High Visual Acuity Retinal Prosthesis: Understanding Limitations and Advancements Toward Functional Prosthetic Vision

Foveal Retinal Ganglion Cells Develop Altered Calcium Dynamics Weeks After Photoreceptor Ablation

Purpose

Physiological changes in retinal ganglion cells (RGCs) have been reported in rodent models of photoreceptor (PR) loss, but this has not been investigated in primates. By expressing both a calcium indicator (GCaMP6s) and an optogenetic actuator (ChrimsonR) in foveal RGCs of the macaque, we reactivated RGCs in vivo and assessed their response in the weeks and years after PR loss.

Design

We used an in vivo calcium imaging approach to record optogenetically evoked activity in deafferented RGCs in primate fovea. Cellular scale recordings were made longitudinally over a 10-week period after PR ablation and compared with responses from RGCs that had lost PR input >2 years prior.

Participants

Three eyes received PR ablation, the right eye of a male Macaca mulatta (M1), the left eye of a female Macaca fascicularis (M2), and the right eye of a male Macaca fascicularis (M3). Two animals were used for in vivo recording, 1 for histological assessment.

Methods

Cones were ablated with an ultrafast laser delivered through an adaptive optics scanning light ophthalmoscope (AOSLO). A 0.5 second pulse of 25 Hz 660 nm light optogenetically stimulated RGCs, and the resulting GCaMP fluorescence signal was recorded using an AOSLO. Measurements were repeated over 10 weeks immediately after PR ablation, at 2.3 years and in control RGCs.

Main Outcome Measures

The calcium rise time, decay constant, and sensitivity index of optogenetic-mediated RGC were derived from GCaMP fluorescence recordings from 221 RGCs (animal M1) and 218 RGCs (animal M2) in vivo.

Results

After PR ablation, the mean decay constant of the calcium response in RGCs decreased 1.5-fold (standard deviation 1.6 ± 0.5 seconds to 0.6 ± 0.3 seconds) over the 10-week observation period in subject 1 and 2.1-fold (standard deviation 2.5 ± 0.5 seconds to 1.2 ± 0.2 seconds) within 8 weeks in subject 2. Calcium rise time and sensitivity index were stable. Optogenetic reactivation remained possible 2.3 years after PR ablation.

Conclusions

Altered calcium dynamics developed in primate foveal RGCs in the weeks after PR ablation. The mean decay constant of optogenetic-mediated calcium responses decreased 1.5- to twofold. This is the first report of this phenomenon in primate retina and further work is required to understand the role these changes play in cell survival and activity.

Financial Disclosures

Proprietary or commercial disclosure may be found in the Footnotes and Disclosures at the end of this article.

Asymmetric Activation of ON and OFF Pathways in the Degenerated Retina

Retinal prosthetics are one of the leading therapeutic strategies to restore lost vision in patients with retinitis pigmentosa and age-related macular degeneration. Much work has described patterns of spiking in retinal ganglion cells (RGCs) in response to electrical stimulation, but less work has examined the underlying retinal circuitry that is activated by electrical stimulation to drive these responses. Surprisingly, little is known about the role of inhibition in generating electrical responses or how inhibition might be altered during degeneration. Using whole-cell voltage-clamp recordings during subretinal electrical stimulation in the rd10 and wild-type (wt) retina, we found electrically evoked synaptic inputs differed between ON and OFF RGC populations, with ON cells receiving mostly excitation and OFF cells receiving mostly inhibition and very little excitation. We found that the inhibition of OFF bipolar cells limits excitation in OFF RGCs, and a majority of both pre- and postsynaptic inhibition in the OFF pathway arises from glycinergic amacrine cells, and the stimulation of the ON pathway contributes to inhibitory inputs to the RGC. We also show that this presynaptic inhibition in the OFF pathway is greater in the rd10 retina, compared with that in the wt retina.

Foveal RGCs develop altered calcium dynamics weeks after photoreceptor ablation

Objective or purpose

Physiological changes in retinal ganglion cells (RGCs) have been reported in rodent models of photoreceptor (PR) loss but this has not been investigated in primates. By expressing both a calcium indicator (GCaMP6s) and an optogenetic actuator (ChrimsonR) in foveal RGCs of the macaque, we reactivated RGCs in vivo and assessed their response in the weeks and years following PR loss.

Design

We used an in vivo calcium imaging approach to record optogenetically evoked activity in deafferented RGCs in primate fovea. Cellular scale recordings were made longitudinally over a 10 week period following photoreceptor ablation and compared to responses from RGCs that had lost photoreceptor input more than two years prior.

Participants

Three eyes received photoreceptor ablation, OD of a male Macaca mulatta (M1), OS of a female Macaca fascicularis (M2) and OD of a male Macaca fascicularis (M3). Two animals were used for in vivo recording, one for histological assessment.

Methods

Cones were ablated with an ultrafast laser delivered through an adaptive optics scanning light ophthalmoscope (AOSLO). A 0.5 s pulse of 25Hz 660nm light optogenetically stimulated RGCs, and the resulting GCaMP fluorescence signal was recorded using AOSLO. Measurements were repeated over 10 weeks immediately after PR ablation, at 2.3 years and in control RGCs.

Main Outcome measures

The calcium rise time, decay constant and sensitivity index of optogenetic mediated RGC were derived from GCaMP fluorescence recordings from 221 RGCs (Animal M1) and 218 RGCs (Animal M2) in vivo.

Results

Following photoreceptor ablation, the mean decay constant of the calcium response in RGCs decreased 1.5 fold (1.6±0.5 s to 0.6±0.3 s SD) over the 10 week observation period in subject 1 and 2.1 fold (2.5±0.5 s to 1.2±0.2 s SD) within 8 weeks in subject 2. Calcium rise time and sensitivity index were stable. Optogenetic reactivation remained possible 2.3 years after PR ablation.

Conclusions

Altered calcium dynamics developed in primate foveal RGCs in the weeks after photoreceptor ablation. The mean decay constant of optogenetic mediated calcium responses decreased 1.5 - 2-fold. This is the first report of this phenomenon in primate retina and further work is required to understand the role these changes play in cell survival and activity.

Differences in the spatial fidelity of evoked and spontaneous signals in the degenerating retina

Vision restoration strategies aim to reestablish vision by replacing the function of lost photoreceptors with optoelectronic hardware or through gene therapy. One complication to these approaches is that retinal circuitry undergoes remodeling after photoreceptor loss. Circuit remodeling following perturbation is ubiquitous in the nervous system and understanding these changes is crucial for treating neurodegeneration. Spontaneous oscillations that arise during retinal degeneration have been well-studied, however, other changes in the spatiotemporal processing of evoked and spontaneous activity have received less attention. Here we use subretinal electrical stimulation to measure the spatial and temporal spread of both spontaneous and evoked activity during retinal degeneration. We found that electrical stimulation synchronizes spontaneous oscillatory activity, over space and through time, thus leading to increased correlations in ganglion cell activity. Intriguingly, we found that spatial selectivity was maintained in rd10 retina for evoked responses, with spatial receptive fields comparable to wt retina. These findings indicate that different biophysical mechanisms are involved in mediating feed forward excitation, and the lateral spread of spontaneous activity in the rd10 retina, lending support toward the possibility of high-resolution vision restoration.

An in-silico analysis of electrically-evoked responses of midget and parasol retinal ganglion cells in different retinal regions

BACKGROUND

Visual outcomes provided by present retinal prostheses that primarily target retinal ganglion cells (RGCs) through epiretinal stimulation remain rudimentary, partly due to the limited knowledge of retinal responses under electrical stimulation. Better understanding of how different retinal regions can be quantitatively controlled with high spatial accuracy, will be beneficial to the design of micro-electrode arrays (MEAs) and stimulation strategies for next-generation wide-view, high-resolution epiretinal implants.

METHODS

A computational model was developed to assess neural activity at different eccentricities (2 mm and 5 mm) within the human retina. This model included midget and parasol RGCs with anatomically accurate cell distribution and cell-specific morphological information. We then performed in silico investigations of region-specific RGC responses to epiretinal electrical stimulation using varied electrode sizes (5 µm - 210 µm diameter), emulating both commercialized retinal implants and recently-developed prototype devices.

RESULTS

Our model of epiretinal stimulation predicted RGC population excitation analogous to the complex percepts reported in human subjects. Following this, our simulations suggest that midget and parasol RGCs have characteristic regional differences in excitation under preferred electrode sizes. Relatively central (2 mm) regions demonstrated higher number of excited RGCs but lower overall activated receptive field (RF) areas under the same stimulus amplitudes (two-way ANOVA, p < 0.05). Furthermore, the activated RGC numbers per unit active RF area (number-RF ratio) were significantly higher in central than in peripheral regions, and higher in the midget than in the parasol population under all tested electrode sizes (two-way ANOVA, p < 0.05). Our simulations also suggested that smaller electrodes exhibit a higher range of controllable stimulation parameters to achieve pre-defined performance of RGC excitation. ..

Implantation and long-term assessment of the stability and biocompatibility of a novel 98 channel suprachoroidal visual prosthesis in sheep

Severe visual impairment can result from retinal degenerative diseases such as retinitis pigmentosa, which lead to photoreceptor cell death. These pathologies result in extensive neural and glial remodelling, with survival of excitable retinal neurons that can be electrically stimulated to elicit visual percepts and restore a form of useful vision. The Phoenix⁹⁹ Bionic Eye is a fully implantable visual prosthesis, designed to stimulate the retina from the suprachoroidal space. In the current study, nine passive devices were implanted in an ovine model from two days to three months. The impact of the intervention and implant stability were assessed using indirect ophthalmoscopy, infrared imaging, and optical coherence tomography to establish the safety profile of the surgery and the device. The biocompatibility of the device was evaluated using histopathological analysis of the tissue surrounding the electrode array, with a focus on the health of the retinal cells required to convey signals to the brain. Appropriate stability of the electrode array was demonstrated, and histological analysis shows that the fibrotic and inflammatory response to the array was mild. Promising evidence of the safety and potential of the Phoenix⁹⁹ Bionic Eye to restore a sense of vision to the severely visually impaired was obtained.

Electronic retinal prosthesis for severe loss of vision in geographic atrophy in age-related macular degeneration: First-in-human use

BACKGROUND

To date there are yet no available approved therapies for Geographic Atrophy (GA) secondary to age-related macular degeneration (AMD).

METHODS

Single site, non-randomized safety and efficacy study presenting the preliminary results in a cohort of five late stage AMD (GA) patients successfully implanted with the Argus II Retinal Prosthesis System (Second Sight Medical Products Inc., Sylmar, CA, USA). Extensive fundus imaging including retinal photographs from which the GA area was measured. A combination of custom and traditional tests designed for very low vision subjects assessed visual function in study subjects. A Functional Low-Vision Observer Rated Assessment was carried out to evaluate the impact of the system on the subject's daily life. In addition, a study to evaluate structural characteristics of the visual cortex of the brain was performed in one subject using magnetic resonance imaging.

RESULTS

Seven device-related adverse events were reported, four of which were classed as serious adverse events. Retinal detachment was reported in three patients and was successfully treated within 12 months of onset. Testing showed an improvement in visual function in three of five patients with the system turned on. Magnetic resonance imaging assessed in one patient after implantation indicates a selective increase in cortical myelin and thickness in visual brain regions 1 year post implantation.

CONCLUSIONS

Epiretinal prostheses can successfully be implanted in those affected by GA secondary to late-stage AMD and can elicit visual percepts by electrical stimulation of residual neuroretinal elements and improve basic visual function in those affected.

Restoring vision at the fovea

In humans high quality, high acuity visual experience is mediated by the fovea, a tiny, specialized patch of retina containing the locus of fixation. Despite this, vision restoration strategies are typically developed in animal models without a fovea. While electrical prostheses have been approved by regulators, as yet they have failed to restore high quality, high acuity vision in patients. Approaches under pre-clinical development include regenerative cell therapies, optogenetics and chemical photosensitizers. All retinal vision restoration therapies require reactivation of inner retina that has lost photoreceptor input and that the restored signals can be interpreted at a behavioural level. A greater emphasis on tackling these challenges at the fovea may accelerate progress toward high quality vision restoration.

Neural engineering: the process, applications, and its role in the future of medicine

OBJECTIVE

Recent advances in neural engineering have restored mobility to people with paralysis, relieved symptoms of movement disorders, reduced chronic pain, restored the sense of hearing, and provided sensory perception to individuals with sensory deficits.

APPROACH

This progress was enabled by the team-based, interdisciplinary approaches used by neural engineers. Neural engineers have advanced clinical frontiers by leveraging tools and discoveries in quantitative and biological sciences and through collaborations between engineering, science, and medicine. The movement toward bioelectronic medicines, where neuromodulation aims to supplement or replace pharmaceuticals to treat chronic medical conditions such as high blood pressure, diabetes and psychiatric disorders is a prime example of a new frontier made possible by neural engineering. Although one of the major goals in neural engineering is to develop technology for clinical applications, this technology may also offer unique opportunities to gain insight into how biological systems operate.

MAIN RESULTS

Despite significant technological progress, a number of ethical and strategic questions remain unexplored. Addressing these questions will accelerate technology development to address unmet needs. The future of these devices extends far beyond treatment of neurological impairments, including potential human augmentation applications. Our task, as neural engineers, is to push technology forward at the intersection of disciplines, while responsibly considering the readiness to transition this technology outside of the laboratory to consumer products.

SIGNIFICANCE

This article aims to highlight the current state of the neural engineering field, its links with other engineering and science disciplines, and the challenges and opportunities ahead. The goal of this article is to foster new ideas for innovative applications in neurotechnology.

In Vivo Photovoltaic Performance of a Silicon Nanowire Photodiode-Based Retinal Prosthesis

Purpose

For more than 20 years, there has been an international, multidisciplinary effort to develop retinal prostheses to restore functional vision to patients blinded by retinal degeneration. We developed a novel subretinal prosthesis with 1512 optically addressed silicon nanowire photodiodes, which transduce incident light into an electrical stimulation of the remaining retinal circuitry. This study was conducted to evaluate the efficacy of optically driving the subretinal prosthesis to produce visual cortex activation via electrical stimulation of the retina.

Methods

We measured electrically evoked potential responses (EEPs) in rabbit visual cortex in response to illumination of the subretinal nanowire prosthesis with pulsed 852-nm infrared (IR) light. We compared the EEP responses to visually evoked potential responses (VEPs) to pulsed 532-nm visible light (positive control) and pulsed 852-nm IR light (negative control).

Results

Activating the devices with IR light produced EEP responses with a significantly higher trough-to-peak amplitude (54.17 ± 33.4 μV) than IR light alone (24.07 ± 22.1 μV) or background cortical activity (23.22 ± 17.2 μV). EEP latencies were significantly faster than focal VEP latencies. Focal VEPs produced significantly higher amplitudes (94.88 ± 43.3 μV) than EEPs. We also demonstrated how an electrode placed on the cornea can be used as a noninvasive method to monitor the function of the implant.

Conclusions

These results show that subretinal electrical stimulation with nanowire electrodes can elicit EEPs in the visual cortex, providing evidence for the viability of a subretinal nanowire prosthetic approach for vision restoration.