, we estimated their spatiotemporal properties from the spike-triggered average responses to photovoltaic binary white noise stimulus with 70-μm pixel size at 20-Hz frame rate. The average photovoltaic receptive field size was 194 ± 3 μm (mean ± SE), similar to that of visual responses (221 ± 4 μm), but response latency was significantly shorter with photovoltaic stimulation. Both visual and photovoltaic receptive fields had an opposing center-surround structure. In the healthy retina, ON RGCs had photovoltaic OFF responses, and vice versa. This reversal is consistent with depolarization of photoreceptors by electrical pulses, as opposed to their hyperpolarization under increasing light, although alternative mechanisms cannot be excluded. In degenerate retina, both ON and OFF photovoltaic responses were observed, but in the absence of visual responses, it is not clear what functional RGC types they correspond to. Degenerate retina maintained the antagonistic center-surround organization of receptive fields. These fast and spatially localized network-mediated ON and OFF responses to subretinal stimulation via photovoltaic pixels with local return electrodes raise confidence in the possibility of providing more functional prosthetic vision. NEW & NOTEWORTHY Retinal prostheses currently in clinical use have struggled to deliver visual information at naturalistic frequencies, resulting in discontinuous percepts. We demonstrate modulation of the retinal ganglion cells (RGC) activity using complex spatiotemporal stimuli delivered via subretinal photovoltaic implant at 20 Hz in healthy and in degenerate retina. RGCs exhibit fast and localized ON and OFF network-mediated responses, with antagonistic center-surround organization of their receptive fields.

Subretinal electrical stimulation reveals intact network activity in the blind mouse retina.

Retinal degeneration (rd) leads to progressive photoreceptor cell death, resulting in vision loss. Stimulation of the inner-retinal neurons by neuroprosthetic implants is one of the clinically approved vision-restoration strategies, providing basic visual percepts to blind patients. However, little is understood as to what degree the degenerating retinal circuitry and the resulting aberrant hyperactivity may prevent the stimulation of physiological electrical activity. Therefore, we electrically stimulated ex vivo retinas from wild-type (wt; C57BL/6J) and blind (rd10 and rd1) mice using an implantable subretinal microchip and simultaneously recorded and analyzed the retinal ganglion cell (RGC) output with a flexible microelectrode array. We found that subretinal anodal stimulation of the rd10 retina and wt retina evoked similar spatiotemporal RGC-spiking patterns. In both retinas, electrically stimulated ON and a small percentage of OFF RGC responses were detected. The spatial selectivity of the retinal network to electrical stimuli reveals an intact underlying network with a median receptive-field center of 350 μm in both retinas. An antagonistic surround is activated by stimulation with large electrode fields. However, in rd10 and to a higher percentage, in rd1 retinas, rhythmic and spatially unconfined RGC patterns were evoked by anodal or by cathodal electrical stimuli. Our findings demonstrate that the surviving retinal circuitry in photoreceptor-degenerated retinas is preserved in a way allowing for the stimulation of temporally diverse and spatially confined RGC activity. Future vision restoration strategies can build on these results but need to avoid evoking the easily inducible rhythmic activity in some retinal circuits.

The impact of calcium current reversal on neurotransmitter release in the electrically stimulated retina

Objective. In spite of intense theoretical and experimental investigations on electrical nerve stimulation, the influence of reversed ion currents on network activity during extracellular stimulation has not been investigated so far. Approach. Here, the impact of calcium current reversal on neurotransmitter release during subretinal stimulation was analyzed with a computational multi-compartment model of a retinal bipolar cell (BC) that was coupled with a four-pool model for the exocytosis from its ribbon synapses. Emphasis was laid on calcium channel dynamics and how these channels influence synaptic release. Main results. Stronger stimulation with anodic pulses caused transmembrane voltages above the Nernst potential of calcium in the terminals and, by this means, forced calcium ions to flow in the reversed direction from inside to the outside of the cell. Consequently, intracellular calcium concentration decreased resulting in a reduced vesicle release or preventing release at all. This mechanism is expected to lead to a pronounced ring-shaped pattern of exocytosis within a group of neighbored BCs when the stronger stimulated cells close to the electrode fail in releasing vesicles. Significance. Stronger subretinal stimulation causes failure of synaptic exocytosis due to reversal of calcium flow into the extracellular space in cells close to the electrode.

Light-controlled biphasic current stimulator IC using CMOS image sensors for high-resolution retinal prosthesis and in vitro experimental results with rd1 mouse.

Retinal prosthetic devices stimulate retinal nerve cells with electrical signals proportional to the incident light intensities. For a high-resolution retinal prosthesis, it is necessary to reduce the size of the stimulator pixels as much as possible, because the retinal nerve cells are concentrated in a small area of approximately 5 mm × 5 mm. In this paper, a miniaturized biphasic current stimulator integrated circuit is developed for subretinal stimulation and tested in vitro. The stimulator pixel is miniaturized by using a complementary metal-oxide-semiconductor (CMOS) image sensor composed of three transistors. Compared to a pixel that uses a four-transistor CMOS image sensor, this new design reduces the pixel size by 8.3%. The pixel size is further reduced by simplifying the stimulation-current generating circuit, which provides a 43.9% size reduction when compared to the design reported to be the most advanced version to date for subretinal stimulation. The proposed design is fabricated using a 0.35 μm bipolar-CMOS-DMOS process. Each pixel is designed to fit in a 50 μ m × 55 μm area, which theoretically allows implementing more than 5000 pixels in the 5 mm × 5 mm area. Experimental results show that a biphasic current in the range of 0 to 300 μA at 12 V can be generated as a function of incident light intensities. Results from in vitro experiments with rd1 mice indicate that the proposed method can be effectively used for retinal prosthesis with a high resolution.

Simultaneous recording of mouse retinal ganglion cells during epiretinal or subretinal stimulation.

We compared response patterns and electrical receptive fields (ERF) of retinal ganglion cells (RGCs) during epiretinal and subretinal electrical stimulation of isolated mouse retina. Retinas were stimulated with an array of 3200 independently controllable electrodes. Four response patterns were observed: a burst of activity immediately after stimulation (Type I cells, Vision Research (2008), 48, 1562-1568), delayed bursts beginning >25ms after stimulation (Type II), a combination of both (Type III), and inhibition of ongoing spike activity. Type I responses were produced more often by epiretinal than subretinal stimulation whereas delayed and inhibitory responses were evoked more frequently by subretinal stimulation. Response latencies were significantly shorter with epiretinal than subretinal stimulation. These data suggest that subretinal stimulation is more effective at activating intraretinal circuits than epiretinal stimulation. There was no significant difference in charge threshold between subretinal and epiretinal configurations. ERFs were defined by the stimulating array surface area that successfully stimulated spikes in an RGC. ERFs were complex in shape, similar to receptive fields mapped with light. ERF areas were significantly smaller with subretinal than epiretinal stimulation. This may reflect the greater distance between stimulating electrodes and RGCs in the subretinal configuration. ERFs for immediate and delayed responses mapped within the same Type III cells differed in shape and size, consistent with different sites and mechanisms for generating these two response types.

Responses to pulsatile subretinal electric stimulation: effects of amplitude and duration

In working to improve the quality of visual percepts elicited by retinal prosthetics, considerable effort has been made to understand how retinal neurons respond to electric stimulation. Whereas responses arising from direct activation of retinal ganglion cells have been well studied, responses arising through indirect activation (e.g., secondary to activation of bipolar cells) are not as well understood. Here, we used cell-attached, patch-clamp recordings to measure the responses of rabbit ganglion cells in vitro to a wide range of stimulus-pulse parameters (amplitudes: 0-100 μA; durations: 0.1-50 ms), applied to a 400-μm-diameter, subretinal-stimulating electrode. The indirect responses generally consisted of multiple action potentials that were clustered into bursts, although the latency and number of spikes within a burst were highly variable. When different parameter pairs representing identical charge levels were compared, the shortest pulse durations generally elicited the most spikes. In addition, latencies were shortest, and jitter was lowest for short pulses. These findings suggest that short pulses are optimum for activation of presynaptic neurons, and therefore, short pulses are more effective for both direct as well as indirect activation.