A model of ganglion axon pathways accounts for percepts elicited by retinal implants.

Degenerative retinal diseases such as retinitis pigmentosa and macular degeneration cause irreversible vision loss in more than 10 million people worldwide. Retinal prostheses, now implanted in over 250 patients worldwide, electrically stimulate surviving cells in order to evoke neuronal responses that are interpreted by the brain as visual percepts ('phosphenes'). However, instead of seeing focal spots of light, current implant users perceive highly distorted phosphenes that vary in shape both across subjects and electrodes. We characterized these distortions by asking users of the Argus retinal prosthesis system (Second Sight Medical Products Inc.) to draw electrically elicited percepts on a touchscreen. Using ophthalmic fundus imaging and computational modeling, we show that elicited percepts can be accurately predicted by the topographic organization of optic nerve fiber bundles in each subject's retina, successfully replicating visual percepts ranging from 'blobs' to oriented 'streaks' and 'wedges' depending on the retinal location of the stimulating electrode. This provides the first evidence that activation of passing axon fibers accounts for the rich repertoire of phosphene shape commonly reported in psychophysical experiments, which can severely distort the quality of the generated visual experience. Overall our findings argue for more detailed modeling of biological detail across neural engineering applications

Subretinal delivery of adeno-associated virus serotype 2 results in minimal immune responses that allow repeat vector administration in immunocompetent mice

Background: Adeno-associated virus serotype 2 (AAV2) vectors show considerable promise for ocular gene transfer. However, one potential barrier to efficacious long-term therapy is the development of immune responses against the vector or transgene product. Methods: We evaluated cellular and humoural responses in mice following both single and repeated subretinal administration of AAV2, and examined their effects on RPE65 and green fluorescent protein transgene expression. Results: Following subretinal administration of vector, splenocytes and T-cells from draining lymph nodes showed minimal activation following stimulation by co-culture with AAV2. Neutralizing antibodies (NAbs) were not detected in the ocular fluids of any mice receiving AAV2 or in the serum of mice receiving a lower dose. NAbs were present in the serum of a proportion of mice receiving a higher dose of the vector. Furthermore, no differences in immunoglobulin titre in serum or ocular fluids against RPE65 protein or AAV2 capsid between treated and control mice were detected. Histological examination showed no evidence of retinal toxicity or leukocyte infiltration compared to uninjected eyes. Repeat administration of low-dose AAV.hRPE65.hRPE65 to both eyes of RPE65-/- mice resulted in transgene expression and functional rescue, but re-administration of high-dose AAV2 resulted in boosted NAb titres and variable transgene expression in the second injected eye. Conclusions: These data, which were obtained in mice, suggest that, following subretinal injection, immune responses to AAV2 are dose-dependent. Low-dose AAV2 is well tolerated in the eye, with minimal immune responses, and transgene expression after repeat administration of vector is achievable. Higher doses lead to the expression of NAbs that reduce the efficacy of repeated vector administration

Non-viral delivery and optimized optogenetic stimulation of retinal ganglion cells led to behavioral restoration of vision

Stimulation of retinal neurons using optogenetics via use of chanelrhodopsin-2 (ChR2) has opened up a new direction for restoration of vision for treatment of retinitis pigmentosa (RP). Here, we report non-viral in-vivo electroporation of degenerated retina of adult RP-mice with ChR2-plasmids and subsequent in-vivo imaging of retina to confirm expression. Further, we demonstrate that in addition to efficient non-viral delivery of ChR2 to a specific retinal layer, threshold level of stimulation light needs to be delivered onto the retina for achieving successful behavioral outcome. Measurement of intensity of light reaching the retina of RP-mouse models along with geometrical optics simulation of light propagation in the eye is reported in order to determine the stimulating source position for optimal light delivery to the retina. The light-guided navigation of mice with ChR2 expressing retinal ganglion cells was found to be significantly improved over a long distance in correlation with stimulation intensity

Topographic mapping of retinal function with the SLO-mfERG under simultaneous control of fixation in Best's disease

Purpose: To introduce the scanning laser ophthalmoscope-evoked mfERG (SLO-mfERG) as a new method to measure focal retinal function. Methods: Sixty-two healthy individuals and 12 patients with Best's disease were examined. mfERGs were recorded using a scanning laser ophthalmoscope as a stimulator and trigger device (He-Neon 632.8 nm) as well as a fundus-monitoring system (infrared 730 nm). Results: Amplitudes in the central concentric area were found to be significantly lower in patients with Best's disease than in healthy controls, while no significant differences were found for the more peripheral areas. Conclusion: SLO-mfERG is a reliable new technique for topographic mapping of retinal function under simultaneous control of fixation

An update on retinal prostheses

Retinal prostheses are designed to restore a basic sense of sight to people with profound vision loss. They require a relatively intact posterior visual pathway (optic nerve, lateral geniculate nucleus and visual cortex). Retinal implants are options for people with severe stages of retinal degenerative disease such as retinitis pigmentosa and age-related macular degeneration. There have now been three regulatory-approved retinal prostheses. Over five hundred patients have been implanted globally over the past 15 years. Devices generally provide an improved ability to localize high-contrast objects, navigate, and perform basic orientation tasks. Adverse events have included conjunctival erosion, retinal detachment, loss of light perception, and the need for revision surgery, but are rare. There are also specific device risks, including overstimulation (which could cause damage to the retina) or delamination of implanted components, but these are very unlikely. Current challenges include how to improve visual acuity, enlarge the field-of-view, and reduce a complex visual scene to its most salient components through image processing. This review encompasses the work of over 40 individual research groups who have built devices, developed stimulation strategies, or investigated the basic physiology underpinning retinal prostheses. Current technologies are summarized, along with future challenges that face the field

Photovoltaic restoration of sight with high visual acuity

Patients with retinal degeneration lose sight due to the gradual demise of photoreceptors. Electrical stimulation of surviving retinal neurons provides an alternative route for the delivery of visual information. We demonstrate that subretinal implants with 70-μm-wide photovoltaic pixels provide highly localized stimulation of retinal neurons in rats. The electrical receptive fields recorded in retinal ganglion cells were similar in size to the natural visual receptive fields. Similarly to normal vision, the retinal response to prosthetic stimulation exhibited flicker fusion at high frequencies, adaptation to static images and nonlinear spatial summation. In rats with retinal degeneration, these photovoltaic arrays elicited retinal responses with a spatial resolution of 64 ± 11 μm, corresponding to half of the normal visual acuity in healthy rats. The ease of implantation of these wireless and modular arrays, combined with their high resolution, opens the door to the functional restoration of sight in patients blinded by retinal degeneration

Using electrical stimulation to enhance the efficacy of cell transplantation therapies for neurodegenerative retinal diseases: Concepts, challenges, and future perspectives

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Zapping the Retina - Understanding electrical responsiveness and electrical desensitization in mouse retinal ganglion cells

The field of science and technology has come a long way since the famous 70’s science fiction series “The Six Million Dollar Man,” where a disabled pilot was transformed into a bionic superhero after receiving artificial implants. What was indeed once a science fiction has now turned into a science fact with the development of various electronic devices interfacing the human neurons in the brain, retina, and limbs. One such advancement was the development of retinal implants. Over the past two decades, the field of retinal prosthetics has made significant advancement in restoring functional vision in patients blinded by diseases such as Retinitis pigmentosa (RP) and Age-related macular degeneration (AMD). RP and AMD are the two leading cause of degenerative blindness. While there is still no definitive cure for either of these diseases, various treatment strategies are currently being explored. Of the various options, the most successful one has been the retinal implants. Retinal implants are small microelectrode or photodiode arrays, which are implanted in the eye of a patient, to stimulate the degenerating retina electrically. They are broadly classified into three types depending on the placement ̶ epiretinal (close proximity to retinal ganglion cells, RGCs) , subretinal (close proximity to bipolar cells, BP) and suprachoroidal (close proximity to choroid). While the ongoing human trials have shown promising results, there remains a considerable variability among patients concerning the quality of visual percepts which limits the working potential of these implants. One such limitation often reported by the implanted patients is “ fading” of visual percepts. Fading refers to the limited ability to elicit temporally stable visual percepts. While, this is not a primary concern for epiretinal implants , it is often observed in subretinal and suprachoroidal implants which use the remaining retinal network to control the temporal spiking pattern of the ganglion cells. The neural correlate of fading is often referred to as “electrical desensitization”, which is the reduction of ganglion cell responses to repetitive electrical stimulation . While much is known about the temporal component of desensitization ( time constant, τ), the spatial aspects (space constant, λ) has not been well characterized. Further, how both these aspects interact to generate spiking responses, remains poorly understood. These crucial questions formed the critical components of my thesis. To address these questions, we stimulated the retinal network electrically, with voltage and current pulses and recorded the corresponding spiking activity using the microelectrode arrays (MEAs). While addressing the primary question of my thesis, we were able to address few idiosyncrasies which has currently stymied the field of retinal prosthetics. At a conceptual level, we have developed an experimental and analysis framework by which one can identify the single stimulus that will activate the most ganglion cells (Chapter 2, Part 1). This stimulus is optimal for ‘blind’ experiments where the specific response properties of each cell are unknown. Furthermore, we attempted to understand the correspondence between the electrical response patterns and visual response types (Chapter 2, Part2). In Chapter 3, we sought to understand better how the visual responses parameters change during ongoing electrical stimulation. We demonstrated that apart from the adaptation occurring due to visual stimulation and invitro experimental conditions, the electrical stimulation alters the RGC visual responses, suggesting the requirement for stimulation-induced changes to be incorporated in the designing of stimulation paradigms for the implant. Finally addressing the primary question (Chapter 4) of my thesis with which we started, we were able to demonstrate, that the electrical desensitization requires the interaction of both time and distance and is not a global phenomenon of the retina. In the final chapter (Chapter 5) we summarize the results of the thesis, discuss the key outcomes and its relevance to the prosthetic field and other vision restoration strategies and the potential future directions of this research. Therefore, in future, to improve the efficacy of retinal prostheses and patient outcomes, it is crucial to have an in-depth understanding of the responsiveness of the retinal circuitry to electrical stimulation

Characterization of Retinal Ganglion Cell Responses to Electrical Stimulation Using White Noise

Retinitis pigmentosa and age-related macular degeneration are two leading causes of degenerative blindness. While there is still not a definitive course of treatment for either of these diseases, there is currently the world over, many different treatment strategies being explored. Of these various strategies, one of the most successful has been retinal implants. Retinal implants are microelectrode or photodiode arrays, that are implanted in the eye of a patient, to electrically stimulate the degenerating retina. Clinical trials have shown that many patients implanted with such a device, are able to regain a certain degree of functional vision. However, while the results of these ongoing clinical trials have been promising, there are still many technical challenges that need to be overcome. One of the biggest challenges facing present implants is the inability to preferentially stimulate different retinal pathways. This is because retinal implants use large-amplitude current or voltage pulses. This in turn leads to the indiscriminate activation of multiple classes of retinal ganglion cells (RGCs), and therefore, an overall reduction in the restored visual acuity. To tackle this issue, we decided to explore a novel stimulus paradigm, in which we present to the retina, a stream of smaller-amplitude subthreshold voltage pulses. By then correlating the retinal spikes to the stimuli preceding them, we calculate temporal input filters for various classes of RGCs, using a technique called spike-triggered averaging (STA). In doing this, we found that ON and OFF RGCs have electrical filters, which are very distinct from each other. This finding creates the possibility for the selective activation of the retina through the use of STA-based waveforms. Finally, using statistical models, we verify how well these temporal filters can predict RGC responses to novel electrical stimuli. In a broad sense, our work represents the successful application of systems engineering tools to retinal prosthetics, in an attempt to answer one of the field’s most difficult questions, namely selective stimulation of the retina

PRIMA subretinal wireless photovoltaic microchip implantation in non-human primate and feline models

Purpose To evaluate the surgical technique for subretinal implantation of two sizes of PRIMA photovoltaic wireless microchip in two animal models, and refine these surgical procedures for human trials. Methods Cats and Macaca fascicularis primates with healthy retina underwent vitrectomy surgery and were implanted with subretinal wireless photovoltaic microchip at the macula/central retina. The 1.5mm PRIMA chip was initially studied in feline eyes. PRIMA implant (2mm,1.5mm sizes) arrays were studied in primates. Feasibility of subretinal chip implantation was evaluated with a newly-developed surgical technique, with surgical complications and adverse events recorded. Results The 1.5mm implant was placed in the central retina of 11 feline eyes, with implantation duration 43-106 days. The 1.5mm implant was correctly positioned into central macula of 11 primate eyes, with follow-up periods of minimum 6 weeks (n = 11), 2 years (n = 2), and one eye for 3 years. One primate eye underwent multi-chip 1.5mm implantation using two 1.5mm chips. The 2mm implant was delivered to 4 primate eyes. Optical coherence tomography confirmed correct surgical placement of photovoltaic arrays in the subretinal space in all 26 eyes. Intraoperative complications in primate eyes included retinal tear, macular hole, retinal detachment, and vitreous hemorrhage that resolved spontaneously. Postoperatively, there was no case of significant ocular inflammation in the 1.5mm implant group. Conclusions We report subretinal implantation of 1.5mm and 2mm photovoltaic arrays in the central retina of feline and central macula of primate eyes with a low rate of device-related complications. The in vivo PRIMA implantation technique has been developed and refined for use for a 2mm PRIMA implant in ongoing human trials