Responses of rabbit retinal ganglion cells to electrical stimulation with an epiretinal electrode

Improvements for recording retinal function with Microelectrode Arrays

A microelectrode array (MEA) is a configuration of multiple electrodes that enables the concurrent targeting of multiple sites for extracellular recording and stimulation. By utilizing light pulses or electrical stimulations, MEA recordings unveil the complex patterns of electrical activities that arise from the signaling processes within the retinal network. Here, we present a stepwise approach for using microelectrode arrays (MEAs) for recording action potentials from the mouse retina in response to electrical and light stimuli. We provide detailed techniques accompanied by description of a custom optical system, example recordings, troubleshooting guidelines, and data processing methods including spike sorting and code resources for analyzing light and electrical responses. The comprehensive nature of this paper aims to guide researchers in utilizing MEAs effectively for investigating retinal functionality. In particular, it can be easy to have a MEA experiment fail, but hard to identify the source of the failure. This paper is meant to demystify that process. It includes:•A description of MEA setup, recording, and spike data validation.•A troubleshooting guide for common failure modes in MEA recordings from mouse retina.•Spike detection and sorting to precisely extract distinctive action potential.

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.

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.

Spatial Resolution of Suprachoroidal-Transretinal Stimulation Estimated by Recording Single-Unit Activity From the Cat Lateral Geniculate Nucleus

Retinal prostheses are devices used to restore visual sensation in patients suffering from photoreceptor degeneration, such as retinitis pigmentosa. Suprachoroidal–transretinal stimulation (STS) is a prosthesis with retinal electrodes located in the sclera. STS has the advantage that it is safer than epiretinal or subretinal prostheses, as the implant is not directly attached to the retinal tissue. We have previously reported feasibility of STS with animal experiments and clinical trials. However, functional evaluation with neurophysiological experiments is still largely missing. To estimate the spatial resolution of STS, single-unit activities in response to STS were recorded from relay cells in the dorsal lateral geniculate nucleus of cats, and the response probability of the units was analyzed in relation to the distance between the stimulus location and the receptive field of each recorded unit. A platinum electrode was attached to the sclera after lamellar resection, and the return electrode was placed in the vitreous. The stimulating current, which ranged from 50 to 500 μA, was applied between these electrodes, and the probability of spike responses occurring just after retinal stimulation was measured. The distance at half-maximum of response was determined from the collected response probabilities as a function of stimulus intensity for all units characterized by their distances from the receptive field center to the stimulation point. As the stimulation became weaker, this distance decreased to 1.8° at 150 and 100 μA. As another estimation, the radius of 25% response probability was 1.4° at 100 μA. The diameter of the stimulated cat retinal area, 3.6° or 2.8°, corresponds to human visual acuity of 0.005 or 0.007, or finger counting. Considering the lower hazard to the retina of STS and its potentially large visual field coverage, STS is an attractive method for retinal prosthetic device development.

Minimizing Iridium Oxide Electrodes for High Visual Acuity Subretinal Stimulation

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

In vivo feasibility of epiretinal stimulation using ultrananocrystalline diamond electrodes

OBJECTIVE

Due to their increased proximity to retinal ganglion cells (RGCs), epiretinal visual prostheses present the opportunity for eliciting phosphenes with low thresholds through direct RGC activation. This study characterised the in vivo performance of a novel prototype monolithic epiretinal prosthesis, containing Nitrogen incorporated ultrananocrystalline (N-UNCD) diamond electrodes.

APPROACH

A prototype implant containing up to twenty-five 120 × 120 µm N-UNCD electrodes was implanted into 16 anaesthetised cats and attached to the retina either using a single tack or via magnetic coupling with a suprachoroidally placed magnet. Multiunit responses to retinal stimulation using charge-balanced biphasic current pulses were recorded acutely in the visual cortex using a multichannel planar array. Several stimulus parameters were varied including; the stimulating electrode, stimulus polarity, phase duration, return configuration and the number of electrodes stimulated simultaneously.

MAIN RESULTS

The rigid nature of the device and its form factor necessitated complex surgical procedures. Surgeries were considered successful in 10/16 animals and cortical responses to single electrode stimulation obtained in eight animals. Clinical imaging and histological outcomes showed severe retinal trauma caused by the device in situ in many instances. Cortical measures were found to significantly depend on the surgical outcomes of individual experiments, phase duration, return configuration and the number of electrodes stimulated simultaneously, but not stimulus polarity. Cortical thresholds were also found to increase over time within an experiment.

SIGNIFICANCE

The study successfully demonstrated that an epiretinal prosthesis containing diamond electrodes could produce cortical activity with high precision, albeit only in a small number of cases. Both surgical approaches were highly challenging in terms of reliable and consistent attachment to and stabilisation against the retina, and often resulted in severe retinal trauma. There are key challenges (device form factor and attachment technique) to be resolved for such a device to progress towards clinical application, as current surgical techniques are unable to address these issues.

Probing and predicting ganglion cell responses to smooth electrical stimulation in healthy and blind mouse retina

Retinal implants are used to replace lost photoreceptors in blind patients suffering from retinopathies such as retinitis pigmentosa. Patients wearing implants regain some rudimentary visual function. However, it is severely limited compared to normal vision because non-physiological stimulation strategies fail to selectively activate different retinal pathways at sufficient spatial and temporal resolution. The development of improved stimulation strategies is rendered difficult by the large space of potential stimuli. Here we systematically explore a subspace of potential stimuli by electrically stimulating healthy and blind mouse retina in epiretinal configuration using smooth Gaussian white noise delivered by a high-density CMOS-based microelectrode array. We identify linear filters of retinal ganglion cells (RGCs) by fitting a linear-nonlinear-Poisson (LNP) model. Our stimulus evokes spatially and temporally confined spiking responses in RGC which are accurately predicted by the LNP model. Furthermore, we find diverse shapes of linear filters in the linear stage of the model, suggesting diverse preferred electrical stimuli of RGCs. The linear filter base identified by our approach could provide a starting point of a model-guided search for improved stimuli for retinal prosthetics.

Effect of interphase gap duration and stimulus rate on threshold of visual cortical neurons in the rat

Stimulation threshold is a key parameter to enable an efficient design for retinal implants. Stimulation parameters such as stimulus pulse duration, pulse amplitude, pulse repetition, pulse shape and polarity have been shown to be the key factors that can influence the efficacy of retinal prosthetics. The effectiveness of these devices should best be evaluated both in the retina and in the visual cortex. Prior electrophysiological studies in the retina have shown that introducing an interphase gap make stimulation more efficient. Previous in vitro studies have also demonstrated the response properties of retinal ganglion cells are frequency dependent. However, the effect of these two stimulus parameters are not well explored at the cortical level where higher visual processing signals are processed. In this study, we examined the response properties of visual cortical neurons under stimulation of retinal ganglion cells in rat using a single-channel electrode of diameter 75 μm. We compared the response strength curves as a function of stimulus current amplitudes under different stimulus pulse duration, interphase gap and stimulus rate. Localized response to single channel epiretinal stimulation was robustly observed in V1 neurons. We found that V1 neurons were more sensitive to longer pulse and stimulus with an interphase gap, similar to previously reported results in the retina. We were also able to examine the effect of stimulus frequency on threshold in the visual cortex. Our results indicate that electrical activation of V1 neurons are more efficient at low frequency.

Comparison of responses of visual cortical neurons in the mouse to intraocular and extraocular electric stimulation of the retina

Retinal implants offered the promise of restoring functional vision to the blind via the delivery of electrical stimulation to the retina. To enhance the efficacy of these devices, stimulation should elicit neural responses that are similar to the responses that occur naturally in the retina as these have the best chance of carrying a robust signal to visual cortex. A corollary of this is that the responses that arise in visual cortical neurons can be used to compare the effectiveness of different stimulation strategies in the retina. Here, we studied how visual cortical neurons in the mouse respond to monophasic cathodal and anodal electric stimulation delivered via a wire electrode positioned on the outer surface of the eye (extraocular) or within the vitreous cavity of the eye (intraocular). Responses of visual cortical neurons were recorded from primary visual cortex on the side contralateral to the stimulatated eye. For both stimulation modalities, response patterns consisted of a brief burst of spikes followed by a 400-500 ms period of inhibition. Both modalities also elicited stronger responses to cathodal stimuli (vs. anodal). The preferential sensitivity to cathodal stimuli is similar to that of epiretinal stimulation (anodal stimuli are more effective with sub-retinal stimulation) suggest the extraocular approach mirrors epiretinal stimulation. Extraocular stimulation also showed some response characteristics that were different from those observed in the retina, e.g., at very strong amplitudes, cathodal and anodal stimulation produced similar responses.

Network-mediated responses of ON ganglion cells to electric stimulation become less consistent across trials during retinal degeneration

Microelectronic retinal prostheses are being developed to restore sight in individuals blinded by outer retinal degenerative diseases such as retinitis pigmentosa and age-related macular degeneration. Unfortunately, the quality of vision restored by these devices is still limited. To improve the quality of elicited vision, our group studies the responses of retinal neurons to electric stimulation. Our previous work showed that responses mediated through the retinal network are reproducible with high temporal precision, even for spikes that occur >100 ms after stimulus onset. Because they arise through the network, it is important to understand whether such reliability changes in the degenerate retina. Here, we examined response variability at several different stages of degeneration: postnatal day 14 (P14), P18, P31 and P60 in a well-established mouse model of degeneration (rd10). Spiking responses of ON alpha RGCs were recorded multiple times to an identical electric stimulus. We found that the trial-to-trial variability increased over the course of retinal degeneration. This finding may help to explain the reported variability in the quality of elicited vision across subjects using these devices.

Non-rectangular waveforms are more charge-efficient than rectangular one in eliciting network-mediated responses of ON type retinal ganglion cells

OBJECTIVE

For individuals blinded by outer retinal degenerative diseases, retinal prostheses would be a promising option to restore sight. Unfortunately, however, the best performance of existing devices is still far removed from normal vision. One possible reason for the shortcoming is thought to be suboptimal stimulation conditions such as the waveform shape of electric stimulus. In this study, we explored the effects of varying waveforms on network-mediated responses arising in retinal ganglion cells (RGCs).

APPROACH

We used a cell-attached patch clamp technique to record RGC spiking activities in the isolated mouse retina. ON alpha RGCs were targeted by soma size and their light responses to stationary spot flashes. Spiking in targeted RGCs was measured in response to an epiretinally-delivered cathodal current pulse in four waveforms: rectangular, center triangular, increasing and decreasing ramp shapes. Each waveform was tested at three durations (20, 10, and 5 ms) with adjusted amplitude for a range of total charges (50-400 nC).

MAIN RESULTS

ON alpha RGCs always generated two bursts of spikes in responses to all stimuli conditions we tested. However, at a given charge, effects of differing waveforms were distinct in the two bursts. For the first burst, the increasing ramp was most effective among the four waveforms (p  <  0.05 for all pairwise comparisons with other waveforms). For example, in responses arising from 20 ms-long stimuli, the increasing ramp evoked ~44% more spikes on average than the rectangular shape which is the typical choice of neural stimulation. Also, the rectangular stimulus evoked the weakest response in the delayed burst arising from pulses of every duration. For instance, 20 ms-long stimuli in the three non-rectangular waveforms showed ~23% or more increment in spike counts compared to response arising from the rectangular one; but there was no statistical difference in response magnitudes across the non-rectangular waveforms.

SIGNIFICANCE

Although the rectangular waveform has been primarily used in retinal prostheses our results indicate that rectangular stimulus is not optimal for network-mediated responses of ON alpha RGCs. Instead, non-rectangular waveforms evoke stronger responses at a given charge, indicating higher charge-efficiency. Therefore, non-rectangular waveforms are expected to enhance clinical efficacy of retinal prostheses.

Upper stimulation threshold for retinal ganglion cell activation

OBJECTIVE

The existence of an upper threshold in electrically stimulated retinal ganglion cells (RGCs) is of interest because of its relevance to the development of visual prosthetic devices, which are designed to restore partial sight to blind patients. The upper threshold is defined as the stimulation level above which no action potentials (direct spikes) can be elicited in electrically stimulated retina.

APPROACH

We collected and analyzed in vitro recordings from rat RGCs in response to extracellular biphasic (anodic-cathodic) pulse stimulation of varying amplitudes and pulse durations. Such responses were also simulated using a multicompartment model.

MAIN RESULTS

We identified the individual cell variability in response to stimulation and the phenomenon known as upper threshold in all but one of the recorded cells (n  =  20/21). We found that the latencies of spike responses relative to stimulus amplitude had a characteristic U-shape. In silico, we showed that the upper threshold phenomenon was observed only in the soma. For all tested biphasic pulse durations, electrode positions, and pulse amplitudes above lower threshold, a propagating action potential was observed in the distal axon. For amplitudes above the somatic upper threshold, the axonal action potential back-propagated in the direction of the soma, but the soma's low level of hyperpolarization prevented action potential generation in the soma itself.

SIGNIFICANCE

An upper threshold observed in the soma does not prevent spike conductance in the axon.

Direct measurement of bipolar cell responses to electrical stimulation in wholemount mouse retina

OBJECTIVE

This in vitro investigation examines the response of retinal bipolar cells to extracellular electrical stimulation.

APPROACH

In vitro investigations characterizing the response of retinal neurons to electrical stimulation have primarily focused on retinal ganglion cells because they are the output neurons of the retina and their superficial position in the retina makes them readily accessible to in vitro recording techniques. Thus, the majority of information regarding the response of inner retinal neurons has been inferred from ganglion cell activity. Here we use patch clamp electrophysiology to directly record electrically-evoked activity in bipolar cells within the inner retina of normal Tg(Gng13-EGFP)GI206Gsat and degenerate rd10 Tg(Gng13-EGFP)GI206Gsat mice using a wholemount preparation.

MAIN RESULTS

Bipolar cells respond to electrical stimulation with time-locked depolarizing voltage transients. The latency of the response declines with increases in stimulation amplitude. A desensitizing response is observed during repeated stimulation with 25 ms biphasic current pulses delivered at pulse rates greater than 6 pps. A burst of long-latency (200-1000 ms) inhibitory postsynaptic potentials are evoked by the stimulus and the burst exhibits evidence of a lower and upper stimulation threshold.

SIGNIFICANCE

These results provide insights into the various types of bipolar cell activity elicited by electrical stimulation and may be useful for future retinal prosthesis stimulation protocols. This investigation uses patch clamp electrophysiology to provide direct analysis of ON-type bipolar cell responses to electrical stimulation in a wholemount retina preparation. It explores the effects of variable stimulus amplitudes, pulse widths, and frequencies in both normal and degenerate retina. The analysis adds to a body of work largely based upon indirect measurements of bipolar cell activity, and the methodology demonstrates an alternative retina preparation technique in which to acquire single-cell activity.

Electric stimulus duration alters network-mediated responses depending on retinal ganglion cell type

OBJECTIVE

To improve the quality of artificial vision that arises from retinal prostheses, it is important to bring electrically-elicited neural activity more in line with the physiological signaling patterns that arise normally in the healthy retina. Our previous study reported that indirect activation produces a closer match to physiological responses in ON retinal ganglion cells (RGCs) than in OFF cells (Im and Fried 2015 J. Physiol. 593 3677-96). This suggests that a preferential activation of ON RGCs would shape the overall retinal response closer to natural signaling. Recently, we found that changes to the rate at which stimulation was delivered could bias responses towards a stronger ON component (Im and Fried 2016a J. Neural Eng. 13 025002), raising the possibility that changes to other stimulus parameters can similarly bias towards stronger ON responses. Here, we explore the effects of changing stimulus duration on the responses in ON and OFF types of brisk transient (BT) and brisk sustained (BS) RGCs.

APPROACH

We used cell-attached patch clamp to record RGC spiking in the isolated rabbit retina. Targeted RGCs were first classified as ON or OFF type by their light responses, and further sub-classified as BT or BS types by their responses to both light and electric stimuli. Spiking in targeted RGCs was recorded in response to electric pulses with durations varying from 5 to100 ms. Stimulus amplitude was adjusted at each duration to hold total charge constant for all experiments.

MAIN RESULTS

We found that varying stimulus durations modulated responses differentially for ON versus OFF cells: in ON cells, spike counts decreased significantly with increasing stimulus duration while in OFF cells the changes were more modest. The maximum ratio of ON versus OFF responses occurred at a duration of ~10 ms. The difference in response strength for BT versus BS cells was much larger in ON cells than in OFF cells.

SIGNIFICANCE

The stimulation rates preferred by subjects during clinical trials are similar to the rates that maximize the ON/OFF response ratio in in vitro testing (Im and Fried 2016a J. Neural Eng. 13 025002). Here, we determine the stimulus duration that produces the strongest bias towards ON responses and speculate that it will further enhance clinical effectiveness.

Survey of electrically evoked responses in the retina - stimulus preferences and oscillation among neurons

Electrical stimulation is an important tool in neuroscience research and clinically. In the retina, extensive work has revealed how the retinal ganglion cells respond to extracellular electrical stimulation. But little is known about the responses of other neuronal types, and more generally, how the network responds to stimulation. We conducted a survey of electrically evoked responses, over a range of pulse amplitudes and pulse widths, for 21 cell types spanning the inner two layers of the rabbit retina. It revealed: (i) the evoked responses of some neurons were charge insensitive; (ii) pulse-width sensitivity varied between cell types, allowing preferential recruitment of cell types; and (iii) 10-20 Hz damped oscillations across retinal layers. These oscillations were generated by reciprocal excitatory / inhibitory synapses, at locations as early as the cone-horizontal-cell synapses. These results illustrate at cellular resolution how a network responds to extracellular stimulation, and could inform the development of bioelectronic implants for treating blindness.

Optimal voltage stimulation parameters for network-mediated responses in wild type and rd10 mouse retinal ganglion cells

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

Patch clamp recordings of retinal bipolar cells in response to extracellular electrical stimulation in wholemount mouse retina

Retinitis pigmentosa is a family of inherited retinal diseases identified by the degeneration of photoreceptors, which leads to blindness. In efforts to restore vision lost to retinitis pigmentosa, retinal prostheses have been developed to generate visual percepts by electrically stimulating the surviving retinal bipolar and ganglion cells. The response of retinal ganglion cells to electrical stimulation has been characterized through direct measurement. However, the response of bipolar cells has only been inferred by measuring retinal ganglion cell activity. This investigation reports on a novel tissue preparation technique facilitating bipolar cell patch clamp recordings in wholemount retina. We find that bipolar cells respond to extracellular electrical stimuli with time-locked voltage spike depolarizations, which are likely mediated by voltage-gated calcium channels.

Electronic approaches to restoration of sight

Retinal prostheses are a promising means for restoring sight to patients blinded by the gradual atrophy of photoreceptors due to retinal degeneration. They are designed to reintroduce information into the visual system by electrically stimulating surviving neurons in the retina. This review outlines the concepts and technologies behind two major approaches to retinal prosthetics: epiretinal and subretinal. We describe how the visual system responds to electrical stimulation. We highlight major differences between direct encoding of the retinal output with epiretinal stimulation, and network-mediated response with subretinal stimulation. We summarize results of pre-clinical evaluation of prosthetic visual functions in- and ex vivo, as well as the outcomes of current clinical trials of various retinal implants. We also briefly review alternative, non-electronic, approaches to restoration of sight to the blind, and conclude by suggesting some perspectives for future advancement in the field.

Probing the functional impact of sub-retinal prosthesis

Retinal prostheses are promising tools for recovering visual functions in blind patients but, unfortunately, with still poor gains in visual acuity. Improving their resolution is thus a key challenge that warrants understanding its origin through appropriate animal models. Here, we provide a systematic comparison between visual and prosthetic activations of the rat primary visual cortex (V1). We established a precise V1 mapping as a functional benchmark to demonstrate that sub-retinal implants activate V1 at the appropriate position, scalable to a wide range of visual luminance, but with an aspect-ratio and an extent much larger than expected. Such distorted activation profile can be accounted for by the existence of two sources of diffusion, passive diffusion and activation of ganglion cells’ axons en passant. Reverse-engineered electrical pulses based on impedance spectroscopy is the only solution we tested that decreases the extent and aspect-ratio, providing a promising solution for clinical applications.

DOI:http://dx.doi.org/10.7554/eLife.12687.001

One of the most common causes of blindness is a disorder called retinitis pigmentosa. In a healthy eye, the surface at the back of the eye – called the retina – contains cells called photoreceptors that detect light and convert it into electrical signals for the brain to process. In people with retinitis pigmentosa, these photoreceptor cells die off gradually, which leads to loss of vision.

The only treatment available for retinitis pigmentosa is to have an artificial retina implanted into the eye. The artificial retina consists of an array of tiny electrodes, which take over from the damaged photoreceptors and generate electrical signals. The person with the implant perceives these electrical signals as bright flashes called “phosphenes”. However, the phosphenes are too large and imprecise to provide the person with vision that is good enough for tasks such as walking unaided or reading.

To find out why artificial retinas produce such poor resolution, Roux et al. compared how a rat’s brain responds to either natural visual stimuli or activation of implanted an array of micro-electrodes. Both the micro-electrodes and the natural stimuli activated the same areas of the brain. However, the micro-electrodes produced larger and more elongated patterns of activation. This is because the electrical currents generated by the micro-electrodes diffused throughout the retinal tissue and activated other neurons besides those intended. To overcome this problem, Roux et al. tested different ways of stimulating the micro-electrodes in order to identify those that induce the desired patterns of brain activity. This approach – known as reverse engineering – did indeed improve the performance of the micro-electrode array.

The next step is to extend these findings, which were obtained in healthy rats, to non-human primates or animal models of retinitis pigmentosa to better understand the condition in humans. In addition, combining the current approach with other existing techniques should further improve the vision that can be achieved with artificial retinas.

DOI:http://dx.doi.org/10.7554/eLife.12687.002

Perspectives on: Materials Aspects for Retinal Prostheses

Retinitis pigmentosa and age-related macular degeneration are both incurable eye diseases that lead to blindness due to photoreceptor degeneration. Electrically stimulating the remaining intact nerve cells may generate some useful vision for patients afflicted with these diseases. Various types of retinal prostheses, sub- and epi-retinal electrode arrays, as well as subretinal microphotodiode arrays are considered from a materials and biocompatibility point of view. Other, more innovative approaches to restoring vision, such as microfluidic pumps and activated nanosystems that deliver neurotransmitters in a controlled way and photodynamic therapy are being developed. This article discusses materials aspects of retinal prostheses that are currently in use or under development.

Temporal properties of network-mediated responses to repetitive stimuli are dependent upon retinal ganglion cell type

OBJECTIVE

To provide artificially-elicited vision that is temporally dynamic, retinal prosthetic devices will need to repeatedly stimulate retinal neurons. However, given the diversity of physiological types of retinal ganglion cells (RGCs) as well as the heterogeneity of their responses to electric stimulation, temporal properties of RGC responses have not been adequately investigated. Here, we explored the cell type dependence of network-mediated RGC responses to repetitive electric stimulation at various stimulation rates.

APPROACH

We examined responses of ON and OFF types of RGCs in the rabbit retinal explant to five consecutive stimuli with varying inter-stimulus intervals (10-1000 ms). Each stimulus was a 4 ms long monophasic sinusoidal cathodal current, which was applied epiretinally via a conical electrode. Spiking activity of targeted RGCs was recorded using a cell-attached patch electrode.

MAIN RESULTS

ON and OFF cells had distinct responses to repetitive stimuli. Consistent with earlier studies, OFF cells always generated reduced responses to subsequent stimuli compared to responses to the first stimulus. In contrast, a new stimulus to ON cells suppressed all pending/ongoing responses from previous stimuli and initiated its own response that was remarkably similar to the response from a single stimulus in isolation. This previously unreported 'reset' behavior was observed exclusively and consistently in ON cells. These contrasts between ON and OFF cells created a range of stimulation rates (4-7 Hz) that maximized the ratio of the responses arising in ON versus OFF cells.

SIGNIFICANCE

Previous clinical testing reported that subjects perceive bright phosphenes (ON responses) and also prefer stimulation rates of 5-7 Hz. Our results suggest that responses of ON cells are weak at high rates of stimulation (> ∼7 Hz) due to the reset while responses of OFF cells are strong at low rates (< ∼4 Hz) due to reduced desensitization, both reducing the ratio of ON to OFF responses. In combination with previous results indicating that responses in ON cells more closely match physiological patterns (Im and Fried 2015 J. Physiol. 593 3577-96), our results offer a potential reason for the user preference of intermediate rates (5-7 Hz).

The Retinal Response to Sinusoidal Electrical Stimulation

Rectangular electrical pulses are the primary stimulus waveform used in retinal prosthetics as well as many other neural stimulation applications. Unfortunately, the utility of pulsatile stimuli is limited by the inability to avoid the activation of passing axons, which can result in the distortion of the spatial patterns of elicited neural activity. Because avoiding axons would likely improve clinical outcomes, the examination of alternate stimulus waveforms is warranted. Here, we studied the response of rabbit retinal ganglion cells (RGCs) to sinusoidal electrical stimulation applied at frequencies of 5, 10, 25, and 100 Hz. Targeted RGCs were restricted to 4 common types: OFF-Brisk Transient, OFF-Sustained, ON-Brisk Transient, and ON-Sustained. Interestingly, response patterns varied between different types; the most notable difference was the relatively weak response of ON-Sustained cells to low frequencies. Calculation of total spike counts per trial revealed that lower frequencies are more charge efficient than high frequencies. Finally, experiments utilizing synaptic blockers revealed that 5 and 10 Hz activate photoreceptors while 25 and 100 Hz activate RGCs. Taken together, our results suggest that while sinusoidal electrical stimulation may provide a useful research tool, its clinical utility may be limited.

Electrically evoked electroretinograms and pupil responses in Argus II retinal implant wearers

PURPOSE

We have recorded the electrically evoked electroretinogram (eERG) and flash ERG in Argus II retinal prosthesis wearers with end-stage retinitis pigmentosa to estimate response properties of the degenerated inner retina to local electrical stimulation. In addition, we have recorded pupil diameters during electrical stimulation.

METHODS

Raw corneal eERGs were recorded at multiple stimulus levels in three subjects. eERG signals were heavily contaminated with various artifacts, including switching artifacts generated by the implant electronics, stimulus, blink, and eye-movement artifacts. Pupil responses were recorded in one subject using a pupil tracker.

RESULTS

eERGs were decontaminated by a variety of techniques, including wavelet transformation and response averaging. The dominant component was a negative wave peaking at approximately 200 ms. eERG amplitudes correlated significantly with stimulus level, but peak latencies did not correlate with stimulus level. Pupil constriction correlated significantly with stimulus level and pupil responses could be accurately used to estimate subjective threshold.

CONCLUSION

eERG recordings hold the potential to be developed further for use as a diagnostic tool for retinal implants. A straightforward approach to increase eERG amplitudes would be the development of intraocular recording methods based on reverse telemetry. The robust pupil response to electrical stimulation in one subject indicates that pupillography can be exploited to assess implant functionality, but reliable pupil recordings could not be obtained in all subjects.

A high-density microelectrode-tissue-microelectrode sandwich platform for application of retinal circuit study

Background

Microelectrode array (MEA) devices are frequently used in neural circuit studies, especially in retinal prosthesis. For a high throughput stimulation and recording paradigm, it is desirable to obtain the responses of multiple surface RGCs initiated from the electrical signals delivered to multiple photoreceptor cells. This can be achieved by an high density MEA-tissue-MEA (MTM) sandwich configuration. However, the retina is one of the most metabolically active tissues, consumes oxygen as rapidly as the brain. The major concern of the MTM configuration is the supply of oxygen.

Methods

We aimed to develop a high density MTM sandwich platform which consists of stacks of a stimulation MEA, retinal tissue and a recording MEA. Retina is a metabolically active tissue and the firing rate is very sensitive to oxygen level. We designed, simulated and microfabricated porous high density MEAs and an adjustable perfusion system that electrical signals can be delivered to and recorded from the clipped retinal tissue.

Results

The porous high-density MEAs linked with stimulation or recording devices within a perfusion system were manufactured and the MTM platform was assembled with a retina slice inside. The firing rate remained constant between 25 and 55 min before dramatically declined, indicating that within certain period of time (e.g. 30 min after habituation), the retina condition was kept by sufficient oxygen supply via the perfusion holes in the MEAs provided by the double perfusion system.

Conclusions

MTM sandwich structure is an efficient platform to study the retinal neural circuit. The material and arrangement of high density microelectrodes with porous design make this MEA appropriate for sub-retina prosthesis. Finding ways to prolong the recording time and reduce the signal-to-noise ratio are important to improve our MTM prototype.

Indirect activation elicits strong correlations between light and electrical responses in ON but not OFF retinal ganglion cells

KEY POINTS

To improve the quality of vision elicited by retinal prosthetics, elicited neural activity should resemble physiological signalling patterns; here, we hypothesized that electric stimulation that activates the synaptic circuitry of the retina would lead to closer matches than that which activates ganglion cells directly. We evaluated this hypothesis by comparing light and electrical responses in different types of ganglion cells. In contrast to the similarity in their light responses, electrical responses in ON and OFF cells of the same type were quite distinct. Further, electrical and light responses in the same cell were much better correlated in ON vs. OFF ganglion cells. Stimuli that activated photoreceptors yielded better correlations than those which activated bipolar cells. Our results suggest that the closer match to physiology in the ON signal transmitted to the brain may help to explain preferential reports of 'bright' phosphenes during earlier clinical trials.

ABSTRACT

To improve the efficacy of microelectronic retinal prosthetics it will be necessary to better understand the response of retinal neurons to electric stimulation. While stimulation that directly activates ganglion cells generally has the lowest threshold, the similarity in responsiveness across cells makes it extremely difficult for such an approach to re-create cell-type specific patterns of neural activity that arise normally in the healthy retina. In contrast, stimulation that activates neurons presynaptic to ganglion cells utilizes at least some of the existing retinal circuitry and therefore is thought to produce neural activity that better matches physiological signalling. Surprisingly, the actual benefit(s) of this approach remain unsubstantiated. Here, we recorded from ganglion cells in the rabbit retinal explant in response to electrical stimuli that activated the network. Targeted cells were first classified into known types via light responses so that the consistency of electrical responses within individual types could be evaluated. Both transient and sustained ON ganglion cells exhibited highly consistent electrical response patterns which were distinct from one another. Further, properties of the response (interspike interval, latency, peak firing rate, and spike count) in a given cell were well correlated to the corresponding properties of the light response for that same cell. Electric responses in OFF ganglion cells formed two groups, distinct from ON groups, and the correlation levels between electric and light responses were much weaker. The closer match in ON pathway responses may help to explain some preferential reporting of bright stimuli during psychophysical testing.

Integration of Perforated Subretinal Prostheses With Retinal Tissue

PURPOSE

To investigate the integration of subretinal implants containing full-depth perforations of various widths with rat and pig retina across weeks of implantation.

METHODS

In transgenic P23H rhodopsin line 1 (TgP23H-1) rats and wild-type (WT) pigs, we examined four subretinal implant designs: solid inactive polymer arrays (IPA), IPAs with 5- or 10-μm wide perforations, and active bipolar photovoltaic arrays (bPVA) with 5-μm perforations. We surgically placed the implants into the subretinal space using an external approach in rats or a vitreoretinal approach in pigs. Implant placement in the subretinal space was verified with optical coherence tomography and retinal perfusion was characterized with fluorescein angiography. Rats were sacrificed 8 or 16 weeks post-implantation (wpi) and pigs 2, 4, or 8 wpi, and retinas evaluated at the light microscopic level.

RESULTS

Regardless of implant design, retinas of both species showed normal vasculature. In TgP23H-1 retinas implanted with 10-μm perforated IPAs, inner nuclear layer (INL) cells migrated through the perforations by 8 wpi, resulting in significant INL thinning by 16 wpi. Additionally, these retinas showed greater pseudo-rosette formation and fibrosis compared with retinas with solid or 5-μm perforated IPAs. TgP23H-1 retinas with bPVAs showed similar INL migration to retinas with 5-μm perforated IPAs, with less fibrosis and rosette formation. WT pig retina with perforated IPAs maintained photoreceptors, showed no migration, and less pseudo-rosette formation, but more fibrosis compared with implanted TgP23H-1 rat retinas.

CONCLUSIONS

In retinas with photoreceptor degeneration, solid implants, or those with 5-μm perforations lead to the best biocompatibility.

Suppression of subthalamic nucleus activity by micromagnetic stimulation

Magnetic stimulation delivered via 0.5-mm diameter coils was recently shown to activate retinal neurons; the small coil size raises the possibility that micromagnetic stimulation ( μMS) could underlie a new generation of implanted neural prosthetics. Such an approach has several inherent advantages over conventional electric stimulation, including the potential for selective activation of neuronal targets as well as less susceptibility to inflammatory responses. The viability of μMS for some applications, e.g., deep brain stimulation (DBS), may require suppression (rather than creation) of neuronal activity, however, and therefore we explore here whether (μMS) could, in fact, suppress activity. While single pulses elicited weak and inconsistent spiking in neurons of the mouse subthalamic nucleus (in vitro), repetitive stimulation effectively suppressed activity in ∼ 70% of targeted neurons. This is the same percentage suppressed by conventional electric stimulation; with both modalities, suppression occurred only after an initial increase in spiking. The latency to the onset of suppression was inversely correlated to the energy of the stimulus waveform: larger amplitudes and lower frequencies had the fastest onset of suppression. These findings continue to support the viability of μMS as a next-generation implantable neural prosthetic.

Inner retinal preservation in rat models of retinal degeneration implanted with subretinal photovoltaic arrays

Photovoltaic arrays (PVA) implanted into the subretinal space of patients with retinitis pigmentosa (RP) are designed to electrically stimulate the remaining inner retinal circuitry in response to incident light, thereby recreating a visual signal when photoreceptor function declines or is lost. Preservation of inner retinal circuitry is critical to the fidelity of this transmitted signal to ganglion cells and beyond to higher visual targets. Post-implantation loss of retinal interneurons or excessive glial scarring could diminish and/or eliminate PVA-evoked signal transmission. As such, assessing the morphology of the inner retina in RP animal models with subretinal PVAs is an important step in defining biocompatibility and predicting success of signal transmission. In this study, we used immunohistochemical methods to qualitatively and quantitatively compare inner retinal morphology after the implantation of a PVA in two RP models: the Royal College of Surgeons (RCS) or transgenic S334ter-line 3 (S334ter-3) rhodopsin mutant rat. Two PVA designs were compared. In the RCS rat, we implanted devices in the subretinal space at 4 weeks of age and histologically examined them at 8 weeks of age and found inner retinal morphology preservation with both PVA devices. In the S334ter-3 rat, we implanted devices at 6-12 weeks of age and again, inner retinal morphology was generally preserved with either PVA design 16-26 weeks post-implantation. Specifically, the length of rod bipolar cells and numbers of cholinergic amacrine cells were maintained along with their characteristic inner plexiform lamination patterns. Throughout the implanted retinas we found nonspecific glial reaction, but none showed additional glial scarring at the implant site. Our results indicate that subretinally implanted PVAs are well-tolerated in rodent RP models and that the inner retinal circuitry is preserved, consistent with our published results showing implant-evoked signal transmission.

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

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

Cortical activation following chronic passive implantation of a wide-field suprachoroidal retinal prosthesis

OBJECTIVE

The research goal is to develop a wide-field retinal stimulating array for prosthetic vision. This study aimed at evaluating the efficacy of a suprachoroidal electrode array in evoking visual cortex activity after long term implantation.

APPROACH

A planar silicone based electrode array (8 mm × 19 mm) was implanted into the suprachoroidal space in cats (ntotal = 10). It consisted of 20 platinum stimulating electrodes (600 μm diameter) and a trans-scleral cable terminated in a subcutaneous connector. Three months after implantation (nchronic = 6), or immediately after implantation (nacute = 4), an electrophysiological study was performed. Electrode total impedance was measured from voltage transients using 500 μs, 1 mA pulses. Electrically evoked potentials (EEPs) and multi-unit activity were recorded from the visual cortex in response to monopolar retinal stimulation. Dynamic range and cortical activation spread were calculated from the multi-unit recordings.

MAIN RESULTS

The mean electrode total impedance in vivo following 3 months was 12.5 ± 0.3 kΩ. EEPs were recorded for 98% of the electrodes. The median evoked potential threshold was 150 nC (charge density 53 μC cm(-2)). The lowest stimulation thresholds were found proximal to the area centralis. Mean thresholds from multiunit activity were lower for chronic (181 ± 14 nC) compared to acute (322 ± 20 nC) electrodes (P < 0.001), but there was no difference in dynamic range or cortical activation spread.

SIGNIFICANCE

Suprachoroidal stimulation threshold was lower in chronic than acute implantation and was within safe charge limits for platinum. Electrode-tissue impedance following chronic implantation was higher, indicating the need for sufficient compliance voltage (e.g. 12.8 V for mean impedance, threshold and dynamic range). The wide-field suprachoroidal array reliably activated the retina after chronic implantation.

A test method for quantification of stimulus-induced depression effects on perceptual threshold in epiretinal prosthesis

PURPOSE

As part of a clinical trial, an investigational epiretinal implant (IMI Intelligent Medical Implant) was implanted in a retinitis pigmentosa patient. The prosthesis was wirelessly controlled by a visual interface containing a microcamera, providing wireless data and energy transmission. Forty-nine (49) electrodes were used for pattern recognition. This study examined the changes of perceptual thresholds over time and its relation to long-term stimulation. The goal of the study was to introduce stimulus-related depression of perceptual threshold (StirDepth) measurements as a method to gain further insight into the safety profile of electrical stimulation.

METHODS

The perceptual threshold was defined as the level of stimulation intensity at which a phosphene perception with a probability of 50% was detected using the Best-PEST method. StirDepth was measured by comparing the threshold changes immediately before and after a stimulation session of using three active electrodes and one passive electrode, which served as control.

RESULTS

The initial threshold of the day remained stable over the observed period. In StirDepth measurement all thresholds raised significantly after the stimulation sessions. The threshold increase of the active electrodes never exceeded that of the inactive control electrode.

CONCLUSIONS

StirDepth measurement is feasible in epiretinal implants. The prolonged stimulation raised no safety concerns in the patient. The threshold increase of both the active electrodes and the control electrode leads one to hypothesise that cognitive or neurophysiological effects are the cause rather than the desensitizing of the retinal network or incipient retinal damage.

Electrical stimulation of inner retinal neurons in wild-type and retinally degenerate (rd/rd) mice

Electrical stimulation of the retina following photoreceptor degeneration in diseases such as retinitis pigmentosa and age-related macular degeneration has become a promising therapeutic strategy for the restoration of vision. Many retinal neurons remain functional following photoreceptor degeneration; however, the responses of the different classes of cells to electrical stimuli have not been fully investigated. Using whole-cell patch clamp electrophysiology in retinal slices we investigated the response to electrical stimulation of cells of the inner nuclear layer (INL), pre-synaptic to retinal ganglion cells, in wild-type and retinally degenerate (rd/rd) mice. The responses of these cells to electrical stimulation were extremely varied, with both extrinsic and intrinsic evoked responses observed. Further examination of the intrinsically evoked responses revealed direct activation of both voltage-gated Na⁺ channels and K⁺ channels. The expression of these channels, which is particularly varied between INL cells, and the stimulus intensity, appears to dictate the polarity of the eventual response. Retinally degenerate animals showed similar responses to electrical stimulation of the retina to those of the wild-type, but the relative representation of each response type differed. The most striking difference between genotypes was the existence of a large amplitude oscillation in the majority of INL cells in rd/rd mice (as previously reported) that impacted on the signal to noise ratio following electrical stimulation. This confounding oscillation may significantly reduce the efficacy of electrical stimulation of the degenerate retina, and a greater understanding of its origin will potentially enable it to be dampened or eliminated.

The response of retinal neurons to high-frequency stimulation

OBJECTIVE

High-rate pulse trains have proven to be effective in cochlear prosthetics and, more recently, have been shown to elicit a wide range of interesting response properties in axons of the peripheral nervous system. Surprisingly, the effectiveness of such trains for use in retinal prostheses has not been explored.

APPROACH

Using cell-attached patch clamp methods, we measured the in vitro response of two rabbit retinal ganglion cell types, OFF-brisk transient (OFF-BT) and ON-OFF directionally selective (DS), to trains of biphasic pulses delivered at 2000 pulses per second (PPS).

MAIN RESULTS

For OFF-BT cells, response onset occurred at ~20 µA, and maximum response occurred at ~40 µA. Interestingly, spiking levels decreased for further increases in amplitude. In contrast, DS cells had a spiking onset at ~25 µA and maintained strong spiking as stimulus amplitude was increased, even at the highest levels tested. Thus, a low-amplitude stimulus train at 2000 PPS (~25 µA) will activate OFF-BT cells strongly, while simultaneously activating DS cells only weakly. In contrast, a high amplitude train (~75 µA) will activate DS cells strongly while suppressing responses in OFF-BT cells.

SIGNIFICANCE

The response differences between cell types suggest some forms of preferential activation may be possible, and further testing is warranted. Further, the scope of the response differences found here suggests activation mechanisms that are more complex than those described in previous studies.

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

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

Responses of retinal ganglion cells to extracellular electrical stimulation, from single cell to population: model-based analysis

Retinal ganglion cells (RGCs), which survive in large numbers following neurodegenerative diseases, could be stimulated with extracellular electric pulses to elicit artificial percepts. How do the RGCs respond to electrical stimulation at the sub-cellular level under different stimulus configurations, and how does this influence the whole-cell response? At the population level, why have experiments yielded conflicting evidence regarding the extent of passing axon activation? We addressed these questions through simulations of morphologically and biophysically detailed computational RGC models on high performance computing clusters. We conducted the analyses on both large-field RGCs and small-field midget RGCs. The latter neurons are unique to primates. We found that at the single cell level the electric potential gradient in conjunction with neuronal element excitability, rather than the electrode center location per se, determined the response threshold and latency. In addition, stimulus positioning strongly influenced the location of RGC response initiation and subsequent activity propagation through the cellular structure. These findings were robust with respect to inhomogeneous tissue resistivity perpendicular to the electrode plane. At the population level, RGC cellular structures gave rise to low threshold hotspots, which limited axonal and multi-cell activation with threshold stimuli. Finally, due to variations in neuronal element excitability over space, following supra-threshold stimulation some locations favored localized activation of multiple cells, while others favored axonal activation of cells over extended space.

Development and evaluation of thin-film flexible microelectrode arrays for retinal stimulation and recording

We have described the development of a flexible microelectrode array with potential applications in the large scale recording of neural signals and in focal electrical stimulation for use as a prosthetic implant in degenerative retinal diseases. The array under test consisted of 61 platinum electrodes of 5 µm diameter with 60 µm spacing connected by 8 µm wide gold tracks encased in a flexible polyimide substrate of 15 µm thickness from which recordings were taken from 16 electrodes. The device was tested on an exposed frog eyecup preparation which is characterized by small retinal ganglion cells of similar dimensions to those present in the human retina. The responses of these cells evoked by photic stimulation consisted of trains of action potentials of high signal-to noise ratio at each of the recording sites. Delivery of cathodal constant voltage pulses and constant current pulses to specific electrodes in the array led to the generation of action potentials in adjacent electrodes, implying that retinal ganglion cells in the proximity had been stimulated. Since prolonged stimulation with supra-threshold voltages impaired neither electrode structure nor retinal function, these results provide a sound basis for scaling up the number of array electrodes to deliver focal electrical pulses to the retina, as would be required by a viable epiretinal prosthesis.

High frequency electric stimulation of retinal neurons elicits physiological signaling patterns

The effectiveness of retinal prosthetics will depend on their ability to elicit patterns of neural activity that can be recognized by the visual cortex. While conventional short-duration pulses activate retinal neurons effectively, many nearby neurons are thought to respond similarly to a given pulse train--a situation that is non-physiological. Use of pulse trains delivered at rates > 1000 pulses per second (PPS) in cochlear prosthetics help to avoid phase-locked responses but have not been evaluated in the retina; here, we explored the response to trains of 2000 PPS. We found that ganglion cells respond robustly to these stimuli but that the properties of the response were highly sensitive to stimulus amplitude. At low amplitudes the response patterns were burst-like while at higher amplitudes elicited spikes had intervals that were more uniform. Because burst responses were insensitive to synaptic blockers, our results suggest that they arise from direct activation. This was surprising because previous studies indicated that burst responses arise only through indirect activation. Thus, our results suggest multiple mechanisms of burst creation may exist. Further, histograms of interspike intervals revealed that the response properties were different in different types of ganglion cells. While further testing is needed, the ability to create different patterns of activity in different types of ganglion cells raises the possibility that more natural spike patterns can be created.

Activation of ganglion cells in wild-type and P23H rat retinas with a small subretinal electrode

Electronic retinal prostheses are being developed for people who become blind due to loss of photoreceptors from the disease retinitis pigmentosa. Previously, we reported on the responses of RGCs in the P23H rat (a model of retinitis pigmentosa) and the Sprague-Dawley (SD) rat to stimulation with a 400-μm diameter electrode (Jensen and Rizzo, 2011). With recent clinical trials now utilizing smaller (50-200 μm) electrodes, I sought to investigate the electrically evoked responses of RGCs in P23H and SD rat retinas with a smaller (125-μm diameter) electrode. Here, I report on the electrically evoked spike activity from RGCs that arose from stimulation of the retinal neural network. With biphasic current pulses of 1 ms per phase, the thresholds for activation of SD rat RGCs ranged from 0.52 to 2.8 μA; thresholds of P23H rat RGCs ranged from 1.2 to 7.8 μA. Median thresholds of RGCs were 1.4 μA in SD rats and 2.5 μA in P23H rats. These thresholds measurements were obtained with the recording electrode placed over the stimulating electrode. I also examined how thresholds of RGCs change as a function of distance (100-500 μm) from the center of the stimulating electrode. The median threshold currents of RGCs were much higher in P23H rats for all distances. What was striking was that the thresholds for activation of RGCs in P23H rat retinas rose much more rapidly. When the recording electrode was only 100-200 μm from the center of the stimulating electrode, the median threshold current of P23H rat RGCs rose by 460%. In contrast, the median threshold current of SD rat RGCs increased only 29%. I also investigated the contribution of photoreceptors to the electrically evoked responses of ON-center RGCs in SD rat retinas by examining the change in RGC thresholds when photoreceptor input to ON bipolar cells was blocked with the mGluR6 antagonist CPPG. At 500-600 μm, CPPG suppressed the light-evoked responses of the RGCs and at the same time increased the amount of current needed to generate an electrically evoked response. Similar to what was observed with SD rat RGCs, CPPG suppressed the light-evoked responses of ON-center P23H rat RGCs. However, the stimulation thresholds were not significantly altered. In conclusion, the data show that the threshold currents for indirect stimulation of both SD and P23H rat RGCs with a 125-μm diameter electrode are much lower than what we found previously with a 400-μm diameter electrode. To achieve high resolution vision with a multielectrode array, the spread of activation of RGCs needs to be limited. Our findings indicate that the spread of activation of RGCs is more confined in the degenerate retina. Lastly, my findings indicate that photoreceptors contribute to the lower stimulation thresholds of RGCs in normal, healthy retinas.

Calcium channel dynamics limit synaptic release in response to prosthetic stimulation with sinusoidal waveforms

Extracellular electric stimulation with sinusoidal waveforms has been shown to allow preferential activation of individual types of retinal neurons by varying stimulus frequency. It is important to understand the mechanisms underlying this frequency dependence as a step toward improving methods of preferential activation. In order to elucidate these mechanisms, we implemented a morphologically realistic model of a retinal bipolar cell and measured the response to extracellular stimulation with sinusoidal waveforms. We compared the frequency response of a passive membrane model to the kinetics of voltage-gated calcium channels that mediate synaptic release. The passive electrical properties of the membrane exhibited lowpass filtering with a relatively high cutoff frequency (nominal value = 717 Hz). This cutoff frequency was dependent on intra-axonal resistance, with shorter and wider axons yielding higher cutoff frequencies. However, we found that the cutoff frequency of bipolar cell synaptic release was primarily limited by the relatively slow opening kinetics of L- and T-type calcium channels. The cutoff frequency of calcium currents depended nonlinearly on stimulus amplitude, but remained lower than the cutoff frequency of the passive membrane model for a large range of membrane potential fluctuations. These results suggest that while it may be possible to modulate the membrane potential of bipolar cells over a wide range of stimulus frequencies, synaptic release will only be initiated at the lower end of this range.

Inner and outer retinal mechanisms engaged by epiretinal stimulation in normal and rd mice

Retinal prosthetic devices are being developed to bypass degenerated retinal photoreceptors by directly activating retinal neurons with electrical stimulation. However, the retinal circuitry that is activated by epiretinal stimulation is not well characterized. Whole-cell patch clamp recordings were obtained from ganglion cells in normal and rd mice using flat-mount and retinal slice preparations. A stimulating electrode was positioned along the ganglion cell side of the preparation at different distances from the stimulated tissue. Pulses of cathodic current evoked action potentials in ganglion cells and less frequently evoked sustained inward currents that appeared synaptic in origin. Sustained currents reversed around E(Cl) and were inhibited by blockade of α-amino-3-hydroxyl-5-methyl-4-isoxazole-proprionate (AMPA)-type glutamate receptors with 2,3-dihydroxy-6-nitro-sulfamoyl-benzo(f)-quinoxaline-2,3-dione (NBQX), γ aminobutyric acid a/c (GABA(a/c)) receptors with picrotoxinin, or glycine receptors with strychnine. This suggests that epiretinal stimulation activates glutamate release from bipolar cell terminals, which in turn evokes release of GABA and glycine from amacrine cells. Synaptic current thresholds were lower in ON ganglion cells than OFF cells, but the modest difference did not attain statistical significance. Synaptic currents were rarely observed in rd mice lacking photoreceptors compared to normal retina. In addition, confocal calcium imaging experiments in normal mice retina slices revealed that epiretinal stimulation evoked calcium increases in the outer plexiform layer. These results imply a contribution from photoreceptor inputs to the synaptic currents observed in ganglion cells. The paucity of synaptic responses in rd mice retina slices suggests that it is better to target retinal ganglion cells directly rather than to attempt to engage the inner retinal circuitry.

Continuous electrical stimulation decreases retinal excitability but does not alter retinal morphology

Retinal prostheses aim to provide visual perception through electrical stimulation of the retina. Hence they have to operate between threshold charge density and maximum safe charge density. To date most studies in the retina have concentrated on understanding the threshold, while stimulation safety has predominantly been studied in structures other than the retina. Toward this end, the present study focuses on determining the effect of continuous electrical stimulation of the retina both on retinal morphology and on the electrically evoked responses in the superior colliculus in a rodent model. The results demonstrate that the retina is able to tolerate 1 h long stimulation with only minor changes evident in retinal histology when examined three to 14 days later, even at charge densities (0.68 mC cm(-2)) above the safe limit of platinum delivered at high stimulus frequency (300 Hz). However, this continuous electrical stimulation causes an elevation in the threshold of the electrically evoked response in the superior colliculus, indicating some form of adaptation to continuous stimulation.