Pattern electrical stimulation of the human retina

Nruthyathi, Jung M, Wang J, Montes VR, Offenhäusser A, Willuweit A, Müller F. Late emergence of pathological oscillatory activity in the retina of the Retinitis pigmentosa model RCS (Royal College of Surgeons) rat

Retinitis pigmentosa (RP) is a leading cause of blindness. The best studied models of human RP are the rd1 and rd10 mouse and the RCS rat (Royal College of Surgeons). In many models after degeneration of the photoreceptors, a pathological rhythmic activity of the retina as well as lowered efficiency of electrical stimulation were observed. In rd10 retina, both events were shown to be intimately linked. Surprisingly, to our knowledge no retinal oscillations have been reported in RCS retina. As oscillations might interfere with the performance of therapeutic approaches to restore vision, e.g., retinal prostheses, it is important to know, whether they are a common feature of retinal degeneration. Electrical activity was recorded in retinae of 3-19 months (M3-19) old RCS rats in vitro using planar and penetrating multi-electrode-arrays. Short deflections in the local field potential resembling those observed in oscillations in rd1 and rd10 retinae were only sporadically found in M3 RCS retinae. Oscillations at appr. 2 Hz occurred more often and were more pronounced the older the animals were. Yet, even at M18-19 oscillatory periods were short and separated by long periods of non-oscillatory activity. In summary, in advanced stages of degeneration, RCS retinae display oscillations similar to rd1 and rd10 retinae. However, in RCS retina oscillatory periods are shorter than in mouse models and may, therefore, have escaped detection in earlier studies. These results together with results observed in non-rodent models suggest that pathological rhythmic activity is a common feature in RP models.

Pre-processing visual scenes for retinal prosthesis systems: A comprehensive review

BACKGROUND

Retinal prostheses offer hope for individuals with degenerative retinal diseases by stimulating the remaining retinal cells to partially restore their vision. This review delves into the current advancements in retinal prosthesis technology, with a special emphasis on the pivotal role that image processing and machine learning techniques play in this evolution.

METHODS

We provide a comprehensive analysis of the existing implantable devices and optogenetic strategies, delineating their advantages, limitations, and challenges in addressing complex visual tasks. The review extends to various image processing algorithms and deep learning architectures that have been implemented to enhance the functionality of retinal prosthetic devices. We also illustrate the testing results by demonstrating the clinical trials or using Simulated Prosthetic Vision (SPV) through phosphene simulations, which is a critical aspect of simulating visual perception for retinal prosthesis users.

RESULTS

Our review highlights the significant progress in retinal prosthesis technology, particularly its capacity to augment visual perception among the visually impaired. It discusses the integration between image processing and deep learning, illustrating their impact on individual interactions and navigations within the environment through applying clinical trials and also illustrating the limitations of some techniques to be used with current devices, as some approaches only use simulation even on sighted-normal individuals or rely on qualitative analysis, where some consider realistic perception models and others do not.

CONCLUSION

This interdisciplinary field holds promise for the future of retinal prostheses, with the potential to significantly enhance the quality of life for individuals with retinal prostheses. Future research directions should pivot towards optimizing phosphene simulations for SPV approaches, considering the distorted and confusing nature of phosphene perception, thereby enriching the visual perception provided by these prosthetic devices. This endeavor will not only improve navigational independence but also facilitate a more immersive interaction with the environment.

Comparison of electrical microstimulation artifact removal methods for high-channel-count prostheses

BACKGROUND

Neuroprostheses are used to electrically stimulate the brain, modulate neural activity and restore sensory and motor function following injury or disease, such as blindness, paralysis, and other movement and psychiatric disorders. Recordings are often made simultaneously with stimulation, allowing the monitoring of neural signals and closed-loop control of devices. However, stimulation-evoked artifacts may obscure neural activity, particularly when stimulation and recording sites are nearby. Several methods have been developed to remove stimulation artifacts, but it remains challenging to validate and compare these methods because the 'ground-truth' of the neuronal signals may be contaminated by artifacts.

NEW METHOD

Here, we delivered stimulation to the visual cortex via a high-channel-count prosthesis while recording neuronal activity and stimulation artifacts. We quantified the waveforms and temporal properties of stimulation artifacts from the cortical visual prosthesis (CVP) and used them to build a dataset, in which we simulated the neuronal activity and the stimulation artifacts. We illustrate how to use the simulated data to evaluate the performance of six software-based artifact removal methods (Template subtraction, Linear interpolation, Polynomial fitting, Exponential fitting, SALPA and ERAASR) in a CVP application scenario.

RESULTS

We here focused on stimulation artifacts caused by electrical stimulation through a high-channel-count cortical prosthesis device. We find that the Polynomial fitting and Exponential fitting methods outperform the other methods in recovering spikes and multi-unit activity. Linear interpolation and Template subtraction recovered the local-field potentials.

CONCLUSION

Polynomial fitting and Exponential fitting provided a good trade-off between the quality of the recovery of spikes and multi-unit activity (MUA) and the computational complexity for a cortical prosthesis.

Effects on Retinal Stimulation of the Geometry and the Insertion Location of Penetrating Electrodes

Retinal implants have been developed and implanted to restore vision from outer retinal degeneration, but their performance is still limited due to the poor spatial resolution. To improve the localization of stimulation, microelectrodes in various three-dimensional (3D) shapes have been investigated. In particular, computational simulation is crucial for optimizing the performance of a novel microelectrode design before actual fabrication. However, most previous studies have assumed a uniform conductivity for the entire retina without testing the effect of electrodes placement in different layers. In this study, we used the finite element method to simulate electric fields created by 3D microelectrodes of three different designs in a retina model with a stratified conductivity profile. The three electrode designs included two conventional shapes - a conical electrode (CE) and a pillar electrode (PE); we also proposed a novel structure of pillar electrode with an insulating wall (PEIW). A quantitative comparison of these designs shows the PEIW generates a stronger and more confined electric field with the same current injection, which is preferred for high-resolution retinal prostheses. Moreover, our results demonstrate both the magnitude and the shape of potential distribution generated by a penetrating electrode depend not only on the geometry, but also substantially on the insertion depth of the electrode. Although epiretinal insertions are mainly discussed, we also compared results for subretinal insertions. The results provide valuable insights for improving the spatial resolution of retinal implants using 3D penetrating microelectrodes and highlight the importance of considering the heterogeneity of conductivities in the retina.

2D Semiconductor Based Flexible Photoresponsive Ring Oscillators for Artificial Vision Pixels

Artificial retina implantation provides an effective and feasible attempt for vision recovery in addition to retinal transplantation. The most advanced artificial retinas ever developed based on silicon technology are rigid and thus less compatible with the biosystem. Here we demonstrate flexible photoresponsive ring oscillators (PROs) based on the 2D semiconductor MoS2 for artificial retinas. Under natural light illuminations, arrayed PROs on flexible substrates serving as vision pixels can efficiently output light-intensity-dependent electrical pulses that are processable and transmittable in the human visual nerve system. Such PROs can work under low supply voltages below 1 V with a record-low power consumption, e.g. only 12.4 nW at a light intensity of 10 mW/cm2, decreased by ∼500 times compared with that of the state-of-the-art silicon devices. Such flexible artificial retinas with a simple device structure, high light-to-signal conversion efficiency, ultralow power consumption, and high tunability provide an alternative prosthesis for further clinical trials.

Nature-inspired saccadic-like electrical stimulation paradigm promotes sustained retinal ganglion cell responses by spatiotemporally alternating activation of contiguous multi-electrode patterns

Objective. Retinal electrical stimulation using multi-electrode arrays (MEAs) aims to restore visual object perception in blind patients. However, the rate and duration of the artificial visual sensations are limited due to the rapid response decay of the stimulated neurons. Hence, we investigated a novel nature-inspired saccadic-like stimulation paradigm (biomimetic) to evoke sustained retinal responses. For implementation, the macroelectrode was replaced by several contiguous microelectrodes and activated non-simultaneously but alternating topologically.Approach.MEAs with hexagonally arranged electrodes were utilized to simulate and record mouse retinal ganglion cells (RGCs). Two shapes were presented electrically using MEAs: a 6e-hexagon (six hexagonally arranged 10µm electrodes; 6e-hexagon diameter: 80µm) and a double-bar (180µm spaced, 320µm in length). Electrodes of each shape were activated in three different modes (simultaneous, circular, and biomimetic ('zig-zag')), stimulating at different frequencies (1-20 Hz).Main results.The biomimetic stimulation generated enhanced RGC responses increasing the activity rate by 87.78%. In the spatiotemporal context, the electrical representation of the 6e-hexagon produced sustained and local RGC responses (∼130µm corresponding to ∼2.5° of the human visual angle) for up to 90 s at 10 Hz stimulation and resolved the electrically presented double-bar. In contrast, during conventional simultaneous stimulation, the responses were poor and declined within seconds. Similarly, the applicability of the biomimetic mode for retinal implants (7 × 8 pixels) was successfully demonstrated. An object shape impersonating a smile was presented electrically, and the recorded data were used to emulate the implant's performance. The spatiotemporal pixel mapping of the activity produced a complete retinal image of the smile.Significance.The application of electrical stimulation in the biomimetic mode produced locally enhanced RGC responses with significantly reduced fading effects and yielded advanced spatiotemporal performance reflecting the presented electrode shapes in the mapped activity imprint. Therefore, it is likely that the RGC responses persist long enough to evoke visual perception and generate a seamless image, taking advantage of the flicker fusion. Hence, replacing the implant's macroelectrodes with microelectrodes and their activation in a topologically alternating biomimetic fashion may overcome the patient's perceptual image fading, thereby enhancing the spatiotemporal characteristics of artificial vision.

Vision recovery with perceptual learning and non-invasive brain stimulation: Experimental set-ups and recent results, a review of the literature

BACKGROUND

Vision is the sense which we rely on the most to interact with the environment and its integrity is fundamental for the quality of our life. However, around the globe, more than 1 billion people are affected by debilitating vision deficits. Therefore, finding a way to treat (or mitigate) them successfully is necessary.

OBJECTIVE

This narrative review aims to examine options for innovative treatment of visual disorders (retinitis pigmentosa, macular degeneration, optic neuropathy, refractory disorders, hemianopia, amblyopia), especially with Perceptual Learning (PL) and Electrical Stimulation (ES).

METHODS

ES and PL can enhance visual abilities in clinical populations, inducing plastic changes. We describe the experimental set-ups and discuss the results of studies using ES or PL or their combination in order to suggest, based on literature, which treatment is the best option for each clinical condition.

RESULTS

Positive results were obtained using ES and PL to enhance visual functions. For example, repetitive transorbital Alternating Current Stimulation (rtACS) appeared as the most effective treatment for pre-chiasmatic disorders such as optic neuropathy. A combination of transcranial Direct Current Stimulation (tDCS) and visual training seems helpful for people with hemianopia, while transcranial Random Noise Stimulation (tRNS) makes visual training more efficient in people with amblyopia and mild myopia.

CONCLUSIONS

This narrative review highlights the effect of different ES montages and PL in the treatment of visual disorders. Furthermore, new options for treatment are suggested. It is noteworthy to mention that, in some cases, unclear results emerged and others need to be more deeply investigated.

From retina to motoneurons: A substrate for visuomotor transformation in salamanders

The transformation of visual input into motor output is essential to approach a target or avoid a predator. In salamanders, visually guided orientation behaviors have been extensively studied during prey capture. However, the neural circuitry involved is not resolved. Using salamander brain preparations, calcium imaging and tracing experiments, we describe a neural substrate through which retinal input is transformed into spinal motor output. We found that retina stimulation evoked responses in reticulospinal neurons of the middle reticular nucleus, known to control steering movements in salamanders. Microinjection of glutamatergic antagonists in the optic tectum (superior colliculus in mammals) decreased the reticulospinal responses. Using tracing, we found that retina projected to the dorsal layers of the contralateral tectum, where the dendrites of neurons projecting to the middle reticular nucleus were located. In slices, stimulation of the tectal dorsal layers evoked glutamatergic responses in deep tectal neurons retrogradely labeled from the middle reticular nucleus. We then examined how tectum activation translated into spinal motor output. Tectum stimulation evoked motoneuronal responses, which were decreased by microinjections of glutamatergic antagonists in the contralateral middle reticular nucleus. Reticulospinal fibers anterogradely labeled from tracer injection in the middle reticular nucleus were preferentially distributed in proximity with the dendrites of ipsilateral motoneurons. Our work establishes a neural substrate linking visual and motor centers in salamanders. This retino‐tecto‐reticulo‐spinal circuitry is well positioned to control orienting behaviors. Our study bridges the gap between the behavioral studies and the neural mechanisms involved in the transformation of visual input into motor output in salamanders.

Improving the spatial resolution of artificial vision using midget retinal ganglion cell populations modelled at the human fovea

OBJECTIVE

Retinal prostheses seek to create artificial vision by stimulating surviving retinal neurons of patients with profound vision impairment. Notwithstanding tremendous research efforts, the performance of all implants tested to date has remained rudimentary, incapable of overcoming the threshold for legal blindness. To maximize the perceptual efficacy of retinal prostheses, a device must be capable of controlling retinal neurons with greater spatiotemporal precision. Most studies of retinal stimulation were derived from either non-primate species or the peripheral primate retina. We investigated if artificial stimulation could leverage the high spatial resolution afforded by the neural substrates at the primate fovea and surrounding regions to achieve improved percept qualities.

APPROACH

We began by developing a new computational model capable of generating anatomically accurate retinal ganglion cell (RGC) populations within the human central retina. Next, multiple RGC populations across the central retina were stimulated in-silico to compare clinical and recently proposed neurostimulation configurations based on their ability to improve perceptual efficacy and reduce activation thresholds.

MAIN RESULTS

Our model uniquely upholds eccentricity-dependent characteristics such as RGC density and dendritic field diameter, whilst incorporating anatomically accurate features such as axon projection and three-dimensional RGC layering, features often forgone in favor of reduced computational complexity. Following epiretinal stimulation, the RGCs in our model produced response patterns in shapes akin to the complex percepts reported in clinical trials. Our results also demonstrated that even within the neuron-dense central retina, epiretinal stimulation using a multi-return hexapolar electrode arrangement could reliably achieve spatially focused RGC activation and could achieve single-cell excitation in 74% of all tested locations.

SIGNIFICANCE

This study establishes an anatomically accurate three-dimensional model of the human central retina and demonstrates the potential for an epiretinal hexapolar configuration to achieve consistent, spatially confined retinal responses, even within the neuron-dense foveal region. Our results promote the prospect and optimization of higher spatial resolution in future epiretinal implants.

BioAdhere: tailor-made bioadhesives for epiretinal visual prostheses

Introduction: Visual prostheses, i.e. epiretinal stimulating arrays, are a promising therapy in treating retinal dystrophies and degenerations. In the wake of a new generation of devices, an innovative method for epiretinal fixation of stimulator arrays is required. We present the development of tailor-made bioadhesive peptides (peptesives) for fixating epiretinal stimulating arrays omitting the use of traumatic retinal tacks. Materials and methods: Binding motifs on the stimulating array (poly[chloro-p-xylylene] (Parylene C)) and in the extracellular matrix of the retinal surface (collagens I and IV, laminin, fibronectin) were identified. The anchor peptides cecropin A (CecA), KH1, KH2 (author's initials) and osteopontin (OPN) were genetically fused to reporter proteins to assess their binding behavior to coated microtiter plates via fluorescence-based assays. Domain Z (DZ) of staphylococcal protein A was used as a separator to generate a bioadhesive peptide. Following ISO 10993 "biological evaluation of medical materials", direct and non-direct cytotoxicity testing (L-929 and R28 retinal progenitor cells) was performed. Lastly, the fixating capabilities of the peptesives were tested in proof-of-principle experiments. Results: The generation of the bioadhesive peptide required evaluation of the N- and C-anchoring of investigated APs. The YmPh-CecA construct showed the highest activity on Parylene C in comparison with the wildtype phytase without the anchor peptide. eGFP-OPN was binding to all four investigated ECM proteins (collagen I, laminin > collagen IV, fibronectin). The strongest binding to collagen I was observed for eGFP-KH1, while the strongest binding to fibronectin was observed for eGFP-KH2. The selectivity of binding was checked by incubating eGFP-CecA and eGFP-OPN on ECM proteins and on Parylene C, respectively. Direct and non-direct cytotoxicity testing of the peptide cecropin-A-DZ-OPN using L-929 and R28 cells showed good biocompatibility properties. Proof-of-concept experiments in post-mortem rabbit eyes suggested an increased adhesion of CecA-DZ-OPN-coated stimulating arrays. Conclusion: This is the first study to prove the applicability and biocompatibility of peptesives for the fixation of macroscopic objects.

Simulating the perceptual effects of electrode-retina distance in prosthetic vision

OBJECTIVE

Retinal prostheses aim to restore some vision in retinitis pigmentosa and age-related macular degeneration blind patients. Many spatial and temporal aspects have been found to affect prosthetic vision. Our objective is to study the impact of the space-variant distance between the stimulating electrodes and the surface of the retina on prosthetic vision and how to mitigate this impact.

APPROACH

A prosthetic vision simulation was built to demonstrate the perceptual effects of the electrode-retina distance (ERD) with different random spatial variations, such as size, brightness, shape, dropout, and spatial shifts. Three approaches for reducing the ERD effects are demonstrated: electrode grouping (quads), ERD-based input-image enhancement, and object scanning with and without phosphene persistence. A quantitative assessment for the first two approaches was done based on experiments with 20 subjects and three vision-based computational image similarity metrics.

MAIN RESULTS

The effects of various ERDs on phosphenes' size, brightness, and shape were simulated. Quads, chosen according to the ERDs, effectively elicit phosphenes without exceeding the safe charge density limit, whereas single electrodes with large ERD cannot do so. Input-image enhancement reduced the ERD effects effectively. These two approaches significantly improved ERD-affected prosthetic vision according to the experiment and image similarity metrics. A further reduction of the ERD effects was achieved by scanning an object while moving the head.

SIGNIFICANCE

ERD has multiple effects on perception with retinal prostheses. One of them is vision loss caused by the incapability of electrodes with large ERD to evoke phosphenes. The three approaches presented in this study can be used separately or together to mitigate the impact of ERD. A consideration of our approaches in reducing the perceptual effects of the ERD may help improve the perception with current prosthetic technology and influence the design of future prostheses.

Hexagonal Stimulation Digital Controller Design and Verification for Wireless Subretinal Implant Device

Significant progress has been made in the field of micro/nano-retinal implant technologies. However, the high pixel range, power leakage, reliability, and lifespan of retinal implants are still questionable. Active implantable devices are safe, cost-effective, and reliable. Although a device that can meet basic safety requirements set by the Food and Drug Administration and the European Union is reliable for long-term use and provides control on current and voltage parameters, it will be expensive and cannot be commercially successful. This study proposes an economical, fully controllable, and configurable wireless communication system based on field-programmable gated arrays (FPGAs) that were designed with the ability to cope with the issues that arise in retinal implantation. This system incorporates hexagonal biphasic stimulation pulses generated by a digital controller that can be fully controlled using an external transmitter. The integration of two separate domain analog systems and a digital controller based on FPGAs is proposed in this study. The system was also implemented on a microchip and verified using in vitro results.

Selective Activation of Retinal Ganglion Cell Subtypes Through Targeted Electrical Stimulation Parameters

To restore vision to the low vision, epiretinal implants have been developed to electrically stimulate the healthy retinal ganglion cells (RGCs) in the degenerate retina. Given the diversity of retinal ganglion cells as well as the difference in their visual function, selective activation of RGCs subtypes can significantly improve the quality of the restored vision. Our recent results demonstrated that with the proper modulation of the current amplitude, small D1-bistratified cells with the contribution to blue/yellow color opponent pathway can be selectively activated at high frequency (200 Hz). The computational results correlated with the clinical findings revealing the blue sensation of 5/7 subjects with epiretinal implants at high frequency. Here we further explored the impacts of alterations in pulse duration and interphase gap on the response of RGCs at high frequency. We used the developed RGCs, A2-monostratified and D1-bistratified, and examined their response to a range of pulse durations (0.1−1.2 ms) and interphase gaps (0−1 ms). We found that the use of short pulse durations with no interphase gap at high frequency increases the differential response of RGCs, offering better opportunities for selective activation of D1 cells. The presence of the interphase gap has shown to reduce the overall differential response of RGCs. We also explored how the low density of calcium channels enhances the responsiveness of RGCs at high frequency.

Real-Time Optimization of Retinal Ganglion Cell Spatial Activity in Response to Epiretinal Stimulation

Retinal prostheses aim to improve visual perception in patients blinded by photoreceptor degeneration. However, shape and letter perception with these devices is currently limited due to low spatial resolution. Previous research has shown the retinal ganglion cell (RGC) spatial activity and phosphene shapes can vary due to the complexity of retina structure and electrode-retina interactions. Visual percepts elicited by single electrodes differ in size and shapes for different electrodes within the same subject, resulting in interference between phosphenes and an unclear image. Prior work has shown that better patient outcomes correlate with spatially separate phosphenes. In this study we use calcium imaging, in vitro retina, neural networks (NN), and an optimization algorithm to demonstrate a method to iteratively search for optimal stimulation parameters that create focal RGC activation. Our findings indicate that we can converge to stimulation parameters that result in focal RGC activation by sampling less than 1/3 of the parameter space. A similar process implemented clinically can reduce time required for optimizing implant operation and enable personalized fitting of retinal prostheses.

Mechanisms underlying activation of retinal bipolar cells through targeted electrical stimulation: a computational study

Objective. Retinal implants have been developed to electrically stimulate healthy retinal neurons in the progressively degenerated retina. Several stimulation approaches have been proposed to improve the visual percept induced in patients with retinal prostheses. We introduce a computational model capable of simulating the effects of electrical stimulation on retinal neurons. Leveraging this computational platform, we delve into the underlying mechanisms influencing the sensitivity of retinal neurons' response to various stimulus waveforms.Approach. We implemented a model of spiking bipolar cells (BCs) in the magnocellular pathway of the primate retina, diffuse BC subtypes (DB4), and utilized our multiscale admittance method (AM)-NEURON computational platform to characterize the response of BCs to epiretinal electrical stimulation with monophasic, symmetric, and asymmetric biphasic pulses.Main results. Our investigations yielded four notable results: (a) the latency of BCs increases as stimulation pulse duration lengthens; conversely, this latency decreases as the current amplitude increases. (b) Stimulation with a long anodic-first symmetric biphasic pulse (duration > 8 ms) results in a significant decrease in spiking threshold compared to stimulation with similar cathodic-first pulses (from 98.2 to 57.5µA). (c) The hyperpolarization-activated cyclic nucleotide-gated channel was a prominent contributor to the reduced threshold of BCs in response to long anodic-first stimulus pulses. (d) Finally, extending the study to asymmetric waveforms, our results predict a lower BCs threshold using asymmetric long anodic-first pulses compared to that of asymmetric short cathodic-first stimulation.Significance. This study predicts the effects of several stimulation parameters on spiking BCs response to electrical stimulation. Of importance, our findings shed light on mechanisms underlying the experimental observations from the literature, thus highlighting the capability of the methodology to predict and guide the development of electrical stimulation protocols to generate a desired biological response, thereby constituting an ideal testbed for the development of electroceutical devices.

Morphological Factors that Underlie Neural Sensitivity to Stimulation in the Retina

Retinal prostheses are a promising therapeutic intervention for patients afflicted by outer retinal degenerative diseases like retinitis pigmentosa and age-related macular degeneration. While significant advances in the development of retinal implants have been made, the quality of vision elicited by these devices remains largely sub-optimal. The variability in the responses produced by retinal devices is most likely due to the differences between the natural cell type-specific signaling that occur in the healthy retina vs. the non-specific activation of multiple cell types arising from artificial stimulation. In order to replicate these natural signaling patterns, stimulation strategies must be capable of preferentially activating specific RGC types. To design more selective stimulation strategies, a better understanding of the morphological factors that underlie the sensitivity to prosthetic stimulation must be developed. This review will focus on the role that different anatomical components play in driving the direct activation of RGCs by extracellular stimulation. Briefly, it will (1) characterize the variability in morphological properties of α-RGCs, (2) detail the influence of morphology on the direct activation of RGCs by electric stimulation, and (3) describe some of the potential biophysical mechanisms that could explain differences in activation thresholds and electrically evoked responses between RGC types.

Effects of Depth-Based Object Isolation in Simulated Retinal Prosthetic Vision

Visual retinal prostheses aim to restore vision for blind individuals who suffer from outer retinal degenerative diseases, such as retinitis pigmentosa and age-related macular degeneration. Perception through retinal prostheses is very limited, but it can be improved by applying object isolation. We used an object isolation algorithm based on integral imaging to isolate objects of interest according to their depth from the camera and applied image processing manipulation to the isolated-object images. Subsequently, we applied a spatial prosthetic vision simulation that converted the isolated-object images to phosphene images. We compared the phosphene images for two types of input images, the original image (before applying object isolation), and the isolated-object image to illustrate the effects of object isolation on simulated prosthetic vision without and with multiple spatial variations of phosphenes, such as size and shape variations, spatial shifts, and dropout rate. The results show an improvement in the perceived shape, contrast, and dynamic range (number of gray levels) of objects in the phosphene image.

Retinal prosthetic vision simulation: temporal aspects

Objective. The perception of individuals fitted with retinal prostheses is not fully understood, although several retinal implants have been tested and commercialized. Realistic simulations of perception with retinal implants would be useful for future development and evaluation of such systems.Approach.We implemented a retinal prosthetic vision simulation, including temporal features, which have not been previously simulated. In particular, the simulation included temporal aspects such as persistence and perceptual fading of phosphenes and the electrode activation rate.Main results.The simulated phosphene persistence showed an effective reduction in flickering at low electrode activation rates. Although persistence has a positive effect on static scenes, it smears dynamic scenes. Perceptual fading following continuous stimulation affects prosthetic vision of both static and dynamic scenes by making them disappear completely or partially. However, we showed that perceptual fading of a static stimulus might be countered by head-scanning motions, which together with the persistence revealed the contours of the faded object. We also showed that changing the image polarity may improve simulated prosthetic vision in the presence of persistence and perceptual fading.Significance.Temporal aspects have important roles in prosthetic vision, as illustrated by the simulations. Considering these aspects may improve the future design, the training with, and evaluation of retinal prostheses.

Ambient Light Rejection Integrated Circuit for Autonomous Adaptation on a Sub-Retinal Prosthetic System

This paper introduces an ambient light rejection (ALR) circuit for the autonomous adaptation of a subretinal implant system. The sub-retinal implants, located beneath a bipolar cell layer, are known to have a significant advantage in spatial resolution by integrating more than a thousand pixels, compared to epi-retinal implants. However, challenges remain regarding current dispersion in high-density retinal implants, and ambient light induces pixel saturation. Thus, the technical issues of ambient light associated with a conventional image processing technique, which lead to high power consumption and area occupation, are still unresolved. Thus, it is necessary to develop a novel image-processing unit to handle ambient light, considering constraints related to power and area. In this paper, we present an ALR circuit as an image-processing unit for sub-retinal implants. We first introduced an ALR algorithm to reduce the ambient light in conventional retinal implants; next, we implemented the ALR algorithm as an application-specific integrated chip (ASIC). The ALR circuit was fabricated using a standard 0.35-μm CMOS process along with an image-sensor-based stimulator, a sensor pixel, and digital blocks. As experimental results, the ALR circuit occupies an area of 190 µm², consumes a power of 3.2 mW and shows a maximum response time of 1.6 s at a light intensity of 20,000 lux. The proposed ALR circuit also has a pixel loss rate of 0.3%. The experimental results show that the ALR circuit leads to a sensor pixel (SP) being autonomously adjusted, depending on the light intensity.

Microstimulation-evoked neural responses in visual cortex are depth dependent

BACKGROUND

Cortical visual prostheses often use penetrating electrode arrays to deliver microstimulation to the visual cortex. To optimize electrode placement within the cortex, the neural responses to microstimulation at different cortical depths must first be understood.

OBJECTIVE

We investigated how the neural responses evoked by microstimulation in cortex varied with cortical depth, of both stimulation and response.

METHODS

A 32-channel single shank electrode array was inserted into the primary visual cortex of anaesthetized rats, such that it spanned all cortical layers. Microstimulation with currents up to 14 μA (single biphasic pulse, 200 μs per phase) was applied at depths spanning 1600 μm, while simultaneously recording neural activity on all channels within a response window 2.25-11 ms.

RESULTS

Stimulation elicited elevated neuronal firing rates at all depths of cortex. Compared to deep sites, superficial stimulation sites responded with higher firing rates at a given current and had lower thresholds. The laminar spread of evoked activity across cortical depth depended on stimulation depth, in line with anatomical models.

CONCLUSION

Stimulation in the superficial layers of visual cortex evokes local neural activity with the lowest thresholds, and stimulation in the deep layers evoked the most activity across the cortical column. In conjunction with perceptual reports, these data suggest that the optimal electrode placement for cortical microstimulation prostheses has electrodes positioned in layers 2/3, and at the top of layer 5.

The impact of synchronous versus asynchronous electrical stimulation in artificial vision

Visual prosthesis devices designed to restore sight to the blind have been under development in the laboratory for several decades. Clinical translation continues to be challenging, due in part to gaps in our understanding of critical parameters such as how phosphenes, the electrically-generated pixels of artificial vision, can be combined to form images. In this review we explore the effects that synchronous and asynchronous electrical stimulation across multiple electrodes have in evoking phosphenes. Understanding how electrical patterns influence phosphene generation to control object binding and perception of visual form is fundamental to creation of a clinically successful prosthesis.

Color and cellular selectivity of retinal ganglion cell subtypes through frequency modulation of electrical stimulation

Epiretinal prostheses aim at electrically stimulating the inner most surviving retinal cells-retinal ganglion cells (RGCs)-to restore partial sight to the blind. Recent tests in patients with epiretinal implants have revealed that electrical stimulation of the retina results in the percept of color of the elicited phosphenes, which depends on the frequency of stimulation. This paper presents computational results that are predictive of this finding and further support our understanding of the mechanisms of color encoding in electrical stimulation of retina, which could prove pivotal for the design of advanced retinal prosthetics that elicit both percept and color. This provides, for the first time, a directly applicable "amplitude-frequency" stimulation strategy to "encode color" in future retinal prosthetics through a predictive computational tool to selectively target small bistratified cells, which have been shown to contribute to "blue-yellow" color opponency in the retinal circuitry. The presented results are validated with experimental data reported in the literature and correlated with findings in blind patients with a retinal prosthetic implant collected by our group.

Restriction of eye motility in patients with RETINA IMPLANT Alpha AMS

PURPOSE

To evaluate the motility of the eye in patients with the RETINA IMPLANT Alpha AMS.

METHODS

Eye motility was determined in eight gaze directions in ten blind retinitis pigmentosa patients, who had received the RETINA IMPLANT Alpha AMS, before implantation of the subretinal implant and at six time-points up to one year after.

RESULTS

The analysis of eye motility showed a restriction in the upgaze and gaze to the temporal side directly after surgery in eight of the nine patients included. The degree of motility restriction decreased continuously with recovery during the observation time. One year after surgery, eye motility was still restricted in the majority of patients, especially in the upgaze to the temporal side at 20° (five of seven patients).

CONCLUSION

Retinal implants with intraorbital parts (e.g. connecting cables) caused restriction in the temporal and superior viewing directions in the majority of patients. Although this restriction might be cosmetically visible, this limitation in eye motility has no effects on the monocular vision and the implant's efficacy for daily use.

[Vision restoration: science fiction or reality?]

Visual prostheses aim at restoring useful vision to patients who have become blind. This useful vision should enable them to regain autonomy in society for navigation, face recognition or reading. Two retinal prostheses have already obtained market authorization for patients affected by retinal dystrophies while a new device is in clinical trials for patients affected by age-related macular degeneration. Various prostheses, in particular cortical prostheses, are currently in clinical trials for optic neuropathies (glaucoma). Optogenetic therapy, an alternative strategy, has now reached the stage of clinical trials at the retinal level while moving forward at the cortical level. Other innovating strategies have obtained proofs of concepts in rodents but require a further validation in large animals prior to their evaluation on patients. Restoring vision should therefore become a reality for many patients even if this vision will not be as extensive and perfect as natural vision.

Blockade of Retinal Oscillations by Benzodiazepines Improves Efficiency of Electrical Stimulation in the Mouse Model of RP, rd10

Purpose

In RP, photoreceptors degenerate. Retinal prostheses are considered a suitable strategy to restore vision. In animal models of RP, a pathologic rhythmic activity seems to compromise the efficiency of retinal ganglion cell stimulation by an electrical prosthesis. We, therefore, strove to eliminate this pathologic activity.

Methods

Electrophysiologic recordings of local field potentials and spike activity of retinal ganglion cells were obtained in vitro from retinae of wild-type and rd10 mice using multielectrode arrays. Retinae were stimulated electrically.

Results

The efficiency of electrical stimulation was lower in rd10 retina than in wild-type retina and this was highly correlated with the presence of oscillations in retinal activity. Glycine and GABA, as well as the benzodiazepines diazepam, lorazepam, and flunitrazepam, abolished retinal oscillations and, most important, increased the efficiency of electrical stimulation to values similar to those in wild-type retina.

Conclusions

Treatment of patients with these benzodiazepines may offer a way to improve the performance of retinal implants in cases with poor implant proficiency. This study may open the way to a therapy that supports electrical stimulation by prostheses with pharmacologic treatment.

Targeted Stimulation of Retinal Ganglion Cells in Epiretinal Prostheses: A Multiscale Computational Study

Retinal prostheses aim at restoring partial sight to patients that are blind due to retinal degenerative diseases by electrically stimulating the surviving healthy retinal neurons. Ideally, the electrical stimulation of the retina is intended to induce localized, focused, percepts only; however, some epiretinal implant subjects have reported seeing elongated phosphenes in a single electrode stimulation due to the axonal activation of retinal ganglion cells (RGCs). This issue can be addressed by properly devising stimulation waveforms so that the possibility of inducing axonal activation of RGCs is minimized. While strategies to devise electrical stimulation waveforms to achieve a focal RGCs response have been reported in literature, the underlying mechanisms are not well understood. This article intends to address this gap; we developed morphologically and biophysically realistic computational models of two classified RGCs: D1-bistratified and A2-monostratified. Computational results suggest that the sodium channel band (SOCB) is less sensitive to modulations in stimulation parameters than the distal axon (DA), and DA stimulus threshold is less sensitive to physiological differences among RGCs. Therefore, over a range of RGCs distal axon diameters, short-pulse symmetric biphasic waveforms can enhance the stimulation threshold difference between the SOCB and the DA. Appropriately designed waveforms can avoid axonal activation of RGCs, implying a consequential reduction of undesired strikes in the visual field.

Creation of virtual channels in the retina using synchronous and asynchronous stimulation - a modelling study

OBJECTIVE

Implantable retinal prostheses aim to provide artificial vision to those suffering from retinal degenerative diseases by electrically stimulating the remaining retinal neurons using a multi-electrode array. The spatial resolution of these devices can be improved by creation of so-called virtual channels (VCs) that are commonly achieved through synchronized stimulation of multiple electrodes. It is largely unclear though if VCs can be created using asynchronous stimulation, which was the primary aim of this study.

APPROACH

A computational model of multi-layered retina and epi-retinal dual-electrode stimulation was developed to simulate the neural activity of populations of retinal ganglion cells (RGCs) using the VC strategy under both synchronous and asynchronous stimulation conditions.

MAIN RESULTS

Our simulation suggests that VCs can be created using asynchronous stimulation. VC performance under both synchronous and asynchronous stimulation conditions can be improved by optimizing stimulation parameters such as current intensity, current ratio (α) between two electrodes, electrode spacing and the stimulation waveform. In particular, two VC performance measures; (1) linear displacement of the centroid of RGC activation, and (2) the RGC activation size consistency as a function of different current ratios α, have comparable performance under asynchronous and synchronous stimulation with appropriately selected stimulation parameters.

SIGNIFICANCE

Our findings support the possibility of creating VCs in the retina under both synchronous and asynchronous stimulation conditions. The results provide theoretical evidence for future retinal prosthesis designs with higher spatial resolution and power efficiency whilst reducing the number of current sources required to achieve these outcomes.

The effect of waveform asymmetry on perception with epiretinal prostheses

Objective

Retinal prosthetic implants have helped improve vision in patients blinded by photoreceptor degeneration. Retinal implant users report improvements in light perception and performing visual tasks, but their ability to perceive shapes and letters is limited due to the low precision of retinal activation, which is exacerbated by axonal stimulation and high perceptual thresholds. A previous in vitro study in our lab used calcium imaging to measure the spatial activity of mouse retinal ganglion cells (RGCs) in response to electrical stimulation. Based on this study, symmetric anodic-first (SA) stimulation effectively avoided axonal activation and asymmetric anodic-first stimulation (AA) with duration ratios (ratio of the anodic to cathodic phase) greater than 10 reduced RGC activation thresholds significantly. Applying these novel stimulation strategies in clinic may increase perception precision and improve the overall patient outcomes.

Approach

We combined human subject testing and computational modeling to further examine the effect of SA and AA stimuli on perception shapes and thresholds for epiretinal stimulation of RGCs.

Main results

Threshold measurement in three Argus II participants indicated that AA stimulation could increase perception probabilities compared to a standard symmetric cathodic-first (SC) pulse, and this effect can be intensified by addition of an interphae gap (IPG). Our in silico RGC model predicts lower thresholds with AA and asymmetric cathodic-first (AC) stimuli compared to a SC pulse. This effect was more pronounced at shorter pulse widths. The most effective pulse for threshold reduction with short pulse durations (≤0.12 ms) was AA stimulation with small duration ratios (≤5) and long IPGs (≥2 ms). For the 0.5 ms pulse duration, SC stimulation with IPGs longer than 0.5 ms, or asymmetric stimuli with large duration ratios (≥20) were most effective in threshold reduction. Phosphene shape analysis did not reveal a significant change in percept elongation with SA stimulation. However, there was a significant increase in percept size (P < 0.01) with AA stimulation compared to the standard pulse in one participant.

Significane

Including asymmetric waveform capability will provide more flexible options for optimization and personalized fitting of retinal implants.

Retinal Prosthetic Approaches to Enhance Visual Perception for Blind Patients

Retinal prostheses are implantable devices that aim to restore the vision of blind patients suffering from retinal degeneration, mainly by artificially stimulating the remaining retinal neurons. Some retinal prostheses have successfully reached the stage of clinical trials; however, these devices can only restore vision partially and remain insufficient to enable patients to conduct everyday life independently. The visual acuity of the artificial vision is limited by various factors from both engineering and physiological perspectives. To overcome those issues and further enhance the visual resolution of retinal prostheses, a variety of retinal prosthetic approaches have been proposed, based on optimization of the geometries of electrode arrays and stimulation pulse parameters. Other retinal stimulation modalities such as optics, ultrasound, and magnetics have also been utilized to address the limitations in conventional electrical stimulation. Although none of these approaches have been clinically proven to fully restore the function of a degenerated retina, the extensive efforts made in this field have demonstrated a series of encouraging findings for the next generation of retinal prostheses, and these could potentially enhance the visual acuity of retinal prostheses. In this article, a comprehensive and up-to-date overview of retinal prosthetic strategies is provided, with a specific focus on a quantitative assessment of visual acuity results from various retinal stimulation technologies. The aim is to highlight future directions toward high-resolution retinal prostheses.

Soft and Ion-Conducting Materials in Bioelectronics: From Conducting Polymers to Hydrogels

Bioelectronics devices that directly interface with cells and tissue have applications in neural and cardiac stimulation and recording, electroceuticals, and brain machine interfaces for prostheses. The interface between bioelectronic devices and biological tissue is inherently challenging due to the mismatch in both mechanical properties (hard vs soft) and charge carriers (electrons vs ions). In addition to conventional metals and silicon, new materials have bridged this interface, including conducting polymers, carbon-based nanomaterials, as well as ion-conducting polymers and hydrogels. This review provides an update on advances in soft bioelectronic materials for current and future therapeutic applications. Specifically, this review focuses on soft materials that can conduct both electrons and ions, and also deliver drugs and small molecules. The future opportunities and emerging challenges in the field are also highlighted.

Stimulation strategies for selective activation of retinal ganglion cell soma and threshold reduction

OBJECTIVE

Retinal prosthetic implants restore partial vision to patients blinded due to outer retinal degeneration, using a camera-guided multielectrode array (MEA) that electrically stimulates surviving retinal neurons. Commercial epi-retinal prostheses use millisecond-scale charge-balanced, symmetric, cathodic-first biphasic pulses to depolarize retinal ganglion cells (RGCs) and bipolar cells (BCs), frequently creating oblong perceptions of light related to axonal activation of RGCs. Stimulation strategies that avoid axonal stimulation and decrease the threshold of targeted neurons may significantly improve prosthetic vision in terms of spatial resolution and power efficiency.

APPROACH

We developed a virus-transduced genetically encoded calcium indicator (GECI) GCaMP6f and microscopy platform for calcium imaging to record the neural activity from RGCs at single-cell resolution in wholemount retinas. Multiple stimulation paradigms were applied through a microelectrode array (MEA) with transparent indium tin oxide electrodes. The evoked neuronal activities were converted to corresponding 2D calcium imaging transient pattern and spatial threshold map to identify the ideal focal response which corresponds to optimal percept in patient.

MAIN RESULTS

The proposed optical system with GCaMP6f is capable of recording from population of mouse RGCs in real time during electrical stimulation with precise location information relative to the stimulation sites. Optimal duration and phase order of pulse were identified to avoid axonal stimulation and selectively activate targeted RGC somas, without requiring a significant increase in stimulation charge. Additionally, we show that reduced stimulus threshold can be achieved with the special design of asymmetric anodic-first pulse.

SIGNIFICANCE

Our findings support the possibility of manipulating the responses of RGCs through varying the stimulation waveform. Focal response can be achieved with relative short duration (⩽120 μs) pulses, and can be improved by reversing the standard phase order. The RGCs threshold can be significantly reduced by 33.3%-50% in terms of charge through applying hyperpolarizing pre-pulses with a 20:1 ratio (pre-pulse:stimulus pulse). The results support the future retinal prosthesis design that potentially forms more ideal shape perception with higher spatial resolution and power efficiency.

Fully Integrated Light-Sensing Stimulator Design for Subretinal Implants

This paper presents a fully integrated photodiode-based low-power and low-mismatch stimulator for a subretinal prosthesis. It is known that a subretinal prosthesis achieves 1600-pixel stimulators on a limited single-chip area that is implanted beneath the bipolar cell layer. However, the high-density pixels cause high power dissipation during stimulation and high fabrication costs because of special process technologies such as the complementary metal-oxide semiconductor CMOS image sensor process. In addition, the many residual charges arising from the high-density pixel stimulation have deleterious effects, such as tissue damage and electrode corrosion, on the retina tissue. In this work, we adopted a switched-capacitor current mirror technique for the single-pixel stimulator (SPStim) that enables low power consumption and low mismatch in the subretinal device. The customized P+/N-well photodiode used to sense the incident light in the SPStim also reduces the fabrication cost. The 64-pixel stimulators are fabricated in a standard 0.35-μm CMOS process along with a global digital controller, which occupies a chip area of 4.3 × 3.2 mm² and are ex-vivo demonstrated using a dissected pig eyeball. According to measured results, the SPStim accomplishes a maximum biphasic pulse amplitude of 143 μA, which dissipates an average power of 167 μW in a stimulation period of 5 ms, and an average mismatch of 1.12 % between the cathodic and anodic pulses.

Retinotopic Responses in the Visual Cortex Elicited by Epiretinal Electrical Stimulation in Normal and Retinal Degenerate Rats

Purpose

Electronic retinal prostheses restore vision in people with outer retinal degeneration by electrically stimulating the inner retina. We characterized visual cortex electrophysiologic response elicited by electrical stimulation of retina in normally sighted and retinal degenerate rats.

Methods

Nine normally sighted Long Evans and 11 S334ter line 3 retinal degenerate (rd) rats were used to map cortical responses elicited by epiretinal electrical stimulation in four quadrants of the retina. Six normal and six rd rats were used to compare the dendritic spine density of neurons in the visual cortex.

Results

The rd rats required higher stimulus amplitudes to elicit responses in the visual cortex. The cortical electrically evoked responses (EERs) for both healthy and rd rats show a dose-response characteristic with respect to the stimulus amplitude. The EER maps in healthy rats show retinotopic organization. For rd rats, cortical retinotopy is not well preserved. The neurons in the visual cortex of rd rats show a 10% higher dendritic spine density than in the healthy rats.

Conclusions

Cortical activity maps, produced when epiretinal stimulation is applied to quadrants of the retina, exhibit retinotopy in normal but not rd rats. This is likely due to a combination of degeneration of the retina and increased stimulus thresholds in rd, which broadens the activated area of the retina.

Translational Relevance

Loss of retinotopy is evident in rd rats. If a similar loss of retinotopy is present in humans, retinal prostheses design must include flexibility to account for patient specific variability.

Taurine Promotes Retinal Ganglion Cell Survival Through GABAB Receptor Activation

Retinal ganglion cell (RGC) degeneration occurs in numerous retinal diseases, either as a primary process like in glaucoma, or secondary to photoreceptor loss and no efficient compound targeting directly RGC neuroprotection is yet available. We previously described that taurine exerts a direct protective effect on RGCs cultured under serum-deprived conditions. Because taurine was known to have an agonist-like activity for GABA/glycine receptors, we investigated here if the taurine-elicited neuroprotective effect may be mediated through the activation of these receptors using selective antagonist ligands. RGCs were purified, seeded in 96-well plate and maintained in culture during 6 days in vitro. Viable cells were labelled with calcein and densities in full-well area were then automatically counted. Here we show that the protective effect of taurine against RGC loss observed under serum deprivation can be mediated through the GABAB receptor stimulation. Hence, two selective agonists, including baclofen, at this metabotropic GABAB receptor were found to reproduce taurine action by enhancing RGC survival in culture. This study suggests that GABAB receptor stimulation provides direct neuroprotection for RGCs. Accordingly, drugs targeting GABAB receptor may represent a new way for the prevention of RGC degeneration.

Argus II retinal prosthesis system: a review of patient selection criteria, surgical considerations, and post-operative outcomes

Retinitis pigmentosa (RP) is a group of heterogeneous inherited retinal degenerative disorders characterized by progressive rod and cone dysfunction and ensuing photoreceptor loss. Many patients suffer from legal blindness by their 40s or 50s. Artificial vision is considered once patients have lost all vision to the point of bare light perception or no light perception. The Argus II retinal prosthesis system is one such artificial vision device approved for patients with RP. This review focuses on the factors important for patient selection. Careful pre-operative screening, counseling, and management of patient expectations are critical for the successful implantation and visual rehabilitation of patients with the Argus II device.

Differential electrical responses in retinal ganglion cell subtypes: effects of synaptic blockade and stimulating electrode location

OBJECTIVE

Visual prostheses have shown promising results in restoring visual perception to blind patients. The ability to differentially activate retinal ganglion cell (RGC) subtypes could further improve the efficacy of these medical devices.

APPROACH

Using whole-cell patch clamp, we investigated membrane potential differences between ON and OFF RGCs in the mouse retina when their synaptic inputs were blocked by synaptic blockers, and examined the differences in stimulation thresholds under such conditions. By injecting intracellular current, we further confirmed the relationship between RGC stimulation thresholds and resting membrane potentials (RMPs). In addition, we investigated the effects of stimulating electrode location on the differences in stimulation thresholds between ON and OFF RGCs.

MAIN RESULTS

With synaptic blockade, ON RGCs became significantly more hyperpolarized (from  -61.8  ±  1.4 mV to  -70.8  ±  1.6 mV), while OFF RGCs depolarized slightly (from  -60.5  ±  0.7 mV to  -58.6  ±  0.9 mV). RGC stimulation thresholds were negatively correlated with their RMPs (Pearson r value:  -0.5154; p-value: 0.0042). Thus, depriving ON RGCs of synaptic inputs significantly increased their thresholds (from 14.7  ±  1.3 µA to 22.3  ±  2.1 µA) over those of OFF RGCs (from 13.2  ±  0.7 µA to 13.1  ±  1.1 µA). However, with control solution, ON and OFF RGC stimulation thresholds were not significantly different. Finally, placement of the stimulating electrode away from the axon enhanced differences in stimulation thresholds between ON and OFF RGCs, facilitating preferential activation of OFF RGCs.

SIGNIFICANCE

Since ON and OFF RGCs have antagonistic responses to natural light, achieving differential RGC activation could convey more natural visual information, leading to better visual prosthesis outcomes.

Implantation of Stimulation Electrodes in the Subretinal Space to Demonstrate Cortical Responses in Yucatan Minipig in the Course of Visual Prosthesis Development

Purpose

During the course of the development of visual prostheses, subretinal stimulation films were implanted in micropigs in order to prove the feasibility of subretinal electrical stimulation with subsequent cortical response. One aim was to demonstrate that epidural recording of visual evoked potentials is possible in the micropig.

Methods

Film-bound stimulation electrode arrays were placed in the subretinal space of micropigs. This enabled the retina to be stimulated subretinally. Since conventional visual evoked potential (VEP) measuring is virtually impossible in the pig from the neurosurgical point of view, epidural recording electrode arrays were positioned over the visual cortex as permanent electrodes.

Results

The feasibility of temporary implantation of film-bound stimulation electrode arrays was successfully demonstrated in the micropig model. On stimulation with monopolar voltage pulses (1000 to 3000 mV), reproducible epidural VEP measurements (5 to 10 μV) were detected.

Conclusions

The feasibility of subretinal stimulation of the retina was demonstrated in a retinal model that is similar to the human retina. This animal model therefore offers a suitable means of studying the tolerability of stimulation situations in the course of visual prosthesis development.

Correlations between specific patterns of spontaneous activity and stimulation efficiency in degenerated retina

Retinal prostheses that are currently used to restore vision in patients suffering from retinal degeneration are not adjusted to the changes occurring during the remodeling process of the retina. Recent studies revealed abnormal rhythmic activity in the retina of genetic mouse models of retinitis pigmentosa. Here we describe this abnormal activity also in a pharmacologically-induced (MNU) mouse model of retinal degeneration. To investigate how this abnormal activity affects the excitability of retinal ganglion cells, we recorded the electrical activity from whole mounted retinas of rd10 mice and MNU-treated mice using a microelectrode array system and applied biphasic current pulses of different amplitude and duration to stimulate ganglion cells electrically. We show that the electrical stimulation efficiency is strongly reduced in degenerated retinas, in particular when abnormal activity such as oscillations and rhythmic firing of bursts of action potentials can be observed. Using a prestimulus pulse sequence, we could abolish rhythmic retinal activity. Under these conditions, the stimulation efficiency was enhanced in a few cases but not in the majority of tested cells. Nevertheless, this approach supports the idea that modified stimulation protocols could help to improve the efficiency of retinal prostheses in the future.

Optic Nerve Stimulation System with Adaptive Wireless Powering and Data Telemetry

To treat retinal degenerative diseases, a transcorneal electrical stimulation-based system is proposed, which consists of an eye implant and an external component. The eye implant is wirelessly powered and controlled by the external component to generate the required bi-polar current pattern for transcorneal stimulation with an amplitude range of 5 μA to 320 μA, a frequency range of 10 Hz to 160 Hz and a duty ratio range of 2.5% to 20%. Power delivery control includes power boosting in preparation for stimulation, and normal power regulation that adapts to both coupling and load variations. Only one pair of coils is used for both the power link and the bi-directional data link. Except for the secondary coil, the eye implant is fully integrated on chip and is fabricated using UMC (United Microelectronics Corporation, Hsinchu, Taiwan) 0.13 μm complementary metal-oxide-semiconductor (CMOS) process with a size of 1.5 mm × 1.5 mm. The secondary coil is fabricated on a printed circuit board (PCB) with a diameter of only 4.4 mm. After coating with biocompatible silicone, the whole implant has dimensions of 6 mm in diameter with a thickness of less than 1 mm. The whole device can be put onto the sclera and beneath the eye's conjunctiva. System functionality and electrical performance are demonstrated with measurement results.

Visual Prosthesis: Interfacing Stimulating Electrodes with Retinal Neurons to Restore Vision

The bypassing of degenerated photoreceptors using retinal neurostimulators is helping the blind to recover functional vision. Researchers are investigating new ways to improve visual percepts elicited by these means as the vision produced by these early devices remain rudimentary. However, several factors are hampering the progression of bionic technologies: the charge injection limits of metallic electrodes, the mechanical mismatch between excitable tissue and the stimulating elements, neural and electric crosstalk, the physical size of the implanted devices, and the inability to selectively activate different types of retinal neurons. Electrochemical and mechanical limitations are being addressed by the application of electromaterials such as conducting polymers, carbon nanotubes and nanocrystalline diamonds, among other biomaterials, to electrical neuromodulation. In addition, the use of synthetic hydrogels and cell-laden biomaterials is promising better interfaces, as it opens a door to establishing synaptic connections between the electrode material and the excitable cells. Finally, new electrostimulation approaches relying on the use of high-frequency stimulation and field overlapping techniques are being developed to better replicate the neural code of the retina. All these elements combined will bring bionic vision beyond its present state and into the realm of a viable, mainstream therapy for vision loss.

Molecular genetics and emerging therapies for retinitis pigmentosa: Basic research and clinical perspectives

Retinitis Pigmentosa (RP) is a hereditary retinopathy that affects about 2.5 million people worldwide. It is characterized with progressive loss of rods and cones and causes severe visual dysfunction and eventual blindness in bilateral eyes. In addition to more than 3000 genetic mutations from about 70 genes, a wide genetic overlap with other types of retinal dystrophies has been reported with RP. This diversity of genetic pathophysiology makes treatment extremely challenging. Although therapeutic attempts have been made using various pharmacologic agents (neurotrophic factors, antioxidants, and anti-apoptotic agents), most are not targeted to the fundamental cause of RP, and their clinical efficacy has not been clearly proven. Current therapies for RP in ongoing or completed clinical trials include gene therapy, cell therapy, and retinal prostheses. Gene therapy, a strategy to correct the genetic defects using viral or non-viral vectors, has the potential to achieve definitive treatment by replacing or silencing a causative gene. Among many clinical trials of gene therapy for hereditary retinal diseases, a phase 3 clinical trial of voretigene neparvovec (AAV2-hRPE65v2, Luxturna) recently showed significant efficacy for RPE65-mediated inherited retinal dystrophy including Leber congenital amaurosis and RP. It is about to be approved as the first ocular gene therapy biologic product. Despite current limitations such as limited target genes and indicated patients, modest efficacy, and the invasive administration method, development in gene editing technology and novel gene delivery carriers make gene therapy a promising therapeutic modality for RP and other hereditary retinal dystrophies in the future.

Retinal electrostimulation in rats: Activation thresholds from superior colliculus and visual cortex recordings

Retinal neuromodulation is an emerging therapeutic approach to restore functional vision to those suffering retinal photoreceptor degeneration. The retina encodes visual information and transmits it to the brain. Replicating this retinal code through electrical stimulation is essential to improving the performance of visual prostheses. In doing so, the first step relies on precise neural recordings from visual centers that allow studying the response of these neurons to electrical stimulation of the retina. This paper demonstrates the feasibility of a rat model to conduct highly reliable electrophysiological studies in the field of retinal neuromodulation. A disc electrode, implanted in the retrobulbar space was used to stimulate the retina of Long-Evans rats. Buzsaki multi-electro arrays were inserted in the superior colliculus (SC) to record electrical activity propagated from the retinal ganglion cells (RGCs). Activation thresholds calculated from local field potentials (visual cortex) and from neural spikes (SC) were contrasted. Both values were comparable to those in humans and in other animal models, and were slightly higher when estimated from SC recordings. However, differences were not statistically significant.

Electronic retinal implants and artificial vision: journey and present

Retinitis pigmentosa and age-related macular degeneration are two significant causes of severe visual dysfunction. In both, the retinal photoreceptors degenerate, preventing successful conversion of light into electrical energy that is interpreted in the visual cortex as visual function. Artificial vision or visual function began over two centuries ago with the idea of creating artificial light pulses, or phosphenes, through cortical stimulation. The pursuit is now on to improve artificial visual function. Two retinal implants appear the most likely to succeed in the future having undergone multicentre human trials: the Argus II electronic epiretinal device (Second Sight Medical Products, CA, USA) and Alpha-IMS electronic subretinal device (Retina Implant AG, Germany). The trial results to date are encouraging with visual improvement and acceptable safety profiles reported for both devices. At present, the visual function generated by either device does not offer high enough resolution or acuity for a patient to regain a fully functional life. Despite this, both devices not only have the potential, but have actually improved the vision-related quality of life in a significant number of patients implanted. With this in mind, the economic argument is clear. Provided device-life is long enough, its cost should be acceptable for the obtained improvement in the quality of life. The aim of this Review Article is to assist those readers that may be considering offering any of these devices as a treatment for blindness in Retinitis Pigmentosa.

Stimulation Strategies for Selective Activation of Retinal Ganglion Cells

Retinal prosthetic implants have shown potential to restore partial vision to patients blinded by retinitis pigmentosa or dry age-related macular degeneration, via a camera-driven multielectrode array that electrically stimulates surviving retinal neurons. Commercial epi-retinal prostheses mostly use charge-balanced symmetric cathodic-first biphasic pulses to depolarize retinal ganglion cells (RGCs) and bipolar cells (BCs), resulting in the perception of light in blind patients. However, previous clinical study for patients with Argus II epiretinal implants reported most percepts evoked by single electrode stimulation were elongated and aligned with estimated axon path of retinal ganglion cells, suggesting the activation of axon bundles. In this project, using an established genetically encoded calcium indicator (GECI), we performed in vitro calcium imaging for different stimulation paradigms, focusing primarily on short duration pulse that can avoid axonal stimulation and selective activate targeted RGC soma. The findings support the possibility to manipulate the responses of RGCs through varying the stimulation waveform, thus potentially forming more ideal shape perception with higher spatial resolution in future retinal prosthesis design.

Neural Probes with Integrated Temperature Sensors for Monitoring Retina and Brain Implantation and Stimulation

Gold (Au) resistive temperature sensors were integrated on flexible polyimide-based neural probes to monitor temperature changes during neural probe implantation and stimulation. Temperature changes were measured as neural probes were implanted to infer the positions of the neural probes, and as the retina or the deep brain region was stimulated electrically. The temperature sensor consisted of a serpentine Au resistor and surrounded by four Au electrodes with 200 and [Formula: see text] diameter (dia.). The Au temperature sensors had temperature coefficient of 0.32%, and they were biocompatible and small in size. In vivo measurements of temperature changes during implantation and stimulation were carried out in the retina and deep brain region in rats. The desired implantation position was reached when temperature measured by the sensor increased to the calibrated level and became stable. There was no temperature increase when low level stimulation current of 8 and [Formula: see text] each for the two 200- and 400- [Formula: see text]-dia. electrodes, respectively, were applied. When higher level stimulation current of 100 and [Formula: see text] each were applied to the two 200- and 400- [Formula: see text]-dia. electrodes, respectively, maximum temperature increases of 1.2 °C in retina and 1 °C in deep brain region were found.